Phthalocyanine Aggregation

Phthalocyanine Aggregation ARTHUR

109

w. SNOW

Chemistry Division, Code 6123, Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, DC 20375, USA

I. II. III. IV.

V.

VI.

VII.

VIII.

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical Aspects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nature of the Aggregation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Electronic Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Monomer-Dimer Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Overall Equilibrium Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Trimers and Tetramers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Fluorescence Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Nuclear Magnetic Resonance (NMR) Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Vapor Pressure Osmometry (VPO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Calorimetry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Other Methods 1. Light Scattering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. X-ray Scattering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . 3. Diffusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. ESR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.·Aqueous Systems B. Nonaqueous Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correlations with Chemical Structure/Composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Complexed Metal Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Peripheral Group Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Beta-Peripheral Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Alpha-Peripheral Substitution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Crown Ether Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Multinuclear Phthalocyanines 1. Binuclear Phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Tetranuclear Phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Avoiding Aggregation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Axial Substitution and Metal Ion Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Bulky Peripheral Beta-Substitution 3. Alpha-Peripheral Substitution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correlations with Physical Environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Electrolytes and pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Optical Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.. Photodynamic Therapy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Optical Limiting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Porphyri n Handbook K.M. Kadish, K.M. Smith, R. Cuilard, Eds. Volume 17/ Phthalocyanines: Properties and Materials

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© 2003 Elsevier Science (USA) All rights reserved ISBN 0-12-393220-3

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I. Introduction Conceptually, aggregation is generally viewed as an association of molecules in solution as represented by the sketch in Figure 1. In much of the early phthalocyanine research, phthalocyanine compounds for the most part had the character of being highly crystalline and insoluble or, in a very few cases (i.e. lithium or magnesium phthalocyanine), of being soluble and noncrystalline. In fact, the first molecular weight measurement on a phthalocyanine compound was performed in Linstead's laboratory as a boiling point elevation experiment on magnesium phthalocyanine dissolved in naphthalene. 1 This measurement was consistent with the magnesium phthalocyanine dihydrate in the monomeric form. As such, aggregation of phthalocyanine compounds was not observed or suspected until soluble phthalocyanines were available and spectroscopically studied. The first generally soluble phthalocyanine compounds were those made watersoluble by peripheral sulfonic acid substitution. This development inducted phthalocyanines into the family of ionic dyes in which aggregation was a known and actively researched phenomenon dating back to the early 1900s. The phthalocyanine dye's unique features of symmetry, metal complex formation, and a relatively strong and narrow long wavelength electronic transition in the visible spectrum make it an especially interesting system for dye aggregation study. Further developments with the nature (neutral, cationic, metal coordinating, etc.), size, substitution position, and number of peripheral substituents have profound influences on aggregation. In the aggregated state, electronic intermolecular interactions alter the physical and chemical properties compared with those of the unaggregated state. These involve changes in color, photodynamics, and catalytic activity. Correlated applications in dyes, optical filters, optical limiters, and photodynamic therapy are of particular importance. The intent of this chapter is to review investigations of phthalocyanine aggregation with the purpose of identifying concepts, measurement methods, correlations with molecular structure, correlations with physical

Monomer

Dimer

Higher Aggregates

Figure 1. Sketch depicting phthalocyanine aggregation.

environment, and impacted applications that may be useful to researchers in this field. In this context, two review sources have been found to be very useful. While no reviews dedicated to phthalocyanine aggregation were found, several reviews of phthalocyanine chemistry have sections of useful information.v ':' Another source of useful information is reviews covering aggregation of . . an d orgaruc . d yes. 14-20 Wh·l1 e th ese reviews porphyrins may not specifically address phthalocyanine compounds, they were found to have valuable examples of aggregation models and experimental methods that are relevant. This chapter is divided into eight sections. Following the introduction, the second section provides a brief historical background on dye aggregation and, in this context, what developments have occurred specific to phthalocyanine compounds. The third section describes the nature of the phthalocyanine aggregation process, how an aggregate is defined and the issues of its structure and bonding. The next section covers various measurement methods used to characterize the aggregate. The fifth section compiles results of phthalocyanine aggregation measurements from the literature. The next two sections attempt to establish correlations of aggregation with phthalocyanine structure and with the physical medium in which it occurs. The final section describes phthalocyanine applications impacted by aggregation.

II. Historical Aspects The phenomenon of organic dye aggregation in solution has been known for a very long time and predates the 1907 discovery of phthalocyanine.r" The observation that a change in the color of a dye in solution as a consequence of solvent or temperature variation could be correlated with an aggregation of dye molecules was first proposed in 1888.22 The spectroscopic characteristics of reversible color changes caused by concentration changes, different alcohol-water combinations, addition of salt, and temperature variation for a variety of water soluble organic dyes were reported in 1908, and it was proposed that these changes were caused by reversible process of solute disaggregation.i" During the ensuing 25 years, studies on hundreds of organic dyes revealed that very few, if any, conformed precisely to . . . 24 Beer's Law over any extensrve range In concentration, The discrepancy from the Beer's Law concentration vs. absorbance plot was usually accompanied by a concentration dependent change in shape of the dye spectrum.

109 / Phthalocyan ine Aggregation

It was during this time in 1927-1929 that the copperv' and irorr'" phthalocyanine compounds were prepared and isolated and Linstead began work on elucidating its structure.r/ Concurrently and during the next 10 years, reports of phthalocyanine compounds made soluble by peripheral substitution appeared in patent literature.i The substituents utilized included sulfonates and carboxylates for solubility in water and alcohol and methoxy, ethoxy, and phenoxy moieties for solubility in less polar organic solvents. The first phthalocyanine visible spectrum was published in 1937,28 and it clearly shows evidence of aggregation although not identified as such. The spectrum of sulfonated zinc phthalocyanine was more quantitatively studied seven years later with the finding of an aggregation dependence on solvent (methanol vs. water), temperature, and pH. 29 It is in this report that the notions of dye aggregate size, structure, and bonding with the concepts of "dimer," "coplanar," and "optical coupling" are first being applied to a phthalocyanine compound. A quantitative understanding of organic dye aggregation appears to have originated with Scheibe's postulate in 1938 that a reversible dimerization is responsible for the growth of a second f3-band and that higher polymer aggregates are responsible for the appearance of a third y-band.i" These bands became stronger relative to the a-band as the dye concentration increased. To test this postulate he applied the law of mass action to the dimerization equilibrium with the assumption that the a-band absorbance was proportional to monomeric dye concentration, [M]: K _ [M 2J_ (1 - (ca/ccxJ)C 2 - [Mf ((ca/ccx:JC)2

(1)

where [M 2] is the dimer concentration, K 2 is the dimerization constant, £a is the extinction coefficient of the a-band, £00 is the extinction coefficient of the a-band at infinite dilution, and C is the analytical concentration of the dye. He was able to obtain the requisite linear logarithmic plot of (l-(£a/£oo))C vs. (£a/£oo)C when a limited concentration range was used. The nonlinear behavior at higher concentrations was attributed to the formation of higher "polymers." This "polymerization" concept represented a significant departure from the viewpoint by Holmes 24 and other early investigators 31,32 that this organic dye spectral anomaly originated from a tautomeric equilibrium. In 1941, Rabinowitch and Epstein significantly advanced the depth and breadth of this "polymerized dye" concept of aggregation.v' Using law of mass action for monomer-dimer

equilibrium model and spectroscopic band assignments, they quantitatively determined dimerization constants and, by variable temperature measurements, related the dimerization constants to the enthalpy and entropy changes for this process. The magnitude of these thermodynamic parameters (-7 kcal/mole and -9 eu) were found to be comparable to those observed for hydrogen bonding. While the dimerization had been postulated to be driven by additive forces of the weak Van der Waal type." it was shown how these are amplified by the dye structure in a coplanar stacked arrangement. These forces were approximated to an oscillator strength· absorption wavelength term (f2.'A 3) in the theory of London dispersion forces from which a very useful potential energy diagram of the dimerization process was derived. The distance of closest approach and the effect of the dielectric constant of the medium could be quantified by the diagram. Another key finding was the quenching of monomer fluorescence by dimer formation. In 1944 Sheppard and Geddes challenged that the use of the simple law of mass action to describe organic dye aggregation was quantitatively inadequate." Using the data of Rabinowitch and Epstein:' along with their own, they demonstrated that, while it' is possible to obtain the linear logarithmic plot discussed in the above paragraph, a rigorous correspondence of the slope to the required value of 2 for the dimerization was not obtained. The values ranged from 1.4 to 1.9 and were dependent on how £00 is determined (two independent methods employed) and the concentration range for the points included in the plot. While this issue may seem to be a small technical detail, it remains a tricky issue to demonstrate, and it continues to plague accurate determinations of equilibrium constants and aggregation numbers by this spectroscopic method. Sheppard and Geddes concluded that, in spite of this quantitative discrepancy to the law of mass action, the dimerization hypothesis offered the best interpretation of the dilution anomaly of organic dyes. They went on to recommend the need for an "activity factor" that is related to solvent-dye interactions for rigorous compliance with the law of mass action. Other researchers also concluded that the simple monomer-dimerization equilibrium was an ovcrsimplificationr" During the next decade, there appeared studies of several organic dyes supporting the existence of dimers by observing conformity with the law of mass action." and two significant papers emphasizing data handling methods.Y Over the past 40 years, many investigations of organic dye aggregation in general and phthalocyanine compounds in specific

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have been conducted. In the remaining paragraphs of this section, a brief listing of selected developments in dimerization constant measurements and in aggregation-inspired peripheral group designs are chronologically presented as historical highlights of this area of phthalocyanine chemistry. In 1961 the first report of phthalocyanine dimerization constants appeared, which involved a series of tetrasulfonated phthalocyanine compounds in water.j" This was followed a year later by a report of constants for successive equilibrations of monomer to dimer to tetramer of a free-base tetrasulfonated phthalocyanine in water.r" The dimerization constants for the aqueous system are on the order of 106 to 107 M- 1, and changing to ethanol as solvent reduces the dimerization constant by a factor of 5000. The tetramerization constant is on the order of 5 x 104 M- 2 . In 1970 the kinetics and thermodynamics of a cobalt tetrasulfonated phthalocyanine-water system were determined by temperature jump and equilibrium spectrophotometric measuremcnts.l" The dimerization constant again approached the 106 to 107 M- 1 range determined by each method, and the enthalpy and entropy reported were -14 kcal/ mole and -18 eu, respectively. These values are close to those generally anticipated from work on organic dyes by Rabinowitch and Epstein'" mentioned above. The first aggregation measurements on a neutral phthalocyanine dye in an organic solvent were conducted in 1972. 4 1 This involved replacement of the sulfonate peripheral group by the octadecylsulfamide group. While this moiety is still a very polar group, the ionic character was removed. The dimerization constant remained high (10 6 M- 1 in carbon tetrachloride) but solvent sensitive (10 4 M- 1 in benzene). Less polar substituents at this position will typically lower the dimerization constant to 103 in nonpolar solvents. While electronic spectroscopy continues to be the primary method used to characterize phthalocyanine aggregation, other methods have been used to study the dimerization process as well as to distinguish higher aggregation from dimerization. These also provide complementary information regarding the structure of the aggregate. These methods include ESR, NMR and fluorescence spectroscopies, mass spectrometry, X-ray scattering, light scattering, calorimetry, diffusion, and vapor pressure osmometry. They will be discussed in a separate section. In the area of innovative phthalocyanine structural design, some very interesting approaches toward influencing or controlling aggregation have been devised, six examples of which are depicted in Figure 2. Binuclear

and multinuclear phthalocyanines were first synthesized in 198542 and 1987. 4 3 These phthalocyanine compounds display varying degrees of intramolecular and intermolecular association depending on structural features of the bridging linkage between phthalocyanine rings and the concentration. In 1986 phthalocyanine compounds with peripheral crown ether substituents were synthesized in an approach to obtain well-defined aggregated structures with controlled polydispersities. 44-46 The aggregate formation and structure are regulated by introduction of alkali metal ions that complex at the peripheral crown ether sites. Approaches toward limiting or eliminating aggregation have included the steric crowding of the phthalocyanine ring by substitution at its a-position peripheral sites, creating a peripheral dendrimer or other bulky peripheral structure to sterically impede association of phthalocyanine rings and blocking of one face of the phthalocyanine ring with a covalently bonded cap. Octa-o-substituted phthalocyanines have been known since 198747 and display diminished tendency to aggregate depending on chain length of the substituent." In 1997 growth of dendrimer substituents at the periphery of the phthalocyanine ring was first reported as an approach to suppress aggregation.l" This approach has had mixed results depending on the identity of the dendrimer. A closely related highly sterically hindered approach using pentaphenylbenzene substituents at four fj-periphery positions has also shown promise for suppressing aggregation. 50 The approach of blocking one face of the phthalocyanine structure with a cap represents a way of limiting aggregation to dimer formation.P'

III. Nature of the Aggregation Process Phthalocyanine aggregation is usually depicted as a coplanar association of rings progressing from monomer to dimer and higher order complexes and driven by nonbonded attractive interactions. A representation of this process is illustrated in Figure 1. As such, it implies assumptions pertaining to chemical structure (both monomer and aggregate) and dynamics. The monomer structure, if incorporating a complexed metal ion, is presumed to be free of covalently bonded axial ligands that would interfere with the close approach of a second monomer. In solution, this presumption generally excludes group IV phthalocyanine complexed ions with octahedral coordination, particularly silicon. In this case the axial ligands block cofacial interactions

133

109 / Phthalocyan ine Aggregation

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Figure 2. Six examples of innovative phthalocyanine structural designs with regard to influencing aggregation. (A) Binuclear phthalocyanines with linking structures of variable lengths and steric orientations.Y (B) Crown ether phthalocyanine dimer with rings held in eclipsed orientation by complexed potassium ions. (C) Octa-a-substituted 47 phthalocyanine wherein adjacent substituents are sterically forced to reside above or below the plane of the ring. (0) Phthalocyanine dendrimer where generations of multiple branched substituents can impede aggregation.l" (E) Pentaphenylbenzene substituted phthalocyanine wherein an orthogonal substituent orientation would hinder aggregation.i" (F). An oxyethylene capped phthalocyanine wherein one face of the phthalocyanine ring is blocked from participating in cofacial association.l '

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and promote an enhanced solubility in common organic solvents. 52 The aggregate structure is a feature of interest in the context of organic dyes in general and phthalocyanine compounds in particular. Depending on the geometry of the stacked arrangement, such structures have been referred to as H- and J-aggregates and will be described below. The aggregation number ranging from dimer to higher aggregates (usually no more than 10-20 units) is an important feature of both the structure and the dynamics. The dynamics depicted in Figure 1 represents two sequential and reversible steps of association, which are usually assumed to follow the law of mass action. In the formation of aggregates larger than the dimer, there is usually a distribution of aggregate sizes, the breadth of which depends on the relative size of the individual aggregate formation constants. In treating such dynamics, the expedient and simplifying practice has been to use a model under appropriate circumstances where only monomers and dimers are present or one where the aggregates of polydisperse size may be treated as one or two single species. In UV-Vis and NMR spectroscopic analysis this data treatment has served quite well. Most organic dyes including phthalocyanines display. anomalous spectral properties in the visible spectrum but have several features in common. These anomalous features appear as concentration dependent bands that have been labeled M, D, H, and J. This nomenclature was derived from the study of cyanine dyes, some members of which display each of these bands at different concentration ranges. As an encompassing example, the absorption spectrum of 1,1/-diethyl-2,2/cyanine chloride is shown in Figure 3 for concentrations where all of these bands are observed. 53 At the lowest concentration the M band is attributed to the monomeric form of the dye. As the concentration increases, this band loses intensity and a new band appears on the short wavelength side of the M band. This is referred to as the D band and is attributed to the dimeric species of the dye. With a further increase in the concentration, a third band or partially resolved shoulder may appear on the short wavelength side of the D band. This band is referred to as the H band ("H" for hypsochromic) and is associated with larger aggregates. These new bands should not be considered intensifications of the shoulders already visible in the M band that result from vibrationally coupled transitions associated with the monomer (although this interpretation was proposed by Sheppard and Geddes 34 ) . At very high concentrations the spectra of certain cyanine dyes display the growth of a narrow intense band on the

-I

···············2 10

---3 ----- 44

,

Figure 3. Absorption spectrum of aqueous solutions of 1,1' -diethyl2,2'-cyanine chloride illustrating monomer M, dimer 0, blueshifted aggregate Hand redshifted aggregate J bands: (1) 1.3 x 10- 5 M; (2) 1.3 x 10-4 M; (3) 7.1 x 10- 3 M; (4) 1.4 x 10- 2 M.53

long wavelength side of the M band. This band is designated the J band (presumably for Jelley - one of the first workers to investigate this redshifted band.") and is attributed to polymeric aggregates involving very large numbers of dye molecules. The appearance of this remarkable J band is accompanied by a strong fluorescence and an increase in viscosity." When the concentrations between the decline and emergence of bands are gradually changed, a family of absorption curves with two or more isosbestic points is obtained indicating that two colored species are in equilibrium. Thus, the progression of structures assigned to these bands are monomer, dimer, H-aggregate (oligomeric), and J-aggregate (polymeric). The H band or H-aggregate is an observation general to many organic dyes including phthalocyanines, but the J band or J-aggregate is observed under a very narrow set of conditions with only selected dyes, a few of which are known for phthalocyanines. The direction of 2wavelength shift of these aggregated bands is a consequence of the relative orientations of the phthalocyanine structures in the aggregate. As illustrated in Figure 4, these aggregates have been depicted as a linear stack of coplanar molecular units with a separation distance of approximately 3.3 A. This is similar to the separation found in crystallographic studies. 55 Using this polymeric structure, an interpretation of blue and the red spectral shifts resulting from the formation of

109 / Phthalocyan ine Aggregation

135

LINEAR POLYMERIC AGGREGATES: ARRANGEMENT OF MOLECULAR LONGAXES

~~~a~oo

a=90°

----~/-~E~

CENTERS H - AGGREGATE (

J - AGGREGATE

SHORT WAVELENGTH) SHIFTED

(

LONG WAVELENGTH) SHIFTED

Figure 4. The orientation of transition dipole moments (or molecular long axes) for ·Iinear H- and J-aggregates. (Reprinted with permission from Emerson, E. S.; Conlin, M. A.; Rosenoff, A. E.; Norland, K. S.; Rodriguez, H.; Chin, D.; Bird, G. R. j. Phys. Chern. 1967, 71, 2396. Copyright 1967, American Chemical Society.)

the H- and J-aggregates was made employing the molecular exciton model. 56 An angular dependent coupling occurs between transition dipole moments (coincident with the long molecular axis) of adjacent dye structures along the stacking axis of the aggregate. This coupling is dependent on the tilt angle a (angle between the long axis of an individual molecule and the axis of the aggregate). The axis of the aggregate is defined by a straight line passing through the centers of each molecule in the aggregate stack. The tilt angle may vary from 90° in an H-aggregate to a value approaching 0° in a J-aggregate, and the transition intensity of the exciton accumulates in the respective blue or redshifted directions as these angular extremes are approached. A qualitative energy level picture for a transition from a monomeric system to a dimeric system is illustrated by Figure 5, where two equivalent dye molecules in a dimer aggregate are configured in these extremes as "sandwich" and as "end-on" dimers.i" While the monomer has a single excited state, the dimers have two possible excited energy states: one in which the transition moments of the two molecules are parallel, and one where they are antiparallel. In the "sandwich" dimer the higher energy state is that where the transition moments are parallel, making it a strongly allowed and blue shifted transition. The reverse situation is the case for the "end-on" dimer making the lower energy transition the more strongly allowed. The magnitude and direction of the dimer transition shift from that of the monomer is dependent on the angle a and has been quantified for a dimer system according to eq. 2:55 ,56 (2)

A

F

Monomer

Dimer Sandwich

End-on

Figure 5. Simple picture correlating energy levels on dimer formation with parallel and antiparallel orientations of molecular transition moments in H- and J-aggregate structures. (Coates, E. j. Soc. Dyers and Col. 1969, 85, 355 - reproduced by permission of the Society of Dyers and Colourists.)

where ~ v is the spectral shift from monomer absorption, h is Planck's constant, r is the separation of molecular centers, and (m 2 ) is the transition dipole moment of the monomer. Accordingly, at a tilt angle of 57° the exciton shift becomes zero. With knowledge of the interplanar spacing and a measurement of ~ v, it is possible to calculate a and r for the aggregate structure using this model. In a very elegant study, this equation has been employed in the analysis of a novel naphthalocyanine system for which both H- and J-aggregates have been observed.i" An increase in the bulkiness of the peripheral groups (methyl-+ ethyl-e- butyl-+ hexyl) resulted in a systematic variation of ~ v from 62 to 18 nm with corresponding variations of a and r from 15.5 to 9.9° and from 15.0 to 23.3 A respectively.

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IV. Measurement Methods Measurements of phthalocyanine aggregation usually address issues of aggregate size, structure, and formation constants. Various methods described in this section address one, two, or all three of these aspects and all have individual limitations. The methods vary from being aggregate specific (e.g. a spectroscopic signature for a monomer, dimer, or higher aggregate) to particle counting (e.g. colligative properties or light scattering) or size/transport discriminating (e.g. a diffusion experiment). The limitations are the ability of a method to discriminate a monomer from a dimer or from trimer, etc. and the concentration window where the method has sensitivity. It is frequently necessary for one method to complement another to get a complete picture of the aggregation. For example, Figure 6 presents three sets of measurements by different techniques on a solution of free-base tetracumylphenoxy phthalocyanine in toluene, which cover a molar concentration range varying six orders in magnitude. The x-axis also provides a correlated weight percent concentration where relevant. The left axis quantifies UV-Vis measurements, the right axis quantifies NMR chemical shift measurements, and the shaded area represents the concentration window for vapor pressure osmometry (VPO) measurements. UV-Vis extinction coefficient measurements of the Q-band maximum at 703 nm cover a range of 10- 7 to 10-2 M and clearly

display a significant change. When data over the 10- 6 to 10- 5 concentration range are worked out according to a monomer-dimcr equilibrium model, reasonably good agreement with the model is found, and an aggregation number of 2 is obtained. The second set of measurements is an NMR measurement of the chemical shift of the cavity proton resonance, DC, over a 10- 5 to a 10- 1 M concentration range in toluene. Clearly, there are quite large changes occurring over this concentration range, and they correlate with an additive ring current effect of a stacked aggregate. However, this is a very different concentration window from the UV-Vis measurements and it is not reasonable to expect the monomer-dimer model to be appropriate. Higher aggregated species should be present, but it is very difficult to quantify their size with the NMR measurement. While VPO does not have a very wide concentration window, in this case it provides a number average aggregate number that is very useful in interpreting the NMR data. Equilibrium models used to interpret the data are usually of two types: sequential aggregation and an overall equilibrium. The sequential aggregation is simply a sequence of equilibria generating dimers, PC2, trimers, PC3, tetramers, PC4, and possibly slightly higher oligomers as illustrated in eqs 3-5. PCl + PCl~

PC2

K _ [PC2] 2 - [PClf

(3)

-8

2.0 p

E0 ~

1.6

€:703 /

/

E L-

:::

-7

Be

<,

a>

/

6

0

-

/

/

E -5 Q.

1.2

Q.

lC)

u

'0 )(

-4 ~

0.8

r<)

0

r-

\.LJ

d

0.4

I

-3

I

-2

10. 3 (.070/0) CONCENTRATION

Molarity and

10- 2 (0.90/0)

10. 1 (9.3%)

(We,ight %)

Figure 6. UV-Vis, NMR, and VPO measurements of free-base tJ-tetracumylphenoxy phthalocyanine (H 2Pc(bOC6H4C(CH3hC6Hs)4) aggregation in toluene solution illustrating effective concentration ranges for each measurement.

137

109 / Phthalocyan ine Aggregation

(4)

are described. Some factors limiting quantitative assessment of the results are also considered.

(5)

A. ELECTRONIC SPECTROSCOPY

In most studies the objective is to obtain a measure of the aggregating tendency of a particular compound under a particular set of conditions, and the dimer formation constant, K 2 , serves this purpose. Formation constants of higher aggregate complexes, K 3 , K 4 , etc., are occasionally measured when conditions permit. However, when K 3 and K 4 are of comparable or greater magnitude than K 2 , an overall aggregation equilibrium model is used as illustrated below. (6)

When this is the case, the aggregation number, n, becomes a more important parameter than the formation constant, K n . In this section various aggregation measurement methods and in some cases data workup techniques

E o

1.5

The observation of phthalocyanine aggregation in solution by electronic spectroscopy usually takes the form of repetitive scans of the Q-band with systematic variations in concentration, temperature, or chemical additive (cosolvent, salt, surfactant, disaggregating agent, acid/base, etc.). The measured absorbance is converted to the extinction coefficient, and a family of spectroscopic curves is generated. Examples of these curves for a series of chloroform solutions of free-base and lead tetracumylphenoxy phthalocyanine, H 2Pc(fJCP)4 and PbPc(fJ-CP)4 respectively, with varying concentrations are displayed in Figures 7a and 7c. As the H 2Pc(fJ-CP)4 solution becomes more concentrated, the narrow Qx (667 nm) and Qy (703 nm) components of the Q-band diminish in intensity, and a broader band at 640 nm emerges and increases in intensity. Within this concentration range, two isosbestic points at 650 and

3.9 X 10-6M

Eo 1.5

3.1 X 10.5 M

(5

8.3x 2.5 X 1.0 X 1.8 X

(c) PbPc(I3-CP)4

5.3 X

CD

CD

(5

E

E

"'i:::::

"'i:::::

~ 1.0

1.6x10-4M

b....

4.9 X 10-3 M 8.0 X 10-3 M 1.3 X 10.2 M 4.3 X 10-2 M

~ 1.0

~

1.3 X 10.3 M 2.4 X 10.3 M 4.0 X 10.3 M

~ 0.5

10-6 M 10.5 M 10-4 M 10-4 M 10-4 M

....

o

~ 0.5

Ol,-_"':::::::~==-_----r_---:::3iiiiii;;;;;=;:::~~=--T

500

600

700

500

800

600

§ 1.5

(b) H2Pc(I3-CP)4

/Monomer

CD

900

E o

1.5

(d) PbPc(I3-CP)4

/Monomer

CD

E

E

~ 0 ....

800

(5

(5

"'i:::::

700 WAVELENGTH (nm)

WAVELENGTH (nm)

"'i:::::

1.0

~

~ ....0

,

Dimer

~ 0.5

1.0

~

~ 0.5 O........,...;;;::;;;;;....-..=::;,...-----r--~-,.----_r'

500

600 700 WAVELENGTH (nm)

800

500

600

700

800

WAVELENGTH (nm)

Figure 7. UV-Vis spectrum concentration dependence and calculated monomer and dimer sp~ctra of free-base tetra-f3-cumylphenoxy phthalocyanine (H 2Pc(f3-CP)4) and lead tetra-f3-cumylphenoxy phthalocyanine (PbPc(f3-CP)4) in chloroform solution at 23 "C.

900

Snow

138

715 nm appear, which are characteristic of a twocomponent equilibrium with overlapping absorption bands. This broad b1ueshifted band is characteristic of an H-aggregated structure and is attributed to a dimer aggregate. The formation of higher aggregates occurs as the concentration is further increased, but the perturbation on the dimer spectrum shape is relatively small and a loss of the well-defined isosbestic points occurs very gradually. This H-aggregate formation is common to most phthalocyanine compounds (metal-substituted as well as free-base), and there are many qualitative observations in the literature similar to the example in Figure 7 depicting this spectroscopic effect as concentration, temperature, or chemical additive causes a phthalocyanine solution to aggregate. The PbPc(,BCP)4 example in Figure 7c depicts a redshift of the Q-band accompanying dimer formation. Although very unusual, there are other examples of phthalocyanine compounds that display an aggregation band redshifted from the monomer Qvband.i" When it is desired to make comparisons of aggregating tendency either between different phthalocyanine compounds or with other organic dyes, a quantitative parameter is needed. The aggregation formation constant, particularly for the dimer, or the aggregate number in cases of very strong aggregation can serve this purpose. Beyond these, it is also possible to determine thermodynamic parameters for the aggregating process, which provide a greater depth of understanding of the chemistry. 1. Monomer-Dimer Model The simple monomer-dimer equilibrium model is described in eq. 3 and applies only where these two species are present. All methods devised for the determination of K 2 are derived from the combination of three basic equations. The first equation is the conservation of mass: (7)

where Co is the analytical concentration of phthalocyanine and [Pc.] and [PC2] are the respective equilibrium concentrations of phthalocyanine monomer and dimer. [Pc.] and [PC2] are often expressed as a single parameter defined as the mole fraction of monomer, x. [Pcd [Co]

x==--

1 - x == 2[PC2] Co

(8)

The second basic equation is the expression for the dimer formation constant (eq. 9).

(9)

The third basic equation is an assumption of the additivity of optical absorbances and a Beer's Law relationship between absorbance and concentration for the monomer and dimer species (eq. 10): (10)

where 8 is the observed analytical molar extinction coefficient per phthalocyanine structural unit, and 81 and 82 are the respective monomer and dimer extinction coefficients per phthalocyanine structural unit. Use of these equations to obtain dimer formation constants may be traced back to Scheibe" and Rabinowitch and Epstein,33 and variant forms have appeared through the years as working equations, which take advantage of iterative computing power. The more generally used equations are described below. The data acquisition is a series of absorbance measurements or spectra over a range of concentrations extending to the highest level of dilution where an accurate measurement can be made. The simplest data handling method is direct use of eqs 9 and 10. This involves a plot of 8 vs. x where x is varied until the best conformity to a straight line is obtained according to a least squares determination. The corresponding values of x and Co may then be used to calculate K 2 from eq. 9. However, scatter in the data at low concentrations and an uncertainty regarding higher aggregate formation at high concentration make this method highly susceptible to inaccuracies. An experimental determination of 81 may be approximated by measurements on very dilute solution or from measurements using a solvent where Beer's law is followed. In the former case, this involves a continuity plot such as that in Figure 6 wherein an extrapolation to zero concentration is made. This sort of determination is attended by some experimental difficulties. To obtain a dilution where at least 95% of the phthalocyanine is in the monomeric form, eq. 9 indicates that for a K 2 of 106 M- 1 the corresponding concentration is 2.8 x 10- 8 M. For most phthalocyanine compounds this condition requires a 10 em path length cell and care that this cell is properly positioned with respect to the focal point of the instrument. Light lost to refractive effects may cause significant error when using cells of this path length. Another experimental difficulty involves phthalocyanine stability and/or surface adsorption at these high degrees of dilution. The second approach, where the monomer

139

109 / Phthalocyan ine Aggregation

extinction coefficient is measured by employing a disaggregating solvent, if available, assumes that the monomer spectrum and absorbance are insensitive to the character of the solvent. Comparison of extrapolated monomer extinction coefficients of organic dyes in aqueous solutions with those directly determined in alcohol solution has indicated that this assumption is generally not validr'" If used, such an assumption should be carefully checked. The most prudent practice appears to be to obtain 81 as extrapolated from the continuity plot as its initial determination. Equations 9 and 10 may then be used a little more effectively to determine the dimerization constant although the formation of higher aggregates may still cause significant error. One method of determining that monomer and dimer are the only significant species in a concentration range for a corresponding data set was devised by West and Pearce.i" This method is an iterative deconvolution of experimental spectra into dimer and monomer components using the spectrum attributed to the monomer species as the determinant of monomer concentration. The difference between the total phthalocyanine and monomer concentrations is assumed to be dimer concentration. A simple logarithmic plot of monomer vs. dimer concentrations must be linear with a slope of 2 to demonstrate that higher aggregate formation is not significant. With a monomer spectrum determined and a monomer-dimer equilibrium demonstrated, both the equilibrium constant and a dimer spectrum may be obtained using eqs 9 and 10. Examples of monomer and dimer spectra for H 2Pc(j3-CP)4 and PbPc(j3-CP)4 are displayed in Figures 7b and 7d respectively. The dimer spectra are not symmetrical bands about a single maximum but consist of two branches: a higher frequency P-branch and a lower energy N-branch, according to West and Pearce.I''' These branches are a result of a splitting of the phthalocyanine excited state when coupled in the coplanar dimer structure (Figure '5), and which branch is the more favored in the electronic transition depends on the relative orientation of the two phthalocyanine rings (Figure 4). For a dimer structure where the tilt angle is 90°, the higher energy transition is allowed and the lower energy is forbidden. Deviations from coplanarity allow weaker transitions to the lower excited state to be observed. The H 2Pc(j3-CP)4 dimer spectrum displays a blueshifted maximum and partially resolved features at the longer wavelengths, which are consistent with this interpretation. The PbPc(j3-CP)4 dimer spectrum displays a redshifted maximum but with significant blueshifted weaker features indicative of a less ordered structure with a smaller tilt angle. This is a

consequence of the large size of the plumbus ion (1.20 A ionic radius) that causes it to reside 0.4 A above the plane of the phthalocyanine ring. In a computerized approach to monomer-dimer equilibrium, Monahan and Blossey have consolidated eqs 7, 9, and 10 into eq. 11 as followsr''' 8 _

-

[l(1 +

8K2CO) 4K o 2C

-

1]

81

+

[1 + 4K

2CO -

4K

Jl

o

2C

+ 8K2CO]

82

(11)

where all terms are the same as previously defined. In this calculation K 2 is varied in a systematic manner and the best-fit 81 and 82 at each wavelength are calculated according to eq. 11 to yield corresponding monomer and dimer spectra. These spectra are combined to yield calculated values of 8 for each wavelength, which can be compared to the experimental 8 values. A minimum standard deviation between calculated and experimental 8 values determines which dimerization constant provides the optimum agreement with experiment. When using this method, two issues should be carefully considered: (1) if a pure monomer spectrum is not experimentally attainable, many calculations may be necessary to converge on a best-fit dimerization constant and (2) a concentration range must be selected where aggregates larger than the dimer are not present in significant quantity to distort the monomer-dimer equilibrium model. In a very careful study of the dimerization of cobalt(II) tetrasulfonated phthalocyanine in water, Yang, Ward, and Seiders provide a good example of handling the issues of obtaining the pure monomer spectrum and of selecting a concentration range where larger aggregates do not interfere with the determination of the dimerization constant.I" They employed eq. 11 and used a nonlinear least-squares program to obtain a best-fit of measured absorbances or extinction coefficients with the corresponding analytical concentrations. The value of 81 was determined independently at high dilution (r-v 10- 7 M) in both water and water-alcohol mixtures and at elevated temperatures (65-75°C). With 81 fixed, 82 and K 2 were allowed to vary to obtain the best-fit values using a set of 18 absorbance-concentration measurements over a concentration of 9 x 10- 8 to 2 X 10- 5 M and a temperature range of 5-75 °C in 10°C intervals. The value of 82 was found to have a temperature dependence attributed to the formation of higher aggregates. This temperature dependence involved a slight but systematic decrease in 82 at the lower temperatures « 25°C) for the higher

140

Snow

concentrations. Values of C2 became consistent when the < 25°C data sets were removed. With values for both C1 and C2 fixed, calculated dimerization constants were obtained for each data set and shown to be independent of concentration at each temperature. Comparison with earlier studies indicated that when this more accurate determination of C2 was not practiced, the dimerization constant is depressed by a factor of 10. It was also noted that isosbestic points continue to be observed at this 2 x 10- 5 M concentration where higher aggregate formation is detected. Thus, the isosbestic point observation does not ensure a pure monomer-dimer equilibrium. Measurement of the enthalpy and entropy of the dimerization process provides further insight and understanding of the magnitude of the chemical interactions and their correlation with molecular structure. The temperature dependence of the dimerization constant has been used to obtain these parameters by way of the traditional In K 2 vs. I/Tplot or by more refined methods such as in eq. 12.64 ,65

single component and the mass balance, equilibrium constant expression and Beer's Law relation analogs to the monomer-dimer model are employed. Mataga derived eq. 13, which is useful for determining an aggregation number via an approximation then subsequently evaluating an aggregation constant. 66

I"'V

where K 2o, ~JfJ and ~cg are the respective dimer formation constant, enthalpy, and heat capacity at To. The free energy of the dimerization is usually dominated by the enthalpy which is modulated by substituent steric and ionic effects. The entropy, while usually negative, frequently provides useful information about solvent coordination with monomer and dimer complexes. 2. Overall Equilibrium Model The overall equilibrium model described in eq. 6 postulates two measurable components, monomer and aggregate (composed of n phthalocyanine units), within the concentration window of observation. When the formation constants of the higher aggregates (e.g. trimer, tetramer, etc.) are comparable to or greater than that for the dimer, larger aggregates by way of successive equilibria form, and it is not possible to isolate the monomer to dimer equilibrium for measurement. The dominant aggregate in some cases may be relatively large (e.g. 10 units). The data treatment for an overall equilibrium model neglects the sequential intermediate equilibria and defines an aggregate number, n, as well as an aggregate formation constant, K n , as indicated in eq. 6. In deriving equations for data treatments, non-monomeric species are regarded as a

(13)

As a first approximation, the cn/nc1 term is considered negligible relative to the c/ C1 term, and a plot of log [(I-(c/c1))CO] vs. log [(c/c1)CO] is used to obtain an initial value of n from the slope. The values of Cn and K; are then determined by iteration from eq. 13. Aggregation numbers up to 8 have been determined for organic dyes." Using this model for naphthalocyanine systems, values approximating 2 were obtained/" Schnabel et al. derived eq. 14 to describe multiple aggregations of free-base tetrasulfonated phthalocyanine in waterr'"

Values for C1 and cn/n need to be known or approximated before the aggregation number and formation constant can be determined. A third approach to the overall equilibrium model does not combine the equilibrium constant expression, mass balance equation, and the Beer's Law relation into a single equation, and seek a linear solution, but it expresses the constraining equations from the model into a polynomial equation, computes solutions to this equation by iterative approximation, and then seeks the best match (determined by standard deviation) with the absorbance-eoncentration experimental data by varying 68 C1, e.; and K; at integral values of n. The mass balance equation for the overall equilibrium model is given by eq. 15. (15)

This equation is combined with eq. 6 to yield the nth order polynomial equation for [Pcn ] (eq. 16) which is combined with Beer's Law to yield eq. 17. nKn[Pclt+[Pcd - Co = 0 A = b(81 [Pc.] + 82Kn[Pcdn)

(16) (17)

141

109 / Phthalocyan ine Aggregation

For absorbance (A), cell path length (b), concentration data sets, eq. 16 is solved by the Newton-Raphson method'" to generate values of [Pcd that are fed into eq. 17 where a nonlinear regression is applied to derive values of 81, e.; and K; for incremental values of n. Values of nand K; are reported for copper tetrasulfonated phthalocyanine in water (2, and 7000 M- 1 respectively) and in methanol (5 and 2800 M- 1) .68

3. Trimers and Tetramers There are very few reports of quantitative measurements of aggregate formation constants beyond that of the dimer. In addition to representing a difficult accomplishment, these studies provide further insight toward the strength of the aggregating tendency after dimer formation, and the spectra of the corresponding trimer and tetramer aggregates serve as valuable information for the interpretation of spectra of phthalocyanine compounds in concentrated solution and possibly the solid state. Trimer formation of a dodecane solution of free-base phthalocyanine with eight branched chain alkoxy substituents at the {j-positions was reported by Schutte et al.7o The dimerization constant and dimer spectrum were determined according to eq. 11. The procedure for determining the trimer formation constant, K 3 , and extinction coefficient, 83, was analogous to that for the dimer. The trimer equilibrium model is described by eq. 4. Terms relating to the trimer specie are added to the mass balance and Beer's Law relation (eqs 18 and 19 respectively).

+ 2[PC2] + 3[PC3] [PCl]cl + 2K2 [PCl f c2 + 3K2K3[PCl]3c3

Co = [Pcd Coc =

(18) (19)

Equation 19 is a solvable third order equation, and values for 83 and K 3 were determined by a linear least squares fit. 70 The concentration range did not exceed 5 x 10- 4 M to avoid interference from higher aggregates, and the 83 and K 3 values were checked by determination at three concentrations. The respective K 2 and K 3 values were determined as 1.5 x 106 M- 1 and 4.9 x 104 M- 1. The distribution ofphthalocyanine in the monomer, dimer, and trimer forms is calculated as a function of concentration and is entered in the upper portion of Table 1. From 10- 6 M to 10- 4 M the dimer is the dominant component, and this is a consequence of K 2 being greater than K 3 by a factor of 30. The trimer spectrum was generated from 83 at the individual wavelengths. The calculated spectra of the monomer, dimer, and trimer along with a Langmuir-Blodgett (LB) film spectrum are reproduced in Figure 8. The broad-

Table 1. Calculated Monomer, Dimer, Trimer and Tetramer Aggregate Distributions of Soluble Phthalocyanines in Organic" and Aqueous " Media Pc Compounds Aggregation Constants

Cone. (M)

0/0 PC1

0/0 PC2

H 2PcCB-X)8

10- 7

80.5

19.4

0.1

X="'O~

10- 6

42.9

55.2

1.9

solvent = dodecane K 2 = 1.5 X 106 M- 1 K 3 = 4.9 X 104 M- 1

10- 5

16.1

75.1

8.8

10-4

4.6

70.3

25.1

H 2Pc(f3-X)4

10- 7

58.7

41.3

0

10- 6

24.6

72.7

2.7

10- 5

7.6

68.9

23.6

10- 4

1.7

35.4

62.9

X =

19 -S-O"Na+ \\

0

solvent = water

K 2 = 6 x 106 M- 1 K 4 = 5 X 104 M- 2

0/0 PC3

0/0 PC4

4

Eo Q)

(5

3

~

...,;

! '9

2

o ~

x

w

500

600

700

800

WAVELENGTH (nm) Figure 8. Calculated spectrum of monomer (-), dimer (+), and trimer (<» of free-base ~-octa(3,7-dimethyloctoxy) phthalocyanine and an experimental LB film spectrum (.~). (Reprinted with permission from Schutte, W. J.; Sluyters-Rehbach, M.; Sluyters, J. H. l- Phys. Chern. 1993, 97, 6069. Copyright 1993, American Chemical Society.)

ening and progressing blue shifting of the aggregated spectra with aggregate size are easily observed. The shift in band center of the monomer (680 nm) to dimer (625 nm) to trimer (610 nm) to LB film (600 nm) was qualitatively interpreted to correlate with the shift in the exciton band as the number of phthalocyanine units in the one dimensional stack incrcases.i" From this trend, the number of phthalocyanine units in an aggregate in the LB film was estimated to be six. Tetramer formation was reported by Schnabel, Nother, and Kuhn for a free-base tetrasulfonated phthalocyanine in waterr'" Dimer formation was observed at dilute solution (10- 6 M) with a pair of

Snow

142

isosbestic points and good conformity with eq. 14 for dimer formation. At significantly higher concentrations (10- 2 M), a further blue spectral shift of the Q-band with a second set of isosbestic points was observed. This was presumed to be tetramer formation based on a calculated [Pc2]/[Pcd ratio of 24 at 10-4 M concentration and modeled by a sequence of eqs 3 and 5 with eq. 20 derived for the dimer to tetramer equilibrium. 109[

(t: - :)Co] = ~lOg[(t:2 X

- 2t:)CoJ+ m

109[t:2 - 2:

] -

(1 -~)

(~)

log[m

2K

m]

(20)

where m was experimentally determined to be 4. The respective K 2 and K 4 values were determined as 6 x 106 M- 1 and 5 x 104 M- 2. As with the trimer above, the distribution of phthalocyanine in the monomer, dimer, and tetramer forms is calculated as a function of concentration in the lower portion of Table 1. From 10- 6 M to 10- 5 M the dimer is the dominant component. This is a consequence of a K 2 being very large. Although K 2 is a factor of 100 greater than K 4 , it would appear that between 10- 6 and 10- 5 M concentration significant tetramer could form.

above. This method has been extended to trimer formation with further experimental results on the zinc tetrasulfonated phthalocyanine-water system. 72 However, when using this method, caution to work only at high dilution should be exercised as readsorption may seriously affect the measured fluorescence.f '

C. NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY The utility of using NMR to investigate aggregation is derived from an association of molecules in solution perturbing the diamagnetic shielding of their constituent nuclei such that detectable changes in chemical shift are observed as a function of concentration. Phthalocyanines as well as porphyrins are especially interesting candidates for study as compared with other organic dyes because of their symmetry and large ring currents. These structural features generate characteristics in the NMR spectrum that may be used for

B. FLUORESCENCE SPECTROSCOPY A method employing fluorescence spectroscopy makes use of the observation that dimerization or aggregation quenches fluorescence. It has the potential advantage of being useful for very strongly aggregating systems (K2 > 106 M) where absorbance measurements of monomer concentration are difficult. A method was reported by Zhang, Xia, and Ma that proposes a simple linear relationship between the fluorescent emission intensity, F, and the monomer concentration as in eq. 21. 71 [Per]

== kF

(21)

where k is a proportionality constant, which would need to be determined at high dilution. The dimerization constant is obtained by direct substitution according to eqs 3 and 7 and rearranging to a polynomial form according to eq. 22. (22)

At high dilution F is measured as a function of Co, and K 2 is determined from a fit to eq. 22. An experimental K 2 of 4.9 x 106 M- 1 for zinc tetrasulfonated phthalocyanine in water is consistent with absorption spectroscopy determined dimerization constants for the free-base and cobalt analogs described

Figure 9. Structure of a silicon phthalocyanine trimer model compound. (Reprinted with permission from Janson, T. R.; Kane, A. R.; Sullivan, J. F.; Knox, K.; Kenney, M. E. ). Am. Chern. Soc. 1969, 91, 5210. Copyright 1969, American Chemical Society.)

143

109 / Phthalocyan ine Aggregation

quantitative assessment of aggregation as well as analysis of aggregate structure. Aggregation effects observed in NMR spectra of phthalocyanine compounds are usually upfield shifts in the resonances of nuclei situated close to the 18-n electron macroring with increasing concentration. When aggregates form the coplanar stacked structure, the ring currents of adjacent structures reinforce the magnitude of the diamagnetic shielding and shift the resonance of nuclei to even further upfield positions in the spectrum. This cumulative ring current effect has been quantitatively studied in a model compound system based on an incremental stacking of phthalocyanine rings in a series of monodisperse oxygen bridged silicon phthalocyanine oligomers (n == 1 through 5) as depicted in Figure 9 for the member with n == 3.74 With an increasing number of phthalocyanine rings, both the terminal methyl resonance and the peripheral 3,6-aromatic protons are shifted to progressively higher fields. A five-loop pair model corresponding to the four benzo and 18-n electron macro ring substructures of the phthalocyanine system has been used to calculate the ring current shielding effect along the fourfold axis normal to the

phthalocyanine ring. When a probe nucleus resides in the phthalocyanine cavity, the chemical shift increment calculated is quite large (r-v9 ppm). For phthalocyanine aggregating systems with NMR observable nuclei in or very near the center of the cavity, the magnitude of chemical shift variation with concentration may be very large. Figure 10 shows series of spectra for free-base and lead phthalocyanine where the chemical shifts of the cavity protons and of the lead ion vary with concentration. A single resonance is observed. The phthalocyanine aggregate, unlike the covalently bonded siloxane phthalocyanine oligomer, is a dynamically equilibrating system with kinetics that are fast on the NMR time scale at room temperature. For the proton and lead cavity ions, the effective 'concentration range for the NMR experiment is 10- 1 to 10- 5 M. Compared with electronic spectroscopy, NMR is much better suited to high concentrations, but at low concentrations its detection limit is a factor of 102 less. In porphyrin research it was recognized very early that this concentration dependence correlated with a coplanar association of structures and could be quantitatively analyzed as dimer forrnation.f In the

'H NMR Pb(C2Hs)4

I 6= 73.5ppm

PbPC(r3-CP)4 I

7.9

~'~~~~

X

10-4M

• •~ .. 1.7x10·3M

. 1.4 X 10-2 M

300 260

260

160

PPM

100

60

Figure 10. 1 Hand 207Pb NMR spectra concentration dependence of free-base tetra-{j-cumylphenoxy phthalocyanine (H 2Pc({j-CP)4) cavity proton and lead tetra-{j-cumylphenoxy phthalocyanine (PbPc({j-CP)4) complexed lead ion resonances in chloroform solution at 23°C.

Snow

144

aggregation process it is assumed that there are unique chemical shifts corresponding to the monomer (81) , dimer (82) , trimer (83) , and so forth and that the observed chemical shift (8) is a number average of the contributions from those molecules in the monomer or aggregate state. Thus, the chemical shift is used in a manner analogous to the extinction coefficient in eq. 10. Equation 23 is the NMR analog to the electronic spectroscopyeq. 10 for a monomer-dimer system and is used analogously with eq. 9 to determine the dimerization constant. (23)

In principle, any nucleus with a measurable concentration dependent chemical shift may be used as was done successfully with cavity and peripheral protons in the porphyrin example." In practice there is significant variation in the magnitude of the concentration dependent chemical shift among different cavity and peripheral phthalocyanine nuclei. Reported results for fJ-substituted octaalkynyl phthalocyanines display relatively modest variations (r-v 1 ppm) for chemical shifts over a 10- 5 to 10- 2 concentration range for both the cavity and peripheral a-aromatic protons.i'' Phthalocyanine compounds with octaalkyl a-substitution display even smaller chemical shift variation of the peripheral fJ-aromatic protons. 77 In the NMR spectra of tetrasubstituted phthalocyanines, the peripheral proton resonances are complicated by the usual mixed isomer nature of these compounds. The cavity protons of

free-base phthalocyanines are less likely to be complicated by the mixed isomer effect, and the ring current usually shifts this resonance upfield ofTMS so that there is little interference from overlapping resonances of other nuclei. When the peripheral substitution is aromatic in character, the cavity proton chemical shift variation can be particularly large as depicted in Figures 6 and 11. The chemical shift data of the cavity proton resonances of free-base fJ-tetracumylphenoxy phthalocyanine in Figure 6 may be analyzed by eqs 23 and 9 which assume a monomer-dimer model. One method is to make a series of rational guesses for the equilibrium constant, calculate the monomer fraction, x, and construct plots for each of the "guessed" equilibrium constants as in Figure 11. If the assumption of the monomer-dimer model is good, the curve that most closely conforms to a straight -line corresponds to the dimerization constant that best fits the data. However, this plot illustrates the effect of higher aggregate formation by the downward curvature at values of x less than 0.2. To properly use the monomer-dimer model, data points from the concentration range where higher aggregate formation is significant must be deleted from the analysis. In Figure 11 data corresponding to monomer fractions of 0.4 or less appear to initiate this higher aggregate effect. The character of the remaining curves illustrates the effects of "guessed" dimerization constants that are too low or too high by a respective upward or downward curvature in the region where x approaches 1. In the data of Figure 11, the dimerization constant corresponding to 3 x 103 M- 1 provides the

O---....--....----.---~--.,..----r-----,--..,.----r-----,

E

c.. c.. "'-"

-2

.....

'+-

:2

en

(ij

-4

• K= 10000 • K =7000

o

·e

A K

Q)

..c:

o

=3000

T K=

-6

1000 • K =500

i/f!

-8 ......--....--.,....---.-----,.---.----.-----,---.----r----I

0.0

0.2

0.4

0.6

0.8

1.0

Monomer Fraction Figure 11. Plot of monomer fraction versus cavity proton resonance chemical shift for the free-base tetra-fJcumylphenoxy phthalocyanine compound (H 2Pc(fJ-CP)4) for a series varying aggregation formation constants.

145

109 / Phthalocyan ine Aggregation

most linear fit with the data. The best fit K 2 (2700M- 1) may be determined by a maximized correlation coefficient for a linear least squares data fit. An aggregation effect test may be made by removing the data point corresponding to the most concentrated solution measurement and obtaining the same K 2 within experimental error. These data illustrate a sensitivity limitation of the NMR method. Chemical shift measurements obtained from 10- 4 to 10- 5 M phthalocyanine solutions require 24-48 h of acquisition time to achieve the signal-to-noise ratio represented in Figure 10. Dimerization constant determinations requiring measurements at solution concentrations < 10- 5 M (i.e. K 2 > 104 M- 1) are outside of the limits of routine NMR capability. However, this method is well suited for concentrated solutions (10- 3 to 10- 1 M) where modest or small dimerization constants (1-1000 M- 1) or the formation of higher aggregates may be studied presuming the phthalocyanine compound has sufficient solubility. Optical spectroscopy is limited at high concentrations by difficulties associated with extremely short path length optical cells (path length precision and cell coplanarity). When the kinetics of the aggregation process are slow on the NMR time scale, resonances of the same nuclei from phthalocyanine compounds in the monomeric and dimeric states having different chemical shifts can be completely resolved. This has recently been quantitatively investigated in a very elegant variable temperature NMR study of methyl rhodium octa-a-pentylphthalocyanine. 77 The individual resonances from the methyl rhodium and aromatic protons have a close proximity to the ring current, and their resonances in the monomer and dimer forms became completely resolved at temperatures below -40°C in toluene solution. The relative monomer and dimer equilibrium concentrations were determined by integration of the spectrum. From these concentrations, the dimerization constant was directly calculated at the temperatures of -60, -80, and -90°C as 2.0 x 102 , 5.7 X 102 , and 1.0 x 103 M- 1 respectively. In a poorer solvent such as chloroform, it was found that a second less intense set of dimer resonances was observed. These resonances were analyzed to be a dimer with a different structure. Twodimensional COSY NMR indicated that all aromatic and pentyl proton resonances were equivalent in the preponderant (87% ) dimer and inequivalent for the lesser dimer. The major dimer was analyzed to be a 45° staggered cofacial structure with D 4d symmetry and the minor dimer to be a slipped cofacial dimer with C2h symmetry.

D. VAPOR PRESSURE OSMOMETRY (VPO) Vapor pressure osmometry is an excellent complementary technique to spectroscopic methods in investigating aggregation of neutral compounds. It can be used to measure the average number of phthalocyanine molecules associated into an aggregate with its concentration range of operation (3 x 10- 3 to 3 X 10- 2 M, see Figure 6 for an example). Spectroscopic methods have great difficulty discriminating dimers from higher aggregates (and practically no capability for determining the 'size of higher aggregates), which makes the VPO measurement very useful. In spite of its utility, only a very limited number of studies have made use of this technique.i'<'" The VPO measurement is schematically illustrated in Figure 12. Two matched thermistors (sensitivity > 10- 4 °C) are situated in a small solvent vapor saturated thermally controlled chamber. A drop of solvent and a drop of phthalocyanine solution are deposited on respective reference and sample thermistors. As solvent vapor driven by a chemical potential toward dilution condenses on the solution drop, heat is generated and a temperature difference between the two thermistors is measured as a function of time. After two to three minutes equilibration time, a steady-state is reached followed by an overall drift back to thermal equilibrium between the two thermistors. The temperaor actually a corresponding ture difference (~T) thermistor voltage difference (~V) measurement is

Solvent

Solution

Figure 12. Schematic representation of the essential features of a hanging drop vapor pressure osmometer (see text for discussion).

146

Snow

made during the steady-state period. The measured temperature difference is related to the number of solute particles in solution and is treatable as a colligative property measurement. While a ~ T approaching a theoretical maximum is never realized, a proportionality relationship exists at high dilution. Equations 24 and 25 are used to convert the ~ V measurements for a series of concentrations, C, to a number average molecular weight, M n , of the aggregate.V (24) (25)

The parameter K is an instrument proportionality constant determined by calibration with a known compound of comparable molecular weight to the unknown, and r 2 and r 3 are second and third virial coefficients related to the solvent and solute interactions. The measurement requires relatively modest amounts of

C,)

Q.

:E

.....

material (10-20 mg), but it must be very pure and, in particular, free of low molecular weight contaminants. The instrument is typically operated at a temperature 50°C below the boiling point of the solvent selected. The data should be worked up by a plot of (~VIC) vs. C for a series of measurements to obtain the (~VIC)o intercept, which can then be converted to a molecular weight. An example of this work up for a series of metal substituted tetra-,B-cumylphenoxy phthalocyanine comaxis pounds is presented in Figure 13 where the (~VIC) is normalized to the aggregation number by dividing by (MpclK) parameter of eq. 25 where M pc, the molecular weight of an individual phthalocyanine unit, is subto zero stituted for M n . The extrapolation of (~VIC) concentration places a mildly conflicting requirement on the determination of the aggregation number. The extrapolation is designed to factor out a solute-solvent interaction, but the concentration range of the data points may also encompass a shift in the degree of aggregation. However, the VPO data concentration

/'

6

.........

0/

PdPcCP

.-.-...

Q ~ 5 .~...., II

u

Q.

~

PtPcCP

NiPcCP

4

.........

:E II

c:

0

3

ii

·0 0 0

~

2

MgPcCP

't-

0

I ~

1 -1---..----.--......-..-----.- PbPcCP

C

o

5

[.0034]

10

[.0067]

15

[.010]

20

[.013]

25

[.017]

30

[.020]

35

[.024]

Concentration (g/Kg) or[M] Figure 13. VPO aggregation measurement results of various metal-substituted ~-tetracumylphenoxy phthalocyain toluene solution at 65 "C. (Reprinted with permission from Snow, A. W.; Jarvis, N. nine compounds (MPc(~-CP)4) L. j. Am. Chern. Soc. 1984, 106, 4706. Copyright 1984, American Chemical Society.)

147

109 / Phthalocyan ine Aggregation

range is relatively narrow (less than one order of magnitude in molarity) and the aggregation changes should be relatively small compared with the spectroscopic methods which bracket several orders in magnitude of concentration. In Figure 13 the steepness slope of the various plots correlates with the extrapolated degree of aggregation. Those with a very small slope (i.e. data corresponding monomer and dimer aggregates) are close to being concentration independent and an aggregation shift is not an issue. Those plots with the steeper slopes (i.e. data corresponding to the highest degrees of aggregation) may have diminishing degree of aggregation, but it would be bracketed between the intercept value and that corresponding to the highest concentration point. For example, the platinum compound aggregation number would be between 4 and 6 at the 10- 2 M concentration. The data in Figure 13 show a clear metal ion effect on aggregation and a correlation with the d8 electronic configuration which are discussed in a later section.

E. CALORIMETRY Calorimetry applied to organic dye aggregation measurement is another nonspectrophotometric method that requires relatively concentrated solutions and measures the heat adsorbed as the aggregate dissociates during a rapid dilution process. The enthalpy of the aggregation process may be determined from such measurements. Like the VPO, only limited use of this technique has been made in the study of phthalocyanine aggregation.v'<'" This method typically requires 0.1-2 ml quantities of 10- 2 M phthalocyanine solutions to be diluted by a factor of 50 depending on the magnitude of the equilibrium constant. The heat of dilution is measured on a series of concentrations, and these data are fit to an equilibrium model, usually a monomer-dimer model. There is an assumption that the dominant contribution to the measured heat of dilution is the heat of disaggregation, which is a reasonably good assumption for neutral dyes in nonaqueous solvents. The data workup involves calculating an enthalpy of dimerization (in the simplest model) from differences in measured heats of dilution at varying initial concentrations of dye according to eq. 26:83 (26)

where ~Hg is the enthalpy of dimerization, ~Hobs is the measured heat of dilution for two different solutions

i and j. The number fraction of phthalocyanine in the dimer state, ([Pc2l/Co) for solution i or j is calculated using a "guessed" dimerization constant according to eq. 9. The calculation iterates over a data set (e.g. eight solutions of varying concentration) until a single dimerization constant yields the same value of ~Hg. The free energy and entropy may be calculated from this determination of the equilibrium constant and the enthalpy. Like the spectrophotometric methods, one of the challenging aspects of the data workup is in treating the issue of higher aggregate formation. Experimental results for the copper tetra-f3-octadecylsulfamido phthalocyanine in benzene determined by calorimetry83,84 are in very good agreement with UV-Vis results on the same system."!

F. OTHER METHODS Other phthalocyanine aggregation characterization methods that have had very limited but potentially useful reports in the literature are described in this section. Their application to organic dyes in general has been broader, and perhaps some unexploited opportunities may be found here. 1. Light Scattering

This method provides a measure of the size of the aggregate and requires measurement of scattered light intensity over a series of solutions. The data are treated according to eqs 27 and 28:85 KC o 1 -==-[1+r2Co+"'] Re M K ==

21{?n~

'A 4 N

•

(~)

dC o

2

(27) (28)

where M is the molecular mass of the aggregate, R e is the reduced scattering intensity, no is the solvent refractive index, A is the wavelength of the incident and scattered light, N is Avogadro's constant and dn/dCo is refractive index increment of the solution. Two difficult requirements for measurement of the aggregate molecular mass are that the compound does not significantly absorb at the wavelength used in the experiment and an accurate measurement of dn/dCo be obtained. While many organic dyes have some absorption across the visible spectrum, wavelengths greater than 700 nm have been successfully used for azo dyes. 86 Phthalocyanine aggregates would require even longer wavelengths. Alternately, it is possible to go to much shorter wavelengths and employ X-rays. A successful measurement of a 105 molecular weight

148

Snow silicon phthalocyanine polymer using this technique has been reported.V

2. X-ray Scattering Small angle X-ray scattering may be used to determine a weight-average aggregation number of phthalocyanine compounds in solution although very few measurements have been reported. A particularly interesting investigation characterized the size and shape of copper tetrasulfonate phthalocyanine in aqueous solution.i" The effects of concentration, temperature, and added salt were remarkably strong as depicted in Figure 14. This method, like VPO, has a higher concentration window than the spectroscopic methods. In neutral water, phthalocyanine aggregates are significantly larger than the dimer at high concentrations (i.e. greater than 10- 3 M). When salt is added, the aggregation number and its dependence on concentration increase substantially. Elevating the temperature reduces the aggregation number as would be expected for a complex association reaction of this type. The temperature dependence was used to calculate an average heat of association of -12 kcal/mole, The aggregate has a cylindrical shape with a slipped plane molecular stacking proposed for its

morphology/" In the solid-state phthalocyanine compounds, which aggregate but do not crystallize retain the stacked coplanar aggregate structure. Structural information about the aggregate may be obtained from a solid-state wide angle X-ray diffraction pattern. When subjected to Cu Ko X-ray scattering, a weak broad reflection in the

diffraction scan is 0 bserved in the 28 region of 26.5-27.5°. This broad reflection is frequently the only resolved feature superimposed on the amorphous halo. Based on an analogous assignment of a similar dominant reflection from a phthalocyanine polymer model system where phthalocyanine rings are covalently constrained to a face-to-face orientation.V these broad reflections are assigned to a cofacial ring periodicity in the aggregate structure. An example of aggregate diffraction patterns forming a series of phthalocyanine compounds with systematic variation in the size and shape of ,B-substituted peripheral groups is shown in Figure 15. There is a systematic variation in sharpness of the interplanar reflection and its position near 28 == 27° that correlate with the nature of the substituent group. Sterically crowded substituents cause phthalocyanine aggregates to have less ordered structure and a very slightly greater interplanar spacing. The corresponding Bragg angles and spacing between rings in the aggregate are entered in Table 2. In amorphous materials, noncrystalline maxima denote the frequent occurrence of particular interatomic distances in a predominantly disordered material, and these distances ' dBragg, are . given only approximately by the Bragg's law equation. Scattering functions for liquids indicate that the true distance in a glass is a factor of 1.2 larger than that for ' . 90 crys t a 11me materials. This example illustrates the effect of substituent structure on molecular order and interring spacing in the aggregate as characterized by X-ray diffraction.

3. Diffusion

.-------

• 0.3% NaCI (21°C)

10

~

E

/

8

z ~

c:

~

6

0>

~

~

4

2

/

0.3% NaCI(50°C)

~.

D==~

.~

..----. .

0>

~

/

6n1]rN

•

~.HP(500C)

............. 0.000

0.005

0.010

0.015

0.020

Concentration (M) Figure 14",SAXS measurement of aggregation number dependence on of a copper ,B-tetrasulfonate phthalocyanine in water illustrating the effects of temperature (e) and salt (_). Data plotted from Ref. 88. ~oncen~ratlon

Another method of assessing the size of a dye aggregate is based on its diffusion properties. The visible color of the dye facilitates this method. The method involves determination of a diffusion coefficient, which is correlated with a hydrodynamic volume ofthe aggregate by way of the Stokes-Einstein relation, eq. 29: (29)

where r is the radius of the hydrodynamic volume and 1] is the viscosity of the medium. The value of r is used to estimate the size of the aggregate. The aggregation of a copper tetra-d-sulfonate phthalocyanine has been analyzed by this method where an apparent diffusion coefficient was measured by the rate of diffusion of this dye into capillaries 1 em or 2 em long, 1 mm in diameter, and in some cases filled with a ge1. 9 1 Molecular volumes consistent with a trimer or tetramer

149

109 / Phthalocyan ine Aggregation

x

10

15

20

25

30

= -O-CH3

35

40

29

x

10

15

20

25

15

20

25

29

10

30

35

29

x

10

=-0-0

30

35

40

10

15

20

25

40

=-0-0+0

30

35

40

30

35

40

29

10

15

20

25

29 Figure 15. Wide angle X-ray scattering patterns of a series free-base ,B-substituted phthalocyanine compounds, H 2Pc(b-X)4, with peripheral substituents of varying size and shape.

Snow

150

Table 2. Effect of Substituents on Phthalocyanine Aggregate Interplanar Bragg Angle Reflection and Spacing from Scattering Patterns in Figure 15

X in H 2Pc(B-X)4

20

dBragg(A)

-O-CH 3 -O-(CH 2 ) 17CH3

27.1 27.1

3.29 3.29

CHs I -O-CH2- C-CHs I CHs

26.8

3.32

27.1

3.29

26.9

3.31

-0-0

-0-0+0

(a) (b)

(c)

(d) (e)

(f)

-o~ were found depending on the density assumed for the aggregate. The effects of salt concentration and urea and thiourea dissaggregating additives were quantitatively studied by this method.

(g)

2600 2800

3000 3200

3400

3600

HIG 4. ESR Spectroscopy

The utility of ESR spectroscopy in investigating aggregation of paramagnetic phthalocyanine compounds (e.g. phthalocyanine complexes of Cu, VO, Co, and Mn ions) originates from an exchange interaction with an identical paramagnetic center in close proximity. This is the situation experienced when a dimer aggregate is formed. This exchange interaction results in the loss of nitrogen and metal hyperfine structure in the spectrum and collapse of the spectrum into a single featureless signa1. Figure 16 presents ESR spectra of copper tetra-,B-sulfonate phthalocyanine in aggregated polymer, dimer, and monomer states. 92 The copper phthalocyanine dissolved in water progresses from a polymeric aggregate to a dimer to a monomer as incremental fractions of a disaggregating agent (dimethylformamide) are· added to the solution. The polymer spectrum is represented by that from the aqueous solution, the dimer spectrum mostly from the 50% dimethylformamide solution, and the monomer spectrum from the 100% dimethylformamide solution. Although overlapping, the monomer and dimer spectra may be simulated for qualitative estimates of the fraction of these components. The resolution of the hyperfine structure is an indicator of the degree of desegregation. Similar observation has been made for the vanadyl derivative. 93 Simulations of the dimer spectra are able to correlate an internuclear distance

Figure 16. ESR spectrum of copper tJ-tetrasulfonate phthalocyanine in aqueous solution at 77° K to which the following amounts of dimethylformamide have been added prior to freezing. (a) 0 % , (b) 10 % , (c) 20 % , (d) 30 % , (e) 50 % , (f) 75 % , (g) 100 % • (De Bolfo, J. A.; Smith, T. D.; Boas, J. F.; Pilbrow, J. R. j. Chern. Soc., Faraday Trans. 2 1976, 72, 481. Reproduced by permission of The Royal Society of Chemistry)

between metal ions of 4.3 and 4.5 nm for the respective copper and vanadyl ions. 9 3 ,94 Simulated ESR spectra of copper phthalocyanine dimers with peripheral crown ether substitution similarly show a 4.1-4.2 A internuclear separation.Pr" Cobalt'" and manganese'" phthalocyanines are more complicated systems due to fast relaxations and multiple spin states but have likewise been investigated.

v.

Experimental Results

The purpose of this section is to provide a compendium of experimental results where quantitative measurements of phthalocyanine aggregation parameters have been reported. These parameters include dimer and higher aggregate complex formation constants, aggregation numbers, and enthalpies and entropies of aggregation. There are many studies where qualitative observations of aggregation are made, usually in the form of displayed spectroscopic effects. While such observations are very useful for correlating relative

109 / Phthalocyan ine Aggregation

conditions and structural effects with their influence on aggregation in a particular study, it is difficult to make comparisons between different investigations. These observations depict important trends, which will be discussed in Sections VI and VII. This section is divided into two parts: one tabulating results for phthalocyanine aggregation in aqueous systems and one for nonaqueous systems. In the sections and tables that follow, there are certain conventions that will be utilized to abbreviate structural designations or to indicate a presumption that has been made regarding conditions of the measurement. Of the 16 peripheral substitution positions on the phthalocyanine ring, those at the eight inner benzo substitution positions (1,4,8,11,15,18,22,25-positions) are designated as the a-positions, and those at the eight outer benzo substitution positions (2,3,9,10,16, 17,23,24-positions) are designated as the ,B-positions. This designation was first used by Linstead'" and accommodates varying degrees of peripheral substitution as well as the mixed isomer nature of the tetra-substituted phthalocyanines. For example, H2Pc(,B-S03Na)4 designates a free-base phthalocyanine with four sodium sulfonate groups peripherally substituted at one of the two ,B-positions of each of the benzo rings. Unless otherwise specified, a mixed isomer distribution with regard to the two possible sites on each benzo ring is presumed. In the reporting of results, occasionally, certain conditions under which a measurement was made are not explicitly stated although they may be inferred from the context of the article. For example, an unspecified temperature presumed to be room temperature and equal to 20°C would be entered in the table with parentheses, (20°C). A. AQUEOUS SYSTEMS Aggregation measurements of water soluble phthalocyanine compounds have mostly involved those with peripheral sulfonate substitution at the ,B-positions. Many of the dimerization constant measurements are summarized in Table 3. In this table are data that illustrate the effects of metal ion substitution, temperature, pH, addition of miscible organic solvents and disaggregating agents, and of a change in the number of peripheral sulfonate groups. While these compounds have been available for almost 40 years and are the best studied of aggregating phthalocyanines, their aggregation behavior is complicated by coulombic, hydrophobic, and strong rr-electron interactions. Different medium conditions and metal substitutions may result

in different aggregate structures with blue or red shifted electronic absorption bands relative to the monomer's Q-band. More recently, the aggregation behavior of phthalocyanines with carboxylate and quaternary amine peripheral substitution is being studied, examples of which are in Table 3. Interestingly, nonionic water soluble phthalocyanine compounds, particularly those with oxyethylene oligomer substitution, are known but to date no quantitative aggregation measurements appear to have been made. In cases where variable temperature measurements are made on the dimerization or aggregate formation constant, the enthalpy and entropy of the process may be quantified. This provides better quantification of the nature of the aggregate bonding and may distinguish temperature and medium effects. Enthalpy and entropy results for phthalocyanine dimerization in aqueous medium are presented in Table 4. B. NONAQUEOUS SYSTEMS The first organic solvent soluble phthalocyanine compounds in quantitative aggregation studies were derived from the water soluble tetrasulfonated phthalocyanines as the octadecylsulfamide analogs. This feature nicely complements the aqueous studies in that a similar electron withdrawing effect is exerted by the peripheral group of the phthalocyanine structure while its ionic character is neutralized. While the peripheral groups effects will be discussed in Section VI in greater detail, it is interesting to compare the first few entries in Table 5 for the octadecylsulfamide phthalocyanine in CC14 with those in Table 3 for the sulfonate phthalocyanine in water. The dimerization constant for each is 2:106 M- 1 , and as solvents of intermediate polarity are employed the aggregation tendency is significantly reduced. This is a good example of very strong attractive forces operating between associating phthalocyanine rings despite peripheral groups, which have a favorable interaction with these extremes in solvents. The organic solvent soluble phthalocyanine compounds have a much greater diversity in peripheral group structure. In addition to differences in complexed metals and solvents, the composition, size, shape, position of substitution, and electron withdrawing or donating nature of the peripheral group are all variations that have influence on dimerization and aggregation. Ideally, Table 5 would be much larger with a more systematic variation between structures. Beyond the examples listed in Table 5, there are many qualitative studies where pertinent observations of aggregation tendency have

151

Snow

152

Table 3. Dimer and Aggregate Formation Constants of MPcX in Aqueous Media

M

X

K2 (M- 1 )

Solvent

Temperature (OC)

Ref.

H2 H2 H2 H2 H2 H2 H2 H2 H2 Co Cu

(,B-S03N a)4 (fi-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a)4 (,B-S03N a), (fi-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a)4 (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a)4 CB-S03Na)4 (,B-S03N a)4 (fi-S03N a)4 (,B-S03N a)4 (,B-S03N a), (,B-S03N a), (,B-S03N a)4 (,B-S03N a), (,B-S03N a)4 (,B-S03N a)2 (,B-S03N a)2 (,B-S03N ah (,B-S03N a)2 (,B-S03N a h (,B-S03N ah (,B-S03N a)2 (,B-S03N ah

6,000,000 1100 K4 == 50,000 148,000,000 117,000,000 93,000,000 76,000,000 62,000,000 10,000,000 200,000 16,000,000 1,000,000 10,000,000 2,000,000 13,000,000 192,000 800,000 470,000 200,000 8,400,000 4,100,000 2,300,000 1,400,000 870,000 640,000 63,000 19,000 22,000 13,000,000 790,000 26,000,000 52,000,000 11,000,000 25,000,000 162,000 286,000 770,000 2,600,000 547,000 214,000 86,000 79,000 51,000 43,000 840 2,500,000 K 3 == 900,000 470,000 200,000 590 2,800 5,000 4,400,000 600,000 970,000 2,300,000 1,100,000 560,000 85,000 50,000 92,000

H 2O 1: 1 EtOH:H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O 15 %MeOH/H2O 15%EtOH/H2O 15 %2-PrOH/H 2O H 2O H 2O 0.006 M NaCI0 4 0.1 M NaCI04 0.1 M NaCI04 H 2O 20%EtOH/H20 20 0/OMeOH/H20 250/0 urea/H 2O 20 %foramide/H20 H 2O H 2O H 2O H 2O H 2O 3M urea 5M urea H 2O H 2O H 2O 2 %CH 3CN/H20 90 %MeOH/H20 900/0 EtOH/H20 20 %MeOH/H20 H 20 pH 2.5 H 20 pH 3.5 H 20 pH 4.8 H 20 pH 5.2 H 20 pH 6.5 H 20 pH 7.5 H 20 pH 8.5 H 20 pH 9.5 H 2O

(20) (20) (20) 14 17 20 23 26 62 58 61 58 (20) (20) (20) 58 38 48 58 25 35 45 55 65 75 25 25 25 25 60 25 25 60 25 25 25 25 25 35 45 55 65 75 35 35 (20) 60 (20) (20) (20) (20) 25 (20) (20) (20) (20) (20) (20) (20) (20) (20)

39 39 39 99 99 99 99 99 38 38 38 38 100 100 100 40a 40b 40b 40b 64 64 64 64 64 64 64 64 64 101 101 101 101 101 102 102 103 103 103 104 104 104 104 104 104 104 72 72 61 61 105 105 105 106 106 106 106 106 106 106 106 107

Zn H2

Zn Cu Co Co Co Co Co Co Co Co Co Co Co Co Co Fe Fe Fe Fe Fe Cu Cu Cu Cu Cu Ni Ni Ni Ni Ni Ni Ni

Zn Zn Zn Zn

Al(Cl) Al(Cl) Al(Cl) Al(Cl,O) Al(Cl,O) Al(Cl,O) Al(Cl,O) Al(Cl,O) Al(Cl,O) Al(Cl,O) Al(Cl,O)

Zn

(,B-COOH)4

(Continued)

153

109 / Phthalocyan ine Aggregation Table 3. Continued. M

X

K2(M- 1 )

Solvent

Temperature (OC)

Ref.

Zn

(f3-COOH)4 (f3-COOHh

26,200 6 K 2 » 10 190,000 11,000 5700 1400 2,100,000 6,000,000

50/0 pyridine/Hjf) H 2O 700/0DMFjH2O 80 0/oDMFjH2O 90 0/oDMFjH2O 100 0/oDMF H 2O H 2O

(20) (20) 25 25 25 25 (20) 20

107 108 109 109 109 109 110 111

H2

Zn Zn Zn Zn Zn Zn

(f3-COOHh(f3-CONH 2h (f3-COOHh(f3-CONH 2)2 (f3-COOH)2(f3-CONH 2h (f3-COOH)2(f3-CONH 2h (f3-[C(CHhhCH 2N(CH3hJ + 1-)4 (f3-[N(CH3hJ+[CH 3S0 4J-)4

Table 4. Dimer Enthalpy and Entropy Measurements of MPcX in Aqueous Media (cal/mole °K)

M

X

~ H (kcal/mole)

~S

Co Co Co Co Co Co Co Cu Cu Cu Cu Cu Cu Cu Cu

(f3-S03N a), (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a), (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a), (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a), (f3-S03N a),

-14 -13 -12 -11 -10 -9.3 -7.4 -13 -19 -17 -16 -9.2 -5.9 -10 -6.3

-18 -13 -9.4 -5.9 -2.4 +1.1 +4.5 -11 -40 -29 -30 -3.7 +6.3 -6.3 +3.3

Cu Cu Cu Cu Cu Cu Ni Ni Ni

(f3-S03N a)4 (f3-S03N a), (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a)4 (f3-S03N a), (f3-S03N a), (f3-S03N a), (f3-S03N a),

-12 -3.3 -5 to -7 -11 -12 -13 -12.3 a -6.6 a -2.7 a

-10 +11

-7.2a -0.13 a +2.3 a

Solvent

H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O 20 % EtOH jH 20 20 0/oEtOHj10- 2M NaCI 20 0/oMeOHjH 2O urea (250 gjliter) urea (500 gjliter) thiourea (100 gjliter) {urea (250 gjliter) } thiourea (100 gjliter) foramide (200 gjliter) foramidej1M CH 3C02K H 2O 0.167 %NaCI 0.30 0/oNaCI 0.50 0/oNaCl H 2O 3 M urea 5 M urea

Temperature (OC)

Ref.

38-58°C 25 35 45 55 65 75 25 25 25 25 25 25 25 25

40b 64 64 64 64 64 64 103 105 105 105 105 105 105 105

25 25 20-50 20-50 20-50 20-50 35-65 35-65 35-65

105 105 88 88 88 88 104 104 104

"Author (M. J. Politi) corrected value by personal communication.

been made. Some of these are very useful in making correlations of relative trends within a single investigation and will be described in later sections. Organic solvent soluble naphthalocyanines, MNcX, are an interesting aromatic extension of the phthalocyanines, and a few quantitative studies of aggregation have also been reported as depicted in Table 6. Finally, a few thermodynamic measurements have been reported for phthalocyanines and naphthalocyanines in organic media which are presented in Table 7.

VI. Correlations with Chemical Structure/ Composition This section summarizes correlations of phthalocyanine aggregation with aspects of chemical structure and

composition. This includes the identity of complexed metal ions and the composition, size, shape, position of substitution, and electron withdrawing or donating characteristics of peripheral groups. These variables are not easy to isolate and quantify. While each of these aspects along with solvent influences may be viewed as separate variables, very few quantitative investigations exist wherein only a single aspect of structure is permitted to vary. In many cases aggregation results are part of a broader study and are reported in the form of qualitative observations. However, there are some general trends tha t can be identified which contribute toward an understanding of the aggregation process. There are also some interesting trends reported in different studies that appear to run counter to each other. These should be regarded as

Snow

154

Table 5. Dimer and Aggregate Formation Constants of MPcX in Organic Media M

X

K2 (M- 1 )

Solvent

Temperature (OC)

Ref.

CU CU CU CU CU VO VO Zn Zn Zn Zn H2 H2 H2

CB-S02NHCI8H37)4 CB-S02NHCI8H37)4 CB-S02NHCI8H37 )4 CB-S02NHCI8H37 )4 CB-S02NHCI8H37)4 (,B-S02N H C 18H37)4 (,B-S02N H CI8H37)4 (,B-S02N H CI8H37)4 (,B-Cl)4

2,970,000 2,400,000 15,800 13,000 141 2,010,000 10,700 1,090,000 6,100 47,000 24,000 13,000 14,000 13,000

CC14 CC14 C 6H6 C 6H6 THF C 6H6 THF C 6H6 DMSO DMSO DMSO CHCb CHC1 3 CHC1 3

20 25 22 25 22 22 22 22 (20) (20) (20) (20) (20) (20)

41 83 41, 112, 113 83 113 112, 113 113 112 114 114 114 115 115 115

13,000

CHCb

(20)

115

H2

(,B-Br)4 (,B-l)4 (,B-OCH3)4 (,B-OC I 8H 37)4 (,B-OCH2C(CH 3h)4

(Ji-o-ot

H2

~-o~),

2,000

CHC1 3

(20)

115

H2

~-o-QtOt

7,000

CHCb

(20)

116

130

CHCb

(20)

116

400

CHC1 3

(20)

115

1,500,000

n-C 12H26

(20)

70

K 3 = 50,000

n-C 12H26

(20)

70

80,000 80,000 300,000 500,000 1,000,000 > 1,000,000 684,000 1,750,000

CC14 THF CH2Ch

C 6H6 CHCb hexanes hexanes

(20) (20) (20) (20) (20) (20) (20) (20)

117 117 117 117 117 117 118 118

25,200

C 6H sCH 3

(20)

119

20,600

C 6H sCH 3

(20)

119

406,000

THF

(20)

119

H2

Pb H2 H2

Pt Pt Pt Pt Pt

Pt TiO TiO Zn

Zn

Zn

(a-o-QtOt ~-o-QtOt (p-O-"'Vl.,~-~)8 (~.O~)8 (,B-OC 12H2 S ) 8 (,B-OC I 2H2 S ) 8 (,B-OC I 2H2 S) 8 (,B-OC12H2S)s

(,B-OC I 2H2 S ) 8 (,B-OC I 2H2 S ) 8 (,B-CH20C sH 11)8 (,B-OC 7H 1S ) 8

~~t

C 6H sCH 3

0

(I3~N~)

2t (pK, NH o

4

Cu

~~O~C12H08

>4,800,000

CHCb

(20)

120

Cu

~~~.C12H2j8

4,800,000

CHC1 3

(20)

120

Cu

(~~O~O.o)8

290

CHCb

(20)

120

Zn

(p'O~O.o)8

1300

CHC1 3

(20)

120

Rh-CH 3

(a-nC SH 11) 8

200

C 6H sCH 3

-60

77

(Continued)

155

109 / Phthalocyan ine Aggregation Table 5. Continued.

M

x

Rh-CH 3 Rh-CH 3

(a-nC SH 11) S (a-nC SH 11) S

H2

Solvent

Temperature (QC)

Ref.

570 1000

C6H sCH 3 C 6H sCH 3

-80 -90

77 77

150

C 6H sCH 3

(20)

121

750

CHC13

(20)

51

Table 6. .Dimer Formation Constants of MNcX in Organic Media M

X

K2 (M- 1 )

Solvent

Temperature (QC)

Ref.

Cu

(fJ- tC 4H g)4 (fJ- tC 4H g )4 (f3- tC 4H g)4 (f3-nC 4H g)4 (f3-SnC4H g)s

10,300 4670 22.4 30,800 272,000

C6H sCH 3 C6H sCH 3 C6H sCH 3 C6H sCH 3 THF

25 25 25 25 (20)

58,67 58,67 58,67 58,67 122

Ni VO VO

Zn

opportunities for refined interpretation or expanded experimentation. Prior to examining individual variables of phthalocyanine structure, it is of interest to consider how this ring system might compare with closely related analogs such as porphyrins, naphthalocyanines, subphthalocyanines, and superphthalocyanines. In his review of porphyrin aggregation.!" White points out that sulfonated and carboxylated phenyl substituted porphyrins in aqueous medium do not dimerize in the absence of ionic strength and that added electrolyte up to 0.1 M causes strong dimerization with constants in the range of 104 to 105 M- 1. This is quite different from sulfonated phthalocyanines, which are strongly dimerized (and aggregated) in neutral water with dimerization constants in the range of 105 to 107 M- 1 (Table 3) with addition of salt causing further enhancement. In organic media neutral porphyrin dimerization constants range from 10°·5 to 102 M- 1 14 while those for phthalocyanines are somewhat larger, ranging from 102 to 106 M- 1 (Table 5). It would appear that a phthalocyanine ring system has a stronger tendency to aggregate. This is likely a consequence of the fused benzene rings in the phthalocyanine structure. When additional benzene rings are fused onto the phthalocyanine ring system to form the naphthalocyanine structure, the degree of aggregation is comparable and substituent effects are the determining factor as to which system has the greater aggregating tendency.122 Contraction or expansion of the phthalocyanine ring to the respective subphthalocyanine'<" or superphthalocyanine'<" results in a non-

planar structure and requires complexed metals having axial substitution. Neither system displays a significant aggregating tendency.

A. COMPLEXED METAL IONS Results of aggregation measurements wherein the metal ion complexed in the phthalocyanine cavity has been varied are presented in Table 8. The intent of this table is to identify complexed metal ions which promote or suppress aggregation. The two types of measurements presented are the aggregation number as determined by VPO and the dimerization constant as determined by spectroscopy. The former relates to a state of a material, while the latter relates to the energetics of a process. The aggregation number is more useful in diagnosing aggregates larger than the dimer and, as such, a ranking of metal ions based on this parameter is biased toward situations of phthalocyanine compound solutions at high concentration where higher aggregates are of interest. It would seem intuitive that a metal ion promoting a high phthalocyanine aggregation number (e.g. Pt or Fe) would also promote a large dimerization constant. However, this may not be the case, as some of the data in Table 8 appear to indicate. The dimerization constant most generally is a dilute solution measurement and discriminates monomer from dimer reasonably well but does not relate to higher aggregate formation. Where changes in electronic structure resulting from aggregation are of interest, most of the change occurs in the monomer to dimer transformation. The VPO results from two studies in the upper part of Table 8 are in reasonable agreement with each other as to the ranking of metal ions by aggregation numbers. The metal ions can be roughly divided into three groups: those promoting higher aggregates (Pt, Fe, Pd, Ni, Cu); dimer aggregates (Co, Zn, H 2, Bi, Mg); those suppressing dimerization (Pb). For complexed metal ions to enhance aggregation, particularly beyond the dimer state, an axial coordination would be a direct mechanism. Such a mechanism has been proposed for the

156

Snow Table 7. Dimer Enthalpy and Entropy Measurements of MPcX and MNcX in Organic Media (cal/mole °K)

M

X

~ H (kcal/mole)

~S

CuPc CuPc CuPc ZnNc ZnNc ZnPc ZnPc H 2Pc

CB-S02NHClSH37)4 (fJ-S02 N H ClSH37)4 (fJ-S02 N H C18H37)4 (fJ-S(CH2CH20)2nC4Hg)s (fJ-nC4Hg)s (fJ-S(CH2CH 20 )2nC4H 9) S (fJ-nC4Hg)s [,8-18 crown 6(BznC lOH 21hl4

-10 -12 -13 -9.2 -6.3 -4.1 -25.1 -29.9

-15 -21 -15

Solvent

Temperature (OC)

Ref.

C 6H6 C 6H6 CCl 4 THF THF THF THF CHC1 3

25 35 25 (20) (20) (20) (20) (20)

83 84 83 122 122 122 122 123

Table 8. Complexed Metal Ion Effects on Phthalocyanine Aggregation VPO measurements

Metal Aggregation number

Pt 4.1

Pd 3.6

Fe 4.1

Ni 3.0 4.0

Cu 2.7 3.3

Ref. 78: MPcX4/toluene

X=

Co 2.0 2.2

Zn 2.0 1.8

H2

Bi 2.0

2.0 2.1

Mg 1.9

Pb 1.0

Ref.

78 126

Ref. 126: MPcX4/toluene

f3'0-O+O

X=

f3'0-Q-C2H s

Spectroscopic observations/dimerization constant measurements

Aqueous Media Cu>H2 > Zn > Co Organic Media VO > Zn > Cu Zn> Cu Cu, Co > Zn Cu > Zn Ni> Cu Zn> Co Cu> Ni > VO

38 MPcX X == (fJ-S02NHClSH37)4/C6H6 MPcX X == (fJ-0(CH 2hOC6Hs)s/CHCI3 MPcX X == (fJ-N02)4(a-N02)4/DMSO MPcX X == (fJ-OCH 2CHOHCH20H)s/DMSO MazaPcX X ==(fJ- 7-helicene)4/3EtOH:CHCl 3 MPcX X == (fJ-S03CH2R F )4/Py MNcX X == (fJ-tC4Hg)4/C6HsCH3

Pt-Pd-Ni group in Krogmann salts (e.g. square planar K 2Pt(CN)4) where a hybridization of the d; and pz orbitals provide an axial pair of orbitals, each containing one electron through which bonding may occur between adjacent metal ions to form a stacked arrangement of complexes. 131 The inclusion of Fe and Cu should relate some way to multiple stacking, which is not clear at this time. The group forming dimer aggregates probably derives this property as a consequence of the phthalocyanine ring structure as indicated by the inclusion of free-base phthalocyanine in this group. In the group suppressing dimerization, the Pb ion appears to be somewhat unique. This may be more a function of its size, which forces it to be displaced 0.4 A from the plane of the phthalocyanine ring and distorts the ring from planarity. 132 As discussed in the following section, a distortion from planar symmetry by peripheral a-substitution also diminishes the dimerization. At very high concentrations, lead phthalocyanine does

112 120 127 128 129 130 58

aggregate and there is a red shift in the Q-band on dimer formation (see Figure 7). The effects of the complexed metal ion on dimerization constants are presented in the lower part of Table 8 as a ranking in the order of diminishing aggregation. There are considerable variations in solvent and peripheral groups as well as particular metal ions, and a distinction has been made between aqueous and organic media. There are no studies encompassing more than three or four different metal ions so these results are quite limited. The results are depicted as a ranking order of metals in single studies because it is not possible to factor out variations of solvent or peripheral group. In the aqueous system there appears to be a very loose correlation with the VPO measurements if copper is considered relative to the other metals. However, caution should be exercised since the peripheral groups are charged, counter ions are present, and water may coordinate with the sulfonate group and/or the central

109 / Phthalocyan i ne Aggregation

metal ion. In the organic media studies, zinc and copper are the two most utilized metals. Unfortunately, studies conflict as to which of these metals is the stronger promoter of aggregation. It could be a consequence of overriding solvent or peripheral group effects, but more extensive work is needed before a definitive effect may be demonstrated. B. PERIPHERAL GROUP SUBSTITUTION

In this section various peripheral groups, which have been attached to the phthalocyanine ring and have some influence on aggregation are examined. Simple peripheral groups with a single attachment to the phthalocyanine ring are designated by the (1- and fJ- convention for the corresponding inner and outer benzo positions as described in Section V. These are discussed individually on the following two subsections. The crown ether substituent is a more complex substituent and a uniquely interesting development as discussed in the third subsection. Finally, an instance where the phthalocyanine aggregation forms the linkage point for a telecheic polymer is described. 1. Beta-Peripheral Substitution

In aqueous solution peripheral groups used to solubilize the phthalocyanine have been ionic in nature. As indicated in Table 3, these groups include predominantly the sulfonate group and also the carboxylate and quaternary amine, and all involve tetra-substitution at the fJ-positions. For the sulfonate substituent at room temperature in neutral water, phthalocyanine dimerization constants range from 108 to 105 M- 1. The carboxylate and ammonium substituents also have very large phthalocyanine dimerization constants. Addition of salts causes the aggregation to increase, and addition of water-miscible organics causes significant disaggregation. When the degree of sulfonylation decreases, some very interesting aggregation results occur. 133,134 A comparative study utilized commercial and specifically synthesized variably sulfonated aluminum phthalocyanines. The commercial phthalocyanines, AIPcS2mix, AIPcS3mix, and AIPcS4mix, had polydisperse distributions of degrees of sulfonate substitution with a respective average of 2, 3, and 4. The custom synthesized phthalocyanines, AIPcS2adj,

157

AIPcS2opp, and AIPcS4, were monodisperse and, in the case of AIPcS2adj and AIPcS2opp, were isomerically pure with regard to positioning the sulfonate on respective adjacent and opposite benzo groups of the phthalocyanine structure. Very significant effects on aggregation were observed.P" A semiquantitative (uncorrected for dimer absorbance) aggregation percentage parameter was defined for a 5 x 10- 7 M solution of these phthalocyanines in a phosphate-buffered saline media by measuring the diminished intensity of the 672 nm monomer absorption maximum relative to that in DMSO solution which is a disaggregating solvent. The results in Table 9 clearly show for monodisperse sulfonated aluminum phthalocyanine compounds that aggregation is greatly enhanced by a lower degree of sulfonation and by a lower degree of symmetry for the AIPcS2adj. The commercial samples are polydisperse mixtures, and this result clearly shows that mixing similar phthalocyanines with different degrees of substitution suppresses aggregation. Another interesting feature of the sulfonated aluminum phthalocyanine system is a red shifted absorption in the aggregate's spectrum particularly when alcohol is added as has been observed in several studies. 59,60,106,133 The axial ligand coordinated is difficult to define. In one case it has been described as an oxygen or chlorine bridge in between a dimer structurc.l'" while in another the dimer structure is depicted as a face-to-face complex with the axial ligands projected outward.P" In organic media peripheral groups used to solubilize phthalocyanine compounds have a much greater diversity structure with polarity, size, shape, and number being variable factors. Examples with associated dimerization constants are entered in Table 5. Regarding polarity, a clear picture does not emerge as to whether polar linkages to the phthalocyanine ring enhance or diminish aggregation. The octadecylsulfamide group solubilizes phthalocyanines in nonpolar solvents, but the polar nature of its attachment to the phthalocyanine ring appears to favor more polar solvents to accomplish disaggregation. For the less polar linked dodecyloxy group in the platinum phthalocyanine series in Table 5, the disaggregating effect of increasing solvent polarity is present but not as strong. Osburn et ale have proposed using substituent constants to describe inductive

Table 9. Aggregation Percentage of AIPcSn in Phosphate-Buffered Saline 13 3 AIPcSn

AIPcS2adj

AIPcS20pp

AIPcS4

AIPcS4mix

AIPcS3mix

AIPcS2mix

Aggregation percent (%)

85±5

75±5

37±5

12±5

11 ± 5

o

158

Snow

and resonance effects of such substituents on the dimerization constant.l/" This approach appears to correlate with their examples using amide, ester, and ether linkages. When the halogen series of CI-Br-I are employed as phthalocyanine substituents, DMSO is required as a solvent, and the more polar CI- substituent results in the lowest dimerization constant (Table 5). The size and shape issue is a little clearer. The methoxy and octadecyloxy substituents in Table 5 appears to have little difference in dimerization constants. When the substituent is changed to a neopentoxy, again little variation in dimerization constant is observed. Changing the methoxy substituent to a phenoxy substituent also results in very little change in the dimerization constant. However, if the phenoxy group is modified to be more bulky by addition of two tert-butyl groups or a cumylphenyl group, the dimerization constant is significantly reduced as further indicated in Table 5. In the series of zinc phthalocyanines with tertiary carbon linkages, substituent size does not seem to have much effect, but the presence of a primary amine group dramatically increases the dimerization constant even as the solvent is changed to THF. This may involve peripheral group hydrogen bonding. Immediately below this example, there may also be a possibility that hydrogen bonding is involved with the ester and amide linkages in the copper phthalocyanine examples with eight, as opposed to four, peripheral groups. In a series of oxypropylene-2,3-glycol octa-substituted phthalocyanine compounds, hydrogen bonding was demonstrated to have a very strong augmenting effect in the formation of higher aggregates, and, when this substituent has the chiral center in the S configuration, the aggregate has a helical structure. 128 Another example of using peripheral group chirality to generate an enhanced effect in the aggregate structure is the fusion of helicene structures to the fJ-positions of an octaazaphthalocyanine system wherein large second order nonlinear optical susceptibility was obtained.V" The next to last example in Table 5 illustrates how aggregation may be reduced by surrounding the ring with polydimethylsiloxane oligomers. A silicone-hydrocarbon incompatibility is thought to be affecting this result. A similar fluoromethylene chain length effect designed to reduce aggregation has also been qualitatively observed.l" Finally, blocking one face of the phthalocyanine structure with a molecular cap (structure in Figure 2F) significantly reduces the dimerization constant and limits the aggregation to the dimer stage. "

2. Alpha-Peripheral Substitution Phthalocyanines substituted in the eight a-positions were first reported by Cook et ale as very unique compounds with an enhanced solubility, a significant red shift in the Q-band, and a formation of liquid crystals." For linear alkyl substituents positioned at the eight a-positions in a series of nine members ranging from methyl to decyl, it was found that these compounds remained unaggregated up to 10-4 M concentration in cyclohexane at which the longest chain member showed initial evidence of aggregation." Physical properties of this octa-o-substitutcd class of phthalocyanines are very sensitive to changes in the substituent structure. In addition to the octa-o-alkyl substituted phthalocyanine series, the octa-o-alkoxy and octa-o-alkoxymethyl series have been investigated with the finding that the alkoxymethyl series is the most strongly aggregated (aggregation observed at 10- 7 M) and the alkoxy series is the least aggregated. 135,136,137 Quantitative dimerization constant measurements of a-substituted phthalocyanines, while scarce, indicate that substitution at this position significantly lowers the aggregating tendency relative to substitution at the fJ-position (see Table 5). The dimerization constant measurements at -60 to -90°C reported for a methyl rhodium phthalocyanine with octa-o-pentyl substitution are quite small and would be even less than 100 M- 1 if measured at room temperature. " These very low values are attributed to both the a-substitution effect and the methyl group axially coordinated to the rhodium. In a direct comparison of a tetra-o-cumylphenoxy substituted phthalocyanine with its tetra-fJ-substituted analog, the a-substitution resulted in a lowering of the dimerization constant from 7000 to 130 M- 1.116 This result is also attributed to an a-substitution effect. This effect is derived from the steric crowding of substituents attached to the inner benzo positions of the phthalocyanine ring. Substituents are sterically constrained to spatial regions above or below the plane of the phthalocyanine ring and do not have free rotation about the bond to the inner benzo position. This steric effect apparently places strain on the phthalocyanine ring system and causes it to warp out of planarity. X-ray crystal structure studies have shown that substitution at all of the a-positions with alkyl138 and alkoxy v'" groups causes the phthalocyanine ring to distort from a planar to a saddle shaped conformation. Both the positioning of the a-substituent and the distortion of the phthalocyanine ring have a diminishing effect on the dimerization constant and a disordering

159

109 / Phthalocyan ine Aggregation

....0

1:20 0.96

x

E o

0.72

"0

~

0.48

~

0.24

.E ...,

0.00 300

400

600

600

700

800

900

700

800

900

700

800

900

Wavelength (nm)

1.20

b....

0.96

E u

0.72

x

41)

0

!

~ ~

0.48

0.24 0.00

300

400

600

500

Wavelength CnmJ 1.50

b...

1.20

E u

0.90

x

G

0

-

~

::

0.60 0.30 0.00 300

400

600

500

Wavelength (nm) Figure 17. UV-Vis spectra of tetra-,B-, tetra-a- and octa-a-ologooxyethylene (n = 8.8) substituted phthalocyanine compounds in dilute chloroform solution and as thin films. Spectra of films do not quantitatively correlate with the molar absorptivity scale. (Snow, A. W.; Shirk, J. 5.; Pong, R. G. S.). Porphyrins Phthalocyanines 2000, 4, 518. Reproduced by permission of J. Wiley & Sons)

effect on the aggregate structure. The effect on the aggregate structure may be observed in the change in shape of the Q-band caused by aggregation. Figure 17 depicts the spectra of a series of oligooxyethylene substituted phthalocyanines wherein ethylene oxide octamers are substituted at four ,B-positions, four a-positions, and eight a-positions both in dilute solution and as neat films. 14o These compounds are isotropic liquids at room temperature, thus the neat film spectra do not exhibit effects of crystallinity. The spectra from dilute solution represent the monomeric species, and those from the neat film represent the aggregated

species. In the progression of spectra from the tetra-d to tetra-a to octa-o-substituted species, the Q-bands have an increasing red shift. The aggregate Q-band shift from its corresponding monomer position progresses from a blue shift for the tetra-d species to a smaller blue shift for the tetra-a species to a red shift for the octa-o species, and its width at half-height becomes larger. According to the exciton model (see Section III), this would indicate that the tilt angle between adjacent phthalocyanine rings is becoming greater (i.e. progressing from an H- to a J-aggregate structure) and that an increasing degree of disorder (deviation from

160

Snow

coplanarity of phthalocyanine rings) is allowing both p- and N-branch transitions to be observed.Y 3. Crown Ether Substituents

The discovery of using crown ether substituents as an accessory to control phthalocyanine aggregation is a remarkable development.YT" The crown ether complexation of alkali metal ions in a manner to reinforce

aggregation is depicted in Figure 2B. Dimerization and aggregation are accompanied by the usual spectroscopic change of a monomer Q-band transformed to a blue-shifted aggregate Q-band. This aggregation can be induced by the alkali metal ions at very low concentrations with K+ claimed to be colorimetrically detectable to 10- 8 M. 45 Various reported crown ether substituted phthalocyanine compounds are depicted in

Table 10. Crown Ether Substituted Phthalocyanine Compounds Structure

MPc(15-crown-5)4

MPc(18-crown-6)4

{

MPc(21-crown-7)4

MPc(18-crown-6-bz(OC lOH2 1h)4

M

Ref.

Cu H2 Zn Ni Co Ag VO Lu

44, 45, 46, 95, 96, 141, 142 95, 142 95, 96 95 95 142 142 143

Cu H2 Zn

141, 144 141, 144 144

Cu

141

H 2 , Si(OHh

123

MPc(OCH 215-crown-5)4

n = 0; MPc(15-crown-5)4

=1; n =2; n

MPc(18-crown-6)4 MPc(21-crown-7)4

Monomer

s~t

effect

Aggregate

Ref.

Py > CHCl 3 < MeOH < acetic acid

44

CRCl 3 < CH 2Cl2 < tol/MeOH

45

CHCl 3 < CH 2CI2 , bz, DMF, DMSO, tol, THF, MeOH

95

CHCl 3 < CH 2Cl2 < py < C 4H90H < EtOH < MeOH

141

CHCl 3 < py < CH 2Cl2 < C4H90H < EtOH < MeOH

142

CHCl3 < DMSO

143

109 / Phthalocyan ine Aggregation

Table 10. In the absence of alkali metal ions, these crown ether phthalocyanines aggregate in solution. While dimerization constants were not reported for the alkali metal free association, its ranking of solvent dependence was observed (Table 10). When alkali metal ions are added, the crown ether: alkali metal ion complex stoichiometry depends on the match of cation diameter with the crown ether ring size. When there is a good fit, a 4 : 4 host: guest complex is formed, and, when the cation diameter exceeds the crown ether ring size, an 8 : 4 crown ether ring: cation complex is formed.'?' The Cul'ct l Svcrown-S), forms 4: 4 complexes with Li+ and Na+ and 8: 4 complexes with K+, Rb+, and Cs+; the Cul'ct l Svcrown-S}, forms 4: 4 complexes with Li+, Na+, and K+ and 8 : 4 complexes with Rb+ and Cs+; and the Cul'cfZl-crown-Z), forms 4: 4 complexes with all of the alkali metal cations. 141 Indirectly, the anionic counter ions also exert some influence. The alkali metal ion induced crown ether phthalocyanine dimer complexes with the 8 : 4 stoichiometry are proposed to form by a two-step mechanism." In the first step one cation binds to two crown ether phthalocyanine molecules, and in the second step the remaining three cations assume their positions complexed to the other crown ether moieties. In the second step for the 8: 4 complexes, the two phthalocyanine rings are locked into an eclipsed cofacial spatial arrangement (Figure 2 B). Spectroscopically, the dimer in the first stage is distinguishable from the dimer of the second stage of this equilibrium. The first stage dimer has a broadened blue shifted Q-band relative to the monomer consistent with the view that a single cation bridges the complex and allows for multiple conformational orientations between the phthalocyanine rings. The second stage dimer has a narrowed blue shifted Q-band indicating very few or only one conformational orientation between the phthalocyanine rings. ESR spectra of the second stage dimer display Cu-Cu coupling, while those of the first stage dimer do not, which is also consistent with the proposed mechanism. When the symmetry is lowered by removing one of the four crown ether peripheral substituents, the two-step mechanism attributes are still observed. 96 Variants of the peripheral crown ether structure have been reported where a benzene ring functionalized with two oxydecyl groups is Joined to each of the four crown groups'<' and where the crown ether group is separated from the phthalocyanine ring by an oxymethylene group145 (Table 10). In the former case a very strong aggregation was observed with an aggregate structure claimed to be composed of stacked

phthalocyanine units into single molecule diameter fibrils many thousands of molecules in length. Placement of silicon in the cavity with a diaxial hydroxyl substitution successfully prevented the large aggregate formation.

C. MULTINUCLEAR PHTHALOCYANINES The synthesis of multinuclear phthalocyanine compounds has added a new dimension to the study of phthalocyanine aggregation; that of intermolecular vs. intramolecular aggregation. Examples of binuclear and tetranuclear phthalocyanines in which aggregation has been investigated are depicted in Table 11. 1. Binuclear Phthalocyanines

A series of linking structures, wherein the length is systematically varied from a chain of zero to five atoms, was studied to examine the effect of the linkage length on the coupling between the two phthalocyanine terminal structures (Table 11). When a sufficient length of the linking moiety is obtained, a cofacial intramolecular association referred to as "clamshell behavior" is observed where a dynamic equilibrium exists between open and closed conformations.l " The electronic spectra display features that relate to the constraints imposed by the length of the linking chain on the coupling between the transition moments of the phthalocyanine rings. When the linkage structure is a single bond, the coupling must occur through this bridge, and, when the linkage structure has a length of five atoms, the coupling occurs through the space between a cofacial association as illustrated in Table 11. Uncoupled phthalocyanines display a spectral line shape of an isolated monomer such as that in Figures 7 or 8. In the analysis of phthalocyanine interactions, the exciton model (see Figures 4 and 5 and eq. 2) has been used to relate the coupling of the transition moments to a correlation of the orientation and separation distance of the phthalocyanine rings with the features of their electronic spectrum. In the series of free-base binuclear phthalocyanines with 0, 1, 2, 4, and 5 atom chain length connecting structures, the corresponding electronic spectra display a broadened blue shifted absorption relative to a mononuclear monomer spectrum with weaker partially resolved peaks at the longer wavelengths corresponding to an uncoupled monomeric phthalocyanine.l'" These spectra represent an intramolecular equilibrium that does not diminish at high dilution. The intensity of the weak monomer peaks is the smallest for the five atom bridged binuclear

161

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162 Table 11. Multinuclear Phthalocyanines

Binuclear Phthalocyanines

X

[Ref.] -

(single bond)

[146]

-0[147] -CH2CH2[148] -CH2CH2CH2CH2[148] R

CH3 I -OCH2-C-CH20 -

= OCH2C(CH3)3

0

0

-

Vo[148]

~o-

VO[148]

I

M = H2 , Cu, Co

CH3 [149]

-o-O+O-oi()-~-o-o-O+O-ol

\'\.J 0

)n = 20,50

[150]

[43]

phthalocyanine indicating that it would have the largest amount of intramolecular dimerization. Quantitative dimerization constants have been reported for the Co mononuclear and the two and five atom binuclear phthalocyanine compounds as 2.6 x 103 M- I , 4.9 X 103 M- I , and 3.3 x 103 M- I respectively in 1,2-dichlorobenzene at 20°C. 152 This appears to indicate that there is little difference between mononuclear and binuclear phthalocyanines with respect to intermolecular aggregation. At the other extreme, a binuclear phthalocyanine with a very long chain linking structure finds use as a telechelic polymer by way of intermolecular aggregation at high concentration. Intermolecular dimerization of terminal

phthalocyanine groups connected by a low molecular weight polymer has the effect of chain extension, and higher aggregation has the effect of forming a crosslinked network. In the phthalocyanine terminated poly(ether sulfone) oligomer example in Table 11, the effect of aggregation was diagnosed as an increase in the glass transition temperature, Tg, which correlates with effective increases in molecular weight and/or crosslink density.P" Using a polysulfone oligomer of 20 repeat units, the Tg increased from 162°C for the unterminated sulfone oligomer to 190°C for the phthalocyanine terminated oligomer. Increasing the polysulfone oligomer chain length to 50 units resulted in a Tg increase from 187°C to 197°C. This isconsistent with a

109 / Phthalocyan ine Aggregation

diluting of the effective crosslink site In the larger oligomer. 2. Tetranuclear Phthalocyanines

The tetranuclear phthalocyanine example in Table 11 uses pentaerythritol as the linking structure and provides a five atom bridge between the phthalocyanine units. 43 The cobalt derivative's dilute solution monomer spectrum consists of an uncoupled phthalocyanine absorption at 676 nm and an intramolecular aggregated absorption at 625 nm. This compound is far more susceptible to intermolecular aggregation than its mononuclear and binuclear analogs. It has an intermolecular dimerization constant of2.4 x 105 M- 1 in 1,2dichlorobenzene at 20°C,43 which is a factor of 100 greater than those for the mononuclear and binuclear analogs (2.6 x 103 and 2.3 x 103 M- 1 respectively'Y). Speculation has suggested that multiple phthalocyanine rings may participate in the intermolecular aggregation, which would result in a stronger interaction and an increased constant.43 D. AVOIDING AGGREGATION

Aggregation causes a perturbation of the phthalocyanine electronic structure, which is very undesirable in some applications that make use of the monomer's optical or catalytic properties. In these technical areas there is interest in structural design features which diminish or ideally eliminate aggregation. Various approaches are summarized in this section with an estimation of their success. The simplest and perhaps the best diagnostic of success in avoiding aggregation is a coincidence of electronic spectra from a phthalocyanine monomer in dilute solution and from the same phthalocyanine compound prepared as a neat thin film. 1. Axial Substitution and Metal Ion Effects

As briefly mentioned in Section III, blocking the coplanar association of phthalocyanine rings with octahedral coordination of the complexed metal ion is one of the most effective ways of not only diminishing aggregation but also of promoting solubility of peripherally unsubstituted phthalocyanine compounds. The pioneering work of Kenney et ale with Group IV metal phthalocyanines has well demonstrated the power of this approach to obtain clean structural characterization data from solutions of these compounds without interference from aggregation effects as well as elegantly documenting coplanar association effects by well-characterized monodisperse single-atom

bridged oligomers (Figure 9).74,89,153 While the axially bonded groups, particularly when silicon is the complexed ion, have frequently been relatively large in size (e.g. triethylsiloxy'<"), there is a most remarkable example where an axial substituent as small as the hydroxyl can be quite effective. One of the most strongly aggregating phthalocyanines known is the MPc(18crown-6-bz(OC 10-H21)2)4 example in Table 10.123 In this example for a free-base phthalocyanine, its crown ether substituents with the appended didecylocybenzo groups promote formation of an extremely large aggregate of up to 104 molecules that forms a gel in chloroform solution. When a silicon with axial hydroxyl groups is substituted for the cavity protons of the freebase, aggregation is reported not to occur in organic solution. Another interesting example involving axial substituted trialkylsilane silicon naphthalocyanine compounds further indicates that, while aggregation is suppressed in solution, it does occur in the solid state. 58,67 Electronic spectra of these compounds in thin film form display red shifted Q-bands indicative of J-aggregation with a small tilt angle (10-15°). Clearly, some ring overlap may occur despite the axial substitution. Other metal ions complexed in the phthalocyanine ring with axial substituents are significantly less effective in suppressing aggregation. Those with a single axial substitution include mainly AI, V, and Ti, and this usually involves a coordinated; chlorine, hydroxyl, or oxygen. When compared with other metals having no axial ligands, these monoaxial coordinated metal phthalocyanines display trends within one study that are reversed in another (see Table 8). Currently, it is not clear whether an aggregate structure of these monoaxial substituted phthalocyanine compounds involve a cofacial association with the axial ligands oriented outward, inward, or in mixed orientations. The lead ion forms a phthalocyanine complex that is exceptional for its low level of aggregation. Both the spectroscopic dimerization constant and VPO aggregation measurements (see Tables 5 and 8) reflect this resistance to aggregation relative to the free-base analog. As discussed above in subsection A, the lead ion's position relative to and distortion of the phthalocyanine ring are contributors to this effect. The large size of the lead ion is responsible for this effect.

2. Bulky Peripheral Beta-Substitution Peripheral group substitution at the j3-position on the phthalocyanine ring is the more prevalent and synthetically the more facile. To diminish aggregation with

163

164

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substituent groups at this position, several general approaches have been followed. These include a steric crowding close to the point of attachment, flexible-chain substituents with long lengths, capping groups and dendrimer substituents. These approaches have had varying levels of success. Structures representative of the steric crowding and the long flexible chain substituents are depicted in Table

12. In the first example eight alkyltosyl-aminomethyl groups occupy the fJ-positions of the phthalocyanine ring. 154 The Cu, Ni, Zn, and Co phthalocyanines with this substitution were reported not to deviate from Beer's Law in chloroform solution. In the second example eight perfluoro-2-propyl groups occupy the eight fJ-positions. 155 An X-ray structure for zinc phthalocyanine with this substitution indicates that the

Table 12. Sterically Crowded and Linear Chain {3-Substituents of Phthalocyanine Compounds

Sterically Crowded Substituents

o~o ~S=O

N

0q ~

.

~S':O

[154]

o [156]

[158] Linear Flexible-Chain Substituents

o

"z;

o~ o

O~O/

~O

;--..../0....-..

O~O~O~ \.---0

~O~O,

I

0""'--

~·o~o~o~o/

~

O~O~O~O,

[160]

I

~

.O~O~O~O~O~O~O~O~O~O/

A

[140]

I:

[121]

o

\/\/\/\/\/\/\/\/\/\/

~Si'O'Si'O'Si'O'Si'o'Si'o'Si'O'Si'O'Si'O'Si'O'Si""""""'"

~

-

109 / Phthalocyan ine Aggregation

phthalocyanine ring is planar, that the orientation of the CF 3 groups in the -CF(CF 3)2 substituents reside above and below the phthalocyanine plane, and that n-at stacking does not occur in the solid state. Aggregation is reported not to occur in a solution based on observations of a concentration independence of NMR and electronic spectra although acetone is coordinated axially to the zinc ion. Zinc tetradibenzobarrelenooctabutoxy phthalocyanine was synthesized with the intent that bulky dibenzobarrelene groups in the tJ-positions would suppress aggregation by the rigid steric orientations of the benzo groups above and below the phthalocyanine rings. I56 The evidence for aggregation suppression is based on observation of Beer's Law behavior of the electronic spectrum from this phthalocyanine in benzene solution up to a concentration of 4.5 x 10-4 M. The presence of the eight o-butoxy groups complicates the assessment of the effectiveness of the tJ-dibenzobarrelene groups. As discussed below, a-sub-substitution is one of the most effective ways to diminish aggregation and these two effects would need to be assessed separately and, preferably, quantitatively. An organic phthalocyanine glass was prepared using amine-epoxy chemistry wherein the peripheral groups encompass amine, hydroxyl, ether, and phenyl moieties in an irregular symmetry designed to disrupt aggregation. I5? While the glass is optically transparent, the thin film solid-state spectrum showed significant aggregation both below and above the 65°C glass transition. A nonionic water-soluble phthalocyanine with glycosylated substituents having four hydroxyl groups pendant to a pentose ring was synthesized with the objective of using steric hindrance to suppress aggregation. 158 This phthalocyanine was found to be monomeric in DMSO solution but aggregated in water. From the. above examples, it is not possible to definitively assess the merit of the bulky group induced steric hindrance approach toward suppressing aggregation without a solid-state spectrum or at least quantitative measurements of an aggregation parameter. The long flexible chain substituent approach to diminishing aggregation is represented by the examples in the lower part of Table 12. The hypothesis of this approach is that a long flexible chain will coil and, if long enough, cloak the phthalocyanine moiety and shield it from an intimate intermolecular association. Long chain substituents have availability as oligomers to certain polymers. As opposed to linear alkane chains, three of these structures have ether linkages and the fourth is a siloxane chain. Long linear alkane chains (dodecyl and octadecyl) have been employed as phtha-

locyanine peripheral substituents and are found to have little aggregation hindering capability (see Table 5). However, the more flexible chains such as those based on oxyethylene and silicone units have a greater tendency to coil and less tendency to pack in an ordered manner. They also confer a liquid character on the phthalocyanine at room temperature as opposed to a solid character for the alkane chains. The first example in the lower part of Table 12 utilizes branched oxyethylene chains of a 12 atom maximum chain length and, as such, combines a steric hindrance with the flexible long chain approach. 159 This system displays a solubility in many solvents ranging from carbon tetrachloride to water, but in many solvents exhibits, to varying degrees, a blueshifted Q-band characteristic of aggregation. By comparison, the second example, which incorporates hybrid oxyethylene-alkane chains of 15 atoms, has a significantly greater aggregating tendency as evidenced by a strong aggregation in polar solvents as well as in a thin film spectrum.l'" In the third example a polydisperse oxyethylene chain with a number average length of 29 atoms are the phthalocyanine substituents. I40 This liquid is soluble in solvents ranging from hexane to water but in neat form is strongly aggregated (see Figure 17). Finally, the polydimethylsiloxane oligomer substituted phthalocyanine is a case where the chain is one of the most flexible known and the substituents occupy 85% of the molecular volume. This phthalocyanine displays a very low dimerization constant (see Table 5) but is aggregated in neat form. I21 It would appear that this long flexible chain substituent approach, like that of the bulky substituent groups, has limited potential for suppressing aggregation. As described earlier, it is possible to restrict aggregation to the dimer stage by blocking one face of the phthalocyanine ring with a capping group. An example using a tetrafunctional oxyethylene cap is illustrated in Figure 2F, and it has a relatively low dimerization constant (last entry in Table 5).51 This represents a difficult synthesis at high dilution and is probably not practical if any significant quantity of material is desired. Theoretically, both phthalocyanine faces could be blocked by two such caps but the synthesis would be very complex. Dendrimer substituents are a special case of steric crowding where the substituent develops as a spacefilling globular polymeric structure that envelops its core as generations of monomeric units are added. The first dendrimer with a phthalocyanine core was synthesized in 1997, and the peripheral structures of four such systems, where observations pertinent to aggregation

165

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166 Table 13. Dendrimer Substituted Phthalocyanines

T

o T

T

T

= -OC2H s , -OLi

G = 1,2

G

G = 1,2,3

= -OC(CH 3)3' -OH·

[161]

[49]

~ O~ G

T

°Yir+T ~~ GT

= -OCH 3 , -OCSH 11, -ONa G = 1,2,3 T

[162]

have been reported, are depicted in Table 13. In the first example three generations of dendrimer were synthesized, and those terminated by the ester group were soluble in organic solvents while those terminated by the lithium carboxylate were water solublc.l" In solutions the three generations of the organic-soluble dendrimers were observed to display spectra resembling the monomeric phthalocyanine while the water-soluble dendrimers displayed aggregation that diminished as the dendrimer increased in size. The second example is a similar but simpler and less branched amide dendrimer.'?' The electronic spectra of thin films of the first and second generation of cobalt phthalocyanine dendrimers appear to indicate that the first generation is highly aggregated and the second generation is mostly nonaggregated which is attributed to increased steric

= -(CH2CH20 )3CH3, -CaHs G = 1,2,3 T

[163]

shielding of the higher generation. The third example is a one, two, and three generation benzyl-phenyl ether based dendrimer with ester and sodium carboxylate terminal groups, which promote respective organic and water solubilities.Y'' Absorption spectra of the ester terminated dendrimers in dilute chloroform solution are reported to be monomeric. The carboxylate terminated dendrimers in dilute aqueous solution display aggregation in their spectra which declines with increasing generation number. Fluorescence quenching correlated with the increased shielding of the higher generation dendrimers. The fourth example is very similar to the third except that the dendrimer repeat unit is reversed (i.e. based on 3,5-dihydroxybenzyl alcohol as opposed to dimethyl 5-hydroxyisophthalate) and the terminal groups are different.l'" Studies of the three generations

167

109 / Phthalocyan i ne Aggregation

of this dendrimer system, which include absorption spectra of solid-state thin films as well as solution spectra, concluded that aggregation is not suppressed by peripheral dendrimer substitution. However, if the dendrimer is bonded to the axial coordination site of silicon in silicon phthalocyanine, coplanar association is blocked. While this is not surprising, it is interesting to note that edge-to-edge interactions still occur with the first generation dendrimer and that the second and third generation dendrimers form solid solutions where short range order induced by interaction between phthalocyanine structures is supprcsscd.l'r'" 3. Alpha-Peripheral Substitution As discussed in subsection B.2 above, peripheral group substitution at the a-position causes the phthalocyanine ring to distort from planarity, and a significant reduction in aggregation may result, particularly if the substituent structure is properly chosen. Very meticulous work by Cook et ala has shown that the alkoxy group is more effective in reducing aggregation than is either the alkyl or the alkoxymethyl group. The alkoxy group also causes a greater red shift in the Q-band. A detracting feature of the alkoxy substituent is that it is a strong electron donating group, and if the number of alkoxy groups is large, it contributes to a photo-

oxidative instability. A useful trade-off in electron donation and aggregation suppression has been found in the fluoroalkoxy substituent. Phthalocyanine compounds with trifluoroethoxy substitution have been found to have remarkable photooxidative stability and to be highly resistant to aggregation in the solid state. 164 Absorption spectra of solid-state thin films closely replicate those of solutions. An example of the spectra of hexadeca(2,2,3,3,3-pentafluoropropoxy) phthalocyanine in dilute solution and as a thin film is depicted in Figure 18.115 Examples with nonfluorinated alkoxy groups are also claimed in the literature. 156 ,163b It is not yet clear whether total peripheral substitution of the phthalocyanine ring or only octa-o-substitution is necessary to achieve a thin film disaggregated absorption spectrum.

VII. Correlations with Physical Environment Physical effects that may alter the state of aggregation of phthalocyanine compounds include variations in solvent, addition of neutral disaggregating agents, presence of electrolytes, and addition of micelle forming surfactants. In practical applications, the selection of solvent and/or additive may be of particular importance. The response of the state of aggregation to these varying

Dilute solution

400

500

600

700

800

Wavelength(nm) Figure 18. UV-Vis spectra of hexadeca(2,2,3,3,3-pentafluoropropoxy) phthalocyanine in dilute acetone solution and as a solid-state thin film. ll s

Snow

168

conditions is a physical way of adjusting aggregation dependent properties to better meet the demands of a particular application.

A. SOLVENT Solubility and aggregation of phthalocyanine compounds have different origins. Solubility resides in substituent groups or coordinated ligands. Peripheral groups are primarily designed to promote solubility in particular solvents so that the phthalocyanine chromophore may be manipulated. Aggregation resides in the phthalocyanine chromophore and results primarily from attractive interactions between two or more of them. Solvents that will have the most pronounced effects on reducing aggregation will be those which compete with the aggregative interaction. While the aggregative interaction is modified by the nature of a complexed ion in the cavity and by the electronic and steric effects of peripheral groups, the interaction remains one of it-a: and Jr-Q' interactions 16S,166 on the frame of an organic molecule. A method of assessing which solvents compete with this interaction is to observe the frequency shift that occurs in the Q-bandof the monomer as the solvent is changed and how it departs from a dependence on refractive index.l'" Phthalocyanine compounds with nonionic peripheral groups in organic solution display a dependence on solvents that is influenced by the peripheral group and correlates with solvent polarity. The best studied example is the CuPC(,B-S02NHC18H37)4, whose dimerization constant in various solvents decreases in the following order: CCl 4 > benzene> toluene> CHCl 3 > dioxane > DMF > THF. 41 A solvent with a lower dielectric constant is less able to screen the n-at interaction from inducing formation of a dimer. 41,113 There are also thermodynamic data for this phthalocyanine system in benzene and CCl 4 solvents (Table 7), where in both the enthalpy and entropy are negative as would be expected. Changing solvent from benzene to CCl 4 increases the enthalpy from -10 to -13 Kcal/mole which may also represent the better screening effect of benzene. In another example of a phthalocyanine compound with nonionic but more polar peripheral groups and a solubility that spans CCl 4 to water, a more complicated dependence on solvent character is observed. In this system based on CuPc(,B-CH20CH(CH20(CH2CH20)3CH3)2)4, the solvent order of decreasing aggregation is: water> methanol > ethanol > CCl 4 > DMF > CHCl 3 > THF. 1S9 In this case, the peripheral groups already provide a significant dielec-

tric screen. A conjecture for this order of aggregation dependence on solvent is that a hydrophobic interaction of phthalocyanine ring toward water and alcohols promotes a very strong aggregation. This is followed by the solvent polarity effect starting with CCI4, which incrementally adds to screening phthalocyanine rings from each other as the solvents become more polar. A final example is the well-studied phthalocyanine tetrasulfonate system (Table 3). This system is complicated by having very small hydrophilic ionic peripheral groups bonded directly to the hydrophobic phthalocyanine ring. In addition to the it-n phthalocyanine attractive forces, there are coulomb repulsive forces from the negative charge of the sulfonate groups. The sulfonate groups promote aqueous solubility of the hydrophobic phthalocyanine ring of the monomer, but this solubilization works equally well with phthalocyanine aggregates. In water, the phthalocyanine tetrasulfonates have some of the highest dimerization constants measured. Addition of a water-soluble alcohol lowers the dimerization constant, and the less polar alcohols are more effective. The alcohol has a weaker hydrophobic interaction with the phthalocyanine and can provide some screening against the dimerization. The thermodynamics of the phthalocyanine sulfonate dimerization are particularly interesting and provide some valuable insight to the roles that water and other additives play in this process. Enthalpies and entropies for this system are tabulated in Table 4. Some very careful and complete data for the entropy of dimerization of the CoPC(,B-S03Na)4 system shows a temperature dependence that is initially unexpected.f" With increasing temperature, the entropy becomes less negative and crosses over to positive values at the higher temperatures. A dimerization process should be negative in entropy. The positive entropy change is proposed to result from release of water molecules coordinated with the monomer during the dimerization process. The additional energy required to release the water molecules also shows up in a diminished enthalpy. Addition of large quantities of water miscible solvents and organic compounds such as DMF, formamide, urea, thiourea, and pyridine as well as aclohols to aqueous solutions of phthalocyanine sulfonates has the effect of lowering the dimerization constant (Table 3). The thermodynamic measurements indicate that both entropy and enthalpy effects are involved (Table 4); the former being concerned with break up of the water structure around the monomer, and the latter being concerned with an interaction and screening action of the organic agent with the phthalocyanine ring.

109 / Phthalocyan ine Aggregation B. ELECTROLYTES AND pH

Increasing the ionic strength by addition of simple salts such as NaCl or NaCI0 4 to aqueous solutions of phthalocyanine tetrasulfonates results in a substantial increase in the dimerization constant (see Table 3) and the aggregation number (see Figure 14).88,101 The added salt has the effect of reducing electrostatic repulsion between sulfonate groups, and the attractive n-ot forces between phthalocyanine rings become more effective in promoting aggregation. This effect parallels that known for charged colloid suspensions where added salts shield electrostatic repulsion between particles to the point where an efficient solid separation can be made. The effect of pH on aggregation of sulfonated zinc phthalocyanine was observed to be small many years ago,29 which would be expected from a substituent as strongly acidic as a sulfonic acid. However, pH variation has a significant effect on the AI(CI,0)Pc(,B-S0 3Na)2 system 106 (see Table 3). Depending on the pH, two types of dimer were observed, and they were proposed to result from different substituents (CI or 0) coordinated to aluminum in the axial position. The effect of pH on aggregation of H 2Pc(COOH)8 system 108 are much stronger as might be expected from a more weakly acidic system. The absorption spectra of a 10- 5 M solution display features with varying pH indicating the following aggregation behavior: extended aggregation in a pH range of 2-5; a D 2h monomer spectrum in the 6-10 pH range; and a change from a monomer D 2h to a D 4h symmetry in the 10-12 pH range. The extended aggregation in the 2-5 pH range is correlated with hydrogen bonding of the peripheral COOH groups. In the 6-10 pH range the aggregates disintegrate as the carboxylate groups ionize. In this context, the phthalocyanine tetrasulfonate analogs are aggregated despite being ionized, so apparently the difference with this system may reside in there being eight peripheral carboxylate groups. At the highest pH range, the very weakly acidic cavity protons of this freebase phthalocyanine are abstracted causing the D 2h to D 4h symmetry change. C. MICELLES

)

Water-soluble phthalocyanine compounds have an important application in photodynamic therapy where in their photocatalytic ability to generate singlet oxygen is being researched as a treatment for cancers and other diseases. However, singlet oxygen generation requires that the phthalocyanine be in the monomeric state in an aqueous medium. A very effective approach has been to

employ surfactants, which form micelles in which phthalocyanines may be disaggregated. The structures of typical micelles that have been used for such work are depicted in Table 14. Most water soluble phthalocyanine compounds have charged groups at their periphery, such as a sulfonate or quaternary amine, and the nature of this charge determines whether an anionic or a cationic surfactant should be selected. In general, phthalocyanines with ionic substituents disaggregate better in micelles formed from surfactants with an opposite charge. Anionic phthalocyanines usually incorporate the sulfonate or the carboxylate group. The disaggregation of CuPc(,B-S0 3Na)4 has been investigated in aqueous systems of CTAB, DDAB, TEAB cationic surfactants, and the SDS anionic surfactant (see Table 14).68 In both the CTAB and DDAB systems at appropriate concentrations, the CuPC(fJ-S03Na)4 could be disaggregated predominantly to the monomer form. The CuPc(fJS03Na)4 remained highly aggregated in aqueous solutions with the TEAB, which lacks the long hydrophobic chains, and with the anioriic SDS. A rational is that the CuPC(fJ-S03Na)4 is bound to the cationic head group of the CTAB or DDAB at the micelle interface, while its hydrophobic long chain interacts with the phthalocyanine ring and screens it from forming aggregates. In another example, ZnPc(fJ-S03Na)4 along with some other sulfonated zinc phthalocyanine analogs were investigated with the nonionic surfactant, Tween 20 (polyoxyethylene(20)-sorbitanemonolaurate, see Table 14).168 Both absorption and fluorescence measurements indicated that the surfactant was effective at a 0.1 to 10/0 concentration. A tetra-substituted carboxylate example, ZnPc(fJ-COOH)4' displayed similar characteristics to the sulfonate analog above with regard to surfactant induced disaggregation. This study investigated cationic CTAB, anionic SDS, neutral Triton X-100, and a zwitterionic n-alkyl nicotinic acid surfactant.l" The anionic surfactant was ineffective; the cationic surfactant very effective; the Triton X-100 has some effect depending on its relative concentration to the phthalocyanine; and the zwitterion became effective when the alkane chain length increased from 12 to 16 and pH was acidic. In a similar finding, the octa-carboxylate substituted phthalocyanine H 2Pc(fJ-COOH)8 was strongly disaggregated by the cationic TOAC but unaffected by the anionic SDBS surfactant. 108 Likewise, carboxylate terminated phthalocyanine dendrimers displayed a disaggregating response to surfactants in the order CTAB > Triton X-100» SDS, AOT. 17o In another particularly interesting study of

169

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170 Table 14. Surfactant Structures

CTAS

~

o 0

DDAB

Na+-o-sto~ n o

o:~ TEAS

o Tween 20

TOAC HO~O/

PEO

carboxylate terminated phthalocyanine dendrimers, the molecular weight effects of a neutral polyethylene oxide disaggregating agent were assessed by absorption and fluorescence spectroscopies and light scattering.T" For polyethylene oxide chains of 400, 2000, and 10,000 molecular weight, it was found that the higher molecular weight was very effective in disaggregating the phthalocyanine by a proposed wrapping of individual phthalocyanine molecules in the PEO chain. Cationic phthalocyanines use the quaternary amine substituent, and the distance that the positive charge is positioned from the phthalocyanine periphery has some importance. The disaggregation of ZnPcCBOCH2CH2N(CH3)3I)4 in an inverse micelle system was investigated using the anionic AOT and cationic CTAB surfactants."" The AOT was very effective in disaggregating the phthalocyanine to the monomeric form, and the CTAB furthered the degree of aggregation. A study in aqueous media of the ZnPc({:1-N(CH3)3CH3S03)4 and ZnPc({:1-N(CH3)2(C6H13)CH3S03)4 systems without

Triton X-100

added surfactant involved a comparison of the effect of the longer chain attached to the quaternary amine. 111 Most interestingly, the long chain appeared to have the effect of a disaggregating surfactant in that it resulted in the phthalocyanine being monomeric while the trimethyl ammonium analog was aggregated in water.

VIII. Practical Applications There are many applications for phthalocyanine compounds such as pigments, xerography, laser printing, optical recording; and most of these are dependent on a solid-state property derived from an intermolecular cooperativity of phthalocyanine molecular elements in an ordered array.l " However, there are some applications that are critically impacted by the phenomenon of aggregation; in particular, those applications that utilize a property derived from the unaggregated state or a state where the aggregation state is very low. Examples

109 / Phthalocyan ine Aggregation

to be discussed here include optical filters, photodynamic therapy, and optical limiting. In these cases it is important that aggregation be suppressed or restricted.

A. OPTICAL FILTERS The photochemical stability and spectral absorption line shape of the monomeric phthalocyanine chromophore are attractive features for optical filter applications. These features are useful when the spectral distribution and intensity from an illumination source must be altered or precisely controlled.l?" The narrow width of the phthalocyanine Q-band is particularly useful in achieving a sharp threshold in wavelength. Filters using' organic dyes typically involve incorporation of the dye as a homogeneous blend in a glass, gelatin, or plastic. Incorporating dyes, which have strong tendencies to aggregate, into these matrices requires a thermodynamic compatibility or a kinetic quench to obtain the dye dispersed in a nonaggregated state. Gelatin filters are used with water-soluble dyes and are generated by simple evaporation of an aqueous co-solution of dye and gelatin followed by encapsulation. 174 With the exception of the water-soluble phthalocyanine dendrimer,49,161,162 most phthalocyanine compounds strongly aggregate under these conditions and do not appear to be good candidates for this sort of filter. The sol-gel process has been used to incorporate phthalocyanine compounds into rigid silica and alumina glassy matrices. 175-177 These studies have involved water-soluble tetra-sulfonated phthalocyanine compounds dispersed in tetramethoxysilane and butoxyaluminoxy triethoxysilane. During the cure of these matrices, an aggregation was observed and attributed to the change in the alcohol to water ratio accompanying curing reaction. Some control over the aggregation could be exercised by regulation of the pH and addition of disaggregating agents. The dispersal of phthalocyanine compounds into thermoplastic and thermoset polymers generates an optical filter that is flexible and can be cut to a desired size and shape. This dispersal also requires some degree of compatibility between the polymer matrix and the phthalocyanine to generate a homogeneous solution of the monomeric dye in the plastic. The phthalocyanine peripheral groups are usually designed to promote this compatibility. For example, the cumylphenoxy group has been found to promote dispersal of the corresponding tetra-substituted phthalocyanine in a polycarbonate matrix. For the thermoplastic polymer, the dispersal

may be accomplished by simple codissolution of the phthalocyanine compound and polymer in a common solvent followed by casting a film. Such filters composed of octa-o-butoxy substituted phthalocyanines blended into vinylacetate-ethylene copolymer and cellulose acetate films have found use in the regulation of plant growth.l " The slow evaporation of the solvent may result in dye aggregation or a phase separation. It may be possible to avoid this by coprecipitating the dye and plastic, then melt processing this dispersed mixture. The higher processing temperature and rapid quench improves the avoidance of aggregation. For a thermoset polymer such as an epoxy, the dye is dissolved in the thermoset monomer or low molecular weight prepolymer, then high molecular weight polymer is formed through a curing reaction into a three-dimensional network that occludes the dye. If the phthalocyanine peripheral groups are designed to participate in the curing reaction by covalently linking to the network, prospects are improved for a uniform dispersal and avoidance of aggregation. An example of such a system is an aminophenoxy tetra-functionalized phthalocyanine incorporated into a polyurethane or epoxy thermoset. 179

B. PHOTODYNAMIC THERAPY Photodynamic therapy is a medical treatment directed against target tissue in which a molecular photosensitizer is selectively adsorbed and then irradiated to catalytically generate singlet oxygen. The localized concentration of singlet oxygen attacks and kills the tissue. The phthalocyanine chromophore has been found to be among the most effective photosensitizers in this application. A large volume of work has been conducted in this area, and this has been the subject of a number of reviews.180-182 The critical features are a selective uptake of the dye into the target tissue or tumor, a strong absorption in the far red or near IR, a long triplet excited state lifetime, a good photooxidative stability, and an excretion from the body after therapy. A selective uptake into the target tissue is a challenge for the design of peripheral groups, which involves considerations of water solubility, lipophilicity, and penetration of cell membranes. A strong absorption in the far red and near IR accommodates a good depth of penetration by the light into the tissue. The phthalocyanine and naphthalocyanine chromophores in the monomeric state meet this feature nicely. The triplet excited state lifetime is an aspect where phthalocyanine aggregation is a very important concern as discussed further below. Photochemical stability correlates with

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the triplet excited state .lifetime as well as with the solvent and electron withdrawing and donating character of peripheral substitucnts.i'" In the process of generating singlet oxygen catalyzed by a phthalocyanine sensitizer, the first step is light absorption by this .chromophore and transition to a singlet excited state. This excited state has a nominal subnanosecond lifetime, and returns to the ground state with fluorescence or undergoes intersystem crossing to the triplet excited state. The lifetime of this triplet state is a factor of 103 to 106 longer, which is sufficient time for an energy transfer reaction with molecular oxygen to convert the oxygen from its ground state triplet to the reactive singlet form. The longer the phthalocyanine excited state triplet lifetime can be extended, the more efficient the generation of singlet oxygen. Singlet oxygen reacts with electron rich tissue components such as cholesterol, unsaturated fatty acids, protein amino acid residues containing cysteine, histidine, and tryptophan, and the guanine and thymine bases of DNA~ 182 Factors that extend the excited state triplet lifetime are the selection of the metal ion and, in particular, a resistance to aggregation. Phthalocyanines complexed with paramagnetic ions have shortened excited state lifetimes and diminished photoactivity compared with those complexed with diamagnetic metal ions. 180 Complexed ions, such as AI, Zn, and Si, are reported to have longer excited state triplet lifetimes.180 Aggregation of phthalocyanines is reported to markedly shorten the excited state lifetimes and reduce the efficiency of singlet oxygen production. Some representative studies include: a comparison of zinc tetrasulfonated phthalocyanine and naphthalocyanine in an aqueous buffer with and without cetyl pyridinium chloride added as a disaggregating agent.l'" measurement of excited state dynamics of free-base and zinc tetrasulfonated phthalocyanine as an aggregate in buffered water and disaggregated in DMSO;185 dimerization, singlet oxygen generation, and cytotoxic effects on He La cells of a series quaternary ammonium substituted zinc phthalocyanines; 111 correlation of dimerization, triplet state lifetime, fluorescence quantum yield, and singlet oxygen generation with a pH dependence for a disulfonated aluminum phthalocyanine.l'" correlation of phthalocyanine uptake into mouse carcinoma cells with variable sulfonate substitution, overall hydrophobicity, and degree of aggregation of an aluminum phthalocyanine series of compounds; 133 investigation of aggregation in tumor tissue, skin and combinations of solvents, buffers, disaggregating surfactants and biomaterials of a series of zinc

phthalocyanine compounds with variable phenylsulfonate substitution.P" All of these studies indicate the importance of disaggregating the phthalocyanine chromophore to the monomeric form in the biological medium to obtain an efficient generation of singlet oxygen for the cytotoxic activity on selected tissue. To obtain phthalocyanine compounds in the monomeric state, several strategies are employed as reviewed earlier. The use of disaggregating agents and surfactants is mentioned above. These approaches have the disadvantage that dilution on administration will result in aggregate formation. The dendrimer approach has been put forward as a way of circumventing this difficulty.l " Another approach, which appears to be enjoying remarkable success, uses silicon axial substitution with a naphthalocyanine chromophorc.P''

C. OPTICAL LIMITING

Optical limiters are nonlinear optical devices that can rapidly attenuate transmitted light to a desirable intensity threshold.I'" They are utilized to protect eye and optical sensing equipment from sudden exposures to high intensities of irradiation from a variety of sources. The nonlinear optical properties of a number of phthalocyanines have been investigated, and there has been an emphasis on the design of materials with large nonlinear absorption coefflcients.P" A reverse saturable absorption mechanism has been assigned to this photochcmistry.l'" This is a sequential two-photon mechanism. The phthalocyanine chromophore is initially excited to the first singlet excited state followed by a rapid intersystem crossing to the first excited triplet state. If this triplet state has a sufficiently long lifetime, its population reaches a threshold where a second photon absorption occurs and results in a transition from the first to the second excited triplet state. This second transition has a very high transition moment and becomes the dominant transition. The onset of this second transition causes a very strong nonlinear attenuation in transmitted light. A very fast response « 1 ns) is essential for eye protection. Like the photodynamic therapy, a critical feature of this mechanism is a long lifetime first triplet excited state. Certain complexed heavy metal ions, such as lead and indium, promote the intersystem crossing to the first triplet state. In most cases dimerization and aggregation of phthalocyanine chromophores cause a substantial shortening of the triplet state lifetime. As such, phthalocyanine compounds designed for optical limiting need features that avoid aggregation.

109 / Phthalocyan ine Aggregation

Devices used for optical limiting usually require that the phthalocyanine component have a very short optical path length. Accordingly, phthalocyanine compounds are incorporated as a confined concentrated solution or as a transparent thin film. In either case, aggregation is an issue that requires control. In the former case, representative studies indicate that axial substitution is sufficient to diminish aggregation in solution such that significant optical limiting is observed. These studies include indium phthalocyanine with axial fluorophenyl and trifluoromethylphenyl substitution 190,191 and titanium phthalocyanine with substituted catechol axial substituents.l'" As solid-state films, electronic absorption spectra of these systems190 display significant aggregation as do other systems on which solution optical limiting studies have been conducted. 192,193 Spectra of thin films of liquid phthalocyanines also display aggregation. 120 However, the hexadeca(perfluoroalkoxy) substituted phthalocyanines with a variety of metal ion substitutions have been shown to be remarkably unaggregated in the form of a solid-state film (see Figure 18), and significant nonlinear absorptions have been reported for this system. 164 ACKNOWLEDGMENTS

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