Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties

114

Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties MICHAEL HANACK and DANILO DINI Institut fur Organische Chemie, Universitat TUbingen, 72076 TUbingen, Germany

I. Introduction II. Synthesis of Stacked Phthalocyanine Polymers A. Nonbridged Stacked Polyphthalocyanines 1. Stacked Unsubstituted Phthalocyanines 2. Stacked Substituted Phthalocyanines 3. Stacked Doped Phthalocyanines B. Bridged Stacked Polyphthalocyanines 1. General ities 2. Bridged Phthalocyanines with Divalent Central Atoms 3. Bridged Phthalocyanines with Trivalent Central Atoms 4. Bridged Phthalocyanines with Tetravalent Central Atoms III. Properties of Bridged Phthalocyanines IV. Conclusions ' References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I. Introduction The verification of columnar arrangcmcnts'
. . . . . . . . . . . . . .

251 253 253 253 255 259 260 260 260 267 269 273 275 276

represent the most effective working units for the accomplishment of fast communication functions, but they also possess the feature of transporting the entity of interest selectively between the extremities of the columnar structure. 28-36 Both features of selectivity and velocity of the transport through the columnar structure of the compound imply the presence of suitable channels whose nature depends basically on the entity to be carried. For example, if the electron is the entity to be carried through the structure, then the suitable channel will be constituted by non-bonding electronic orbitals which overlap along the direction of the columnar structure. 16-20 On the other hand, the channels for the transport of molecular species must fulfil the requirements of a tube whose walls possess coordinating centers for the molecule to be carried, and with a diameter sufficiently large to allow the passage of the molecule in its different conformations. 33,37-40 Suitable channels for the transmission of photons should be based on those materials having high transmittance and suitable refractivity in correspondence of the photons energy, and, additionally, the necessary processability for the creation . I mo Iecu Iar wavegui'd es.22,41-43 of quasi. one- diimensiona The realization of such devices with highly anisotropic © 2003 Elsevier Science (USA) All rights reserved ISBN 0-12-393220-3

251

252

Hanack and Dini

properties is strictly related to the possibility of synthesizing functional materials 44-50 which can be grown,51,52 polymenzed.V''" or assembled 6,61,62 in the desired columnar fashion. Among the various classes of molecular compounds assembling in columnar structures,3-7,63-68 the metalmacrocycle complexes derived from combination of multivalent central atoms M and coordinating conjugated systems like porphyrins (Pors), tetraazaporphyrins (TAPs), phthalocyanines (Pes), naphthalocyanines, , (Ncs), and their derivatives (Figure 1) represent a special case because they can stack in a columnar fashion through a large variety of metal-macrocycle-Iigand combinations (Figures 2 and 3).1,69 It is expected that this kind of arrangement leads to the formation of highly anisotropic systems having electron delocalization through overlap of the rr-orbitals belonging to adjacent macrocycles and/or bonds connecting the central atoms. The central atoms can be either linked directly or bridged through bidentate ligands L (Figure 3). The existence of such interactions between different metalmacrocycle units leads to the creation of new molecular entities which have to be considered as polymers 70 constituted by stacked monomers and possessing

specific chemical and physical properties. Illustrative examples of the evolution of spectral, electrical, and electronic features in passing from a single macrocycle to piled structures of macrocycles can be found in several reviews.69,71-75 The verification of stacked arrangements whose basic units are constituted by macrocyclic compounds like those presented in Figure 1, is basically due to the quasi-planar geometry of the involved macrocycles" The planarity of the macrocycle is a prerequisite for the achievement of onedimensional stacked polymers77 but not all planar macrocyclic systems arrange in such a fashion. 1 This is because the molecular packing obtained by the overlapping of corresponding parts from adjacent metalmacrocycle complexes can give rise to one-dimensional stacked structures with relatively high energy due to electrostatic repulsion between corresponding parts which possess the same partial charge.v' Generally, the central atom represents the positively charged center of the complex whereas the conjugated macrocycle is the part of the complex in which the negative charge resides. On these premises the molecular packing which minimizes the electrostatic energy in those systems with marked charge separation will be either ri- or {3-

~ N" :Q

~N ~

N ~

(a)

§

~

~

N((M)) ~\

~

N

(b)

~

§

(c)

(d) Figure 1. Basic structures of a metal-porphyrine (PorM) (a), a metal-tetraazaporphyrine (TAPM) (b), a metalphthalocyanine (PcM) (c), and a metal-naphthalocyanine (NcM) (d).

114/ Stacked Polymeric Phthalocyan ines: Synthesis and Structu re-Related Properties ,,

I

-N--M--N-

I

-N--~--N-

,

-N--M--N-

-N--~--N-

,

I

-N--M--N-

-N--~--N-

,

I

-N--M--N-

-N--~--N-

,

I

-N--M--N-

-N--~--N-

,

I

(a)

(b)

Figure 2. Possible stacked arrangements of metal-macrocycle complexes: (a) one-dimensional arrangement and (b) r:J.,- or ~-crystal structures. The full line here indicates the plane of the coordinating macrocycle.

M

L

M

L

M

L-

Figure 3. General structure of a metal-macrocycle complex in the stacked arrangement with the central atoms connected via a bidentate ligand L. The square indicates the coordinating macrocycle.

modification (Figure 2b) in which the distances between corresponding parts of different metal-macrocycle complexes are largest. One-dimensional arrangement like that shown in Figure 2a can be directly achieved from inclined stacked structures (Figure 2b) by means of redox processes involving the oxidation or the reduction of the macrocycle or the central atom. This has been demonstrated with PcNi which crystallizes in the ,B-modification in its neutral form and, upon oxidation, in a one-dimensional stacked arrangement as schematically depicted in Figure 2a. 78-81 Such a structural rearrangement is usually accompanied by dramatic changes of some physical properties like conductivity,82-85 photoconductivity.f" or optical absorption/ due to modification of the interactions between electronic orbitals from neighboring complexes. 71,74,87 The interest in the design and synthesis of onedimensional polymeric materials in which the relative orientation of the constituent conjugated macrocycle units can be controlled by the introduction of bonds stronger than the weak intermolecular forces present in macrocycles of relatively low molecular mass, relies upon the possibility of creating robust materials with unique electrical and mechanical properties. 88-9o Different from network 91-97 and side-chain98-101 polymers, the one-dimensional stacked polymers whose constitutive units are metal-conjugated macrocycle complexes, represent the most convincing candidates

for the practical realization of organic metals 71,73,102-112 due to the high experimental values of the electronic conductivity (> 102 S cm- 1)78,79,82 combined with the exceptional chemical stabili tyl13 and the typical structural versatility of macrocycles like Pes or Pors. 114-117 The present chapter will firstly deal with the preparative chemistry of stacked polymers whose units are constituted by Pes with the aim of updating and enriching the content of previous reviews on the same topic from our working group.l,2,118 The second part of the chapter will be mostly devoted to an analysis of the chemical and physical properties of the stacked Pc polymers. For an exhaustive description of the chemistry and the physical properties of stacked polymers made by conjugated macrocycles other than Pc, the reader is referred to some recent literaturel19-124 and other relevant works published before 1990.78,80,125-132

II. Synthesis of Stacked Phthalocyanine Polymers A. NONBRIDGED STACKED POLYPHTHALOCYANINES

1. Stacked Unsubstituted Phthalocyanines

Nonbridged stacked one-dimensional PcM are rarely found when metal-phthalocyanine (PcM) is in the neutral state. The most important example of this case is given by neutral monoclinic PcPb which crystallizes in a columnar fashion with the molecules stacked along the direction of the c axis (Figure 4).77 The preparation of PcPb crystals is accomplished through the reaction of PbO with phthalonitrile in refluxing high-boiling point solvents followed by sublimation of the dry product in nitrogen atmosphere at 250°C.133 Other methods for the synthesis of PcM can be adopted and a survey of the most used ones is given in Scheme 1.134-137 In Scheme 1, one can recognize two substantially different pathways for the formation of PcM involving on one side the central atom substitution of a preformed macrocycle like PcLi or PcR 2 by reaction with metal halides, and, on the other side, the direct metalmacrocycle complex formation from phthalodinitrile, phthalic anhydride, and their derivatives combined with the corresponding metal salts or the pure metal.i Typical catalysts are usually weak bases whose most used representative is 1,8-diazabicyclo[5.4.0]undec-1-ene (DBU).138,139 Another case of neutral metallophthalocyanine complex with stacked columnar structure is represented by

253

Hanack and Dini

254

PC3Bi2140 in which the molecular units constituted by the triple-decker bismuth phthalocyanine are packed in a quasi-columnar fashion in the triclinic system (Figure 5).140 The synthesis of PC3Bi2 is achieved by heating under vacuum Bi2Se3 with an excess of phthalodinitrile at 220°C for 24 h. 140

From the examples ofPcPb and PC3Bi2, one can figure out that PcMs having the central atoms with high atomic number N A (let us rule N A > 80), will tend to crystallize in a columnar fashion. This can be explained in terms of

Figure 4. Columnar structure of PcPb. The image refers to a section of the molecular column (adapted from Ukei, K. Acta Cryst. 1973, 829,

Figure 5. Structure of triple-decker PC3Bi2 (image adapted from Janczak, J.; Kubiak, R.; Richter, J.; Fuess, H. Polyhedron 1999, 18,

2290).

2775).

o: I.

,&

CN

eN MX21 solvent

PcH2

MX21quinoline

------.

~

MX21solvent

room temperature

)

MX2 formamide

~:H. o

~

PcL~

MX2 1 u rea

. catalyst,solvent 475 K

~:

Scheme 1. Synthetic routes for the general preparation of PcM.

0

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties

variation of the charge separation between the conjugated macrocycle (usually with negative charge -2) and the positively charged ~entral atom induced by the change of polarizability due to the size of the central atom in the complex metal-macrocycle.l'" The larger the electronic cloud surrounding the central atom M, the higher is the polarizability of the resulting PcM. In particular, this has been verified by means of the optical determination of the susceptibility and hyperpolarizability of a series of PcMs whose third-order electronic hyperpolarizability values increased in the order PcCo < PcNi < PcCu < PcZn < PcPt. 142 As a consequence of the PcM polarizability increase, the charge separation between the central atom and the macrocycle is reduced. Therefore, the stacking of analogous parts belonging to neighbor PcM complexes is not prevented by repulsive electrostatic forces in these cases. In addition, the interplanar distance between macrocycles is made greater with the the increasing atomic number of the central atom, thus contributing to the further diminution of possible repulsive electrostatic forces between stacked units. 143 On the other hand, complexes having higher charge separation between macrocycle and central metal due to the higher charge density in smaller central ions will not arrange in columnar structures.i In fact, the shrinking of the electronic cloud volume accompanying the diminution of the atomic number leads to the decrease of the electronic polarizability with consequent increase of the electronic stiffness of the central atom toward the electric fields produced by the negative charge of the macrocyclic ligand. 144

2. Stacked Substituted Phthalocyanines The formation of one-dimensional stacks constituted by neutral metal-macrocycle complexes can be also obtained with discotic phthalocyanines possessing liquid · . .In t h e can d ense d p h ase. 145-150 crysta11me properties Discotic liquid crystalline Pes can be achieved generally through the octasubstitution at the positions 2, 3, 9, 10, 16, 17, 23, 24 of the Pc ring with alkyl, alkoxy, or thioalkyl groups havi avmg at Ieast five C atoms (Fiigure 6) .146 ' 151-163 The synthesis of liquid crystalline Pes with columnar structure is analogous to the preparation of the unsubstituted Pes (Scheme 1) when the corresponding precursors, i.e., 4,5-disubstituted phthalonitriles, are available. In Schemes 2, 3, 4, and 5 the syntheses of a series of precursors, respectively 4,5-dialkyl,158 dialkylalkoxy.l " dialkoxy.i'" and dithioalkyI159)62,164,165 phthalonitriles, which are used in the preparation of columnar liquid crystalline Pes are schematically described.

The synthesis of a 4,5-dialkylphthalonitrile I58 (Scheme 2) starts with the iodination of phthalimide 1 in sulphuric acid at 70 °C to give a mixture of 4,5diiodophthalimide 2a and 4,5-diiodophthalic acid 2b. The compound 2a is treated with concentrated ammonia to give 4,5-diiodophthalamide 3 which is then converted into 4,5-diiodophthalonitrile 4 by trifluoroacetic anhydride in pyridine. 166 The phthalonitrile 4 is coupled with l-hexyne in triethylamine at 110°C in presence of a catalytic amount of diacetatobis(triphenylphosphine)palladiumffl l'" with resulting formation of 4,5-bis(1hexynyl)phthalonitrile 5. The last step involves the catalytic hydrogenation of 5 with hydrogen over Pd/C catalyst for the production of the target species dihexylphthalonitrile 6. The synthesis of a 4,5-bis(dialkoxyalkyl)phthalonitrile 157 involves as first step the bromination of o-xylene 7 with bromine and the resulting formation of 4,5-dibromo-o-xylene 8. 168 In the second step, the methyl groups of 8 are brominated with N-bromosuccinimide via a mechanism involving radical formation and which gives the tetrabromo derivative 9 as main product. 169 The addition of the paraffinic dodecyl chain to 9 is achieved through the reaction of 9 with sodium dodecylate in t-butanol by substituting the bromine atoms on the methyl carbons with the dodecyloxy group. The resulting 1-(4,5-dibromo-2-dodecyloxymethyl-benzyloxy)-dodecane 10 is treated with cuprous cyanide in dimethylformamide to give 4,5-bis(dodecyloxymethyl)phthalonitrile 11.170-173 The synthesis of 4,5-dialkoxyphthalonitrile is presented in Scheme 4 with catechol 12 as starting material.l'" The first step is represented by the dibromination of catechol in the positions 4 and 5 with bromine in acetic acid which leads to the formation of 4,5dibromo-catechol l L'{" Subsequently, 13 is O-alkylated in dimethylformamide with octylbromide in the presence of the base sodium methylatc'{" to give 1,2-dibromo-4,5bis(octyloxy)benzene 14. The last step is the cyanation of 14 with cuprous cyanidel70-173 with the resulting formation of 4,5-bis(octyloxy)phthalonitrile 15. For the preparation of the thio-analogues of 15, the starting material is phthalic acid 16 (Scheme 5) which is firstly chlorinated into 4,5-dichlorophthalate in basic medium.i'" The 4,5-dichlorophthalate is then converted into the corresponding 4,5-dichlorophthalic acid 17 by hydrolysis in hydrochloric acid solution. The 5,6dichloro-phthalic acid anhydride 18 is obtained from the reaction of 17 with acetic anhydride and then it is converted into 5,6-dichloro-phthalimide 19 through reaction with formamide.l'" The successive step is the

255

Hanack and Dini

256

R

R

R

R R

R

(a)

R

M

ref.

-(CH 2)2(OC2H s)CH3

2H

-(CH2)2(OC2Hs)7CH3

2H

158 156 146 157 157 157

-(CH2)2(OC2Hs)14CH3

2H

157

-SC aH17

2H,Cu

159

-SC 10H 21

2H,Cu

159

-SC 12H 2S

2H,Cu

159

-SC 16H33

2H,Cu

159

-C SH 11 2Li,Zn -OC SH 11 Ni,Pd,Pt -CH20C12H2S 2H,Cu 2H -CH(OC 12H 2S)2

(b)

Figure 6. (a) 2,3,9,1 O,16,17,23,24-0ctasubstituted metal-phthalocyanine complex; (b) liquid crystalline columnar structure of (a).

transformation of 19 into 4,5-dichloro-l,2-benzenedicarboxamide 20 in ammonia solution followed by the cyanation of 20 with thionyl chloride in dimethylformamide at 0 °C.165 The product of the cyanation of 20 is 1,2-dichloro-4,5-dicyanobenzene 21 which is further converted into 4,5-bis( dodecylthio)phthalonitrile 22 with dodecylthiol in dimethylsulfoxide in the presence of potassium carbonate. 159 ,162

The possibility of achieving columnar discotic mesophases (Figure 6b) whose molecular constituents are octasubstituted 2,3,9,10,16,17,23,24 Pes (Figure 6a) is mostly related with the tendency of analogous submolecular groups to segregate and form chemically homogeneous subdomains.l" In the case of Pes derived from the precursors 6, 11, 15, 22, or analogous phthalodinitriles, the rigid conjugated macrocycles

Ix» InCO,H

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties

C<:H

H2SO4

3

H

1.0

I

I

2a 0

0

1

0

..

12

•

pyridine

.0

C0 2H

· 2a

NH40H

•

InC02N~ I

2b

CN In I

I

1.0 3

eN

5

4

5

Scheme 2. Synthesis of 4,5-dihexylphthalonitrile from phthalimide.

~CH3

~CH

Sr2

..

srn I

h-

Sr

3

CH3

- - - - - -... N-bromosuccinimide

CH3

8

7

C

12H 2SONa 9 ---:..=.,;...-==-----.... •

t-butanol

Scheme 3. Synthesis of 4,5-bis(dodecyloxymethyl)phthalonitrile from a-xylene.

~OH

Br~ U

~OH

Br

I.&-

OH OH

NaOCH 3,C aH 17 Br

a

dimethylformamide

13

12

CuCN

14

C02N~

...

dimethylformamide

Scheme 4. Synthesis of 4,5-bis(octyloxy)phthalonitrile from catechol.

257

Hanack and Din;

258

C0 2H ~ r COH c 2

1) CI2,KOH.. ClnC02H 2) HCI

~

CI

17

16

18

HCONH2

..

°

CI

NH

...

C0 2H

CI CI

NH4OH/H 2O

CI

NH2 NH2

~

CI 0

19

20

C 1 n CN

SOCI2

•

dimethylformamide CI

~

I

°

18 0

CI

20

(CH3CO)20

CN

0

I

C12H25SH K2CO3 C12H25SXXCN I

dimethYISulfOxid: C H S 12 25

~

CN

22

21 Scheme 5. Synthesis of 4,5-bis(dodecylthio)phthalonitrile from phthalic acid.

tend to pile up whereas the paraffinic chains mix together. This phenomenon occurs on the basis of the empirical principle similia similibus solvuntur which expresses the tendency of groups with similar character to aggregate together according to the combinations polar-polar, apolar-apolar or hydrophilic-hydrophilic and hydrophobic-hydrophobic. Some authors tried to quantify this aggregation tendency of chemical groups sometimes called sympathy-antipathy effect 150 through the definition of group molar attraction constants based on the heats of vaporization or vapor pressures of the groups themselves. 150 ,176,177 The usefulness of these theoretical treatments resides in the possibility of predicting the packing properties of a molecule in the condensed phase through the simple analysis of the constituents of the molecule. This is equivalent to say that the examination of the single molecule structure can give insights on the possible arrangement of the molecule in condensed phases through the evaluation of those thermodynamic properties of the condensed phase which are strongly dependent on intermolecular forces. 176,177 In the case of the octasubstituted Pes presented in Figure 6a, the occurrence of columnar stacked arrangements (Figure 6b) is found to be easily predictable because of the confinement of the different chemical groups in well-defined parts of the structure geometry. 178 From the structure in Figure 6a, it can be easily recognized that the presence of a rigid central core constituted by the conjugated macrocycle which is

surrounded by flexible paraffinic chains radially orientated with respect to the central rigid core. In these systems, there is a strong tendency for the molecules to segregate into chemically homogeneous sub domains with piled conjugated cores and entangled paraffinic chains. Such tendency prevents the formation of disordered structures constituted by mixtures between aromatic cores and alkyl chains due to the substantially different shapes and sizes of the two molecular subunits.l " In thermodynamic terms, thefree Gibbs energy of mixing ~GM (eq. 1) between different parts of the same molecule is the relevant parameter to evaluate for the prediction of the arrangement in the condensed phase. (1)

In eq. 1, ~HM, ~SM and Trepresent respectively the enthalpy of mixing, the, entropy of mixing, and the temperature. The two boundary cases are represented respectively by segregated ordered structures (stacked arrangements) and disordered mixtures (dispersions of aromatic rings in alkyl chains). The enthalpy ~HM associated with the mixing of aromatic macrocycles and alkyl chains is positive and then unfavorable to the spontaneous mixing of chains with aromatic macrorings. ~HM is evaluated by considering Van der Waals interactions as the sole kind of electrostatic interaction between these nonbonded molecular units and results: 150 (2)

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties

where rl and r2 are the characteristic linear lengths respectively of the paraffinic chain and the macroeycle, and n is an exponential whose value depends on the type of electrostatic forces considered, i.e., dipole-dipole, induced dipole-dipole or induced dipole-induced dipole. From eq. 2 it is evident that ~HM > 0 when rl i=- r i- The latter condition corresponds to the case of molecular subunits having different characteristic lengths. Therefore ~HM is favorable to the formation of segregated piled structures in those cases of octasubstituted Pes reported in Figure 6a. On the other hand, the entropy of mixing ~SM is always positive due to the increased degree of disorder associated with nonsegregated mixtures and will prevail with the rising of temperature T. At room temperature, it is found that ~GM > 0 150 for the group of compounds of Figure 6a and consequently stacked piled structures have to be expected with these molecular systems.

(a)

259

(b)

Figure 7. (a) ex-modification of Ge(ll)Pc (inclination angle 26.5° with respect of the vertical axis); (b) ,B-modification of Ge(ll)Pc (inclination angle 45.8° with respect of the vertical axis).

3. Stacked Doped Phthalocyanines In the two previous paragraphs describing the synthesis of nonbridged stacked columnar phthalocyanines, it was tacitly implied that the PcM considered were electrically neutral, being Pc in the oxidation states -2 and M in that oxidation state which fully compensates the -2 charge of the macrocycle without any intervention of external charged species acting as dopants.180-182 This paragraph will deal with those examples of stacked PcM structures whose formation follows the occurrence of solid state redox reactions, altering either the oxidation state of the Pc ring183-186 or, less often, the coordinated central metal M. 187 As previously mentioned, the complexes with macrocycles like Pes generally crystallize in the r:t- or ,B-modification in the absence of a bridging ligand L, giving rise to molecular crystals with inclined stacked arrangements (Figure 2b). Many cases are reported on the existence of both r:t and ,B phases for solid PCM,188-196 which is dependent on the temperature or the preparation conditions of the molecular crystal. 143 Among others, one example of polymorphism is given by PcGeII which exhibits both r:t- and ,B-modifications respectively at room temperature and above 640 K (Figure 7).196 Stacked columnar structures (Figure 2a) can be obtained from inclined tx- or ,B-arrangements of PcM (Figure 2b) upon chemical or electrochemical oxidation/ reduction.f The latter process represents a solid state redox reaction l97,198 analogous to the doping of conjugated polymers,199,200 which gives rise to dramatic

changes of those chemical-physical properties of the PcMs depending on their arrangement in the solid state. 2,82-85,201 The chemical oxidation of PcNiII ,B-phase202 with 12 vapors and the resulting formation of columnar compounds with the general formula (PcNi)(I) x 125,180,203,204 is one of the best known examples of structural rearrangement of a PcM induced by a redox reaction. In Figure 8, the representation of PcNi structural rearrangement from neutral ,B-phase to the iodinated columnar structure of oxidized nickel phthalocyanine is shown together with a schematic picture of the PcNil crystal structure. The method used for the synthesis of polycrystalline PcNil is based upon the simultaneous slow diffusion of finely powdered PcNi and iodine solutions in chlorobenzene.i'' The preparation of single crystals of PcNil can be accomplished by diffusing together NiPc and iodine solutions in 1-chloronaphthalene and 1,2,4-trichlorobenzene. 78 The electrochemical preparation of columnar stacked PcNi(BF 4 )x has been also rcported.i'" In particular, needle-like PcNi(BF4)0.33 crystals have been grown galvanostatically on Pt anodes with currents in the range 10- 1-100 u.A, The electrolyte for the electrochemical growth of PcNi(BF4)0.33 was constituted by PcNi and [(C4H9)4N]+(BF4)- in 1-chloronaphthalene and the galvanostatic deposition of PcNi(BF 4)0.33 was accomplished at temperatures higher than 100°C in absence of oxygen and water. 185 The same species PcNi(BF 4)0.33

260

Hanack and Dini

(a)

(b)

(c)

Figure 8. Transformation of the (a) neutral Ni(II)Pc {3-modification into the (b) oxidized columnar structure being X- = 13/ 15 the charge compensating species. (c) Top view from the c-axis of the NiPcI unit cell. For sake of simplicity 1- anions were not represented in (c).

can be also prepared via the chemical oxidation of PcNi in dichloromethane using the oxidizing salt (NO)+(BF 4)-.20s

B. BRIDGED STACKED POLYPHTHALOCYANINES

1. Generalities

The term bridged polyphthalocyanines usually refers to those compounds with the general molecular formula [PcM(L)]n, and whose schematic representation is given in Figure 3. The bridged species [PcM(L)]n are systems prone to a high compositional variability due to the possible modification of the nature of the macrocycle, of the central metal, and of the bridging ligand L. 1,2,S3,S4 A list of various bridged [PcM(L)]n, obtained by varying the central atoms, axial ligands, and Pc macrocycles is presented in Table 1 just to give an idea of the possible different combinations that this class of polymerized Pc can offer. The analysis of the compounds listed in Table 1 leads to some general considerations concerning the possible combinations of central atom-ligand M-L, which are present in the neutral [PcM(L)]n polymers. The case of [PcM(L)]n where M is divalent (e.g. M(ll) == Mn(ll), Fe(ll), Co(II), or Ru(II)), presents the M(II) atoms linked together through bidentate ligands like pyrazine, 4,4'-bipyridine, diisocyanobenzene, or 1,4-diazabicyclooctane, whose electron-rich extremities bridge the M(II) atoms belonging to two different macrocycles thus originating a polymeric

structure held together only by coordination bonds. The case of [PcM(L)]n where M is trivalent [e.g. M(lll) == AI(III), Cr(III), Mn(III), Fe(III), Co(III), or Ga(III)], presents the M(III) atoms linked together through monovalent ligands having coordinating properties like F, CN, SCN, or N 3.1,206 The type of [PcM(L)]n involves the combination of tetravalent central atoms M(IV) being M(IV) == Si, Ge, or Sn, and divalent ligands like 0, S, or C=C.207-210 Two covalent bonds link the bridging ligand to two central atoms and constitute the support necessary for inferring stability to the resulting polymeric structure. 1 2. Bridged Phthalocyanines with Divalent Central Atoms

This group of stacked polymeric phthalocyanines is characterized by the presence of coordination bonds as links which bridge the metal-macrocycles adducts via the ligand. In the following, the preparation of various [PcMII(L)]n differing for the nature of the axial bidentate ligand will be described. The reason for such distinction criterium relies upon the fact that the properties of interest of [PcMII(L)]n mostly depend on the nature of L.

a. Pyrazine and Derivatives as Bridging Ligands Pyrazine (pyz) and its derivatives methylpyrazine (Mepyz), dimethylpyrazine (Me2PYZ), ethylpyrazine tert-butylpyrazine (t- Bupyz), and (Etpyz), Chloropyrazine (Clpyz) have been successfully used as

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties Table 1. Combinations of Various Bridged Polymeric Complexes [PcM(L)]n Macrocycle

M

L

Pc

Al Si Si Cr Cr Mn Mn Mn

F

Fe Fe Fe

F

o CN

N3

CN SCN

N3

CN SCN CNVNC

Fe

j=\

~

Fe Co Co Ga Ge Ge Ru

t-~

'"N-NI)

CN SCN F F

o

t-~

\\

I)

N-N

Rh Rh Sn Sn Os

CN SCN F

o

(=~ N-N

Fe Ru Si

(H17CSOCH2)SPC 2,3-Nc

Ge Sn Co

Fe Fe

CNVNC CNVNC

o o

o CN CNVNC t-~ ~ I) N-N

bidentate axial ligands for the formation of the bisadducts PcM II(RxpYZ)2 whereas polymeric compounds [PcMII(Rxpyz)]n could be prepared only with unsubstituted pyrazine due to sterical hindrance of the substituents at the pyrazine ring. 1,211-215 The latter consideration is useful when bis-adducts have to be isolated and the occurrence of polymerization is not a desired process. In [PcMII(pyz)]n, the central atoms are usually the divalent Fe(II), Ru(II), Os(II), Co(II), or Rh(II).214,216-218 The preparation of the prototype compound [Pcf'erpyzj], (Figure 9) can be easily achieved by heating f3-PcFe with an excess of pyrazine in chlorobenzene at 135°C.212 The resulting product of polymerization (a dark violet solid) shows strong absorption bands in the UV-vis range at the wavelengths 790, 725, and 715 nm whereas the precursor bis-adduct PcFe(pyz)2 absorbs at 650 and 340 nm in benzene solutions. 1 The thermal stability of [Pcf'etpyzj], has been tested through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), with the verification of no mass losses and heat exchange up to 300°C.212,219-221 From the IR spectrum of [Pcf'cipyzj], pressed pellets, a degree of polymerization n :::: 20 was found. 222 In passing from the monomer Pcl-etpyz}, to the polymer [FePc(pyz)]n, the IR spectrum shows the vanishing of the bands at 1580, 1230, and 695 em-I, which correspond respectively to the pyrazine ring stretch, in-plane C-H bending from pyrazine ring, and deformation of the pyrazine ring.223-225 Such spectral modifications arise from the symmetry change associated with the relative increase of the number of bidentate bridging pyrazine rings with D 2h symmetry in the [PcFe(pyz)]n, with respect to the number of the terminal monodentate pyrazine rings in PcFe(pyz)2 having local C 2v symmetry.

~N~-IW---

~I---N

\=::=/

Figure 9. Schematic representation of the pyrazine-bridged polymer [PcFe(pyz)]n·

261

262

Hanack and Dini

The three above-mentioned IR transitions are allowed in terminal monodentate pyrazine rings whereas are symmetry-forbidden for bridging bidentate pyrazine rings. From the analysis of the absorption intensities at about 1580 ern-1, it was found that the ratio bridging bidentate : terminal monodentate pyrazines was higher than 10 and therefore a value of n ~ 20 was determined. Analogous considerations are valid also for determining the degree of polymerization in [PcCo(pyz)]n. 222 An indication of the occurrence of polymerization comes from the analysis of the absorption at about 800 cm- 1 associated with the out-of-plane C-H bending from pyrazine ring, which shifts at higher wavenumbers in passing from the monomeric adduct Pcf'etpyzj, to the polymer [Pcf'erpyzjj.; This is because such an absorption is sensitive to the strength of the coordination bond between aromatic amines and central metal atom. 226 Moreover, in the far IR region (150-200 cm- 1), the absorption associated with the angular deformation mode of the group Npc-M-Npc shows a splitting in passing from the hexacoordination of the bridged central metal to pentacoordination of the terminal central metal respectively with one and two pyrazine rings. Pyrazine can bridge also substituted iron-phthalocyanine and naphthalocyanine complexes with formula RxPcM like CI16PcFe, (CH3)8PcFe,(CH30)8PcFe, 2,3NcFe giving rise to the formation of [RxPcFe(pyz)]n through the same synthetic pathway of [PcFe(pyz)]n. 1,2,227,228 The thermal stability of the species [CI 16PcFe(pyz)]n, [(CH30)8PcFe(pyz)]n, and [(CH3)8PCFe(pyz)]n has been studied and it was found that the thermal loss of base molecules in these polymers occurs in the temperature range of 280-350 °C, with the onset of an endothermic process corresponding to pyrazine removal at about 350°C.1 The presence of electronwithdrawing groups in the substituted macrocycles, e.g., chloro, leads to the increase of thermal stability of the resulting [CI16PcFe(pyz)]n with respect to the unsubstituted [PcFe(pyz)]n. 1 Other examples of polymeric substituted iron-phthalocyanines bridged by pyrazine are [(C2Hs)4PcFe(pyz)]n, [(CN)4PcFe(Pyz)]n, {[(C2Hs)C6HI20]4PcFe(pyz)}n, {[(C2Hs)C6H120]8Pc-Fe(pyz)}n, and [2,3_NcFe(pyz)]n. 1,2,227,228 Preparation of pyrazine bridged PeRu can be realized by heating the adduct PcRU(PYZ)2 at 300°C, thus leading to a splitting off of one pyrazine molecule with the resulting formation of [PcRu(pyz)]n.214 A comparison of the thermal stabilities of [Pck.utpyzj], and [Pcl-erpyzj], shows that the first species has a higher stability because it decomposes at temperatures not lower than 600°C whereas [Pcf'etpyzj], shows the onset

of mass losses at about 310 °C.212 Polymers of pyrazine bridged ruthenium-substituted phthalocyanines can be prepared in a similar fashion to the unsubstituted analogs and the species [(t-BU)4PcRu(pyz)]n, [(C2Hs)4Pckufpyzj], have been synthesized.2 27,228 The preparation and characterization of [PcOs(pyz)]n have been also reported by US. 229 [PcOs(pyz)]n can be prepared by heating PcOS(PYZ)2 at 320°C under nitrogen. The starting PcOS(PYZ)2 is obtained as pulverized violet melt from a mixture of PcOs and pyrazine in excess (molar ratio 1 : 100) which are heated at 80°C for 24 h. 229 Polymeric cobalt phthalocyanines with pyrazine as bridging ligand can be prepared by simple stirring CoPc at room temperature in an excess of pyrazine for two months. 1 The synthesis of the polymers [(t-BU)4PCCO(pyz)]n and [(N02)4PCCO(pyz)]n obtained by bridging the tetrasubstituted macrocycles (t-BU)4PCCO and (N02)4PCCO with pyrazine has been also reported.l'<'" The presence of ring substituents like t-butyl and nitro groups generally lowers the temperature at which the corresponding pyrazine-bridged polymeric species start to decompose with respect to the polymer with an unsubstituted ring. For the series [PcCo(pyz)]n, [(t-BU)4PCCO(pyz)]n, and [(N02)4PC Cotpyzj], respectively the values of temperature at which the onset of decomposition occurs were found to be 120°C, 100 °C, and 90°C.1

b. Triazine, Tetrazine, and Derivatives as Bridging Ligands 1,3,5-Triazine (tri) and 1,2,4,5-Tetrazine (tz) are heterocycles containing more than two nitrogen atoms which can be successfully used as bidentate ligand for the preparation of bridged polymeric complexes having PcFe, PeRu, PcOs, 2,3-NcFe and their derivatives as macrocyclic units. 1,2,216,227,228,230,231 Few examples of 1,3,5-triazine-bridged polyphthalocyanines are available and these include [PcFe(tri)]n, [PcRu(tri)]n, and the substituted [(C2Hs)4PcFe(tri)]n and [(C2Hs)4PcRu(tri)]n.1,2,228 On the other hand, the number of polymeric complexes with tetrazine as bridging ligand is larger and synthesis are reported for the following species [PcFe(pyz)]n,230 [PcRu(tz)]n,230 [PcOs(tz)]n,229 [2,3-NcFe(tz)]n,232 {PcFe[(CH 3)2- tZ]}n,233-236 {PcRu[(NH 2)2tZ]}n,228 [PcRu(CI 2tz)]n,228 {PcRu[(CH 3)2tZ]}~,233-236 and the ring substituted [(CH 3)8 PCFe(tz)]n,233-236 [(CN)4PcFe(tz)]n,233-236 [(t-BU)4 PCFe(tz)]n,227,228 [(C2Hs)4PcFe(tz)]n,233-236 {[(C2Hs)C6HI20]4PcFe(tz)}n,227,228 [(t-BU)4PcRu(tz)]n227,228 and [(C2Hs)4PcRu(tz)]n.227,228

114/ Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties The synthesis of [Pcf'ettzj], can be accomplished under the same conditions of [Fef'ctpyzj], by substituting tetrazine with pyrazine in the synthesis solution.v''' The species [PcRu(tz)]n,216 [PcOs(tz)]n,2 and [2,3NcFe(tz)]n231 are prepared via a heterogeneous reaction of respectively PcRu, PcOs, and 2,3-NcFe in a slight excess of tetrazine at 70°C in chlorobenzene. The X-ray diffraction patterns of [Pcf'ettzj], and [PcRu(tz)]n powders have shown that these two compounds are isomorphous.i'" Differently from substituted pyrazines, the disubstituted 2,5-tetrazines can bridge metalphthalocyanine complexes to produce [PcM(R 2tz)]n. The synthesis of species like {PcFe[(CH3)2tZ]}n, {PcRu[(CH3)2tZ]}n, {PcRu[(NH2)2tZ]}n, and [PcRu(CI 2tzj], has been reported by the authors.2 27,228,233-236 In Scheme 6 the synthesis of the polymer originated by the

,N=(

CHa

j.-I't

~N~

excess

H3C

~~R~~~

)

263

bridging of the PcRu complex units with 3,6-dimethyl1,2,4,5-tetrazine (CH3)2tz is presented.233-236 The species {PcRu[(CH3)2tZ]}n (24) is simply obtained from prolonged stirring of PcRu (23) in a chloroform solution of (CH3)2tZ at room temperature.233-236 The product 24 has the appearance of a violet powder which is unsoluble in the reaction solvent and shows UV-Vis absorption at the wavelengths 666, 600, 325, and 210 nm.233-236 Among the tetrazine-bridged ring-substituted polyphthalocyanines [RxPcM(II)(tz)]n, the synthesis of [(CN)4PcFe(tz)]n is described (Scheme 7).233-237 The starting material for the preparation of [(CN)4PcFe(tz)]n (28) is 4-aminophthalodinitrile (25). The latter is converted into 4-cyanophthalodinitrile (26) through the cyanation of -NH 2 via diazonium salt formation

~1U 1~ ~[

CH

~N1U 1~, ~[

CH,

~1U 1~, ,~[

~ -N~ CHCI 3 ,2 days

I N\

IN \

23

IN \

24 Scheme 6. Synthesis of {PeRu[(CH 3btz]}n (24) from PeRu (23).

NaNoiH2S0\ CuS04/ KCN

NCYyCN

~CN 26

25

-----.~ C~~CN Fe(CO)s

1~)[

1-Chloronaphthalene

N

~N~

250'C

CN

,N=='\

N

L

27

Fe N

N

N'

)

Chlorobenzene 48 hrs.

28 Scheme 7. Synthesis of [(CN)4PeFe(tz)]n (28) from 4-amminophthalodinitrile (25).

27

CN

Hanack and Dini

264

and following Sandmeyer rcaction.v'" Compound 26 was successively tetramerized in chloronaphthalene in the presence of pentacarbonyliron to give (CN)4PcFe (27) as a product of the template reaction. The polymerization of 27 with bridging tz was realized in chlorobenzene at room temperature by stirring 27 and tz together with a molar ratio of 27: tz close to 1 for 48 h.233-236 The solid product was then extracted with CH 30H and acetone and successively dried at 100°C to give 28. The UV-vis absorption peaks of 28 are located at 1190, 850, 695, 355, and 235 nm.233-236

PcRu(dabco); from a 1,2-dichlorobenzene solution of PcCo and dabco in excess which are heated at 80°C for 24 h. The hexacoordinated Ru-complex, PcRu(dabco )2, is then suspended in 1,2-dichlorobenzene at room temperature and stirred for 24 h to give the final product [Pclcutdabcojj.; d. Diisocyanobenzene and Derivatives as Bridging Ligands

In passing from pyz, tri, tz, and dabco bidentate ligands to 1,4-Diisocyanobenzene (dib), there is a slight increase in the distance between the two donor centers bridging the macrocycles. Moreover in dib and its derivatives (Figure 11), the coordinating centers are constituted by the terminal electron-rich C atoms. As a consequence, the dib-bridged polyphthalocyanine will undergo a greater separation between adjacent macrocycles and their structural stability will depend mainly on the degree of charge transfer between the central atom M and the coordinating C. 216 For the latter reason, this group of bridged polymeric phthalocyanines has to be regarded as a class of organometallic compounds.v" The dib-bridged polymers which have been synthesized with phthalocyanine macrocycles units are listed in ,Table 2. Preparation of [Pcf'etdibj], and [Pckutdibj], was accomplished by refluxing f3-PcFe or PcRu with an excess of dib (100/0 in excess) for two days in acetone.v'" Peripherally substituted dib-bridged polyphthalocyanines like [RxPcFe(dib)]n could be also prepared by mixing RxPcFe with a twofold amount of dib in refluxing acetone for two days.l,2 The latter procedure was used in the preparation of [(CH3)8PcFe(dib )]n, [(CH30)8PcFe(dib)]n, and [CI16PcFe(dib)]n' The structural analysis of these polymers bridged by isocyanidecontaining ligands was made possible by the strong IR

c. Diazabicyclooctane as Bridging Ligand 1,4-Diazabicyclo[2.2.2]octane (dabco) represents another bidentate ligand which is characterized by the absence of n-electrons between the two donor nitrogen atoms at the extremities of the ligand (Figure 10). The only reported examples of polymeric phthalocyanines held together by dabco bridges are [PcFe(dabcoj].; [Pckutdabcojj.; and [PcCo(dabco)]n. 211,212 [Pcf'efdabcoj], which were prepared by the authors by dissolving equimolar amounts of f3-PcFe and dabco in chloroform and stirring for several days. The completion of the polymerization reaction can be detected through the turning of the solution color from violet to turquoise green. The dried product [Pcf'ctdabcoj], is a scaly greenish powder and presents UV-Vis absorption peaks at 680, 660, 420, and 360 nm. 212 [PcRu( dabco )]n can be synthesized in two steps.211 The first step involves formation of the bis-adduct

Figure 10. Structure of the ligand 1,4-diazabicyclo[2.2.2]octane (dabco).

CN-Q-NC CN-Q (a)

(b)

CN-0-0-NC

NC

CI

CI

CN*NC CN*NC CI

CI

(d)

F

F (e)

(0)

~XH3 CN~NC H3C

CH3 (f)

Figure 11. (a) 1,4-Diisocyanobenzene (dib) and its derivatives (b) 1,3-Diisocyanobenzene (rn-dib), (c) diisocyanobiphenyl (phdib), (d) Cladib, (e) F4dib, and (f) (CH 3)4 d ib.

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties Table 2. dib-Bridged Polymeric Phthalocyanines

Table 3. Thermal Stabilities of dib-Bridged Polymeric Phthalocyanines '

Compound

Reference

[PcFe(dib )]n [Pckutdibj], [(CH3)8PcFe(dib )]n [(CH30)8PcFe(dib )]n [Cl 16PcFe(dib )]n [2,3- NcFe(dib )]n [1,2- NcFe(dib )]n [(t- BU)4PcRu( dib )]n {PcFe[(CH3)4dibUn {PcRu[(CH3)4dib]}n {(n-CsH 11)8PcFe[(CH3)4dibUn

212 214 216 216 216 234 216

{[(2-C2Hs)-C6H120]4PcFe[(CH3)4dib]}n {(t-BU)4PcRu[(CH3)4dibUn {PcRu[F4dib]}n {PcFe[C14dibUn {PcRu[CI4dib [(CH3)8PcFe(C14dib )]n [(CH 30hPcFe(CI4dib )]n [PcFe(dibph)]n [PcRu(dibphj], [PcFe(m-dib )]n [PcRu(m-dib )]n [(CH3)8PcFe(m-dib)]n [(CH 30hPcFe(m-dib )]n [Cl 16PcFe(m-dib )]n

n,

265

2

235 235 2 2 2

227 235 235 236 235 235 235 216 214 216 216 216

absorption of the isocyanide groupNC in the region 2050-2150 cm "..' An important modification of IR absorption associated with the NC stretching in passing from the free dib to the phthalocyanine-coordinated dib was the shift of the frequency to higher values indicating the occurrence of a-donation from the donor C to the central atom M and the consequent decrease of negative charge density on the coordinating C atom. I In peripherally substituted dib-bridged polyironphthalocyanines [RxPcFe( dib )]n, electron releasing substituents e.g., R == CH 3 or CH 30 , provoke a shift of the NC stretching frequency to lower values whereas electron withdrawing substituents, e.g., R == CI, induce a shift to higher frequencies for the same bond vibration. I This finding could be explained on the basis of the consequent increase (decrease) of the electron density at the central atom due to the presence of electron releasing (electron withdrawing) groups in the macrocycle structure. The increase (decrease) of the electron density at M augments (reduces) the extent of electron back-donation from M to C, thus weakening (strengthening) the NC bond which vibrates at lower (higher) frequencies with respect to NC in [PcM(dib)]n- 1 Polymeric phthalocyanines can be also bridged by substituted dib like m-dib, phdib, Cladib, F4dib, and (CH 3)4dib (Figure 11 and Table 2).1 The stacked

Temperature of the onset of ligand

dlssociation/C

Compound

[Cl 16PcFe(dib )]n {PcFe[(CH 3)4dib {PcFe[C14dib [(CH3)8PcFe(dib)]n [PcFe(dib )]n [PcRu(dib)]n

n,

n,

290 260 260 235 220 210

polymers {PcFe[(CH3)4dib}n and [Pcf'e/Cladibj], are produced by a heterogeneous reaction of fJ-PcFe respectively with the ligands (CH3)4dib and Cl.dib in the stoichiometric ratios 1: 1 and I: 1 in acetone or chlorobenzene at room temperature or under reflux for 24 h. The thermal stabilities of the various dib-bridged phthalocyanines are reported in Table 3. In the latter table the compounds have been listed in decreasing order of dissociation onset temperature. Dib-bridged polyirontetrachlorophthalocyanine shows the highest thermal stability probably due to the existence of interactions between CI atoms belonging to different macrocycles and the considerable electronic effects of the chloro atoms. The separation between stacked rings due to the presence of the dib ligand does not prevent possible inter-ring interactions via relatively large phthalocyanine substituents like Cl. On the other hand, the substitution in the axial ligand probably affects the thermal stability of the resulting bridged polyphthalocyanine through the size effect, not electronic, of the substituent as demonstrated by the comparable stability of {PcFe[(CH3)4dib}n and [PcFe(CI4dib )]n. 1 e. Phenylenediamine as Bridging Ligand The bidentate ligand p-phenylenediamine (ppd) can be used as bridging ligand for the polymerization of metal-phthalocyanines being the nitrogen atom of the amine groups the coordinating donor centers. Following the procedure presented in Scheme 8,240 ppd has been used to bridge PcRu (29) with the resulting formation of the species [PcRu(ppd)]n (30). According to the latter scheme, 30 is prepared by mixing 29 and ppd in toluene in an equimolar ratio and refluxed for 24 h. Species like 30 can possess interesting electrical properties for the possible involvement of ligand oxidation in the processes of [Pclvltppdj], doping. 24o,241

Hanack and Dini

266

)

Toluene, reflux 24 hrs.

30

29 Scheme 8. Synthesis of [PcRu(ppd)]n (30) from PeRu (29).

N;)-O (a)

(c)

(b)

Figure 12. (a) 4,4 /-Bipyrid ine (bpy) and its derivatives (b) 2,2' -dimethvl-s.a'
f. Bipyridine and Derivatives as Axial Ligands Another class of bidentate bridging ligands forming [PcM(L)]n is constituted by 4,4'-bipyridine (bpy) and the related species 2,2' -dimethyl-4,4'-bipyridine [(CH3)2bpy], bipyridylacetylene (bpyac), azobispyridine (azobpy) (Figure 12). These ligands coordinate the central atoms of the metal-phthalocyanine complex with the nitrogen atoms of the two linked heteroaromatic systems. It is expected that the a-donation tendency of these coordinating nitrogen atoms is lower with respect to a nitrogen atom of an amine, like ppd, because of the reduced basicity of a heteroatom belonging to an aromatic system. With respect to pyz, tri, tz, and dabco, the bpy-based ligands introduce a larger interplanar distance in the respective bridged polyphthalocyanines with typical distances in the order of 1100 pm versus 700 pm imposed by the presence of pyz- and dabco-based Iigands.i'" Table 4 presents a list of some [PcM(bPY)]n.

Table 4. bpy-Bridged Polymeric Phthalocyanines Compound

Reference

[Pcf'efbpyj],

214 216 216 216 243 243 216 216 242

[PcRu(bPY)]n [2,3- NcFe(bpY)]n {PcFe[(CH 3)4 bPY]}n [PcFe(bipyac )]n [PcRu(bipyac)]n [(CH3)8PcFe(bipyac)]n [Cl 16PcF e(bi pyac)]n [(t-BU)4PcFe(azobPY)]n

The species [PcM(bPY)]n are prepared by refluxing the respective PcM with a slight excess of the ligand in chloroform, chlorobenzene, or toluene for at least 24 h. An alternative route to [PcM(bPY)]n is the conversion of the hexacoordinated PcM(bpY)2 into the final polymer via the thermal splitting off of one L molecule in PcM(bpY)2 and the consequent polymerization. 214,216

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties Table 5. Thermal Stabilities of bpy-Bridged Polymeric Phthalo-

267

Table 6. Various Polymeric Phthalocyanines of the [PcM11I(L)]n Type

cyanines '

Compound

Temperatu re of the onset of ligand dissociation (OC)

[PcRu(bPY)]n [PcFe(bipyac )]n {PcFe[(CH 3)4 bpy]) n [PcFe(bpY)]n

450 285 220 220

The hexacoordinated bis-adducts are directly prepared putting PcM in a melt of the ligand. 216,242,243 The composition of the various species [PcM(bPY)]n is usually determined by the simultaneous detection of mass losses and heat exchanges with thermogravimetric analysis and differential calorimetry. 1 The controlled-rate heating of the [PcM(bPY)]n is generally characterized by well defined endothermic peaks associated with the progressive loss of bpy molecules. 1 The thermal stability is .highest for [Pckutbpyj], which show the onset of thermal losses at 450°C (Table 5). From the data in Table 5, the comparison of [Pckurbpyj], and [Pcf'etbpyj], shows a remarkable stabilizing effect of the central Ru atom with respect to Fe. Another interesting aspect is the relatively good stability of the bipyac-bridged polyPcFe with respect to [Pcl etbpyj], (Table 5). The alkynyl group in [Pcf'etbpyacl], conjugates the pyridyl rings and does not allow the relative rotation of the pyridyl planes, thus inferring a major stiffness to the junction linking the positions 4 and 4' of the pyridyls. In fact, systems with higher conformational rigidity generally present a better stability toward temperature rise. Like in the case of pyz-bridged polymeric metalphthalocyanines, the group of bpy-bridged analogues can be successfully analyzed with IR spectroscopy.Y' In fact, analysis of IR spectra of the compounds containing bpy as ligand can make possible the distinction between bpy in bridged polymers, bis-adducts, or monocoordinated macrocycles.Y' In the bis-adducts PcM(bpY)2' the IR absorptions of the ligand are located at the wavenumbers 800, 1070, 1215, 1400, 1485, and 1590 em -1.2 44,245 The critical region of the IR spectrum for the detection of the occurrence of bpy-bridged polymerization is the range 700-800 em-I. In the latter, the absorption of the out-of-plane C-H bending has a decrease of intensity and a concomitant shift at higher energies upon occurrence of polymerization. 1 Possible evaluation of the degree of polymerization in [PcM(bPY)]n is also possible through comparison of IR spectra. 222

Compound

Reference

[PcCr( CN)]n [PcMn(CN)]n [PcFe(CN)]n [PcCo(CN)]n [PcRh(CN)]n [2,3-NcFe(CN)]n [2,3-NcCo(CN)]n [2,3-NcRh(CN)]n [(CH 3)sPcCo(CN)]n [(t-BU)4PCCo(CN)]n [(C7H 1S)sPcCo(CN)]n [(t- Buh2,3- NcCo(CN)]n [PcCrN 3]n [PcMnN 3]n [PcAIF]n [PcCrF]n [PcGaF]n [PcMnSCN]n [PcFeSCN]n [PcCoSCN]n

246 246 246 246 54 227 118 254 118 118 118 118 248 248 206 206 206 248 248 249

3. Bridged Phthalocyanines with Trivalent Central Atoms Metal-phthalocyanine complexes with M in the oxidation state +3 can exist if a monovalent ligand L is bonded with the central atom. In general, PCM1IIL species do not form stacked structures unless the conditions for the formation of an additional coordination bond between Land M are present. In this case, PCMII1L can assemble in a columnar fashion giving rise to structures like those depicted in Figure 3 which can be formulated as [PcMIII(L)]n.II8 In the latter species, the central atom M(III) can be Fe(III), Co(III), Rh(III), Cr(III),246-255 or AI(III) and Ga(III) ,206,256-260 whereas ligands L have to be monovalent and monodentate with' donor properties. A list of the most common examples of [PcMII1(L)]n type stacked polymers is reported in Table 6. It can be noticed that Al and Ga atoms are axially coordinated by the sole F ligand whereas among fiuoro-coordinated polyphthalocyanines, Cr(III) represents the only central atom which can form stacked polymeric complexes also with ligands other than F (Table 6).

a. Cyano-bridged Polyphthalocyanines Species with the general formula [PcM(CN)]n (35) where M == Cr, Mn, Fe, Co, Rh, have been prepared by the authors through several steps using PcM (31) as starting material (Scheme 9).54,246 The critical step is represented by the selective oxidation of the central atom M in 31 with M passing from the oxidation state +2 to +3. 249,250 This can be accomplished using SOCl2

Hanack and Dini

268

or

i

"

35

Scheme 9. Synthetic routes for the preparation of generic [PcM(CN)]n' M' represents an alkaline metal, e.g., K or Na.

or O 2 as oxidizing agents. The products of PcM oxidation are the bis-adduct PcM(CI)2 (32) in which also the Pc ring has been oxidized to Pc( -1), the monocoordinated PcMCI (33) and the salt M'+[PcM(CN)2]- (34) with M' == Na, K (Scheme 9).54 The oxidized species 32, 33, and 34 are converted into 35 respectively via chlorine elimination and CN addition (ring reduction must occur),253 ligand exchange, and splitting off of one CN group per 34 molecule. "

In the IR region, the analysis of the CN-stretching vibration in the various CN-containing compounds offers the opportunity of determining the occurrence of CN-bridged polymerization through the verification of the CN-stretching wavenumber shift at higher values (about 20-30 cm- I ) in passing from 34 to 35. 54,222 X-ray diffraction on powders of 35 revealed that all of these compounds are isostrucutral, 54 Thermal stabilities

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties Table 7. Thermal Stabilities of Cyano-Bridged Polyphthalocyanines54/261

Compound

Temperature of the onset of ligand dissociation (OC)

[PcMn(CN)]n [PcCo(CN)]n [PcRh(CN)]n [PcCr(CN)]n [PcF e(CN)]n [(CH 3)sPcCo(CN)]n [(CsH 17 0 CH 2)SPCCO(CN)]n

250 220 200 170 170 150 -100

of some [RxPcM(CN)]n have been analysed and results are reported in Table 7. 54,261

b. Thiocyanate-bridged Polyphthalocyanines Thiocyanate-bridged polymers [PcM(SCN)]n with M == Cr, Mn, Fe, Co are known. 248 The preparation of these compounds can be realized according to the routes presented in Scheme 10. The species 32 and 33 are treated with KSCN in water to give directly [PcM(SCN)]n (38). An alternative route is the initial reaction of 32 with a tenfold excess of KSCN in ethanol to form the bis-adduct salt K+[PcM(SCN)2](36). The latter is then dissolved in water to remove one molecule of KSCN and produce insoluble 38 (Scheme 10). Another useful precursor is the trichloroacetate of the metal-phthalocyanine PcM(OCOCCI 3) (37) which can be transformed into 38 under the same reaction conditions of the transformation 32 --+ 36 (Scheme 10). The tendency of disassembling the polymeric chain in presence of other donor ligands like pyz or pyridine (py) is quite evident in [PcCo(SCN)]n or [PcFe(SCN)]n which give respectively PcCo(SCN)pyz or PcFe(pY)2 in presence of pyz or py in solution. 248,249 X-ray diffraction of powders has established that [PcM(SCN)]n are all isostructural with bridged stacked arrangement as depicted 38 in Scheme 10.262,263 The thermal properties of some [PcM(SCN)]n are reported in Table 8.248,249

Table 8. Thermal Stabilities of Thiocyanate- and Azido-Bridged Polyphthalocyaninesv'"

Compound

Temperature of the onset of ligand dissociation (OC)

[PcMn(SCN)]n [PcMn(N 3)]n [PcCr(N 3)]n [PcFe(SCN) ]n [PcCo(SCN)i,

350 260 210 155 140

thermal properties of some [PcM(N 3)]n are also reported in Table 8. 248

d. Fluorine-bridged Polyphthalocyanines In the stacked fluorine- bridged complexes [PcMF]n, the central atom M can be AI(III) and Ga(III).206,256,259,264 The compounds [PcAIF]n and [Pcxiaf'], are synthesized from chloroaluminium-phthalocyanine and chlorogallium-phthalocyanine. PcAICI and PcGaAI are. prepared via a template reaction between phthalonitrile and respectively, AICl 3 and GaCI 3. The PcAICI and PcGaCI thus formed are successively treated with NH 40H or pyridine to give the hydroxilated systems PcAIOH and PcGaOH. The hydroxilated phthalocyanines are then treated with aqueous HF to give the species [PcAIF]n and [PcGaF]n. These fluorine-bridged polymeric phthalocyanines can be obtained as needle-like crystals by sublimation under vacuum at about 500°C.264 Evidences for a stacked columnar structure in [PcAIF]n and [PcGaF]n are based upon the anisotropy of the X-ray reflections, the strong dichroism, and the dendritic character of the . crys t a I growt. separation between PeAl umts h 256 '264 T hee senarati is in the order of 400 pm as determined from X-ray analysis.T" The polymer [PcCrF]n can be prepared in an analogous fashion by treating PcCrIIIOH250,265,266 with concentrated HF. 1

4. Bridged Phthalocyanines with Tetravalent Central Atoms

c. Azido-bridged Polyphthalocyanines AzidofNjl-bridged polyphthalocyanines with the formula [PcM(N 3)]n have been prepared with M == Cr and Mn. 248 Two routes lead to the formation of [PcM(N 3)]n (40). In both procedures, the two starting materials are 33 and the acetate of the metal-phthalocyanine PcM(OCOCH 3) (39) (Scheme 11).248 Both the precursors 33 and 39 are treated with a salt of the azido anion in water at room temperature to form 40. The

Tetravalent atoms like Si, Ge, and Sn can be coordinated in the cavity of the Pc macroring if the exceeding divalence of the central atom M is compensated through the further coordination of two monovalent axial groups.133,267,268 The tetravalence of the atoms belonging to the IV group of the periodic table gives rise to bis-axially substituted PcML 2 with the ligands arranged in a trans configuration with respect to the .macrocycle plane (Figure 13).

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270

32

KSCN,

33

20

36

38

38 Scheme 10. Routes for the preparation of generic [PcM(SCN)] n

Such trans configuration allows the stacking of the PcML2 units to give [PcML'Jn once the axial ligands are transformed from monovalent L to divalent L' with the latter being shared by two overlapped PCMS. 133,267,268 On the other hand, metallic central atoms with the same oxidation number 4+ like

titanium or vanadium, produce PcML2 with the ligands arranged in the cis configuration as demonstrated in the case of PcTiC12,269 PcTi(02 C6H4),270,271 or PcVC12,272 and, therefore, do not arrange in stacked polymeric structures upon simple ligand exchange.

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties

40 Scheme 11. Routes for the preparation of generic [PcM(SCN)] n

a. Oxo-bridged Polyphthalocyanines Oxo-bridged [PcMO]n with M == Si, Ge, Sn (Figure 14) can be prepared from the starting dichlorides PcMCl 2 which are firstly hydrolyzed to PcM(OH)2 by 273 and succesa basic mixture of pyridine and NaOH sively condensed133,267,268,274,275 by dehydration at 300-400 DC under vacuum, to give [PcMO]n. The oxo-bridged phthalocyanines can be also prepared by refluxing the dihydroxy PcM(OH)2 in l-chloronaphthalene.' The degree of polymerization of

[PcMO]n has been estimated to be in the range 70-150 as evaluated from IR end-group analysis, tritium labeling, and laser light scattering. 1 The average molecular weight of [PcMO]n can be increased with the augmentation of the condensation time or by raising the temperature of dehydration.i/" Structural informations on oligomers are based on single-crystal X-ray diffraction and give the following values for the interplanar distances: 333, 353, and 382 pm respectively for [PcSiO]n, [PcGeO]n, and [PcSnOln-1 The occurrence of polymerization is verified

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272

in the IR spectrum of [PcMO]nwith the gradual extinction of the absorption due to the OH-stretching vibration and the simultaneous increase of the absorption corresponding to the asymmetrical bending of M-O-M at 980, 870, and 820 cm- 1 respectively for M == Si, Ge and Sn. 1 The introduction of substituents R in the macrocylic units constituting the stacked polymer [RxPcMO]n 207 allows the solubilization of the systems in the common organic solvents and the possibility of analyzing the UV-Vis properties in solutions of these species. An increase of the energies at which the UV-Vis absorption of the polymer is found to occur with respect to the parent dichloride or dihydroxide phthalocyanine. Such a finding has been interpreted in terms of exciton coupling which is operating mostly in [RxPcSiO]n and [RxPcGeO]n. 277 b. Sulphur-bridged Polyphthalocyanines

Examples for the synthesis of sulphur-bridged polyphthalocyanines with tetravalent central atoms 208 [PcMS]n are less recurrent than the oxo-bridged analogues. It is possible to prepare [PcGeS]n from PcGe(OH)2 in two different ways as reported by Fischer and Hanack.i'" In the first route PcGe(OH)2

Figure 13. Trans configuration of a bis-axially coordinated metalphthalocyanine PcML 2 complex with tetravalent central atoms.

-fIlt+----Q----"I--

reacts with H 2S at 130°C under pressure to give [PcGeS]n after 2 h. 208 The second route involves the reaction of PcGe(OH)2 with triphenylsilanthiol (C6Hs)3SiSH in cholorobenzene under reflux for 24 h. 208 The latter procedure can be adopted also for the preparation of the polymer 1,4-benzoldithiolato-bridged PcGe (Figure 15) if (C6Hs)3SiSH is substituted with dithiohydroquinone.r'" In contrast to the oxo-bridged polymers prepared from PcM(OH)2, the analogous species PcGe(SR)2 do not represent useful precursors for the synthesis of sulfur-bridged polyphthalocyanines.r'"

c. Alkynyl-bridged Polyphthalocyanines Stacked polymeric phthalocyanines with tetravalent central atoms can also be assembled through the divalent alkynyl group (Figure 16).210 The species polyethynylphthalocyaninatosilicon [PcSiC:=C-]n has been prepared by the authors with a quantitative yield from the reaction of PcSiCl 2 with the same amount of bis-bromomagnesium acetylene (BrMgC:=CMgBr) in refluxing tetrahydrofuran.V'' The polymer [PcSiC:=C-]n can be characterized with Raman spectroscopy which detects the presence of the triple bond C:=C through the band at 2150 em-1. 1 In order to prevent the disadvantages associated with poor solubility of the resulting systems [PcMC:=C-]n, the addition of ring substituents has been adopted in the starting R xPcMCl 2 [M == Si and Ge, R ==-C(CH 3)3 and -Si(CH 3)3], as effective approach to increase the concentration of reactive material in solution with BrMgC:=CMgBr under Grignard reaction conditions. 1,2

- ....- - - - Q - - -.....+--

Figure 14. Columnar structure of oxo-bridged polyphtalocyanines with tetravalent central atoms.

114/ Stacked Polymeric Phthalocyan ines: Synthesis and Structu re-Related Properties

Figure 15. Columnar structure of 1,4-benzoldithiolato-bridged polygermaniumphtalocyanines.

Figure 16. Alkynyl-bridged polyphtalocyanines with tetravalent central atoms.

III. Properties of Bridged Phthalocyanines Anisotropy, directionality, one-dimensionality. These three words which are going to be defined in the following, constitute the necessary and basic glossary for the description of the properties and the correlations existing between these and the structure, in molecular materials which are assembled in columnar arrangements like the face-to-face linked metal-phthalocyanines considered here. Anisotropy is a feature typical of a system whose, properties have substantially different . 0 f exammauon. . . 279-281 values depending on the diirection Directionality is characteristic of those materials possessing properties, especially those related with the transport of matter or particles through the structure, which are markedly pronounced along specific directions.282-284 The term one-dimensionality is used, instead, for the description of a molecular arrangement characterized by the development of the structure in one

dimension and refers basically to structure geometries with linear symmetry.285-288 The interest in the creation of quasi-one-dimensional compounds has been originated by the search for the realization of high-temperature superconductors with the belief that the preparation of quasi-one-dimensional systems could be effective in reaching this goal. 289 In the present context quasi-one-dimensional system as far as electrical conductivity is concerned, represents a system in which the electronic average free path along a specific direction is comparable to or greater than the lattice constant in that dircction.t'" This is in contrast with the length of the electronic average free path along the perpendicular direction to the chosen one, which is much less than the lattice constant in that direction. Such specification is made necessary to explain the diverse origin of conduction in the different crystallographic directions. More precisely the transverse intercolumnar conduction is diffusive and practically ncgligiblef" whereas the

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274

parallel intracolumnar conduction is wavelike and predominant. 291 The prototypical material prepared for the first attempt of realization of high-temperature superconductor was the complex tetrathiafulvalene (TTF)-tetracyanoquinodimethane (TCNQ), which reached a conductivity (a) value of 2 x 104 Scm- 1 at 60 K, comparable with that of Pb and Sn at the same temperature. 104,112 Such an example represented the first case of an organic material with stable extended electronic interactions in the solid state and the remarkable result made possible the introduction of terms like one-dimensional metal"! and organic metal 105 among the concepts of the solid-state physics. The reasons for such facile charge conduction in TTFTCNQ have to be found in the crystal structure of the material, which consists of columnar stacks of TTF and TCNQ slightly tilted with respect to the stacking axis, with an interplanar stacking distance of 362 and 317 pm respectively for TTF and TCNQ stacks. 292,293 The existence of such a short separation allows the overlap of the nonbonding or antibonding zr-molccular orbitals situated on adjacent molecules and the realization of the maximum electron delocalization along the stacking crystallographic axis. Another prerequisite for the achievement of metal-like charge transport in stacked structures is the uniform presence of donor-acceptor couples constituted by adjacent molecules with different oxidation states. 294 In this situation the non-bonding electrons of conduction can pass or hop from a partially occupied orbital of the reduced molecule (TCNQ acceptor) to an unoccupied one of the oxidized molecule (TTF donor) with the realization of alternating oxidation-reduction between nearest-neighbor molecules. In quantum terms this corresponds to the electron tunneling between adjacent molecules. 295,296 The occurrence of effective charge-transfer through the mechanism of Table 9. Conductivity Values of Some Nonbridged Phthalocyanines at Room Temperature in the Undoped State Compound

ex-PeR2 fJ-PeR 2 PeMg PeTiO PeMn PeFe fJ-PeCo ex-PeNi fJ-PeNi ex-PcCu fJ-PcCu ex-PeZn fJ-PeZn

a/S cm- 1

1.3 x 2.4 x 1.0 x 5.0 x 2.5 x 2.0 x 1.0 x 7.1 x 2.5 x 1.0 x 5.0 x 2.1 x 2.3 x

Reference 10

1010- 15 10- 6 10- 10 10- 7 10- 10 10- 10 10- 9 10- 15 10- 7 10- 14 10- 8 10- 12

305 305 306 307 308 243 308 305 305 305 305 305 305

alternating oxidation-reduction requires the presence of distinct sites with comparable energy within tunneling distance.297-300 The outline presented on the mechanism of charge conduction in stacked molecular materials fully apply for polymeric columnar phthalocyanines as well, and it is expected that closely packed phthalocyanines possessing mixed-valence propcrties/'" will form the most effective conductors onthe basis of the above mentioned considerations.301-303 Nonstacked phthalocyanines in the a- and ,B-structures are intrinsic insulating materials in the neutral state being usually a < 10- 5 S cm- 1 at room temperature as shown in Table 9. 304-308 In 1948 Eley304 for the first time determined the semiconducting behavior of the phthalocyanines showing that crystals of molecular species possessing a network of conjugated electronic orbitals have qualitatively the same behavior of inorganic covalent semiconductors as far as electronic conductivity is conccrned.Y' In fact, it was found that the temperature dependence of the conductivity of Pes followed the well-known Arrhenius-type trend.v'" (3)

where ao (preexponential factor) is the conductivity value when liT tends to zero, k is the Boltzmann constant (1.38 x 10- 23 J K- 1), and Ea is the activation energy. The polymerization of the phthalocyanines into stacked columnar structures improves noticeably the Table 10. Conductivity Values of Undoped Polymeric Stacked Phthalocyanines at Room Temperature/ Compound

a/S cm- 1

[2,3NeFe(tz)]n [2,3NeCo(CN)]n [PcFe(tz)]n [PeRu(tz)]n [(CH 3hPeFe(tz)]n [PcRu(tz)]n [PcOs(tz)]n [PcFe(CN)]n [2,3NcFe(dib )]n [2,3NeFe(CN)]n [PcFe( dib )]n [PcMn(CN)]n [PeSiO]n [PcRu(dib)]n [PeFe(pyz)]n [PcRu(pyz)]n [(t-BU)4PCSiO]n [PcSnO]n [PcSiO]n [(t-Bu)4PcGeO]n

3.0 X 1.0 X 2.0 X 2.0 X 1.0 X 1.0 X 1.0 X 6.0 X 2.0 X 1.0 X 2.0 X 1.0 X 5.5 X 2.0 X 1.0 X 1.0 X 8.0 X 1.2 X 2.2 X 8.0 X

10- 1 10- 1 10-2 10- 2 10- 2 10- 2 10- 2 10- 3 10- 3 10- 3 10- 5 10- 5 10-6 10- 6 10- 6 10- 7 10- 9 10- 9 10- 10 10- 11

114/Stacked Polymeric Phthalocyanines: Synthesis and Structure-Related Properties

electronic transport properties as verified by the increase of conductivity in stacked configurations at room temperature (Table 10).2 It is evident from Table 10 that the polymerization of Pc through cyano- and tetrazine-bridging is particularly effective for the rising of conductivity in neutral Pes. Such an effect can be ascribed to the existence of new additional intermolecular interactions through space (n orbital-or orbital interactionsj.''" and through the bond linking the bridged macrocyclcs.Y'' which both modify sensibly the electronic structure of the resulting stacked . . etermincd array. 302 Th e interactions t h roug h space are determi basically by the physical separation existing between cofacially stacked macrocycles. The separation between stacked macrocycles is controlled by the size and the orientation of the axial ligand and the latter can provoke negative effects on the conductivity of the resulting complex if no electronically conjugated path exists through the ligand. The examples of dabco-bridged phthalocyanines are illustrative of this effect whose nature is basically physical.227,228 On the other hand, the chemical nature of the ligand L in stacked bridged polyphthalocyanines plays a significant role on the conductivity of the resulting bridged complexes [PcM(L)]n' 227,228 This is especially true when tz is the bridging ligand and possesses an electronically conjugated path. The reason for that has to be ascribed to the energy difference between the lowest unoccupied molecular orbital (LUMO) of the bridging ligand acting as an acceptor, and the highest occupied molecular orbital (HOMO) of the metal-macrocycle complex acting as a donor.311-313 The difference between the energy levels of LUMO and HOMO constitutes the energy bandgap of the bridged polyphthalocyanine, which corresponds to the activation energy for the occurrence of charge carrier generation. A suitable combination of metalmacrocycle complex and bridging ligand for the attainment of good intrinsic semiconducting properties will be realized with macrocycles with high-lying HOMO and ligands with low-lying LUM0.2 27,228 Calculations based upon the functional density theory have shown that the intrinsic semiconducting properties of [PcM(L)]n depend mainly on the frontier bands. 312,313 The valence band is composed largely of the transition metal orbitals with symmetry Dx y [M == Fe(II), Ru(II), Os(II)]; whereas, the conduction band is constituted by a mixture of macrocycle and bridging ligand orbitals (L == pyz, tri, bpy, acbpy)?12 In the particular case of tetrazinebridged polyphthalocyanines, the conduction band is mostly formed by the rr-systcm of the ligand?12 The cyano-bridged polyphthalocyanines have the conduc-

275

Table 11. Values of Conductivity for Doped Columnar Polymeric Phthalocyanines at Room Temperature

Compound

PcNi PcNi PcPt PcNi [PcAl(F)]n [PcAl(F)]n [PcAl(F)]n [PcAl(F)]n [PcAl(F)]n [PcSi(O)]n [PcSi(O)]n

[Pcf'etpyzl], [PcCo(CN)]n [PcGa(F)]n [PcGe(O)]n

Dopant (moles per Pc mole)

BF 4(0.33) 1(1) 1(0.93) 1(0.56) 1(3.3) BF 4(0.44) BF 4(0.90) PF 6(0.38) PF6(0.59) Br(1.12) 1(1.55) 1(2.54) 1(1.6) 1(2.1) 1(2.0)

a/5 cm- 1

Reference

103 6.0 X 10- 2 2.4 0.7 5.1 3.0 x 10- 2

85 320 180 180 259 258

1

4.2 X 10- 3 0.3 9.5 x 10- 1 1.4 2 x 10- 1 0.6 0.15 1 x 10-2

321 258

321 303 303 54 54 259 54

tion band formed by a mixture of orbitals localized between the central metal and the ligand when M == Co(III), or by the antibonding orbitals of the macrocycle when M == Ru(III).313 In chemical terms the metal-macrocycle complex should be capable of reducing the ligand to form piled donor-acceptor couples along the stacking axis even in the neutral state. The HOMO-LUMO bandgap can be evaluated electrochemically with the determination of the oxidation potential of the macrocycle and the reduction potential of the bridging ligand. 314,315 The. conductivity of polymeric stacked phthalocyanines can be further improved through doping processes,180-182,316-321 which lead to the increase of the charge carrier number as a consequence of the partial oxidation or reduction of the molecular entities constituting the polymeric complex. In Table 11 the a values of some doped stacked phahtalocyanines are reported including also those cases in which stacking is a consequence of the doping process, e.g., nonbridged iodinated PcNi (Figure 8).125,180,202,204,320,321

IV. Conclusions The obtaining of columnar structures at a molecular level represents a noteworthy achievement in materials science because it can allow the realization of one-dimensional systems with high efficiency in the fast transport of entities like electrons, phonons, or photons in the micron or nanometer range. Among various classes of materials, the stacked polymeric metal-phthalocyanines complexes represent one of the most suitable for the realization of mono-dimensional structures with relevant conduction

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properties. This is because of the presence of interactions between nonbonding orbitals belonging to different macrocyclic units which allow the creation of a quasimetallic electronic structure. A survey of the most important synthetic methods for the preparation of stacked columnar polyphthalocyanines in both bridged and nonbridged arrangements is here presented with the aim of showing the versatility of the phthalocyanine chemistry in preparing molecular systems of new conception with predefinite chemical-physical properties. ACKNOWLEDGMENTS

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