Photophysics in amphiphilic polyelectrolyte systems

Prog. Polym. Sci., Vol. 15, 949-997, 1990 Printed in Great Britain. All rights reserved.

0079-6700/90 $0.00 + .50 © 1990Pergamon Press plc

PHOTOPHYSICS IN AMPHIPHILIC POLYELECTROLYTE SYSTEMS YOTAROMORISHIMA Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan CONTENTS 1. Introduction 2. Properties of amphiphilic polyelectrolytesin aqueous solution 2.1. General considerations 2.2. Poly(methacrylicacid) and related polymers 2.3. Maleic acid copolymers 2.4. Quaternized poly(vinylpyridine) 3. Photophysicsof hydrophobicchromophoresisolated in polyelectrolytes 3.1. Poly(methacrylicacid) loaded with polycyclicaromatic chromophores 3.2. Poly(acrylicacid) loaded with polycyclicaromatic chromophores 3.3. Polycationsloaded with pyrene 3.4. Poly(2-acrylamido-2-methylpropanesulfonicacid) loaded with polycyclic aromatic chromophores 4. Photophysicsof chromophores in self-organized hydrophobicmicrodomains 4.1. Phenanthrene aggregates in amphiphilic random copolymers 4.2. Naphthalene aggregates in amphiphilic random copolymers 5. Photophysicsof amphiphilic alternating copolymers 6. Photophysicsof amphiphilic block copolymers References

949 950 950 951 953 957 958 958 962 968 969 973 973 978 982 989 995

1. INTRODUCTION There has been a considerable interest in polyelectrolytes covalently functionalized with hydrophobic chromophores such as polycyclic aromatic groups. A number of investigations have focused on the photophysics of such polyelectrolyte-bound chromophores and have demonstrated that the photophysical behavior is strongly dependent on the nature of the microenvironments generated by the amphiphilic polyelectrolytes to which the chromophores are covalently confined. Such functionalized amphiphilic polyelectrolytes have a variety of interesting features worthy of study. First, they "solubilize" hydrophobic chromophores in water because of their covalent bonding to the polymer backbone. This provides an unusual opportunity to study photophysics of such otherwise water-insoluble chromophores in aqueous solution. Secondly, the hydrophobic chromophores

949

950

Y. MORISHIMA

may self-aggregate to form a microphase structure that consists of clusters of the chromophores and charged interfaces. This provides an interesting photophysical system to study energy transfer and migration through the chromophore clusters. Studies reported so far have mostly concentrated on copolymers of various vinylic polycyclic aromatics and electrolyte monomers with varying compositions. These studies can be divided into two cases. In the first case the chromophore loadings on the copolymers are limited to a low level so as to avoid chromophore-chromophore interactions that often lead to complexity in photophysical behavior. In the second case the chromophore loadings are high such that self-aggregation of the chromophores occurs in aqueous solution. In the following, we first review the properties of some common amphiphilic polyelectrolytes in aqueous solution. Then, we concern ourselves with the microenvironmental effects of amphiphilic polyelectrolytes on the photophysical behavior of some polycyclic aromatic chromophores covalently attached to the polyelectrolytes.

2. P R O P E R T I E S

OF AMPHIPHILIC POLYELECTROLYTES IN AQUEOUS SOLUTION

2.1. General considerations

Water molecules around hydrophobic residues in amphiphilic polymers in aqueous solution are known to have an ice-like structure] The solvent structuring occurs because the water molecules surrounding the hydrophobic residues have a tendency to form the largest possible number of hydrogen bonds. If these hydrophobic residues encounter one another in aqueous solution, they tend to aggregate such that they have the smallest possible contact with water by releasing the structured water molecules into free water in the bulk phase. The principal driving force for this hydrophobic aggregation is a large increase in entropy which is sufficient to overcome a positive enthalpy change due to the breakup of the hydrogen bonds. 2'3 Since the hydrophobic attractive force balances the electrostatic repulsive force in an amphiphilic polyelectrolyte molecule, the degree of ionization of the polyelectrolyte is a critical factor to determine whether or not the hydrophobic aggregation occurs. Therefore, in the case of amphiphilic polycarboxylic acids, the transition of the polymer conformation may be induced by pH. These molecules tend to adopt a highly coiled compact structure at a low degree of ionization, while with increasing pH the conformation may be transformed into an extended structure at a certain pH. This transition from a compact to an extended structure takes place sharply within a narrow range of pH, which is typical of cooperative processes and is often viewed as a two-state transition.

PHOTOPHYSICS IN P O L Y E L E C T R O L Y T E SYSTEMS

951

The compact structure of amphiphilic polyelectrolytes shows a close analogy to that of well known surfactant micelles. Amphiphilic small molecules, when added to an aqueous solution of an amphiphilic polyelectrolyte, tend to be bound to the polymer through hydrophobic interaction. The addition of organic compounds that destroy water structure, such as alcohols and urea, breaks up the compact structure. These properties are important in connection with biological phenomena such as substrate binding by naturally occurring enzymes, the intercalation of carcinogenic substances into DNA strands, and the denaturation of proteins and DNA. Therefore, amphiphilic polyelectrolytes can provide a simple model system for physicochemical studies of these biological phenomena. Furthermore, the effect of the medium around the hydrophobic residues is also an important factor from the point of view of enzyme-like reactivity control. For example, it is known that the acidity of carboxylic acids surrounded by hydrophobic amino acid residues in an enzyme is reduced. This is because the hydration of protons is less favorable in the hydrophobic environment (owing to the structured water molecules) than in the bulk phase. Moreover, since the local dielectric constant in the hydrophobic microenvironment is considerably lowered, ion-ion and dipole-dipole interactions are strengthened. In the following sections we are concerned with some properties of common amphiphilic polyelectrolytes in aqueous solution. 2.2. Poly(methacrylic acid) and related polymers Poly(methacrylic acid) (PMA), unlike poly(acrylic acid) (PAA), behaves as an amphiphilic polycarboxylic acid and shows a pH-induced conformation transition in aqueous solution. 4"5 The thermodynamic parameters of the transition are evaluated from potentiostatic data. The non-electrostatic free energy change accompanying the conformation change from a compact to an extended form, or the free energy change of the hypothetical transition at zero charge, AGo , was estimated to be 711 J/mol at 2 5 ° C . 6 This suggests the degree of stabilization of the compact structure is not large. The temperature dependence of AG° revealed that the transition from the extended to compact form was accompanied by a decrease in both enthalpy (AHt° = + 1255 J/mol for the change from the compact to extended form) and entropy (AS° = + 0.44 e.u.). 6 This is apparently the opposite of what is expected if the hydrophobic interaction is an essential driving forceY Mandel et al. concluded from these experimental results that the stabilization of the compact structure of PMA was not primarily due to hydrophobic bonding but rather to direct stabilization of certain chain conformations by van der Waals interactions between the s-methyl groups. 6 They also studied partially esterified PMA and demonstrated that the replacement o f - C O O H by the more hydrophobic -COOH3 did not affect the stability of the compact structure.7

952

Y. M O R I S H I M A

On the other hand, Delben e t al. have shown that calorimetric data allow the direct evaluation of the enthalpy of the conformational transition of PMA in aqueous solution. 8-~° With the aid of AGo derived from conventional potentiometric titrations, the entropy change could also be evaluated. They studied the temperature dependence of the transition of atactic PMA and found that A/4~ values are 1029 and 1397 J/mol at 25°C and 45°C, respectively: AHt° becomes more positive at a higher temperature. 9 Such a temperature dependence is common to phenomena that are driven by hydrophobic interaction. 2 On this ground they have claimed that the compact form of atactic PMA is essentially stabilized by hydrophobic bonding. 9'~° When the behavior of PMA is compared with that of PAA, it is evident that the effect of the methyl groups together with the hydrophobicity of the main chain is critical to force the PMA chain to assume a compact structure at lower degrees of ionization. This is clear when one compares the AGo value for poly(ethacrylic acid) (PEA) and that of PMA as reported by Joyce and Kurucsev. ~ The contribution of an additional methylene group in the ~-alkyl group to the stabilization of the uncharged compact form is about 1012J/mol in terms of AG°. This was estimated by comparing the AGo values for PMA and PEA. ~ On the other hand, it should be noted here that poly(N-methacryloyl-L-alanine) (1), CH3

I --~-c.~-- ~-~C-'--O

I I CH ~

NH CH 3 - -

COOH

a polycarboxylic acid with the same main chain structure as PMA, shows no conformational transition. ~2'~3 The introduction of hydrophilic amide groups may cause a shift in the hydrophobic-hydrophilic balance in the chemical structure. If N-methacryloyl-L-alanine is copolymerized with N-phenylmethacrylamide, copolymers (2) with over 19mo1% phenyl residues show a pHinduced conformational transition as a result of the hydrophobic aggregation of the phenyl residues. ~2 From all these facts one can draw the conclusion that the pH-induced conformational transitions of PMA and related polymers are the result of a subtle balance of hydrophobic-hydrophilic nature in the whole polymer structure and are not solely due to the hydrophobic nature of the s-methyl group. One should keep these facts in mind when one considers the modification of PMA

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

CH3

I

---fcn~--~ )l--x

953

CH 3

I

( CH~C--)--

C~O

C=O

I NH

NH

I

CH3----CH--COOH

with various functional groups. The subtle balance may be quite easily destroyed by chemical modification. It is known that the compact structure of uncharged PMA in aqueous solution solubilizes alkanes and aromatic compounds. These are preferentially located in the hydrophobic microdomains in the interior of the tightly coiled PMA chain. ~4-~7Effects of the degree of ionization, the concentration, and the molecular weight of PMA on the solubility of pyrene and 3,4-benzopyrene in PMA aqueous solutions were studied by Barone and co-workers] 5The authors observed that the solubility of the polycyclic hydrocarbons is critically dependent on the degree of neutralization of PMA; i.e. the solubilizing power of PMA aqueous solutions is abruptly and almost completely lost at a degree of neutralization of ca. 0.1. The solubility of pyrene and 3,4-benzopyrene showed an increase with an increase in the molecular weight of PMA and was found to level off at a viscosity-average molecular weight of about 105. The solubilizing power of the uncharged PMA in aqueous solution toward 3,4-benzopyrene was close to that of native DNA on the basis of the repeating unit of each strand. The authors reported, however, that the UV absorption spectra of pyrene and 3,4-benzopyrene in the uncharged PMA solutions showed only slightly redshifted bands as compared to the spectra in cyclohexane (red-shifted by 4 nm). With the native DNA, on the other hand, a considerable red shift of the absorption spectra for the intercalated pyrene and 3,4-benzopyrene is known to occur; i.e. the bands are red-shifted by ca. 11 nm as compared with those in cyclohexane solution] s'~9 This is considered to be due to the electrostatic perturbation of the inserted aromatic molecules exerted by the unsymmetrical charge distribution of the nucleic acid base units in the DNA strand. There are no such specific features in the solubilization in the uncharged PMA aqueous solution. 2.3. Maleic acid copolymers Alternating copolymers of maleic acid and hydrophobic monomers are also a common amphiphilic polyelectrolyte. These copolymers are obtained by

954

Y. MORISHIMA

hydrolysis of the parent copolymers of maleic anhydride and hydrophobic monomers. Strauss and co-workers synthesized a series of alternating copolymers (3) of maleic acid with methyl, ethyl, butyl, and octyl vinyl ethers. 2°'2~ In --(- CH a - - C H - - C H - -

CH-)--

I

[

0

COOH COOH

I x

I

R

common with other polycarboxylic acids, the properties of the aqueous solutions of these maleic acid copolymers are strongly dependent on pH because the charge density on the polymer chain varies with the degree of ionization of the carboxyl groups. Based on potentiometric titration and viscometric data, Strauss and co-workers demonstrated that the copolymers with butyl, hexyl, and octyl vinyl ethers exist in a "hypercoiled" compact structure at low degrees of ionization. 2°'zl The butyl and hexyl copolymers showed a pH-induced conformational transition. Data for the octyl copolymers, however, showed that the compact structure was not fully disrupted even though the pH was raised such that the degree of ionization was 1.3 ([COO ]/[monomer residues]), conformation being slightly dependent on solution pH. The contribution of a methylene group to the stability of the uncharged compact form was estimated to be 1675J/tool. 2j The thermodynamic parameters AHt° and ASt° for the conformational transition were both negative for the butyl copolymers (AH° = - 2 9 0 0 _+ 400J/mol and ASt° = - 3 . 4 _+ 0.5e.u. at 25°C). The compact structure was destabilized by urea added to the aqueous solution. 22These results were supported by the calorimetric studies done by Crescenzi and co-workers. ~3 Copolymers of maIeic acid and styrene are also common amphiphilic polyelectrolytes, of which properties in aqueous solution were first studied by Ferry and co-workers 24 in 1951. Sugai and co-workers revealed that the maleic acidstyrene copolymers showed a pH-induced conformational transition. 25Based on the fact that the absorption spectra due to the phenyl groups in the copolymers reflect the compactness of the polymer coil, the authors spectroscopically studied the kinetics of the conformational transition by a pH-jump technique. 26 The relaxation time constants involved in the transition process were shown to be distributed in a wide range from about 80 to 400msec or longer. The conformation transition is considerably slower than the helix-coil transition of poly(~-amino acids) but faster than the denaturation process of globular proteins such as lysozyme. From thermodynamic data obtained by potentiometric and calorimetric studies, it was concluded that the compact structure was stabilized by the hydrophobic interaction of the phenyl residues in aqueous solution. 27

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

955

The ionization equilibria of the pairs of adjacent carboxyl groups in maleic acid copolymers are characterized by a two-step dissociation with an intrinsic pK value for each step. Bianchi and co-workers pointed out that the pH values for both the first and the second step dissociations tend to be shifted in an opposite direction as a result of the hydrophobic effect of the comonomer unitsJ 8 They prepared maleic acid copolymers (4) with olefins of various carR1

I

--(- C H 2 - - C - R2

C H - - CH COOH COOH

4 R 1 = H,

R2 = H

; e t h y l e n e - m a l e i c anhydride p o l y a n i o n copolymer (EMA)

R 1 =H,

R 2 = CH 3 ; p r o p y l e n e - m a l e i c a n h y d r i d e polyanion copolymer ( P M A )

RI = CH3, R2 = CH3 ; i s o b u t y l e n e - m a l e i c a n h y d r i d e polyanion copolymer (IBMA) RI = CH3, R2 = Call 7 ; 2 - m e t h y l p e n t e n e - 1 - m a l e i c anhydride polyanion copolymer (MMA) R 1 =H,

R2=C6Hs;styrene--maleic anhydride p o l y a n i o n copolymer (SMA)

bon numbers; i.e. ethylene, propylene, isobutylene and 2-methyl-l-pentene. The intrinsic pK values (pK0) of the dicarboxyl groups were calculated from the plots of the apparent pK against the degree of ionization curves. The results show that, as shown in Fig. 1, the pK0 for the first step dissociation decreases while that for the second step dissociation increases with increasing number of carbon atoms in the alkyl side chain of the olefin comonomer. The authors explained these phenomena in terms of the effect of local dielectric constants on ion-dipole interactions for the first step and ion-ion interactions for the second step dissociations. Namely, with decreasing local dielectric constant, the attractive interaction between a carboxylate anion and a dipole on an uncharged carboxyl group would be strengthened, and thereby the first step dissociation is more favored with increasing number of carbon atoms in the hydrophobic side chain. On the other hand, the prevailing interaction involved in the second step dissociation is the electrostatic repulsion between the carboxylate anions, which is strengthened with decreasing local dielectric constant. Therefore, the second step dissociation is disfavored on increasing the number of the carbon atoms. It is to be noted that the values of

956

Y. MORISHIMA

1

2

3

|

i

i

4

5

~

6

10

/

8'

~, ,.,E~

7

6

N

i

2~I"

(2)

5 4

3 x3.Q 2 1 I

I

I

I

I

0 1 2 3 4

n

FIG. 1. Variation of p K 0 (the intrinsic pK) with the number of carbon atoms (n) in the side chain of the olefin c o m o n o m e r in (4). N ( = n + 2) is the total number of carbon atoms in the olefin comonomer. 28

pK0 extrapolated to n = 0 (4.5 and 5.5) are in good agreement with the values for succinic acid. 28 The rather peculiar viscosity behavior of maleic acid copolymers in aqueous solution may be mentioned at this point. The reduced viscosities of the copolymers increase as the degree of dissociation of the dicarboxylic acid groups increases. This is the typical electrostatic expansion behavior of weak polyacids. However, the viscosities tend to decrease as the degree of dissociation further increases beyond 50%. For example, Kitano and co-workers recently reported that the intrinsic viscosity of a maleic acid copolymer with isobutylene (PIM) in a 0.1 N NaC1 aqueous solution showed a clear maximum as shown in Fig. 2. 29 These phenomena are qualitatively in agreement with the data reported on the maleic copolymers with alkyl vinyl ethers 2~ and with styrene. 3° It should be noted that the viscosity behavior as a function of the degree of dissociation of these maleic acid copolymers is also quite similar to that of poly(maleic acid) and its stereoisomer, poly(fumaric acid). 31 In most cases the viscosity shows a maximum value at 50% dissociation, though the maximum tends to shift to a higher degree of dissociation when the hydrophobicity of the comonomer unit is high. Obviously the presence of the maximum viscosity at a certain degree of dissociation is not at all related to the amphiphilic nature of the copolymers. So far there has been no plausible explanation for these phenomena except for a few speculations proposed; i.e. (1) an increase in the

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

957

.8

a

.2

0

0.5 C~

1

FIG. 2. Relationship between the intrinsic viscosity and the degree of dissociation for P I M in 0.100-N NaC1 aqueous solutJon.-

rotational freedom of the polymer main chain arising from the lack of hydrogen bonding at complete dissociation, 3° (2) a "local salting out" effect at high charge densities, 31 and (3) a strong interaction between a polyion having a high local charge density and counterions around it. 29 Consequently, the question of the exact structures or conformations of maleic acid copolymers in aqueous solution, especially the structures of the highly dissociated copolymers, remains open.

2.4. Quaternized poly(vinylpyridine) In 1951 Strauss and Jackson first synthesized amphiphilic polycations by quaternization of poly(2-vinylpyridine) with n-dodecyl bromide and they called them "polysoaps". 32 In their pioneering work on amphiphilic polyelectrolytes Strauss and co-workers revealed that the long aliphatic hydrocarbon side chains attached to the partially quaternized poly(vinylpyridine) tend to aggregate in aqueous solution. The polymers tend to remain in compact con-

958

Y. MOR1SHIMA

formations when the mole fraction of hydrophobic side chain is higher than a certain critical value. The authors prepared poly(4-vinylpyridine) derivatives by quaternizing the 4-vinylpyridine units partially with n-dodecyl bromide and the remainder with ethyl bromide. 33 The intrinsic viscosities of the aqueous solutions of the polymers with high contents of n-dodecyl groups (28.5 and 37.5mo1%) were extremely low, an order of magnitude smaller than those estimated for random coils. The sudden decrease in the intrinsic viscosity was observed between 6.7 and 13.6mo1% content of the n-dodecyl groups in the polymers. These authors first showed experimentally the intramolecular micellation of amphiphilic polymers and the existence of a critical content of the hydrophobic residues, analogous to the critical micelle concentration of ordinary surfactant micelles. They also studied the solubilization of hydrocarbon small molecules such as benzene, 1-heptanol and n-decane by the aqueous solutions of these copolymers. They revealed that the solubility is dependent on the content of the n-dodecyl residues in the polymer. The maximum solubilizing power was observed for the 37.9 mol% dodecyl polymer. 34 Decane solubilization sharply dropped offas the dodecyt content decreased and reached a value of zero for the 13.6 mol% dodecyl polymer. Although amphiphilic polycations that have appeared so far in papers are fewer in number than amphiphilic polyanions, it is meaningful to cite Strauss and co-workers because they introduced a pioneering study on amphiphilic polyelectrolytes. 3. PHOTOPHYSICS OF HYDROPHOBIC CHROMOPHORES ISOLATED IN POLYELECTROLYTES The photophysics of chromophores covalently incorporated into polyelectrolytes at small mole fraction is relatively simple because of the lack of interaction between the chromophores within the polymer. Therefore, a number of studies have focused on amphiphilic polyelectrolytes covalently bonded with a small mole fraction of polycyclic aromatics.

3.1. Poly(methacrvlic acid) loaded with polycyclic aromatic chromophores Chu and Thomas prepared a copolymer of methacrylic acid and 1-pyreneacrylic acid (5) with not more than one pyrene unit included per polymer chain. 3~ The mole ratio of pyrene to methacrylic acid unit was 1 to 1390 and the molecular weight of the copolymer was 1.2 x 105. At p H 3 the steady-state fluorescence spectra of (5) were those typical of a substituted pyrene in a nonpolar environment. It is known that the third to first vibrational fine structure ratio, 13/L, of the pyrene fluorescence is dependent on local environmental polarity, the ratio being 1.65 in hexane and 0.64 in water. 36 With increasing pH, the polymer coil becomes open and the pyrene units come into contact with the

PHOTOPHYSICS IN POLYELECTROLYTE

I I I--x %

CH3 -4---CHz--C )

SYSTEMS

959

COOH

( CH2--C

)

COOH

a q u e o u s phase, leading to a s h a r p decrease in the I3/I1 r a t i o (Fig. 3). T h e fluorescence intensity a n d lifetime s h o w a s h a r p decrease at p H > 4 as the pyrene moieties experience m o r e p o l a r e n v i r o n m e n t s at higher p H s as s h o w n in Fig. 3. T h e plots o f the fluorescence intensity a n d lifetime as a function o f p H show a clear t r a n s i t i o n with a m i d p o i n t at p H 5.7, reflecting the p H - i n d u c e d c o n f o r m a t i o n a l t r a n s i t i o n o f the p o l y m e r . T a b l e 1 c o m p a r e s the rate c o n s t a n t s o f the p y r e n e fluorescence q u e n c h i n g in (5) for v a r i o u s quenchers w h e n the p o l y m e r coil is c o m p l e t e l y closed a n d when it is c o m p l e t e l y open. 35 The rate c o n s t a n t s t e n d to increase as the p o l y m e r c o i l opens. Positively c h a r g e d quenchers such as t h a l l i u m a n d cupric ions exhibit a high q u e n c h i n g efficiency when the p o l y m e r coil is open. This is due to a greater access o f these ionic quenchers to the p y r e n e units in the loose coil a n d is also due to the fact that these c a t i o n s are m o r e s t r o n g l y b o u n d to the negatively c h a r g e d p o l y m e r . I n the case o f iodine, on the o t h e r h a n d , the a n i o n i c q u e n c h e r tends to be repelled b y the negative charges. W i t h n e u t r a l quenchers, the rate TABLE I. Quenching of bound pyrene in 5 for various quenchers Quencher T1NO3

pH

kq, M-1Sec i

2.5

4.3 x 107 2.4 × 10l° 1.1 × 108 1.2 × 10~° 5.4 × 108 3.4 × 10 9 7.2 × 10 6 5.2 X 10 7 5.4 × 108 7.0 x 10 9 1.6 x 10l° 1.9 × 10t°

11.8 CuSO 4

02

2.2 7.2 2.5 11.8

NaI CH3NO 2 CPC*

2.5 11.8 4.0 7.0 2.2 7.2

*Cetylpyridinium chloride. t = kq (uncoiling)/kq (coiling~l.

At 5 × 102 1

×

6.3 6.8 13 1.2

10 2

960

Y. M O R I S H I M A

(a)

(b) 4

PH

N~

3.0

/~-PH ,.o

250

:/ //'" ,.o >i-

>b-

z uJ i.z

Z W IZ

150

-J ...... 370

418

WAVELENGTH

I

nm

2

4

6

O

10

12

PH

FIG. 3. Fluorescencecharacteristics of a 10-2 Mmethacrylicacid-l-pyreneacrylicacid copolymer in aqueous solution: (a) emission spectra of deaerated copolymer at various pHs; (b) relative intensity and lifetime of bound pyrene in copolymer as a function of pH in aerated solutions.35

constants for oxygen and nitromethane show an increase of about 6 and 13 times, respectively, on opening of the polymer coil. These facts show the hydrophobic protection of the c h r o m o p h o r e by the compact form of the chain. The hydrophobic environment of the polymer chain denies entry of ionic quenchers to the pyrene sites in the polymer. The same protective nature of the compact polymer coil is observed in the quenching of the pyrene triplet state by oxygen. 35 At p H 3 a fast O~ quenching of the pyrene triplet occurs at a rate constant (.kq) of 2.13 x 108 u ~sec- l followed by a much slower rate with kq being 0.41 x 108M ~sec -1. Both of these quenching rate constants are significantly smaller than those observed in homogeneous solutions. In time-dependent fluorescence experiments, two distinct exponential decays were observed at p H 3, while the decay could be fitted to a single exponential function at p H 10. These results indicate that some pyrene units are located close to the surface of the coiled polymer, while others are in the interior. Similar results were also reported on an amphiphilic methacrylic acid copolymer with 9,10-diphenylanthracene (DPA) as a chromophore. Webber and co-workers synthesized a copolymer of methacrylic acid with a small mole fraction ( < 1 tool%) of vinyldiphenylanthracene, (6), and studied the fluorescence spectroscopy of the 9,10-diphenylanthracene moiety in aqueous solution, a7 The absorption and fluorescence spectra of (6) with x = 0.073 in aqueous solutions at p H 2 and 11 and of D P A in cyclohexane are compared in Fig. 4. The absorption spectra are all very similar except for a small red shift for the copolymer on the order of 4 nm at p H 2, decreasing to 2 nm at p H 11. The fluorescence spectrum of the copolymer at p H 2 is similar to that of D P A in

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

961

CH 3

I -"(- CH 2 - - i

)l--x

( CH2~UH )

COOH

cyclohexane, but there is a significant red shift and a change of shape at pH 1 1. The emission maximum is plotted as a function of pH in Fig. 5. 37 In the same figure, the diffusion coefficient D of poly(methacrylic acid) (PMA), that was calculated from light scattering data, is also plotted vs p H on the same scale. Upon increasing pH there is an abrupt drop in the D value around pH 5.5. There is also a clear change in the emission maximum between pH4.5 and 6.5. The fluorescence lifetime also shows a significant shortening at higher pH as shown in Fig. 5 (insert). The rapid changes in the emission maximum and the fluorescence lifetime occur in the same pH range. In the pH range of 4.5-6, the polymer exhibits a conformational transition from a compact coil in acidic medium to an extended chain in basic medium as carboxyl groups become ionized. The fluorescence characteristics (both spectrum and lifetime) of the DPA moieties in the copolymer in acidic aqueous solution

FL i'-

ABS

OD

o

i

,

350

~

400

450 nm

500

'

0

FIG. 4. Absorption and fluorescence spectra of D P A and (6) (x = 0.073) in different solvents: ( . . . . ) DPA in cyclohexane; ( ) (6) in water (pH 2.0); (- - -) (6) in water (pH 10); [DPA] (residue) = 2 × 10 -5 M. The excitation wavelength = 355 nm. 37

962

Y. MORISHIMA

l

I

l

I

I

~,rn

-7

7/i

i 4

2

i i 6H8

i - T 10 12

440

log' D

nm

"8 I

2

I

I

4

l

I

I~

6

I

8

I

!

10

I

I

430

pH

FIG. 5. Variation with pH of the logarithm of tee diffusion coefficientD (in cm2 sec 1) of pure polymer PMA (e) and of the maximum wavelength of emission of DPA in (6) (x = 0.073) (I). Insert: variation with pH of the fluorescence lifetime r v (in nsec) of DPA in (6),~7 suggest that the c h r o m o p h o r e s are in a less polar environment than water. On the other hand, they are in intimate contact with water in basic aqueous solution. In fact, the enhancement o f efficiency o f fluorescence quenching o f the D P A moieties u p o n changing the p H from acidic to basic occurs largely for this reason. F o r example, the quenching rate constant for a neutral quencher, nitromethane, increases from 4.8 x 108 M i sec I to 3.7 x 109 M l sec l as the p H is changed from 3.4 to 6.4. The effect o f p H on the quenching is m u c h larger for cationic quenchers such as p a r a q u a t or methyl viologen ( M V 2+ ) and Cu 2+ ions because these ions are electrostatically b o u n d with polyanions, leading to a high local concentration a r o u n d the c h r o m o p h o r e . F o r example, a concentration o f 4 x 10 -2 M o f M V 2+ was necessary to reduce the fluorescence intensity by a factor o f 2 at p H 2.3, but a concentration o f 10 4M was adequate to effect the same reduction at p H 8.5. The apparent second-order rate constant, on the basis o f the initial slope o f a S t e r n - V o l m e r plot in the basic medium, was estimated to be 6.6 x 101°M l sec-~. 3.2.

Poly(acrylic acid) loaded with polycyclic aromatic chromophores

T u r r o and A r o r a synthesized a pyrene-tagged poly(acrylic acid) (PAA) (7) and used the fluorescence from the pendant pyrene to investigate the interactions o f (7) with c o m p l e m e n t a r y polymers in aqueous solution. 3s'39 In spite o f a low pyrene content (1.5 mol %) in (7), a considerable intensity o f excimer emission was observed in an acidic aqueous solution (IM/IE = 1.4 at p H = 4.63). The copolymerization m e t h o d for (7) suggests a r a n d o m distribution o f the pyrene

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

963

COO- Na + ---"("CH 2 - - C H

)

I

( CH2--C

)

I l~x COO-Na +

chromophores in the polymer, where each pyrene group is separated by about 99 acrylic acid units on average. Upon ionization of (7), the IM/IE ratio tends to increase but the polymer still shows a significant excimer fluorescence (IM/IE = 4.9 at pH > 8). The excimer emission maximum for the ionized (7) (P(-)) is 486 nm, whereas that for the protonated (7) (P(H)) is 480 nm. This bathochromic shift for P ( - ) is due to more hydrophobic environments and increased charge density of P ( - ) . From these observations one can easily see a strong tendency for hydrophobic interactions of the pyrene groups that are brought into neighboring positions by coiling of the polymer chain. This facilitates and stabilizes excimer formation even when the pyrene groups are separated by large acrylic acid blocks in the polymer chain. Such spectroscopic properties of the covalently tagged pyrene groups provide information about intrachain interactions of the pyrene groups and the local environments around the chromophores, which, in turn, can be related to the polymer structure in solution. Intermolecular complexes between complementary polymers formed in solution are often insoluble and can cause precipitation or turbidity. This can be avoided, however, when (7) is used to investigate the polymer complexation, because the fluorescence spectroscopy requires very low concentrations of polymers. Under these dilute conditions the interpolymer complexes do not cause an immediate precipitation. The intrachain interactions of pyrene groups separated by large acrylic acid blocks can be reduced by interpolymer complex formation. Poly(vinylamine hydrochloride) (PVAm) (8) associates with (7) mainly via -4- c . 2 - - c . - ~ xI

+

NH3 Ct

_

electrostatic interactions.38As shown in Table 2 the IMlIE values for P(--) tend to decrease in the presence of an equimolar or an excess amount of a low

964

Y. MORISHIMA

O

o +

g

0

-

? I

" e~

0

~x::l

X

b-

c~ ~

~

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

965

molecular weight model compound, 3-aminopentane hydrochloride (M(t)) (9). 39 As indicated by a reduction in P values in Table 2, the presence of M(t) also causes a broadening of the absorption spectrum of P ( - ) , which suggests ground state interactions between amino and pyrene groups. The increase in the intensity of the red emission could be caused by a contribution from the emission of an exciplex formed between excited pyrene and amino groups. 4°

CH 3 CH2 CH CH 2 CH 3

I NH3+CL -

The effects of PVAm on the absorption and emission spectra of P ( - ) are considerably different from those of M(t). The IM/IE ratio for P(--) was strongly reduced in the presence of PVAm. The maximum of the red structureless emission in the emission spectra of P ( - ) showed a hypsochromic shift in the presence of PVAm. The presence of PVAm also caused a broadening of the absorption spectrum of P ( - ) as reflected in lower P values. All these effects are enhanced by an increase in the amount of PVAm added. This behavior could be attributed to ground and excited state interactions (exciplex) of the amino and pyrene groups. Cooperative effects involved in the polymer-polymer complexation lead to stronger exciplex interactions. The same authors have studied the interpolymer complexation of (7) with poly(ethylene oxide), poly(1-vinyl-2-pyrrolidone), poly(1-aminoacrylic acid), and poly(1-acetylaminoacrylic acid) by use of spectroscopic information from the tagged pyrene probes in dilute aqueous solution) 8'39 Turro and co-workers have investigated the interaction of (7) with phospholipidsf The interactions of a phospholipid and a synthetic polymer are interesting as a model for studying the interaction of a biological macromolecule and membranes. Photophysical parameters of (7) were measured in the presence of dipalmitoylphosphatidylglycerol (DPPG) in aqueous solution at pH 4. At pH 4, (7) exists essentially in the protonated form, P(H). The intrachain interactions of pyrene groups in P(H) in the presence of DPPG are greatly reduced by the interactions of P(H) and the phospholipids. This is evidenced by the fact that the IM/IE values in the absence and presence of DPPG are 1.52 and 6.22 at 30°C, respectively. The interaction of P(H) and DPPG is considered to arise from the hydrophobic interactions of P(H) and the lipid chains of phospholipids as well as hydrogen bonding between carboxyl groups in P(H) and glycerol head groups of DPPG at low pHs. Therefore, the pyrene groups in P(H) are considered to be located in the lipophilic interior of the "mixed micelles" of P(H) and DPPG

966

Y. MORISHIMA

P(H)

p(_)

• PYRENE • DPPG

P{H): DPPG FIG. 6. Schematic representation of (7) at pH 4 [P(H)] and pH 7 [P( - )]. The mixed micelles considered to be formed in a solution of (7) and phospholipids at pH 4 are shown for P(H) : DPPG. 41

as conceptually illustrated in Fig, 6. The emission maximum of the excimer fluorescence from the mixed micelles is blue-shifted by 7 nm, consistent with the hydrophobic local environment for the chromophores. This model for the mixed micelle was supported by data on the fluorescence quenching of pyrene by nitromethane. In the absence of D P P G the quenching rate constant for P(H) is 8.77 x 109 M I s e c 1. However, the quenching rate constant in the mixed micelles was reduced to 2.55 × 109M I s e c i. Turro and co-workers also used (7) as a fluorescence probe to investigate the interactions of (7) with a cationic surfactant, dodecyltrimethylammonium bromide (DTAB) in aqueous solution. ~2 Oppositely charged polymers and long-chain surfactants can interact strongly due to electrostatic interactions and the hydrophobic self-aggregation of the surfactant. Therefore, such polyelectrolytes can

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

967

0.91 g/I PAA 0.03M NaCI

0.8 m m


0.7. 0

IE

0.6'

0

0.5

. . . . . . . .

0

i

10 -4

. . . . . . . .

i

. . . . . . . .

10-3

[DTAB],

=

10.2

. . . . . . . .

10 "1

M

FIG. 7. Ratio of third to first vibrational peak of the emission spectra for pyrene dissolved with dodecyltrimethylammoniumbromide (DTAB) in the presence of sodium polyacrylate. 42

induce a micelle-type aggregation of the surfactants at concentrations well below the CMC (critical micelle concentration). Figure 7 shows the change in the third (384nm) to first (374 nm) vibrational peak ratio in the fluorescence spectra of pyrene dissolved with DTAB in the presence of poly(acrylic acid) (PAA) as a function of the concentration of DTAB at pH6.4. The sharp decrease in micropolarity, which is indicated by a sharp increase in the 13/11 ratio, occurs at a DTAB concentration well below the CMC (1.3 x 10 -2 M) in the presence of PAA. The onset of this polymer-induced association was referred to as the critical aggregate concentration (CAC). The emission and excitation spectra and the excimer fluorescence decay profiles observed for (7) i n t h e presence of DTAB at concentration below and above the CAC revealed the following facts. At DTAB concentrations below the CAC, an increase in excimer formation was observed as a result of polymer coiling, and the existence of ground-state pyrene dimers or aggregates was evidenced. Thus. in this case the excimer formation is a static process. By contrast, at DTAB concentrations above the CAC, the ground-state aggregates dissociate and the absorbing species are the isolated pyrene moieties. Thus the excimer formation is decreased. The process is dynamic in nature with a characteristic build-up observed in the timedependent fluorescence data. The surfactant aggregates provide nonpolar environments in which the pyrene groups in (7) can be "solubilized" so that ground-state pyrene aggregates dissociate. From the experiments at different pHs, the ease of DTAB aggregation was shown to depend on the conformational flexibility of the system. With decreasing pH, the tendency for coiling of the polymer-surfactant complex increases. A schematic illustration of the interactions of (7) and DTAB is shown in Fig. 8. Thomas and co-workers used a copolymer of acrylic acid and 1-pyreneacrylic

968

Y. MORISHIMA HIGH DH

LOW DH

a)pH dependent e x p a n s i o n / c o n t r a c t i o n of p o l y m e r : more e x c i m e r under c o i l e d c o n d i t i o n s (low pH).

b ) a b s o r p t i o n of c a t i o n i c s u r f a c t a n t leads to increased excimer formation due to c o i l i n g : greater c o i l i n g at l o w e r pH.

1

DT&B

t

DT&B

c)formation of polymerbound s u r f a c t a n t a g g r e g a t e s : hydrophobic microenvironment f o r p y r e n e : ] e s s ground state aggregation.

FIG. 8. Schematic representation of the ~rmation of polymer-sur~ctant complexes ~r (7) and D T A B . 4z acid as a fluorescence probe for studying the chain conformation when absorbed on CaCO3 particles. 43 The adsorption of polymers onto solid surfaces and their effect on the properties of particulate dispersions have received considerable attention in recent years. Time resolved fluorescence of a deaerated colloidal dispersion of the pyrene-labeled copolymer adsorbed on CaCO3 revealed that the pyrene moieties exist mainly in two different environments in the adsorbed state, one that is similar to that in solution, and the other that is close to, and quenched by, the surface. The degree of polarization (P) of the fluorescence of pyrene in the copolymer when adsorbed on CaCO3 in water was P = 0.168, whereas P = 0 in solution. This is indicative that the pyrene groups are fairly rigidly confined with only a limited range of molecular motion. 3.3. Polycations loaded with pyrene Herkstroeter and co-workers synthesized a polycation (10) with covalently linked pyrene moieties. 44 The fluorescence spectra of (10) in aqueous solution showed a considerable amount of excimer emission even when the concentration was extremely low. The intensity ratio of excimer to m o n o m e r fluorescence (IE[IM) was 1.05 when the residue concentration of the pyrene moieties was 5.4 × 10 -6 M. The concentration is low enough to rule out the possibility of

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

CH a

CH 3

--4-CH~CI )

I ( CH~ - - C--I--

Ii--x

I x

C----O

C "-"O

I I

I I

0

CHa ~

N --

969

O

CH a

I CI-I 3

cH~------~so~

x = 0.04

10

intermolecular excimer formation. The copolymer (10) showed two distinguishable excitation spectra when monitored at wavelengths that correspond to the fluorescence of pyrene monomer and of pyrene aggregate, respectively. The excimer emission progressively increased with an increase in the concentration of an added salt (sodium p-toluenesulfonate) (p-TS). This is an expected observation because polyelectrolytes become more coiled as the ionic strength of the solution is increased. If, however, sodium dodecyl sulfate (SDS) is added in place of p-TS, the results are different. The net effect of SDS is to break up weak ground-state interactions between the pendant substituents of (10). With a pyrene residue concentration of 4 x 10-SM, SDS was added at a level of 2 x 10-5M to decrease the 1E/1M ratio to 0.7. The addition of SDS causes a displacement of p-TS counterion with SDS ions. When such a displacement takes place at neighboring sites to the pyrene pendants, the hydrophobic chromophores may "dissolve" in the long hydrophobic tails, thus reducing the driving force for the weak ground-state interactions between the pendant pyrene groups. This SDS-induced micellization of (10) is schematically illustrated in Fig. 9.

3.4. Poly(2-acrylamido-2-methylpropanesulfonic acid) loaded with polycyclic aromatic chromophores Morishima and co-workers prepared amphiphilic copolymers of 2-acrylamido2-methylpropanesulfonic acid (AMPS) with 9-vinylphenanthrene (9VPh) and

970

Y, MORISHIMA

® ® 9 (9

(

@

UU_

o

FIG. 9. Micellization of (10) induced by the addition of SDS. 44

with 1-vinylpyrene (1VPy). The segments of AMPS in the amphiphilic polyelectrolytes are effective in solubilizing the sequences of the hydrophobic monomer units in water; i.e. both the copolymers of 9VPh (APh-x, x represents mol% content of 9VPh in the copolymer) (11) and 1VPy (APy-x, x represents mol% ~ C H 2 - - C H -~----

- - ( - C H 2 ~ CH

I

100~x

C'~-O

I I C [

NH CH3 ~

CH 3

Ctt2

!o~

Na+

APh--x

11 content of 1VPy in the copolymer) (12) were soluble in water when x < ca. 50mo1%. In Fig. 10 the relative intensity of the fluorescence of APh-x in various solvents is plotted as a function of the phenanthrene mole fraction in the copolymer. 4~The fluorescence intensity decreases markedly in aqueous solution with increasing phenanthrene content. This is obviously due to chromophorechromophore interactions leading to excimer formation or "self-quenching". The decrease in the steady-state fluorescence intensity in organic solution with increasing phenanthrene content is more significant in MeOH than in DMF.

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

971

%

--(-CH2--iH )lO0--x ( CH2-- CH")---C~--O

I I C ~ CH 3 I CH2 I SO7 Na+

NH CH3~

APy-- x 12

This suggests that the shrinkage of the copolymer in MeOH, a poor solvent, facilitates self-quenching. Figure 11 shows the fluorescence spectra of APy-x with various copolymer compositions in aqueous solution. The fluorescence of the copolymer with x = 32 shows a strong excimer emission at about 490 nm. The copolymer with x = 16 still shows a significant excimer emission. Even a copolymer with a very low pyrene content (x = 0.6) exhibits a sign of very weak excimer emission. These data imply that since the polycyclic aromatics such as phenanthrene and pyrene have a strong tendency to self-interact in aqueous solution, 46 the content of these hydrophobic chromophores should be limited to a very low mole fraction for them to be isolated from one another in a polymer coil in aqueous solution. Figure 12 compares fluorescence decay curves of APh-2 (APh-x with x = 2) and APh-41 in aqueous solution. The decay curves are obviously straighter for APh-2 than for APh:41. It is to be noted here that, even though the phenan-

~, 1"0 "~ 0.8 c_. Q6 .-~ 0.& 0.2

o'1 olz o13 o'.4 ols o'.s fP~ FiG. 10. Relative fluorescence intensity o f APh-x in various solvents as a function o f

phenanthrene mole fraction in the copolymers:(o) in water; (zx)in methanol; (D) in DMF.45

972

Y. MORISHIMA

o

400

500 Wavelength

600

I

nrn

FIG. 11. Fluorescence spectra of A P y - x with various copolymer compositions in a q u e o u s solution; ( ) x = 32, ( - - - - ) x = 16, ( . . . . ) x = 0.6; excitation wavelength = 346nm. Fluorescence intensities are normalized at peaking emission at 380 nm. 45

threne content is sufficiently low, the fluorescence decay of APh-2 is not a single exponential but is best fitted to a three-exponential function. This suggests that local environments for the chromophores in the amphiphilic polyelectrolyte are not uniform with respect to each chromophore moeity as might be expected for

, e o . 2 O ~ o

4.0.1'0

I

1

I

I

~e

I

iGe•

-o.lo

104 '

"

t

~

h,I 10.1

~o~4_._.p(-!~j,,.:.

/k

"1,.I I

O.238eIp(-I/24.0n|)4. OJ48exp(-I/4 ?.Ins)

"~

| "

!

!

0

,

$0

I

Io0

!

~0

! 2OO

I O~O

SO

IO0

ISO

TIId£/N.kJdOSir CONO

FIG. 12. Fluorescence decay traces for APh-2 and APh-41 in a q u e o u s solution.

200

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

973

a microheterogeneous system. This seems to be a general feature for amphiphilic polyelectrolytes with a small mole fraction of hydrophobic chromophores Covalently loaded. 4. P H O T O P H Y S I C S O F C H R O M O P H O R E S IN SELF-ORGANIZED HYDROPHOBIC MICRODOMAINS

Photophysical behavior of the polycyclic aromatic chromophores in the clusters of self-organized hydrophobic microdomains is rather complex. This complexity mostly stems from the electronic interactions between chromophores, which involve excimer formation, self-quenching through excited state-ground state interactions, energy transfer and migration, excited state annihilation and so on.

4.1. Phenanthrene aggregates in amphiphilic random copolymers Figure 13 compares the steady-state fluorescence spectra of APh-x with various phenanthrene contents and the related monomer model (AM) (13)

%

CH 2 SO~"Na+

AM

13 in aqueous solution.47 As the phenanthrene content increases, the spectral shape becomes broad with considerable tailing in a long-wavelength region. This implies that the electronic interaction between the phenanthryl groups increases with increasing phenanthrene content in the copolymer. Although, in general, hydrophobic interactions between aromatic chromophores are expected to facilitate excimer formation, no clear excimer emission was observed in the longer wavelength region even for APh-41. This is in contrast with the APy-x case. In fact, phenanthrene is known as a chromophore that would not show an excimer emission peak in the solution state. 48 It has been speculated, however, that poly(9-vinylphenanthrene) may form an excimer with a fluorescence quantum yield that is so low that no clear excimer emission appears. 49 The phenanthrene groups covalently incorporated in the hydrophobic microdomain in the amphiphilic polyelectrolytes are considered to exist under highly favorable conditions for excimer formation. Considering the facts that

974

Y. MOR1SHIMA

\

~,

",

\~:,.

,7

\ "\i"-.

350

400

/,50

Wavelength

500

550

Inm

FIG. 13. N o r m a l i z e d fluorescence spectra m e a s u r e d in a q u e o u s solution at r o o m temperature: - - - , A M ; . . . . , APh-2; . . . . . . . , APh-19; . . . . APh-41: residue concent r a t i o n o f the p h e n a n t h r y l g r o u p , 5.0 × 10 -4 M; excitation w a v e l e n g t h = 332 n m . 47

phenanthrene crystals show a broad excimer emission at 440 nm under high pressure 5° and that phenanthrene groups tethered together on short methylene chains exhibit an excimer-like fluorescence which is less red-shifted relative to the monomer fluorescence than for other excimers of aromatic chromophores, 51 the emission tailing in the longer-wavelength region in Fig. 13 suggests a contribution from excimer emission. It should be noted, again, that the fluorescence intensity in water tends to decrease sharply with increasing phenanthrene content as shown in Fig. 10. As mentioned in the previous section, this is attributed to the excimer contribution as well as "self-quenching" of the excited phenanthrene occurring as a result of the packing of the phenanthrene rings via hydrophobic interaction. The phenanthrene groups in APh-2 are considered to exist in isolation from one another in aqueous solution. Therefore, the difference in the fluorescence spectra between APh-2 and AM in Fig. 13 is simply attributed to the difference in the microenvironments such as local electrostatic potential, local polarity, etc. The fluorescence from the photoexcited phenanthrene groups in APh-x is effectively quenched by an amphiphilic quencher, bis(2-hydroxyethyl)terephthalate (BHET) (14), in aqueous solution. The half-wave reduction potential for B H E T is estimated to be - 1.42 V v s S C E . 45 This quencher molecule acts as an electron acceptor toward the singlet-excited state of the phenanthrene group, of which oxidation potential is estimated to be - 2 . 0 7 V v s S C E . 45 Since the lowest-energy absorption band of B H E T appears at a shorter wavelength than

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

975

+

COOCH2 CH2 OH

COOCH~ CH 2 OH BHET

14

that of phenanthrene, the possibility of energy transfer from the photoexcited phenanthrene to BHET is precluded. An interesting observation is that the efficiency for the phenanthrene quenching by BHET is strongly dependent on the content of the phenanthrene groups (the value of x) in APh-x. 47 Some examples of the relationships between the relative fluorescence intensity Io/I (Io and I are the emission intensity of the phenanthrene group in the absence and in the presence of BHET, respectively) and the quencher concentration are given in Fig. 14. The fluorescence quenching data for APh-2 and APh- 19 obey the Stern-Volmer relation as in the case of the related monomer model (AM). A remarkable feature to be noted is that with APh-41 quenching is extremely effective and the plot shows a downward curvature. The apparent Stern-Volmer constant for APh-41 was calculated from the initial slope in Fig. 14 to be 888 M ] as listed in Table 3. Using the

3

--

2

1

0

10

20

30

40

{BHET} I 10.4 M

FIG. 14. Stern-Volmer-type plots for the fluorescence quenching of phenanthryl group in the presence of B H E T in aqueous solution; the residue concentration of the phenanthryl group is 5.0 × 10 4 M: O, APh-41; e , APh-19; II, APh-2; 13, A M Y

976

Y. MORISHIMA

TABLE 3. Parameters obtained from the Stern-Volmer analyses of fluorescence quenching of the amphiphilic copolymers and the monomer model by BHET Systema

Ksvb/102 M-1

AM APh-2 APh-19 APh-41

1.33 0.60 1.41 8.88 e (K~v,o)

zC/nsec 40 43 38 f 34 f

k~/109 M I sec-I 3.33 1.40 3.71 26.1 g (kq,app)

aResidue concentration of the phenanthryl group; 5.0 x 10 4 M.

bStern-Volmerconstant. CNatural lifetimeof the fluorescence. dSecond-orderrate constant for the fluorescencequenching. eCalculated from the initial slope of the Stern-Volmer plots. fEstimated as a half-lifesince the fluorescencedecay contained an initial fast component. gApparent rate constant calculated from the Stern-Volmerequation.

lifetime of the phenanthrene fluorescence of APh-41, an apparent second-order equivalent rate constant was estimated to be kq,app = 2.61 x 10~°M l sec-~, which significantly exceeds the diffusion limit. In contrast, the quenching of APh-2 is extremely ineffective compared with that of APh-41. In fact, APh-2 shows even less effective quenching than does AM. Similar phenomena are also known for other systems; i.e. if the quenching occurs in a completely dynamic process without any associative binding of the quenchers, the rate of quenching tends to be faster for the small-molecule model systems than for the polymer systems. 45'52's3 This difference may be due to the hindering effect of the polymer segments on the diffusion of the quencher molecules inside the polymer coils: 4,55 Another feature for the APh-41 quenching with BHET is that the quenching efficiency depends on the residue concentration of the phenanthrene groups. 47 As shown in Fig. 15, the lower the phenanthrene concentration in aqueous solution, the more efficient is the quenching with BHET. This is not the case for AM. No such concentration dependence was observed for APh-2 and also for APh-19. This effective quenching and the anomalous behavior in the quenching observed for APh-41 are rather general features for the fluorescence quenching with B H E T due to the hydrophobic binding of the amphiphilic quencher molecules with the hydrophobic microdomains of the chromophore self-aggregates in the amphiphilic copolymer aqueous solution. A kinetic model that is based on the stepwise association model has been proposed to quantitatively explain the fluorescence quenching for this system. 56 The following assumptions have been made in the model. All the hydrophobic chromophores in the polymer are clustered in the hydrophobic microdomains with an average aggregation number of n. Amphiphilic quencher molecules are incorporated into the microdomains via hydrophobic association according to

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

977

the multistep equilibrium: ka

M,,i_~ + Q ~

ik a

" Mn,i,

K,. =

i = 0, 1,2 . . . .

(1) (2)

ka/ik_ a

where Mn,~ denotes the hydrophobic microdomain of average aggregation number or associated with i quencher molecules, Q the quencher in the bulk aqueous phase, ka the rate constant for the association of the quencher molecule and ik_a the rate constant for the ith quencher escape, respectively. It is also assumed that the excitation is delocalized via energy migration over a cluster of the chromophores and thus the intradomain quenching process is described by the first-order rate constant kq,~ that is independent of the number of quenchers incorporated. In this situation a photoexcited chromophore that is buried in the microdomain and is shielded from direct contact with quencher molecules is also susceptible to quenching. The term "static quenching" here means quenching that takes place with quenchers already bound with the chromophore aggregates. This does not necessarily imply, therefore, the presence of a ground-state charge-transfer interaction that could lead to the most effective quenching. 57 On the basis of above assumptions the quenching process can be represented by the following combination of dynamic and static contributions: (3)

M*o kq'2[Q]+1# ) Mn,o

M,, i

kq.l+l/¢

Mn.i '

i=

1,2 . . . .

(4)

where kq, 2 is the second-order rate constant for diffusional quenching, and z the unperturbed lifetime of the singlet excited state. The steady state solution for these schemes leads to Io/I

=

(1 + vkq.,)(A + B[Q]o)/[A + B[Q]0 + "r(Akq., - nkq,2[Q]o) exp { - ( n K , / A ) [ Q ] o } ]

(5)

where A = K~[M] + n and B = n'rkq.2. The initial slope of the curve for eq. (5) in the I o / I v s [Q]0 coordinates, Ksv.0, is related to the total residual concentration of chromophores as follows: 1

Ksv,0 -

K1

1

n(zkq,2 + K~) [M] + zkq,2 + Kt"

(6)

In the extreme case where the quenching process is entirely static; i.e. K~ >> "ckq.2, eq. (6) is approximated as 1

Ksv.0

-

1

1 [M] + - - . n K1

(7)

In another extreme case where the quenching occurs only dynamically, i.e. "ckq,2 >~ K1, the Stern-Volmer relation Ksv = zkq,2 holds.

978

Y. MORISHIMA

170 . -

• •

•

160 78

~

74

~

7o

~

LJ

12~

14 10 0

5'

10 '

15 '

2'0

[Phlresidue I lO-4M

FIG. 15. Dependence of the reciprocal of K,v.0 on the residue concentration of phenanthryl groups for the APh-2 (m), A M (t2), APh-19 (o), and APh-41 (o) systernsY

In the case of the APh-41 quenching by BHET, the plot of the reciprocals of the apparent Stern-Volmer constants against the residue concentration of the phenanthrene groups yields a linear relation obeying eq. (6) (Fig. 15). Using kq = 3.33 x 109M-1 sec i for AM and z = 34nsec for APh-41, one may roughly estimate the term rkq.~ in eq. (6) for APh-4I to be 112M 1. Therefore, the quenching of the APh-41-BHET system can approximately be regarded as all static in nature (K~ >> rkq). Thus, the plots in Fig. 15 can be fit with eq. (7). From the slope and intercept, K~ = 1.1 x 103 M - j and n = 2.6 are estimated, respectively. This means that in a phenanthrene cluster in APh-41 photoexcited energy is delocalized over 2.6 phenanthrene groups on the average. This number does not necessarily mean the actual aggregation number of the phenanthrene groups, but reflects the number of chromophores over which energy migration could occur. The kinetic parameters, kq.l and kq.2, w e r e determined by the computer best fit of the experimental data with eq. (5) (Fig. 16). As listed in Table 4, the value of kq. 2 = 1.8 × 109M l s e c -1 thus obtained seems to be reasonable compared with the kq value for APh-2 listed in Table 3. The value of kq. 1 --= 5.5 × 107 sec-l is similar to those reported for the surfactant micellar system. 58'59 4.2. Naphthalene aggregates in amphiphilic random copolymers Guillet and co-workers have also studied amphiphilic polyelectrolytes loaded with large mole fractions of aromatic chromophores. 6° They have proposed a

PHOTOPHYSICS IN POLYELECTROLYTESYSTEMS

3.0

o

2.0

979

//

1°6

;.o

2:o

#.o

4:o

[BHET]IIO-3M FIG. 16. Simulationusing eq. (5) with the optimally fit values o f k q , 1 = 5.5 × 107sec i and k q 2 = 1.8 X 109M I sec I for the APh-41-BHET system: [Phi (residue) = 5.0 x i0-4M, z = 3.4 x 10-Ssec, Kl = 1.1 × 103M-1, and n = 2.6 were used: plots in the figure represent experimental data. 47 " p s e u d o m i c e l l a r " m o d e l or " h y p e r c o i l e d " structure for polyelectrolytes cont a i n i n g a r o m a t i c c h r o m o p h o r e s such as n a p h t h a l e n e ; i.e. in dilute a q u e o u s solution the a r o m a t i c groups are clustered near the center o f the p o l y m e r coil, while the charged groups are located o n the exterior. 6L62 Guillet a n d co-workers have prepared a n amphiphilic polyelectrolyte with naphthalene chromophores, poly(2-vinylnaphthalene-co-2-vinylnaphthalene sulfonic acid) (SP2VN) by partial s u l f o n a t i o n of p o l y ( 2 - v i n y l n a p h t h a l e n e ) (P2VN). 62The s u l f o n a t i o n was performed by treating P 2 V N with chlorosulfonic acid in 1 : 1 c a r b o n t e t r a c h l o r i d e - d i c h l o r o m e t h a n e solution a n d the degree o f s u l f o n a t i o n for the p o l y m e r was d e t e r m i n e d by t i t r a t i o n with a q u e o u s N a O H to be 77%. The fluorescence spectra o f S P 2 V N (Fig. 17) showed almost entirely

TABLE4. Parameters for the fluorescence quenching of the APh-41 system obtained by analysis using the kinetic model Parameters KI,M -l n

1.1 x 103* 2.6*

kq. 1, see -1 kq,2, M -1 s e c 1

5.5 × 1077 1.8 x 1 0 9 t

*Calculated from the data in Fig. 15 by using eq. (7). fEstimated by computer best fitting using eq. (5) (Fig. 16).

980

Y. MORISHIMA

•

i

i

i

i

m

"~ ~ i1:

.,°.,"

350

400

450

500

Wavelength (nrn) FIG. 17. Fluorescence emission spectra of SP2VN (4.8 × 10-3M) on 310-nm excitation at 22°C: ( .... ) in T H F - C H 3 O H (4:1); ( - - - ) in H20; ( ) in N a O H - H 2 0 (pH 13). 62

excimer emission in aqueous solution at r o o m temperature. This result is consistent with the proposed pseudomicellar model, where the naphthalene groups are located in close proximity to facilitate excimer formation and energy migration. The aqueous solution of SP2VN showed almost entirely excimer even at 77 K, suggesting that excimer forming sites are already formed that do not require any further molecular motion. The authors compared the delayed emission of SP2VN in organic solvent (4 : 1 tetrahydrofuran-methanol) with that in water or dilute N a O H . A s shown in Fig. 18, in organic solution SP2VN shows a normal phosphorescence emission with an emission m a x i m u m of 500 nm and a significant delayed fluorescence at 300 nm attributable to naphthalene monomer. By contrast, in aqueous solution, almost all of the delayed emission is from the excimer at 400nm. The phos-

~rQ,

' :.

' .-. .....

' .--

'

'"'

"..\'

-_

'

.~.~.

:__~.~_.~--... 1

350

I

I

I

400 450 500 Wavelength (nrn)

I

I

550

600

FIG. 18. Phosphorescenceand delayed emission spectra of SP2VN solid glass at 77K on 310-nm excitation (4.8 x 10 3M): ( - - - ) in NaOH-H20 (0.125N); (.... ) in THF-CH3OH (4: 1); (--.--) in H:O. ( ) P2VN in THF. 62

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

981

phorescence peak is red-shifted in aqueous solution by about 12nm. These results are also consistent with the proposed pseudomicellar structure where photon energy absorbed anywhere in the polymer chain would undergo rapid singlet energy migration to excimer forming sites. It was shown by the same authors that perylene could be trapped in the hydrophobic core of the SP2VN coil in dilute NaOH. 62 Emission bands at 450 and 480 nm due to perylene are visible when the naphthalene groups are selectively excited at 310 nm in aqueous solution. They presumed that the excitation reaches the perylene trap via singlet migration and transfer, representing an "antenna" effect. Guillet and co-workers have also demonstrated an efficient "antenna" effect in the intramolecular energy transfer from naphthalene to anthracene end groups in the aqueous solution of amphiphilic copolymers of acrylic acid and 1-naphthylmethyl methacrylate (NMMA) ( 1 5 ) . 61 The tagging of the copolymer CH3

1 COOH

C~

0

I I CH2

O

x = 0 . 0 6 9 - 0.22

15 ends with anthracene moieties was performed by using 9-bromomethylanthracene as a chain transfer agent in the free radical copolymerization of acrylic acid and NMMA. The content of the NMMA units in the copolymers studied ranged from 6.9 to 22 mol%. In dioxane solution the efficiency of the energy transfer from naphthalene to anthracene ranged from 12 to 17%. However, in aqueous solution the fluorescence was predominantly from anthracene when naphthalene was excited. The energy transfer efficiency was estimated to be up to 70%, even though the copolymer contained only 7.6 mol% of naphthalene units in (15) and 0.25 mol°/0 of terminal anthracene units. In order for such high efficiencies to be possible most of the naphthalene groups must exist within a distance less than the F6rster distance (R0) for the trap (R0 ~ 24 A for naphthalene and anthracene). One would think that a copolymer containing only 7.6 mol°/0 NMMA groups would assume an extended conformation in dilute alkaline solution due to the prevailing mutual electrostatic repulsion of the carboxylate anions. However, it is unlikely that the energy

982

Y. M O R I S H I M A

c O

Ol CO~

"°Oc~,~

C0 0 I l

I _ - ,Ca-- ./-coO-ooc~-/;%00:, ;o

-Ooc .ooc

A

.'~

-OOC

_ - - 4-L,

c,oo-

"r~-- j

, cO0°

.

C o o / o _

-

~

"

.

FIG. 19. Proposed hypercoiled structure of anthracene end-trapped copolymers of acrylic acid and NMMA in dilute alkaline solution showing F6rster radii for various energy-transfer processes.6~

migration takes place along the extended copolymer chain with 7.6mo1% NMMA units. Therefore, the authors have proposed that these copolymers with such low contents of the hydrophobic chromophore units adopt a "hypercoiled" structure even in dilute alkaline solution, in which the naphthalene groups form the interior of the coil and the carboxylate groups form an outer "shell" as conceptually illustrated in Fig. 19. 5. PHOTOPHYSICS OF AMPHIPHILIC ALTERNATING COPOLYMERS The photophysical behavior of chromophores is complex when the chromophore loading on a polymer is high, as reviewed in the previous chapter. This is due to the electronic interaction of the chromophores, often leading to "self-quenching" and/or excimer formation. To avoid this photophysical complexity while maintaining high loadings of chromophore on the polymers, attention has been directed to alternating copolymers in which the chromophores are equally separated with a comonomer unit. The photophysics of alternating copolymers consisting of chromophore units has been investigated mostly in organic solvents. Fox and co-workers demonstrated that excimer formation was essentially eliminated in the alternating copolymer of 2-vinylnaphthalene (2VN) and methyl methacrylate (MMA). 63 This has been confirmed by recent work of Webber and co-workers who compared the singlet-state processes of excimer formation and energy migration in alternating and random copolymers of 2VN and MMA. 64 Singlet energy migration rates were estimated for the alternating and random copolymers by the method of comparative quenching using carbon tetrachloride as a fluorescence quencher in tetrahydrofuran. The estimated

PHOTOPHYSICS IN POLYELECTROLYTE

SYSTEMS

983

energy migration rate for the alternating copolymer was found to be faster than that for the random copolymer with roughly comparable mole fraction of the 2VN units in the copolymer.64 No excimer emission was reported to be observed in alternating copolymers of vinylcarbazole with diethyl fumarate and fumaronitrile.65On the other hand, Wang and Morawetz studied the alternating copolymers of maleic anhydride (MAn) with stilbene and acenaphthylene and concluded that these exhibited strong excimer fluorescence. 66 Bai and Webber prepared the copolymers of acenaphthylene and methacrylic acid or maleic acid and found that these displayed strong excimer fluorescence in organic solvents.67 Therefore, the design of an alternating copolymer is not always a solution to avoid excimer formation. Amphiphilic alternating copolymers show very different photophysical behavior in aqueous solution compared with random copolymers with a comparable content of the same pendant chromophore groups. There are only a few publications on the amphiphilic alternating copolymers. Morishima and co-workers prepared water-soluble amphiphilic alternating copolymers of 2VN and maleic acid (MAA) (16) and studied the fluorescence --(-- C H 2 ~

CH ~

CH ~

CH -)--

COOH

COOH

16

behavior in aqueous solution in comparison with that of a random copolymer (17) of 2VN and acrylic acid with a 2VN content of 44mo1%. 68 Figure 20 -----~CH2 ~

CH ~

(

CHz~CHA--COOH

x=0.04

, 0.44

17 compares the fluorescence spectra of the alternating copolymer (prepared from the 1 • l 2VN/MAn feed ratio) with those of the random copolymer in aqueous

984

Y. MORISHIMA

pH

pH

pH

10C 0

E.4

3~ 0 C:

~so

o;

r:-;..-" ...... .......... "::.'.x'}i ....

3OO

3S0

~00

4so

...

A

soo"3oo

3so

Wavelenglh

~oo I

~so

soo 300

~ 3so

~oo

nm

FIG. 20. Fluorescence spectra for the alternating (16) and r a n d o m (17) copolymers at various pHs in aqueous solution; [phenanthrene residue] = 0.3mM; excitation wavelength = 290 nm. (a) Alternating copolymers (16); (b) r a n d o m copolymer (17) with x = 0.44; (c) r a n d o m copolymer (16) with x = 0.04. 68

solution at various pHs. A striking feature for the fluorescence of the alternating copolymer is that the excimer emission is predominant and its intensity is insensitive to the change in pH. In this copolymer, the monomer fluorescence, noticeable only as a small peak at higher pHs, tends to decrease with decreasing pH and almost completely disappears at acidic pHs. In contrast, the fluorescence spectra of the random copolymer with 44 mol% naphthalene units showed a considerable dependence on pH. A critical decrease of the monomer fluorescence intensity occurred in a narrow pH range of 8.2-7.5, suggesting a conformational transition. At acidic pHs the random copolymer displayed almost exclusively excimer emission. However, the intensity of this excimer emission is much lower than that of the alternating copolymer. A 2VN-acrylic acid random copolymer with 4mo1% 2VN content exhibited only monomer fluorescence, the intensity of which is insensitive to pH. The total fluorescence quantum yields for the alternating copolymer at pH 9 and 6 are estimated to be 0.28 and 0.26, respectively. These values are considerably higher than those estimated for the random copolymer with 44 mol% 2VN; i.e. 0.099 and 0.056 at pH 9 and 6, respectively. These facts indicate that a considerable self-quenching process is occurring in the random copolymer. A significant difference in the fluorescence behavior between the alternating and random copolymers was also observed in the concentration dependence. In the case of the random copolymer the concentration markedly affects the monomer-to-excimer fluorescence ratio. The intensity of excimer emission increases relative to the monomer fluorescence intensity with an increase in the polymer concentration as shown in Fig. 21. The total fluorescence quantum

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

[ Naph ]resiauelmM

985

INaphlresi~ue/mM

30

o'.~

(b)

(a)

,.s

0,3 0.03

o

i

I

300

350

~0o

450

5oo 3o0

Wavelenglh

350 I nm

4O0

i

450

5O0

FIG. 21. Comparison of the normalized fluorescence spectra of the alternating (16) and random (17) copolymers at pH 10 at various concentrations; excitation wavelength = 290 nm. (a) Alternating copolymer (16); (b) random copolymer (17) with x = 0.44. 68

yield decreases with the increase in the excimer emission at high concentrations. These facts indicate that the intermolecular association of the random copolymers contributes to the excimer formation. By contrast, the fluorescence spectral shape of the alternating copolymer prepared from 1 : 1 2VN/MAn feed ratio is virtually independent of the polymer concentration. These facts strongly suggest that excimer formation is an intramolecular process in the alternating copolymer. The excimer fluorescence decay profiles for both the alternating and random copolymers are not single exponentials. This is not surprising because the configurational constraints are likely to result in excimers with varying stability and hence varying lifetimes. The different macromolecular environments for the excimers may also cause photophysical complexity that is reflected in the lifetime. At lower pHs the excimer fluorescence of the random copolymer tends to decay faster, suggesting a considerable self-quenching in the random copolymer in acidic media. A question arises as to what are the excimer forming sites in alternating 2VN-MAA copolymers. In order to answer this question 2VN-MAA copolymers were prepared with various 2VN/MAn monomer feed ratios and the characterization of the copolymers including the sequence distribution was carried o u t . 69 The 2VN-MAn copolymerization shows an alternating tendency. However, the copolymers deviate slightly from the equimolar composition when either monomer is in excess in the feed as shown in Fig. 22. The composition curve given in Fig. 22 is the best-fit curve with the monomer reactivity ratios given in the figure caption. If only propagation steps are taken into consideration, the number fraction of the alternate dyads, MIM 2 and M2M I for 2VN (M1) and

986

Y. MORISHIMA

o

.2"

/ d

O

f

,

,

o

,

,

,

i

I

i

i

o,5 M1 FY~c-tlon In-I~eecl

I.O

FIG. 22. Relationship between the mole fraction of 2VN (M~) in copolymer and that in the monomer feed for the copolymerization of 2VN and MAn in benzene at 70°C. Plots: experimental data. Solid line: the best fit curve for r~ = 0.07 ___ 0.018 and r2 = 0.034 + 0 . 0 0 6 . 6 9

MAn (Me), is given by Fi2,21

=

2/(2 +

rlu +

r2/u)

u = [M,]/[M2].

(8) (9)

Similarly, the consecutive dyad fraction for M~ M~ and M2M 2 is given by Fu

=

r,u/(2 + rlu + r2/u)

(10)

F=

=

(r2/u)/(2 + rlu + r2/u).

(1l)

Table 5 lists the results of the calculations. The equimolar feed yields a copolymer with the equimolar composition and with a number fraction of the alternate 2VN-MAn dyads of 0.951. However, 2VN-2VN and MAn-MAn dyad fractions of 0.033 and 0.016 are present, respectively, as "structural defects" in the alternating copolymer. Figure 23 compares the ~3CNM R spectra of the methyl-esterified 2VN-MAA copolymers prepared from different monomer feed ratios of 2VN and MAn. The intensities of all resonance peaks are normalized by the most intensive peak at 127.5 ppm due to naphthalene ring carbons. The peak intensities at 42.5, 51.5, and 170 ppm, due to methine, methyl, and carbonyl carbons On the dimethyl maleate (DMM) unit, respectively, decrease as the 2VN/MAn feed ratio increases. This indicates that the copolymer composition is slightly dependent on the monomer feed ratio consistent with the copolymerization composition data in Fig. 22. A change in the sequence distribution is slightly reflected in the peak of the C~ carbon in the naphthalene ring. The C~ peak tends to become a multiplet as the 2VN/MAn feed ratio increases, reflecting an increase in the 2VN-2VN dyad fraction in the copolymer sequence. Small peaks at 140 and

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

987

TABLE 5. Composition and sequence data for the 2VN-MAn copolymers Sample

u*

Fit

2VN-MAn-1 2VN-MAn-2 2VN-MAn-3 2VN-MAn-4

0.111 0.429 1.00 2.33

0.44 0.48 0.50 0.54

Fll 0.004 0.014 0.033 0.075

F,,2

F12.21

F,/(FII + F22)

LI2.21

/~n$

0.132 0.038 0.016 0.007

0.864 0.948 0.951 0.918

0.025 0.269 0.673 0.915

8.26 19.5 20.3 12.9

9 160 12 200 15700 29 500

*Mole ratio of 2VN/MAn in monomer feed. t2VN mole fraction in the copolymers. :~Polystyrene equivalent number-average molecular weights determined by GPC for the methyl esters of the copolymers.

-CHz-CH-- (~H-(~H-

.~, ~--oc,--o

''

' ' ~H3 ~H3 C2.3.10

I

-

CH3

C5-8 C4'9

2VN/MAn ~i 0

-O-I-(DMM}

' t "CH2*

PPM

FIG. 23. 100-MHz ~3C NMR spectra for the 2VN-DMM copolymers in CDC13 at 60°C. The 2VN/MAn monomer feed ratios for the preparation of the precursor copolymers are indicated in the figure. 69

988

Y. MORISHIMA

).s

/

/tJ-N !~."~'.~ tl :

~1.0 :

Os

'~, \

•

o/"

;// 3

5

317

\,,,

?

9

11

13

pl.,I

FIG. 24. pH dependence of the reduced viscosity of the 2 V N - M A A copolymers in aqueous solution: C = 0.2 g/dL at 30°C. The 2 V N / M A n monomer feed ratios for the preparation of the precursor copolymers are indicated in the figure. 69

141.5 ppm are attributable to C1 carbons in the 2VN-2VN dyad sequence. The multiplicity of the methylene carbon peaks also tends to increase with an increase in the 2VN/MAn feed ratio. Consequently, it is confirmed that the 2VN-MAA copolymer prepared from the equimolar feed, though it may practically be viewed as an alternating copolymer, contains a small amount of the 2VN-2VN dyad sequence. The 2VN-MAA copolymers are still to be referred to as "alternating" copolymers in this article, in the sense of copolymers with a strongly alternating tendency. The effects of hydrophobic and electrostatic interaction on macromolecular conformations in aqueous solution are well reflected in viscosity behavior. The viscosities of the alternating 2VN-MAA copolymers increase at a pH between 6 and 7 due to an expansion of the polymer coil from a compact to a loose form as the carboxyl groups are ionized, as shown in Fig. 24. Consistent with a rather peculiar feature known to maleic acid copolymers as mentioned in the previous chapter, the viscosities showed a decrease with further increasing pH in the basic region. Structural models for these copolymers at complete dissociation have not yet been proposed. In the case of a 2VN-acrylic acid random copolymer with roughly comparable 2VN content and comparable molecular weight, the reduced viscosity is similar to that of the 2VN-MAA alternating copolymer at pH 6, but is much higher at pH 9. The random copolymer forms a microphase structure with hydrophobic aggregates of the naphthalene moieties at pH 7 or lower, while the polymer

PHOTOPHYSICSIN POLYELECTROLYTESYSTEMS

989

chain assumes an extended chain conformation at pH 9. Namely, the random copolymer undergoes a conformational transition from a compact to a loose or extended form at a pH between 7 and 9. This is reflected in the sharp decrease in the monomer fluorescence intensity observed in the pH range 8.2-7.5 shown in Fig. 20. The molecular weight and molecular weight distributions of the alternating and random copolymers, which are estimated by GPC of their methyl ester derivatives, are similar. Therefore, the large difference in the viscosity at pH 9 between the alternating and random copolymers is not attributed to the differences in molecular weight, but rather to the difference in the chain conformation. The ionized alternating copolymer assumes a much more compact conformation than does the ionized random copolymer. The conformation of the fully ionized alternating copolymer is an open question. These studies strongly suggest that the 2VN-2VN dyad sequences provide excimer forming sites. Since the number of excimer forming sites is very small in the alternating copolymers, energy migration through the alternating sequences is an important process in obtaining a large population of excimer in aqueous solution. As the conformation of the alternating copolymers is not highly extended even at basic pHs and is crowded with the naphthalene residues, geometrical requirements for the F6rster-type energy migration are likely to be satisfied in the clusters of the naphthalene moieties. 6. PHOTOPHYSICS OF AMPHIPHILIC BLOCK COPOLYMERS Another interesting class of amphiphilic polyelectrolytes are those consisting of block hydrophobic and charged sequences. It is well known, in general, that block copolymers exhibit a mucJa clearer microphase separation in "selective" solvents than do random copolymers. Block copolymers in such a solvent consist of aggregates of less soluble blocks of one monomer and a dispersed phase of soluble blocks of another monomer unit. Therefore, amphiphilic block copolymers are expected to behave very differently compared with random copolymers with the same sequence units in aqueous solution. Namely, block copolymers may show (1) a larger size of hydrophobic microdomains, (2) more efficient binding of hydrophobic fluorescence quenchers, (3) a longer range of energy migration, and therefore, (4) an efficient quenching of fluorescence from the hydrophobic chromophores. For the preparation of well defined block copolymers, living anionic polymerization techniques are generally employed. A disadvantage of these techniques, however, is that none of the "electrolyte monomers" can be directly employed. Trimethylsilyl methacrylate (TMSM) is a useful monomer susceptible to living anionic polymerization. The trimethylsilyl ester groups in the resulting polymers can be easily hydrolyzed under mild conditions to yield methacrylic acid (MA) seq uences.70

990

Y. M O R I S H I M A

The A-B-A type amphiphilic block copolymers of MA and 9-vinylphenanthrene (9VPh) (18) were synthesized by a two step process of living anionic CHs Li+

Li+ CH - - CH2 r v x / ' x f v CH~ - - CH- Li +

CH2=! COO Si (CH~):

THF,- 78°C

CH3

THF,-78°C ~

CHs

CHs

~--c.~ ~ 2 ~ - - ~ .~-~c~--~ ~

_-

CH3

-~c--c~ ~-~ ~ - - c~~--~CH~--~m

18

polymerization followed by a complete hydrolysis of the trimethylsilyl ester groups. 71 The degree of polymerization of the phenanthrene block sequences was estimated by GPC analysis of the 9VPh living polymer that was sampled out during the polymerization immediately before the second monomer (TMSM) was added. For the block copolymers DMF is a good "nonselective" solvent, while water is a selective solvent. The block copolymers are soluble in water, but cannot directly be dissolved in water. A clear aqueous solution can be obtained when the block copolymers are dissolved into a small amount of DMF first and then the solution is injected into water. The fluorescence from the excited phenanthrene groups in the amphiphilic block copolymer is quenched by BHET in aqueous solution far more effectively than that in the random copolymer R-VPh-21 (19). 72 Fluorescence quenching CH3

~-

CH 2 - -

I

C-~---(-

COOH

x=0.21

19

CH 2 - -

CH -~

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

991

~t (o) 1

0

5

10 15 20 [BHET]

I

i

i

J

i

0

I

2

3

I.

25x10.SM

I

5 x10.3M

[BHEI] FIG. 25. Stern Volmer plots for the fluorescence quenching by BHET at pH 9 in aqueous solution: (a) block copolymer (o) B-VPh-23, (o) B-VPh-7 (see Table 7); (b) random copolymer R-VPh-21. 72

f o r t h e b l o c k c o p o l y m e r s a n d t h e r a n d o m c o p o l y m e r is c o m p a r e d i n Fig. 25 a n d t h e q u e n c h i n g d a t a a r e s u m m a r i z e d i n T a b l e 6. A l l t h e S t e r n - V o l m e r p l o t s f o r the block copolymers showed downward curvature. Therefore, the quenching c o n s t a n t s , Ksv, i n T a b l e 6 w e r e t h o s e c a l c u l a t e d f r o m t h e i n i t i a l s l o p e o f t h e Stern-Volmer plots. The random copolymer showed a linear relation in the Stern-Volmer plot. The large difference in the quenching constants between the b l o c k a n d r a n d o m c o p o l y m e r s is d u e i n p a r t t o t h e m o r e efficient b i n d i n g of BHET with the hydrophobic microdomains of the block copolymers, TABLE 6. Rate constants for fluorescence quenching by BHET at pH 9 and 7 Sample

B-VPh-43 B-VPh-23 B-VPh- 16 B-VPh-7 R-VPh-21

DPMA-DPvr,h-DPMAa Z (nsec)b

77-43-77 c 54-23-54 c 53-16-53 c 23-7-23 d -

12 20 18 25

pH 9e

pH 7f

K~v(M-' )

kq(M lsec - t )

Ksv(M -1)

17000 g 14000g 195

8.5 x 1011 7.8 x 1011 7.8 X l0 9

38000 g 29000 g 22 000g 21000 g 830

kq(M-'sec 1) 3.2 X 1012 1.5 × 1012 1.2 x 1012 3.3 x 10I°

"Degree of polymerization of each block sequence. bMeasured in pure water. CCalculated from GPC data for VPh living polymer and the MA/VPh ratio determined by NMR. dCalculated from GPC data for VPh living polymer and the methyl ester of the block copolymer. eBorate buffer. fPhosphate buffer. gCalculated from the initial slope of the Stern-Volmer plot.

992

Y. MORISHIMA

10~

5 [M] 1lO"~M

10

FIG. 26. Dependence of the reciprocal of K~v,o On the residual concentration of phenanthrene groups in the block copolymer B-VPh-23 in aqueous solution. 56

and also to the more efficient energy migration within the phenanthrene block sequences. These conclusions can be reached by kinetic analysis of the fluorescence quenching of the block copolymers. The apparent quenching constants, Ksv.0,calculated from the initial slopes of the Stern-Volmer plots are dependent on the concentration of the chromophore group. Figure 26 shows the dependence of the reciprocal of Ksv,0on the residue concentration of the phenanthrene groups at pH 7 for a block copolymer (B-VPh-23). 56 Applying eq. (7) (given in the previous chapter), zkq. 2 -k- Kl = 3.7 x 104 M-~ was obtained from the intercept. The value of the phenanthrene excited state lifetime for the block copolymer is roughly estimated to be shorter than 2 × 10 -8 sec, and the kq, 2 value cannot exceed the diffusion-limiting value (7 × 10 9 M - l sec -I).73 Therefore, the theoretical upper limit for the zkq, 2 'term can be estimated to be smaller than 1.4 × 102M 1. The contribution of the dynamic quenching with BHET present in the bulk phase can virtually be neglected. Consequently, K~ -- 3.7 × 104u 1 can be estimated. This/£1 value for the block copolymer is about 30 times larger than that for a random copolymer of 9VPh and AMPS (APh-41) described in the previous chapter. From the slope for the plot in Fig. 26, n = 26 can also be estimated. Although the estimation of the slope contains somewhat larger error, this n value is fairly close to the degree of polymerization of the phenanthrene sequence in the block copolymer used for the experiment. These results suggest that in the concentration range employed in this experiment the block copolymer forms a "unimolecular micelle" in aqueous solution and the energy migration is taking place throughout the hydrophobic microdomain. The MA sequences in the block copolymers are not completely dissociated at pH 7. Unlike poly(acrylic acid), undissociated MA sequences are hydrophobic in nature as discussed in the previous chapter. Therefore, the undissociated portions of the MA sequences may play a role in facilitating hydrophobic binding of BHET around the phenanthrene groups and also in stabilizing

PHOTOPHYSICS

IN POLYELECTROLYTE

SYSTEMS

993

TABLE7. Rate constants for fluorescence quenching by BHET in DMF solution* Sample

D-'--PvPh'~

T (nsec)

Ksv(M l)

B-VPh-43 B-VPh-7 R-VPh-21

43 7

14 21 24

25 26 30

-

-

kq (M l sec-I) 1.8 1.2 1.3

×

10 9

×

109

×

109

*Measured without deaeration; [VPh]residue = 3 × 10-4 M. tDP of VPh sequence in the block copolymer. self-aggregates of the hydrophobic chromophores in aqueous solution. This seems to be a reason that the quenching constants are larger at p H 7 than at p H 9, especially in the case of the random copolymer. In D M F , a "nonselective" solvent, the effect of hydrophobic interactions is nil. Reference data obtained in this solvent are listed in Table 7. 72 Several interesting facts can be obtained from the comparison of the fluorescence quenching by B H E T in aqueous and in D M F solutions. First, the quenching constants for the block copolymer in D M F solution are m a n y orders of magnitude smaller than those observed in aqueous solution. Second, the Stern-Volmer plots for the block copolymers in D M F give straight lines, and the second-order quenching rate constants, kq, fall in the range of the diffusion-limiting value. Thirdly, there is no difference in the magnitude of the kq values between the block and random copolymers in D M F . These results strongly support the interpretation of the unusually effective fluorescence quenching of the block copolymers in aqueous solution. More recently, Webber and co-workers studied the solvent dependence of the fluorescence and energy trapping of the same block copolymers by using D M F and methanol as a nonselective and selective solvent, respectively. 74 Fluorescence spectra of B-VPh-43 and B-VPh-7 in D M F and in M e O H are shown in Fig. 27.

"•/•L" a )

- --.-....

•

B-Wh-43 I~VPh-7 D~fferenco Iplctrum Of mod+l nnd B-VPh-43

b

--

B-VPh-43

""'~'~'~k~. DMF

350

400

450 nrn

500 350

400

450 nm

FIG. 27. Fluorescence spectra of B-VPh-43 and B-VPh-7 normalized at 358 nm and the difference spectra: solvent, (a) DMF+ (b) methanol.TM

500

994

Y. MORISHIMA

O-

F -1-

$

-3-

-420

40

60

80

ns

FIG. 28. Fluorescence decay for B-VPh-7, B-VPh-16, and B-VPh-43 (from upper to lower trace for solvent indicated). All curves for a given solvent have been superimposed at their maxima for ease of comparison (IRF = instrument response function; wavelength observed = 358nm). TM

The spectra are normalized at 358 nm. Fluorescence spectra of the block copolymers are slightly broadened compared with those of the random copolymer, although no clear excimer emission in the longer wavelength region is observed in any solvent. It is obvious that in M e O H the fluorescence spectra of the block copolymers are broadened and the relative intensity at longer wavelength is higher than those in D M F . For the random copolymer, R-VPh-21, there is no significant change in spectra, but the spectra tend to red-shift by l - 2 n m in M e O H . For the block copolymers no red-shift is observed. A new broad emission spectrum having a maximum around 390nm was obtained by subtracting the R-VPh-21 spectra from the block copolymer spectra. The intensity of this new emission decreases in the order of B-VPh-43 > B-VPh-16 > B-VPh-7 and M e O H > M e O H / D M F (1:1) > D M F . This lower energy emitting species is assigned to be a weakly bound excimer state which is less red-shifted relative to the monomer than are many other "classical" aromatic excimers. These excimer forming sites are believed to act as traps for the mobile monomeric excitation. Figure 28 shows fluorescence decay for the block copolymers with various phenanthrene block units in D M F and in MeOH. The fluorescence decay curves were monitored at 358 nm since the excimer makes essentially no contribution at this wavelength. The fluorescence decay is nonexponential for all samples, including the random copolymer. It can be seen from Fig. 27 that the m o n o m e r fluorescence decay is faster for the longer chain block copolymer, and that the

PHOTOPHYSICS IN POLYELECTROLYTE SYSTEMS

995

poorer the solvent quality the faster is the monomer fluorescence decay. In D M F there is only a small difference between the decay curves but these differences are accentuated in MeOH. It is interesting to see how the extent of energy trapping and energy transfer with the phenanthrene blocks is dependent on the nature of a solvent. Webber and co-workers have attempted to ascertain whether cross-chain energy migration occurs in the block copolymer system by comparing the photophysical data with a simple lattice model of the polymer chain. 74 They carried out this comparison by using a "fractal" expression for the fluorescence decay function

I(t) =

exp ( - a t a/2 + bt a -

t/Zo)

(12)

where d is the "spectral" dimension and z0 the unperturbed lifetime of the chromophore. This expression is based on a random walk to traps distributed on a lattice. 75 The spectral dimension may vary between 1 (ideal l-D) and 2 (ideal 3-D). For the case of B-VPh-43, d was found to vary from 1.08 to 1.4 as the solvent is changed from D M F to methanol. This variation is consistent with the idea that the phenanthrene block undergoes a transition from a random coil to a compact coil of higher effective dimensionality. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. l l. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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