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Intramolecular excimer formation in macromolecules—III
European Polymer Journal, Vol. 15, pp. 925 to 929
0014-3057/79/1001-0925502.00/0
© Pergamon Press Ltd 1979. Printed in Great Britain
INTRAMOLECULAR EXCIMER FORMATION IN MACROMOLECULES--III ENERGY
MIGRATION AND EXCIMER FORMATION IN COPOLYMERS OF VINYLNAPHTHALENE AND METHYL ACRYLATE R. A. ANDERSON,R. F. REID and I. SOUTAR Chemistry Department, Heriot-Watt University, Riccarton, Currie, Edinburgh EH14 4AS, U.K.
(Received 5 April 1979) Al~a'aet--Energy migration and intramolecular excimer formation have been examined in copolymers of 1-vinyinaphthalene and methyl acrylate. The degree of depolarization of fluorescence, p- 1 from glassy solutions of the copolymers in 2-methyltetrahydrofuran at 77 K was used to characterize the extent of intramolecular energy migration which is described by the mean sequence length of naphthalene chromophores, 1., The proportionality of the ratio of excimer to monomer intensities of emission to the function l.'fnn implies that the excimer site concentration is proportional to the fraction of naphthalene pairs, f.., in the copolymer. The same function has been shown previously to describe intramolecular excimer formation in l-vinylnaphthalene/methyl methacrylate and other copolymers [20, 21]. Comparison of the data pertinent to the methyl acrylate copolymers with those of the methyl methacrylate series shows that (a) there is no evidence for long range intramolecular excimer formation in copolymers of vinylaromatic chromophores even in the relatively flexible methyl acrylate series; (b) intramolecular excimer formation, occurs more readily in the less stericaUy constrained methyl acrylate series consequent upon the fact that rotation into the excimer conformation will be less restricted and that (c) energy migration is more extensive in the methyl acrylate copolymers. It has been demonstrated that comparison of excimer formation efficiency in different copolymer systems is more meaningful when a common value of the function descriptive of the formation is used as opposed to a common chromophore concentration. Reactivity ratios of 1-vinylnaphthalene and methyl acrylate were determined as 1.18 and 0.39 respectively. INTRODUCTION
Intramolecular excimer formation has been studied in a variety of homopolymer systems [1-9]. Information regarding the parameters governing excimer formation in macromolecules have been sought through studies of the influence of temperature upon the relative emissions of excimer and monomer [7, 10-13]. Recently, the emergence of time resolved emission spectroscopic investigations has provided valuable information regarding the importance of excimer dissociation in the description of polymer photophysics [14-16]. The study of copolymers has extended understanding of the mechanisms of excimer formation in macromolecules [17-23]. Reid and Soutar [20-22] have argued that intramolecular excimer formation in macromolecules shows a greater degree of similarity to intermolecular excimer formation than to the intramolecular phenomenon for low molar mass species. Energy migration in the polymer plays an analogous role in population of potential excimer sites to that adopted by material diffusion in creation of excimer configurations in an intermolecular interaction. Consequently, the kinetic scheme of Birks [24] has been modified to model excimer formation in copolymers containing a variety of vinylaromatic [20, 21] and acenaphthylene [22] chromophores respectively. Methyl methacrylate was the spectroscopically inactive monomer in all the copolymer systems studied. In particular it was demonstrated that, in the copolymers investigated to
date, excimer formation can be described in terms of the functions which characterize energy migration and excimer site concentration. It has been proposed that fluorescence depolarization data can be used, under favourable circumstances, to quantify the extent of energy migration in a copolymer. In the copolymers of methyl methacrylate with 1- and 2-vinylnaphthalenes or styrene, the depolarization of emission emitted in a frozen glass matrix was a linear function of the mean sequence length of aromatic, 1o [20, 21]. Since excimer formation is expected to result from nearest-neighbour interactions in such systems, the concentration of potential excimer sites is expected to be proportional to the mole fraction of aromatic pair sequences, f,~ [19]. Proportionality between the excimer to monomer intensity ratio Iv/Iu and the product lo'f,~ was evident as predicted by the model [20, 21], More recently, excimer formation in acenaphthylene/methyl methacrylate copolymers has been described in terms of the mol fraction of chromophore, fa, characteristic of the energy migration and a complex sum term of pentad sequences which describes the next to nearest neighbour excimer interactions in acenaphthylene polymers [22]. The present paper describes excimer formation in copolymers of 1-vinyinaphthalene with methyl acrylate. The aims of the work are as follows. (i) To test the validity of the model of Reid and Soutar [20, 21] in the description of copolymer of a vinylaromatic species with a "spectroscopically inac-
925
R.A. ANDERSON, R. F. REID and I. SOUTAR
926
tive" monomer which is less sterically constrained than methyl methacrylate. The validity of the function la'f,~ in description of excimer formation of the methyl methacrylate/vinylaromatic copolymers implies the absence of significant contributions from long-range interactions to the excimer concentration. If such interactions between species distant along the polymer chain are prevalent in vinylaromatic polymers, it would be expected that they would increase in significance as the flexibility of the copolymer chain is increased. (ii) To investigate and quantify the effects of the structure of the "non-chromophoric" monomer upon the excimer emission relative to monomer. Wang and Morawetz [23] have observed that, in acenaphthylene copolymers of comparable aromatic content, In/IM is greater with methyl methacrylate as comonomer relative to copolymers incorporating methyl acrylate. The opposite trend in In/IM is apparent from examination of the data of David et al. [19] for copolymers of styrene with methyl methacrylate or methyl acrylate. However, a direct comparison of excimer formation efficiency requires examination of photophysical data pertinent to a common value of the function descriptive of excimer formation rather than reference to a common aromatic content. (iii) To study the effects of comonomer structure upon intramolecular energy migration efficiency.
Table 1. Composition data for 1-vinylnaphthalene/methyl acrylate copolymers
EXPERIMENTAL
where r. and r= are reactivity ratios of 1-vinylnaphthalene and methyl acrylate respectively, and [Mi] is molar feed concentration of species i.
Materials Distilled 1-vinylnaphthalenc (Koch-Light) and methyl acrylate (B.D.H.) were prepolymerized and fractionally distilled under vacuum prior to use. Copolymers of 1-vinylnaphthalene and methyl acrylate were prepared by bulk polymerization at 70°C using 10-3 M AIBN as initiator under high vacuum conditions. Polymerization was terminated at less than 5% conversion. Excess monomer was removed by multiple reprecipitation of the copolymers. Solvents were subjected to fractional distillation and their purities checked by fluorimetry. 2-Methyltetrahydrofuran was purified by refluxing over and distillation from lithium aluminium hydride.
Sample
[M.] [Mm]
1 2 3 4 5 6 7 8 9
0.003 0.063 0.136 0.267 0.369 0.525 0.823 1.243 2.226
f. 0.02* 0.13 0.23 0.36 0.43 0.52 0.58 0.64 0.77
R
f.n
l.
-24.3 39.7 53.0 57.3 59.5 58.1 53.0 41.7
-0.010 0.030 0.092 0.148 0.218 0.287 0.370 0.562
-1.07 1.16 1.31 1.43 1.62 1.97 2.46 3.62
* Denotes composition calculated from reactivity ratios. ameters which are significant to the subsequent analysis of the photophysical data. A brief description of the relevant functions is given below.
Run number, R The run number, described by Harwood and Ritchey [26], is a useful function for generation of functions characteristic of the statistical composition of a copolymer. R is given by R =
200 2 + rn[Mn]/[Mm] + r . [ M m ] / [ M . ]
(1)
Fraction of pairs of naphthyl chromophores, f~, The fraction of bonds between naphthyl species is given by f " = f"
R 200
(2)
where f , is the mol fraction of naphthyl chromophore in the copolymer.
Mean sequence length of naphthyl species, In The average length ofnaphthyl sequences is given by
Techniques Fluorescence intensity and polarization data were obtained on a suitably modified Perkin-Elmer MPF3L spectrofluorimeter. Solutions employed for fluorescence measurements were 10- 5 M in naphthalene chromophore. Emission spectra were not corrected for wavelength dependence of instrument response. Excimer to monomer intensity ratios were estimated by curve fitting procedures as previously described [21]. Copolymer compositions were determined by nuclear magnetic resonance (Jeol 100 MHz) and elemental analyses. RESULTS AND DISCUSSION
Copolymer composition Monomer feed and copolymer composition data upon application of a Kelen--Tiidos [25] analysis yielded excellent linear plots independent of monomer "identity inversion". The reactivity ratios obtained were 1.18 and 0.39 for I-vinyl naphthalene and methyl acrylate respectively. The basic copolymerization data may be converted into statistical functions descriptive of chain microcomposition. Table 1 details the par-
Energy mioration The randomization of the transition vector of emission which results from energy migration may be detected in the degree of polarization of the emitted radiation. The depolarization, p-a, produced by a mean number t] of migrative jumps is related to the intrinsic polarization, Po, of the fluorescent species by ( p - t _ 1/3)= (po t - 1/3)(1 + Ch)
(4)
where C is a constant whose value is dependent upon the dominant energy transfer mechanism. The application of luminescence depolarization techniques to the study of energy transfer requires elimination of the depolarizing effects of micro Brownian rotations of the chromophores. Depolarization measurements of naphthalene emission wore made upon solutions of the copolymers in M T H F at
Intramolecular excimer formation in macromolecules--llI 77 K. The depolarization, p - l , following excitation with polarized excitation is estimated from the parallel, I11 and perpendicular, Ii components of emission intensity as p - i = III + I± (5) Iii -
927
10
°o o--'"
ID
IM
I L"
The perpendicular intensity was corrected to compensate for instrumental distortions according to the method of Azumi and McGlynn [27]. The depolarization of emission, which yields information regarding relatively short range energy transfer effects, has been shown to be proportional to the mean sequence length of aromatic species, /,, in the vinylaromatic polymers studied to date [20, 21], The applicability of the average sequence length of naphthalene chromophores, 1,, to description of the energy migration in the I-vinyl naphthalene/methyl acrylate copolymers is demonstrated in Fig. 1. This implies that the average number of migrations, ~, is proportional to the average length of chromophore sequence irrespective of whether methyl methacrylate or methyl acrylate is employed as partially reflecting barrier to migration. Comparison of the data of Fig. 1 with those obtained in the 1-vinyl naphthalene/methyl methacrylate series [20,21] reveals a more significant migration when methyl acrylate is comonomer. In other words the proportionality_constant governing the relationship between h and I, is greater when the less stericaily hindered monomer is present. (The gradient of the graph of p-~ against [, for the methyl acrylate series is more than twice that for the methyl methacrylate series), The more efficient energy migration in the methyl acrylate copolymer series may be a consequence of several effects, the two most important of which are: (i) the chain conformation of the methyl acrylate copolymers in the frozen glass may be such as to produce a more compact coil leading to more exten-
1 "0
2g
~nf~
Fig. 2. ID/IM as a function of ld',~ (dichloromethane solvent: 298 K). sive migration than observed in the methyl methacrylate copolymers; (ii) the methyl acrylate may function as a less efficient barrier to migration than the methyl methacrylate. These effects of the comonomer are not entirely separable since the chain conformation will affect the ability of the exciton to migrate across the polymer coil to a chain segment which is sufficiently close. The value of the intrinsic polarization, Po 1, estimated from the data in Fig. 1 by extrapolation to 1. = 1 is 5.9 which agrees well with that of 5.4 obtained from fluorescence quenching experiments in fluid solutions of 1-vinylnaphthalene labelled poly methyl methacrylate [28] and of 4.9 obtained by extrapolation to 7. = 1 for energy migration experiments on the methyl methacrylate copolymer series [20, 21]. This agreement of polarization data indicates the relative insignificance of possible distortions due to stresses in the solvent glass, etc.
Spectroscopic data
/
10C
/
p-~
J
Under photostationary state conditions, the kinetic scheme of Birks [24] applicable to intermolecular excimer formation reduces to the following expression for excimer to monomer intensity ratio at constant temperature
ID IM
0
2.0
3'0
T. Fig. 1. Depolarization as a function of mean path length of naphthyl chromophores in the copolymer (2-methyltetrahydrofuran solvent; 77 K). E.p.J. 1 5 / 1 0 ~
-
k kDu[M]
(6)
where k is a composite rate coefficient (involving rate constants of fluorescence of monomer and excimer in addition to those of radiationless deactivation pathways), knu is the rate coefficient of excimer formation and [M] is the concentration of fluorescent species. The photophysical scheme has been modified [20--22] to encompass intramolecular excimer formation in polymers, A function descriptive of energy migration is incorporated into Eqn (5) as a component of kDM and a function descriptive of the concentration of excimer sites is used in place of the [M] relevant to bimolecular excimer formation. If excimer formation is a consequence of interactions between nearest-neighbour chromophores on
928
R. A: ANDERSON,R. F. REIDand I. SOUTAR
the polymer chain, the appropriate concentration term will be the fraction of aromatic pairs in the copolymer, f , . Consequently, if the energy migration in fluid solution is characterized by the same function as in a rigid glass, Eqn (5) evolves into the form: ID = k l . f . . . IM
(7)
The validity of such a relationship has been demonstrated for some vinylaromatic/methyl methacrylate copolymer series [20,21]. Figure 2 illustrates the applicability of Eqn (6) to the description of excimer behaviour in the 1-vinylnaphthalene/methyl acrylate copolymer series. It is apparent that the function In'f,, adequately describes the excimer formation over a fair composition range of the copolymers (certainly up to 64mo1% naphthalene content, corresponding to l,'f,, = 1.1). Deviation from linearity is evident in the datum point for the copolymer of 77 mol % naphthyl chromophore content. The breakdown of models incorporating an energy migration term and an excimer site concentration term is to be expected when migration becomes sufficiently efficient to guarantee population of potential excimer sites. It is to be expected under these conditions that ID/IM will depend solely upon the excimer site concentration. Excimer site population is extremely efficient in the eopolymer of 77 mol % naphthyl species. The value of ID/IM of 9.7 is of comparable magnitude to that of poly-l-vinylnaphthalene (loAM = 10.8 for identical solvent/temperature conditions). It is obvious that the model must break down prior to the homopolymer since intramolecular excimer formation is not concentration dependent for all but extremely low molar mass homopolymers. Further justification for the inapplicability of the model in this system at high aromatic contents is to be found in examination of the triad composition of the 77 mol % naphthalene copolymer. 94% Of the naphthyl chromophores exist in triads of the form NNM or N N N (where N and M represent units derived from 1-vinylnaphthalene and methyl acrylate respectively). Furthermore, about 98% of the methyl acrylate species exist as isolated units in the polymer. Under such conditions energy migration is expected to be very efficient. The ability of the function 1, .f,, to describe ID/IM in the 1-vinylnaphthalene/methyl acrylate copolymer series implies that interactions other than of a nearest-neighbour form are insignificant in description of the assemblage of excimer sites. Despite the introduction of the more flexible (relative to methyl methacrylate) methyl acrylate units into the chain, the influence of excimer formation through chromophores rendered in the correct alignment through the chain folding back upon itself is not evident. Such long-range interactions have been supposed as possible interactions in many polymer systems and are suggested as dominant in polymers of naphthyl methacrylate [e.g. 9]. However, such interactions do not appear to be significant in the vinyl aromatic copolymers studied to date. Comparison of In/IM of the 1-vinylnaphthalene/ methyl acrylate copolymers with those of 1-vinylnaphthalene/methyl methacrylate copolymers of similar naphthyl content reveals that excimer formation is
more efficient in the former system. However, the true magnitude of the difference in excimer formation efficiency in the two series of polymers is apparent only through comparison of macromolecules of similar l,.f~,. Under these circumstances it is apparent that ID/IM values of the methyl acrylate series are about 6 times those of the methyl methacrylate series in dichloromethane solution. The increased efficiency is a consequence of two effects: (i) increased energy migration in the methyl acrylate series leading to increased efficiency in population of the available sites; (ii) the greater flexibility of the methyl acrylate species will influence the relative energies of the excimer conformation and the pre-excimer rotamer, leading to a greater exeimer site concentration in the methyl acrylate series. CONCLUSIONS Intramolecular excimer formation in 1-vinylnaphthalene/methyl acrylate copolymers is: (i) resultant upon interactions between nearest-neighbour chromophores; (ii) about six times more efficient than in 1-vinylnaphthalene/methyl methacrylate copolymers, as estimated from comparison of data pertaining to polymers of identical 7,'f,,. Intramolecular energy migration in glass solutions of the 1-vinylnaphthalene/methyl acrylate copolymers is characterized by 1, and is more extensive than in the corresponding methyl methacrylate copolymer. Acknowledgements--The authors are pleased to acknowledge financial assistance from the S.R.C. in the form of an equipment grant (to I.S.) and a studentship (to R.F.R.). REFERENCES
1. S. S. Yanari, F. A. Bovey and R. Lumry, Nature 200, 242 (1963). 2. F. Hirayama, J. chem. Phys. 42, 3163 (1965). 3. M.T. Vala, J. Haebig and S. A. Rice, J. chem. Phys. 43, 886 (1965). 4. W. Kl6pffer, Ber. Bunsenoes. Physik. Chem. 73, 864 (1969). 5. W. Kltpffer, J. chem. Phys. 50, 2337 (1969). 6. C. David, W. Demarteau and G. Geuskens, Eur. Polym. J. 6, 1397 (1970). 7. R. B. Fox, T. R. Price, R. F, Cozzens and J. R. McDonald, J. chem. Phys. 57, 534 (1972). 8. C. David, M. Lempereur and G. Geuskens, Eur. Poly~ J. 8, 417 (1972). 9. A. C. Somersall and J. E. GuiUet, Macromolecules 6, 218 (1973). 10. C. David, M. Piens and G. Geuskens, Eur. Polym. J. 8, 1019 (1972). 11. L. A. Harrah, J. chem. Phys. 56, 385 (1972). 12. C. W. Frank, J. chem. Phys. 61, 2015 (1974). 13. I. Mclnally, I. Soutar and W. Steedman, J. Polym. Sci. Polyr~ Chem. Ed. 15, 2511, (1977). 14. K. P. Ghiggino, A. J. Roberts and D. Phillips, J. Photochem. 9, 301 (1978). 15. K. P. Ghiggino, R. D. Wright and D. Phillips, Chent Phys. Lett. 53, 552 (1978). 16. K. P. Ghiggino, R. D. Wright and D. Phillips, J. Polynt Sci. Polynt Phys. Ed. 16, 1499 (1978). 17. R. B. Fox, T. R. Price, R. F. Cozzens and W. H. Echols, Macromolecules 7, 937 (1974).
Intramolecular excimer formation in macromolecules--III 18. M. Yokoyama, T. Tamamura, M. Atsumi, M. Yoshimura, Y. Shirota and H. Mikawa, Macromolecules 8, 101 (1975). 19. C. David, M. Lempereur and G. G-euskens, Eur. Polym. J. 9, 1315 (1973). 20. R. F. Reid and I. Soutar, J. Polym. Sci. Polym. Lett. Ed. 15, 153 (1977). 21, R. F. Reid and I. Soutar, J. Polym. Sci. Polym. Phys. Ed, 16, 231 (1978). 22, R. F. Reid and I. Soutar, J. Polym. Sci. Polym. Phys. Ed. In press.
929
23. Y.-C. Wang and H. Morawetz, Makromalek. Chem. Suppl. 1, 283 (1975). 24. J. B. Birks, Photophysics of Aromatic Molecules. Wiley, New York (1970). 25. T. Kelen and F. Ttidos, J. Macromolec. Sci. A 9, 1 (1975). 26. H. J. Harwood and W. M. Ritchey, J. Polym. Sci. B 2, 601 (1964). 27, T. Azumi and S. P. McGlynn, J. chem. Phys. 37, 2413 (1962). 28. G. J. Kettle and I. Soutar, Eur. Polym. J. 14, 892 (1978).
0014-3057/79/1001-0925502.00/0
© Pergamon Press Ltd 1979. Printed in Great Britain
INTRAMOLECULAR EXCIMER FORMATION IN MACROMOLECULES--III ENERGY
MIGRATION AND EXCIMER FORMATION IN COPOLYMERS OF VINYLNAPHTHALENE AND METHYL ACRYLATE R. A. ANDERSON,R. F. REID and I. SOUTAR Chemistry Department, Heriot-Watt University, Riccarton, Currie, Edinburgh EH14 4AS, U.K.
(Received 5 April 1979) Al~a'aet--Energy migration and intramolecular excimer formation have been examined in copolymers of 1-vinyinaphthalene and methyl acrylate. The degree of depolarization of fluorescence, p- 1 from glassy solutions of the copolymers in 2-methyltetrahydrofuran at 77 K was used to characterize the extent of intramolecular energy migration which is described by the mean sequence length of naphthalene chromophores, 1., The proportionality of the ratio of excimer to monomer intensities of emission to the function l.'fnn implies that the excimer site concentration is proportional to the fraction of naphthalene pairs, f.., in the copolymer. The same function has been shown previously to describe intramolecular excimer formation in l-vinylnaphthalene/methyl methacrylate and other copolymers [20, 21]. Comparison of the data pertinent to the methyl acrylate copolymers with those of the methyl methacrylate series shows that (a) there is no evidence for long range intramolecular excimer formation in copolymers of vinylaromatic chromophores even in the relatively flexible methyl acrylate series; (b) intramolecular excimer formation, occurs more readily in the less stericaUy constrained methyl acrylate series consequent upon the fact that rotation into the excimer conformation will be less restricted and that (c) energy migration is more extensive in the methyl acrylate copolymers. It has been demonstrated that comparison of excimer formation efficiency in different copolymer systems is more meaningful when a common value of the function descriptive of the formation is used as opposed to a common chromophore concentration. Reactivity ratios of 1-vinylnaphthalene and methyl acrylate were determined as 1.18 and 0.39 respectively. INTRODUCTION
Intramolecular excimer formation has been studied in a variety of homopolymer systems [1-9]. Information regarding the parameters governing excimer formation in macromolecules have been sought through studies of the influence of temperature upon the relative emissions of excimer and monomer [7, 10-13]. Recently, the emergence of time resolved emission spectroscopic investigations has provided valuable information regarding the importance of excimer dissociation in the description of polymer photophysics [14-16]. The study of copolymers has extended understanding of the mechanisms of excimer formation in macromolecules [17-23]. Reid and Soutar [20-22] have argued that intramolecular excimer formation in macromolecules shows a greater degree of similarity to intermolecular excimer formation than to the intramolecular phenomenon for low molar mass species. Energy migration in the polymer plays an analogous role in population of potential excimer sites to that adopted by material diffusion in creation of excimer configurations in an intermolecular interaction. Consequently, the kinetic scheme of Birks [24] has been modified to model excimer formation in copolymers containing a variety of vinylaromatic [20, 21] and acenaphthylene [22] chromophores respectively. Methyl methacrylate was the spectroscopically inactive monomer in all the copolymer systems studied. In particular it was demonstrated that, in the copolymers investigated to
date, excimer formation can be described in terms of the functions which characterize energy migration and excimer site concentration. It has been proposed that fluorescence depolarization data can be used, under favourable circumstances, to quantify the extent of energy migration in a copolymer. In the copolymers of methyl methacrylate with 1- and 2-vinylnaphthalenes or styrene, the depolarization of emission emitted in a frozen glass matrix was a linear function of the mean sequence length of aromatic, 1o [20, 21]. Since excimer formation is expected to result from nearest-neighbour interactions in such systems, the concentration of potential excimer sites is expected to be proportional to the mole fraction of aromatic pair sequences, f,~ [19]. Proportionality between the excimer to monomer intensity ratio Iv/Iu and the product lo'f,~ was evident as predicted by the model [20, 21], More recently, excimer formation in acenaphthylene/methyl methacrylate copolymers has been described in terms of the mol fraction of chromophore, fa, characteristic of the energy migration and a complex sum term of pentad sequences which describes the next to nearest neighbour excimer interactions in acenaphthylene polymers [22]. The present paper describes excimer formation in copolymers of 1-vinyinaphthalene with methyl acrylate. The aims of the work are as follows. (i) To test the validity of the model of Reid and Soutar [20, 21] in the description of copolymer of a vinylaromatic species with a "spectroscopically inac-
925
R.A. ANDERSON, R. F. REID and I. SOUTAR
926
tive" monomer which is less sterically constrained than methyl methacrylate. The validity of the function la'f,~ in description of excimer formation of the methyl methacrylate/vinylaromatic copolymers implies the absence of significant contributions from long-range interactions to the excimer concentration. If such interactions between species distant along the polymer chain are prevalent in vinylaromatic polymers, it would be expected that they would increase in significance as the flexibility of the copolymer chain is increased. (ii) To investigate and quantify the effects of the structure of the "non-chromophoric" monomer upon the excimer emission relative to monomer. Wang and Morawetz [23] have observed that, in acenaphthylene copolymers of comparable aromatic content, In/IM is greater with methyl methacrylate as comonomer relative to copolymers incorporating methyl acrylate. The opposite trend in In/IM is apparent from examination of the data of David et al. [19] for copolymers of styrene with methyl methacrylate or methyl acrylate. However, a direct comparison of excimer formation efficiency requires examination of photophysical data pertinent to a common value of the function descriptive of excimer formation rather than reference to a common aromatic content. (iii) To study the effects of comonomer structure upon intramolecular energy migration efficiency.
Table 1. Composition data for 1-vinylnaphthalene/methyl acrylate copolymers
EXPERIMENTAL
where r. and r= are reactivity ratios of 1-vinylnaphthalene and methyl acrylate respectively, and [Mi] is molar feed concentration of species i.
Materials Distilled 1-vinylnaphthalenc (Koch-Light) and methyl acrylate (B.D.H.) were prepolymerized and fractionally distilled under vacuum prior to use. Copolymers of 1-vinylnaphthalene and methyl acrylate were prepared by bulk polymerization at 70°C using 10-3 M AIBN as initiator under high vacuum conditions. Polymerization was terminated at less than 5% conversion. Excess monomer was removed by multiple reprecipitation of the copolymers. Solvents were subjected to fractional distillation and their purities checked by fluorimetry. 2-Methyltetrahydrofuran was purified by refluxing over and distillation from lithium aluminium hydride.
Sample
[M.] [Mm]
1 2 3 4 5 6 7 8 9
0.003 0.063 0.136 0.267 0.369 0.525 0.823 1.243 2.226
f. 0.02* 0.13 0.23 0.36 0.43 0.52 0.58 0.64 0.77
R
f.n
l.
-24.3 39.7 53.0 57.3 59.5 58.1 53.0 41.7
-0.010 0.030 0.092 0.148 0.218 0.287 0.370 0.562
-1.07 1.16 1.31 1.43 1.62 1.97 2.46 3.62
* Denotes composition calculated from reactivity ratios. ameters which are significant to the subsequent analysis of the photophysical data. A brief description of the relevant functions is given below.
Run number, R The run number, described by Harwood and Ritchey [26], is a useful function for generation of functions characteristic of the statistical composition of a copolymer. R is given by R =
200 2 + rn[Mn]/[Mm] + r . [ M m ] / [ M . ]
(1)
Fraction of pairs of naphthyl chromophores, f~, The fraction of bonds between naphthyl species is given by f " = f"
R 200
(2)
where f , is the mol fraction of naphthyl chromophore in the copolymer.
Mean sequence length of naphthyl species, In The average length ofnaphthyl sequences is given by
Techniques Fluorescence intensity and polarization data were obtained on a suitably modified Perkin-Elmer MPF3L spectrofluorimeter. Solutions employed for fluorescence measurements were 10- 5 M in naphthalene chromophore. Emission spectra were not corrected for wavelength dependence of instrument response. Excimer to monomer intensity ratios were estimated by curve fitting procedures as previously described [21]. Copolymer compositions were determined by nuclear magnetic resonance (Jeol 100 MHz) and elemental analyses. RESULTS AND DISCUSSION
Copolymer composition Monomer feed and copolymer composition data upon application of a Kelen--Tiidos [25] analysis yielded excellent linear plots independent of monomer "identity inversion". The reactivity ratios obtained were 1.18 and 0.39 for I-vinyl naphthalene and methyl acrylate respectively. The basic copolymerization data may be converted into statistical functions descriptive of chain microcomposition. Table 1 details the par-
Energy mioration The randomization of the transition vector of emission which results from energy migration may be detected in the degree of polarization of the emitted radiation. The depolarization, p-a, produced by a mean number t] of migrative jumps is related to the intrinsic polarization, Po, of the fluorescent species by ( p - t _ 1/3)= (po t - 1/3)(1 + Ch)
(4)
where C is a constant whose value is dependent upon the dominant energy transfer mechanism. The application of luminescence depolarization techniques to the study of energy transfer requires elimination of the depolarizing effects of micro Brownian rotations of the chromophores. Depolarization measurements of naphthalene emission wore made upon solutions of the copolymers in M T H F at
Intramolecular excimer formation in macromolecules--llI 77 K. The depolarization, p - l , following excitation with polarized excitation is estimated from the parallel, I11 and perpendicular, Ii components of emission intensity as p - i = III + I± (5) Iii -
927
10
°o o--'"
ID
IM
I L"
The perpendicular intensity was corrected to compensate for instrumental distortions according to the method of Azumi and McGlynn [27]. The depolarization of emission, which yields information regarding relatively short range energy transfer effects, has been shown to be proportional to the mean sequence length of aromatic species, /,, in the vinylaromatic polymers studied to date [20, 21], The applicability of the average sequence length of naphthalene chromophores, 1,, to description of the energy migration in the I-vinyl naphthalene/methyl acrylate copolymers is demonstrated in Fig. 1. This implies that the average number of migrations, ~, is proportional to the average length of chromophore sequence irrespective of whether methyl methacrylate or methyl acrylate is employed as partially reflecting barrier to migration. Comparison of the data of Fig. 1 with those obtained in the 1-vinyl naphthalene/methyl methacrylate series [20,21] reveals a more significant migration when methyl acrylate is comonomer. In other words the proportionality_constant governing the relationship between h and I, is greater when the less stericaily hindered monomer is present. (The gradient of the graph of p-~ against [, for the methyl acrylate series is more than twice that for the methyl methacrylate series), The more efficient energy migration in the methyl acrylate copolymer series may be a consequence of several effects, the two most important of which are: (i) the chain conformation of the methyl acrylate copolymers in the frozen glass may be such as to produce a more compact coil leading to more exten-
1 "0
2g
~nf~
Fig. 2. ID/IM as a function of ld',~ (dichloromethane solvent: 298 K). sive migration than observed in the methyl methacrylate copolymers; (ii) the methyl acrylate may function as a less efficient barrier to migration than the methyl methacrylate. These effects of the comonomer are not entirely separable since the chain conformation will affect the ability of the exciton to migrate across the polymer coil to a chain segment which is sufficiently close. The value of the intrinsic polarization, Po 1, estimated from the data in Fig. 1 by extrapolation to 1. = 1 is 5.9 which agrees well with that of 5.4 obtained from fluorescence quenching experiments in fluid solutions of 1-vinylnaphthalene labelled poly methyl methacrylate [28] and of 4.9 obtained by extrapolation to 7. = 1 for energy migration experiments on the methyl methacrylate copolymer series [20, 21]. This agreement of polarization data indicates the relative insignificance of possible distortions due to stresses in the solvent glass, etc.
Spectroscopic data
/
10C
/
p-~
J
Under photostationary state conditions, the kinetic scheme of Birks [24] applicable to intermolecular excimer formation reduces to the following expression for excimer to monomer intensity ratio at constant temperature
ID IM
0
2.0
3'0
T. Fig. 1. Depolarization as a function of mean path length of naphthyl chromophores in the copolymer (2-methyltetrahydrofuran solvent; 77 K). E.p.J. 1 5 / 1 0 ~
-
k kDu[M]
(6)
where k is a composite rate coefficient (involving rate constants of fluorescence of monomer and excimer in addition to those of radiationless deactivation pathways), knu is the rate coefficient of excimer formation and [M] is the concentration of fluorescent species. The photophysical scheme has been modified [20--22] to encompass intramolecular excimer formation in polymers, A function descriptive of energy migration is incorporated into Eqn (5) as a component of kDM and a function descriptive of the concentration of excimer sites is used in place of the [M] relevant to bimolecular excimer formation. If excimer formation is a consequence of interactions between nearest-neighbour chromophores on
928
R. A: ANDERSON,R. F. REIDand I. SOUTAR
the polymer chain, the appropriate concentration term will be the fraction of aromatic pairs in the copolymer, f , . Consequently, if the energy migration in fluid solution is characterized by the same function as in a rigid glass, Eqn (5) evolves into the form: ID = k l . f . . . IM
(7)
The validity of such a relationship has been demonstrated for some vinylaromatic/methyl methacrylate copolymer series [20,21]. Figure 2 illustrates the applicability of Eqn (6) to the description of excimer behaviour in the 1-vinylnaphthalene/methyl acrylate copolymer series. It is apparent that the function In'f,, adequately describes the excimer formation over a fair composition range of the copolymers (certainly up to 64mo1% naphthalene content, corresponding to l,'f,, = 1.1). Deviation from linearity is evident in the datum point for the copolymer of 77 mol % naphthyl chromophore content. The breakdown of models incorporating an energy migration term and an excimer site concentration term is to be expected when migration becomes sufficiently efficient to guarantee population of potential excimer sites. It is to be expected under these conditions that ID/IM will depend solely upon the excimer site concentration. Excimer site population is extremely efficient in the eopolymer of 77 mol % naphthyl species. The value of ID/IM of 9.7 is of comparable magnitude to that of poly-l-vinylnaphthalene (loAM = 10.8 for identical solvent/temperature conditions). It is obvious that the model must break down prior to the homopolymer since intramolecular excimer formation is not concentration dependent for all but extremely low molar mass homopolymers. Further justification for the inapplicability of the model in this system at high aromatic contents is to be found in examination of the triad composition of the 77 mol % naphthalene copolymer. 94% Of the naphthyl chromophores exist in triads of the form NNM or N N N (where N and M represent units derived from 1-vinylnaphthalene and methyl acrylate respectively). Furthermore, about 98% of the methyl acrylate species exist as isolated units in the polymer. Under such conditions energy migration is expected to be very efficient. The ability of the function 1, .f,, to describe ID/IM in the 1-vinylnaphthalene/methyl acrylate copolymer series implies that interactions other than of a nearest-neighbour form are insignificant in description of the assemblage of excimer sites. Despite the introduction of the more flexible (relative to methyl methacrylate) methyl acrylate units into the chain, the influence of excimer formation through chromophores rendered in the correct alignment through the chain folding back upon itself is not evident. Such long-range interactions have been supposed as possible interactions in many polymer systems and are suggested as dominant in polymers of naphthyl methacrylate [e.g. 9]. However, such interactions do not appear to be significant in the vinyl aromatic copolymers studied to date. Comparison of In/IM of the 1-vinylnaphthalene/ methyl acrylate copolymers with those of 1-vinylnaphthalene/methyl methacrylate copolymers of similar naphthyl content reveals that excimer formation is
more efficient in the former system. However, the true magnitude of the difference in excimer formation efficiency in the two series of polymers is apparent only through comparison of macromolecules of similar l,.f~,. Under these circumstances it is apparent that ID/IM values of the methyl acrylate series are about 6 times those of the methyl methacrylate series in dichloromethane solution. The increased efficiency is a consequence of two effects: (i) increased energy migration in the methyl acrylate series leading to increased efficiency in population of the available sites; (ii) the greater flexibility of the methyl acrylate species will influence the relative energies of the excimer conformation and the pre-excimer rotamer, leading to a greater exeimer site concentration in the methyl acrylate series. CONCLUSIONS Intramolecular excimer formation in 1-vinylnaphthalene/methyl acrylate copolymers is: (i) resultant upon interactions between nearest-neighbour chromophores; (ii) about six times more efficient than in 1-vinylnaphthalene/methyl methacrylate copolymers, as estimated from comparison of data pertaining to polymers of identical 7,'f,,. Intramolecular energy migration in glass solutions of the 1-vinylnaphthalene/methyl acrylate copolymers is characterized by 1, and is more extensive than in the corresponding methyl methacrylate copolymer. Acknowledgements--The authors are pleased to acknowledge financial assistance from the S.R.C. in the form of an equipment grant (to I.S.) and a studentship (to R.F.R.). REFERENCES
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