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Intramolecular excimer formation in polystyrene model molecules
European Polymer Journal, Vol. 13. pp. 921 to 924. Pergamon Press 197% Printed in Great Britain.
INTRAMOLECULAR EXCIMER FORMATION IN POLYSTYRENE MODEL MOLECULES LILIANE BOKOBZA, BRUNO JASSE and LUCIEN MONNERIE Laboratoire de Physico-chimie Structurale et Macromolrculaire ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
(Received 29 April 1977) Akstraet--Intramolecular excimer formation has been investigated for three polystyrene model molecules: 2,4-diphenylpentane, 2,4,6-triphenylheptane and 2,4,6,8-tetraphenylnonane. The results show that isotactic conformations are more favourable for excimer formation than syndiotactic. A high efficiency of excimer sampling is observed for the isotactic forms of the three and four-units polystyrene model molecules with respect to the two-unit model compound. Rt~sum#----On 6tudie la formation d'excimrres intramol~culaires dans trois molrcules modrles du polystyrrne: diphrnyl-2,4 pentane, triphrnyl-2,4,6 heptane et trtraphrnyl-2,4.6,8 nonane. Les rrsultats montrent que les conformations isotactiques sont plus favorables ~ la formation d'excim~re que les conformations syndiotactiques. D'autre part, le rendement quantique de formation d'excim~re est plus important dans les formes isotactiques du triphrnylheptane et trtraphrnylnonane que dans celle du diphrnylpentane.
\
INTRODUCTION
Intramolecular excimer formation has been observed in many vinyl aromatic polymers. This intramolecular excimer is an electronically excited complex formed in molecules containing two identical aromatic groups separated by a three carbon chain. Since analysis of solution behaviour of polymers is complex, it is useful to consider appropriate model molecules. So, in order to gain insight into local conformation and molecular motion in polystyrene, we first describe excimer formation in dilute solutions of three polystyrene model compounds: 2,4-diphenylpentane, 2,4,6-triphenylheptane and 2,4,6,8-tetraphenylnonane. 2,4-Diphenylpentane is the simplest polystyrene model molecule; its two diastereoisomers, meso and racemic (dl), can be considered as the first steps of the isotactic and syndiotactic chains respectively. 2,4,6-triphenylheptane presents three isomers, viz. the isotactic, syndiotactic and heterotactic forms; 2,4,6,8-tetraphenylnonane has six isomers, viz. the isotactic, syndiotactic and four heterotactic forms. The results concerning the heterotactic compQunds will not be presented here since separation of the pure stereo-isomers has not yet been accomplished [1]. Of the polystyrene model molecules, only 2,4-diphenylpentane has been previously studied in terms of intramolecular excimer formation [2].
Emission spectra were automatically corrected for instrumental response; the excitation wavelength was about 260 nm; the solutions were concentration matched in order to have an optical density of 0.1 at 260nm. Measurements were made on solutions carefully degassed by a repetitive freeze-pump-thaw cycle. A Dupont Curve Resotver (Model 310) was used to analyze the fluorescence spectra. RESULTS AND DISCUSSION
Figure 1 shows the absorption spectra at room temperature of methylene chloride solutions of dl and meso 2,4-diphenylpentanes. The u.v. spectra vary with configuration: the maximum absorption hands are different for the two compounds. Similar deviation also occurs between the syndiotactic and isotactic forms of the two other polystyrene model molecules. Figure 2 shows the fluorescence spectra of the samples in methylene chloride at 25 °. They consist of two emission bands appearing at 283 and 330 nm. The first band corresponds to the fluorescence of the monomer phenyl group, the lower energy band is ascribed to the fluorescence of the intramolecular excimer. Two important features of the emission spectra should be noted. (1)The meso 2,4-diphenylpentane presents a greater ratio of excimer to normal fluorescence intensity than the dl isomer; further the spectral patterns of the two compounds differ significantly from those of the literature [2]. (2) There is close EXPERIMENTAL PART correspondence between the emission spectra of all 2,4-Diphenyipentane was prepared by the method of the syndiotactic forms but no correspondence Overberger and Bonsignore f3]. 2,4,6-Triphenylheptane between meso 2,4-diphenylpentane and the isotactic and 2,4,6,8-tetraphenylnonane were prepared by a method forms of the two other polystyrene model molecules. developed by Jasse ['4]. The study was carried out in methIn order to characterize excimer sampling in each ylene chloride supplied by Merck and used without further compound, the ratio of excimer to monomer emission purification. Absorption spectra were made on a Cary Model 15 spectrometer. Emission spectra were recorded intensities (lo/IM) and the quantum efficiency of on a Fica Model 55 MK II spectrofluorometer equipped excimer formation (¢) are reported in Table I. lo/lu with a 450 xenon lamp and a R 212 photomultiplier tube. ratio, which is an approximate measure of the relative 921 e.P.J 13/12--A
922
LILIANE BOKOBZA, BRUNO JASSE and LUCIEN MONNERIE
spectively the intensity of the monomer phenyl group and that of isopropylbenzene under identical experimental conditions.
di isomer meso isomer
(o)
dt
/-
/
II II ii It
.c o n,
¢
/ I
/ /
/
I
J
I
// \
~s
/
250
300
3.50
Wovelength, x\ \ II 230
I
240
I
250
I
260
Wovelengt"h,
I 270
400
nm
Ib) s
I
280
nm
Fig. 1. Ultra-violet absorption spectra of dl and meso 2,4-diphenylpentanes in methylene chloride at room temperature. efficiency of the excimer formation mechanism, is calculated from band areas; since the two bands overlap, the monomer phenyl group is assumed to have the same spectral distribution as an alkylbenzene. Analysis of absorption spectra of different alkylbenzenes leads to the choice of the appropriate model: the position in space of the phenyl group with respect to the carbon chain seems to control the spectral pattern of the u.v. spectrum [5]. Figure 3 shows absorption spectra of ethylbenzene, isopropylbenzene and tertbutylbenzene. Similarity in the shapes and spectral distribution exists between isopropylbenzene and dl isomer (Figs. 1 and 3); the shape of the absorption spectrum of the meso diphenylpentane is intermediate between isopropylbenzene and tert-butylbenzene. Thus isopropylbenzene is a more appropriate model for polystyrene than ethylbenzene, generally considered as the best model. Figure 4 represents the total emission spectrum of d/ diphenylpentane and that of isopropylbenzene under identical conditions. The two spectra have been normalized with regard to the monomer band and subtracted. The contribution of the intramolecular excimer fluorescence is seen as a structureless band peaking at about 330 nm. Each value of ¢' is calculated according to Hirayama's kinetic treatment [6] which assumes that dissociation of excimers into phenyl groups is negligible at room temperature and that the quantum yield of the excited monomer in the absence of the excimer formation process is about the same as that of an alkylbenzene in the same solvent; ¢, is defined by the expression: l --.IM/IO, where IM and 1o represent re-
~ 250
wl///
~ ~ "~'~ ~,~
300
Wovelength,
35O
40O
nm
(c) S
_j/ 250
I
I
500
350
Wovelength,
400
nm
Fig. 2. Corrected fluorescenc¢ spectra of degassed methylene chloride solutions at 25 ° of: (a) 2,4-diphenylpentanes
dl = dl isomer, m = meso isomer; (b) 2,4,6-triphenylheplanes s = syndiotactic isomer, i - isotactic isomer; (c) 2,4,6,8-tet raphenylnonanes.
Intramolecular excimer formation
923
Table I. Ratio of excimer to monomer fluorescence intensity and quantum efficiency of excimer formation at 25° 2,4-diphenylpentane dl meso Iollu
0.19 0.40
/
2,4,6-triphenylheptane syndio iso
1.37 0.84
0.27 0.40
I
I
1
t /ii i ; ,) f-J /
./
_-."
,/
/
I ~
.I
i \"',,.. ..... \
\, I
I,
I
I
I
I
2150
240
250
260
270
280
Wavelength ~ nm
Fig. 3. Ultra-violet absorption spectra o f ethylbenzene. ---- isopropylbenzene, and - - - - - tert-butylbenzene in methylene chloride at room temperature.
250
300
350
Wovelength,
nm
400
Fig. 4. Normalized emission spectra of d/2,4-diphenylpentane and isopropylbenzene . . . . , in methylene chloride at 25" and their difference - - - - -
5.35 0.95
2.4,6,8-tetraphenylnonane syndio iso 0.28 0.40
4.20 0.94
A particularly interesting result is the rather large difference in Io/I~ for the two diphenylpentanes, the meso isomer exhibiting a ratio approximately seven times greater than that of d! isomer. This low efficiency of excimer sampling in dl 2,4-diphenylpentane is not surprising since conformational analysis by Gorin and Monnerie [7] shows that, in the case of a syndiotactic dyad, the rotational process between the ground state conformation (tt or g - g - ) and the higher energy excimer conformation g+t (or tg +) can hardly be reached. The formation of the sandwich-like excimer conformation from g - g - , the other preferred conformation for a d! dyad, is less probable than the formation from tt state since it requires a double rotational motion. For the meso isomer, theoretical analysis [7, 8] shows that the tt state corresponding nearly to the excimer site has a conformational energy of the order of magnitude of that of the ground state tg + (or y-t). Moreover, ultrasonic relaxation study [9] reveals an amount of 5 ~ for the tt state at room temperature. So the rotational isomerism between tg + and tt conformations of meso isomer seems easier than between tt and g+t conformations of dl isomer. Apparent activation energies related to the rotational barrier between the ground and excimer states, deduced from measurements of lo/IM as a function of temperature, are different for the two isomers; the values of 2.0 and 4.6 kcal mole-I respectively for meso and dl diphenylpentanes also support the conclusion that the meso dyad is more favourable for excimer formation than the dl. No great evolution has been found in the fluorecence spectra when one or two syndiotactic sequences are added to dl 2,4-diphenylpentane. The intensities of the monomer components are the same for the three syndiotactic forms; the increase of the excimer emission band may indicate a higher quantum yield of excimer fluorescence in syndiotactic triphenylheptane and tetraphenylnonane than in dl diphenylpentane. The most striking feature of the data is the important efficiency of excimer formation observed in the isotactic forms of the three and four-chromophore polystyrene model molecules compared to the twochromophore model analog: it may be'the result of a high delocalized excitation energy in an isotactic triad and tetrad. These are preliminary results; we feel that additional data concerning especially the heterotactic isomers, to be supplied in the near future, will further advance understanding of local conformational behaviour. REFERENCES
1. B. Jasse, F. Laupr~tre and L. Monnerie, Die Makromol. Chem. to be published. 2. J. W. Longworth and F. A. Bovey, Biopolym. 4, 1115 (1966).
924
LILIANE BOKOBZA,BRUNOJASSEand LUCIEN MONNERIE
3. C. G. Overberger and P. V. Bonsignore, d. Art chem. Soc. 80. 5427 (1958). 4. B. Jasse, Bull. Soc. chirt Fr. to be published. 5. C. Noel and L. Monnerie, to be published. 6. F. Hirayama, J. chert Phys. 42, 3163 (1965). 7. S. Gorin and L. Monnerie, J. Chirt phys. Physicochim. Biol. 6/, 869 (1970).
8. D. Y. Yoon, P. R. Sundararajan and P. J. Flory, Macromolecules 8, 776 (1975). 9. B. Froelich, C. No~l, B. Jasse and L. Monnerie, Chera. Phys. Lett. 44, 159 (1976).
INTRAMOLECULAR EXCIMER FORMATION IN POLYSTYRENE MODEL MOLECULES LILIANE BOKOBZA, BRUNO JASSE and LUCIEN MONNERIE Laboratoire de Physico-chimie Structurale et Macromolrculaire ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
(Received 29 April 1977) Akstraet--Intramolecular excimer formation has been investigated for three polystyrene model molecules: 2,4-diphenylpentane, 2,4,6-triphenylheptane and 2,4,6,8-tetraphenylnonane. The results show that isotactic conformations are more favourable for excimer formation than syndiotactic. A high efficiency of excimer sampling is observed for the isotactic forms of the three and four-units polystyrene model molecules with respect to the two-unit model compound. Rt~sum#----On 6tudie la formation d'excimrres intramol~culaires dans trois molrcules modrles du polystyrrne: diphrnyl-2,4 pentane, triphrnyl-2,4,6 heptane et trtraphrnyl-2,4.6,8 nonane. Les rrsultats montrent que les conformations isotactiques sont plus favorables ~ la formation d'excim~re que les conformations syndiotactiques. D'autre part, le rendement quantique de formation d'excim~re est plus important dans les formes isotactiques du triphrnylheptane et trtraphrnylnonane que dans celle du diphrnylpentane.
\
INTRODUCTION
Intramolecular excimer formation has been observed in many vinyl aromatic polymers. This intramolecular excimer is an electronically excited complex formed in molecules containing two identical aromatic groups separated by a three carbon chain. Since analysis of solution behaviour of polymers is complex, it is useful to consider appropriate model molecules. So, in order to gain insight into local conformation and molecular motion in polystyrene, we first describe excimer formation in dilute solutions of three polystyrene model compounds: 2,4-diphenylpentane, 2,4,6-triphenylheptane and 2,4,6,8-tetraphenylnonane. 2,4-Diphenylpentane is the simplest polystyrene model molecule; its two diastereoisomers, meso and racemic (dl), can be considered as the first steps of the isotactic and syndiotactic chains respectively. 2,4,6-triphenylheptane presents three isomers, viz. the isotactic, syndiotactic and heterotactic forms; 2,4,6,8-tetraphenylnonane has six isomers, viz. the isotactic, syndiotactic and four heterotactic forms. The results concerning the heterotactic compQunds will not be presented here since separation of the pure stereo-isomers has not yet been accomplished [1]. Of the polystyrene model molecules, only 2,4-diphenylpentane has been previously studied in terms of intramolecular excimer formation [2].
Emission spectra were automatically corrected for instrumental response; the excitation wavelength was about 260 nm; the solutions were concentration matched in order to have an optical density of 0.1 at 260nm. Measurements were made on solutions carefully degassed by a repetitive freeze-pump-thaw cycle. A Dupont Curve Resotver (Model 310) was used to analyze the fluorescence spectra. RESULTS AND DISCUSSION
Figure 1 shows the absorption spectra at room temperature of methylene chloride solutions of dl and meso 2,4-diphenylpentanes. The u.v. spectra vary with configuration: the maximum absorption hands are different for the two compounds. Similar deviation also occurs between the syndiotactic and isotactic forms of the two other polystyrene model molecules. Figure 2 shows the fluorescence spectra of the samples in methylene chloride at 25 °. They consist of two emission bands appearing at 283 and 330 nm. The first band corresponds to the fluorescence of the monomer phenyl group, the lower energy band is ascribed to the fluorescence of the intramolecular excimer. Two important features of the emission spectra should be noted. (1)The meso 2,4-diphenylpentane presents a greater ratio of excimer to normal fluorescence intensity than the dl isomer; further the spectral patterns of the two compounds differ significantly from those of the literature [2]. (2) There is close EXPERIMENTAL PART correspondence between the emission spectra of all 2,4-Diphenyipentane was prepared by the method of the syndiotactic forms but no correspondence Overberger and Bonsignore f3]. 2,4,6-Triphenylheptane between meso 2,4-diphenylpentane and the isotactic and 2,4,6,8-tetraphenylnonane were prepared by a method forms of the two other polystyrene model molecules. developed by Jasse ['4]. The study was carried out in methIn order to characterize excimer sampling in each ylene chloride supplied by Merck and used without further compound, the ratio of excimer to monomer emission purification. Absorption spectra were made on a Cary Model 15 spectrometer. Emission spectra were recorded intensities (lo/IM) and the quantum efficiency of on a Fica Model 55 MK II spectrofluorometer equipped excimer formation (¢) are reported in Table I. lo/lu with a 450 xenon lamp and a R 212 photomultiplier tube. ratio, which is an approximate measure of the relative 921 e.P.J 13/12--A
922
LILIANE BOKOBZA, BRUNO JASSE and LUCIEN MONNERIE
spectively the intensity of the monomer phenyl group and that of isopropylbenzene under identical experimental conditions.
di isomer meso isomer
(o)
dt
/-
/
II II ii It
.c o n,
¢
/ I
/ /
/
I
J
I
// \
~s
/
250
300
3.50
Wovelength, x\ \ II 230
I
240
I
250
I
260
Wovelengt"h,
I 270
400
nm
Ib) s
I
280
nm
Fig. 1. Ultra-violet absorption spectra of dl and meso 2,4-diphenylpentanes in methylene chloride at room temperature. efficiency of the excimer formation mechanism, is calculated from band areas; since the two bands overlap, the monomer phenyl group is assumed to have the same spectral distribution as an alkylbenzene. Analysis of absorption spectra of different alkylbenzenes leads to the choice of the appropriate model: the position in space of the phenyl group with respect to the carbon chain seems to control the spectral pattern of the u.v. spectrum [5]. Figure 3 shows absorption spectra of ethylbenzene, isopropylbenzene and tertbutylbenzene. Similarity in the shapes and spectral distribution exists between isopropylbenzene and dl isomer (Figs. 1 and 3); the shape of the absorption spectrum of the meso diphenylpentane is intermediate between isopropylbenzene and tert-butylbenzene. Thus isopropylbenzene is a more appropriate model for polystyrene than ethylbenzene, generally considered as the best model. Figure 4 represents the total emission spectrum of d/ diphenylpentane and that of isopropylbenzene under identical conditions. The two spectra have been normalized with regard to the monomer band and subtracted. The contribution of the intramolecular excimer fluorescence is seen as a structureless band peaking at about 330 nm. Each value of ¢' is calculated according to Hirayama's kinetic treatment [6] which assumes that dissociation of excimers into phenyl groups is negligible at room temperature and that the quantum yield of the excited monomer in the absence of the excimer formation process is about the same as that of an alkylbenzene in the same solvent; ¢, is defined by the expression: l --.IM/IO, where IM and 1o represent re-
~ 250
wl///
~ ~ "~'~ ~,~
300
Wovelength,
35O
40O
nm
(c) S
_j/ 250
I
I
500
350
Wovelength,
400
nm
Fig. 2. Corrected fluorescenc¢ spectra of degassed methylene chloride solutions at 25 ° of: (a) 2,4-diphenylpentanes
dl = dl isomer, m = meso isomer; (b) 2,4,6-triphenylheplanes s = syndiotactic isomer, i - isotactic isomer; (c) 2,4,6,8-tet raphenylnonanes.
Intramolecular excimer formation
923
Table I. Ratio of excimer to monomer fluorescence intensity and quantum efficiency of excimer formation at 25° 2,4-diphenylpentane dl meso Iollu
0.19 0.40
/
2,4,6-triphenylheptane syndio iso
1.37 0.84
0.27 0.40
I
I
1
t /ii i ; ,) f-J /
./
_-."
,/
/
I ~
.I
i \"',,.. ..... \
\, I
I,
I
I
I
I
2150
240
250
260
270
280
Wavelength ~ nm
Fig. 3. Ultra-violet absorption spectra o f ethylbenzene. ---- isopropylbenzene, and - - - - - tert-butylbenzene in methylene chloride at room temperature.
250
300
350
Wovelength,
nm
400
Fig. 4. Normalized emission spectra of d/2,4-diphenylpentane and isopropylbenzene . . . . , in methylene chloride at 25" and their difference - - - - -
5.35 0.95
2.4,6,8-tetraphenylnonane syndio iso 0.28 0.40
4.20 0.94
A particularly interesting result is the rather large difference in Io/I~ for the two diphenylpentanes, the meso isomer exhibiting a ratio approximately seven times greater than that of d! isomer. This low efficiency of excimer sampling in dl 2,4-diphenylpentane is not surprising since conformational analysis by Gorin and Monnerie [7] shows that, in the case of a syndiotactic dyad, the rotational process between the ground state conformation (tt or g - g - ) and the higher energy excimer conformation g+t (or tg +) can hardly be reached. The formation of the sandwich-like excimer conformation from g - g - , the other preferred conformation for a d! dyad, is less probable than the formation from tt state since it requires a double rotational motion. For the meso isomer, theoretical analysis [7, 8] shows that the tt state corresponding nearly to the excimer site has a conformational energy of the order of magnitude of that of the ground state tg + (or y-t). Moreover, ultrasonic relaxation study [9] reveals an amount of 5 ~ for the tt state at room temperature. So the rotational isomerism between tg + and tt conformations of meso isomer seems easier than between tt and g+t conformations of dl isomer. Apparent activation energies related to the rotational barrier between the ground and excimer states, deduced from measurements of lo/IM as a function of temperature, are different for the two isomers; the values of 2.0 and 4.6 kcal mole-I respectively for meso and dl diphenylpentanes also support the conclusion that the meso dyad is more favourable for excimer formation than the dl. No great evolution has been found in the fluorecence spectra when one or two syndiotactic sequences are added to dl 2,4-diphenylpentane. The intensities of the monomer components are the same for the three syndiotactic forms; the increase of the excimer emission band may indicate a higher quantum yield of excimer fluorescence in syndiotactic triphenylheptane and tetraphenylnonane than in dl diphenylpentane. The most striking feature of the data is the important efficiency of excimer formation observed in the isotactic forms of the three and four-chromophore polystyrene model molecules compared to the twochromophore model analog: it may be'the result of a high delocalized excitation energy in an isotactic triad and tetrad. These are preliminary results; we feel that additional data concerning especially the heterotactic isomers, to be supplied in the near future, will further advance understanding of local conformational behaviour. REFERENCES
1. B. Jasse, F. Laupr~tre and L. Monnerie, Die Makromol. Chem. to be published. 2. J. W. Longworth and F. A. Bovey, Biopolym. 4, 1115 (1966).
924
LILIANE BOKOBZA,BRUNOJASSEand LUCIEN MONNERIE
3. C. G. Overberger and P. V. Bonsignore, d. Art chem. Soc. 80. 5427 (1958). 4. B. Jasse, Bull. Soc. chirt Fr. to be published. 5. C. Noel and L. Monnerie, to be published. 6. F. Hirayama, J. chert Phys. 42, 3163 (1965). 7. S. Gorin and L. Monnerie, J. Chirt phys. Physicochim. Biol. 6/, 869 (1970).
8. D. Y. Yoon, P. R. Sundararajan and P. J. Flory, Macromolecules 8, 776 (1975). 9. B. Froelich, C. No~l, B. Jasse and L. Monnerie, Chera. Phys. Lett. 44, 159 (1976).