Photochemistry of Liquid-Crystalline Polymers - ACS Symposium

Chapter 2

Photochemistry of Liquid-Crystalline Polymers 1

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David Creed , Richard A. Cozad , Anselm C. Griffin , Charles E. Hoyle , Lixin Jin , Petharnan Subramanian , Sangya S. Varma , and Krishnan Venkataram 1

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Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 15, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch002

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Department of Chemistry and Biochemistry and Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406

Several aspects of the photophysics and photochemistry of liquid crystalline polymers containing aryl cinnamate and stilbene chromophores are reported. UV-VIS spectra of these materials in various phases in thinfilmsare perturbed relative to spectra in solution. These perturbations are attributed to chromophore aggregation that is dependent on the phase type. Fluorescence and fluorescence excitation spectra of a stilbene polyester show the presence of emissive aggregates. The photochemical reactions of these materials are also dependent on phase. In polyarylcinnamates the ratio of 'dimer' or 'dimerlike'to photo-Fries photoproducts depends both on temperature (phase type) and wavelength of irradiation. 'Dimer' or 'dimer like' products are formed preferentially under conditions where chromophore association and/or excitation is maximized. In the stilbenes a photocycloaddition is observed that is photoreversible at elevated temperature. Parallel studies are reported on small molecule model compounds.

The photochemistry of liquid crystalline (LC) materials is of interest both as a means of modifying the properties of these potentially useful materials and because they have the unique combination of one or two dimensional order and translation^ mobility. There have been many reports of the photochemistry of small molecule LC materials (1) but comparatively few reports (2-14) of the photochemistry of LC polymers. We have already reported the synthesis and some aspects of the photochemistry and photophysics of nematic (2,3,4) and smectic (5,6) LC polycinnamates such as 1 and 2, and preliminary observations (7) of the photochemistry of nematic stilbene polyesters such as 3, that were originally synthesized by Jackson and Morris (15). We now report further observations on the photophysics and photochemistry of these polymeric LC materials, paying particularly attention to ground state chromophore association and its unique photophysical and photochemical consequences.

0097-6156/94/0579-0013$08.00/0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS


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Photochemistry ofLiquid-Crystalline Polymers 15


Experimental Polymers and small molecule model compounds were synthesized and characterized as described previously (2,5,8,15). The keto-coumarin triplet sensitizer 8 was synthesized as described elsewhere (16). UV-VIS spectra were obtained using a Perkin Elmer Lambda 6 spectrophotometer with a variable temperature cell as described elsewhere (4). Correctedfluorescenceandfluorescenceexcitation spectra were obtained using a Spex Fluorolog spectrophotometer. Polymer films were cast from chloroform, methylene chloride, or tetrachloroethane on to quartz plates or cuvettes using an EC101D series photoresist spinner P/N 8-13242 from Headway Research Inc. In variable temperature experiments, cuvettes coated on one outside face with the polymer film were filled with an optically transparent, high boiling point fluid, such as ethylene glycol, to facilitate heat transfer from the variable temperature cell to the film. Most irradiations were conducted in the spectrophotometer cavity using a Bausch and Lomb SP200 high pressure mercury lamp and monochromator (6.4 nm bandpass) together with appropriate cut-off filters. In some cases irradiations were conducted at room temperature with a Hanovia 450W medium pressure mercury lamp and cut-off filters. Results and Discussion Chromophore Aggregation in L C Polymers. We have observed perturbations of the UV-VIS spectra that we attribute to chromophore association or aggregation in thin (ca. 0.5-2.0 um) films of all the LC polymers we have studied to date. These perturbations are present in all phases offilmsof the polymers. Thus the spectrum of an 'as cast' film of a main-chain LC (MCLC) polyarylcinnamate, lb, is broader than the spectrum in solution (Fig. 1). Much more pronounced effects are observed on the UV-VIS spectra of the LC phases and especially in the crystalline phase of the sidechain LC (SCLC) polyarylcinnamate, 2. Spectra of LC films of both MCLC and SCLC polyarylcinnamates show enhanced blue-shifted absorption and weak enhancement of absorption to the red of that of the 'isolated' chromophore (2,5), ie., the polymer in a good solvent or a small molecule model compound in solution. Spectral effects attributed to formation of these aggregates are quite dependent on the phase type, annealing, and the molecular weight of the sample. Data for low (M « ca. 10 ) and higher (M * ca. 5 χ 10 ) molecular weight glassy nematic samples of l a prepared by cooling of the isotropic melt are shown in Figure 2. Spectral perturbations attributed to chromophore association are most dramatic (5,6) for the side-chain substituted polymer, 2, particularly in 'as cast', partially crystalline films (Fig. 3) and in smectic Β films, and are less apparent in less ordered nematic and isotropic films. The spectra of 'as cast'filmsstrongly resemble those of 50% dispersions of the model compound 4 (vide infra) in poly(methyl methacrylate), (PMMA). In the case of thin films of MCLC stilbene polyesters such as 3, temperature dependent chromophore association can also be observed (7). UV-VIS spectra of 'as cast'filmsof 3 at room temperature show enhanced absorption both to the blue and to the red of the spectra of the same polymer in a good solvent such as chloroform or a model compound in solution. Perturbed spectra analogous to those of the polymer films can be obtained using the small molecule model compound, 5, n

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Figure 1. UV-VIS Spectra of Polymer lb, (a) as an 'As Cast' Thin Film, and (b) in Chloroform.

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Figure 2. UV-VIS Spectra of Thin Films of MCLC Polymer l a Cooled from the Isotropic Melt (HMW = high molecular weight, LMW = low molecular weight), (a) 'As Cast' Film at Room Temperature, (b) HMW Film, Fast Cool, (c) HMW Film, Slow Cool, (d) LMW Film, Fast Cool.


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dispersed in PMMA (vide infra) where aggregation readily occurs. Perturbed spectra of all three polymers, 1,2, and 3, can also be obtained by first dissolving the polymers in 'good solvents such as chloroform, methylene chloride, or ethanol and then adding 'poor' solvents such as cyclohexane or water. An example is shown (Fig. 4) of the spectrum of a low molecular weight sample of l a in chloroform, ethanol, and 50 % aqueous ethanol at very low concentrations. Presumably the hydrophobic effect in aqueous non-solvents leads to inter-chromophore associations that are absent when the polymers are dissolved in good solvents. Others (9,11) have published UV-VIS spectra of cinnamate containing polymers in LC phases. Keller (8) reported the strongly perturbed spectrum of a film of an aryl cinnamate substituted SCLC polysiloxane but did not comment on the perturbation or its origin. Noonan and Caccamo (11) attributed perturbed spectra of a PMMA substituted with a 4methoxycinnamate chromophore in its side chain, to alterations of the conformations of the chromophores as a result of enhanced side chain packing. We have begun to explore the fate of singlet excitation energy using the fluorescence of stilbene polyesters such as 3. Such experiments are not possible with the polyarylcinnamates 1 and 2 since they are weakly fluorescent and by the time the emission spectrum is recorded (using narrow excitation and broad emission slits) the material has undergone significant photolysis. On the other hand, the model stilbene and stilbene polyesters such as 3 are highlyfluorescent.The structured fluorescence spectra (Fig. 5) andfluorescenceexcitation spectra of 3 in dilute solution in good solvents resemble those of model compounds such as 5 in dilute solution. However, thefluorescencespectra of films of 3 display the broad, red shifted, structureless emission characteristic of excimers or excited aggregates (Fig. 5), with little or no contribution from emission of 'isolated' stilbene chromophores. Fluorescence excitation spectra (Fig. 6) offilmsof 3 are quite different fromfluorescenceexcitation spectra of solutions of 3, but are very similar to absorption spectra of films of 3. The implications of these observations for the photochemistry of 3 and related compounds are currently under investigation.


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Chromophore Aggregation in Model Compounds. In order to improve our understanding of the aggregation process in the polymers and its photophysical and photochemical consequences, we have investigated the behavior of small molecule model compounds such as 4-pentyloxyphenyl-4'-pentyloxycinnamate, 4, and diethyl stilbene dicarboxylate, 5, that serve as models for the chromophores in the cinnamate and stilbene polymers, respectively. We do not observe aggregate spectra in concentrated solutions (up to 0.1 M) of these model compounds in organic solvents. However, most of the spectral changes attributed to chromophore association in the polymers can be duplicated using dispersions of these model compounds in PMMA. For example, UV-VIS spectra of 50 % dispersions of 4 in PMMA (Fig. 7), which are partially opaque and therefore almost certainly contain microcrystalline 4, strongly resemble spectra (Fig. 3) of 'as cast' thinfilmsof partially crystalline SCLC polymer 2. Spectra at intermediate concentrations (eg. 25 % of 4) resemble spectra of the films of MCLC polymer 1 (eg. Fig. 1), although the polymer spectra are not as structured as those of the model compound. It is also intriguing to note that the shape of the intermediate (25 % of 4) spectrum can be duplicated almost perfectly (Fig. 8) by



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Figure 3. Normalized UV-VIS Spectra of SCLC Polymer, 2, (a) in Methylene Chloride, (b) as a Smectic Β Film at 70 °C, and (c) as an 'As Cast' Film at Room Temperature.

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Figure 4. UV-VIS Spectra of Low Molecular Weight Polymer l a in (a) Chloroform, (b) Ethanol (b), and (c) 1:1 Ethanol:Water at Room Temperature.



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Figure 5. Fluorescence Spectra of an 'As Cast* Film of MCLC Polymer 3 Excited at (a) 330, (b) 290, and (c) 370 nm.

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Figure 6. Fluorescence Excitation Spectra of Polymer 3, (a) in Solution in Chloroform and (b) as an 'As Cast' Film at Room Temperature.




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addition of the spectra of the concentrated (50 %) and dilute (5 %) dispersions in a 2:1 ratio. This strongly suggests that only two chromophores, corresponding to 'isolated' and 'aggregated' species, are present in dispersions of the model compound. We have also compared spectra of SCLC polymer 2 with spectra of mixtures of the 5 and 50 % dispersions of its methacrylate monomer. We hope ultimately to make reasonable estimates of the fraction of light absorbed by isolated and aggregated chromophores in different phases of the polymer. Obtaining this information is a necessary pre requisite to obtaining quantum yields of reaction of both isolated and aggregated chromophores. Photochemistry of Polymers and Model Compounds. Long term irradiation (313 nm) of 'as cast'filmsof polyarylcinnamates 1 and 2 leads to insolubilization and spectroscopic changes attributed to saturation of the cinnamate double bond and to photo-Fries rearrangement (2,5). Solid state C nmr shows that saturation of the double bond involves 2 + 2 photocycloaddition in both 1 and 2, although other reactions leading to saturation of the cinnamate double bond may also occur (17). The photochemical behavior is strongly dependent on phase type. In the case of low molecular weight samples of fluid nematic 1, irradiation leads initially to hyperchromism and a change of the spectral shape indicative of generation of 'isolated' chromophores and loss of chromophore association in the initial phase of the reaction (2). However, this initial hyperchromism is not observed in the higher molecular weight sample, suggesting it is dependent on the viscosity of the film (Fig. 9). Since there is a large difference in absolute optical densities at 313 and 390 nm, the OD data in Figure 9 are shown as percentages of the maximum OD obtained for each sample. Interestingly, a small hyperchromic effect is observed in the glassy nematic phase of high molecular weight samples of 1. In the SCLC polymer 2, hyperchromism and spectral changes are most dramatic in highly organized smectic Β films but greatly diminished in the less organized smectic A and nematic phases (5,6). These effects are most likely due to photocycloadduct formation since they occur upon excitation at wavelengths (366 or >380 nm) where aggregates preferentially absorb and little or no photo-Fries rearrangement occurs and upon triplet sensitization when photo-Fries rearrangement does not occur (vide infra). Spectral changes that occur after this initial hyperchromism are due to saturation of the cinnamate double bond and to photo-Fries rearrangement (2,5). Aggregation also leads to wavelength dependent photoproduct formation (18) from polyarylcinnamates such as 1 and 2. Thus, when aggregates are specifically excited at long wavelengths (eg. above 380 nm), spectral changes attributed to [2 + 2] photocycloaddition are observed, whereas, when both aggregates and unassociated chromophores are excited at shorter wavelengths (eg. 313 nm), spectral changes attributed to both photocycloaddition and photo-Fries rearrangement are observed. The ratio of 'dimers' (saturated products) to photo-Fries rearrangement products upon 313 nm irradiation of low molecular weight samples of 1 has been investigated as a function of phase type at different temperatures at both low (11-13 %) and high (>90 %) conversion (Table I). This ratio was estimated from the simple assumption that only three types of chromophores contribute to the UV-VIS spectrum of the irradiated polymer: the aryl cinnamate type (represented by 4), a 'dimer* type (represented by 6), and the photo-Fries type (represented by 7). In both l3




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the amorphous and glassy nematic phases at 25 °C there is a strong preference for 'aimer' over 'Fries' type products at low conversions. This preference is greatly diminished in the isotropic melt at 128 °C, presumably because of both reduced chromophore aggregation and increased fluidity of this phase. Data for high Table I. DimerrFries Ratios (D/F)fromIrradiation (313 nm) of Thin Films of Low Molecular Weight lb at Low (L) and High (H) Conversions T,°C

Phase

25 25 25 25 88 88 128 128

Amorphous Amorphous Nematic glass Nematic glass Nematic Nematic Isotropic Isotropic

Conversion L H L H L H L H

D/F >2.5 1.8 >4.5 2.0 a 0.5 0.55 0.85

"cannot be obtained because of initial hyperchromism of irradiated fluid nematic samples. conversions indicate a preference for 'photo-Fries' products (low D/F ratios). Presumably, aggregated chromophores are consumed in the early stages of the reaction to give 'dimer' products. The remaining unaggregated chromophores are more likely to undergo the unimolecular photo-Fries rearrangement leading to the lower D/F ratios observed at high conversions. To confirm this possibility we exhaustively irradiated films of 1 at 366 nm to convert all aggregates into 'dimers. We then changed the irradiation wavelength to 313 or 334 nm and observed spectral changes consistent with conversion of the remaining chromophores to photo-Fries products. Absolute quantum yields for these reactions and those of the stilbene polyesters 3 cannot be obtained because of the heterogeneity of chromophores in the absorbing material. Triplet quenching and sensitization experiments on both 1 and model compounds suggest that the formation of photo-Fries rearrangement products from aryl cinnamates occurs from the singlet or possibly an upper triplet state (19). Thus we are able to suppress photo-Fries product formation and formation of radical fragments [observed by flash photolysis (19)] from 4 by triplet sensitization (4,19). We have used ketocoumarin triplet sensitizers (20) in this work because they have a convenient 'window* in their UV-VIS absorption spectra between ca. 230 and 320 nm that enables us to observe cinnamate photochemistry without interference from the absorbance of the sensitizer or its photolysis products. However, we had to synthesize a modified ketocoumarin sensitizer, 8, with a long alkoxy 'tail' because the commercially available ketocoumarins phase-separate (4) from the glassy nematic and nematic phases of 1. Prehminary results using 8 are most promising and suggest there is extensive triplet energy migration in films of polyarylcinnamates such as 1. We have not yet begun to study triplet sensitization of the reactions of the SCLC polyarylcinnamate 2 or stilbene polyesters such as 2L. Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Irradiation (313 nm) of the stilbene polyester, 3, in solution or in cast films leads to disappearance of the long wavelength absorption due to the irans-stilbene chromophore and appearance of a new absorption band at ca. 250 nm (6). We believe the major products of this reaction are cyclobutane dimers (eg. 9). In the case of the film irradiations, the shape of the UV-VIS spectrum changes as reaction proceeds until it more closely resembles the solution spectrum. This suggests that aggregates are preferentially consumed in the early stages of reaction. When an exhaustively 313 nm irradiated solution (methylene chloride) or film at 65 °C is irradiated at 254 nm there is recovery of absorption above 300 nm but with a broader, blue-shifted absorption consistent with regeneration of a mixture of trans- and cw-stilbenes by photochemically allowed cleavage of the cyclobutane dimers, such as 9, believed to be generated by longer wavelength irradiation. The two isomers presumably arise from the two possible cleavage pathways A and Β (Scheme) for 9. The 313 nm disappearance/254 nm recovery behavior exhibited by films irradiated at 65 °C is not observed at room temperature. This observation, together with changes in the UVVIS spectra of films that occur at about 40 °C (vide supra) suggest the possibility of a T for 3 at about 40 °C, although, to date, we have not been able to observe such a T by DSC. This type of photoreversible photoreaction does not occur with the cinnamate polymers 1 and 2 either in films or in solution. The photochemistry of dispersions of model compounds in PMMA differs appreciably from that of the polymer films. In general, dispersions of model compounds at high loadings at room temperature (below the T of PMMA), where there is extensive aggregation, are relatively unreactive compared to the polymer films. We estimate quantum yields of reaction are at least an order of magnitude less for the model compounds relative to polymer films of comparable optical density. We also attempted to obtain cyclobutane dimers of the model compounds by irradiations (>300 nm) of powdered crystals of 4 and 5. However these materials are also quite unreactive under these conditions which suggests that the intermolecular distance and/or orientations in crystals of these model compounds do not permit dimerization to occur. This would be an example of the well-known phenomenon of topochemical control (21) of a solid state dimerization reaction. In dilute dispersions the situation is quite different. A 5 % dispersion of the cinnamate model compound 4 in PMMA when irradiated at 313 or 366 nm undergoes a clean reaction, as evidenced by the UVVIS spectrum (Fig. 10), to afford a spectrum characteristic of the photo-Fries rearrangement product, 7. A similar effect is observed upon irradiation of 4 in solution. We consider the observation of an isosbestic point upon irradiation in PMMA to be significant to the question of whether the polymers undergo any transcis isomerization upon irradiation. The isosbestic point indicates that, for 4, photoFries rearrangement is the only unimolecular photochemical reaction occurring in the glassy polymer medium. We believe that this observation and the absence of bands due to the d$-isomer in irradiated films of 1 and 2 indicate trans-cis isomerizations are not major photochemical reactions of either the polyarylcinnamates or the model compounds that we have studied. In contrast to the relatively clean reaction of 4, the stilbene 5, in dilute dispersions in PMMA, undergoes irreversible photolysis under 313 nm irradiation perhaps due to ester fragmentation and consequent saturation of the stilbene double bond. Isomerization, as in the case of the cinnamates, does not g

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Figure 10. OD Changes During Irradiation (313 nm) of a Dilute (5 %) Dispersion of Model Compound 4 in PMMA for (a) 0, (b) 30, (c) 90, and (d) 170 min.


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occur. W e have not attempted any further characterization of the products o f irradiation of dilute dispersions of 5.


Acknowledgments. This work was supported by the National Science Foundation ( E P S C o R program), the State of Mississippi, and the University of Southern Mississippi. W e thank Drs. Richard Weiss and Joachim Stumpe for helpful discussions.

Literature Cited. 1. Weiss, R. G. Tetrahedron., 1988, 44, 3413-3475. 2. Creed, D.; Griffin, A. C.; Gross, J. R. D.; Hoyle, C. E.; Venkataram, K. Mol. Cryst. Liq. Cryst., 1988, 155, 57-71. 3. Haddleton, D. M.; Creed, D.; Griffin, A. C.; Hoyle, C. E.; Venkataram, K. Makromol. Chem. Rapid Commun., 1989, 10, 391-396. 4. Subramanian, P.; Creed, D.; Hoyle C. E.; Venkataram, K. Proc. SPIE - Int. Soc. Opt. Eng. (Photopolym. Device Phys. Chem. Appl. 2), 1991, 1559, 101-109. 5. Singh, S.; Creed, D.; Hoyle, C. E. Polymer Preprints, 1993, 34, 743-744. 6. Singh, S.; Creed, D.; Hoyle, C. E. Proc. SPIE - Int. Soc. Opt. Eng., 1993, 1774, 2-11. 7. Creed, D.; Cozad, R. Α.; Hoyle, C. E.; Morris, J. C.; Jackson, Jr., W. J. Proc. SPIE -Int. Soc. Opt. Eng., 1993, 1774, 69-73. 8. Koch, T.; Ritter, H.; Buchholz, N.; Knochel, F. Makromol. Chem., 1989, 190, 1369-1377. 9. Keller, P. Chem. Mater., 1990, 2, 3-4. 10. Ikeda, T.; Lee, C. H.; Sasaki, T.; Lee, B.; Tazuke, S. Macromolecules, 1990, 23, 1691-1695. 11. Noonan, J. M.: Caccamo, A. F. In Liquid Crystalline Polymers; Editors, Weiss, R. Α.; Ober C. K. American Chemical Society Symposium Series, American Chemical Society, Washington, DC, 1990, Vol. 435, 144-157. 12. Stumpe, J.; Muller, L.; Kreysig, D.; Hauck, G.; Koswig, D.; Ruhmann, R.; Rubner, J. Makromol. Chem. Rapid Commun., 1991,12,81-87. 13. Whitcombe, M. J.; Gilbert, Α.; Mitchell, G. R.; J. Polym. Sci., (A), Polym, Chem., 1992, 30, 1681-1691. 14. Gangadhara, Kishore, K. Macromolecules, 1993, 26, 2995-3003. 15. Jackson, Jr., W. J.; Morris, J.C. J. Appl. Polym. Sci: Appl. Polym. Symp., 1985, 41, 307-326. 16. Woods, L. C.; Fooladi, M. J. Chem. Eng. Data, 1963, 2, 624-627. 17. Egerton, P. L.; Pitts, E.; Reiser, A. Macromolecules, 1981, 14, 95-100. 18. Creed, D.; Griffin, A. C.; Hoyle, C. E.; Venkataram, K. J. Amer. Chem. Soc., 1990,112,4049-4050. 19. Subramanian, P.; Creed, D.; Griffin, A. C.; Hoyle, C. E.; Venkataram, K. J. Photochem. Photobiol. A: Chem., 1991, 61, 317-327. 20. Specht, D. P.; Martic, P. Α.; Farid, S. Tetrahedron, 1982, 38, 1203-1211. 21. Schmidt, G. M. J. Pure Appl. Chem., 1971, 27, 647-657. RECEIVED July 12, 1994 Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Photochemistry of Liquid-Crystalline Polymers - ACS Symposium

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