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Photoreaction and Molecular Reorientation in a Nanoscaled Film of Poly(methyl 4-(methacryloyloxy)cinnamate) Studied by Two-Dimensional FTIR and UV Correlation Spectroscopy Boknam Chae,† Seung Woo Lee,‡ Moonhor Ree,*,‡ Young Mee Jung,† and Seung Bin Kim*,† Department of Chemistry, BK21 Functional Polymer Thin Film Group, Laboratory for Vibrational Spectroscopy, Center for Integrated Molecular Systems, and Polymer Research Institute, Pohang University of Science & Technology, San 31, Hyoja-dong, Nam-gu, Pohang 790-784, Republic of Korea Received May 15, 2002. In Final Form: October 31, 2002 Photoreaction and photoinduced molecular reorientations in nanoscaled films of poly(methyl 4-(methacryloyloxy)cinnamate) (PMMCi) were investigated through two-dimensional (2D) correlation analyses of their infrared (IR) and ultraviolet (UV) absorption spectra. In the PMMCi film the presence of three distinct chromophore types was evident: isolated trans-isomeric cinnamoyl moieties and parallel and end-to-end cinnamoyl aggregates. Their photoinduced reorientations take place in the following sequence: isolated trans isomeric chromophores f parallel chromophore aggregates f end-to-end chromophore aggregates. It turns out that they react photochemically at different rates. The PMMCi polymer molecules were found to undergo both photoisomerization and photodimerization upon UV irradiation, with the trans-cis isomerization of the isolated trans isomeric cinnamoyl moieties occurring more rapidly than the photodimerization of the cinnamoyl aggregates. Of the photodimerization processes, the reorientation of the parallel cinnamoyl aggregates precedes that of the end-to-end cinnamoyl aggregates. The isolated chromophores may partly involve in the photodimerization, but its fraction is expected small because of the film in a confined geometry. The cinnamoyl moiety was found to be present as two rotational conformers, trans-s-trans and trans-s-cis conformers. Upon UV irradiation, the trans-s-trans conformer changes before the trans-s-cis conformer. The cinnamoyl moieties undergo a cooperative reorientation with the methacryloyloxy units during photoreaction. The photoinduced molecular reorientations occur in the following sequence: trans isomer f phenyl ring f trans-s-trans conformer f trans-s-cis conformer f photodimer.
Introduction The photochemistry of poly(vinyl cinnamate) (PVCi) and its derivatives has been extensively investigated due to its potential applications in liquid crystal (LC) display technology.1-3 PVCi and its derivatives induce the alignment of LCs in contact with the film surface when they are irradiated by linearly polarized ultraviolet light (LPUVL).4 It has also been reported that LC alignment on the film surface is known to be governed by the anisotropic interaction between liquid crystal molecules and chromophores and/or photoproducts in polymer films irradiated with LPUVL.5,6 Therefore, elucidation of the molecular orientation and photochemical processes in PVCi and its derivatives is essential for a better understanding of the mechanism of LC alignment. Previously, it has been proposed that the only photochemical process in dilute solutions of PVCi is the trans* To whom all correspondence should be addressed. Tel: +8254-279-2106 (S.B.K.); 279-2120 (M.R.). Fax: +82-54-279-3399. E-mail:
[email protected];
[email protected]. † Laboratory for Vibrational Spectroscopy. ‡ Center for Integrated Molecular Systems and Polymer Research Institute. (1) (a) Rehab, A. Eur. Polym. J. 1998, 34, 1845. (b) Rajesh, K.; Ram, M. K.; Jain, S. C.; Samanta, S. B.; Narliker, A. V. Thin Solid Films 1998, 325, 251. (2) (a) Allen, S. D. M.; Almond, M. J.; Bruneel, J.; Gilbert, A.; Hollins, P.; Mascetti, J. Spectrochim. Acta, Part A 2000, 56, 2423. (b) Ghosh, M.; Chakrabarti, S.; Misra, T. N. J. Raman Spectrosc. 1998, 29, 263. (3) Reiser, A.; Egerton, P. L. Photogr. Sci. Eng. 1979, 23, 144. (4) Kim, H.; Lee, J.; Sung, S.; Park, J. Polym. J. 2001, 33, 9.
cis isomerization and that this process is favored in the early stages of UV light irradiation.7 Further, it has been suggested that photoisomerization of the cinnamoyl group is negligible in films of PVCi since it requires rotation of the CdC double bond so photodimerization of the cinnamoyl groups is the dominant photoprocess in these films.8 However, Egerton et al.9 showed that trans-cis isomerization occurs in films of PVCi but its fraction is less than 10%. Moreover, Ichimura et al.10 reported that the fraction of trans-cis isomerization shows a marked dependence on the regioisomerism of the cinnamoyl group in the films of PVCi and its derivatives and that photodimerization contributes to the enhancement of the thermal stability of LC photoalignment. According to the reports of Egerton et al.11 and Hirayama,12 cinnamoyl (5) (a) Creed, D.; Griffin, A. C.; Hoyle, C. E.; Venkataram, K. J. Am. Chem. Soc. 1990, 112, 4049. (b) Murase, S.; Kinosita, K.; Horie, K.; Morino, S. Macromolecules 1997, 30, 8088. (c) Kawatsuki, N.; Ono, H.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Macromolecules 1997, 30, 6680. (6) Schadt, M.; Schmitt, K.; Kozinkov, V.; Chigrinov, V. Jpn. J. Appl. Phys. 1992, 31, 2155. (7) (a) Kawatsuki, N.; Matsuyoshi, K.; Hayashi, M.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Chem. Mater. 2000, 12, 1549. (b) Rennert, J. J. Photogr. Sci. Eng. 1971, 15, 60. (c) Graley, M.; Reiser, A.; Roberts, A. J.; Phillips, D. Macromolecules 1981, 14, 1752. (8) Chakrabartk, S.; Maity, A. K.; Misra, T. N. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1625. (9) Egerton, P. L.; Trigg, J.; Hyde, E. M.; Reiser, A. Macromolecules 1981, 14, 95. (10) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayashi, Y. Macromolecules 1997, 30, 903.
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groups in poly[1,4-bis(ethylenyloxy)cyclohexane p-phenylenediacrylate] and PVCi derivatives are known to form 11 stereoisomeric products in the photodimerization; the distribution of the stereoisomeric products is nonuniform, with a preponderance of regioisomeric head-to-head dimers over regioisomeric head-to-tail dimers. More recently, Perny et al.13,14 suggested that photodimerization induces a reorientation of the phenyl axis of the cinnamoyl moiety. These reports lead to the conclusion that in films of PVCi and its derivatives, photodimerization is the major photoprocess, while trans-cis photoisomerization is the minor photoprocess. However, the sequence of reorientation of the chromophore and its photoproducts during photoreaction, which is critical information for understanding the alignment mechanism of LCs at the surface of films irradiated with UV light, has not yet been investigated. Two-dimensional (2D) correlation spectroscopy has been generalized through rigorous research for the past decade and applied extensively to the analysis of spectral data obtained during the observation of a system under some external perturbation.15-17,22 Because of a wide range of applications of this technique, it has become one of the standard analytical techniques to interpret numerous spectroscopic data for analytical chemistry, complex reaction kinetics, electrochemistry, photochemistry, etc.15-17,22 The details of this technique are described elsewhere.15-17 In this study, 2D correlation spectroscopic analysis has been extended to the photochemistry of poly(methyl 4-(methacryloyloxy)cinnamate) (PMMCi) (see the chemical structure shown in Figure 1) that was chosen as a PVCi analogue. Using 2D correlation analyses of the Fourier transform infrared (FTIR) and ultraviolet-visible (UV-visible) spectroscopy, we have extensively examined the photochemistry of the PMMCi in films and determined the reorientation sequence of the chromophore and its photoproducts during photoreaction in the films. Experimental Section Materials. All chemicals, including methacrylic acid, 1,3dicyclohexylcarbodiimide (DCC), 4-hydroxycinnamic acid, 4-(dimethylamino)pyridine (DMAP), and azobis(isobutyronitrile) (AIBN), (11) Egerton, P. L.; Pitts, E.; Reiser, A. Macromolecules 1981, 14, 100. (12) Hirayama, F. J. Chem. Phys. 1965, 42, 3163. (13) Perny, S.; Le Barny, P.; Delaire, J.; Buffeteau, T.; Sourisseau, C. Liq. Cryst. 2000, 27, 341. (14) Perny, S.; Le Barny, P.; Delaire, J.; Buffeteau, T.; Sourisseau, C.; Dozov, I.; Forget, S.; Martinot-Lagarde, P. Liq. Cryst. 2000, 27, 329. (15) (a) Noda, I. Appl. Spectrosc. 1993, 47, 1329. (b) Noda, I. Appl. Spectrosc. 2000, 54, 994. (16) The program can be obtained from Prof. Yukihiro Ozaki of Kwansei Gakuin University, Japan. (17) Nakano, T.; Shimada, S.; Saitoh, R.; Noda, I. Appl. Spectrosc. 1993, 47, 1337. (18) (a) Dong, J.; Ozaki, Y. Macromolecules 1997, 30, 286. (b) Dybal, J.; Krimm, S. Macromolecules 1990, 23, 1301. (19) Barber, F.; Bormann, D.; Warenghem, M.; Khelifa, B.; Kurios, Y.; Reznikov, Y.; Simoni, F. Mater. Chem. Phys. 1998, 55, 202. (20) (a) Shin, H. S.; Jung, Y. M.; Oh, T. Y.; Chang, T.; Kim, S. B.; Lee, D. H.; Noda, I. Langmuir 2002, 18, 5953. (b) Jung, Y. M.; Shin, H. S.; Czarnik-Matusewicz, B.; Noda, I.; Kim, S. B. Appl. Spectrosc. 2002, 56, 1568. (21) (a) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: San Diego, CA, 1990; Chapter 9. (b) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic: San Diego, CA, 1991; Chapter 6. (22) (a) Noda, I.; Dowrey, A. E.; Marcott, C.; Story, G. M. Appl. Spectrosc. 2000, 54, 236A. (b) Czarnecki, M.; Okretic, S.; Siesler, H. W. J. Phys. Chem. B 1997, 101, 374. (c) Buffeteau, T.; Pezolet, M. Macromolecules 1998, 31, 2631. (d) Nakano, T.; Shimada, S.; Saitoh, R.; Noda, I. Appl. Spectrosc. 1993, 47, 1377.
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Figure 1. Chemical structure of poly(methyl 4-(methacryloyloxy)cinnamate) (PMMCi). were purchased from Aldrich Chemical Co. and used without further purification. Tetrahydrofuran (THF) and dichloromethane (MC) were distilled over calcium hydride under a nitrogen atmosphere. Methyl 4-(Methacryloyloxy)cinnamate. 4-Hydroxycinnamic acid (2.8 g, 17.1 mmol), absolute methanol (50 mL), and concentrated sulfuric acid (0.2 mL) were refluxed for 12 h. After being cooled to room temperature, the reaction mixture was poured into 500 mL of water and adjusted to pH 7 using a sodium bicarbonate solution, producing a white solid. The white solid was washed thoroughly with water and then collected by filtration. The crude product was dried in a vacuum, yielding methyl 4-hydroxycinnamate (yield: 82%). Methyl 4-hydroxycinnamate (2.2 g, 12.3 mmol), methacrylic acid (1.06 g, 12.3 mmol), and DCC (2.6 g, 12.3 mmol) were dissolved with DMAP catalyst (0.15 g, 1.23 mmol) in dry MC, and the reactant mixture was stirred at ambient temperature for 12 h. After stirring, the reaction solution was filtered and the solvent removed by rotary evaporation. The residue was purified using column chromatography [SiO2, MC/ethyl acetate (45:1 in volume)] to yield methyl 4-(methacryloyloxy)cinnamate (yield: 74%). The product methyl 4-(methacryloyloxy)cinnamate was determined in dimethyl-d6 sulfoxide (DMSO-d6) using a proton nuclear magnetic resonance (1H NMR) spectrometer (Bruker Aspec 300 MHz). 1H NMR (300 MHz, DMSO-d6): δ ) 7.82 (d, 2H, Ar H), 7.72 (d, 1H, Ar-CHd), 7.26 (d, 2H, Ar H), 6.68 (d, 1H, Ar-CHdCH), 6.30 (s, 1H, CH2d C), 5.93 (s, 1H, CH2dC), 3.74 (s, 3H, -OCH3), 2.01 (s, 3H, C-CH3). Poly(methyl 4-(methacryloyloxy)cinnamate). A solution of methyl 4-(methacryloyloxy)cinnamate (1.5 g, 6.1 mmol) and AIBN (0.05 g, 0.3 mmol) in dried THF was heated at 60 °C overnight. After being cooled to room temperature, the reaction mixture was poured into 300 mL of hot methanol. The precipitate was filtered out, washed with hot methanol several times, and dried in a vacuum (yield: 71%). The polymer product was determined in CDCl3 to be poly(methyl 4-(methacryloyloxy)cinnamate) (PMMCi) by 1H NMR spectroscopy. 1H NMR (δ, CDCl3): 7.63 (d, 1H, -CHdCHCOO-), 7.41 (d, 2H, Ar H), 7.08 (d, 2H, Ar H), 6.38 (d, 1H, -CHdCHCOO-), 3.79 (s, 3H, -COOCH3), 3.26 (s, 2H, -CH2C), 1.15 (s, 3H, -CCH3). 13C NMR (δ, CDCl ): 175 (-COO-Ar), 167 (-COOCH ), 152 3 3 (-O-Ar), 143 (-CHdCOOCH3), 133 (O-Ar), 129 (O-Ar), 118 (-CHdCHCOO-), 52 (-COOCH3), 46 (-CH2C(CH3)-), 20.54 (CH2C(CH3)-). Measurements. The molecular weight of the polymer PMMCi was determined by a gel permeation chromatography (GPC) system (Polymer Labs model PL-GPC 210) equipped with a set of four columns (Alltech Jordi 100A, 1000A, 10000A, and 100000A), which was calibrated by a series of polystyrene standards of 2800-1 260 000 Mw (weight averaged molecular weight). The GPC measurements were carried out with THF eluent at a flow rate of 1.0 mL/min. The polymer product was measured to have a Mw of 37 000 and a polydispersity of 2.51. Glass transition temperature Tg and decomposition temperature Td were determined with 5.0 K/min ramping rate under nitrogen atmosphere using a differential scanning calorimeter (DSC) (Seiko DSC 220CU) and a thermal analyzer (Seiko TG/DTA6300), respectively. The PMMCi was determined to have 128 °C Tg and 225 °C Td. A solution in THF (2 wt % solid) of the PMMCi was made. The PMMCi solution was spin-coated onto quartz substrates (25 mm diameter × 1.5 mm thick) for UV-absorption spectroscopy or onto NaCl substrates (25 mm diameter × 2 mm
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Langmuir, Vol. 19, No. 3, 2003 689 Table 1. Characteristic Infrared Bands of Poly(methyl 4-(methacryloyloxy)cinnamate) (PMMCi) frequency (cm-1) 1D
2Da
1749 1735 1721 1718 1708 1640 1637 1630 1600 1506 1276 980 887 a
Figure 2. (a) FTIR spectra and (b) UV absorption spectra measured from a nanoscaled PMMCi film irradiated with unpolarized UV light with varying exposure energy. thick) for FTIR spectroscopy, followed by drying at 35 °C for 30 min under nitrogen flow. The dried films were further baked at 120 °C for 24 h under vacuum. The PMMCi films were irradiated with UV light of wavelength 260-380 nm using a high-pressure Hg lamp system of 1.0 kW (ALTECH, model ALHg-1000) with an optical filter (Milles Griot, model 03-FCG-179). The exposure dose was measured using an International light photometer (model IL1350) with a sensor (model SED 240). The thickness of the polymer films was about 100 nm. UV-visible absorption spectra were obtained as a function of the exposure dose using a Hewlett-Packard 8453 spectrophotometer. FTIR spectra were recorded at a spectral resolution of 4 cm-1 using a Bomem DA8 spectrometer with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector under vacuum (2 Torr). Interferograms were accumulated over 256 scans. 2D correlation analyses were performed using an algorithm based on the numerical method developed by Noda.15 The 2D correlation analyses were carried out after baseline correction of the FTIR spectra. A subroutine named KG2D16 written in Array Basic language (GRAMS/386; Galatic Inc., NH) was employed for the 2D correlation analyses.
Results and Discussion Photochemistry in Film. As shown in Figure 1, the polymer PMMCi has one cinnamoyl moiety in the side group per chemical repeat unit. The nanoscaled films of PMMCi were exposed to unpolarized UV light for varying exposure doses and then examined by FTIR and UV spectroscopy to investigate the photoreactions and photoproducts of the polymer in detail. Figure 2a shows the FTIR spectra of the PMMCi films irradiated with UV light for exposure doses varying in the range 0-0.3 J/cm2. All observed vibrational bands in the spectra can be assigned in accordance with the results reported previously.2,13,14,17,18 In particular, the band at 1749 cm-1 corresponds to the CdO stretching vibration in the methacryloyloxy unit, while the bands at 1718, 1637,
assgnt description ν(CdO) in the methacryloyloxy unit nonconjugated CdO str vibration ν(CdO) of conjugated trans-s-cis conformer conjugated CdO str vibr in 1D IR spectrum ν(CdO) of conjugated trans-s-trans conformer ν(CdC) of vinylene trans-s-trans conformer vinylene CdC str vibr in 1D IR spectrum ν(CdC) of vinylene trans-s-cis conformer ν(CdC) in benzene ring ν(CdO) in benzene ring CH3 deformation mode in the methacryloyloxy unit trans-vinylene CH deformation cis-vinylene CH deformation
Frequencies obtained from the 2D IR correlation analysis.
and 980 cm-1 correspond to the conjugated CdO stretching vibration, the vinylene CdC stretching vibration, and the trans-vinylene C-H deformation in the cinnamoyl moiety, respectively. Additional bands at 1600 and 1506 cm-1 originate from the vibrational modes of the benzene ring in the cinnamoyl moiety, and the band at 1276 cm-1 is due to the CH3 deformation vibration in the methacryloyloxy unit. The IR spectral data of the PMMCi are summarized in Table 1 with assignments proposed from references and observations in this study.2,13,14,17,18 As seen in Figure 2a, the intensities of the bands at 1637 and 980 cm-1 (due to the vinylene CdC stretching vibration and the trans-vinylene C-H deformation vibration in the cinnamoyl moieties, respectively) decrease as the exposure dose increases. The intensity drop in the vinylene CdC stretching band might result from the loss of cinnamoyl moieties due to photodimerization, while the intensity drop in the trans-vinylene C-H deformation vibrational band could be attributed to the consumption of the trans-vinylene linkage in the cinnamoyl moiety due to trans-cis photoisomerization and photodimerization. Moreover, as the exposure dose increases, the intensity of the band at 1718 cm-1 (due to the conjugated CdO stretching vibration of the cinnamoyl moiety) decreases and its position shifts to higher wavenumber. This intensity drop and position shift of the conjugated CdO stretching band could be attributed to two possible factors: the loss of π-conjugation due to photodimerization of the cinnamoyl moieties and/or the photoisomerization of the cinnamoyl moieties from trans isomers to cis isomers. According to the report of Chakrabartk et al.,8 the position shift of the conjugated CdO stretching band due to trans-cis photoisomerization is small. In fact, such a small position shift cannot be resolved in this study because of the extensive overlapping of the band at 1718 cm-1 with the CdO stretching vibrational band of the methacryloyloxy unit at 1749 cm-1. What is actually observed is that at higher exposure doses a new band, presumably originating from the nonconjugated CdO stretching vibration, is discerned at around 1735 cm-1; its band intensity increases with increasing exposure dose. This result supports the fact that the intensity drop and the position shift of the conjugated CdO stretching band originate principally from the photodimerization of the cinnamoyl moieties and possibly in part from the transcis photoisomerization of the cinnamoyl moieties. Recently Liao et al.23 found that the cis-vinylene C-H deformation vibration in poly[(m-phenylenevinylene)-alt(23) Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules 2001, 34, 6756.
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(1,4-dibutoxy-2,5-phenylenevinylene)] appears at about 880 cm-1. On the basis of this result, we expect that the cis-vinylene C-H deformation vibration in the PMMCi appear around 880 cm-1. In addition, the CH3 and CH2 deformation bands, as well as the C-H deformation of benzene ring are known to appear in the similar region.21 The ring deformation of substituted cyclobutane is also known to appear in this region.21 As seen in the inset of Figure 2a, a broad, weak band centered at 887 cm-1 is detected from a film of the PMMCi before UV exposure. In fact, the population of the cis-vinylene unit might be none or very small in the unexposed film. Further, no cyclobutane ring unit presents in the unexposed film. Therefore, the vibrational band observed from the unexposed film might be originated mainly from the deformations of CH3, CH2, and benzene ring C-H in the PMMCi. However, the intensity of the band drastically increases and the band maximum position slightly shifts to 885 cm-1 when the film is first irradiated to the UV light with an exposure dose of 0.05 J/cm2. As exposure dose increases, the band intensity turns to slightly drop but the band maximum position varies very little. These substantial changes in the intensity and position of the band might be attributed to both the C-H deformation vibration of cis-vinylene units and the ring deformation of substituted cyclobutane units formed in the film by the UV exposure. In addition, such variations in the band may be partially attributed to intensity changes in the deformation vibrations of CH3, CH2, and benzene ring units that are caused by the UV light irradiation. However, their stretching bands [νas(CH3) at 2950 cm-1, νas(CH2) at 2929 cm-1, and ν(benzene ring C-H) at 3051 cm-1] are rather reduced in intensity by the UV exposure. From this fact, the intensities of the deformation vibrations of CH3, CH2, and benzene ring units are expected to slightly decrease through the UV irradiation, negatively contributing to the intensity increase of the band centered at 885 cm-1. Indeed the intensity changes of the band (885 cm-1) due to the UV exposures might be attributed mainly to the formation of cis-vinylene units via the trans-cis photoisomerization of the cinnamoyl moieties and/or the formation of cyclobutane rings via the photodimerization of the cinnamoly moieties. Therefore, these FTIR spectroscopic results lead to the qualitative conclusion that both photodimerization and trans-cis photoisomerization of the cinnamoyl moieties take place in PMMCi films when the films are irradiated with UV light, as reported previously for films of PVCi and its derivatives.4,10,11,13,14 Figure 2b shows UV absorption spectra of the PMMCi film upon UV light irradiation. The PMMCi film strongly absorbs UV light over the range of 240-340 nm as PVCi and its derivatives absorb.4,10,11,13,14 A strong, broad band appears, which is centered at 275 nm. This band is extensively overlapped with the other two bands discerned as shoulder peaks. The intensities of all these absorption bands apparently decrease as the exposure dose increases. These absorption bands could not be resolved quantitatively because of their large overlaps. To resolve these overlapped bands, we applied a 2D mapping technique to the UV spectra. This technique makes it possible to interpret the complex spectral changes that occur during the photoreactions induced by UV light irradiation. We have previously introduced a new method for 2D data representation that depicts the intensities of the first derivatives of the absorbance with respect to temperature on a plot of wavenumber versus temperature.20 Here we employ the same technique, but use UV exposure energy instead of temperature. Figure 3a displays the first
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Figure 3. (a) 2D map representation of first derivatives of UV absorption spectra with respect to exposure energy. Solid and dashed lines indicate +∆A/∆E and -∆A/∆E at a given wavenumber, respectively; here, ∆A and ∆E denote absorption difference in the spectrum and UV exposure energy difference, respectively. (b) Fractions of trans and cis isomers and photodimers in a nanoscaled film as a function of UV exposure energy.
derivatives of the UV absorption spectra with respect to exposure energy plotted at each wavelength against exposure energy. As seen in Figure 3a, the three absorption bands over the range of 240-340 nm are distinguishable in the 2D map; one band is centered at 275 nm, and the other two bands are centered at 255 and 307 nm, respectively. Several research groups studied PVCi films.5,9,11,13,14 They found that isolated cinnamoyl chromophores in the trans form absorb light of 275 nm and undergo isomerization to the cis form.11,13,14 It was additionally reported that the cyclic products of PVCi arise in large part through the dimerization of cinnamoyl groups and the ratio of regioisomeric head-to-head and head-to-tail dimers is about 10:1.9 On the contrary, polyesters synthesized from p-phenylenediacrylic acid were found to undergo similar photodimerizations as the PVCi does but the ratio of regioisomeric head-to-head and head-to-tail dimers was about 5:1.11 Moreover, it was reported that cinnamoyl aggregates are present in some poly(aryl cinnamates) and their UV absorption shifts to the lower wavelength region by about 30 nm, compared to that of isolated cinnamoyl moieties.5 Kasha24 theoretically studied optical absorption behaviors of chromophore aggregates and proposed a generalized feature that parallel aggregates [i.e., head-to-head aggregates (so-called H-aggregates) and/or head-to-tail ag(24) (a) Kasha M. Radiat. Res. 1963, 20, 55. (b) Kasha, M.; Ashraf El-Bayoumi, M.; Rhodes, W. J. Chem. Phys. 1961, 58, 916.
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gregates (so-called K-aggregates)] show a strong allowed blue-shifted band and a weak forbidden red-shifted band while end-to-end aggregates (so-called J-aggregates) show a strong allowed red-shifted band and a weak forbidden blue-shifted band, compared to isolated chromophores. These studies lead conclusions as follows. Chromophores of PVCi and its derivatives in films are present both in the isolated state and in the aggregate states. The tendency of chromophores to aggregate depends on the structure of polymer molecules, and the fractions of parallel and endto-end aggregates are different for different polymers, although they have same chromophores. Adopting the results of the previous works described above, we have further assigned the three absorption bands in the 2D map of Figure 3a as follows. The strong band centered at 275 nm is assigned to absorption of isolated cinnamoyl chromophores presented in the PMMCi film, according to the results reported previously.11,13,14,25 On the other hand, taking the proposal of Kasha24 into account, the band centered at 255 nm is attributed to a strong allowed absorption of the parallel aggregated cinnamoyl chromophores while the other band centered at 307 nm is due to a strong allowed absorption of the end-to-end aggregated cinnamoyl chromophores. The appearance of these two bands is evidence that both types of aggregates of cinnamoyl chromophores coexist in the PMMCi film. From these band assignments the 2D map of Figure 3a indicates the differences between the photoreactivities of the isolated and paired cinnamoyl chromophores. The intensities of the absorption bands at 255, 275, and 307 nm, which correspond to absorptions of parallel aggregated, isolated, and end-to-end aggregated cinnamoyl moieties, respectively, decrease abruptly in the early stages of photoreaction and then change only slowly at exposure energies above 0.1 J/cm2. In particular, the bands centered at 255 and 307 nm display comparable change in intensities. At these energies, the isolated cinnamoyl moieties in the trans form favorably isomerize to cis isomers while the aggregated cinnamoyl moieties favorably undergo dimerization. Beyond the trans-cis photoisomerization the isolated trans isomeric chromophores may undergo photodimerization, depending on their neighbored chromophores; the fraction of their photodimerization is, however, expected small in the film, a confined geometry. Therefore, the 2D correlation analysis leads to the conclusion that both photodimerization and trans-cis photoisomerization processes take place in the PMMCi film irradiated by UV light exposure, producing regioisomeric head-to-tail and head-to-head dimers as well as cis isomers of the cinnamoyl moieties. These photoreactions in the PMMCI film were further examined by estimating fractions of the photoproducts. The fractions of the photoproducts were determined from the measured UV spectra shown in Figure 2b using the absorption extinction coefficients of trans and cis isomer measured from PVCi derivatives by Ichimura et al.10 As displayed in Figure 3b, the fraction of the trans isomer decreases rapidly at the early stages of photoreaction and then turns to slowly decrease at exposure energies above 0.1 J/cm2, as indicated from the result of 2D map above. On the contrary, the fraction of the dimers produced from the photochemical consumption of the aggregated trans isomers increases rapidly at the early stages of exposure energy and turns to slowly increase with further increasing exposure energy. The fraction of the cis isomer produced also increases with further increasing exposure energy. (25) Seki, T.; Ichimura, K. J. Phys. Chem. 1990, 94, 3769.
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Figure 4. First derivatives of FTIR spectra with respect to exposure energy. Solid and dashed lines indicate +∆A/∆E and -∆A/∆E at a given wavenumber, respectively; here, ∆A and ∆E denote absorption difference in the spectrum and UV exposure energy difference, respectively.
Overall the fraction of the photoisomerized cis isomer product is relatively much lower than that of the photochemically formed dimers over the range of exposure energy studied here. The low fraction of the cis isomer product might be due to several factors as follows. As suggested in the literature,9,13,14 the trans-cis photoisomerization of the isolated chromophores may be restricted in a confined geometry such as a solid film, compared to in the solution, leading to a low reaction yield. As seen in Figure 1, the PMMCi polymer has one chromophore per chemical repeat unit so that the chromophore is present as a high concentration in the film. The large number of the chromophores in the film may tend to aggregate more rather than to isolate, consequently resulting in a low fraction of the cis isomer product in the photoreaction. If the isolated chromophores are involved in part in photodimerization, as mentioned earlier, this also reduces the fraction of the cis isomer product. It is worthwhile to further evaluate the UV absorption spectra in Figure 2b and their 2D maps against exposure energy in Figure 3a with respect to the photoreactivity and populations of the paired cinnamoyl chromophores. In Figure 3a we see that the change in intensity of the band at 255 nm is analogous to that of the band at 307 nm. These results suggest that the parallel arrangement of cinnamoyl chromophores (255 nm) is as reactive as the end-to-end arrangement of cinnamoyl chromophores (307 nm). On the other hand, the intensity (or area) of the band at 255 nm is comparable to that of 307 nm, as is clear from Figure 3a. This result can be interpreted as measuring the relative populations of the parallel and end-to-end chromophores, if their absorption bands are assumed to have the same or comparable extinction coefficients. With this assumption, the UV spectroscopic results suggest that in PMMCi films the population of the parallel pairing of cinnamoyl moieties is comparable to that of the end-to-end pairing of cinnamoyl moieties. To further examine the photoreactivity of the cinnamoyl moieties, we calculated the first derivatives of the FTIR spectra with respect to UV exposure energy, which are plotted in Figure 4as 2D map of exposure energy versus wavenumber. As seen in the 2D plot of UV absorption (Figure 3a), the IR absorption bands also change rapidly in the early stages of photoreaction. Most of the bands due to the cinnamoyl moiety (for example, 1718 and 1637 cm-1) show changes in intensity during the early stages of photoreaction (up to an energy of 0.1 J/cm2), but their
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Figure 5. (a) Synchronous and (b) asynchronous 2D IR correlation spectra in the region 1800-970 cm-1 generated from the IR spectra of a nanoscaled PMMCi film irradiated with unpolarized UV light with varying exposure energy. Solid and dashed lines indicate positive and negative cross-peaks, respectively.
intensities change little at the later stages of photoreaction above 0.15 J/cm2 exposure energy. These results coincide with the spectral changes observed in the 2D map of UV absorption spectra with respect to exposure energy. In particular, the band at 1735 cm-1 due to the nonconjugated CdO stretching vibration increases monotonically with increasing UV exposure energy, confirming that this band is associated with the photodimerization of cinnamoyl moieties. As described above, the 2D maps of the UV spectra and the FTIR spectra with respect to UV exposure dose provide information about the photoreactions of cinnamoyl moieties in the film but do not enable determination of the sequence of reorientation of the chromophore and its photoproducts during photoreaction. In the next section both the FTIR spectra and the UV spectra are further analyzed by 2D correlation analysis to obtain information about the sequence of reorientation of the chromophore and its photoproducts as well as of other segments of the polymer. 2D Correlation Analysis. Figure 5 shows synchronous and asynchronous 2D correlation IR spectra of a PMMCi film irradiated with various UV exposure doses, measured in the range 1800-970 cm-1. The power spectrum extracted along the diagonal line of the synchronous spectrum is shown at the top of Figure 5a. Here, the 2D
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correlation spectra were constructed from seven spectra measured at 0.05 J/cm2 intervals over the exposure energy range 0 to 0.3 J/cm2. As shown in Figure 5a, the synchronous 2D correlation spectrum reveals a positive cross-correlation peak between the vinylene CdC stretching vibration (1637 cm-1) and the conjugated CdO stretching vibration (1718 cm-1). This implies that the intensities of the two units in the cinnamoyl moiety itself decrease together; the vinylene CdC and conjugated carbonyl CdO units in the cinnamoyl moiety reorient as a single unit when the polymer film is irradiated with UV light. A positive cross-correlation peak is also observed between the vinylene CdC stretching vibration and the CH3 deformation vibration (1276 cm-1) in the methacryloyloxy unit, showing that the bands associated with the cinnamoyl moiety and the methacryloyloxy unit change in intensity in the same direction. This strongly suggests that the cinnamoyl moiety undergoes a cooperative reorientation with its linked methacryloyloxy unit. In contrast, the nonconjugated CdO stretching vibration (1735 cm-1) due to photodimerization shows strong negative cross-correlation peaks with the vibrations of the cinnamoyl moiety (1718, 1637, and 1600 cm-1) and methacryloyloxy unit (1276 cm-1), which is in a good agreement with the first derivatives of the FTIR spectra with respect to exposure energy shown in Figure 4. This result indicates that the intensity changes of the photodimerized cinnamoyl products are opposite to those of the cinnamoyl moiety and the methacryloyloxy unit. These photoinduced molecular reorientation behaviors are also evident in the asynchronous 2D correlation spectra shown in Figure 5b. Moreover, the asynchronous 2D correlation spectrum shows the enhanced spectral resolution. The 1800-1550 cm-1 region of the 2D correlation spectra in Figure 5 has been magnified and plotted in Figure 6 to examine some specific vibrational bands of that region in detail. The power spectrum extracted along the diagonal line of the synchronous spectrum is shown at the top of Figure 6a. In Figure 6a, the synchronous 2D correlation spectrum again confirms positive cross-correlation peaks between the stretching vibration bands associated with the cinnamoyl group (1718 and 1600 cm-1 and 1718, 1637 cm-1) and negative cross-correlation peaks between the nonconjugated CdO stretching vibration at 1735 cm-1 due to photodimerization and the vibrational bands of the cinnamoyl moiety (1600, 1637, and 1718 cm-1). In contrast, the asynchronous 2D correlation spectrum in Figure 6b gives new information as follows. In the 2D IR correlation spectrum of Figure 6b, we see that the band of the conjugated CdO group at 1718 cm-1 actually splits into two peaks located at 1708 and 1721 cm-1. In addition, the vinylene CdC group at 1637 cm-1 also splits into two peaks at 1630 and 1640 cm-1. These peaks may originate from the two rotational conformers possible for the R,βunsaturated ketone in the side group containing the cinnamoyl moiety, as shown in Figure 7. Since the CdOand CdC bands in the s-cis-ketone are far apart in comparison to those in the s-trans-ketone,19,21 the bands at 1721 and 1630 cm-1 are assigned to the CdO stretching vibration and the vinylene CdC vibration of the trans-s-cis conformer of the side group, respectively. The bands at 1708 and 1640 cm-1 are assigned to the CdO stretching vibration and the vinylene CdC stretching vibration of the transs-trans conformer of the side group, respectively. Therefore, the IR spectroscopic results presented here suggest that two types of cinnamoyl conformers, the trans-s-cis and trans-s-trans conformers, are present together with their dimerized photoproducts in the UV-irradiated film.
Nanoscaled Film of PMMCi
Figure 6. (a) Synchronous and (b) asynchronous 2D correlation spectra in the region 1800-1550 cm-1 generated from the IR spectra of a nanoscaled PMMCi film irradiated with unpolarized UV light with varying exposure energy. Solid and dashed lines indicate positive and negative cross-peaks, respectively.
Figure 7. Rotational conformers of the conjugated ketone linkage in the cinnamoyl moiety of the side group.
From the 2D asynchronous correlation spectrum in Figure 6b, the sequences of reorientation of the segments of the cinnamoyl moiety and its photoproducts were determined according to a rule proposed by Noda.15-17,22 As seen in the cross-peaks of Figure 6b, the intensity variation of the CdO stretching band (1718 cm-1) in the cinnamoyl moiety with increasing exposure energy indicates that the photoinduced molecular reorientations occur in the following sequence of spectral changes: 1708 f 1721 f 1735 cm-1 (see Table 1). The intensity change of the vinylene CdC stretching band (1637 cm-1) with UV exposure dose shows that the photoinduced molecular reorientations occur in the following sequence of spectral changes: 1640 f 1630 cm-1. These results indicate that
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the trans-s-trans conformers change before the trans-scis conformers and that the photodimers grow in population after these changes. In addition, the cross-peaks of the phenyl ring vibration (1600 cm-1) with the carbonyl bands (1708 and 1721 cm-1) in the cinnamoyl moiety indicate that the phenyl ring changes before the trans-s-trans conformer. The crosspeaks of the phenyl ring vibration (1600 cm-1) with the vinylene CdC stretching bands (1630 and 1640 cm-1) also indicate that the phenyl ring changes before the transs-trans conformer. Further, the trans-cis photoisomerization of the cinnamoyl moiety via rotation of the CdC bond is known to induce rotation of the phenyl ring.13,14 Therefore, the prior change of the phenyl ring might be attributed in part to the reorientation of the phenyl rings in the trans isomeric cinnamoyl moieties being photochemically transformed into cis isomers as well as the reorientation of the phenyl rings in the resultant cis isomers, although unfortunately the cis isomeric cinnamoyl moiety, as the resultant isomer of such trans-cis photoisomerization, could not be identified in our 2D IR correlation analysis. These observations collectively lead the conclusion that the photoinduced molecular reorientations occur in the following sequence: trans isomer f phenyl ring f transs-trans conformer f trans-s-cis conformer f photodimer. As discussed in earlier sections, the presence of isolated chromophores and chromophore aggregates has been noticed in the 2D mapping method of UV spectra with respect to the exposure energy as shown in Figure 3. Therefore, our synchronous and asynchronous 2D correlation analysis was extended to the UV spectra to obtain more information about the reorientation sequence of the isolated chromophores and of the photodimeric stereoisomers and about the trans-cis photoisomerization. Figure 8 shows the synchronous and asynchronous 2D UV correlation spectra for the region 205-340 nm. The power spectrum extracted along the diagonal line of the synchronous spectrum is shown at the top of Figure 8a. As for the 2D IR correlation spectra, the 2D UV absorption correlation spectra were constructed from seven UV absorption spectra measured at 0.05 J/cm2 intervals over the energy range 0-0.3 J/cm2. All of the cross-peaks in the synchronous correlation spectrum are positive, indicating that all intensity changes are in the same direction. This implies that as the photoreaction precedes the intensity changes of the isolated chromophores and photodimeric stereoisomers decrease together. On the other hand, the absorption band at 275 nm in Figure 8a is resolved into cross-peaks with the absorption bands at 255 and 307 nm as shown in the asynchronous spectrum (Figure 8b). In particular, the band of the isolated chromophores at 275 nm and both the bands at 255 nm (parallel chromophore arrangements) and 307 nm (endto-end chromophore arrangements) show that the photoinduced molecular reorientations occur in the following sequence of spectral changes: 275 f 255 f 307 nm. This result indicates that the isolated cinnamoyl chromophores change more rapidly than the paired cinnamoyl chromophores. Moreover, the parallel chromophore arrangements change fast, compared to the end-to-end chromophore arrangements. Collectively, the above 2D IR and 2D UV correlation analyses indicate that the isolated chromophores in the trans form undergo more rapidly trans-cis photoisomerization than the aggregated chromophores undergo photodimerization, even though the trans-cis photoisomerization of the isolated chromophores may be suppressed in a confined geometry such as a solid film, compared to
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Figure 8. (a) Synchronous and (b) asynchronous 2D UV correlation spectra in the region 205-340 nm generated from the UV absorption spectra of a nanoscaled PMMCi film irradiated with unpolarized UV light with varying exposure energy. Solid and dashed lines indicate positive and negative cross-peaks, respectively.
in the solution, as suggested in the literature.9,13,14 These correlation analyses additionally inform that the parallel aggregated chromophores undergo faster photodimerization than the end-to-end aggregated chromophores. To ascertain the photoreaction mechanism of the PMMCi in the nanoscaled films, we carried out 2D heterospectral UV-IR correlation analysis. Figure 9a shows the synchronous 2D heterospectral UV-IR correlation spectrum. A positive cross-peak in the synchronous 2D heterospectral UV-IR correlation spectrum means that two bands sharing the cross-peak have the same origin, while a negative cross-peak means that two bands sharing the cross-peak have the different origins. Thus, the negative cross-peak (275 nm and 1735 cm-1) shows that the band assigned to isolated chromophores at 275 nm in the UV spectra is assigned to the different origin from that assigned to the nonconjugated CdO stretching vibration of cinnamoyl moieties at 1735 cm-1 in IR spectra, whereas the positive cross-peaks (275 nm, 1718 cm-1; 275 nm, 1637 cm-1; 275 nm, 1600 cm-1) indicate that band at 275 nm in UV spectra and bands at 1718, 1637, and 1600 cm-1 in IR spectra arise from the same origin (the cinnamoyl moieties). Figure 9b depicts the asynchronous 2D heterospectral UV-IR correlation spectrum. From Figure 9b it is noted that the band at 275 nm decreases in the intensities first and then the intensities of IR bands at 1718 and 1637 cm-1 decrease before that of IR band at 1735 cm-1 increases, again confirming that the trans-cis isomerization of the isolated cinnamoyl
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Figure 9. (a) Synchronous and (b) asynchronous 2D heterospectral UV-IR correlation spectra of a nanoscaled PMMCi film irradiated with unpolarized UV light with varying exposure energy. Solid and dashed lines indicate positive and negative cross-peaks, respectively.
chromophores occur more rapidly than the photodimerization of the aggregated cinnamoyl chromophores. Conclusion 2D correlation analyses of FTIR and UV spectra were used to probe the photoreaction as well as the reorientation sequence of the photosensitive cinnamoyl moiety and its photoproducts in nanoscaled PMMCi films. The PMMCi molecules in the films were found to undergo both photoisomerization and photodimerization upon UV irradiation. Strong evidence was found for the presence of three distinct chromophore types in the film: isolated; parallel aggregated (H- and/or K-aggregated); end-to-end aggregated (J-aggregated) cinnamates. The analysis of the 2D UV absorption correlation spectra showed that the three chromophore types photochemically reorient at different rates, which are associated with their photoreactions at different rates. The photoisomerization of the isolated chromophores occurs faster than the photodimerization of the aggregated chromophores. This fact was also confirmed in the 2D heterospectral UV-IR correlation analysis. In the photodimerization processes, the parallel aggregated chromophores were found to react more rapidly than the end-to-end aggregated chromophores. The analysis of the 2D IR correlation spectra led to the identification
Nanoscaled Film of PMMCi
of two rotational conformers of the conjugated ketone linkage in the cinnamoyl moiety. Both the trans-s-trans and trans-s-cis conformers of the cinnamoyl group are present, and the trans-s-trans conformer changes before the trans-s-cis conformer. Our 2D IR correlation analysis also revealed that the cinnamoyl moieties undergo a cooperative reorientation with the methacryloyloxy units (i.e., polymer backbone units) during photoreaction. Overall, The photoinduced molecular reorientations occur in the following sequence: trans isomer f phenyl ring f
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trans-s-trans conformer f trans-s-cis conformer f photodimer. Acknowledgment. This study was supported in part by the Center for Integrated Molecular Systems founded by KOSEF, by the Ministry of Industry & Resources and the Ministry of Science & Technology (G7 Project Program), and by the Korea Research Foundation (BK21 Project). LA020453C
