Individualized optically induced orientation of photochemical isomers

5 February 1999

Chemical Physics Letters 300 Ž1999. 421–428

Individualized optically induced orientation of photochemical isomers Hidekazu Ishitobi, Zouheir Sekkat ) , Satoshi Kawata Department of Applied Physics, Osaka UniÕersity, Suita, Osaka 565-0871, Japan Received 28 September 1998; in final form 16 November 1998

Abstract We present evidence of individualized optically induced orientation in photoisomers of diarylethene and spiropyran photochromic chromophores. The light-induced orientation studied on the entire UV–vis spectrum of the chromophores shows spectral dependencies which inform about the relative orientation of the directions of the ultra-violet and visible photochemical transitions of the chromophores. q 1999 Elsevier Science B.V. All rights reserved.

The growing interest that has arisen in photosensitive chromophores containing polymeric films in the past few years has been driven by requirements in the areas of optoelectronics, photonics and optical signal processing. Such materials have been used for holography, optical data storage, integrated optics, molecular switching, all-optical modulation and second- and third-order nonlinear optical effects w1x. Reorientation of photo-excited chromophores alleviates centrosymmetry and isotropy, thereby inducing anisotropy and quadratic and cubic optical nonlinearities. For fundamental understanding of the reorientation process itself and its correlation with the microcogent environment of the chromophore, much of the optically induced reorientation studies have focused on azo dye containing materials w2x. However, despite this activity, studies of light-induced orientation of chromophores other than azobenzene derivatives and azobenzene-type molecules have rarely been reported w3x. In this Letter we individualize optically

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Corresponding author. Fax: q81 6 6879 7876

oriented photoisomers of diarylethene and spiropyran derivatives in thin films of a poly-methyl-methacrylate ŽPMMA. polymer. Such photochromic chromophores have been extensively studied, not only from a photochemical point of view w4–6x, but also for use in near-field and three-dimensional optical data storage w7–9x. Besides the technological interest into the photoisomerization of diarylethene and spiropyran derivatives in polymeric thin films, studies of the optical orientation in such systems will improve the comprehension of the reorientation process of photochromic chromophores within isomerization reactions. Indeed, in contrast to azobenzene derivatives, one of the two photochemical isomers of diaryletehene- and spiropyran-type chromophores can be spectrally distinguished, a feature which allows for the individualization of the photo-induced orientation processes in the photo- and thermal isomerization reactions. In this Letter, we separate the optical orientation processes of both photoisomers of each of the diarylethene and spiropyran chromophores studied, by using the polarized UV–vis spectroscopy technique, and by taking advantage of the natural

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 3 9 0 - 6

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H. Ishitobi et al.r Chemical Physics Letters 300 (1999) 421–428

Fig. 1. Chemical structures and isomerization of Žtop. diarylethene and Žbottom. spiropyran isomers.

spectral differences exhibited by the photoisomers in the UV–vis region Žvide infra.. In particular, it will be shown that the apparent optically induced orientation depends on the UV and visible photochemical transition bands. The structure formula of 1,2-dicyano-1,2-bisŽ2,4,5-trimethyl-3-thienyl.ethene, and 6-nitro1X ,3X ,3X-trimethylspirow2H-1-benzopyrane-2,2X-indolinex that we studied, referred to in the text as DE and SP, respectively, and their photochemical isomers are shown in Fig. 1. The DE and SP chromophores have two photochemical isomers, a stable isomer and a thermally unstable isomer, namely the open- and closed-ring forms for DE, and the spiropyran and photomerocyanine for SP. The stable and thermally unstable isomers are henceforth referred to in the text as the A and B isomers, respectively. Light irradiation produces photoreaction in both the A ™ B and A § B directions, and the thermal reaction proceeds in the A § B direction. In contrast to the colored photomerocyanine form which usually fades after several minutes at room temperature w5,9x, the colored closed-ring form of DE is stable for more than 3 months at 808C w6x. The photochromism of

DE and SP derivatives has been studied extensively for a long time, and spectroscopic characterization of DE- and SP-type molecules is performed by observing a low-energy band in the visible region of the absorption spectrum and a high-energy band in the UV region. Both of the A and B isomers of DE and SP can absorb UV light and simultaneously induce the A ™ B and A § B photoisomerizations. When either of DE or SP is irradiated with visible light, only the B isomer can appreciably absorb light and induce the A § B photoisomerization. In other words, the A and B forms of the chromophores studied and the related photochemical and photophysical processes can be individualized inasmuch as the spectral features of the photochemical isomers can be distinguished. We have studied the photoisomerization and the photo-induced orientation in films of PMMA Žglass transition temperature, Tg s 1058C, and molecular weight, MW s 40 000. doped with DE or SP. Guest–host polymer films were prepared by spincasting from a 2-butanone solution on cleaned glass substrates. The weight ratio of the chromophore relative to the polymer was 1.7% for SP and 10% for DE to ensure an absorbance that is large enough in the visible region of the spectrum. Residual solvent was removed from the films by heating for 1 h at 1108C. The films were removed from the oven and allowed to slowly cool to room temperature. Irradiation was performed by linearly polarized UV Ž365 " 15 nm; glass filter, UV-D33S from Irie. and green Ž546 " 5 nm; interference filter, VPF-25C-10-5054610 from Sigma. lights from a high-pressure mercury lamp ŽNikon. to induce the A l B and A § B photoreactions, respectively. A UV–vis spectrometer ŽModel UV-1600 from Shimadzu. was used to record linearly polarized spectra of the film samples. The polarized spectra were recorded for probe light linearly polarized either parallel ŽAbs 5 . or perpendicular ŽAbs H . to the polarization of the irradiation light.

Fig. 2. UV–vis absorption spectra of PMMA films containing: Ža. diarylethene and Žb. spiropyran before Ž1. and after Ž2, 3. linearly polarized UV irradiation. The probe light was also linearly polarized, and spectra were obtained for both parallel, Abs 5 , and perpendicular, Abs H , orientations. The insets in Ža. are expanded views of both the UV and visible absorption of the diarylethene chromophore. Identical spectra were obtained for both Abs 5 and Abs H prior to UV irradiation. For both of the diarylethene and spiropyran chromophores, inasmuch as the OHB and OR processes occur with Abs H ) Abs 5 within the irradiation wavelength band, note the inversion of the sign of the dichroism between the UV and visible bands of the spectra.

H. Ishitobi et al.r Chemical Physics Letters 300 (1999) 421–428

The spectra were recorded after the end of the irradiation. Dark conditions were employed to avoid the influence of the room light on the isomerization reaction.

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Photoisomerization of both DE and SP clearly occur in films of PMMA. Spectra Žnot shown. taken before, and after several amounts of UV and green irradiation exhibit shape changes and isosbestic points

H. Ishitobi et al.r Chemical Physics Letters 300 (1999) 421–428

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at about 329, 377 and 429 nm for DE and 317, 339 and, less pronounced, 440 nm for SP, clearly demonstrate the forth A ™ B and back A § B photoisomerization reactions. The isosbestic points shifted slightly when film samples are irradiated with green vs. UV lights. We could notice no spectral changes during the A § B thermal back reaction of the DE chromophore over 24 h, e.g. the closed-ring form Žthe B isomer. of the DE chromophore is stable at room temperature for this time period in agreement with the finding of Irie et al. w6x Žvide infra.. The colored photomerocyanine isomer Žthe B isomer. of the SP chromophore persists for several minutes at room temperature. A biexponential fading ŽA § B thermal recovery. of this form at room temperature was observed with rate constants Ž k . of k 1 s 0.00125 sy1 and k 2 s 0.00009 sy1 and weighting factors Žamplitudes. 0.5157 for k 1 and 0.4017 for k 2 in reasonable agreement with previous results w5,9x. Although the polymer free volume, the free volume distribution, or both, could influence the fading rate constants, in the absence of aggregation, the biexponential behavior is, however, usually attributed to the dual form of the photomerocyanine, e.g. the quinonic and zwitterionic forms. The assessment of these photo- and thermal isomerization features is necessary for the light-induced orientation studies reported in this Letter. Fig. 2 shows the dichroic spectra observed in films of DErPMMA ŽFig. 2a. and SPrPMMA ŽFig. 2b.. The insets in Fig. 2a are expanded views of both

the UV and visible absorptions of the DE chromophore. These spectra were obtained 30 s after polarized UV irradiation Žirradiation dose: 78 mJrcm2 .. It is clear that Abs 5 and Abs H are different, in other words the irradiated samples show anisotropic Ždichroic. absorbance upon polarized UV irradiation. Identical spectra were recorded for Abs 5 and Abs H prior to UV irradiation, demonstrating that the samples were in-plane isotropic at that time. The order parameter S, ŽAbs 5 y Abs H .rŽAbs 5 q 2Abs H ., and the dichroic ratio R, Abs 5rAbs H , achieved under polarized irradiation are summarized in Table 1 for all the experiments described in this Letter. It is particularly remarkable from Fig. 2 that Abs 5 is higher than Abs H in the visible band where the absorption of the A form is negligibly small and only the B form exhibits an appreciable absorption. This result was further confirmed by real-time dichroism experiments we performed. This clearly shows that when the DE and SP chromophores undergo the A l B photoreaction under polarized UV light, the B isomer is formed with its Õisible transition dipole moment parallel to the UV polarization. Abs H is higher than Abs 5 in the UV band; a feature which shows that the UV transition dipole moment of the DE and SP chromophores is perpendicular to the UV polarization after cycles of the A l B photoisomerization. This shows that the direction of the UV transition of the mixture of the A and B isomers and the Õisible transition of the B isomer of the

Table 1 Order parameter, S, and dichroic ratio, R, of diarylethene- and spiropyran-containing PMMA films achieved under polarized UV and green light irradiation Irradiation wavelength Žnm. and dose ŽmJrcm2 . Chromophore Analysis wavelength Žnm.

Order parameter, S Dichroic ratio, R Diarylethene

Spiropyran

360

520

345

570

365 and 78

y0.03 0.93

q0.07 1.22

y0.07 0.81

q0.10 1.32

365 and 401

y0.04 0.90

q0.05 1.16

q0.07 1.22

546 and 117 a , and 72 b

q0.01a 1.03 a

y0.06 a 0.83 a

y0.04 b 0.89 b

S and R are defined in the text.

H. Ishitobi et al.r Chemical Physics Letters 300 (1999) 421–428

oriented chromophores are perpendicular to each other. Further evidence of this phenomenon will be given later in this Letter. In Fig. 3 we have plotted the absorbance of linearly polarized probe light Žat 360 and 520 nm for DE, and 570 nm for SP. at various angles, C , between the polarizations of the probe and UV lights. The UV irradiation dose was 401 mJrcm2 . Sinusoidal behavior is clearly shown, and demonstrates the nonpolar orientational distribution of the isomers, rigorously the polarizability tensor of the isomers, in both the UV and visible bands. Perpendicularly oriented transitions are clearly shown for the UV and visible bands Žvide infra.. In the UV region, the A and B isomers have comparable extinction coefficients, and the apparent orientation can be due to both isomers. In the visible region, the orientation observed is only due to the B isomer since the visible-band observed absorption is due to the B isomer only. While the B isomer of the DE chromophore is practically stable at room temperature during the amount of time required to perform this experiment, e.g. 1 h 56 min, the B isomer of the SP chromophore is unstable during the time of the experiment performed on the SP chromophore, e.g. 1 h 37 min Žvide infra.. The data shown in Fig. 3 Žright.

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have been obtained after normalization by the biexponential recovery of the A form during the A § B thermal back reaction with the rate constants and weighting factors given above. The value of the rate constant which corresponds to the long-time relaxation processes, e.g. k 2 , had to be increased to 0.00014 sy1 due to rotational diffusion which tends to randomize the chromophores orientation. The small drift Žsmearing-out of the modulation. in the absorption data of DE is also due to the rotational diffusion of the chromophores. The light polarization sensitivity of the photoisomerization of DE and SP derivatives originates from the anisometric shape of these chromophores. Rigorously, the tensor of the chromophore’s linear polarizability is anisometric, and light absorption is proportional to the cosine square of the angle between the transition dipole moment and the polarization of the exciting light. Within a photochemical transition band, a long molecular axis ŽLMA. can be assigned along the direction of the transition dipole moment, and the chromophores that are most likely to be excited are the ones of which the LMAs are oriented along the polarization of the exciting light. This selective depletion creates anisotropy by burning a hole into the initial isotropic orientational distribu-

Fig. 3. Dependence of the absorbance of PMMA films containing Žleft. diarylethene and Žright. spiropyran on the angle, C , between UV irradiation and probe beam polarizations. The normalized absorbance is defined as ŽAbs y Abs min .rŽAbs max y Abs min .; where Abs max and Abs min are the maximum and minimum absorbances, respectively. The 5 and H signs stand for the directions parallel and perpendicular to the UV polarization, respectively. The markers are experimental data points, and the full and dashed lines are cos 2 C theoretical fits. The analysis wavelengths are indicated. Note the apparent pr2 angle-shift between the UV and visible orientational distributions of the diarylethene chromophore.

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tion of the chromophores Žorientational hole burning, OHB.. In principle, isotropy can be restored by rotational diffusion. Reorientation of the molecular axis of the chromophore during the photoisomerization reaction Žorientational redistribution, OR. will also contribute to the observed anisotropy, since the chromophores that are oriented along the polarization of the exciting light present the highest probability of being excited and reoriented. DE and SP chromophores exhibit orientational features that are not observed for previously studied photochemical chromophores. So, the term reorientation includes both the physical processes of the reorientation of the chromophore as a block, or block reorientation, and the reorientation due to the isomeric shape change without any block rotation, or isomeric reorientation. The block and isomeric reorientation processes are due to the reorientation of the chromophore’s LMA after a full A ™ B ™ A or B ™ A ™ B cycle and a A ™ B or B ™ A isomerization, respectively. When only one isomer, say the more stable isomer A, can absorb the excitation light the apparent anisotropy which results from pure OHB processes is measured at the irradiation wavelength, or at a wavelength where only A absorbs. The anisotropy observed for the less stable photoisomer B which is inherent to the excitation light, measured at a wavelength where only B absorbs, informs about the reorientation of the chromophore’s axis within the A ™ B photoisomerization. This behavior has been observed for azobenzene molecules introduced into a polyglutamate polymer w10x. For the DE and SP chromophores studied in this Letter, both of the A and B photoisomers absorb UV light, and the UVband anisotropy results from both of the OHB and OR processes which lead to a net orientation of the chromophore axis perpendicular to the polarization of the excitation light Žsee the anisotropic absorbance of the UV-band in Fig. 2.. The contribution of the isomeric reorientation of the chromophore’s axis

within the thermal A § B recovery to the UV-band anisotropy is negligible for the anisotropy observed at early decay times, a feature which holds true for the dichroic spectra presented in this Letter. Reorientation of the chromophore’s axis can also be observed within the A § B photoisomerization. Such an observation was made for both DE and SP chromophores, and is depicted in Fig. 4. This figure shows the sample spectra at a photostationary state attained after unpolarized UV light irradiation Žirradiation dose: 1.026 and 0.864 Jrcm2 for DE and SP, respectively., and the dichroic spectra obtained after a subsequent linearly polarized green irradiation Žirradiation dose: 117 and 72 mJrcm2 for DE and SP, respectively.. Identical spectra were recorded for both parallel, Abs 5 , and perpendicular, Abs H , absorbances prior to green irradiation at the photostationary state. Since the photomerocyanine form of the SP chromophore is thermally unstable, the data of Fig. 4b have been normalized by the double-exponential fading of the photomerocyanine with the rate constant k 2 s 0.00014 sy1 that also accounts for rotational diffusion. Therefore, only the data of the visible region of the spectra are shown in Fig. 4b, since in this region the observed absorbance is directly proportional the concentration of the photomerocyanine. For both of the DE and SP chromophores, Abs H is higher than Abs 5 in the absorption band of the B isomer; a finding which proves that the chromophores visible transition dipole moment is oriented perpendicular to the green polarization. This observed anisotropy is due to OHB in the orientational distribution of the B isomer. Here also, the apparent anisotropy observed in the UV region of the DE chromophore ŽFig. 4a. is opposite in sign to that observed in the visible region in agreement with the results of Figs. 2 and 3. The observed anisotropy in the UV region can be due to both the OHB of the B isomer and anisotropic orientation of the A isomer processes; each as a result of the 546 nm green

Fig. 4. UV–vis absorption spectra of PMMA films containing: Ža. diarylethene and Žb. spiropyran before Ž1. and after Ž2, 3. linearly polarized green irradiation. The probe light was also linearly polarized, and spectra were obtained for both parallel, Abs 5 , and perpendicular, Abs H , orientations. The insets in Ža. are expanded views of both the UV and visible absorption of the diarylethene chromophore. The green irradiation followed unpolarized UV irradiation after which identical spectra were obtained for both Abs 5 and Abs H . For both of the diarylethene and spiropyran chromophores, the OHB process occurs with Abs H ) Abs 5 within the irradiation wavelength band. Note the inversion of the sign of the dichroism between the UV and visible regions of the spectra for the diarylethene chromophore.

H. Ishitobi et al.r Chemical Physics Letters 300 (1999) 421–428

irradiation induced B ™ A photoreaction and isomeric reorientation. The optically induced reorientation of the chromophore’s axis on going from one

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isomer to the other, within a single isomerization reaction, is attributed to the differing structures of the A and B isomers Žisomeric reorientation. Žvide

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infra.. In contrast to azobenzene derivatives, it seems that the photo-induced ring-closing Žphotocyclization. and opening in the chromophores studied leads to an opposite UV and visible electronic polarization Žmolecular polarizability. of the A and B forms of the chromophores. Polarized light irradiation orients both of the photochemical isomers of diarylethene and spiropyran derivatives in polymeric thin films. The individualized UV and green light induced orientation that is studied on the entire UV–vis spectrum of the chromophores suggests that the UV and visible transitions of the chromophores studied are perpendicular. We believe that such experimental findings will trigger studies to reveal the symmetry of molecular polarizability tensors of photochromic isomers, and elucidate the true nature of the related photochemical transitions.

Acknowledgements The authors are indebted to M. Irie of Kyushu University for providing the chromophores, and thank

A. Toriumi and O. Nakamura for helpful discussions. This work is supported by the Japanese Society for the Promotion of Science under the Research for the Future Program.

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