Photoinduced Control over the Self-Organized Orientation of

Photoinduced Control over the Self-Organized Orientation of Amorphous Molecular Materials Using Polarized ... Publication Date (Web): December 31, 200...
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J. Phys. Chem. B 2010, 114, 1227–1232

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Photoinduced Control over the Self-Organized Orientation of Amorphous Molecular Materials Using Polarized Light Masuki Kawamoto,*,† Takafumi Sassa,† and Tatsuo Wada‡ RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ReceiVed: July 14, 2009; ReVised Manuscript ReceiVed: NoVember 3, 2009

Novel chiral amorphous molecular materials containing photoresponsive azobenzene moieties were designed and synthesized. These materials led to the formation of smooth and uniform thin films without grain boundaries. Photoirradiation of the thin film with linearly and elliptically polarized light caused trans-cis photoisomerization of the azobenzene moieties and resulted in an anisotropic orientation of the materials. Maximum change in a value of birefringence, ∆n, after irradiation of the linearly polarized light is about 0.08 with a response time of 10 s. However, when irradiation was ceased after photoinduced orientation, the value of ∆n decreased due to relaxation of the azobenzene moieties. Furthermore, these materials exhibited a high efficiency of a photoinduced polarization rotation over 30 deg µm-1 after irradiation with the elliptically polarized light for 60 s. We also found that the efficiency depends on ellipticity of the incident light and on the thickness of the sample. The origin of the large change in the molecular orientation is the anisotropic arrangement of the azobenzenes in the terminal groups upon irradiation with the linearly and elliptically polarized light. Introduction Complex functions of naturally occurring molecules arise from their precise self-organized structures. For example, helical structures such as the double helix of DNA and the R-helix of proteins possess fundamental properties in cells and play important roles in biological activity.1 Artificial self-organized materials are currently of interest because of their molecular response to external stimuli such as electric field, temperature, pH, ionic strength, and light, which leads to changes in their molecular orientation. These materials are expected to find application in the fields of catalysis, optics, molecular recognition and molecular devices.2 Photoresponsive materials have received a great deal of attention because they exhibit selective excitation upon irradiation with incident light. Photochromic compounds are promising in view of their reversible, rapid response to light. For example, azobenzene changes its shape upon photoirradiation,3 showing a large change in length upon photoisomerization, with the distance between the 4- and 4′-carbons decreasing from 9.0 Å (trans form) to 5.5 Å (cis form). Furthermore, the trans form of azobenzene has a π-π* transition dipole moment approximately parallel to the molecular axis and shows angulardependent absorption of polarized light. Todorov et al. demonstrated that azobenzene derivatives form a photoinduced self-organized structure through selective excitation with polarized light.4 When the transition dipole moment of trans-azobenzene is parallel to the polarization direction of polarized light, the molecule is excited to effectively cause trans-cis photoisomerization. In contrast, molecules with a transition dipole moment perpendicular to the polarization direction of actinic light are inactive toward isomerization. After repeated trans-cis-trans isomerization cycles, the molecule * To whom correspondence and requests for materials should be addressed. E-mail: [email protected]. Tel: 81 48 467 2745; Fax: 81 48 467 9389. † These authors contributed equally to this work. ‡ Retired.

Figure 1. The change in alignment of azobenzene moieties by irradiation with LPL known as the Weigert effect.

becomes perpendicular to the polarization direction of the actinic light, making it inactive toward polarized light. The phenomenon is interpreted in terms of a photoinduced change in orientation of the moiety perpendicular to the polarization direction of the incident light and is known as the Weigert effect (Figure 1).5 Many groups have reported photoinduced changes in the molecular orientation of azobenzene materials with linearly polarized light (LPL) for optical recording and switching devices.6 Furthermore, it has been demonstrated that photoinduced polarization rotation, or photoinduced chirality,7 of azobenzene polymers is achieved by irradiation with circularly and elliptically polarized light. Nikolova et al. reported the first example of photoinduced polarization rotation.8 They found that the mechanism of photoinduced orientation involves anisotropic rotation, which allows selective excitation of azobenzenes by polarized light. They also observed that photoinduced rotation depends on differences in the morphological characteristics of amorphous and liquid-crystalline (LC) polymers.8d The film containing an amorphous polymer was irradiated with circularly polarized light (CPL), which caused a very small rotation angle. On the other hand, films containing an LC polymer exhibited large changes in the rotation angle. This significant difference is because of a greater number of anisotropic domains in the LC film. In the LC film, the CPL is influenced by the anisotropic structure. Because the structure possesses an optical axis and birefringence, each domain can convert the CPL to elliptically polarized light (EPL), which exhibits a propagation direction. EPL can induce changes in the anisotropic rotation not only in

10.1021/jp906625a  2010 American Chemical Society Published on Web 12/31/2009

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the LC film but also in the amorphous film.7 Natanthon9 and Takezoe10 et al. investigated the photoinduced chirality of LC materials with CPL in detail. Recently, we designed and synthesized novel light-driven chiral materials containing a photoresponsive azobenzene moiety and a chiral binaphthyl moiety in a single species.11 The binaphthyl moiety showed changes in the dihedral angle between the two naphthalene rings without racemization. It was found that the chiral materials exhibited photoinduced changes in the dihedral angle of the binaphthyl moiety by the means of trans-cis isomerization of the azobenzene moiety. We have also demonstrated photoswitching of dextro/levo rotation11a and the photomodulation of a chiral nematic LC phase11b,c caused by photoinduced molecular twisting of binaphthyl moieties. Furthermore, some of the chiral compounds exhibited glassforming properties and showed molecular twisting upon irradiation with UV and visible light, even in neat films.11c Amorphous molecular materials are promising in view of their ability to form high quality films without the need for binder polymers. Shirota and Nakano et al. have reported novel photoresponsive amorphous molecular materials containing azobenzene moieties.12 These readily prepared materials form high-quality films, are processable, and exhibit good thermal stability. Furthermore, they demonstrated that surface relief gratings (SRGs), which are used in photonic devices, are formed on the surfaces of amorphous films upon irradiation with two coherent laser beams. We propose that, if materials showing the glassy state are irradiated with LPL and EPL to cause photoinduced orientation, it could be a novel and effective approach to realize photoresponsive self-organized orientation of molecules in thin films. Furthermore, if the materials show a large change in the photoinduced orientation with a fast response, they will show promise as photoresponsive materials for photonic applications. Experimental Section Materials and Methods. Unless otherwise noted, materials and solvents were purchased from commercial suppliers and were used without further purification. (R)-2,2′-Dihydroxy-1,1′binaphthyl, (S)-2,2′-dihydroxy-1,1′-binaphthyl, and azodicarboxylic acid diethyl ester (40% in toluene solution) were obtained from Tokyo Kasei Kogyo Co. Ltd. Disperse Red 1 (DR1) was purchased from Sigma-Aldrich Co., Ltd. Triphenylphosphine and anhydrous tetrahydrofuran were purchased from Kanto Kagaku Co. Ltd. The synthesized compounds were identified by means of 1H NMR spectroscopy (JEOL AL400), fast atom bombardment mass spectroscopy (FAB-MS; JEOL JMS-700), and elemental analysis. Absorption spectra were measured using a JASCO V-530 spectrometer. Circular dichroism (CD) spectra were obtained using a JASCO J-720 spectrometer. The thermal behavior of the materials was evaluated using differential scanning calorimetry (DSC; Perkin-Elmer DSC7, heating and cooling rate: 10 °C min-1) and X-ray diffractometry (Rigaku RINT 2100). (R)-2,2′-Bis[3-{4′-(4-nitrophenylazo)phenoxy}propoxy]1,1′-binaphthyl (RBDR1). To a solution of (R)-2,2′-dihydroxy1,1′-binaphthyl (0.28 g, 1.0 mmol), DR1 (0.64 g, 2.0 mmol), and triphenylphosphine (0.58 g, 2.2 mmol) in anhydrous tetrahydrofuran (25 mL), azodicarboxylic acid diethyl ester (1.6 mL, 3.5 mmol) was slowly added under nitrogen. The reaction mixture was heated at 50 °C for 48 h. The solvent was removed under reduced pressure, and the solid residue was purified by column chromatography on silica gel with gradient elution from 0 to 2% acetone in toluene. The second fraction, containing a mixture of mono- and disubstituted azobenzene derivatives, was

Kawamoto et al.

Figure 2. Optical setup for investigating photoinduced orientation behavior with LPL (a) and EPL (b), respectively. The incident angle (θ) of the probe light is 6° in panel a. The ellipticity (ein) of the incident light is defined as a ratio of b/a. S: sample; M: mirror; P: polarizer; A: analyzer; BSC: Babinet-Soleil compensator; PD: photodiode.

collected and purified by preparative size exclusion chromatography (Shodex K-2001 + K2002). The first fraction was collected and evaporated to dryness to afford RBDR1 (0.38 g, 43%). 1H NMR (400 MHz, CDCl3): δ (ppm) 0.75 (t, 6H, CH3-), 2.58 (m, 4H, CH3CH2-), 3.31 (t, 4H, -OCH2CH2N-), 4.05 (t, 4H, -OCH2CH2N-), 6.35 (d, 2H, aromatic rings), 7.13-7.37 (m, 8H, aromatic rings), 7.71 (d, 4H, aromatic rings), 7.86 (d, 2H, aromatic rings), 7.89-7.93 (m, 6H, aromatic rings), 8.32 (d, 4H, aromatic rings). FAB-MS (m/z): 879.3 [M+H]+. Anal. Calc. for C52H46N8O6: C, 71.06; H, 5.27; N, 12.75. Found: C, 70.80; H, 5.19; N, 12.70. (S)-2,2′-Bis[3-{4′-(4-nitrophenylazo)phenoxy}propoxy]1,1′-binaphthyl (SBDR1). SBDR1 was prepared from (S)-2,2′dihydroxy-1,1′-binaphthyl using a method similar to that described for RBDR1. FAB-MS (m/z): 879.3 [M+H]+. Anal. Calc. for C52H46N8O6: C, 71.06; H, 5.27; N, 12.75. Found: C, 70.80; H, 5.14; N, 12.76. Fabrication of Thin Films. Thin films were fabricated by spin-coating 1 wt % solution of the molecule in chloroform at a rate of 1000 or 2500 rpm for 30 s. The films were annealed at 100 °C for 30 min and then cooled to form a glassy state. The sample thickness was checked using a Dektak surface profiler. The orientational behavior of the films was determined using a polarizing microscope (Nikon E600WPOL) equipped with a digital camera (Nikon CoolPix995). Optical Setup for Measuring Photoinduced Orientation Behavior. Figure 2a,b shows the optical setups for measuring the photoinduced orientation behavior caused by LPL or EPL, respectively. In the experiments using LPL, the sample was irradiated at a wavelength of 532 nm with a linearly polarized frequency-doubled Nd:YAG pump laser (COHERENT Compass315M-100; 24 mW cm-2). The intensity of a 633 nm probe light from a He-Ne laser transmitted through a pair of crossed polarizers containing the sample film between them was measured with a photodiode (Hamamatsu Photonics S2386-8K). The pumping light was incident normal to the surface of the sample film, whereas the probe light was incident with a slight angle of 6° to the pumping beam (Figure 2a). The response of the materials to EPL was investigated by the apparatus shown in Figure 2b. The polarization direction of the light from the frequency-doubled Nd:YAG pump laser (24 mW cm-2) was set 45° to one optical axis of a Babinet-Soleil compensator using a wavelength plate. EPL with required ellipticity (ein: b/a) and a direction of rotation, left or right, was set by tuning retardation of the compensator before irradiation with the incident light. The polarization of the pumping light after traveling through the film was checked using a rotating analyzer by measuring the transmitted power with a photodiode.

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Figure 3. Chemical structures of RBDR1 and SBDR1.

Results and Discussion Figure 3 shows the chemical structures of RBDR1 and SBDR1. DR1 was chosen as the azobenzene moiety because it contains strong donor (D)-acceptor (A) pairs.13 With such D-A type azobenzenes, very fast recovery (several milliseconds in solution) of the trans form is achieved.3 RBDR1 and SBDR1 were obtained simply using the Mitsunobu reaction.14 However, the pure products could not be isolated by column chromatography on silica gel. The compounds were successfully purified using size exclusion chromatography. RBADR1 and SBADR1 gave the same results for 1H NMR spectroscopy, mass spectrometry, and elemental analysis. This is expected, as the molecules are identical except for their opposite chirality. The maximum absorption wavelengths, λmax (ε M-1cm-1), of RBDR1 and SBDR1 in tetrahydrofuran were 230 (143 000), 283 (33 700), 340 (10 700), and 488 nm (70 800). The 1Bb band at 230 nm corresponds to the long axis of a naphthyl group, while the 1La band at 283 nm corresponds to a short axis. The trans form of the azobenzene moiety in RBDR1 exhibited absorption maxima at 488 nm caused by a π-π* transition. DR1 displays a π-π* transition at the same λmax (ε at 488 nm: 32 300). This indicates that strong dipole-dipole interactions between the two D-A azobenzenes in RBDR1 does not occur. The CD spectra of the compounds in tetrahydrofuran exhibited symmetrical structures with two exciton couplets. The peak at 238 nm is caused by the 1Bb transitions of the two naphthyl moieties and the peak at around 480 nm is derived from the π-π* transitions of the two azobenzene moieties, respectively.15 We found the exciton couplet of the azobenzene moieties reflects the configuration of the binaphthyl moiety, following the rules of exciton chirality.16 The thermal behavior of the materials was investigated by DSC at a heating rate of 10 °C min-1 (Figure S1). On the first heating cycle, an endothermic peak (∆H ) 40 kJ mol-1) caused by melting was observed at 180 °C. Upon cooling to room temperature, the compound formed a transparent glass via a supercooled liquid state. We detected an endothermic event corresponding to the glass transition temperature at 90 °C on the second heating cycle. The formation of the glassy state was also evidenced by the X-ray diffraction patterns of the samples (Figure S2). Before thermal treatment, the sample exhibited sharp diffraction peaks because of the crystalline nature of the molecule. After thermal treatment above the melting point, the diffraction pattern of the sample showed only a broad halo. We have reported that binaphthyl derivatives containing azobenzene groups exhibit glassy states, because the inherent steric hindrance of the two naphthalene rings causes these derivatives to have unique morphological characteristics.11 In general, amorphous molecular materials are capable of forming uniform amorphous thin films by spin coating the pure compound. We prepared neat films with a thickness of 120 nm

Figure 4. Absorption spectra of RBDR1 (a) and CD spectra of RBDR1 (red) and SBDR1 (blue) (b) in thin films. Film thickness: 120 nm.

Figure 5. Change in the birefringence of RBDR1 in a thin film upon irradiation with LPL (532 nm, 24 mW cm-2) at room temperature. Film thickness: 300 nm.

by spin coating solutions of RBDR1 and SBDR1 in chloroform. These samples exhibited isotropic, homogeneous properties without grain boundaries under a polarizing microscope. Figure 4 shows the absorption and CD spectra of the thin films. The λmax of RBDR1 were at 230 and 476 nm, which are attributed to the 1Bb band of the binaphthyl moiety and π-π* transition of the azobenzene moiety, respectively (Figure 4a). Furthermore, the superimposed CD spectra of the films of RBDR1 and SBDR1 formed a symmetrical image, providing evidence of the opposite chirality of the binaphthyl moieties, with 1Bb transitions at 243 nm ((526 mdeg µm-1) and a weak exciton couplet between 400 and 600 nm ((40 mdeg µm-1) (Figure 4b). To investigate the photoinduced orientation of RBDR1 caused by trans-cis photoisomerization upon irradiation with LPL, we evaluated the change in birefringence of the material in a thin film (Figure 5a). As described above, 4,4′-disubstituted azobenzenes possess a rod-like shape in the trans form. An optical axis tends to lie along the long axis of the azobenzene moiety, and the birefringence associated with the alignment of the chromophore can be expressed in the form of birefringence, ∆n ) ne - no, where ne and no indicate the refractive index of a medium against extraordinary and ordinary light, respectively. One can estimate the value of ∆n when a sample is placed between a pair of crossed polarizers as follows:6

T ) sin2

( πd∆n λ )

where d is the thickness of the film, T is the transmittance, and λ is the wavelength of the probe light. Before irradiation, the

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Figure 6. Angular-dependent transmittance of a RBDR1 film (a) and a racemic film (b) consisting of equimolar amounts of RBDR1 and SBDR1, after irradiation with left- (A) and right- (B) handed EPL(ein: 0.87, 24 mW cm-2) for 60 s at room temperature: (open square) incident light, (closed circle) transmittance of incident light through analyzer.

polydomain RBDR1 film was opaque, and the value of ∆n was almost zero when it was evaluated with the polarizing microscope equipped with crossed polarizers. After irradiation with LPL at 532 nm, a clear change in the ∆n of the film was obtained (Figure 5). During photoirradiation at 532 nm, RBDR1 in the thin film exhibits trans-cis-trans isomerization cycles, because azobenzene derivatives containing electron donating and accepting groups revert to the initial state through fast cis-trans thermal back-isomerization. At the end of the cycles, the Weigert effect means a net population of the azobenzene moiety is aligned perpendicular to the light polarization. The response time for the change in ∆n was defined as the time required to increase the transmittance of the probe light to 80% of the saturated value. The magnitude of modulations of ∆n during photoirradiation was approximately 0.08 with a response time of 10s. This phenomenon was observed with similar ∆n in photochemical reactions of amorphous polymers containing the same azobenzene moiety.17 When photoirradiation was ceased after photoinduced orientation, ∆n decreased because of relaxation of the chromophore to an equilibrium distribution.18 However, the ∆n (∼0.05) induced in the irradiated site remained unchanged at room temperature. If RBDR1 was repeatedly irradiated with LPL at 532 nm, ∆n would increase again at the irradiated site (Figure 5). The maximum ∆n could be achieved repeatedly by irradiation with the pumping light, causing reorientation of the azobenzene moieties through further trans-cis-trans isomerization cycles. It is also interesting to note that RBDR1 exhibits a rapid change in ∆n with the response time of 500 ms during repeated photoexcitation.19 The fast response of RBDR1 is an encouraging result from the viewpoint of application in photonic devices, even though relaxation takes place after irradiation. Figure 6a shows the angular-dependent transmittance of the RBDR1 film after irradiation with right- and left-handed EPL at room temperature. We selected incident light (open square) with an ellipticity, ein, of 0.87 (Figure 2b). Upon photoirradiation at 532 nm, the change in the optical axis of the thin film was found to take place in 60 s, and the change in the polarization rotation angle, φ (closed circle), is about 30 deg µm-1. When the sample was irradiated with right- or left-handed EPL, the right- or left-handed polarization rotation could be induced with

Kawamoto et al. essentially the same value of |φ| over the same time scale.20 Photoinduced switching was achieved after several cycles. These results indicate that the thin film of RBDR1 exhibits a rapid, large change in φ compared with those of amorphous polymers previously reported. Only in an exceptional case does an azobenzene copolymer containing rod-like moieties possess large birefringence.21 This copolymer exhibits large rotation angles when irradiated at 457.9 nm (φ: 41 deg µm-1, ein: 0.5) and 514.5 nm (φ: 31 deg µm-1, ein: 0.5) with EPL for 300s. These results indicate that the origin of the large φ is the anisotropic arrangement of the rod-like moieties caused by the photoisomerization of the azobenzene moiety. However, the energy consumption of the incident light by the copolymer was 6 times higher than that of our materials. We believe the large φ and its fast response can be ascribed to the absence of segmental motions in the main chain of the polymers. Refractive index modulation by means of changes in alignment depends on the orientational relaxation of chromophores. In general, polymer materials possess a high viscosity, so the orientational relaxation of azobenzenes requires a relatively long time. Shirota et al. have elucidated differences in the photoinduced formation of SRGs in amorphous molecular materials and polymers.12f Comparative studies of SRG formation using azobenzene derivatives and the corresponding vinyl polymer containing the same chromophore as a pendant group showed that SRGs were formed more rapidly in the amorphous molecular material than in the corresponding vinyl polymer. They concluded that the more rapid response in the amorphous molecular material was caused by the fast trans-cis photoisomerization of the azobenzene moiety, allowing faster formation of SRGs relative to the vinyl polymer. Furthermore, the photoinduced rotation of SBDR1 gave rise to changes in |φ| (35 deg µm-1, ein: 0.87) upon irradiation with right- or left-handed EPL (Figure S5). RBDR1 and SBDR1 possess a binaphthyl moiety, which shows a stable chiral configuration without racemization. We considered that, if the photoinduced molecular rotation is affected by the chirality of the materials, it would exhibit differences in photochromic reactions to incident light. To check this, we investigated the photoresponsive behavior of a racemic film consisting of equimolar amounts of RBDR1 and SBDR1. A 1 wt % equimolar solution of the materials in chloroform was prepared, and then the racemic film was obtained by spin coating the solution onto a fused silica substrate at a rate of 1000 rpm for 30 s (thickness: 200 nm). Figure 6b shows the angular-dependent transmittance of the racemic film after irradiation with rightand left-handed EPL at room temperature. It was found that the changes in |φ| were the same (33 deg µm-1, ein: 0.87) when the film was irradiated with right- and left-handed EPL for 60 s. We also checked the CD spectra in the region of the 1Bb band of the binaphthyl moiety at around 230 nm before and after irradiation (Figure S6). Before irradiation, there was no CD signal because of the racemic nature of the film. On the other hand, the sample irradiated with left-handed EPL exhibited a slightly smaller ellipticity than that achieved upon irradiation with right-handed EPL. However, the superimposed CD spectra formed a symmetrical image. The intensity of the 1Bb band at 230 nm is dependent on the dihedral angle between two naphthyl segments of the binaphthyl moiety. In our previous reports, we found that changes in the dihedral angle of the binaphthyl moiety can be observed when the intensity of the 1Bb band is changed.11 Before irradiation, RBDR1 and SBDR1 should possess the same dihedral angle in the racemic film.22 After irradiation with EPL, photoinduced orientation of the azobenzene moieties is driven

Photoinduced Orientational Control of Amorphous Molecular Materials TABLE 1: Dependence of the Thickness of Films of SBDR1 on the Photoinduced Orientation after Irradiation with Right-handed EPL (ein ) 0.87) sample film thickness (nm) absorbance at 532 nm φ (deg µm-1) F1 F2 F3

110 210 360

0.66 1.21 2.01

35 34 21

by trans-cis-trans isomerization cycles, which induces an anisotropic arrangement perpendicular to the polarization of the actinic light. At the same time, photoisomerization would cause a change in the dihedral angle of the binaphthyl moiety, because the alkyl linkage allows communication between the azobenzene moiety and the binaphthyl moiety. Although the photoinduced changes in the ellipticities are not large, one can induce the opposite sense upon irradiation using either right- or left-handed EPL. As a result, we could not observe the effect of the chirality of the materials. Photoirradiation of the materials with EPL leads to isomerization of the azobenzene moieties. They undergo trans-cis-trans isomerization cycles and tend to reorient perpendicular to a major axis of the EPL, resulting in the introduction of anisotropy with an optical axis on the surface of the film. Once anisotropy is induced, the optical axis rotates in a clockwise (upon irradiation with right-handed EPL) or counterclockwise (upon irradiation with left-handed EPL) direction, with the help of trans-cis isomerization of the azobenzene moieties by the Weigert effect. When the polarization rotation is extended into a bulk area of the film, the optical axis gradually rotates along the propagation direction.8d In addition, the effects of film thickness on the photoinduced orientation behavior were explored when the material was irradiated with right-handed EPL (ein: 0.87) (Table 1). As the thickness of the sample increased, φ decreased. This was especially obvious in F3, which had a thickness of 360 nm. This means that the photoinduced orientation behavior depends on the thickness of the film. To examine the photoinduced orientation behavior of F3, we investigated the rotation angle at various ellipticities (Figure 7). When ein was zero, an optical rotation could not be induced because of the LPL, as mentioned in Figure 5. φ increased as the value of ein increased, and the maximum value of φ was reached when the value of ein was 0.75. If the sample was irradiated with EPL with a value of ein above 0.93, photoinduced polarization rotation was not observed. We believe that the incident light can only penetrate a surface area because of the large extinction coefficients of the azobenzene moieties. Because trans-cis photoisomerization mainly takes place on the surface of the film, the bulk molecules tend to become inactive toward EPL. As a result, thicker films exhibit a smaller φ than other films. When EPL (ein > 0) is employed,

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it is expected that azobenzenes orient effectively perpendicular to the major axis, a, of the polarization ellipse. Thus, one can control the direction of alignment of the materials through the Weigert effect to form a helical structure. In this work, the device parameters such as the irradiation conditions and molecular structures were not optimized. According to the results of Table 1 and Figure 7, the photoinduced rotation behavior was affected by penetration depth of the light into the thin film. In these materials, because of the large extinction coefficients of the azobenzenes, the depth profile for the absorption of photons could be controlled by choosing suitable wavelengths of actinic light. Furthermore, the photochromic reactions of azobenzenes are very fast, occurring on a time scale of picoseconds. If an ultrafast laser is used as a pumping light for photoinduced orientation, the materials might show rapid changes in molecular orientation upon trans-cis isomerization. Furthermore, photoinduced birefringence of the material after trans-cis isomerization decreased because of relaxation of the chromophore. To overcome this problem, amorphous molecular materials with high Tg are required to increase the stability of the anisotropic orientation by restricting their mobility. RBDR1 and SBDR1 are new photoresponsive amorphous molecular materials that exhibit rapid, large changes in photoinduced orientation in the thin films. A further detailed investigation of this novel and interesting phenomenon is now in progress. Conclusions Anisotropic orientation of photoresponsive amorphous molecular materials was induced by irradiation with LPL and EPL. Changes in the birefringence, ∆n, of the samples during irradiation with LPL are about 0.08 with a response time of 10 s. However, when irradiation was ceased after photoinduced orientation had occurred, ∆n decreased because of relaxation of the azobenzene moieties. The materials exhibited a large photoinduced polarization rotation, |φ|, of over 30 deg µm-1 after irradiation with EPL for 60 s. We also found that |φ| depends on the ellipticity, ein, of the incident light and on the thickness of the sample. Moreover, the effect of the chirality of the materials on the photoinduced orientation behavior was explored. When a racemic film of the compounds was irradiated with left- and right-handed EPL, one could induce opposite orientations with the same value of |φ|, indicating that the anisotropic change in the molecular orientation of the azobenzenes did not affect the inherent chirality of the materials. These results suggested that the origin of the large change |φ| is the anisotropic arrangement of the terminal azobenzene groups upon irradiation with EPL. Supporting Information Available: Experimental and analytical details and spectral data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. Photoinduced rotation angles of SBDR1 in a thin film after irradiation with right-handed EPL at various ellipticities. Film thickness: 360 nm.

(1) Lough, W. J., Wainer, I. W., Eds. Chirality in Natural and Applied Science; Blackwell Science: Oxford, England, 2002. (2) For reviews, see (a) Hembury, G. A.; Borovkov, V. V.; Inoue, Y. Chem. ReV. 2008, 108, 1. (b) Ikeda, T.; Mamiya, J.; Yu, Y. Angew. Chem., Int. Ed. 2007, 46, 506. (c) Yashima, E.; Maeda, K.; Nishimura, T. Chem.sEur. J. 2004, 10, 42. (d) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661. (e) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483. (f) Rebek, J., Jr. Acc. Chem. Res. 1999, 32, 278. (g) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38, 3418. (3) For reviews, see: (a) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (b) Ichimura, K. Chem. ReV. 2000, 100, 1847, and references therein.

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