2806
J. Phys. Chem. B 1997, 101, 2806-2810
Distinct Photochemical Phase Transition Behavior of Azobenzene Liquid Crystals Evaluated by Reflection-Mode Analysis Atsushi Shishido, Osamu Tsutsumi, Akihiko Kanazawa, Takeshi Shiono, and Tomiki Ikeda* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Naoto Tamai Department of Chemistry, School of Science, Kwansei Gakuin UniVersity, Uegahara, Nishinomiya 662, Japan ReceiVed: NoVember 18, 1996; In Final Form: February 6, 1997X
Photochemical phase transition behavior of photochromic azobenzene liquid crystals (LCs) was explored by means of reflection-mode analysis. On pulse irradiation at 355 nm, which causes trans-cis isomerization of the azobenzene moiety, these LCs underwent nematic (N) to isotropic (I) phase transition in 100 µs as probed by change in reflectivity at the interface between the sample and glass substrate. The N-I phase transition was confirmed by calculation of the refractive index of the sample before and after pulse irradiation on the basis of the reflectivity. The decay that represents the recovery of the initial N phase occurred in 2-3 ms and remarkably faster than that observed in the transmission-mode analysis in which transmittance of the probe light through crossed polarizers, with the sample between them, was measured as a function of time. In the reflection-mode analysis, the N phase was assumed to be restored through diffusion of the cis form, followed by reorientation of the trans form that replaced the cis form in the interface region, while in the transmission-mode analysis the N phase recovered through the cis-trans thermal back-isomerization process.
Introduction Photonics, which can control light by light as a stimulus, has become of interest as future technology because of the advantage in high-speed processing.1 In photonics, switching devices play an important role in the control of light and change their own physical properties with the stimulus light. Liquid crystals (LCs) are convenient to control the light because LCs show large optical anisotropy due to the anisotropy in molecule shape and the responsiveness to electric field. Therefore, many studies have been reported to construct the optical switching and image storage devices by the use of the LCs.2 At present, LCs are used only as active media in display devices with the response time of several milliseconds because the response of LCs to change in electric field is slow. However, if the response becomes fast enough with light as a stimulus, we will be able to use LCs not only in display devices but for various photonics applications such as optical switching, optical image storage, optical display, and optical computing. To drive LCs by light, such photochromic molecules as azobenzene and spiropyran have been used which change their molecular shape on photoirradiation.3 This property is applicable to control the orientation of the LCs.4-11 For instance, the trans form of the azobenzene derivatives stabilizes the phase structure when dispersed in nematic LCs (NLCs) because of its rodlike shape, whereas its isomer, the cis form, tends to destabilize the N phase due to the bent shape. Therefore, the trans-cis photoisomerization of the azobenzene guest molecules in the N phase can disorganize the phase structure of NLCs, resulting in N-I isothermal phase transition (photochemical phase transition). When a small amount of the azobenzene derivatives was dispersed in NLCs (1-5 mol %), the photochemical phase transition was induced in 50-200 ms.8,12,13 Recently it was found that the monomer and polymer LCs * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 15, 1997.
S1089-5647(96)03839-4 CCC: $14.00
possessing the azobenzene moiety in each mesogen caused the photochemical phase transition in 200 µs on pulse irradiation. The important feature of these azobenzene LCs is that they show the LC phase only in the trans form of the azobenzene moiety while they never show any LC phase at any temperature when the azobenzene moiety is in the cis form. This enables the azobenzene LCs to undergo the LC-I phase transition instantaneously when the trans-cis isomerization is induced with a short laser pulse.14 The photochemical phase transition of LCs has been analyzed most conveniently by the transmission-mode analysis.8-14 Since LCs show birefringence, the N-I phase transition can be monitored easily by the measurement of transmittance of, say, a He-Ne laser through a pair of crossed polarizers, with the LC sample between them.13 When the LC sample is in the N phase, the transmittance is high, while no transmitted light is detected when the sample is in the I phase. Although the transmission-mode analysis is a very convenient method, there is a drawback; the phase transition can be detected only after the LC sample becomes an I state completely across the sample. This is in some sense not convenient especially for the investigation of the photochemical phase transition of thick samples. If photons are absorbed entirely in the surface region because of a high extinction coefficient of the sample, the N-I phase transition is induced only in the surface region with the rest of the sample remaining in the N phase. Under these circumstances, it is very difficult to analyze the phase transition behavior precisely in the transmission-mode analysis. Contrary to the transmission-mode analysis, events occurring only in the surface region can precisely be explored by the reflection-mode analysis.15,16 In this mode of analysis, the probe light incident upon the interface between the sample and the substrate can penetrate only the surface region of the sample, depending on the incident angle, and provides information on the surface of the sample if reflected probe light is monitored carefully. This mode of analysis can, therefore, be a powerful tool especially © 1997 American Chemical Society
Phase Transition Behavior of Azobenzene LCs
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2807
Figure 1. Structures of LCs used in this study.
TABLE 1: Thermodynamic Properties of LCs Used in This Study phase transition temperature, °Ca BMAB 8AB8 a
K 30 N 45 I (heating) K 8 N 45 I (cooling) K 98 N 111 I (heating) K 92 S 94 N 110 I (cooling)
∆H, kJ/mol
∆S, J/(mol‚K)
0.46
1.42
1.7
4.4
K, crystal; S, smectic; N, nematic; I, isotropic.
when the photochemical phase transition behavior of photochromic LCs with high extinction coefficients is investigated. In this study, we explored the phase transition behavior and the optical switching behavior of azobenzene LCs by the use of reflection-mode analysis, and especially investigated the quantitative change in the refractive index of the sample due to the photochemical phase transition of the LCs in detail. Experimental Section Materials. Structures of the LCs used in this study are shown in Figure 1. 4-Butyl-4′-methoxyazobenzene (BMAB) and 4,4′di(octyloxy)azobenzene (8AB8) were synthesized and purified as reported previously.6,17 Characterization of LCs. Liquid-crystalline behavior and phase transition behavior were examined on an Olympus Model BH-2 polarizing microscope equipped with Mettler hot-stage models FP-90 and FP-82. Thermotropic properties of LCs were determined with a differential scanning calorimeter (Seiko I&E SSC-5200 and DSC220C) at a heating rate of 0.5 °C/min. At least three scans were performed for each sample to check reproducibility. The thermodynamic properties of the LCs are given in Table 1. Refractive indices of the azobenzene LC (BMAB) in trans and cis forms were measured on an Abbe refractometer (ATAGO, 2T high refractive index type) at a cooling rate of 0.5 °C/min, where a small portion of lecithin solution in ethanol (1 wt %) was cast on a prism of the Abbe refractometer to align BMAB molecules in a homeotropic manner. In the trans form, the azobenzene LC molecules show the birefringence in N phase (N trans), which arises from the molecular shape itself. As shown in Figure 2, the refractive index parallel to the long axis of molecule, ne, is different from that perpendicular to the short axis of molecule, no. On the other hand, the LC molecules in the trans form show no birefringence in the I phase (I trans), since the molecular direction is random in the I phase. In this case, the refractive index of the LC molecules is n′. Similarly, the LC molecules in the cis form show only an I phase (I cis); birefringence has never been observed because the shape of the cis form is bent. Thus, the refractive index of the cis form is n, which is nearly equal to n′. The following relation is in general obtained: ne > n ∼ n′ > no. Therefore, it is expected that the refractive index of the sample changes from no or ne to n, depending on
Figure 2. Birefringence of azobenzene LCs: N trans, nematic phase in the trans form of the azobenzene moiety; I trans, isotropic phase in the trans form; I cis, isotropic phase in the cis form. Birefringence was observed for N trans, whereas there was no birefringence in I trans and I cis; no, ordinary refractive index in the N trans; ne, extraordinary refractive index in the N trans; n′, refractive index in the I trans; n, refractive index in the I cis.
Figure 3. Schematic illustration of principle of reflection-mode analysis: na, nb, refractive indices of two materials; θi, incident angle; θr, refractive angle; R, reflectivity, which is a fraction of light reflected at the interface. In this study, na and nb correspond to the refractive indices of the quartz substrate and the sample, respectively.
the polarization of the probe light, when N-I phase transition is induced by trans-cis photoisomerization of the azobenzene moiety. In this study, we investigated the change in the refractive index of the azobenzene LCs by the reflection-mode analysis as a result of the photochemical phase transition. Principle of Reflection-Mode Analysis. In the reflectionmode analysis, we measured the intensity of the reflected light from the interface between the sample and the glass substrate to estimate the refractive index of the sample, as shown in Figure 3. Reflectivity, which is a fraction of light reflected at the interface, changes with the change in the refractive index of the sample, and their relation can be given by eqs 1 and 218
Rs )
Rp )
( (
) )
na cos θi - nb cos θr na cos θi + nb cos θr
2
nb cos θi - na cos θr nb cos θi + na cos θr
2
(1)
(2)
where Rs and Rp represent reflectivities of light in s-polarization and p-polarization, na and nb are refractive indices of two materials, and θi and θr denote the incident angle and the refractive angle. Reflectivity depends on refractive indices of two materials, incident angle, refractive angle, and the polarization. In this study, we measured the reflectivity at a fixed incident angle of 70°, at which the reflectivity and its change on the phase transition were significantly large. Measurements in Reflection-Mode Analysis. Optical setup for the reflection-mode analysis is shown in Figure 4. The sample was irradiated with a single pulse of a Nd:YAG laser (Spectron, SL805 laser system; the third harmonic, 355 nm; 10 ns, fwhm) where the light at 355 nm was separated with dichroic mirrors from lights at 532 and 1064 nm. Intensity of the probe light (NEC, GLC5370 He-Ne laser; 633 nm; 1 mW) reflected from the interface between the sample and the glass substrate was measured with a Hamamatsu R-928 photomultiplier as a function of time and recorded with a storage scope (Iwatsu, DS8631). The probe light was passed through a pinhole (200 µm) and a polarizer and was incident upon the quartz block. The samples were prepared on the quartz block substrate that had
2808 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Shishido et al.
Figure 6. Calculation of reflectivity as a function of refractive index at a fixed angle of 70°.
Figure 4. Schematic diagram of optical setup: DM, dichroic mirror; M, mirror; P, polarizer; PH, pinhole (200 µm); S, sample; A, analyzer; F, filter; PM, photomultiplier; MM, monochromator.
Figure 7. Refractive index of BMAB at 633 nm measured with the Abbe refractometer: (0), ne for the N phase with the trans form (N trans); (O), no for the N phase with the trans form (N trans); (4), n′ for the I phase with the trans form (I trans); (b), n for the I phase with the cis form (I cis). The measurement of the refractive indices in the cis form was performed after photoirradiation at 366 nm.
Figure 5. Change in reflectivity as a function of incident angle of the probe light at 633 nm at 30 °C: (A), s-polarization; (B), p-polarization. The sample is BMAB, and the values of the refractive index used in the calculation were those obtained with the Abbe refractometer at 633 nm at 30 °C.
been rubbed into one direction to align LC molecules and thermostated to show the N phase. Results and Discussion Calculation of Refractive Index. Figure 5 shows results of simulation of change in reflectivities from the interface
between the sample (BMAB) and the glass substrate as a function of the incident angle. The results obtained for s-polarized and p-polarized light are shown in parts A and B of Figure 5, which were calculated by eqs 1 and 2, respectively. The reflectivity of light at 633 nm increases as the incident angle increases for both s-polarized and p-polarized light. Furthermore, the reflectivity increases as the refractive index of the sample increases, but the change in the reflectivity with the refractive index is larger for the s-polarization particularly at high incident angles. As eqs 1 and 2 indicate, the value of the refractive index of the sample (nb in Figure 3) is necessary for the calculation of the reflectivity, and we used the values of BMAB determined at 633 nm at 30 °C with the refractometer. Figure 6 shows the change in the reflectivity of light at 633 nm in s-polarization as a function of the refractive index of the sample at a fixed incident angle of 70°. It is evident that the reflectivity is proportional approximately linearly to the refractive index. Accordingly, we can estimate the refractive index of the sample by measurement of the reflectivity from the interface between the sample and the glass substrate. Refractive Index Measurements. Figure 7 shows refractive index of BMAB at 633 nm as a function of temperature; the measurements were performed with the Abbe refractometer. An LC phase appeared in “N trans”, which means an N phase in the trans form, and hence birefringence could be observed at temperatures in the N phase. High and low refractive indices below 44 °C correspond to the extraordinary refractive index
Phase Transition Behavior of Azobenzene LCs
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2809
(ne) and the ordinary refractive index (no), respectively. It is worth noting here that the value of ne decreases with temperature while the no value increases. This temperature dependence of ne and no may be interpreted in terms of the change in an order parameter of the LC; the order parameter (S) which represents the degree of the order of the LC molecules is defined by eq 319
S)
1
N
∑ N i)1
(3 cos2 θi - 1) 2
1 ) 〈3 cos2 θ - 1〉 2
(3)
where θ is the angle between the direction of the long axis of a molecule and the average direction of the long axis of molecules (director). For instance, the value of S equals 1 when all molecules are aligned completely to one direction and 0 when the long axes of the molecules direct randomly. S changes with orientation of the LC molecules, which depends on temperature. Therefore, we can interpret that the decrease in ne with temperature shown in Figure 7 is due to decrease in S with temperature and mixing of the component of no with ne. Similarly, the decrease in S and the mixing of the ne component with no resulted in increase in no with temperature. However, no birefringence can be seen in the I phase (I trans). These profiles are characteristic of LC materials. Photoirradiation at 366 nm brings about trans-cis photoisomerization of BMAB molecules, and the BMAB molecules in the cis form show no LC phase at any temperature and thus show no birefringence (I cis). Under this circumstance, the refractive index is n, which is nearly equal to n′ as described before. Difference in the refractive index between n and no or n and ne is of the order of 10-1, and hence it is expected that photochemical N-I phase transition produces a large change in the refractive index of BMAB from no or ne to n. On the other hand, difference in the refractive index between n (I cis) and n′ (I trans) is much smaller, of the order of 10-3. This is due to the difference in molecular volume; the molecular volume of the trans form is smaller than that of the cis form.20-22 The refractive index of BMAB can also be determined from the reflectivity by the use of eqs 1 and 2. We calculated the refractive index of BMAB from the reflectivity measured in the reflection-mode analysis and compared with those determined with the refractometer. In Figure 8, we plotted the values of the refractive index determined from the reflectivity and those determined with the refractometer as a function of temperature. It is observed that both values are in good agreement at any temperature in any state of N trans (no), I trans (n′), and I cis (n). These results indicate that the refractive index of LC molecules can be evaluated by the reflectivity obtained by means of the reflection-mode analysis. N-I Phase Transition Behavior. Figure 9 shows the result of the time-resolved measurement in the reflection-mode analysis on the change in the refractive index of BMAB on pulse irradiation at 355 nm. To discuss the phase transition behavior and the optical switching behavior quantitatively, we defined the response time (τr) as the time required to raise the intensity of the reflected light to 90% of the maximum value. It was found that the refractive index of BMAB rose up from 1.564 to 1.632 on pulse irradiation; in this experiment the BMAB molecules were aligned in a homogeneous manner with the molecular long axis oriented parallel to the probe light (spolarization) as shown in Figure 9. The value of 1.564 corresponds to the refractive index of no in the N phase where the azobenzene moiety is in the trans form, while the value of 1.632 indicates the refractive index of n in the I phase. The change in the refractive index of the sample demonstrates that
Figure 8. Refractive index of BMAB calculated from reflectivity that was obtained by the reflection-mode analysis: (O), no for the N phase with the trans form (N trans); (4), n′ for the I phase with the trans form (I trans); (b), n for the I phase with the cis form (I cis); (s), refractive indices determined with the Abbe refractometer. The measurement of the refractive indices in the cis form was performed after photoirradiation at 366 nm. The refractive indices obtained by the reflection-mode analysis are in good agreement with those measured with the refractometer at any temperature.
Figure 9. Time-resolved measurement of change in the refractive index of BMAB on pulse irradiation (355 nm, 10 ns fwhm) at 34 °C (nematic phase).
the N-I phase transition was induced isothermally by the transcis photoisomerization of the azobenzene moiety on pulse irradiation. To examine if the change in the refractive index is due to heat evolved by the laser pulse irradiation, reference experiments were performed. The sample was irradiated with the laser pulse in the I phase, either in I cis or in I trans, and it was found that the refractive index of the sample never changed on pulse irradiation at 355 nm at any temperature in the I phase. These results clearly indicate that the refractive index is insensitive to the heat generated by the laser pulse, if any. Therefore, the change in the refractive index is evidently due to the N-I phase transition induced by the trans-cis photoisomerization of the azobenzene moiety in BMAB, so that this phenomenon results from the photon-mode process but not from the heat-mode process. It was found that the N-I phase transition occurred in 100 µs, and this value is the same as those obtained in the
2810 J. Phys. Chem. B, Vol. 101, No. 15, 1997 transmission-mode measurements (200 µs).14 The response time of 100 µs is faster by 1-2 orders of magnitude than those of nematic LCs to the change in the electric field. I-N Phase Transition Behavior. To discuss the optical switching behavior, it is necessary to evaluate not only the response time, which means the N-I phase transition, but also the decay time, which corresponds to I-N phase transition. Therefore, we also defined the decay time (τd) as the time required to decrease the intensity to 10% of the maximum value. It was found that the I-N phase transition, namely the recovery of the initial N phase, was completed only in 2 ms. This value was smaller by 6 orders of magnitude than that obtained in the transmission-mode analysis. It is worth noting that τd was much different between the reflection-mode measurement and the transmission-mode measurement, while no difference in τr was observed between these two methods. It was reported that the recovery process in the transmission-mode measurements mainly depended on the cis-trans thermal backisomerization of azobenzene moiety.14 The thermal backisomerization of the azobenzene moiety is very slow at low temperatures. For instance, it took several hours at room temperature in BMAB. However, in the reflection-mode analysis, the small values of τd of several milliseconds were observed in both BMAB and 8AB8, although these samples showed the N phase at different temperatures. Therefore, these results indicate that the recovery mechanism is different in the reflection-mode measurements. The molar extinction coefficients of the azobenzene moieties at 355 nm are so large (∼104) that the pumping light at 355 nm was absorbed entirely in the surface. Consequently, the trans-cis photoisomerization was also induced near the surface, and the N-I phase transition occurred only in the surface, leaving the bulk area intact as an N phase. In the reflection-mode analysis, the probe light can penetrate only in the surface region. So if the molecules in the cis form produced in the surface by photoirradiation diffuse into the bulk phase and the molecules in the trans form in the bulk phase replace them and reorient, the initial N phase can recover, not through the slow cis-trans back-isomerization process. Since the diffusion and reorientation processes are much faster than the cis-trans back-isomerization, the quicker optical switching has been achieved in the reflection-mode measurements. Conclusions The photochemical phase transition behavior of the azobenzene LCs was investigated by the use of the reflection-mode
Shishido et al. analysis. The azobenzene LCs show the N phase in the trans form, while no LC phase in the cis form. It was found that the refractive index of the sample changed from no to n on pulse irradiation and the N-I phase transition was induced simultaneously. The initial N phase recovered in 2-3 ms, which was faster by 6 orders of magnitude than those in the transmissionmode measurements. Switching mechanism in the reflectionmode analysis is due to the diffusion and reorientation processes of the LC molecules and is quite different from that in the transmission-mode analysis in which the decay depends on the cis-trans thermal isomerization of the azobenzene moiety. The rapid optical switching has been achieved successfully at room temperature by the use of the reflection-mode analysis, which may be applicable to optical switching devices. References and Notes (1) Balkanski, M.; Lallemand, P. In Photonics; Gauthier-Villars: Montreal, 1975. (2) Kelker, H.; Hatz, R. In Handbook of Liquid Crystal; Verlag Chemie: Weinheim, 1980. (3) Kobayashi, T.; Degenkolb, E. O.; Rentzepis, P. M. J. Phys. Chem. 1979, 83, 2431-2434. (4) Ogura, K.; Hirabayashi, H.; Uejima, A.; Nakamura, K. Jpn. J. Appl. Phys. 1983, 58, 1154-1158. (5) Odulov, S. G.; Reznikov, Yu.A.; Soskin, M. S.; Khizhnyak, A. I. SoV. Phys. JETP 1983, 58, 1154-1158. (6) Tazuke, S.; Kurihara, S.; Ikeda, T. Chem. Lett. 1987, 911-914. (7) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214-1217. (8) Kurihara, S.; Ikeda, T.; Sasaki, T.; Kim, H.-B.; Tazuke, S. J. Chem. Soc., Chem. Commun. 1990, 1751-1752. (9) Kurihara, S.; Ikeda, T.; Tazuke, S.; Seto, J. J. Chem. Soc., Faraday Trans. 1991, 87, 3251-3254. (10) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361, 428-430. (11) Sasaki, T.; Ikeda, T.; Ichimura, K. J. Am. Chem. Soc. 1994, 116, 625-628. (12) Ikeda, T.; Sasaki, T.; Kim, H.-B. J. Phys. Chem. 1991, 95, 509511. (13) Sasaki, T.; Ikeda, T.; Ichimura, K. Macromolecules 1992, 25, 38073811. (14) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873-1875. (15) Knoll, W. Makromol. Chem. 1991, 192, 2827-2856. (16) Hamai, S; Tamai, N; Masuhara, H. J. Phys. Chem. 1995, 99, 49804985. (17) Weygand, C.; Gabler, R. J. Prakt. Chem. 1940, 155, 322-341. (18) Born, M.; Wolf, E. In Principles of Optics, 2nd ed.; Pergamon Press: Oxford, 1964; pp 38-51. (19) de Gennes, P. G. In the Physics of Liquid Crystals; Clarendon Press: Oxford, 1974; pp 23-24. (20) Tanio, N; Irie, M. Jpn. J. Appl. Phys. 1994, 33, 1550-1553. (21) Tanio, N; Irie, M. Jpn. J. Appl. Phys. 1994, 33, 3942-3946. (22) Tachibana, H.; Azumi, R.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Chem. Lett. 1992, 173-176.
