Optothermal Switching of Cholesteric Liquid Crystals: A Study of Azobenzene Derivatives and Laser Wavelengths

The laser-initiated thermal (optothermal) switching of cholesteric liquid crystals (CLCs) is characterized by using different azobenzene (Azo) derivatives and laser wavelengths. Under 405-nm laser irradiation, Azo-doped CLCs undergo phase transition from cholesteric to isotropic. No cis-to-trans photoisomerization occurs when the 405-nm laser irradiation is blocked because only a single laser is used. The fast response of Azo-doped CLCs under the on–off switching of the 405-nm laser occurs because of the optothermal effect of the system. The 660-nm laser, which cannot be used as irradiation to generate the trans–cis photoisomerization of Azo, is used in Anthraquinone (AQ)-Azo-doped CLCs to examine the optothermal effect of doped Azo. The results show that the LC-like Azo derivative bearing two methyl groups ortho to the Azo moiety (A4) can greatly lower the clearing temperature and generate large amount of heat in AQ-A4-doped CLCs.

wavelength must be used to initiate the back photoisomerization. The optical properties of the Azo dye-doped LC, such as optical polarization, could be on-off switching under alternative UV and visible light irradiation. Our previous studies have proven that the use of a 405-nm laser could also initiate the photoisomerization of Azo-LCs (A1, BMAB), as shown in Figure 1a, even though the Azo-LC has weak absorption at a 405-nm wavelength. Without the use of the second light irradiation, the optical properties of Azo-LC-doped CLCs could be on-off switching under on-off exposure of a 405-nm laser (80 mW/cm 2 ). Figure 1b shows the POM image of A1-doped CLCs under 405-nm laser light exposure. The light-initiated bend-like structure of A1 (cis-A1) disrupts the CLCs and causes the LC cell to be transparent. When the laser light was switched off, the CLCs recovered, and a focal conic CLC phase was formed, as shown in Figure 1c. Figure 1d shows the transmittance change of A1-doped CLCs before and under 405-nm laser irradiation, and an 80% transmittance change could be achieved under on-off laser exposure.
Materials 2015, 8 3 must be used to initiate the back photoisomerization. The optical properties of the Azo dye-doped LC, such as optical polarization, could be on-off switching under alternative UV and visible light irradiation. Our previous studies have proven that the use of a 405-nm laser could also initiate the photoisomerization of Azo-LCs (A1, BMAB), as shown in Figure 1a, even though the Azo-LC has weak absorption at a 405-nm wavelength. Without the use of the second light irradiation, the optical properties of Azo-LC-doped CLCs could be on-off switching under on-off exposure of a 405-nm laser (80 mW/cm 2 ). Figure 1b shows the POM image of A1-doped CLCs under 405-nm laser light exposure. The light-initiated bend-like structure of A1 (cis-A1) disrupts the CLCs and causes the LC cell to be transparent. When the laser light was switched off, the CLCs recovered, and a focal conic CLC phase was formed, as shown in Figure 1c. Figure 1d shows the transmittance change of A1-doped CLCs before and under 405-nm laser irradiation, and an 80% transmittance change could be achieved under on-off laser exposure. The same experiments have been performed in A2, A3, and A4 compounds. The use of A2 involves behavior similar to that of A1, as shown in Figure 2. Under 405-nm laser irradiation, the generated cis-A2 changes the LC phase from cholesteric to isotropic. Unlike A1 with LC like, linear shape molecular structure, the ability of generating cis-form Azo from A2 (bulky and dentrilic structure) is lower than A1. The use of A3 involves behavior different from that involved in the use of A1 and A2. The same experiments have been performed in A2, A3, and A4 compounds. The use of A2 involves behavior similar to that of A1, as shown in Figure 2. Under 405-nm laser irradiation, the generated cis-A2 changes the LC phase from cholesteric to isotropic. Unlike A1 with LC like, linear shape molecular structure, the ability of generating cis-form Azo from A2 (bulky and dentrilic structure) is lower than A1.
The use of A3 involves behavior different from that involved in the use of A1 and A2. The absorption spectrum of A3 remains the same before and under 405-nm laser irradiation, as shown in Figure 3a.
The lack of C 4 H 9 chain prohibits the occurrence of trans-cis photoisomerization from A3 compound. Because the cis-form of A3 was not generated under 405-nm laser irradiation, the CLC phase of planner texture in the A3-doped CLC sample remains unchanged, as shown in Figure 3b,c. No focal conic texture was formed in A3-doped CLCs under 405-nm laser irradiation. The transmission spectrum of the sample changed slightly under 405-nm laser exposure, as shown in Figure 3d. Figure 4 shows the experimental results from the A4-doped CLC sample. Under 405-nm laser irradiation, the π´π˚band slightly decreases, whereas the π´π˚also slightly decreases. Compared with the effects resulting from the use of A1 and A2, the effect of photoisomerization under 405-nm laser exposure was very weak in the A4-doped CLC sample. However, a CLC phase of focal conic texture was formed in the A4-doped CLC sample after 405-nm laser exposure, as shown in Figure 4c. Because the 405-nm laser could not initiate the trans-cis photoisomerization of the A4 compound, the phase change between the focal conic and isotropic phases may be caused by the optothermal effect that the effect was verified by measuring the sample's temperature. The A4-doped CLC sample showed the highest temperature (38.9˝C) under 405-nm laser exposure.
Materials 2015, 8 4 The absorption spectrum of A3 remains the same before and under 405-nm laser irradiation, as shown in Figure 3a. The lack of C4H9 chain prohibits the occurrence of trans-cis photoisomerization from A3 compound. Because the cis-form of A3 was not generated under 405-nm laser irradiation, the CLC phase of planner texture in the A3-doped CLC sample remains unchanged, as shown in Figure 3b,c. No focal conic texture was formed in A3-doped CLCs under 405-nm laser irradiation. The transmission spectrum of the sample changed slightly under 405-nm laser exposure, as shown in Figure 3d. Figure 4 shows the experimental results from the A4-doped CLC sample. Under 405-nm laser irradiation, the    band slightly decreases, whereas the    also slightly decreases. Compared with the effects resulting from the use of A1 and A2, the effect of photoisomerization under 405-nm laser exposure was very weak in the A4-doped CLC sample. However, a CLC phase of focal conic texture was formed in the A4-doped CLC sample after 405-nm laser exposure, as shown in Figure 4c. Because the 405-nm laser could not initiate the trans-cis photoisomerization of the A4 compound, the phase change between the focal conic and isotropic phases may be caused by the optothermal effect that the effect was verified by measuring the sample's temperature. The A4-doped CLC sample showed the highest temperature (38.9 °C) under 405-nm laser exposure.       As shown in Figure 5, the transmittance dependent on the heating temperature was recorded to verify the optothermal effect from the Azo-doped CLCs used in this study. For example, regarding A1, before the temperature of the heating plate reaches 55˝C, the transmittance of the sample remains unchanged, indicating that the CT is higher than 55˝C. The A3 compound has a higher CT than A1 does because the transmittance increases when the temperature of the heating plate reaches 68˝C. When the CT is defined as the temperature required to cause the entire sample to be transparent, the CT-lowering ability is A4 (50˝C) > A2 (60˝C) > A1 (65˝C) > A3 (75˝C), where A4 has the lowest CT, which induces a stronger optothermal effect. Figure 6 shows the sample temperature recorded by the thermal imager (Fluke Ti30) before and under 405-nm laser exposure, and the A4-doped CLCs show that the highest temperature was 38.9˝C under laser exposure. The order of temperature induced by the Azo compound is A4 (7.4˝C) > A2 (5.2˝C) > A1 (4.3˝C) > A3 (2.4˝C). The photo-induced tuning mechanism of using single laser (405 nm) to carry out the switching effect contains two processes. Under 405 nm laser exposure, the trans-to-cis photoisomerization occurs and the cis-form Azo disrupts the CLC and change the LC phase from cholesteric to isotropic. Once the cis-form of Azo is generated, back cis-to-trans photoisomerization occurs only if another laser of longer wavelength is used. Because 405-nm wavelength laser cannot efficiently trigger the photoisomerization comparing to UV light, which is located in the peak absorption of Azo, most of the laser energy converts to thermal energy. The temperature of sample increases under laser exposure, as shown in Figure 6. Turning off 405-nm laser irradiation changes the LC phase from isotropic to focal conic texture due to the back thermal isomerization.

Materials 2015, 8 6
As shown in Figure 5, the transmittance dependent on the heating temperature was recorded to verify the optothermal effect from the Azo-doped CLCs used in this study. For example, regarding A1, before the temperature of the heating plate reaches 55 °C, the transmittance of the sample remains unchanged, indicating that the CT is higher than 55 °C. The A3 compound has a higher CT than A1 does because the transmittance increases when the temperature of the heating plate reaches 68 °C. When the CT is defined as the temperature required to cause the entire sample to be transparent, the CT-lowering ability is A4 (50 °C) > A2 (60 °C) > A1 (65 °C) > A3 (75 °C), where A4 has the lowest CT, which induces a stronger optothermal effect. Figure 6 shows the sample temperature recorded by the thermal imager (Fluke Ti30) before and under 405-nm laser exposure, and the A4-doped CLCs show that the highest temperature was 38.9 °C under laser exposure. The order of temperature induced by the Azo compound is A4 (7.4 °C) > A2 (5.2 °C) > A1 (4.3 °C) > A3 (2.4 °C). The photo-induced tuning mechanism of using single laser (405 nm) to carry out the switching effect contains two processes. Under 405 nm laser exposure, the trans-to-cis photoisomerization occurs and the cis-form Azo disrupts the CLC and change the LC phase from cholesteric to isotropic. Once the cis-form of Azo is generated, back cis-to-trans photoisomerization occurs only if another laser of longer wavelength is used. Because 405-nm wavelength laser cannot efficiently trigger the photoisomerization comparing to UV light, which is located in the peak absorption of Azo, most of the laser energy converts to thermal energy. The temperature of sample increases under laser exposure, as shown in Figure 6. Turning off 405-nm laser irradiation changes the LC phase from isotropic to focal conic texture due to the back thermal isomerization.   A 660-nm laser diode of 100 mW/cm 2 intensity and AQ dye were used to further verify the optothermal ability of A4 molecules in Azo-doped CLCs. AQ dye has been used to generate optothermal effects in AQ dye-doped CLCs under 660-nm laser exposure [27]. However, a large Kr + laser (647 nm) and high intensity of 250 mW/cm 2 were used to initiate the switching effect that limits the application of optothermal systems in dye-doped CLCs. Figure 7 shows the phase and transmittance change of CLCs containing both Azo (A4) and AQ dye. An isotropic and focal conic phase could be switched under on-off single 660-nm laser exposure, as shown in Figure 7a. Figure 7b shows the transmission spectrum of the sample before and under 660-nm laser exposure. Before laser exposure, a focal conic CLC texture scatters the light, and a nearly 0% transmittance was observed. Under laser exposure, the transmittance increases and reaches nearly 100% (wavelength higher than 800 nm). The transmission spectrum of the A 660-nm laser diode of 100 mW/cm 2 intensity and AQ dye were used to further verify the optothermal ability of A4 molecules in Azo-doped CLCs. AQ dye has been used to generate optothermal effects in AQ dye-doped CLCs under 660-nm laser exposure [27]. However, a large Kr + laser (647 nm) and high intensity of 250 mW/cm 2 were used to initiate the switching effect that limits the application of optothermal systems in dye-doped CLCs. Figure 7 shows the phase and transmittance change of CLCs containing both Azo (A4) and AQ dye. An isotropic and focal conic phase could be switched under on-off single 660-nm laser exposure, as shown in Figure 7a. Figure 7b shows the transmission spectrum of the sample before and under 660-nm laser exposure. Before laser exposure, a focal conic CLC texture scatters the light, and a nearly 0% transmittance was observed. Under laser exposure, the transmittance increases and reaches nearly 100% (wavelength higher than 800 nm). The transmission spectrum of the laser-on sample shows a deep notch between 550 nm and 750 nm, indicating the absorbance from the AQ dye. The control experiments were performed to verify the role of AQ and Azo dye (A4). Figure 8 shows the transmission spectra from the CLC sample, which separately contains only A4 (Figure 8a) and only AQ dye (Figure 8b). Both samples show no transmittance change under 660-nm laser exposure. The addition of AQ dye assists in the absorption of a 660-nm laser, whereas the addition of A4 decreases the CT that assists in lowering the irradiated laser intensity. Figure 9 shows the time-dependent transmittance change of the sample at 800 nm with and without laser irradiation. The transmittance could be switched between 0% and 100% at an 800-nm wavelength under on-off laser exposure, as shown in Figure 9a. The response time is separately 100 s and 10 s under on-off switching laser exposure, as shown in Figure 9b,c. laser-on sample shows a deep notch between 550 nm and 750 nm, indicating the absorbance from the AQ dye. The control experiments were performed to verify the role of AQ and Azo dye (A4). Figure 8 shows the transmission spectra from the CLC sample, which separately contains only A4 (Figure 8a) and only AQ dye (Figure 8b). Both samples show no transmittance change under 660-nm laser exposure. The addition of AQ dye assists in the absorption of a 660-nm laser, whereas the addition of A4 decreases the CT that assists in lowering the irradiated laser intensity. Figure 9 shows the time-dependent transmittance change of the sample at 800 nm with and without laser irradiation. The transmittance could be switched between 0% and 100% at an 800-nm wavelength under on-off laser exposure, as shown in Figure 9a. The response time is separately 100 s and 10 s under on-off switching laser exposure, as shown in Figure 9b,c.   laser-on sample shows a deep notch between 550 nm and 750 nm, indicating the absorbance from the AQ dye. The control experiments were performed to verify the role of AQ and Azo dye (A4). Figure 8 shows the transmission spectra from the CLC sample, which separately contains only A4 (Figure 8a) and only AQ dye (Figure 8b). Both samples show no transmittance change under 660-nm laser exposure. The addition of AQ dye assists in the absorption of a 660-nm laser, whereas the addition of A4 decreases the CT that assists in lowering the irradiated laser intensity. Figure 9 shows the time-dependent transmittance change of the sample at 800 nm with and without laser irradiation. The transmittance could be switched between 0% and 100% at an 800-nm wavelength under on-off laser exposure, as shown in Figure 9a. The response time is separately 100 s and 10 s under on-off switching laser exposure, as shown in Figure 9b,c.    Other Azo derivatives, A1, A2, and A3, have also been incorporated into the optothermal system of AQ-dye-doped CLCs, as shown in Figures 10-12. No obvious transmittance change was observed under 660-nm laser exposure from the sample containing A1 and A3 that had a higher CT, as shown in Figure 5, and low optothermal effect, as shown in Figure 6. For the sample containing A2, 60% transmittance was observed, and the results were consistent with the observations from Figures 5 and 6, which show that the CT of A2 is lower than those of A1 and A3, and the temperature change from the A2-doped sample is larger than those of the A1-and A3-doped samples.  Other Azo derivatives, A1, A2, and A3, have also been incorporated into the optothermal system of AQ-dye-doped CLCs, as shown in Figures 10-12. No obvious transmittance change was observed under 660-nm laser exposure from the sample containing A1 and A3 that had a higher CT, as shown in Figure 5, and low optothermal effect, as shown in Figure 6. For the sample containing A2, 60% transmittance was observed, and the results were consistent with the observations from Figures 5 and 6 which show that the CT of A2 is lower than those of A1 and A3, and the temperature change from the A2-doped sample is larger than those of the A1-and A3-doped samples. Other Azo derivatives, A1, A2, and A3, have also been incorporated into the optothermal system of AQ-dye-doped CLCs, as shown in Figures 10-12. No obvious transmittance change was observed under 660-nm laser exposure from the sample containing A1 and A3 that had a higher CT, as shown in Figure 5, and low optothermal effect, as shown in Figure 6. For the sample containing A2, 60% transmittance was observed, and the results were consistent with the observations from Figures 5 and 6, which show that the CT of A2 is lower than those of A1 and A3, and the temperature change from the A2-doped sample is larger than those of the A1-and A3-doped samples.

Experimental Section
The NLC (Merck E-series, MDA 3461, ne = 1.77, n0 = 1.51 at 589 nm) (Merck Taiwan, Taipei, Taiwan) was used as the LC host, and the CT of MDA 3461, which is defined as the temperature when the LC phase changes from nematic to isotropic, is 92 °C. The left-handed (ZLI 811) and right-handed (S 811) chiral molecule of fixed 20 wt% were dissolved in the NLC host to produce a cholesteric phase. The LC-like Azo (A1, A2, A3, and A4) were synthesized according to the reported paper [28]. The structure of the Azo derivatives used in this study are shown in Figure 13. Transmission-mode analysis was used to measure the CT of the CLC samples. The intensity of the probe beam (5 mW, 633 nm, He-Ne laser) (Edmund Optics Taiwan Branch, Taichung, Taiwan) passing normally to the samples was monitored by a power meter while the sample was heated by a transparent hot plate. The transmission spectra were recorded using a fiber-based UV-VIS spectrometer (Ocean Optics, HR4000HCG) (Dunedin, FL, USA). The phototunable properties of the Azo-doped CLC cells with or without the AQ dye were characterized by monitoring the optical transmittance change of the CLC mixture in the light "on" and "off" states. The switching behavior of the CLC cell was characterized by

Experimental Section
The NLC (Merck E-series, MDA 3461, ne = 1.77, n0 = 1.51 at 589 nm) (Merck Taiwan, Taipei, Taiwan) was used as the LC host, and the CT of MDA 3461, which is defined as the temperature when the LC phase changes from nematic to isotropic, is 92 °C. The left-handed (ZLI 811) and right-handed (S 811) chiral molecule of fixed 20 wt% were dissolved in the NLC host to produce a cholesteric phase. The LC-like Azo (A1, A2, A3, and A4) were synthesized according to the reported paper [28]. The structure of the Azo derivatives used in this study are shown in Figure 13. Transmission-mode analysis was used to measure the CT of the CLC samples. The intensity of the probe beam (5 mW, 633 nm, He-Ne laser) (Edmund Optics Taiwan Branch, Taichung, Taiwan) passing normally to the samples was monitored by a power meter while the sample was heated by a transparent hot plate. The transmission spectra were recorded using a fiber-based UV-VIS spectrometer (Ocean Optics, HR4000HCG) (Dunedin, FL, USA). The phototunable properties of the Azo-doped CLC cells with or without the AQ dye were characterized by monitoring the optical transmittance change of the CLC mixture in the light "on" and "off" states. The switching behavior of the CLC cell was characterized by

Experimental Section
The NLC (Merck E-series, MDA 3461, n e = 1.77, n 0 = 1.51 at 589 nm) (Merck Taiwan, Taipei, Taiwan) was used as the LC host, and the CT of MDA 3461, which is defined as the temperature when the LC phase changes from nematic to isotropic, is 92˝C. The left-handed (ZLI 811) and right-handed (S 811) chiral molecule of fixed 20 wt% were dissolved in the NLC host to produce a cholesteric phase. The LC-like Azo (A1, A2, A3, and A4) were synthesized according to the reported paper [28]. The structure of the Azo derivatives used in this study are shown in Figure 13. Transmission-mode analysis was used to measure the CT of the CLC samples. The intensity of the probe beam (5 mW, 633 nm, He-Ne laser) (Edmund Optics Taiwan Branch, Taichung, Taiwan) passing normally to the samples was monitored by a power meter while the sample was heated by a transparent hot plate. The transmission spectra were recorded using a fiber-based UV-VIS spectrometer (Ocean Optics, HR4000HCG) (Dunedin, FL, USA). The phototunable properties of the Azo-doped CLC cells with or without the AQ dye were characterized by monitoring the optical transmittance change of the CLC mixture in the light "on" and "off" states. The switching behavior of the CLC cell was characterized by recording variations of transmittance in the transmission spectra at a wavelength of 800 nm. The polarization optical microscopic (POM) images were recorded using a polarized microscope (Olympus IX 71, Olympus Taiwan Co., Ltd., Taichung, Taiwan) equipped with a CCD camera under the observation of crossed polarizers. A 405-nm wavelength laser diode (80 mW/cm 2 ) was used to characterize the PLC system using only Azo dye as light absorbing material. A 660-nm wavelength laser diode (100 mW/cm 2 ) was used to characterize the PLC system using AQ dye as light-absorbing material. The laser exposure time was 2 min. All PLC samples were prepared in 12-µm-thick sandwiched glass cells without orienting substrates. All measurements were performed at room temperature. g variations of transmittance in the transmission spectra at a wavelength of 800 nm tion optical microscopic (POM) images were recorded using a polarized microscope (O lympus Taiwan Co., Ltd., Taichung, Taiwan) equipped with a CCD camera under the obse ed polarizers. A 405-nm wavelength laser diode (80 mW/cm 2 ) was used to characterize th using only Azo dye as light absorbing material. A 660-nm wavelength laser diode (100 mW d to characterize the PLC system using AQ dye as light-absorbing material. The laser ex s 2 min. All PLC samples were prepared in 12-μm-thick sandwiched glass cells without or es. All measurements were performed at room temperature. lusions optothermal system of Azo-and AQ-doped CLCs was characterized, and the results sho of suitable Azo helps to decrease the CT of CLCs and further facilitates the phase chang adiation. For the A4-doped CLCs, no obvious phase change was observed under 405-n on. Because no trans-to-cis photoisomerization occurred in the sample under 405-nm e, the slight transmittance change was caused by the laser-induced thermal effect. The AQ ntaining A4 molecules further proves the strong optothermal effect of the A4 molecules. Th ture change and lower CT caused partially by the A4 molecules facilitates the phase ch sample containing both AQ and A4 molecules under 660-nm laser exposure. The demon mal system can be applied in LC-based, light-switchable micro-or nano-devices.

Conclusions
The optothermal system of Azo-and AQ-doped CLCs was characterized, and the results show that the use of suitable Azo helps to decrease the CT of CLCs and further facilitates the phase change under laser irradiation. For the A4-doped CLCs, no obvious phase change was observed under 405-nm laser irradiation. Because no trans-to-cis photoisomerization occurred in the sample under 405-nm laser exposure, the slight transmittance change was caused by the laser-induced thermal effect. The AQ-doped CLCs containing A4 molecules further proves the strong optothermal effect of the A4 molecules. The large temperature change and lower CT caused partially by the A4 molecules facilitates the phase change in the CLC sample containing both AQ and A4 molecules under 660-nm laser exposure. The demonstrated optothermal system can be applied in LC-based, light-switchable micro-or nano-devices.