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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 4 — Feb. 24, 2014
  • pp: 4404–4411
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Photo- and electro-isomerization of azobenzenes based on polymer-dispersed liquid crystals doped with azobenzenes and their applications

Yen-Chen Liu, Ko-Ting Cheng, Hsin-Fu Chen, and Andy Ying-Guey Fuh  »View Author Affiliations


Optics Express, Vol. 22, Issue 4, pp. 4404-4411 (2014)
http://dx.doi.org/10.1364/OE.22.004404


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Abstract

We report the photo-isomerization and electro-isomerization effects in azobenzenes-doped polymer-dispersed liquid crystals during the switching of the liquid crystal (LC) device between transparent (cis-isomers dominant) and scattering states (trans-isomers dominant). The isothermal phase transition, which is a result of the illumination of UV light and the application of DC voltage, was the main mechanism to switch the LC device between transparency, scattering, and gray scales. This study discusses in detail the variations in the population of cis-isomers as functions of the period and the amplitude of the applied DC voltage.

© 2014 Optical Society of America

1. Introduction

In recent decades, polymer-dispersed liquid crystals (PDLCs) have been extensively developed because of their high scattering, electrically and optically switchable properties, simple fabrication, and so on [1

1. J. W. Doane, N. A. Vaz, B. G. Wu, and S. Zumer, “Field controlled light scattering from nematic microdroplets,” Appl. Phys. Lett. 48(4), 269–271 (1986). [CrossRef]

5

5. P. S. Drzaic, Liquid Crystal Dispersions (World Scientific, 1995).

]. PDLCs possess high potential to fabricate many liquid crystal (LC) devices, such as light switches [6

6. H. Ren and S. T. Wu, “Reflective reversed-mode polymer stabilized cholesteric texture light switches,” J. Appl. Phys. 92(2), 797–800 (2002). [CrossRef]

], light shutters [7

7. D. K. Yang, L. C. Chien, and J. W. Doane, “Cholesteric liquid crystal/polymer dispersion for haze-free light shutters,” Appl. Phys. Lett. 60(25), 3102–3104 (1992). [CrossRef]

, 8

8. R. Bao, C. M. Liu, and D. K. Yang, “Smart bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express 2(11), 112401 (2009). [CrossRef]

], smart cards/windows [9

9. C. M. Lampert, “Large-area smart glass and integrated photovoltaics,” Sol. Energ. Mat. Sol. 76(4), 489–499 (2003). [CrossRef]

, 10

10. C. M. Lampert, “Smart switchable glazing for solar energy and daylight control,” Sol. Energ. Mat. Sol. 52(3-4), 207–221 (1998). [CrossRef]

], applications of photonic crystals [11

11. S. T. Wu, M. S. Li, and A. Y. G. Fuh, “Unusual refractions in photonic crystals based on polymer-dispersed liquid crystal films,” Appl. Phys. Lett. 91(25), 251117 (2007). [CrossRef]

, 12

12. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tomlin, and T. J. Bunning, “Holographic formation of electro-optical polymer-liquid crystal photonic crystals,” Adv. Mater. 14(3), 187–191 (2002). [CrossRef]

], and others. Generally, the structures of PDLCs are the micron-size LC droplets distributed in continuous polymer networks fabricated by phase separation processes [13

13. S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001).

, 14

14. A. Y. G. Fuh, C. C. Chen, C. K. Liu, and K. T. Cheng, “Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals,” Opt. Express 17(9), 7088–7094 (2009). [CrossRef] [PubMed]

]. To obtain high performance light shutters using PDLCs, the refraction index of the selected polymers (np) should conform to the ordinary refractive index (no) of the used LCs. Considering the scattering state (voltage off), the boundaries between the polymer networks and LC droplets will produce light scattering because of the refractive index mismatch. On the other hand, when the directors of the LC droplets (∆ε > 0) are aligned along the applied electric field, the transparent state can be generated because of the matching refractive index of polymer and LCs (no = np). However, in general, the polymer walls also produce strong surface anchoring effect, which increases the driving voltage. The operating voltage of a traditional PDLC scattering mode light shutter is extremely high [1

1. J. W. Doane, N. A. Vaz, B. G. Wu, and S. Zumer, “Field controlled light scattering from nematic microdroplets,” Appl. Phys. Lett. 48(4), 269–271 (1986). [CrossRef]

, 13

13. S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001).

, 14

14. A. Y. G. Fuh, C. C. Chen, C. K. Liu, and K. T. Cheng, “Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals,” Opt. Express 17(9), 7088–7094 (2009). [CrossRef] [PubMed]

]. Therefore, the high driving voltage, the low contrast ratio, the absence of bistability, and so on comprise the main disadvantages of PDLC devices. In particular, with regard to the large size of PDLC device, the concern of high energy consumption should be significantly reduced by multiple stabilizations or the reductions of the driving voltage [15

15. G. Z. Liu, D. L. Xia, W. J. Yang, and Z. Q. Huang, “The surface rubbing effect on morphologies of LC droplets and electro-optic properties of flexible PDLC films,” Sci. China, Ser. Biol. Chem. 52, 2329–2335 (2009).

, 16

16. K. J. Yang and D. Y. Yoon, “Electro-optical characteristics of dye-doped polymer dispersed liquid crystals,” J. Ind. Eng. Chem. 17(3), 543–548 (2011). [CrossRef]

]. Moreover, both long-term stabilization and low operation energy properties for the applications of PDLCs can be achieved in this study.

In addition, the well-known azobenzene liquid crystals (azo-LCs), which have key functions in this study, are used to enhance the performances of PDLCs. Briefly, azo-LCs, presenting the combined properties of azobenzene dyes, and nematic LCs, have two isomers of stably rod-like trans-isomers and unstably bent cis-isomers [17

17. L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, and C. Umeton, “Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching,” Adv. Mater. 22(21), 2316–2319 (2010). [CrossRef] [PubMed]

22

22. H. Hervet, W. Urbach, and F. Rondelez, “Mass diffusion measurements in liquid crystals by a novel optical method,” J. Chem. Phys. 68(6), 2725–2729 (1978). [CrossRef]

]. Both can be transferred between each other by light illumination with different wavelengths (photo-isomerization) [17

17. L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, and C. Umeton, “Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching,” Adv. Mater. 22(21), 2316–2319 (2010). [CrossRef] [PubMed]

20

20. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Photoinduced isotropic state of cholesteric liquid crystals: novel dynamic photonic materials,” Adv. Mater. 19(20), 3244–3247 (2007). [CrossRef]

]. Moreover, the unstable cis-isomers can spontaneously transfer back to stable trans-isomers through a process called dark relaxation. The energy level of cis-isomers is higher than that of trans-isomers. The external applied energy, including thermal treatment, light illumination, specifically electric field, and others can be used to stimulate the isomerization process. The comparison of trans- and the cis-azo-LCs showed that the former are much similar to the common nematic LCs because of their physical properties, including dielectric anisotropy, optical anisotropy, and elastic properties [23

23. F. M. Leslie, “Continuum theory for nematic liquid crystals,” Contin. Mech. Thermodyn. 4(3), 167–175 (1992). [CrossRef]

]. Moreover, according to the absorption spectra, photo-isomerization of the used azo-LCs in this study from trans- to cis-isomers can be initiated by UV light illumination [24

24. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Optical tuning of the reflection of cholesterics doped with azobenzene liquid crystals,” Adv. Funct. Mater. 17(11), 1735–1742 (2007). [CrossRef]

]. Then, the order parameter can be reduced because of the formation of bent cis-isomers in situ as well as the orientation of the entire LCs [18

18. N. Tabiryan, U. Hrozhyk, and S. Serak, “Nonlinear refraction in photoinduced isotropic state of liquid crystalline azobenzenes,” Phys. Rev. Lett. 93(11), 113901 (2004). [CrossRef] [PubMed]

20

20. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Photoinduced isotropic state of cholesteric liquid crystals: novel dynamic photonic materials,” Adv. Mater. 19(20), 3244–3247 (2007). [CrossRef]

]. Therefore, the isothermal phase transition of LCs in the whole LC cell will occur with the increase of the population of the cis-azo-LCs [25

25. A. Y. G. Fuh, Y. C. Liu, K. T. Cheng, C. K. Liu, and Y. D. Chen, “Isomerization-induced phase separation of a mixture of monomer, azobenzene, and liquid crystals,” Sci. Adv. Mater. 6(1), 37–42 (2014). [CrossRef]

]. By contrast, the unstable cis-azo-LCs can be recovered back to the stable trans-azo-LCs by illumination with green light. The transformation rate from cis- to trans-isomers is much higher than that of dark relaxation [26

26. Y. C. Liu, K. T. Cheng, Y. D. Chen, and A. Y. G. Fuh, “All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films,” Opt. Express 21(15), 18492–18500 (2013). [CrossRef] [PubMed]

].

Moreover, it was also proposed that the transformation from cis- to trans-isomers of azobenzenes can be sped up by electric field application [27

27. Z. F. Liu, K. Hashimoto, and A. Fujishima, “Photoelectrochemical information storage using an azobenzene derivative,” Nature 347(6294), 658–660 (1990). [CrossRef]

29

29. T. Enomoto, H. Hagiwara, D. A. Tryk, Z. F. Liu, K. Hashimoto, and A. Fujishima, “Electrostatically induced isomerization of azobenzene derivatives in langmuir-blodgett films,” J. Phys. Chem. B 101(38), 7422–7427 (1997). [CrossRef]

]. This effect is called electro-isomerization. Liu et al. first reported the electrochemical effect of the azobenzene materials and showed the three status systems to demonstrate the electrochemical effect in the DC voltage system [27

27. Z. F. Liu, K. Hashimoto, and A. Fujishima, “Photoelectrochemical information storage using an azobenzene derivative,” Nature 347(6294), 658–660 (1990). [CrossRef]

]. Tong et al. studied the fast electro-isomerization from cis- to trans-isomers of azobenzene materials and showed the chemical reaction equations during the DC application. The chain reaction of the electron and the unstable cis-isomers indicate the fast isomer transition by application of low DC voltages. Briefly, some parts of ions existing in the materials of azobenzene-doped LCs can diffuse forward and aggregate onto the indium-tin-oxide (ITO) surfaces when an electric field (DC field) is applied onto the cell. After that, the aggregate ions can generate an internal field, which is higher than the applied field, at the azobenzene solution and the electrode interface, indicating that the operating voltage for switching the cis- to trans-isomers can be reduced [28

28. X. Tong, M. Pelletier, A. Lasia, and Y. Zhao, “Fast cis-trans isomerization of an azobenzene derivative in liquids and liquid crystals under a low electric field,” Angew. Chem. Int. Ed. Engl. 47(19), 3596–3599 (2008). [CrossRef] [PubMed]

]. However, so far, no direct evidence has been proposed to explain that the interaction of the cis-isomers and the generated internal field results in such rapid isomerization processes from cis- to trans-isomers. Moreover, the polymer networks in such polymer-dispersed liquid crystals doped with azobenzenes provide the high scattering performance in this system.

2. Experiments

The nematic LCs used in the present study was MDA-00-3461 (ne = 1.7718, no = 1.5140, TC = 92 °C) and purchased from Merck. The monomer [1,6-hexanediol diacrylate (HDDA), nHDDA = 1.456] and azo-LC (1205; Δn ≈0.21, nematic phase from 8 °C to 59 °C) were purchased from Alfa-Aesar and BEAM Corp., respectively. Two non-rubbed ITO-coated glass substrates were combined to fabricate an empty cell, whose cell gap was 12 μm. The nematic LCs (MDA-00-3461), azo-LCs (1205), monomer (HDDA), and thermal initiator with a weight ratio of 58:22:18:2 were homogeneously filled into the empty cell. Notably, the azo-LCs account for 27.5% of the total LCs (nematic LCs and azo-LCs). The edges of the LC cell were sealed with epoxy. Finally, the LC cell was baked in an oven at 90 °C for 90 minutes for completing polymerization. After cooling, the fresh scattering PDLC cell was fabricated.

The absorption spectra of trans-azo-LCs (1205) show a strong maximum absorption near 350 nm [24

24. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Optical tuning of the reflection of cholesterics doped with azobenzene liquid crystals,” Adv. Funct. Mater. 17(11), 1735–1742 (2007). [CrossRef]

]; thus, UV derived from Ar+ laser (λ = 365 nm) was selected to achieve the photo-isomerization from trans- to cis-isomers in this study. Consequently, excitation by UV light resulted in the increase of cis-isomer population and the reduction of order parameter. Notably, the isothermal phase transition from nematic state to isotropic state can be initiated as the concentration of the cis-azo-LCs in the entire LC cell attains higher than the critical concentration, approximately 23%. Experimentally, the life time of cis-azo-LCs was confirmed to be longer than 10 hours [24

24. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Optical tuning of the reflection of cholesterics doped with azobenzene liquid crystals,” Adv. Funct. Mater. 17(11), 1735–1742 (2007). [CrossRef]

26

26. Y. C. Liu, K. T. Cheng, Y. D. Chen, and A. Y. G. Fuh, “All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films,” Opt. Express 21(15), 18492–18500 (2013). [CrossRef] [PubMed]

]. Moreover, as previously described, the cis- to trans-isomerization process can be sped up by illumination with blue-green light because of the red-shifted absorption spectrum of cis-azo-LCs [24

24. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Optical tuning of the reflection of cholesterics doped with azobenzene liquid crystals,” Adv. Funct. Mater. 17(11), 1735–1742 (2007). [CrossRef]

]. The electric field (DC voltage) was also demonstrated to speed up not only the isomerization from cis- to trans-isomers but also the isothermal phase transition from isotropic to LC states, resulting from the variation of the concentration of trans- and cis-isomers.

3. Results and discussion

Fig. 1 Dynamic variations in the transmittance of an azo-LCs-doped PDLC scattering cell with the duration of UV illumination under various UV intensities of (a) 46, (b) 39, (c) 25, (d) 18, and (e) 7 mW/cm2
Figure 1 shows the dynamic transmittance variations of a fresh scattering LC cell illuminated with different UV intensities. The probed beam was a He–Ne laser (λ = 632.8 nm), whose wavelength was not in the absorption band of the azo-LCs (1205). Initially, the fresh scattering PDLC cells presented extremely low transmittance. The higher intensities and longer duration of UV laser (Ar+ laser, λ = 365 nm) irradiation onto the cell resulted in the increase of transmittance and reduction of order parameter because of phase transition and photo-isomerization. Notably, the high transmittance, resulting from isothermal phase transition by UV laser illumination, should be stable for at least 10 hours (data not shown). In general, the transition from transparent state to scattering state can be sped up by green light illumination or thermal treatment [30

30. J. F. Rabek and G. W. Scott, Photochemistry and Photophysics (Taylor & Francis, 1989)

]. However, electro-isomerization effect was used in this study to accelerate the phase transition from isotropic state (cis-isomer, transparent state) to nematic state (trans-isomer, scattering state). Experimentally, both of the continuous DC voltages with different amplitudes and the periodically pulsed voltages were adopted to accelerate the switching from transparent to scattering states.

With regard to the switch from transparent (cis-isomer dominant) to scattering states (trans-isomer dominant), electro-isomerization was used to accelerate the isomerization processes. Additionally, the life time of the cis-isomers (dark relaxation) can also be influenced by the polymer networks [31

31. T. J. White, R. L. Bricker, L. V. Natarajan, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Polymer stabilization of phototunable cholesteric liquid crystals,” Soft Matter 5(19), 3623–3628 (2009). [CrossRef]

, 32

32. C. Wang and R. G. Weiss, “Thermal cis to trans isomerization of covalently attached azobenzene groups in undrawn and drawn polyethylene films. Characterization and comparisons of occupied sites,” Macromolecules 36(11), 3833–3840 (2003). [CrossRef]

]. White et al. reported that the polymer networks can obviously reduce the life time of photo-induced isotropic state (cis-isomers). The local interactions of azobenzene materials and the polymer networks speed up the restoration of cis-isomers. Regarding our experiments, shown in this paper, the polymer network also provides the capability to reduce the life time of the isotropic state (cis-isomers). Experimentally, the life time can be reduced from 12 to 10 hours. However, the life time reduced by the polymer stabilization in this system is not quick enough, comparing with the response of electro-isomerization effect. Restated, the mechanism of electro-isomerization dominates the properties of the switching. Theoretically, the electro-isomerization from cis- to trans-isomers can be initiated by applying a DC voltage but not by an AC voltage [27

27. Z. F. Liu, K. Hashimoto, and A. Fujishima, “Photoelectrochemical information storage using an azobenzene derivative,” Nature 347(6294), 658–660 (1990). [CrossRef]

29

29. T. Enomoto, H. Hagiwara, D. A. Tryk, Z. F. Liu, K. Hashimoto, and A. Fujishima, “Electrostatically induced isomerization of azobenzene derivatives in langmuir-blodgett films,” J. Phys. Chem. B 101(38), 7422–7427 (1997). [CrossRef]

].
Fig. 2 Dynamic variations in the transmittance of an azo-LCs-doped PDLC cell with the application of DC voltage (20 V). Insets show the observations of the LC cell (voltage off states) at (I) transparent state (centered circle, isotropic state with high population of cis-isomers) and (II) scattering state (nematic state with low population of cis-isomers) after the DC voltage was switched off.
Figure 2 shows the variations of dynamic transmittance of the initially transparent LC cell, prepared by irradiating with UV laser (46 mW/cm2) for 25 s [as the transparent spot shown in the inset (I) of Fig. 2], during the electro-isomerization by applying a DC voltage (20 V). The highly transmissive LC cell can be switched to low transmissive LC cell within a few tens of seconds. Compared with the dark relaxation process, the required time to transfer the isomers from cis- to trans-states using such an electro-isomerization technique was much shorter than the dark relaxation (10 hours). As shown in Fig. 2, the transfer time to stable state by electro-isomerization was 40 s. Notably, the residual transmittance resulted from the electrical orientation of LC molecules so that a little leakage of light would remain. However, when the applied DC voltage was switched off, the transmittance of the LC device suddenly decreased to extremely low value (dark-state) because of the mismatch of refractive indices of LCs and polymers [inset (II) of Fig. 2]..

Fig. 3 Time requirement of the azo-LCs-doped PDLC cell switching from transparent state to scattering state as a function of applied DC voltage. Each initially transparent state was obtained by illumination of UV laser (46 mW/cm2 for 25 s).
Figure 3 shows the required duration for completing electro-isomerization from cis- to trans-states by applying different amplitudes of continuous DC voltage. Clearly, the higher the amplitude of the applied DC voltage is, the shorter the duration is required. Therefore, the decreasing ratio of the cis-isomer population in the mixing LCs would correspond to the rising DC voltage amplitude. The cell thickness also determined the operation voltage of the electro-isomerization (including the threshold field that could start the isomerization effect) [28

28. X. Tong, M. Pelletier, A. Lasia, and Y. Zhao, “Fast cis-trans isomerization of an azobenzene derivative in liquids and liquid crystals under a low electric field,” Angew. Chem. Int. Ed. Engl. 47(19), 3596–3599 (2008). [CrossRef] [PubMed]

]. The threshold field in this experiment was approximately 8 V. In this system, the optimized amplitude of the applied DC voltage was approximately 15 V, which depended on the mixing ratio of nematic LCs and azo-LCs. Moreover, the required time (switching time) would not be shortened obviously when the applied DC voltage was higher than 15 V.

Considering the populations of trans- and cis-isomers, which can be tuned electrically, in the PDLCs doped with azobenzene LC cell, the variations in the population (%) of cis-isomers as a function of the period of DC voltage (15 V) application were calculated according to their absorption spectra, as shown in Fig. 4(a).
Fig. 4 Population variations of cis-isomers as a function of the period of applied DC voltage (15 V) and (b) stable populations of the cis-isomers versus the amplitudes of applied DC voltage.
Additionally, the stable populations (%) of cis-isomers, defined as the populations of cis-isomers when the electrically tuned transmittance reaches stability, versus the amplitudes of the applied DC voltage is also calculated [Fig. 4(b)]. Theoretically, the populations of trans- and cis-isomers depend on the optical absorbance [28

28. X. Tong, M. Pelletier, A. Lasia, and Y. Zhao, “Fast cis-trans isomerization of an azobenzene derivative in liquids and liquid crystals under a low electric field,” Angew. Chem. Int. Ed. Engl. 47(19), 3596–3599 (2008). [CrossRef] [PubMed]

]. The selected wavelength in the spectrum for calculating the populations of cis-isomers was green light (λ = 532 nm) so that the population can be obtained by mathematically conversing the optically measured absorption spectra. Notably, regarding the materials in cells, as shown in Experiments Section, the employed azo-LCs (1205) is the only component absorbing the optical energy at the wavelength of 532 nm. The difference between the energy absorbed by trans- and cis-isomers from their absorption spectra can be used to obtain the population of cis-isomers directly. In this paper, the 100% (maximum population) of cis-isomer population is defined as the conversion of the absorption spectrum of the used materials after the LC cell is illuminated with UV laser (46 mW/cm2 for 25 s), resulting in photo-isomerization. The termination of red-shifting in the absorption spectrum was used to confirm the saturation of cis-isomers generated by the illumination of UV laser. As previously mentioned, the high transparent state can be obtained at this state (cis-isomer dominant). Moreover, the 0% (minimum population) of cis-isomer population is defined as the conversion of the absorption spectrum of the used materials after the dark relaxation is completed. The trans-isomers should dominate the azobenzene materials at this state. Notably, the population of cis-isomers indicates the percentage of the cis-azo-LCs in the azo-LC (1205; azobenzene), including trans- and cis-isomers, rather than the cis-isomers in the total LCs (azo-LCs 1205 and MDA-00-3461).

Figure 4(a) shows the reduction of cis-isomer population during the application of DC voltage (15 V). Clearly, the reduction of the population of cis-isomers stopped at approximately 35% within 75 s. According to the calculation, the cis-isomer concentration accounted for approximately 10% of the total LCs in the cell [35% multiplied by 27.5% (concentration of azo-LCs in the total LCs) was approximately 10%]. Experimentally, the critical concentration of the cis-azo-LCs in the total LCs to initiate isothermal phase transition at room temperature (25 °C) was measured to be approximately 23%. The calculated cis-isomer concentration (10%) was low enough to obtain isothermal phase transition from isotropic to nematic states. As previously described, the phase transition from isotropic (cis-isomer dominant, transparency) to nematic (trans-isomer dominant, scattering) states can be achieved by applying a DC voltage (15 V). However, the isomers cannot be completely transferred from cis-isomers back to trans-isomers by applying 15 V of a DC voltage via electro-isomerization effect, but the transmittance can be switched between transparency and scattering. This finding is reasonable because the scattering state results from the mismatch of refractive indices of LCs and polymers. Therefore, if the isotropic (nematic) state, induced by high enough percentage of cis-isomers (trans-isomers), can be achieved, the transparent (scattering) state can be obtained, as shown in Figs. 1 and 2. Figure 4(b) shows the stable population (%) of cis-isomers versus the amplitudes of the applied DC voltage. Obviously, the cis-isomers are difficult to be transferred back to trans-isomers via electro-isomerization effect when the applied DC voltage was lower than 8 V, which is consistent with the results shown in Fig. 3. Moreover, according to the reduction of the stable population of cis-isomers, the electro-isomerization effect can be clearly enhanced by increasing the amplitude of the applied DC voltage. Again, the saturated population of cis-isomers transferred by 15 V of a DC voltage was approximately 35% of azo-LCs. This result indicates the incomplete transfer of azo-isomers to trans-isomers by DC voltage application via electro-isomerization effect [28

28. X. Tong, M. Pelletier, A. Lasia, and Y. Zhao, “Fast cis-trans isomerization of an azobenzene derivative in liquids and liquid crystals under a low electric field,” Angew. Chem. Int. Ed. Engl. 47(19), 3596–3599 (2008). [CrossRef] [PubMed]

].

Fig. 5 Variations in transmittance with the application of pulsed voltages (40 V, 2 s duration). Seven cycles are repeated. An initial high transmittance (isotropic state) was obtained by illumination of UV laser (46 mW/cm2 for 25 s).
The aforementioned experimental results indicated that the electro-isomerization effect can be used to accelerate the isomerization from cis- to trans-isomers. However, several points should be considered in this system as described below. First, during the processes of electro-isomerization effect, the transformation of cis- to trans-isomers, as well as phase transition from isotropic to nematic states, are spontaneously occurred. It indicates that the transmittance cannot be tuned to minimum without switching off the applied DC voltage because of the electrical orientation of LCs, as shown in Fig. 2. Moreover, the DC voltage application into LCs will produce the surface charge accumulation or the so-called ion-charge effect [33

33. H. Mada and K. Osajima, “Time response of a nematic liquid‐crystal cell in a switched dc electric field,” J. Appl. Phys. 60(9), 3111–3113 (1986). [CrossRef]

]. The disadvantage of holding voltage will reduce the performance of the LC devices. Accordingly, periodically pulsed voltage is much better for driving the LC devices that clearly reduce the surface charge accumulation. Besides, the LC devices applied with pulsed driving voltage can be switched between bright state, dark state, and gray scales by applying different numbers of pulsed voltages. Figure 5 shows the variations in transmittance with the applications of pulsed voltages. The initially high transmittance (isotropic state) was obtained by UV laser illumination that was defined as 100%. Experimentally, each application of pulsed voltages with amplitude of 40 V and pulsed width of 2 s could be used to achieve the corresponding stable transmittances (gray scales) of 81%, 69%, 56%, 43%, 32%, and 20%. The contrast ratio between the two switched states, transparent and scattering states, was measured to be approximately 120. Importantly, no significant thermal effect, which can induce isomerization from cis- to trans-isomers, was observed after the LC cell was illuminated (photo-isomerization) and applied with DC voltages (electro-isomerization) in these experiments.

4. Conclusion

In conclusion, an optically (photo-isomerization) and electrically (electro-isomerization) switchable PDLC light modulator was demonstrated with long-term stabilization. The isothermal phase transition by UV illumination was used to transfer scattering PDLC cell to the transparent state. The illumination of green light and the application of DC voltage can speed up the isothermal phase transition from isotropic (bright state) to nematic state (dark state). The variations in cis-isomer population in the LC cell as functions of the period and the amplitude of the applied DC voltage were discussed. The gray scales were also demonstrated by the application of pulsed DC voltages. To our knowledge, this study is the first to report that DC voltage application was adopted to control the phase transition processes in azobenzene-doped PDLCs, as well as to demonstrate a PDLC light modulator. Given the long life time of cis-azo-LCs, the LC device exhibited long-term stability at transparent state and gray scales and permanently stable at scattering state. The contrast ratio was approximately 120.

Acknowledgment

The authors would like to thank the National Science Council (NSC) of Taiwan for financially supporting this research under Grant Nos. NSC 101-2112-M-006-011-MY3 and NSC 102-2112-M-008-016. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center. Correspondences about this paper can be addressed to Prof. Ko-Ting Cheng at chengkt@dop.ncu.edu.tw or Prof. Andy Ying-Guey Fuh at andyfuh@mail.ncku.edu.tw.

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J. H. Liu and H. Y. Wang, “Optical switching behavior of polymer-dispersed liquid crystal composite films with various novel azobenzene derivatives,” J. Appl. Polym. Sci. 91(2), 789–799 (2004). [CrossRef]

4.

A. Y. G. Fuh, M. S. Tsai, L. J. Huang, and T. C. Liu, “Optically switchable gratings based on polymer-dispersed liquid crystal films doped with a guest-host dye,” Appl. Phys. Lett. 74(18), 2572–2575 (1999). [CrossRef]

5.

P. S. Drzaic, Liquid Crystal Dispersions (World Scientific, 1995).

6.

H. Ren and S. T. Wu, “Reflective reversed-mode polymer stabilized cholesteric texture light switches,” J. Appl. Phys. 92(2), 797–800 (2002). [CrossRef]

7.

D. K. Yang, L. C. Chien, and J. W. Doane, “Cholesteric liquid crystal/polymer dispersion for haze-free light shutters,” Appl. Phys. Lett. 60(25), 3102–3104 (1992). [CrossRef]

8.

R. Bao, C. M. Liu, and D. K. Yang, “Smart bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express 2(11), 112401 (2009). [CrossRef]

9.

C. M. Lampert, “Large-area smart glass and integrated photovoltaics,” Sol. Energ. Mat. Sol. 76(4), 489–499 (2003). [CrossRef]

10.

C. M. Lampert, “Smart switchable glazing for solar energy and daylight control,” Sol. Energ. Mat. Sol. 52(3-4), 207–221 (1998). [CrossRef]

11.

S. T. Wu, M. S. Li, and A. Y. G. Fuh, “Unusual refractions in photonic crystals based on polymer-dispersed liquid crystal films,” Appl. Phys. Lett. 91(25), 251117 (2007). [CrossRef]

12.

V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tomlin, and T. J. Bunning, “Holographic formation of electro-optical polymer-liquid crystal photonic crystals,” Adv. Mater. 14(3), 187–191 (2002). [CrossRef]

13.

S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001).

14.

A. Y. G. Fuh, C. C. Chen, C. K. Liu, and K. T. Cheng, “Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals,” Opt. Express 17(9), 7088–7094 (2009). [CrossRef] [PubMed]

15.

G. Z. Liu, D. L. Xia, W. J. Yang, and Z. Q. Huang, “The surface rubbing effect on morphologies of LC droplets and electro-optic properties of flexible PDLC films,” Sci. China, Ser. Biol. Chem. 52, 2329–2335 (2009).

16.

K. J. Yang and D. Y. Yoon, “Electro-optical characteristics of dye-doped polymer dispersed liquid crystals,” J. Ind. Eng. Chem. 17(3), 543–548 (2011). [CrossRef]

17.

L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, and C. Umeton, “Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching,” Adv. Mater. 22(21), 2316–2319 (2010). [CrossRef] [PubMed]

18.

N. Tabiryan, U. Hrozhyk, and S. Serak, “Nonlinear refraction in photoinduced isotropic state of liquid crystalline azobenzenes,” Phys. Rev. Lett. 93(11), 113901 (2004). [CrossRef] [PubMed]

19.

T. Ikeda, S. Horiuchi, D. B. Karanjit, S. Kurihara, and S. Tazuke, “Photochemically induced isothermal phase transition in polymer liquid crystals with mesogenic phenyl benzoate side chains. 2. Photochemically induced isothermal phase transition behaviors,” Macromolecules 23(1), 42–48 (1990). [CrossRef]

20.

U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Photoinduced isotropic state of cholesteric liquid crystals: novel dynamic photonic materials,” Adv. Mater. 19(20), 3244–3247 (2007). [CrossRef]

21.

W. M. Gibbons, P. J. Shannon, S. T. Sun, and B. J. Swetlin, “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351(6321), 49–50 (1991). [CrossRef]

22.

H. Hervet, W. Urbach, and F. Rondelez, “Mass diffusion measurements in liquid crystals by a novel optical method,” J. Chem. Phys. 68(6), 2725–2729 (1978). [CrossRef]

23.

F. M. Leslie, “Continuum theory for nematic liquid crystals,” Contin. Mech. Thermodyn. 4(3), 167–175 (1992). [CrossRef]

24.

U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Optical tuning of the reflection of cholesterics doped with azobenzene liquid crystals,” Adv. Funct. Mater. 17(11), 1735–1742 (2007). [CrossRef]

25.

A. Y. G. Fuh, Y. C. Liu, K. T. Cheng, C. K. Liu, and Y. D. Chen, “Isomerization-induced phase separation of a mixture of monomer, azobenzene, and liquid crystals,” Sci. Adv. Mater. 6(1), 37–42 (2014). [CrossRef]

26.

Y. C. Liu, K. T. Cheng, Y. D. Chen, and A. Y. G. Fuh, “All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films,” Opt. Express 21(15), 18492–18500 (2013). [CrossRef] [PubMed]

27.

Z. F. Liu, K. Hashimoto, and A. Fujishima, “Photoelectrochemical information storage using an azobenzene derivative,” Nature 347(6294), 658–660 (1990). [CrossRef]

28.

X. Tong, M. Pelletier, A. Lasia, and Y. Zhao, “Fast cis-trans isomerization of an azobenzene derivative in liquids and liquid crystals under a low electric field,” Angew. Chem. Int. Ed. Engl. 47(19), 3596–3599 (2008). [CrossRef] [PubMed]

29.

T. Enomoto, H. Hagiwara, D. A. Tryk, Z. F. Liu, K. Hashimoto, and A. Fujishima, “Electrostatically induced isomerization of azobenzene derivatives in langmuir-blodgett films,” J. Phys. Chem. B 101(38), 7422–7427 (1997). [CrossRef]

30.

J. F. Rabek and G. W. Scott, Photochemistry and Photophysics (Taylor & Francis, 1989)

31.

T. J. White, R. L. Bricker, L. V. Natarajan, S. V. Serak, N. V. Tabiryan, and T. J. Bunning, “Polymer stabilization of phototunable cholesteric liquid crystals,” Soft Matter 5(19), 3623–3628 (2009). [CrossRef]

32.

C. Wang and R. G. Weiss, “Thermal cis to trans isomerization of covalently attached azobenzene groups in undrawn and drawn polyethylene films. Characterization and comparisons of occupied sites,” Macromolecules 36(11), 3833–3840 (2003). [CrossRef]

33.

H. Mada and K. Osajima, “Time response of a nematic liquid‐crystal cell in a switched dc electric field,” J. Appl. Phys. 60(9), 3111–3113 (1986). [CrossRef]

OCIS Codes
(160.3710) Materials : Liquid crystals
(160.5470) Materials : Polymers
(230.0230) Optical devices : Optical devices

ToC Category:
Optical Devices

History
Original Manuscript: November 26, 2013
Revised Manuscript: January 29, 2014
Manuscript Accepted: January 31, 2014
Published: February 19, 2014

Citation
Yen-Chen Liu, Ko-Ting Cheng, Hsin-Fu Chen, and Andy Ying-Guey Fuh, "Photo- and electro-isomerization of azobenzenes based on polymer-dispersed liquid crystals doped with azobenzenes and their applications," Opt. Express 22, 4404-4411 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-4-4404


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References

  1. J. W. Doane, N. A. Vaz, B. G. Wu, S. Zumer, “Field controlled light scattering from nematic microdroplets,” Appl. Phys. Lett. 48(4), 269–271 (1986). [CrossRef]
  2. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, T. J. Bunning, W. W. Adams, “Volume holographic image storage and electro-optical readout in a polymer-dispersed liquid-crystal film,” Opt. Lett. 20(11), 1325–1327 (1995). [CrossRef] [PubMed]
  3. J. H. Liu, H. Y. Wang, “Optical switching behavior of polymer-dispersed liquid crystal composite films with various novel azobenzene derivatives,” J. Appl. Polym. Sci. 91(2), 789–799 (2004). [CrossRef]
  4. A. Y. G. Fuh, M. S. Tsai, L. J. Huang, T. C. Liu, “Optically switchable gratings based on polymer-dispersed liquid crystal films doped with a guest-host dye,” Appl. Phys. Lett. 74(18), 2572–2575 (1999). [CrossRef]
  5. P. S. Drzaic, Liquid Crystal Dispersions (World Scientific, 1995).
  6. H. Ren, S. T. Wu, “Reflective reversed-mode polymer stabilized cholesteric texture light switches,” J. Appl. Phys. 92(2), 797–800 (2002). [CrossRef]
  7. D. K. Yang, L. C. Chien, J. W. Doane, “Cholesteric liquid crystal/polymer dispersion for haze-free light shutters,” Appl. Phys. Lett. 60(25), 3102–3104 (1992). [CrossRef]
  8. R. Bao, C. M. Liu, D. K. Yang, “Smart bistable polymer stabilized cholesteric texture light shutter,” Appl. Phys. Express 2(11), 112401 (2009). [CrossRef]
  9. C. M. Lampert, “Large-area smart glass and integrated photovoltaics,” Sol. Energ. Mat. Sol. 76(4), 489–499 (2003). [CrossRef]
  10. C. M. Lampert, “Smart switchable glazing for solar energy and daylight control,” Sol. Energ. Mat. Sol. 52(3-4), 207–221 (1998). [CrossRef]
  11. S. T. Wu, M. S. Li, A. Y. G. Fuh, “Unusual refractions in photonic crystals based on polymer-dispersed liquid crystal films,” Appl. Phys. Lett. 91(25), 251117 (2007). [CrossRef]
  12. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tomlin, T. J. Bunning, “Holographic formation of electro-optical polymer-liquid crystal photonic crystals,” Adv. Mater. 14(3), 187–191 (2002). [CrossRef]
  13. S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays (Wiley, 2001).
  14. A. Y. G. Fuh, C. C. Chen, C. K. Liu, K. T. Cheng, “Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals,” Opt. Express 17(9), 7088–7094 (2009). [CrossRef] [PubMed]
  15. G. Z. Liu, D. L. Xia, W. J. Yang, Z. Q. Huang, “The surface rubbing effect on morphologies of LC droplets and electro-optic properties of flexible PDLC films,” Sci. China, Ser. Biol. Chem. 52, 2329–2335 (2009).
  16. K. J. Yang, D. Y. Yoon, “Electro-optical characteristics of dye-doped polymer dispersed liquid crystals,” J. Ind. Eng. Chem. 17(3), 543–548 (2011). [CrossRef]
  17. L. De Sio, S. Serak, N. Tabiryan, S. Ferjani, A. Veltri, C. Umeton, “Composite holographic gratings containing light-responsive liquid crystals for visible bichromatic switching,” Adv. Mater. 22(21), 2316–2319 (2010). [CrossRef] [PubMed]
  18. N. Tabiryan, U. Hrozhyk, S. Serak, “Nonlinear refraction in photoinduced isotropic state of liquid crystalline azobenzenes,” Phys. Rev. Lett. 93(11), 113901 (2004). [CrossRef] [PubMed]
  19. T. Ikeda, S. Horiuchi, D. B. Karanjit, S. Kurihara, S. Tazuke, “Photochemically induced isothermal phase transition in polymer liquid crystals with mesogenic phenyl benzoate side chains. 2. Photochemically induced isothermal phase transition behaviors,” Macromolecules 23(1), 42–48 (1990). [CrossRef]
  20. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, T. J. Bunning, “Photoinduced isotropic state of cholesteric liquid crystals: novel dynamic photonic materials,” Adv. Mater. 19(20), 3244–3247 (2007). [CrossRef]
  21. W. M. Gibbons, P. J. Shannon, S. T. Sun, B. J. Swetlin, “Surface-mediated alignment of nematic liquid crystals with polarized laser light,” Nature 351(6321), 49–50 (1991). [CrossRef]
  22. H. Hervet, W. Urbach, F. Rondelez, “Mass diffusion measurements in liquid crystals by a novel optical method,” J. Chem. Phys. 68(6), 2725–2729 (1978). [CrossRef]
  23. F. M. Leslie, “Continuum theory for nematic liquid crystals,” Contin. Mech. Thermodyn. 4(3), 167–175 (1992). [CrossRef]
  24. U. A. Hrozhyk, S. V. Serak, N. V. Tabiryan, T. J. Bunning, “Optical tuning of the reflection of cholesterics doped with azobenzene liquid crystals,” Adv. Funct. Mater. 17(11), 1735–1742 (2007). [CrossRef]
  25. A. Y. G. Fuh, Y. C. Liu, K. T. Cheng, C. K. Liu, Y. D. Chen, “Isomerization-induced phase separation of a mixture of monomer, azobenzene, and liquid crystals,” Sci. Adv. Mater. 6(1), 37–42 (2014). [CrossRef]
  26. Y. C. Liu, K. T. Cheng, Y. D. Chen, A. Y. G. Fuh, “All-optically controllable and highly efficient scattering mode light modulator based on azobenzene liquid crystals and poly(N-vinylcarbazole) films,” Opt. Express 21(15), 18492–18500 (2013). [CrossRef] [PubMed]
  27. Z. F. Liu, K. Hashimoto, A. Fujishima, “Photoelectrochemical information storage using an azobenzene derivative,” Nature 347(6294), 658–660 (1990). [CrossRef]
  28. X. Tong, M. Pelletier, A. Lasia, Y. Zhao, “Fast cis-trans isomerization of an azobenzene derivative in liquids and liquid crystals under a low electric field,” Angew. Chem. Int. Ed. Engl. 47(19), 3596–3599 (2008). [CrossRef] [PubMed]
  29. T. Enomoto, H. Hagiwara, D. A. Tryk, Z. F. Liu, K. Hashimoto, A. Fujishima, “Electrostatically induced isomerization of azobenzene derivatives in langmuir-blodgett films,” J. Phys. Chem. B 101(38), 7422–7427 (1997). [CrossRef]
  30. J. F. Rabek and G. W. Scott, Photochemistry and Photophysics (Taylor & Francis, 1989)
  31. T. J. White, R. L. Bricker, L. V. Natarajan, S. V. Serak, N. V. Tabiryan, T. J. Bunning, “Polymer stabilization of phototunable cholesteric liquid crystals,” Soft Matter 5(19), 3623–3628 (2009). [CrossRef]
  32. C. Wang, R. G. Weiss, “Thermal cis to trans isomerization of covalently attached azobenzene groups in undrawn and drawn polyethylene films. Characterization and comparisons of occupied sites,” Macromolecules 36(11), 3833–3840 (2003). [CrossRef]
  33. H. Mada, K. Osajima, “Time response of a nematic liquid‐crystal cell in a switched dc electric field,” J. Appl. Phys. 60(9), 3111–3113 (1986). [CrossRef]

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