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Photo-Reversible Liquid Crystal Alignment using Azobenzene-Based Self-Assembled Monolayers: Comparison of the Bare Monolayer and Liquid Crystal Reorientation Dynamics Guanjiu Fang, Yue Shi, Joseph E. Maclennan, and Noel A. Clark* Department of Physics and the Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States
Matthew J. Farrow and David M. Walba Department of Chemistry and Biochemistry and the Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States Received July 12, 2010. Revised Manuscript Received September 20, 2010 Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.
Introduction Photoalignment is a technologically appealing, noncontact approach for achieving liquid crystal (LC) alignment that avoids the problems of debris, static charges, and contamination typically associated with rubbing spin-coated surfaces, and can reduce the complexity of fabricating multidomain alignment substrates used to improve the viewing angle of LC displays.1 Although rubbed polymer surfaces cause alignment by mechanically induced grooves and partially oriented polymer chains, photobuffed surfaces align LC molecules via light-induced orientational ordering of the alignment surface. This can be achieved through the mechanisms of photodecomposition,2,3 photodimerization,1,4,5
and photoisomerization,6-23 only the last of which is reversible and offers the possibility of dynamic realignment of LC cells. Azobenzene and its derivatives are photoisomerizable materials that undergo a reversible transformation between trans and cis isomers in the presence of light. The trans form can be converted into cis by illuminating with light, and cis to trans isomerization can be induced either thermally or optically. Under the irradiation of linearly polarized light (LPL) of appropriate wavelength, photoisomerization is an angle-dependent process, resulting eventually in the photoselection of azobenzene dye orientations that are preferentially perpendicular to the polarization of the actinic light24 and with orientational anisotropy capable of aligning LCs.6-23
*To whom correspondence should be addressed. E-mail: Noel.Clark@ Colorado.edu.
(13) Corvazier, L.; Zhao, Y. Macromolecules 1999, 32, 3195. (14) Park, B.; Jung, Y.; Choi, H.-H.; Hwang, H.-K.; Kim, Y.; Lee, S.; Jang, S.-H.; Kakimoto, M.-A.; Takezoe, H. Jpn. J. Appl. Phys.Part1 1998, 37, 5663. (15) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214. (16) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007. (17) Kamezaki, H.; Kawanishi, Y.; Ichimura, K. Jpn. J. Appl. Phys. 1995, 34, 1550. (18) Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ikeda, T.; Ishizuki, N. Appl. Phys. Lett. 1993, 63, 449. (19) Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ishizuki, N. Langmuir 1993, 9, 3298. (20) Ichimura, K.; Akiyama, H.; Kudo, K.; Ishizuki, N.; Yamamura, S. Liq. Cryst. 1996, 20, 423. (21) Yi, Y. W.; Furtak, T. E.; Farrow, M. J.; Walba, D. M. J. Vac. Sci. Technol., A 2003, 21, 1770. (22) Yi, Y. W.; Furtak, T. E. Appl. Phys. Lett. 2004, 85, 4287. (23) Yi, Y. W.; Farrow, M. J.; Korblova, E.; Walba, D. M.; Furtak, T. E. Langmuir 2009, 25, 997. (24) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309.
(1) Schadt, M.; Seiberle, H.; Schuster, A. Nature 1996, 381, 212. (2) Nishikawa, M.; Taheri, B.; West, J. L. Appl. Phys. Lett. 1998, 72, 2403. (3) Matsuie, N.; Ouchi, Y.; Oji, H.; Ito, E.; Ishii, H.; Seki, K.; Hasegawa, M.; Zharnikov, M. Jpn. J. Appl. Phys. 2003, 42, L67. (4) Schadt, M.; Schmitt, K.; Kozinkov, V.; Chigrinov, V. Jpn. J. Appl. Phys. 1992, 31, 2155. (5) Yamaguchi, R.; Sato, A.; Sato, S. Jpn. J. Appl. Phys. 1998, 37, L336. (6) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R. I.; Ichimura, K. Langmuir 1993, 9, 211. (7) Sch€onhoff, M.; Mertesdorf, M.; L€osche, M. J. Phys. Chem. 1996, 100, 7558. (8) Ignes-Mullol, J.; Claret, J.; Albalat, R.; Crusats, J.; Reigada, R.; Romero, M. T. M.; Sagues, F. Langmuir 2005, 21, 2948. (9) Park, M.-K.; Advincula, R. C. Langmuir 2002, 18, 4532. (10) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature 1991, 351, 49. (11) Iimura, Y.; Kusano, J.-I.; Kobayashi, S.; Aoyagi, Y.; Sugano, T. Jpn. J. Appl. Phys. 1993, 32, L93. (12) Palffy-Muhoray, P.; Kosa, T.; Weinan, E. Mol. Cryst. Liq. Cryst. 2002, 375, 577.
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Figure 2. Sealed azo-SAM cell, with the active surfaces (shown in red) bathed in an argon atmosphere.
Figure 1. Chemical structure of dMR, the precursor used to prepare self-assembled monolayers. dMR is 2-(4-dimethylamino-phenylazo)N-(3-triethoxysilane-propyl)-benzamide.
Azobenzene dye surfaces previously ordered in this way can be orientationally randomized either by illumination with circularly polarized light (CPL) or through thermal relaxation. In addition to liquid crystal alignment,6-23 optical control of the molecular orientation enables many applications such as optical data storage,24 surface relief gratings,25 and photoinduced motion.26 In azobenzene systems, both the ratio of trans to cis isomer populations and the orientational distribution of the trans isomers change during illumination of the sample with linearly polarized light, effects which can be monitored by measuring the absorption and birefringence of the photoactive surfaces. Azobenzene-based coatings for LC alignment have been prepared previously by depositing Langmuir-Blodgett (LB) films,6-8 using layer-by-layer deposition techniques,9 spin-coating dye-doped or dye-functionalized polymers,10-14 and by forming covalently bonded self-assembled monolayers (SAMs) on glass substrates.15-23 Light-induced orientational order can be stabilized to some degree by the polymer matrix in spin-coated polymers,11,13,14 in which photoisomerization may be suppressed by the interactions between the chromophores and the matrix.7,27-29 SAMs used as active layers that can dynamically change the LC alignment are less constrained by such interactions. Azobenzene-based SAMs (azo-SAMs), in particular, represent an ideal system for studying photoselection via photoisomerization under illumination with LPL and for probing the interaction between the azobenzene and mesogens that leads to alignment of the LC director in cells. In 1988, Ichimura reported an azo-SAM that aligns LCs either homeotropic or planar depending on the wavelength of the exciting light.15-17 In his system, the long axis of trans molecules orients perpendicular to the substrate in the absence of exciting light, yielding homeotropic LC alignment, but when irradiated with UV light at 365 nm, some of the trans isomers transform to cis and the LCs then align parallel to the substrate (planar alignment). If subsequently illuminated with visible light at 440 nm, the LC alignment can be changed back to homeotropic. The alignment does not depend on the polarization of the exciting light in Ichimura’s system but rather on its wavelength, with the trans and cis isomers being absorptive in the UV and visible light, respectively. Azo-SAMs have, however, also been designed to give planar alignment in the absence of light by attaching the photochromic group perpendicular to the main chain (side-on type attachment),18-23 a geometry which allows the preferred LC alignment direction in the surface plane to be controlled by varying the polarization direction of the exciting light. The methyl red-based SAM precursor, dMR, shown in Figure 1, bonds covalently to glass to form side-on (25) (26) (27) (28) (29)
Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136. Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624. Paik, C. S.; Morawetz, H. Macromolecules 1972, 5, 171. Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558. Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403.
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attachment azo-SAMs that are excellent planar alignment layers for nematic liquid crystals, which enable photocontrol of the azimuthal orientation of the planar alignment with polarized light.22,23 In this application, dMR monolayers provide reversible LC alignment with the highest measured photosensitivity, azimuthally reorienting the LC when illuminated with 450 nm LPL with a fluence of only 5.5 mJ/cm2 (∼1.3 absorbed photons/ molecule).23 In this paper, we make the first detailed comparison of the reorientation dynamics of a bare dMR-SAM monolayer with the reorientation dynamics induced in nematic LC contacting the SAM surface. The experiments are made possible by a highly sensitive polarimeter that probes the orientational order of the bare SAM by measuring in-plane SAM birefringence as small as Δn ∼ 0.001. We compare both the short-time transient response dynamics and the long-term stability of the bare SAM alignment with the induced LC alignment in cells. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: although a SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC can maintain the director distortion in a twist cell permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We propose that the short-time LC reorientation is limited by the SAM anchoring strength and the LC viscosity.
Experiment 1. Azo-SAM Preparation. The methyl red derivative shown in Figure 1 is synthesized through a 1,3-dicyclohexylcarbodiimide (4.77 g) coupling between methyl red (5.75 g) and 3-aminopropyltriethoxysilane (4.73 g) in solvent of dichloromethane (60 mL). The dichloromethane is distilled from CaH. The reaction is carried out overnight, stirring under argon. The resulting solution is filtered and concentrated to form a dark red oil. Dark red crystals are obtained by purifying this oil using silica gel column chromatography (50% ethyl acetate, 50% hexane).21 The preparation of the self-assembled monolayers is carried out as follows: 272 mg of dMR is dissolved in 70 mL of toluene with 0.25 mL of n-butylamine as a catalyst. The resulting solution is heated to about 45 C while being sonicated to form a homeogeneous solution. A clean, dry glass substrate is put into the solution, which is further sonicated to prevent molecules physically attaching to the glass. After 90 min, the substrate is taken out of solution and rinsed with toluene until the toluene does not absorb any more color from the glass substrate. Finally the substrate is blown dry with pure nitrogen and cured at 115 C in vacuum for 1 h. How well the substrates are cleaned beforehand is critical in determining the quality of the azo-SAMs. We first cleaned the glass slides in piranha solution (1:1 concentrated H2SO4/30% H2O2), soaking them for 1 h. They were then sonicated and rinsed several times in distilled or deionized water and then blown dry with pure nitrogen gas. The resulting substrates are extremely hydrophilic, with hydrolyzed surfaces that form covalent bonds with the triethoxysilane group in dMR. Yi et al. have characterized the basic properties of formed dMRSAMs on glass.23 AFM measurements reveal very uniform and smooth monolayers, with the dimethylaminoazobenzene chain DOI: 10.1021/la102788j
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Figure 3. Liquid crystal alignment by azo-SAMs. Schematic illustration of liquid crystal alignment by azo-SAMs: chromophores, initially )
with random orientational distribution (a), orient perpendicular to the polarization direction P of the exciting light shown in blue during photobuffing (b), and liquid crystal molecules oriented approximately parallel to the chromophores (c). The optical textures of the liquid crystal cell viewed between crossed polarizers: with a uniform state (d) when actinic light polarization P^R and a twisted state (e) when P R. This cell, composed of a rubbed nylon surface with a rubbing direction (R) and an azo-SAM surface, is 5 μm thick, and is filled with E31. The exciting light for this experiment is UV radiation centered at 365 nm.
tilted on average 55 from the glass normal as determined from SHG measurements. 2. Azo-SAM Cells. Azo-SAMs degrade if they are exposed to air and light concurrently.22 We therefore constructed sealed cells from pairs of SAM-coated substrates (see Figure 2), with the azo-SAMs on the inner glass surfaces bathed in an inert argon atmosphere. Epoxy was used to seal the periphery, and the azoSAM monolayers on the outer glass surfaces then removed using piranha solution. Such cells were employed to measure the reorientation dynamics of bare monolayers using the precision polarimeter described below. 3. Planar LC Cells. To study the planar alignment of LCs on dMR-SAMs under conditions where SAM anchoring in the presence of surface torques arising from twist of the LC director could be probed, test cells were made by sandwiching the LC Merck E31 (nematic-to-isotropic transition temperature TNI = 61.5 C) between a rubbed nylon substrate and an azo-SAM. The nylon surface was prepared by spin coating a nylon solution (0.5 wt % nylon in methanol) for 30 s at 3000 rpm. The cell was filled in the isotropic phase through the capillary effect. Both substrates align LCs parallel to the glass, that is, promote planar alignment. The alignment properties were characterized in a polarized light microscope. 4. Hybrid LC Cells. Hybrid LC cells 5 μm thick were made by combining two substrates coated respectively with octadecyltriethoxysilane (OTE)30 and a dMR-SAM and filling the gap with the nematic LC 5CB (Cr-24C-N-35C-Iso). The alignment on the OTE is homeotropic, a boundary condition that enables the measurement of the LC reorientation in response to changes in the active azo-SAM surface in the absence of spurious (30) Walba, D. M.; Liberko, C. A.; Korblova, E.; Farrow, M.; Furtak, T. E.; Chow, B. C.; Schwartz, D. K.; Freeman, A. S.; Douglas, K.; Williams, S. D.; Klittnick, A. F.; Clark, N. A. Liq. Cryst. 2004, 31, 481.
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Figure 4. Thermal relaxation at room temperature of an azo-SAM initially optically aligned using LPL with different writing times (shown in the legend). The writing light intensity is 50 mW/cm2 at 514.5 nm, and the initial molecular orientation is at θ = þ45, maximizing the transmission of the 632.8 nm probe beam at t = 0.
azimuthal anchoring effects from the passive substrate. The reorientation dynamics of this cell were measured using the polarimeter described below. 5. Photobuffing and Optically Probing the Cells. A high extinction polarimeter has been designed to measure the tiny birefringence changes in the SAMs induced by exposure to polarized light.31 The polarimeter uses a 0.7 mW helium-neon (31) Fang, G..; Maclennan, J. E.; Clark, N. A. Langmuir 2010, 26, 11686.
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Figure 5. Room temperature textures in a rubbed nylon/azo-SAM twist cell filled with nematic LC observed in polarized light after heating the sample above the clearing point for different periods: (a) original photoaligned twist cell at 25 C; (b) after 30 min at 125 C; (c) after 60 min at 125 C; (d) after 90 min at 125 C; (e) the same cell after a further 19 h at 25 C. The black loop in (a) is an inversion wall separating leftand right-handed twist domains. The cell is 5 μm thick and filled with the liquid crystal E31. The initial photobuffing polarization P (with λ = 488 nm light: shown in blue) was parallel to the nylon rubbing direction R. The initial orientation of the liquid crystal director near the nylon and azo-SAM surfaces is indicated in green. laser focused to a 40 μm diameter spot on the sample as the probe beam and has an extinction ratio of 2.4 10-10. For photowriting the SAM or SAM/LC cells, two 514 nm pump beams, one of which is linearly polarized and the other circularly or linearly polarized, are incident on the cell with an angle of 10 with respect to the probe beam. The pump beams can be turned on or off individually with 40 μs switching time by applying square waveforms to ferroelectric liquid crystal optical shutters. The pump beams are 1.67 mm in diameter, which is much wider than the probe beam, so that the probed region is essentially uniformly illuminated. For an anisotropic organic monolayer illuminated with visible light, the retardance is very small and the transmission T is well approximated by31 ( " #)2 πΔnd 4nm 2 ð1Þ T = sin 2θ λ ðng þ1Þ2
)
where a principal optic axis is along the mean in-plane molecular orientation of the monolayer (n), θ is the angle between n and the polarization of the probe light, d is the film thickness, ng is the index of refraction of the glass substrate, and nm = (n þ n^)/2 is the mean refractive index of the film. For a monolayer with anisotropy typical of a liquid crystal (nm =1.6, ng =1.5, and Δn= 0.13) we have (4nm/(ng þ 1)2) ≈ 1, and with θ = 45 to get the maximum transmission T = (πΔnd/λ)2.
Results and Discussion 1. Reversibility of Liquid Crystal Alignment on AzoSAMs. When a disordered azo-SAM substrate (Figure 3a) is irradiated with LPL, the SAM molecules reorient due to photoisomerization (Figure 3b), the final azimuthal orientation distribution being peaked around a direction perpendicular to the polarization of the exciting light, the result of orientational hole (32) Pedersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam, P. S.; Hvilsted, S. J. Opt. Soc. Am., B 1998, 15, 1120.
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burning in the direction parallel to the light polarization.32 LC molecules in contact with the aligned SAM surface orient preferentially parallel to the chromophores (Figure 3c). A planaraligned nematic LC cell prepared as described above is used to investigate the reversibility of the LC alignment on dMR-SAMs. When the polarization of the writing beam is set perpendicular to the rubbing direction of the nylon-coated surface of the cell, the LC director field in the cell is uniform and we obtain a dark state under crossed polarizers (Figure 3d). If the polarization is then oriented parallel to the nylon rubbing direction, a twisted nematic (TN) state is soon obtained and the cell becomes bright (Figure 3e), with dark lines showing the boundaries between left- and righthanded twist domains. This kind of experiment confirms that the azo-SAMs are capable of aligning LCs in situ using LPL and that the photoalignment direction is continuously variable, as also reported by Yi and Furtak.21-23 Although very stable, this optical alignment is clearly not immutable, as it is readily modified by exposing the sample to LPL with a different orientation, If desired, however, permanent optical alignment can be achieved by chemically deactivating the azo-SAM molecules by exposing them to intense LPL, a process that simultaneously aligns and oxidizes the azo-SAM molecules,22 rendering them photoinactive. 2. Thermal Stability of Liquid Crystal Alignment on AzoSAMs. Polymeric azobenzene-based systems may be rendered anisotropic by exposure to LPL. Such films are typically below the glass transition so the photoinduced alignment can only partially relax, making them potentially useful as information storage media.24 To determine the thermal stability of liquid crystal alignment on an azo-SAM surface, we first investigated the thermal stability of the orientation of bare azo-SAMs. Using a sealed azo-SAM cell of the kind shown in Figure 2, the photoactive layers are first written to an orientation of θ = þ45 by illuminating with LPL at θ = -45, with the photoisomerization inducing birefringence in the SAMs. At time t = 0, the pump beam is then shut off, and the transmission signal is recorded as DOI: 10.1021/la102788j
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Figure 7. Reorientation dynamics of liquid crystal in a hybrid OTE/ azo-SAM cell compared with the response of an azo-SAM alone when the writing light polarization orientation changes by 45. The cell is 5 μm thick and filled with 5CB. The linearly polarized pump and probe light beams are at 514 and 632.8 nm respectively. The intensity both for the initial writing and for rewriting starting at t = 0 is 50 mW/cm2, with the initial writing time either 5 or 100 s as indicated. The transmission of the liquid crystal cell, scaled here to match the initial transmission of the azo-SAM, is retarded relative to that of the SAM. The data are selected from Figure 6.
Figure 6. Liquid crystal and bare azo-SAM reorientation dynamics when the writing light polarization direction is changed by 45: (a) nematic liquid crystal 5CB in a hybrid azo-SAM/OTE-SAM cell 5 μm thick; (b) bare azo-SAM on glass. Two linearly polarized 50 mW/cm2, 514 nm pump beams are used to prepare the azo-SAMs (with initial writing times shown in the legends) and then to reorient them starting at t = 0. The order parameter at t = 0 depends on the initial writing time. The polarimeter probe beam is at 632.8 nm and all experiments were carried out at room temperature.
a function of time. The subsequent decay of the induced birefringence (shown in Figure 4) indicates that the oriented SAM molecules relax thermally by rotational diffusion to an orientational distribution of trans isomers that is isotropic in two dimensions. The decay rate depends on how hard the active surface was previously written: the higher the initial writing dose, the slower the subsequent thermal relaxation. Significant relaxation of bare azoSAMs at room temperature was observed to occur on a time scale of minutes to hours. We found, in contrast, that the LC alignment in a planar LC cell with azo-SAM and rubbed nylon substrates such as the one shown in Figure 3 is very stable, being unchanged in a cell kept at room temperature for 4 years. The optically induced order, while relaxing away when the SAM is isolated, must therefore persist when the monolayer is in contact with LC, since surface anisotropy is required to provide the torque maintaining the twist in the LC direction field. Heating the LC in the cell into the isotropic phase, however, compromises the alignment, as seen by heating a nylon/azo-SAM twist cell of E31 up to 125 C, well above the clearing point of the liquid crystal. When cooled back to the nematic phase after 30 min at elevated temperature, dark areas 17486 DOI: 10.1021/la102788j
appeared in the originally bright texture (compare Figure 5b with 5a) implying that the amount of twist of the LC director was substantially reduced in these areas. The twist was further reduced after heating for 60 min (Figure 5c), and completely disappeared, rendering a uniform planar aligned cell, after keeping the cell at an elevated temperature for a total of 90 min (Figure 5d). Twist was still absent in most areas of the cell even after then being left for many hours at room temperature (Figure 5e). In contrast, a twist cell of E31 with two rubbed nylon surfaces heated in the same way still showed good alignment after cooling back down to room temperature. Since an aligned bare azo-SAM also relaxes to a randomly distributed state at room temperature, the long-term orientational stability of LC molecules observed in nylon/azo-SAM cells at room temperature must arise from interactions between the SAM and the LC molecules, with LC molecules locally ordered by the optically aligned SAM in turn stabilizing the SAM molecule orientation and preventing it from relaxing. When the LC is heated to the isotropic phase, however, the cooperative orienting torques applied on the SAM molecules by the LC disappear and the SAM molecules relax to an orientationally random state as an isolated SAM does. It appears that our aligned SAM surface does not form a stabilizing “smectic-like layer” between the SAM and the neighboring LCs as proposed in Ichimura’s system,18-20 where the alignment of a photobuffed cell does not change even after heating the cell well into the isotropic phase for 1 h. The azoSAM molecule used in Ichimura’s experiment is longer and more symmetric than ours, which may give a longer interaction length along the alignment direction and hence stronger interaction between the SAM and the LC molecules, resulting in liquid crystalline ordering of the LC near the SAM surface even above the bulk clearing point. However, in studies of rubbed polyimide (PI) surfaces with the nonmesogenic 4CB (4-n-butyl-4-cyanobiphenyl) evaporated on them, Shioda et al. found no evidence of preferential alignment of the 4CB molecules,33 which implies that nonmesogenic molecules are not aligned by an oriented surface even if they (33) Shioda, T.; Okada, Y.; Takanishi, Y.; Ishikawa, K.; Park, B.; Takezoe, H. Jpn. J. Appl. Phys. 2005, 44, 3103.
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Figure 9. Liquid crystal cell response when the azo-SAM alignment layer is reoriented through 45. The curves show the change in optical transmission for different rewriting intensities incident on a cell previously written for 5 s with 50 mW/cm2 LPL. The hybrid liquid crystal cell is made with OTE-SAM and azo-SAM-coated substrates spaced 5 μm apart and filled with 5CB. The pump and probe beams are at 514 and 632.8 nm respectively.
Figure 8. Simulated liquid crystal reorientation dynamics in a hybrid azo-SAM/OTE-SAM cell when the writing light polarization direction is changed by 45: (a) Azimuthal orientation at the SAM-coated surface of a model 5CB 5 μm thick nematic liquid crystal vs time when the preferred orientation follows that measured for the bare azo-SAM (black squares), for different azimuthal anchoring energies γ1 = 6 10-3 erg/cm2 (cyan), 3 10-3 erg/cm2 (magenta), and 6 10-4 erg/cm2 (green) respectively; (b) Optical transmission vs time, scaled to be equal at t = 0. The dynamic model of a hybrid LC cell with weak azimuthal anchoring at the azo-SAM surface (γ1 = 6 10-4 erg/cm2, green curve) predicts an optical response similar to that measured experimentally (red squares). As the curves in (a) would imply, the response of the model cell is increasingly retarded at early times with respect to that of the bare azo-SAM (black squares) as the surface anchoring of the LC on the SAM is reduced.
are anisotropic in shape. This suggests that the thermal stability of the LC alignment on Ichimura’s SAM may originate mainly from the intrinsic orientational stability of the SAM itself rather than from any “smectic-like layer” formation. We are not aware of measurements of the long-term orientation stability of Ichimura’s SAMs alone, which could provide useful additional information on their stabilizing mechanism. Whether or not “smectic-like layers” are present, the fact is that the family of long and symmetric sideon azo-SAMs gives thermally stable alignment of nematics, suggesting that molecular structure is an important factor in obtaining thermally stable, photoactive alignment layers for LC cells. 3. Reorientation Dynamics of Liquid Crystals on AzoSAMs. The LC reorientation dynamics was measured in a hybrid nematic LC cell with OTE and azo-SAMs as the alignment surfaces, giving, respectively, planar and homeotropic alignment. The transmission of the cell is measured as a function of time in the polarimeter,31 with a neutral density filter placed after the analyzer to prevent the light transmitted by the thicker LC layer from saturating the PMT. One linearly polarized laser pump beam is used to orient the azo-SAM initially at þ45 with respect to the polarizer of the probe beam, following which the second linearly polarized laser pump beam is turned on to rewrite the Langmuir 2010, 26(22), 17482–17488
azo-SAM surface to þ90, allowing us to observe the bulk reorientation dynamics of the LC as the average orientation of the alignment layer changes by 45. The response of the LC depends on the initial writing times: the longer the exposure to the first pump beam, the slower the subsequent reorientation, as shown in Figure 6a, an effect similar to what is observed with bare monolayers, as shown in Figure 6b. The dependence of the initial transmission of the LC cell on the previous writing time thus mimics the behavior of bare azo-SAMs. The LC molecules at the surface clearly respond to the orientational order parameter of the photobuffed azo-SAMs and align accordingly, in a manner analogous to the alignment of nematic monolayers on a PI surface with main-chain-substituted azobenzene chromophores, where the order parameter of the monolayer of LC molecules was found to be equal to that of the photobuffed PIs.34 The effective azimuthal anchoring energy of the azo-SAM surface is a function of the orientational order parameter of the SAM, which determines the strength of the interaction between the azo-SAM and LC molecules. At long times, the transmission of the LC cell falls to the same low level independent of initial writing times, giving a very high contrast between the two uniform states (up to 3150:1 for the 100 s initial writing case), the highest achieved for a nematic on a photoreversible surface. We have seen that in the steady state, the LC director is aligned with that of the azo-SAM. We would like to understand in more detail how the dynamic response of LCs compares with that of azo-SAMs alone when the LC cell is exposed to LPL. Figure 7 shows selected bare SAM and LC cell response curves plotted together for two values of initial writing time. The optical response of the hybrid azo-SAM/OTE nematic LC cell is significantly retarded with respect to that of the bare azo-SAM at early times, as shown in Figure 7. We propose that this is the result of weak azimuthal coupling between the reorienting azo-SAM and the nematic director field in the cell. Modeling of the director reorientation dynamics in a hybrid nematic cell with homeotropic anchoring on the top surface and planar anchoring with azimuthal anisotropy on the bottom shows that, when the preferred (34) Usami, K.; Sakamoto, K.; Uehara, Y.; Ushioda, S. Appl. Phys. Lett. 2005, 86, 211906.
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Figure 10. Simulated optical transmission vs fluence of a hybrid azo-SAM/OTE-SAM cell illuminated with different light intensities when the polarization direction of the writing light is changed by 45. The dynamic model of a hybrid LC cell with weak azimuthal anchoring at the azo-SAM surface (γ1 =610-4 erg/cm2, green curve) predicts an optical response similar to that measured experimentally (see Figure 8b). The blue and red curves show the response of the LC cell when the azo-SAM is illuminated with actinic light, respectively, five times less and more intense than in the green model. The model cell is prepared in exactly the same way for each run using LPL, so that the initial values of the transmitted intensity are identical. The transmission of the model LC cell at small fluence becomes increasingly retarded as the incident light intensity during reorientation increases.
azimuth at the bottom is forced to match the angular reorientation measured in a bare azo-SAM under the same illumination conditions (that is, assuming strong anchoring strength), the entire LC cell switches essentially with these fast dynamics. When weak anchoring strength is assumed, however, with a magnitude comparable to that measured by Yi et al. for a dMR-SAM in a nematic twist cell,23 the azimuthal response of the director at the bottom of the model cell lags behind that of the azo-SAM at short times, with the delay increasing as the azimuthal anchoring energy is reduced as shown in Figure 8a. This causes a corresponding lag in the optical response of the cell, shown in Figure 8b. Numerical simulations show that, when a bare azo-SAM reorients in response to a 45 change in the orientation of linearly polarized writing light, the orientational order parameter, derived from the angular probability distribution of the trans isomer, is always temporarily reduced (see Supporting Information).35 This would lead to a corresponding decrease in the effective azimuthal anchoring strength of the azo-SAM during reorientation36 and a further retardation of the LC response. As we have seen, the temporal response of the LC cells depends on the exciting light intensity. The optical transmission as a function of the rewriting fluence (the total incident energy per area) is shown in Figure 9. Although the reorientation rate for bare SAMs (35) Fang, G.; Shi, Y.; Maclennan, J. E.; Clark, N. A.; Farrow, M. J.; Walba, D. M. unpublished. (36) Sonin, A. A. The Surface Physics of Liquid Crystals; Gordon and Breach Publishers: 1995; Ch. 9, p 74.
17488 DOI: 10.1021/la102788j
Fang et al.
is linearly proportional to the exciting light intensity37 (see also, Supporting Information) in the range from 50 to 1000 mW/cm2, the same degree of reorientation of the LC (i.e., the same reduction in transmission) appears to require more photons when the rewriting beam is more intense. This result is in direct contrast to our observations of bare azo-SAMs and seems rather counterintuitive. When the incident energy on the azo-SAM is the same, should not the LC reorientation progress by the same amount? Simulations of the LC director dynamics and of the optical response of the cell were performed assuming a SAM orientation rate linearly proportional to the incident light intensity, with results shown in Figure 10. The model predicts a delay of the LC response at early times that increases with incident light intensity, as is observed experimentally. The LC response is limited by its orientational viscosity. Even in the limit of high light intensity and strong anchoring, the orientation must diffuse into the bulk LC from the surface. As the surface anchoring is weakened, this LC response is further delayed by the transient orientational decoupling of the LC and the azo-SAM, as Figure 8a shows. Thus, both the weak anchoring and the LC viscosity affect the LC response. In the limit of vanishing LC viscosity and low light intensity, the LC reorientation will track that of the azo-SAM with no time lag.
Conclusions Azo-SAMs photobuffed using linearly polarized light can be used to control the azimuthal alignment of nematic liquid crystals in situ. The chromophores in the SAM align preferentially perpendicular to the polarization of the exciting light, and this in turn orients the liquid crystal director. The twisted alignment induced in an azo-SAM/rubbed nylon nematic cell is very stable at room temperature, but is degraded if the LC is heated into the isotropic phase for an hour or more, where the orientation field of the SAM is not stabilized by the liquid crystalline order. The reorientation rate both of liquid crystal in hybrid azo-SAM cells and of bare azo-SAMs on glass depends on the initial SAM writing time: the longer the initial photobuffing, the slower the subsequent reorientation. The photoinduced reorientation of the liquid crystal lags behind that of bare azo-SAMs, and, paradoxically, when the rewriting beam is more intense, a larger fluence of exciting light is required to achieve the same reorientation at the early times. Acknowledgment. We are grateful to Youngwoo Yi and Thomas Furtak for useful discussions and for providing some of SAMs. We acknowledge support from NSF MRSEC grant DMR0820579 and NSF grant CHE-0079122. Supporting Information Available: Reference 35 showing that the order parameter of azo-SAMs decreases in the beginning of reorientation through 45. Reference 37 showing that the reorientation rate of azo-SAMs is linearly proportional to the incident light intensity. This material is available free of charge via the Internet at http://pubs.acs.org. (37) Fang, G.; Shi, Y.; Maclennan, J. E.; Clark, N. A.; Farrow, M. J.; Walba, D. M. unpublished.
Langmuir 2010, 26(22), 17482–17488
