We report concurring phase and anchoring transitions of chiral azo-dye doped nematic liquid crystals. The transitions are induced by photo-stimulation and stable against light and thermal treatments. Photochromic trans- to cis-isomerization of azo-dye induces an augmented dipole moment and strong dipole-dipole interaction of the cis-isomers, resulting in the formation of nano-sized dye-aggregates. Consequent phase separation of the aggregates of a chiral azo-dye induces phase transition from a chiral to nonchiral nematic phase. In addition, the deposition of dye-aggregates at the surfaces brings about anchoring transition of LC molecules. The stability and irreversibility of the transition, together with no need of pretreatments for LC alignment, provide fascinating opportunity for liquid crystal device applications.
© 2013 Optical Society of America
The alignment control is an essential process for liquid crystal (LC) device applications. Although various techniques have been developed, the current method adopted for mass production is to use special alignment layers such as polyimide films. In this case, the coating and curing of polymer alignment layers are prerequisite processes for device fabrications .
Two different types of noncontact alignment methods, so called photoalignment, based on the photoresponsive azo-dye have been reported [2–4]. In the pioneering works achieved by Ichimura et al., the azobenzene derivatives are covalently attached to either in the inorganic surfaces or the polymer side chains and used as LC alignment layers . In these cases, photochromic trans- to cis-isomerization is responsible for the anchoring transition of LC molecules at the surface. Gibbson et al. have reported the azo-dye doped anisotropic films, formed by the polarized-light stimulated rotation of the dye molecules, for LC alignment layer by irradiating intensive polarized laser light . The reversible anchoring transition of azo-LC or azo-dye doped LCs has also been reported and interpreted by the molecular adsorption of cis-form isomer at surfaces through dipole-dipole and hydrogen bonding interactions .
Most of these methods require pretreatments of the surface for LC alignment. It was not until recent years that a few different approaches have been attempted to simplify the process without using pretreatments of substrates for LC alignment [5–9]. Nano-particle doped nematic LCs exhibit spontaneous homeotropic alignment in the confined electro-optic (E.O.) cells without any alignment layer . Some photo-responsive or reactive additives doped to the nematic host have been developed to instigate a vertical alignment of LCs upon UV-light irradiation without a pretreated alignment layer [6–9].
On the other hand, the photochromic isomerization induced phase transition from the nematic to isotropic phase is also well documented for the azo-dye doped LCs . However, all above mentioned anchoring and phase transitions induced by the trans/cis-isomerization are characterized by their reversibility.
In this report, we describe the simultaneous phase and anchoring transitions of the nematic LCs doped with a liquid crystalline chiral azo-dye, stimulated by UV-light irradiation. The chiral nematic to nematic phase transition and planar to homeotropic anchoring transition are concurring under UV-light irradiation. The resulting transitions are stable against light and thermal treatments. The stability and irreversibility of the reported phenomenon, together with no need of pretreatments for LC alignment, provide a great potential for practical device applications.
2.1 Sample preparation
Nematic liquid crystals with either positive (E7, Merck) or negative (MLC 6608, Merck) dielectric anisotropy have been used as a host liquid crystal. The photochromic chiral azo-dye shown in Fig. 1 has been added to the LC host with the ratio of 0.5 ~3.0 wt%. Synthetic scheme for the derivatives can be found in . The details on synthesis and characterization will be described separately. For homogeneous mixing, the mixture has been melted by stirring at a few degrees above the nematic-to-isotropic transition temperature (TNI) of host LCs.
E.O. cells have been fabricated by using indium-tin oxide (ITO) coated glass without any pretreatment for LC alignment. The cell thickness was maintained by 10 μm-thick tape spacers. The dye-doped LC mixture has been loaded into the cells with a capillary action at 5 degree above the TNI and cooled to an ambient temperature. The cells were then exposed to UV-light with 20 mW ~500 mWcm−2 intensity peaked at 378 nm wavelength for 10 ~30 minutes either at room temperature or 2°C above the TNI. The Spot Cure Model SP-9 (Ushio Inc.) has been used for UV-source. The 378 nm wavelength was selected by using a bandpass filter with 58 nm of full width at half maximum.
Polarized optical microscope (POM) and conoscopy images have been taken using a Nikon Eclipse LV 100 POL equipped with a Nikon DS-Ri1 CCD camera and Instec HCS 402 hot stage with a Instec STC 200 temperature controller. The UV-Vis absorption spectra of dye solutions in LC or chloroform were measured by a UV-Vis spectrophotometer (Jasco, ARSN-733). For particle size analyses, count rate and particle size of the dye-solutions in chloroform contained in a quartz cuvette have been measured by using a Brook Haven BioPlus particle size analyzer (Brook Haven Inc.). Electro-optical switching behaviors are characterized by applying square-wave voltage at 1 kHz using an Agilient 33521A function generator.
3. Results and discussion
The E.O. cell loaded with chiral dye-doped nematic LCs exhibits a random planar texture as expected. The untreated ITO surface generally enforces a tangential anchoring of LC molecules at the surface with no macroscopic orientation. Figure 2 shows depolarized optical images. As presented in Figs. 2(a) and 2(b), both macroscopic and microscopic images exhibit random planar state of the E.O. cell with 1.0 wt% azo-dye in the MLC 6609 prior to UV-light irradiation. After 30 minutes exposure with 365 nm UV-light with 500 mWcm−2 intensity at 25 °C, however, the exposed area of the cell turns into a completely dark state under crossed polarizers as shown in Fig. 2(c).
The corresponding POM images, Fig. 2(d) and Fig. 2(e), represent border and inner areas of the exposed circle, respectively. It is evident that UV-irradiated area has been turned to a homeotropic state of nematic LCs with no chiral nature. The conoscopic figure, shown in the inset of Fig. 2(e), further confirms a vertical orientation of the uniaxial optic axis to the substrate plane.
During the process, the changes are in two folds: The anchoring transition in LC alignment and phase transition from the chiral nematic to nematic LC phase. It is interesting to observe that both transitions occur simultaneously upon irradiation of UV-light. This has been evidenced by the existence of cholesteric fingerprint texture, typically observed for the cholesteric LCs with a homeotropic boundary condition, in the midst of irradiation process. Figure 3(a) displays cholesteric streaks observed from the cell with 1.0 wt% chiral azo-dye, irradiated at 50 mW/cm2 for 30 minutes. The streaks are slowly annihilated by applying a pulsed mechanical stress and turn to a uniform dark state with some bright defects as in Fig. 3(b), indicating a vertically aligned uniaxial optic axis. For the mixture with a higher dye concentration at 3.0 wt%, however, the initial planar state shown in Fig. 3(c) has been transferred to the fingerprint texture in Fig. 3(d) after irradiation at 50 mW/cm2 for 60 minutes. The changes strongly indicate the anchoring transition to a homeotropic state at the surfaces. The extended UV-exposure gradually lengthens a cholesteric pitch. For the highly doped LC mixtures, the anchoring transition occurs before the chiral nematic to nematic phase transition is completed. For dye content below 0.1 wt%, the anchoring transition has not been observed in the UV-irradiation condition given above.
Once chiral azo-dye molecules are depleted in the LC host and the phase separation is completed, the bulk LC lost its chirality turns to the nematic phase. Therefore, the uniformly aligned uniaxial optic axis is aligned vertical to the surface due to the homeotropic anchoring conditions. Based on our experimental results, the both transitions proceed and are completed simultaneously for the mixture with a low chiral azo-dye content. For a high content dye mixture, however, the anchoring transition is accomplished prior to a complete phase transition. As a result, the cholesteric LC exhibits fingerprint texture under a homeotropic boundary condition. Prolonged UV-light exposure up to three hours ultimately exhausts chiral additives and completes both anchoring and phase transitions for 3.0 wt% dye-doped mixture.
In fact, the phase transition between nematic (or cholesteric) and isotropic phases has been previously reported . However, all these are known as a reversible process. The trans- to cis-isomerization of azo-dye molecules and consequent configuration change to bent-shaped nonmesogenic molecules disrupt LC order of the host due to their impure nature. Therefore isomerization of azo-dye stimulated by UV-light induces a phase transition of the LC host from nematic (or cholesteric) to isotropic phase. The manipulation of a helical twisting power (HTP) of photoresponsive chiral dopants has been also reported [10–12]. In this case, the conformational change induced by photochromic isomerization alters its chiral activity and thus results in a variation of a cholesteric pitch. In most of cases, however, the original LC phase or HTP is recovered by removal of UV-light or the irradiation of visible light. The cis-isomer relaxes back to energetically more favorable trans-isomer by heat or visible-light treatments, subsequently resulting in a recovery of the LC phase.
However, in our case, the chiral nematic to nematic transition is observed in addition to the reversible isotropic transition under UV-irradiation. More interestingly, the process is irreversible and very stable against thermal and light treatments. On the other hand, the cis-form azo-benzene has a much enhanced dipole moment due to its broken symmetry compared to its trans-form isomer. As a result, they have a strong dipole-dipole interaction and form nano-sized aggregates, finally resulting in a phase separation from the host [9,13,14]. Indeed, it has been confirmed that the azo-dye, used for our study, dissolved in chloroform results in a particle formation after UV-light irradiation. For the 0.01 wt% solution in chloroform, the aggregates with approximately 300 nm diameter have been detected by the particle size analyzer. For UV-Vis absorption spectra, the very intense peak at 365 nm quickly diminishes and relatively weak absorption at 450 nm is significantly enhanced upon a 365 nm UV-light irradiation. In addition, relatively sharp and strong peak at 290 nm is gradually intensified as a function of exposure time. These spectral changes are not recovered even after 24 hours in a dark state (i.e., irreversible). The spectral peak for the trans-isomer has been recovered after drying and melting the aggregates by heating, as previously reported in the similar condition . This clearly indicates that the photo-stimulated process is attributed to a physical aggregation rather than irreversible chemical reaction such as a polymerization.
The homeotropic dark state shown in Fig. 2(e) transforms to a uniform light state as in Fig. 4(a) upon applying a vertical electric field. To confirm a surface mediated anchoring transition, the host LC has been completely removed by soaking the cell in hexane for eight hours. After thorough rinsing and drying, the fresh LC has been reloaded without adding any chiral additive. Figure 4(b) shows the resulting optical state of the reloaded LC cell. The UV-exposed area retains its homeotropic anchoring condition while the unexposed region exhibits random planar alignment. It is explicit that the anchoring transition has been facilitated by the surface rather than changes in bulk components. The modified surface is stable against solvent treatment. Both cells with original and reloaded fresh LCs perform uniform electro-optic switchings, indicating homogeneous surfaces, as shown in Fig. 4(a) at 3.0Vpp and Fig. 4(c) at 5.0 Vpp, respectively.
Based on all these considerations, we conclude that no simple trans- to cis-isomerization directly causes phase and anchoring transitions simultaneously. The photochromic isomerization encourages a phase separation of the chiral azo-dye from a host LC. The augmented dipole moment of photo-stimulated cis-isomers instigates to form nano-aggregates of chiral azo-dye and subsequently the dye aggregates are phase separated from the host LCs, inducing a chiral nematic to nematic phase transition. In addition, the phase-separated aggregates are adsorbed at the LC-ITO interface and modify the ITO-surfaces. This topographical modification is mainly responsible for the anchoring transition of LCs to a homeotropic state at the surfaces . In these regards, our observation on concurring phase and anchoring transitions is distinct from the conventional nematic to isotropic phase transition of the azo-dye doped LCs, driven by populating nonmesogenic cis-isomers stimulated by UV-light irradiation.
We have demonstrated the concurring phase and anchoring transitions of the nematic LCs doped with a liquid crystalline chiral azo-dye, stimulated by UV-light irradiation. The simultaneous phase and anchoring transitions are originated from the photochromic trans- to cis-isomerization and subsequent augmented dipole moment of a cis-form isomer. The strong dipole-dipole interaction of cis-isomers instigates to form nano-sized aggregates and thus induce a phase separation of the chiral azo-dye from a host LC. Consequently, the chiral nematic phase gradually transforms to a nematic phase. Simultaneously, the aggregates of cis-from dye molecules is adsorbed at the LC-ITO interface and modify topography of the surfaces. As a result, the anchoring transition occurs and the uniaxial optic axis of a nematic host is aligned vertical to the surface. The stability and irreversibility of the reported transitions, together with no prerequisite treatments for LC alignment, provide a great potential for practical device applications.
This research was supported by Brain Korea PLUS Project through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. Authors thank Prof. Myong-Hoon Lee and Prof. Seung Hee Lee for their valuable discussion and supports. Authors also thank the Merck Advanced Technologies in Korea for their kind support of LC materials.
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