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In this paper, we demonstrate a controllable optical activity emerging as electric-field-dependent circular dicroism (CD) and optical rotatory dispersion (ORD). Such controllable optical activity may offer fascinating possibilities for the production of novel optical materials/devices. The result also supports our previous result about the formation of the chiral superstructure of the 5CB molecules in the BX phase of the 5CB/P8-O-PIMB mixture system.
©2011 Optical Society of America
1. Introduction
In liquid crystals (LCs), introduction of chirality affects their structural and/or physical properties [1]. For instance, formation of helical superstructures of cholesteric (Ch*), tilted chiral smectic C (SmC*), and twist grain boundary (TGB*) phases is a well-known example. Besides, unique phase structures such as blue phases (BP*) emerge due to the frustration of helical structures in some chiral LC systems. In addition, the introduction of chirality removes the mirror symmetry of the systems, so that physical properties of LC phases can be modified. The most typical example is the induction of spontaneous polarization in the SmC* phases. Thus, ferroelectricity/antiferroelectricity can be often realized by designing LC molecules to take a suitable phase structures and boundary conditions.
Already, more than a dozen of years have past since the discovery of the polar switching in bent-core liquid crystals [2]. Nevertheless, still they have attracted much attention mainly due to their polarity and chirality [3]. For example, in the tilted smectic phase of the bent-core molecules (so-called the B2 phase), polar order is realized by closed packing of bent-core molecules due to hindered free rotation of molecules in each smectic layer. This is the distinct difference from that in the SmC* phase, where the polar order originates from the removal of the mirror symmetry by introducing chiral molecules [4]. In the B2 phase, on the other hand, chirality appears by tilting of non-chiral bent-core molecules with respect to the layer normal; in addition to two axes, bent (polarization) direction and layer normal, the tilt sense defines the chirality [5]. So far, many new phenomena related to chirality have been reported such as enhanced chirality [6, 7] and induced blue phases by doping chiral systems with achiral bent-core molecules [8].
So far eight characteristic phases (so-called banana phases, B1∼B8) have been reported [3], although the structures of some of them have not been clarified yet. In addition, biaxial nematic Nb [9, 10] and non-tilted SmA analogous phases have been reported; i.e., SmAdPA [11–13], SmAPA [14, 15], SmAPF [16], SmAPR [17, 18], and SmAPAR [19]. We focus on the B4 phase in this paper. The B4 phase often emerges over a wide temperature range including room temperature below B2 and sometimes B3 phases of bent-core LCs, and appears as a smooth glassy state without fluidity nor electric-field induced molecular reorientation [20]. Usually, its polarizing microscopy image exhibits uniform and bluish domains in thin LC cells. Additionally, optical (chiral) segregation (deracemization) can be recognized by slightly decrossing polarizers in a polarizing microscope; i.e., two bright and dark domains appear and the brightness interchanges by decrossing the polarizers to the opposite sense. Such a strong optical activity in thin LC cells cannot be driven only by molecular conformational chirality but can also be due to macroscopic chirality such as helix formation. In fact, Hough et al. recently suggested the ”helical nanofilament” structure, in which spiraling layers are assembled in a periodic nano-scale filament, by electron- and x-ray diffraction measurements, and freeze-fracture transmission electron microscopy (FFTEM) observation [21].
Recently, we successfully demonstrated a set of precise measurements of circular dichroism (CD) spectra for the mixtures of the conventional rod-core and bent-core molecules using ultra-thin cells [7]. The rod- and bent-core molecules used were n-pentylcyanobiphenyl (5CB, Fig. 1(a) right) and 1,3-phenylene bis[4-(4-8-alkoxyphenyliminomethyl)-benzoates] (P8-O-PIMB, Fig. 1(a) left). The use of ultrathin cells enables us to reduce strong bulk contributions and thus allows us to discuss their spectral shapes even for the near-UV region. This work mainly discussed on two results: (1) the origin of CD signals is due to chiral-segregated bent-core molecules in the B4 phase, where 5CB is in the isotropic phase; (2) the enhanced CD signal is detected in the BX phase, where 5CB is in the nematic phase. From these facts, we concluded that 5CB molecules in the BX phase are embedded in the network of helical nanofilaments formed by P8-O-PIMB and form some kind of chiral superstructures, resulting in the enhanced CD signals in the BX phase. Since the nematic liquid crystals are sensitive to external stimuli, such as electrical potential, we naturally expect the controllable optical activity by an external stimulation. Such controllable optical activity may offer some possibilities for the production of novel optical devices.
Fig. 1 (a) Chemical structures of the bent-core (P8-O-PIMB) and rod-core (5CB) liquid crystalline compounds used in this study. (b) Phase diagram of a binary mixture system of P-8-OPIMB and 5CB, determined by polarization optical microscopy (reproduced from Ref. [7]).
2. Experimental
The samples prepared for this study were the same as before, the binary mixtures of P8-O-PIMB and 5CB. The phase transition sequences of P8-O-PIMB and 5CB are Iso-174 °C-B2-152 °C-B3-140 °C-B4 and Iso-35 °C-N-22.5 °C-Cryst, respectively. These compounds were weighed, and dissolved in chloroform to mix well by sonication. We obtained sample mixtures after evaporating the solvent by slowly and gently heating in a jet oven. The sample cells were sandwich-shaped ones consisting of two smooth fused-silica substrates, having transparent electrodes of indium-tin-oxide sputtered on either inner surface. Cell gaps were estimated by optical simulation (SCOUT, W. Theiss Hard- and Software) upon transmittance spectrum measurement with a multichannel monochromator (USB4000, Ocean Optics) and a polarizing optical microscope (OPTIPHOT-POL, Nikon). Each sample mixture was injected into the cell with capillary action at the isotropic temperature. Temperature-dependent CD and optical rotatory dispersion (ORD) spectra were taken under precise temperature control of the sample cell using a specially designed furnace settled in a CD spectrometer (J-720WI with ORD option, Jasco). To detect the signal from a small region, a focal reducing optics was built up in the optical path in the CD spectrometer (Fig. 2).
Fig. 2 Optical setup built up for the micro-monodomain CD spectrum measurement. This setup allows to detect the signal from a small spot of several 10s μm diameter.
3. Results
First, we checked their textures by polarizing microscopy under varying temperatures. For all the sample mixtures, it was confirmed that the phase transitions took place consistently as previously reported in Refs. [6,7] (phase diagram of P8-O-PIMB/5CB system is shown in Fig. 1(b)). Among them, the mixtures of higher 5CB fraction than 50 wt% showed the B4-BX transition. Figure 3 shows photomicrographs taken for the mixture of 60 wt% 5CB in an ultra-thin cell (cell gap = 628 nm) cooled from the iso phase at −1 °C/min. In the B4 phase cooled to 40 °C, the texture showed dark smooth domains with a very low birefringence (Fig. 3(a)). Slight rotation of the analyzer visualized two regions of the segregated enantiomeric domains. By further cooling, the B4-BX phase transition occured. In the BX phase at 30 °C, the texture slightly brightened up with an emergence of slightly-birefringent needle-like domains as shown in Fig. 3(b). Even in this phase, the chiral segregation was conserved almost in the same shape. Temperature dependence of the CD spectra taken for the chiral monodomain in 60 wt% 5CB is shown in Fig. 4. The CD peak is enhanced and blue-shifted towards the resonance wavelength of 5CB with decreasing the temperature. These observations clearly indicate the formation of the chiral superstructure of the 5CB molecules in the BX phase.
Fig. 3 Photomicrographs of the textures of the (a) B4 and (b) BX phases. Optical segregation can be clearly observed by slightly decrossing the polarizers.
Fig. 4 Influence of the thermal change on the CD spectral shape for the chiral monodomain of 60 wt% 5CB sample.
After the above confirmation, we applied an alternating sinusoidal electric field (100 Hz) along the cell surface normal. Shown in Fig. 5 is a comparison of the textures of 90 wt% 5CB mixture in a 12-μm-thick cell with and without field application. First in the B4 phase (Figs. 5(a) and 5(b)), its appearance scarcely changed by the application of an electric field. No further change was observed even for higher electric fields at least by polarizing microscopy. Because of the very thick cell, bright birefringent domains appeared at the B4-BX phase transition in the absence of a field (Fig. 5(c)). However, by field application, these bright domains disappeared, and then the overall texture turned alike the B4 phase. By decrossing the polarizers, optical segregation could be observed still in the same region in the same shape as in the B4 phase, as shown in Fig. 5(d). By terminating the field, the texture returns back to the original BX state with birefringent domains. The reason of this texture change in the BX phase is attributed to the electric-field-induced reorientation of the 5CB molecules. As mentioned above, 5CB molecules in the BX phase are forming some kind of chiral superstructures, while they are in the isotropic liquid state in the B4 phase. In the present case, the vertical electric field unwinds the chiral superstructure of 5CB molecules and aligns them homeotropically in the cell. Since the nanofilaments of P8-O-PIMB are considered to be fixed in the matrix, the B4 phase and the field-applied BX phase are remarkably alike in their appearances. Then, the CD spectrum would also show the same tendency. Figure 6 reveals CD spectra for the monodomain of the 60 wt% 5CB sample in the ultra-thin cell (∼410 nm gap) under various electric fields at 30 °C. The CD intensity gradually decreased and the spectrum shape approached to that of the B4 phase as increasing the electric field (compare with the result at 40 °C in Fig. 4). Hence, this result strongly supports the above-suggested mechanism.
Fig. 5 Variation of the textures of 90 wt% 5CB sample in a thick cell (∼12-μm gap) with and without field application.
Fig. 6 Variety of CD spectra measured for the chiral monodomain of 60 wt% 5CB sample upon several different voltages.
Figure 7(a) shows plots of CD spectra taken for 90 wt% 5CB mixture in the B4 and BX phases in an ultra-thin cell (∼700 nm gap). According to the extrapolation of CD intensity originated from the helical nanofilaments to that in 90 wt% 5CB mixture, CD intensity due to the helical nanofilaments in B4 is about 0.25 deg/μm [7], which is consistent to the present experiment. By contrast, CD intensity due to the 5CB chiral superstructure was more than 5.5 deg/μm. The difference arises from small amount of P8-O-PIMB (low density of helical nanofilament) and large chiral superstructure made of 5CB. To see more drastic optical response in the visible range, ORD measurement was carried out. The results are shown in Fig. 7(b). Due to the limitation of the CD spectrometer, ORD spectrum was taken only between 350 nm and 800 nm. Since in this mixture the fraction of P8-O-PIMB is too small (∼10 wt%), the ORD signal almost vanishes in the B4 phase. The BX phase has a huge optical rotation (OR) due to the enhancement by the formation of the chiral superstructure of 5CB. Since the visible region is already far enough out of the resonance condition, the ORD spectrum shows a monotonic decrease on the wavelength and hence has no peak. To confirm the consistency between CD and ORD, we calculated ORD spectra by means of the Kramers-Kronig transform from the measured CD spectra. The calculated results shown in Fig. 8 qualitatively agree with the experimental ones; i.e., ORD monotonically decreases with increasing wavelength above 350 nm. This comparison confirms that the contribution of birefringence is sufficiently low, although slight nonzero baseline may attribute to the birefringence.
Fig. 7 (a) CD spectra and (b) ORD spectra measured for 90 wt% 5CB sample in the B4 and BX phases in an ultra-thin cell (∼700-nm gap).
To examine possible device application, ON-OFF switching of the ORD signal was measured using the same sample of 90 wt% 5CB cell by repeated application of elevated voltages; i.e., the applied electric field was alternately turned on and off every minute, and its amplitude increased stepwise as indicated by the black thick line in the plot. The results are plotted in Fig. 9(a), where blue, green, and red lines represent the signals for three different wavelengths, λ = 450 nm, 550 nm, and 650nm, respectively. According to the stepwise field and the relatively long time-constant of the CD spectrometer, each ORD signal looks saw-like. In Fig. 9(b), the saturated ORD signal for λ = 550 nm is plotted against the applied voltage. As shown in this result, the OR signal continuously drops down to 0 from about 0.7 deg/μm. Thus, the electric-field-controllable optical activity is realized in this range. This result provides a potential mechanism for a novel device controlling optical activity, differently from a liquid crystal variable retarder which controls phase retardation of the polarized light.
Fig. 9 (a) ON-OFF switching behavior of the OR signal from 90 wt% 5CB sample in the BX phase, confirming an electric-field-controllable optical activity. (b) Continuous change in the OR signal at λ = 550 nm plotted as a function of applied voltage.
4. Conclusion
In this paper, we demonstrated a controllable optical activity emerging as electric-field-dependent circular dicroism and optical rotatory dispersion. The result also supports our preceding result about the formation of the chiral superstructure of the 5CB molecules in the BX phase of the 5CB/P8-O-PIMB mixture system. Such a controllable optical activity may offer attractive possibilities for the production of novel optical devices.
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