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A new cyclic chiral azobenzene compound Azo-o-Bi was synthesized, which exhibits photochemically reversible cis-trans isomerization in both organic solvents and liquid crystal hosts. When doping into a nematic liquid crystal, it displays high helical twisting power (HTP, β) and a large change in HTP (Δβ) due to photoisomerization. Therefore, we are able to reversibly tune the reflection colors from near-IR to blue using irradiation with light. For weak UV light irradiation, a broadband reflection film with pitch gradients can be obtained with superior stability in the dark. This stability is associated with the cyclo-shape of the Azo-o-Bi and the high viscosity of the host liquid crystal. The latter reduces the diffusion contribution of isomers to a negligible level. The spatial and temporal control of the light irradiation can be used to photoaddress the CLC with RGB reflection colors and white-on-black reflective displays.
© 2016 Optical Society of America
1. Introduction
Cholesteric liquid crystals [1–3] (CLCs), which are also known as chiral nematic liquid crystals (N*-LC), have attracted significant interest due to their periodic helical superstructures. These superstructures can be characterized with two structural parameters, the helical pitch and the twist sense [4, 5]. The pitch length (p) corresponds to the distance over which the director of the rod-like molecules rotates a full turn, and its handedness describes the direction in which the molecular orientation rotates about the helical axis [1]. The resulting macroscopic helical superstructure can selectively reflect a band of incident circularly polarized light with the same handedness (chirality) as its helix, whereas the band with the opposite twist sense is transmitted. Outside the reflection band, both polarization states are simply transmitted [6]. The central wavelength λ0 of the selective reflection is related to p via λ0 = n × p, for normal incidence, where n is the average refractive index of the liquid crystal (LC). Moreover, the weak intermolecular interaction in LCs enables the possibility for the helical superstructure to be adjusted by external stimuli, such as heat [7, 8], light [9, 10], electric field [11, 12], and mechanical stress [13]. In recent years, the tune-ability of a helical superstructure has been applied to optical switching [14, 15], reflective displays [2, 16], information storage, tunable lasers [17, 18], molecular sensors [19], and biomedical applications [20, 21]. External stimulation through light is particularly interesting because of its potential for easy addressability, a fast response time, and (remote, spatial, and temporal) controllability in a wide range of ambient environments. Photoresponsive CLCs can be achieved by doping an achiral nematic liquid crystal (NLC) host with photochromic chiral dopants [1]. The ability of a chiral molecule to induce a helix structure is defined as the helical twisting power (HTP, β). It is expressed as β = (p × c)−1, where c is the chiral dopant concentration. The reversible photoisomerization of dopants upon light irradiation can control the HTP and the reflection wavelength (λ0) of the cholesteric phase, which provides the opportunities of CLC material applications in tunable color filters and reflectors [16], and optically addressed displays that require no driving electronics [2].
The reflection bandwidth of a single-pitch CLC is related to the birefringence Δn (Δn = ne-no) and the pitch p via Δλ = (ne-no) × p = Δn × p, where ne, no and Δn are the extraordinary refractive index, ordinary refractive index, and birefringence of the LC, respectively [13]. Since the value of Δn of LC is typically below 0.3, the bandwidth of a single-pitch CLC is limited to a few tens of nanometers in the visible region [6]. This limit is desired for many applications like sensors, narrow-band polarizers and filters [22]. However, the bandwidth must be dramatically increased for new applications such as full-color or white-on-black reflective displays, broadband polarizers, optical data storages, smart switchable reflective windows, and stealth technologies. Many theoretical and experimental studies have demonstrated that broadband reflection films can be achieved by adjusting the pitch of the CLCs to get a pitch gradient distribution or a non-uniform pitch distribution, as reviewed by Mitov [1]. Among methods like photo-induced pitch gradient distribution [23], temperature-induced non-uniform pitch distribution [24], nanoparticle-induced non-uniform pitch distribution and non-uniform pitch distribution [25] introduced by mixing the CLCs with different pitch, light-directed pitch gradient distribution is widely used because of the simple and practical manufacturing process. Broer et al. have obtained a broadband reflective polarizer with a pitch gradient by controlling the kinetics of the photo-polymerization reaction [26]. However, their reflection bandwidths cannot be adjusted due to the permanent solid structure.
Dynamic variation of the selective reflection properties (λ0 and Δλ) of CLCs has long been known and used. High HTP and large change in HTP (Δβ) are of essential for the performance of photoresponsive CLC materials. Low HTP requires higher dopant concentrations to meet the specific pitch length, which often leads to phase separation, coloration and deterioration of the desired physical properties of the NLC host. Increasing the Δβ will increase the phototuning range of the photoresponsive CLC materials. To date, various kinds of photochromic chiral dopants have been reported, such as azobenzenes, overcrowdedalkenes, diarylethenes [27, 28], spirooxazines and fulgides [29, 30]. Among these established dopants, azobenzenes are particularly promising because the azo group has the unique property of a typical trans-cis isomerization upon light irradiation, which can result in large conformation and polarization changes in the molecules [26].
Here, we report a new cyclic chiral azobenzene compound Azo-o-Bi that exhibits both high HTP and a large change of HTP during a trans-cis photoisomerization – see in Fig. 1. This work describes a new approach to adjust the reflective wavelength and control the bandwidth of induced CLC films. The photo-induced isomerization gradient can be stabilized by decreasing the diffusion contributions of the isomers to a negligible level. Our study contributes to broadening of the application of cholesteric liquid crystals in the information storage and display areas.
2. Experimental
Materials and methods
The nematic liquid crystal host HNG (HNG715600-100, abbreviated HNG) was purchased from Hecheng Chemical Materials Co. Ltd. All chemicals, solvents and reagents were obtained from commercial sources and used as purchased. Column chromatography was carried out on silica gel (60–200 mesh). Analytical thin layer chromatography (TLC) was performed on commercially coated 60 mesh F254 glass plates. 1H and 13C NMR spectra were measured on an Agilent VNMR 600 (600 MHZ). Chemical shifts are reported in δ (ppm) relative to tetramethylsilane as internal standard. UV-vis absorption spectra of the solutions and selective transmission spectra of the cells were performed using a UV/Vis double beam spectrophotometer (Shimadzu UV2550). CD spectra were recorded using a JASCO J-1500 CD spectrophotometer. Mass spectra were obtained from an Agilent Q-TOF 6540 mass spectrometer. The cano’s lines were observed under crossed polarizers using a polarized optical microscope (Leica DM2500 M), at room temperature.
Measurement of the helical pitch
The cholesteric liquid crystal composite was prepared by mixing the chiral dopant Azo-o-Bi (0.1 wt%) and the host liquid crystal HNG homogeneously followed by injection into a wedge cell by capillary force at room temperature. The pitch was determined by measuring the distance between the Cano’s lines on the surfaces of the wedge cells. The pitch was determined according to the equation p = 2a × tanθ, where a represents the distance between the Cano’s lines, and θ is the wedge angle of the wedge cells (tanθ = 0.01).
3. Results and discussion
A cyclic chiral dopant Azo-o-Bi consists of an axially chiral binaphthyl moiety [(R)-( + )-1,1′-bis(2-naphthol)] bonded to the meta positions of a photoresponsive azobenzene through oxyalkyl linkages- see Fig. 7. The compound was successfully synthesized by reduction of the corresponding phenolic hydroxyl groups using the Williamson reaction for the dilute condition [28]. The crude products obtained were purified using column chromatography on silica gel. The synthetic route and structure analysis are described in the supporting information.
The optical properties of Azo-o-Bi in THF solution were examined using UV-Vis and CD absorption spectra. UV-Vis absorption spectrum of the trans-isomer of Azo-o-Bi shows three distinct regions (one band below 260 nm, another one between 270 and 400 nm and the last one above 400 nm), which indicates that the chiral dopant Azo-o-Bi has both naphthalene and azobenzene chromophores in the structure – see in Fig. 2(a) [31, 32]. The strong absorption band below 260 nm corresponds to the long-axis polarization(1B transition) of the naphthalene chromophore. The band at 270-400 nm due to the absorption of the π-π* transitions of the azobenzene and the short axis polarizations (1Lb and 1La) of the naphthalene moiety. The weak absorption band above 400 nm is observed and corresponds to the n-π* transition of the azobenzene unit [33]. Upon UV light irradiation (365 nm, 1.0 mW/cm2), there is a decrease in the absorption of the band that corresponds to the π-π* transition bands accompanied by a small increase in the n-π* band around 440 nm, which means a trans-cis isomerization is induced by UV light. A photostationary state (PSSUV) is reached within 64 s, but additional irradiation does not induce further changes in absorption. Subsequent visible light irradiation at 440 nm (2.5 mW/cm2) results in the recovery of the spectrum to its other photostationary state (PSSVis). The PSSVis state is reached in 54 s. An isosbestic point maintains at 270 nm throughout the whole trans-cis isomerization process. The ratios of E/Zisomers are found to be 7: 93 and 52: 48 for the PSSvis and PSSUV states, respectively [22]. These were calculated from the 1H NMR spectra – see in Fig. 8 in the Appendix. The photoinduced E/Z isomerization was repeated for several cycles without a sign of fatigue – see in Fig. 9 in the Appendix.
Fig. 2 (a) Changes in the absorption spectra of Azo-o-Bi in THF solution (10 μM) upon irradiation with UV (365 nm, 1.0 mW/cm2) and visible (440 nm, 2.5 mW/cm2) light at room temperature. (b) CD spectra of Azo-o-Bi in THF solution (10 μM) at PSSUV (black line) and PSSVis (red line).
CD spectroscopy was performed to investigate the chiroptical properties of the Azo-o-Bi in organic solvent at room temperature. As shown in Fig. 2(b), the Azo-o-Bi solution exhibits CD signals and displays three distinct regions. A large positive and negative cotton effect for the 1B (210-260 nm) transitions and positive cotton effects for the 1La (275-310 nm) and 1Lb (310-350 nm) transitions correspond to the binaphthyl moiety [34]. In fact, no cotton effect was observed in the visible region that corresponds to the absorption band of the azobenzene unit. The intensive positive couplet at 225 nm associated with a negative couplet at 235 nm, reflects the absolute R configuration of the binaphthyl moiety in the molecule [35]. The influence of trans-cis isomerization on the chiro-optical properties is evident from the change in the intensity of the CD bands. UV light irradiation does not switch the overall R chiral configuration of Azo-o-Bi. Upon UV irradiation, the 1La and 1Lb transitions change slightly. However, the exciton couplets for the 1B transitions at 225 and 235 nm increased, indicating a change in the dihedral angle of the binaphthyl moiety. Binaphthyl derivatives can rotate along the carbon-carbon bond between the 1 and 1′ positions within a certain range. The intensity change of the 1B transitions is a sign of the variation of the dihedral angle (θ) between two naphthalene rings, which impacts on the chiro-optical properties of the compound [36]. According to the Rosini correlation between the Δε of the low energy component of 1B exciton couplet (at 235 nm) and the dihedral angle of binaphthyl derivatives, the θ for the trans-isomer is estimated to lie around 77° [37, 38]. The increase of Δε indicates that the dihedral angle may slightly decrease during photoisomerization, which is consistent with the simulated result based on the density functional theory. As shown in Fig. 10 in the Appendix, the θ for the trans- and cis-isomer were calculated to be 79° and 74.5°, respectively. Upon irradiation with 440 nm light, fast reverse CD spectral change occurred. The chiroptical properties of Azo-o-Bi and their changes during the photoisomerization process are mainly attributed to the binaphthyl units and the variation of the dihedral angle [39].
When a chiral dopant is dissolved into an achiral NLC, its chiral information can be effectively transferred to the whole nematic phase, amplification due to the elastic property of the NLC and the entire system becoming chiral [40]. This has demonstrated that the dihedral angle of binaphthyl derivatives plays a key role in their induced cholesteric property, not merely identifying the twist sense of the induced cholesteric phase, but also confirming the HTP [38–41]. CLCs were prepared by adding a small amount of photoresponsive cyclic chiral compound Azo-o-Bi as a chiral dopant into a commercially available nematic liquid crystal HNG. The induced CLC phase was verified as a fingerprint texture (long pitch) and Granjean (short pitch) texture using a polarized optical microscope. The twist-sense of the CLC was determined through the contact-test based on the miscibility between two LC compounds. The method depends on the mixing area between the induced CLC and the standard LC. When the screw sense of the CLC is the same as that of the standard LC, the mixing area will be continuous. Otherwise, it will be discontinuous. The result of the contact test is shown in Fig. 11 in the Appendix. The twist sense of the Azo-o-Bi containing CLC is left-handed, which is due to the (R)-cisoid conformation (θ<90°) of binaphthyl moieties in the structure.
The helical pitch of induced CLCs and the HTP of the chiral dopant can be measured using the Grandjean-Cano’s method [42, 43]. When a CLC mixture containing 0.1 wt% Azo-o-Bi in HNG was filled into a wedge cell, the photoinduced variation in HTP values and the pitch were directly observed as a change in distances between Cano’s lines with a polarized optical microscope under UV (365 nm) or visible light (440 nm) irradiation – see in Fig. 3. During the UV irradiation process, the distance between the lines was found to decrease considerably, and it reached a PSSUV state within 25 min. This occurred because of the shortening of the helical pitch due to the increase in HTP of the dopant following trans-cis isomerization. Irradiation with 440 nm light reversed the pitch toward the PSSVis state. According to the relationship between the HTP and the pitch length, the HTPs of Azo-o-Bi in HNG were calculated to be −59.1 and −89.5 μm−1 in the PSSVis and the PSSUV state, respectively.
Fig. 3 The change of Cano’s line of 0.1 wt% Azo-o-Bi dissolved in host HNG in the wedge type cell upon UV or visible light irradiation at room temperature.
Inspired by the high HTP, as well as the pronounced change in HTP value during trans-cis isomerization, we tuned the reflection wavelength of the induced CLC in the visible region [33, 36]. A mixture of 4.0 wt% Azo-o-Bi doped HNG was filled into a 20 μm thick planar cell using capillary action. Figure 4(a) shows the transmittance spectra of the cell during the photoisomerization process. There is a decrease of all transmittance spectra with a wavelength below 470 nm due to the absorption through the n–π* transition of the azobenzene unit and cell substrates [43–47]. Surprisingly, reversible RGB reflection colors could be achieved in the cell. At the initial state, the reflective band of the cell is located in the near-IR region with a peak at 780 nm, and no reflection color is observed visually. Upon UV irradiation (365 nm, 1.0 mW/cm2), a blue shift of the reflection band is observed. The reflection band shifts to 630 nm in 5 min, and the cell displays red (R) color. Further irradiation tunes the reflection band to 570 nm in 6 min, and the cell exhibits a green (G) color. The prolonged illumination shifts the reflection band further towards shorter wavelength and reaches the PSSUV state in 25 min with the reflection band at 480nm (blue color). The reverse process is achieved with 440 nm visible light irradiation (2.5 mW/cm2), and it takes about 15 min for the CLC mixture to reach the PSSVis state (~780 nm). As shown in Fig. 4(b), the reflection color can be controlled through the irradiation times. The tuning time can be improved by increasing irradiation intensity. Obviously, the 4 wt% Azo-o-Bi containing CLC is enough to induce the RGB reflection colors reversibly.
Fig. 4 (a) Reversibly change in the transmittance spectra of 4 wt% Azo-o-Bi dissolved in host HNG upon UV or visible light irradiation at room temperature. (b) The tunability of reflection wavelength as a function of exposure time. (c) Images of the real cells that show reflection colors corresponding to the transmittance spectra.
The maximum absorption of azobenzene occurs in the UV region, which indicates that azobenzene is a good UV absorber to form a pitch gradient distribution in the cell. As shown in Fig. 5(a), a broadband reflection CLC is obtained after weak UV light irradiation (365nm, 0.4mW/cm2) for 8 min, which suggests left-hand circularly-polarized light within the visible region. The effective irradiation intensity for trans-cis isomerization depends on the penetration depth of the UV light [48–50]. Since the chiral Azo-o-Bi molecule strongly absorbs UV light, the UV light intensity decays exponentially as the path length increases in the LC medium. As a result, the population of cis-isomers with a high HTP is higher near the UV irradiation side than at the far side. This population gradient of isomers and corresponding HTP gradient in UV-propagation direction creates subsequent pitch distribution – see in Fig. 5(b) [50–53]. The formation of this broadband reflection state is related to the intrinsic nature of its molecular structure, and the dramatic geometrical change upon photoisomerization. It is worth mentioning that this broadband reflection state has good stability when the CLC is stored in the dark – see in Fig. 12 in the Appendix. This behavior might be related to the cyclo-shape of the Azo-o-Bi, and the high viscosity of the host NLC HNG (170 mm2·s−1), which reduces the diffusion contributions of the isomers to a negligible level. In addition, the broadband reflection state can be returned to the selective reflection state with visible light (440 nm, 2.5 mW/cm2) irradiation.
Fig. 5 (a) The transmittance spectra of 4 wt% Azo-o-Bi dissolved in host HNG in the broadband reflection (red line) and selective reflection (black line) states. (b) The mechanism of the formation of broadband reflection state. (c) The photographs illustrating the reflection/transmission of the broadband reflection and the selective reflection state.
The ability of dynamic switching between the broadband reflection (white) and selective reflection (near-IR, transparent) is shown in Fig. 5(c). A paper printed with the image of a school badge of “Hefei University of Technology” was placed under the device to identify transparent. A pattern of “HFUT” containing RGB colors was placed above the device to characterize the reflection band in the visible light region. In the selective reflection state, the reflective band of the cell is outside the visible wavelength region. As a result, the image on the paper underneath the cell can be seen clearly through the cell, and the cell cannot reflect the color letters [22]. While at the broadband reflection state, the reflection band of the cell is broadened to cover the entire visible wavelength range. Accordingly, the image is visible behind the cell, and the colored letters “HFUT” are reflected.
The spatial and temporal control of light irradiation was used to photoaddress the azobenzene containing CLC through a mask. Three primary RGB colors could be observed simultaneously in a single thin film when using different UV light (365 nm, 1.0 mW/cm2) irradiation times. Figure 6(a) shows masking for different areas: black, no irradiation; red, irradiated for 5 min; green, irradiated for 8 min; blue, irradiated for ∼25min. As for Fig. 6(b), at the initial state, the reflective band of the cell is located in the near-IR region, and no reflection color is observed visually. Hence the cell shows a black color when put it above the black background. A white-on-black reflective display cell can be obtained within 8 min through a photomask by a weak UV light (365nm, 0.4 mW/cm2) irradiation. As a result, the UV irradiated region goes to the broadband reflection state and the UV un-irradiated region remains the initial state. Moreover, the optically addressed images can be erased by visible light (440 nm, 2.5 mW/cm2) irradiation when desired, and the cell is rewriteable many times due to excellent fatigue resistance.
4. Conclusion
A new chiral cyclic azobenzene compound Azo-o-Bi with high helical twisting power was synthesized. Its photoresponsive properties were investigated in both organic solvents and a liquid crystal host. When introduced into nematic liquid crystals as a photochromic chiral dopant, it exhibits a large change in HTP during trans-cis photoisomerization upon light irradiation. This work presents a novel approach to passively control the reflective wavelength and bandwidth of the induced CLC films. The reflection color can be reversibly tuned by photo irradiation to generate blue, green, and red colors. Moreover, by controlling the intensity of the UV light, we successfully created a broadband reflection film with superior stability in the dark. To the best of our knowledge, this is the first report to not only reversibly shift the reflection band, but also broaden the reflection band in a same CLC purely by controlling the intensity of the exposure light. This study aims to aid the development of light-driven chiral molecular switches or motors with enhanced functionalities for practical device applications.
Appendix
1. Synthesis of Azo-o-Bi
Synthesis of compound 1
Compound 1 was synthesized by the following steps. An excess of compound Tosyl chloride (19.92 g, 104.48 mmol), NaOH (4.77 g, 103.82 mmol) was dissolved in THF (50 mL). The diethylene glycol (5 mL, 52.67 mmol) was slowly added and the mixture was stirred for 12 hours at room temperature under a nitrogen atomosphere. Then, the reaction mixture was purified by column chromatography (n-hexane: ethyl acetate = 1:2). The white power 1 was acquired after dried in vacuum (18.60 g, 95% yield).
1H NMR (600 MHz, CDCl3): δ (ppm) = 2.45 (s, 6H), 3.60 (t, 4H), 4.09 (t, 4H), 7.34 (d, 4H), 7.77 (d, 4H).
13C NMR (150 MHz, CDCl3): δ (ppm) = 147.6, 135.5, 132.5, 130.6, 71.6, 71.4, 24.3.
Synthesis of compound 2
A suspension of 1 (5.80 g, 14.01 mmol), 2,2-Dihydroxyazobenzene (1.00 g, 4.67 mmol), Cs2CO3 (4.56 g, 13.99 mmol), Diphenyl-18-crown-6-ether (0.504 g, 1.40 mmol) and DMF(50 mL) reflux for 15 hours at 80 °C under nitrogen atmosphere. Then the solvent was evaporated and the resulting crude product was purified by column chromatography (n-hexane: ethyl acetate = 4:1) to afford 2 as an orange solid (7.14 g, 73% yield).
1H NMR (600 MHz, CDCl3): δ (ppm) = 2.39 (s, 6H), 3.84 (t, 4H), 3.88 (t, 4H), 4.16 (t, 4H), 4.27(t, 4H), 7.02 (t, 2H), 7.06 (d, 2H), 7.27 (d, 4H), 7.40 (t, 2H), 7.57 (d, 2H), 7.75(d, 4H).
13C NMR (150 MHz, CDCl3): δ (ppm) = 159.0, 147.4, 145.8, 135.6, 134.9, 132.4, 130.6, 124.2, 119.8, 117.9, 72.6, 72.5, 72.1, 71.8, 24.2.
HRMS m/z: calcd for C34H39N2O10S2 [M+H]+: 699.1968; found: 699.2083.
Elemental Analysis: C 58.39, H 5.51, N 4.03, S 9.20.
Synthesis of Azo-o-Bi
A mixture of 2 (1.00 g, 1.43 mmol), R-1,1’-2-isonaphthol (0.50 g, 1.50 mmol), Cs2CO3 (1.56 g, 4.79 mmol), Diphenyl-18-crown-6-ether (0.17 g, 0.47 mmol) and N,N-dimethylformamide (50 mL) was stirred for 3 days at room temperature under a nitrogen atmosphere. The color of the reaction turned light orange gradually. Then, the solvent was removed under the reduced pressure. The crude product was re-dissolved in CH2Cl2 (100 mL). The organic solvent was filtered and concen-trated in vacuo. The residue was purified by column chromatography (n-hexane: ethyl acetate = 3:2) to afford Azo-o-Bi as an orange solid (0.20 g, 19.7%).
1H NMR (600 MHz, CDCl3): δ (ppm) = 3.35 (m, 2H), 3.42 (m, 2H), 3.48 (m, 2H), 3.56 (m, 2H), 3.85 (m, 2H), 3.92 (m, 2H), 3.98 (m, 2H), 4.11 (m, 2H), 7.03 (d, 2H), 7.09 (t, 4H), 7.16 (m, 4H), 7.29 (t, 2H), 7.42 (t, 2H), 7.49 (d, 2H).
13C NMR (150 MHz, CDCl3): δ (ppm) = 157.4, 157.0, 147.3, 136.7, 134.0, 132.0, 131.8, 130.4, 128.7, 128.2, 126.2, 124.1, 123.2, 122.4, 118.7, 118.4, 72.8, 72.6, 72.5, 71.9.
HRMS m/z: calcd for C40H36N2O6 [M+H]+: 641.2573; found: 641.2647; C40H36N2O6 –Na [M+Na]+: 663.2471; found: 663.2493, C40H36N2O6 –K [M+K]+: 679.2210; found: 679.2244.
Elemental Analysis: C 75.01, H 5.64, N 4.22.
2. Photoisomerization of Azo-o-Bi in organic solvent
2.1 Changes in 1H NMR of Azo-o-Bi in the PSSUV and PSSVis
The change of Azo-o-Bi in CDCl3 solution upon light irradiation could be clearly observed by monitoring the 1H NMR spectra – see in Fig. 8 [54]. As the alkoxy protons near the light-driven part of Azo-o-Bi placed in fairly different chemical environments, the 1H NMR spectra can effectively monitor the process of Z-E isomerization under light illumination. The amount of E-isomer achieved the maximum value at the PSSUV accompanied with a minimum value of the Z-isomer. According to the proton peaks of alkoxy chain in PSSUV and PSSVis in 1H NMR spectra, the resulting E/Z conversion ration of Azo-o-Bi is calculated to be 7: 93 and 52: 48 at the PSSvis and PSSUV states.
2.2 The stability of Azo-o-Bi in organic solvent
Fig. 9 Cyclical absorbance of compound Azo-o-Bi in THF solution at 337 nm (black line) and 433 nm (red line) as the solution is repeatedly irradiated with UV light (365 nm) and visible light (440 nm) respectively.
3. Geometry structure of trans- and cis-isomer of Azo-o-Bi
In order to gain a better understanding of the photoisomerization of Azo-o-Bi, the geometry structures of trans- and cis-isomer were optimized by density function theory (DFT) using Gaussian 09 software [55] package at B3LYP/6-31G level. As shown in Fig. 10, the dihedral angle (θ) between two naphthalene rings for the cis- and trans-isomer are calculated to be 74.5° and 79°, respectively.
Fig. 10 The configuration of trans- and cis-isomer of Azo-o-Bi and the dihedral angle between two naphthalene rings.
4. The twist sense of Azo-o-Bi containing CLC
The twist sense of the helix was determined through the contact test based on the miscibility between two LC compounds. The chiral dopant R811, S811 known to be right- and left-handed respectively. The LC containing wt = 4% R811 (R-standard) and wt = 4% S811 (S-standard) were used as LC standards. This method is based on the observation of the mixing area between the two kind of CLCs using polarized optical microscopy (POM). When the screw of the Azo-o-Bi containing CLC is the same as the standard, the mixing area will be continuous. Otherwise, it will be discontinuous.
The mixture of wt = 0.1% Azo-o-Bi containing CLC and the R-standard gave a discontinuous boundary – see in Fig. 11(a) however when the CLC is contacted with the S-standard it gave a continuous boundary – see in Fig. 11(b). Which indicates that the screw sense of the Azo-o-Bi containing CLC is the same as the S-standard but opposite of R-standard. That is the CLC is left-handed.
Fig. 11 Contact tests between the CLC including Azo-o-Bi and (a) R811 (right- handed screw) or (b) S811 (left-handed screw) in the same LC host HNG.
5. The stability of the broadband reflection state
Acknowledgments
This work was mainly supported by the National Natural Science Foundation of China (grant No. 61107014, 51573036), and Program for New Century Excellent Talents in University (Grant No. NCET-12-0839) for the financial support.
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