Precisely Tuning Helical Twisting Power via Photoisomerization

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Precisely Tuning Helical Twisting Power via Photoisomerization Kinetics of Dopant in Chiral Nematic Liquid Crystals Dongxu Zhao, Yuan Qiu, Weinan Cheng, Shuguang Bi, Hong Wang, Qin Wang, Yonggui Liao, Haiyan Peng, and Xiaolin Xie Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03786 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Precisely Tuning Helical Twisting Power via Photoisomerization Kinetics of Dopant in Chiral Nematic Liquid Crystals Dongxu Zhao†, Yuan Qiu†, Weinan Cheng†, Shuguang Bi‡, Hong Wang†, Qin Wang†, Yonggui Liao*,†,§, Haiyan Peng*,†, Xiaolin Xie†,§ †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry

of Education. Hubei Key Laboratory of Material Chemistry and Service Failure. School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. ‡

Hubei Biomass Fibers and Eco-dyeing & Finishing Key Laboratory, College of

Chemistry and Chemical Engineering, State Key Laboratory of New Textile Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan 430073, China. §

National Anti-counterfeit Engineering Research Center, Huazhong University of

Science and Technology, Wuhan 430074, China.

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ABSTRACT It has been paid much attention to improve the helical twisting power (β) of dopants in chiral nematic liquid crystal (CLC), however, the correlations between the β value and the molecular structures as well as the interaction with nematic LCs are far from clear. In this work, a series of reversibly photo-switchable axially chiral dopants with different length of alkyl or alkoxyl substituent groups have been successfully synthesized through nucleophilic substitution and thiol-ene click reaction. Then, the effect of miscibility between these dopants and nematic LCs on the β values, as well as the time dependent decay/growth of the β values upon irradiations, have been investigated. The theoretical Teas solubility parameter shows that the miscibility between dopant and nematic LCs decreases with increasing of the length of substituent group from dopant 1 to dopant 4. The β value of chiral dopants in nematic LCs decreases from dopant 1 to dopant 4 both at visible light photostationary state (PSS) and at UV PSS after UV irradiation. With increasing of the length of substituent group, the photoisomerization rate constant of dopants increases for trans-cis transformation upon UV irradiation and decreases for the reverse process upon visible light irradiation either in isotropic ethyl acetate or in anisotropic LCs, although the constant in ethyl acetate is several times larger than the corresponding value in LCs. Also, the color of the CLCs could be tuned upon light irradiations. These results enable the precise tuning of the pitch and selective reflection wavelength/color of CLCs, which paves the way to the applications in electro-optic devices, information storage, high-tech anti-counterfeit, etc. 2 / 25


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1. INTRODUCTION Variable colors are highly desirable for applications in displays, sensors, cosmetics, and so on.1–4 Chiral nematic liquid crystal (CLC) has become one of the most important materials with variable colors since it was found by Reinitzer in 1888.5 Colors of the CLCs derive from the unique optically tunable helical superstructure.6–8 When an incident polarized light propagates through the CLCs media, it selectively reflects the light with a specific wavelength according to Bragg’s law. The average wavelength λ of the selective reflection is defined as = , where p is the pitch of helical structure and is the average refractive index of the material. In general, there are two methods to fabricate CLCs. The first way is to synthesize CLC molecules, but such single molecular system has a narrow range for pitch tuning often.9 Moreover, the synthesis of CLCs molecules is not easy and time-consuming. The second and most used method is to dope a small amount of chiral dopant into an achiral nematic LC, which can self-organize into a helical superstructure.10,11 For the latter one, the CLCs mixture contains the nematic LCs as host and the chiral dopant as guest. The nematic LCs can be induced into CLCs by the chiral template of dopant, in which the miscibility between dopant and nematic LCs is particularly important. Loading a stimuli-response dopant into a nematic LCs is an efficient strategy to achieve CLCs with a wide range of controllable pitch, because the structure and chirality of these dopants can be tuned easily using the external stimuli.12–14 Stimuli including heat,15 light,16 pH,17 host-guest interactions,18 have been applied in CLCs. Among these dopants, the photo-responsive dopant has been used widely to induce 3 / 25


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nematic LCs into CLCs due to its remotely controllable and quickly switchable capabilities. Since the isomerization of azobenzene group was successfully used to modulate the pitch of the induced CLCs mixtures in 1971,19 some other photo-responsive dopants containing spiropyran,20 fulgide,21 and diarylethene,22 have been used to vary the reflection colors of CLCs. When a photo-responsive chiral dopant is loaded into a nematic LCs, the ability of the dopant to twist the achiral nematic LCs phase into CLCs phase, i.e., the helical twisting power (β) can be expressed by the equation, β = (pc)-1, where c is the concentration of dopant. Upon a light with specific wavelength range and certain power, the isomerization of the chiral dopant happens, leading to a variation of its chirality and polarity, as well as the interaction between the dopant and LCs media. With the irradiation with another wavelength, the reversible process occurs. Therefore, the pitch and corresponding color of the induced CLCs can be easily tuned upon irradiation (Figure 1a, 1b). Grandjean-Cano method is a typical non-spectroscopic technique to measure the pitch (p) of helical structure using a wedge cell, where the alignment is planar and substrates are rubbed parallel, p = 2Rtanθ, R represents the distance between the Cano’s lines and θ is the wedge angle (Figure 1c).7 Azobenzene-containing chiral dopant is the most popular for the fabrication of photo-responsive CLCs due to the following reasons. Firstly, the azobenzene dopant has the advantages of cheap raw material, mild synthetic conditions, easy chemical modification, good fatigue resistance, and so forth. Secondly, the photo-isomerization of azobenzene moiety between the trans and cis isomers is reversible, easy to realize, and free from side 4 / 25


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reactions.23–26 Thirdly, the changes of molecular size and polarity between the trans and cis isomers are large, which is capable to tune the pitch of induced CLCs in a wide range.11,27–30 Combining azobenzene moiety with the axially chiral 1,1'-binaphthyl group,3 Li and his coworkers have done a series of pioneered studies on the doped CLCs with reversible wide range color-tuning, even a dynamic color photo-tuning in full visible light range.31–34 Meanwhile, the wavelength of stimuli light varies from ultraviolet (UV) to near infrared of 980 nm via loading upconversion nanoparticles into CLCs.11,35 In order to endow the doped CLCs photo- and thermal responses concurrently, a mixture of hydrogen-bonded two-component chiral dopants has been used to construct CLCs with full visible light range color, where the dopant mixture contains a proton-acceptor of binaphthyl azobenzene and a proton-donor of tetrahedral chiral acid.8

Figure 1. (a) A schematic mechanism of the reflective wavelength of photo-responsive azobenzene chiral dopant in achiral nematic. LCs medium was reversibly and dynamically tuned by light. (b) Schematic illustration of the color change of selective reflection for the doped CLCs upon different irradiations. (c) Schematic illustration of a Grandjean-Cano cell for the helical twisting power measurements of CLCs. Disclination lines are pointed out with arrows and the thickness change between two domains is marked as p/2.

Most researches have focused on how to improve the β value of dopants so far,36 however, the correlations between this value and the molecular structure as well as the interaction with nematic LCs are far from clear. On the other hand, the quantitative 5 / 25


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kinetics of the β variations from one isomer to another upon irradiation is also in absence. In this work, a series of binaphthyl azobenzene dopants with different length of alkyl or alkoxyl substituent groups have been synthesized successfully. Then, the effect of miscibility between these dopants and nematic LCs on the β values, as well as the time dependent decay/growth of the β values upon irradiations, have been investigated, which enables the precise tuning of the pitch and corresponding selective reflection wavelength and color of CLCs.

2. EXPERIMENTAL SECTION 2.1. Materials. The mixture nematic liquid crystals P0616A with a clear point TIN about 58 ºC were purchased from Shijiazhuang Chengzhi Yonghua Display Material Co., Ltd. The detailed information of the components was listed in Table S1. 2,2-Dimethoxy-2-phenylacetophenone (DMPA) was served as photo-initiator which could be triggered at 365 nm.37 All the starting materials were obtained from commercial supplies and used as received except that the solvents were treated by anhydrous sodium sulfate before used.

2.2. Syntheses of Photo-switchable Axially Chiral Dopants. In this work, four reversibly photo-switchable axially chiral dopants were synthesized through nucleophilic substitution and thiol-ene click reaction.38–40 Their chemical structures were shown in Scheme 1. The detailed synthetic routes were illustrated in Scheme S1. Their characterizations were also shown in Supporting Information.

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Scheme 1. The chemical structures of four dopants with different substituent groups.

2.3. Preparation of Chiral Nematic Liquid Crystals (CLCs). The CLCs were obtained by dissolving the dopant into P0616A, the mixture was heated to 120 ºC and kept for 30 min, then cooled to room temperature, finally capillary-filled into a cell. Two types of cells, i.e., the wedge cell of KCRK-07 with a wedge angle of 1.12º (tanθ = 0.0183) and the parallel rubbed cell of KSRO-05/B211M1N2205-RO, were used to measure the pitch and to observe the color of CLCs, respectively. Both cells were purchased from Score Asia Technology Ltd. 2.4. Characterization. The CD spectra were recorded at room temperature with a Jasco J-810 spectropolarimeter (Tokyo, Japan). The CD spectra from 575 to 275 nm were recorded at a scan speed of 500 nm/min with a cell path length of 0.1 cm. The data pitch was 1 nm with a band width of 1 nm. Each spectrum represents the average of three measurements. The samples were prepared by dissolving dopant in ethyl acetate (EtOAc) at a concentration of 5×10-4 mol/L. The irradiation wavelength and intensity of UV light were 365 nm and 1.2 mW/cm2. UV-Vis absorption spectra were recorded at room temperature on a diode-array spectrophotometer (Shimadzu UV-2550) with a cell path length of 1.0 cm. The 7 / 25


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samples were prepared by dissolving dopant in EtOAc at a concentration of 8×10-5 mol/L. The irradiation wavelength and intensity of UV light were 365 nm and 3.6 mW/cm2. The time dependent CD and UV-Vis measurements were carried out upon an ultraviolet lamp (ELC-500, light exposure system electrolyte corporation, US) at 365 nm and 1.2 mW/cm2 for trans-cis isomerization of dopant, as well as on a xenon lamp (Perfect Light, PLS-SXE300) through a filter at 550±13 nm and 30.6 mW/cm2 for cis-trans photo-isomerization of dopant. Differential scanning calorimetry (DSC) measurements were conducted with a DSC Q2000 (Thermal Analyst Co., TA Instruments) to determine the TCI of the CLCs. All experiments were performed from 30 ºC to 115 ºC at a heating rate of 10 ºC/min, kept at 115 ºC for 10 min, subsequently cooling at a rate of 10 ºC/min to 15 ºC in an argon atmosphere at 50 mL/min. Transition temperatures were taken at peak values from the cooling ramp. The distance between Cano’s lines and its variation process for CLCs upon visible light irradiation at 550±13 nm and an intensity of 52.8 mW/cm2, as well as UV irradiation at 365 nm and an intensity of 3.6 mW/cm2 at 25 ºC, were observed by a Carl Zeiss Jena polarized optical microscope (POM).

3. RESULTS AND DISCUSSION 3.1. Chirality and Isomerization of Dopants in EtOAc Solution. Circular dichroism (CD) and UV-Vis spectra have been used to characterize the chirality and trans-cis isomerization of azobenzene dopants in EtOAc solution, separately. Figure 8 / 25


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2a and 2b show the CD spectra of dopant 2 under UV and visible lights, respectively. The bands at 340 nm (a negative band) and 390 nm (a positive band) are attributed to the coupling of the long-axis polarized π-π* transition of the trans azobenzene chromophore, while the band at 454 nm corresponds to the n-π* transition of the cis azobenzene isomer. Upon irradiation with UV light, the mdeg values at 340 nm and 390 nm decrease 90% within 102 s; at the same time, the mdeg value at 454 nm increases about 6 times. These results indicate that the chirality of dopant 2 rapidly decreases upon UV irradiation. Moreover, this chirality is reversible. As shown in Figure 2b, the mdeg values at 330 nm and 390 nm increase from 8 mdeg to 60 mdeg in 134 s under visible light. These values only recover 73% in comparison with the original value before UV irradiation in Figure 2a, which implies that the dopant has not completely reached the photo-stationary state (PSS) at this time. Figure 2c and 2d show the UV-Vis spectra of dopant 2 under UV and visible lights, respectively. The band at 350 nm corresponds to a typical trans azobenzene absorption spectrum with an intense π-π* transition, while the band at 460 nm is assigned to the n-π* transition of the cis azobenzene isomer. Upon irradiation with UV light, the absorbance decreases remarkably, i.e., 42% within 51 s. The trans azobenzene recovers under visible light, recovers 92% within 264 s in comparison with the original value before UV irradiation in Figure 2c. The other azobenzene dopants with different length of substituent groups also show similar chirality and trans-cis isomerization responses (Figures S1 - S3).

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Figure 2. Time-dependent circular dichroism spectra (a, b) and UV-Vis spectra (c, d) of dopant 2 in EtOAC upon UV irradiation (a, c) and visible light irradiation (b, d). The concentrations of dopant 2 were 5×10-4 mol/L for circular dichroism spectra and 8×10-5 mol/L for UV-Vis spectra, respectively.

To investigate the effect of molecular structure on the photo-responsive behavior of the dopant in isotropic solution, the photo-isomerization process of dopant is evaluated quantitatively by fitting the experimental data to first order kinetics as following.41 ln

=

(1)

Where A∞, A0, and At are absorbance at 350 nm at time of PSS, time zero and time t, respectively, k is the photo-isomerization rate constant. The value of A∞ has been obtained when the absorbance does not change anymore upon UV irradiation. The experimental data for all four dopants obey the first-order behavior of trans-cis and cis-trans isomerization very well in Figures 3. The k values for the trans-cis isomerization of dopants 1 to 4 are 0.088 s-1, 0.100 s-1, 0.106 s-1 and 0.115 s-1, respectively. The corresponding k values for the cis-trans isomerization are 0.035 s-1, 0.032 s-1, 0.031 s-1, 0.028 s-1, separately. The rate of trans-cis / cis-trans 10 / 25


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photo-isomerization of dopants slightly increases / decreases with the length of alkyl or alkoxyl of substituent group. In general,

three mechanisms on the

photo-isomerization of azobenzene have been proposed,42,43 i.e., by rotation along the N–N bond with rupture of π bond, by inversion of the N=N bond with a semilinear hybridized transition state and intact π bond, or by concerted motion of the nitrogens and phenyl rings. It has been proven that most azobenzene molecules undergo the inversion mechanism for their photo-isomerization, since this mechanism has a much smaller free volume requirement than rotation mechanism. In this work, the optimized structures for the trans and cis dopants can be obtained using the quantum mechanical calculation in Figure S4. The dihedral angles of the two naphthyl planes (α) are 98.9°, 99.1°, 99.2°, 99.6° for trans-isomers 1 to 4, as well as 90.1°, 86.1°, 85.8°, 85.4 for their corresponding cis-isomers, respectively. Firstly, a slight increasing α value for trans isomers leads to a little decreasing of steric hindrance between substituent groups and thus a little increasing of free volume for isomerization. Secondly, as the difference between dihedral angles of the two-isomers naphthyl planes for trans and cis forms increases from 8.8° for dopant 1 to 14.2° for dopant 4, the driven force for the transformation of dopants from trans to cis should become larger and larger. As a result, although the energy of ground state and transition state might simultaneously increases / decreases on the potential energy surface,44 their corresponding barrier decreases gradually, indicative of an increasing k value. On the other hand, the cis-trans photo-isomerization should undergo a very different path on the potential energy surface.43 The tendency of the energy barrier between ground state and 11 / 25


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transition state for cis isomer is able to increase from dopants 1 to 4. More detailed calculations are being studied for quantitative analyses.

Figure 3. Time dependence of ln[(A∞-At)/(A∞-A0)] on UV irradiation at 350 nm (a) and then visible light irradiation (b) for different dopants in EtOAc.

3.2. Miscibility between Dopants and P0616A. Hansen solubility parameter, which can estimate the competition between the solute–solvent interaction and the solute–solute interaction in the case of a molecularly dispersed solution, has been successfully used for several decades to select solvents for coatings and other applications.45–47 As mentioned in the introduction section, a good miscibility between chiral dopant and nematic LCs host is of great importance for the formation of CLCs. Herein, the theoretical Teas parameter, one kind of Hansen solubility parameters, and the isotropic-chiral liquid crystal transition temperature TCI by DSC experiment, are employed to further understand the interaction between dopant and nematic LCs. The Teas parameters are estimated by the following equations.41 = ⁄

(2)

= ⁄

(3)

= ⁄

(4)

Where fd, fp, and fh represent the dispersion, polar, and hydrogen-bonding components for the fractional cohesion, respectively. Their corresponding solubility parameters δd, 12 / 25


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δp and δh are calculated from groups contribution as follows.48 = ∑ ⁄

(5)

= ∑

(6)

= !∑ " ⁄

(7)

Where V is the molar volume, Fdi, Fpi and Ehi are contributions of dispersion force, polar force and hydrogen bonding of each group. These values are listed in Table S2. Usually, the interaction between LCs and dopant decreases with increasing of their difference of Teas parameters, ∆f.49, 50

∆ = ∑$%,'( %,)*+, -

(8)

Where the subscript j represents d, p or h. As shown in Figure 4a, the gaps between all dopants and P0616A are less than 0.2, indicating a good miscibility between all dopants and P0616A. The use of the Teas parameters for the system is based on the rule of chemistry “like dissolves like”.51 The biphenyl group in P0616A is a rod-like structure. With the help of substituent alkyl or alkoxyl groups, the fraction of rod-like binaphthyl group in dopant decreases from 1 to 4, leading to an increasing ∆f from 0.056 to 0.191 and a decreasing miscibility between the dopant and LCs hosts.

Figure 4. (a) Teas plot of calculated solubility parameters of P0616A and four dopants. (b) DSC cooling curves for the P0616A/dopant 2 mixtures with different dopant content. The TCI of the mixture decreased with the increase of dopant concentration. 13 / 25


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In addition, the solutions of dopants in P0616A above TCI (phase temperature from CLCs state to isotropy state) are transparent at high concentration. For instance, the loading of dopant 2 in P0616A can reach as high as 25 wt%. Even if the solutions are cooled down to room temperature, no any precipitates appear for more than one year. Moreover, DSC cooling curves only exhibit one exothermal peak for all mixtures of dopant 2 and P0616A with different dopant content (Figure 4b). The peak values, i.e., the transition temperatures of TCI gradually decreases from 58 °C for the neat P0616A to 50 °C for the mixture with 6 wt% dopant 2. The cooling curves for other dopants in P0616A show similar behavior in Figure S5. Bunning et al have reported that the doping of a closed spiropyran lowered the TIN of liquid crystal host 5CB.52 In another case, we have found that the loading of alcohols also lowered the TIN of the syrups for polymer dispersed liquid crystals (PDLCs).53 Both decreasing of TNI can be ascribed to the diluent effect of the additives on the LC hosts, where the additives disrupt the original order of LCs. In other words, the LC hosts and the additives, i.e., P0616A and the dopants in this work, possess a good miscibility. 3.3 Tuning Helical Twisting Power of Dopants in CLCs via Photoisomerization Kinetics. A hot mixture of 2.0 wt% different dopant in LCs P0616A is injected into a wedge cell with a wedge angle θ of 1.12º by capillarity. After the mixture is cooled to room temperature, the Cano’s lines of induced CLCs within the wedge cell can be clearly observed through a polarized optical microscope. Their representative snapshots of POM images upon UV and visible light irradiations are shown in Figure 5 and Figures S6-S8. For instance, the distance between Cano’s lines (R) of the CLCs 14 / 25


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with dopant 2 increases from 62 µm to 219 µm in 5 min upon UV irradiation (Figure 5a-5c). On the contrary, when the mixture is irradiated by visible light, the R value reversibly decreases from 219 µm to 117 µm within 18 min. These changes can be ascribed to the reversible photoisomerization between trans and cis azobenzenes under UV and visible light irradiations. In comparison with the rod-like trans azobenzene, the bent-shaped cis form is less miscible with the rod-like LCs hosts. Notably, there is no any precipitate upon different irradiation conditions as shown in Figure S9, indicative of good miscibility of both trans dopants and cis ones in the LCs. As a result, with the increase of UV irradiation time, the increasing cis azobenzene loosens the compact helical structure of CLCs formed by trans azobenzene, which leads to a larger R value and a wider pitch p for CLCs. On the contrary, the p and R values decrease with the increasing of trans-form content upon visible light irradiation. According to the relationship of = , the reflection wavelength λ will red shift upon UV irradiation and blue shift upon visible light irradiation, respectively.

Figure 5. (a-c) POM images of 2.0 wt% dopant 2 in P0616A upon UV irradiation at 0 min (a), 0.7 min (b), and 5 min (c). (f-j) POM images of 2.0 wt% dopant 2 in P0616A upon visible irradiation after (c) at 6 min (d) 7.5 min (e) and 10.5 min (f). The scale bar is 100 µm.

The helical twisting power β can be calculated from its definition of β = (pc)-1. The 15 / 25


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molar β values for all dopants in P0616A at photo-stationary states (PSS) before and after UV irradiations are summarized in Table 1. For the mixture of chiral dopants in P0616A, the β values decrease from 82.6 µm-1 for dopant 1 to 20.9 µm-1 for dopant 4 at visible PSS and from 30.8 µm-1 to 12.4 µm-1 at corresponding UV PSS, respectively. As shown in Figure 4a, the gap of Teas parameters between dopants and LCs hosts increases from dopant 1 to dopant 4, which implies their miscibility decreases. With the increase of length of alkyl or alkoxy substituent group, the fraction of rod-like trans azobenzene group in the dopant decreases. So, more amount of dopant is required to be used as template for a given pitch in CLCs. Namely, the helical twisting power of dopant decreases from dopant 1 to dopant 4. On the other hand, the effect of light irradiation on the β value is related to the change of miscibility between the chiral dopant and the host LC molecule, which originates from the variety of molecular orientation and the decrease of fraction of rod-like trans structure in the dopant via trans-cis photoisomerization.54 The changing kinetics of β values for different dopants from one PSS to another upon light irradiations is investigated in this work because it is of great importance for precise tuning the corresponding reflection wavelength or color. Figure 6a shows the time dependent helical twisting power of different dopants in P0616A with 2.0 wt% dopant upon UV light. Within 330 s of UV irradiation, the β value gradually decreases from 82 µm-1 to 32 µm-1, from 56 µm-1 to 26 µm-1, from 38 µm-1 to 15µm-1, and from 21µm-1 to 12 µm-1 for dopant 1 to 4 in P0616A, respectively. Then, a plateau of β values appears, which indicates that the dopants in CLCs mixtures reach their UV 16 / 25


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PSS. Interestingly, the change of β values obeys the exponential decay behavior very well. By fitting the experimental data, the trans-cis photoisomerization rate constant k in CLCs increases from 0.011 s-1 for dopant 1 to 0.026 s-1 for dopant 4. The corresponding k value for cis-trans transformation in CLCs upon visible light irradiation decreases from 0.0051 s-1 for dopant 1 to 0.0019 s-1 for dopant 4 (Figure 6b). These data are also summarized in Table 1. Obviously, although the light intensities for exposure to CLCs are higher than those used in EtOAc solutions, these k values in anisotropic LC hosts are 4 ~ 8 times for trans-cis photoisomerization and 8 ~ 15 times for cis-trans transformation smaller than those in isotropic EtOAc solution. First, the sizes of LC molecules are larger than EtOAc, leading to a larger steric hindrance effect on the dopant motion during the trans-cis and cis-trans transformations. Second, the rod-like dopant is regarded as a template to induce the alignment of rod-like nematic LC molecules along the major axis of azobenzene group. As a result, it forms a more ordered superstructure of CLCs, where the dopant molecule is coated by a suit of impact armor of oriented LC molecules. Consequently, it is required more time to destroy and reconstruct the ordered superstructure of CLCs. Third, the viscosity for P0616A (31.5 mPa·s) is much higher than that for EtOAc (0.45 mPa·s) at room temperature, which also slows down the inversion of azobenzene group during photoisomerization. It is noteworthy that the change trend of k values for dopants is consistent both in isotropic EtOAc solution and in anisotropic CLCs. In other words, with increasing of the length of substituent group from dopant 1 to dopant 4, the k value increases during trans-cis transformation and decreases 17 / 25


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during the cis-trans transformation. The photoisomerization kinetics for the CLCs of 2 wt% dopant 2 in another nematic LC 5CB and different concentrations of dopant 1 in P0616A also obeyed well to the fitted equation upon UV/Vis irradiations as showed in Figures S10-S12. These results indicate that the quantitative analyses in isotropic solutions can provide a qualitative guidance in anisotropic media sometimes, which is able to simplify some experimental designs and measurements.

Figure 6. Time dependent helical twisting power of different dopants in P0616A with 2.0 wt% dopant upon UV irradiation (a) and then visible light irradiation (b). Table 1. Characteristic parameters of dopants and their mixtures with EtOAc or P0616A. dopant αtrans /

o *a

αcis / o *a -1

βtrans / µm -1

βcis / µm

dopant 1

dopant 2

dopant 3

dopant 4

98.9

99.1

99.2

99.6

90.1

86.1

85.8

85.4

82.6

55.2

37.6

20.9

30.8

26.5

15.4

12.4

-1 *b

0.088

0.100

0.106

0.115

kcis-trans,EtOAc / s-1 *b

ktrans-cis,EtOAc / s

0.035

0.032

0.031

0.028

-1 *b

0.011

0.013

0.017

0.026

-1 *b

0.0051

0.0041

0.0041

0.0019

0.056

0.141

0.175

0.191

ktrans-cis,P0616A / s kcis-trans,P0616A / s ∆f

*c

* a) α, the dihedral angle of the two naphthyl planes; b) k, the photo-isomerization rate constant; c) ∆f, the difference of Teas parameters between the dopant and liquid crystal P0616A. Subscripts of trans, cis, trans-cis, cis-trans, EtOAc, and P0616A represent the trans-dopant, cis-dopant, isomerization from trans form to cis form upon UV irradiation, isomerization from cis form to trans form upon visible light irradiation, the solution in EtOAc, the mixture with P0616A, respectively. 18 / 25


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Also, the color and its change of the CLCs with different concentrations of dopant 2 upon UV irradiation can be tuned as shown in Figure 7. Figure 7a shows that the colors of CLCs red shift from green, yellow, and colorless when the dopant concentration decreases from 10 wt%, 6 wt%, to 4 wt% and 2wt%. The colors are in good agreement with the equations p = 2Rtanθ and λ = np. More interestingly, upon UV irradiation, the colors for CLCs with dopant concentrations more than 6.0 wt% red shift quickly to colorless and transparent within 60 s. For example, Figure 7b shows the color changes from yellow, orange, red, to colorless for the CLC with 6.0 wt% dopant, while it changes from green to yellow, orange, red, to colorless for the CLC with 10 wt% dopant in Figure 7c. Obviously, these colorless and transparent CLCs are in that the selective reflection light shifts to the near infrared region. These colors and their changes upon UV irradiation have been verified by the reflection spectra through the UV-Vis transmission mode in Figure S13. Thus, through the consequence discussed, we can selectively control the reflection wavelength as well as the color of the CLC systems.

Figure 7. (a) Photos of CLCs with different concentrations of dopant 2 in P0616A within parallel orientation cells with a gap of 5 µm. (b) Photos of CLC with 6 wt% dopant 2 in P0616A upon UV irradiation for 0 s, 3 s, 9 s, 15 s and 45 s. (c) Photos of CLC with 10 wt% dopant 2 in P0616A upon UV irradiation for 0 s, 3 s, 15 s, 27 s, and 57 s. All photos were taken for the cells (25 mm × 18 mm) on a black table. 19 / 25


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4. CONCLUSIONS In summary, a series of reversibly photo-switchable axially chiral dopants with different length of alkyl or alkoxyl substituent groups have been successfully synthesized through nucleophilic substitution and thiol-ene click reaction. The theoretical difference of Teas solubility parameters between dopant and nematic LCs increases from 0.056 for dopant 1 to 0.191 for dopant 4, indicative of a decreasing miscibility between them with increasing of the length of substituent group. The β value of chiral dopants in nematic LCs decreases from dopant 1 to dopant 4 both at visible light photostationary state (PSS) and at UV PSS after UV irradiation. With increasing of the length of substituent group, the photoisomerization rate constant of dopants increases for trans-cis transformation upon UV irradiation and decreases for the reverse process upon visible light irradiation either in isotropic ethyl acetate or in anisotropic LCs, although the constant in ethyl acetate is 4-15 times larger than the corresponding value in LCs. These results enable the precise tuning of the pitch and selective reflection wavelength/color of CLCs, which paves the way to the applications in electro-optic devices, information storage, high-tech anti-counterfeit, etc. ASSOSIATED CONENT Supporting Information General synthetic procedures for chiral dopants. Detailed information for P0616A. UV-vis and CD spectra for dopants. Optimal structures for trans and cis chiral dopants. Solubility parameters for dopants and P0661A. TCI and typical picture for the CLC mixture. Reversible R upon UV and visible light irradiations of P0616A/dopant 20 / 25


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mixtures with 2.0 wt% dopant under UV-Vis irradiation. Photoisomerization kinetics of dopant 1 in 5CB. Concentration dependent photoisomerization kinetics of dopant 2 in P0616A. The transmission spectra of CLCs upon UV irradiation. The Supporting Information is available free of charge on the ACS Publications website at DOI: …… AUTHOR INFORMATION Corresponding Authors *

(Y.L.) Tel: +86-27-8755-8194. Fax: +86-27-8754-3632. E-mail:

[email protected]. *

(H.P.) Tel: +86-27-8754-0053. Fax: +86-27-8754-3632. E-mail:

[email protected]. ORCID Yonggui Liao: 0000-0003-2943-1501 Haiyan Peng: 0000-0002-0083-8589 Notes The authors declare no competing financial interest. Acknowledgements We acknowledge the financial support of the National Natural Science Foundation of China (51773070, 51373060, and 51433002). We greatly appreciate the helpful discussion on Gaussian09 calculation with Professor Rongzhen Liao. We also thank the HUST Analytical and Testing Center for CD Measurements. References 1.

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Precisely Tuning Helical Twisting Power via Photoisomerization

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