Enhancement of Photosensitivity of Photorefractive ... - ACS Publications

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Enhancement of Photosensitivity of Photorefractive Ferroelectric Liquid Crystal Blends to Green and Red Wavelength Regions Using Oligothiophene Photoconductive Dopants Takeo Sasaki,*,† Shouta Morino,† Azusa Sumiya,† Yuta Yamamoto,† Masaya Nakano,† Khoa Van Le,† Yumiko Naka,† and Takafumi Sassa‡ †

Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Advanced Device Laboratory, RIKEN (The Institute of Physical and Chemical Research), Cooperation Center, South Area, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

‡

ABSTRACT: The photorefractive effect can be used to generate rewritable holograms in a material, and it is utilized in various devices, including three-dimensional displays, optical tomography units, novelty filters, phase conjugate wave generators, and optical amplifiers. Ferroelectric liquid crystal (FLC) blends composed of a smectic liquid crystalline mixture, a chiral photoconductive dopant, and an electron trap reagent exhibit significant photorefractivity together with rapid responses. In previous studies, terthiophene derivatives were used as chiral photoconductive dopants and the resulting photorefractive effects were examined at 488 nm. In order to utilize the photorefractive ferroelectric liquid crystals in practical devices such as photoacoustic interferometers and ultrasound imaging units, the wavelength sensitivity of the material should be extended to longer regions. In the present work, chiral dopants possessing quarter-thiophene chromophores, sexithiophenes, and other photoconductive compounds with larger molecular structures were prepared and the photorefractive effect was examined in FLC blends containing these compounds, at longer wavelengths.

1. INTRODUCTION The photorefractive effect in mesophases has been studied extensively in the past 20 years.1−3 The photorefractive effect exhibited by ferroelectric liquid crystals (FLCs) has been previously investigated,2,3 and FLC blends composed of liquid crystalline compounds and chiral photoconductive dopants have shown significant photorefractivity. Since it is difficult to obtain both high transparency and good performance using a single FLC compound, mixtures of various liquid crystalline and chiral compounds are often employed for display applications.4,5 As an example, the amplification of dynamic optical signals by photorefractive FLC blends has been reported.6,7 The photorefractive effect results in the formation of a dynamic refractive index grating within a medium, based on changes in the refractive index of the medium resulting from photoinduced electric field and electrooptic effects.8−10 Figure 1 shows the mechanism of the photorefractive effect in FLCs doped with photoconductive compounds. A space-charge field is formed at the interference fringe in the FLC medium, and the direction of the spontaneous polarization of the FLC is changed by this field. In most cases, the change in direction of the spontaneous polarization is small; however, since the birefringence of the FLC is very large, a relatively large change in apparent refractive index is induced. The asymmetric energy exchange, in which the energy of one of the interfering laser beams is transferred to the other (thus increasing its intensity), is the most characteristic aspect of the photorefractive effect.11,12 This asymmetric energy exchange can © XXXX American Chemical Society

be employed for the purposes of optical signal amplification. As noted, the FLCs used in practical applications are typically mixtures of several liquid crystalline compounds and chiral dopants, while a photoconductive compound may also be added in order to obtain a photorefractive material. However, in most cases, the photoconductive compounds are not liquid crystals, so the addition of these chemicals to the FLC mixture disturbs the alignment of the liquid crystalline compounds and increases the degree of light scattering in the FLC medium. To avoid this problem, photoconductive compounds that also possess chiral structures have been synthesized,13,14 allowing the formulation of photorefractive FLCs simply by combining a chiral photoconductive compound with an FLC mixture. Our own group has previously reported that photorefractive FLC blends containing chiral photoconductive dopants exhibit both significant photorefractivity and fast response times.6,7 In prior studies, terthiophene compounds were employed as chiral photoconductive dopants (Figure 2, 3T-2MB) and, because the absorption of terthiophenes occurs at wavelengths shorter than 500 nm, a 488 nm laser was used to induce the photorefractive effect. In the present study, quarter-thiophenes (Figure 2, C8-4T-2MB and 2EH-4T-2MB) were synthesized and mixed with smectic liquid crystals to form an FLC, and the photorefractive Received: June 16, 2017 Revised: July 19, 2017 Published: July 24, 2017 A

DOI: 10.1021/acs.jpcc.7b05921 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 2. Structures of chiral photoconductive dopants, smectic LCs, and electron trap reagent (TNF).

Figure 1. Schematic illustration of the mechanism of the photorefractive effect in an FLC. (a) Two laser beams undergo interference in the surface-stabilized state of the FLC/photoconductive compound mixture, (b) charge generation occurs in the bright areas of the interference fringes, (c) electrons are trapped at trap sites in the bright areas, while holes migrate by diffusion or drift in the presence of an external electric field to generate an internal electric field between the bright and dark positions, and (d) the orientation of the spontaneous polarization vector (i.e., the orientation of mesogens in the FLC) is altered by the internal electric field.

dopants 5-hexyl-5‴-((((5-hexylthiophen-2-yl)methyl)thio)methyl)-2,2′:5′,2″:5″,2‴-quarter-thiophene (6T), 2,4-bis[4(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine (SQ), MK-2 dye (MK-2), 2,3,9,10,16,17,23,24-octakis(octyloxy)29H,31H-phthalocyanine (PC), pentacene (P), and 2,5-di(2ethylhexyl)-3,6-bis(5″-n-hexyl-[2,2′,5′,2″]terthiophene-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (SM) were obtained from Tokyo Chemical Industry Co., Ltd. The chiral dopant 4′-alkoxybiphenyl-4-yl (2S,3S)-2-chloro-3-methylpentanoate (3M2CPOOB) was synthesized according to the literature.15 The base-LC, photoconductive chiral dopant (or photoconductive compound and chiral dopant), and TNF were dissolved in chloroform and the solvent was evaporated. The mixture was stored in a vacuum at room temperature for 72 h. The resulting mixture was injected into a glass cell (EHC Co. Ltd.) equipped with an indium tin oxide (ITO) electrode and a polyimide alignment layer to produce a 10 μm thick FLC layer. 2.1.1. Synthesis of the Photoconductive Chiral Dopant 2EH-4T-2MB. 2.1.1.1. 5-(2-Ethylhexyl)-2,2′:5′,3-terthiophene. A quantity of 2,2′:5′,3-terthiophene (3.03 g, 12.2 mmol) was dissolved in 100 mL of dry tetrahydrofuran (THF) in a 500 mL three-necked flask filled with N2 and cooled to −78 °C. Subsequently, n-butyl lithium in n-hexane (5.57 mL, 14.5 mmol) was added dropwise to the solution. After stirring for 1 h, the mixture was heated to 0 °C and stirred for a further 10 min. The solution was then cooled back to −78 °C, and a THF solution of potassium tert-butoxide (12.0 mL, 12.0 mmol) was added dropwise, followed by the dropwise addition of 1-bromo-2ethylhexane (2.50 mL, 14.5 mmol). The reaction mixture was then heated to room temperature and stirred for 12 h. An aqueous solution of HCl (1 N, 20 mL) was added, and the

properties of this mixture at longer wavelengths were investigated. In order to lengthen the working wavelength, larger photoconductive compounds such as sexithiophene were also examined.

2. EXPERIMENTAL SECTION 2.1. Samples. The structures of the compounds used in this study are shown in Figure 2. The synthetic route for the quarterthiophene chiral photoconductive dopant 2EH-4T-2MB is summarized in Figure 3. The photoconductive chiral dopant C8-4T-2MB was synthesized in the same manner. Liquid crystalline compounds 2-(4-hexyloxyphenyl)-5-octylpyrimidine (8PP6), 2-(4-octyloxyphenyl)-5-octylpyrimidine (8PP8), and 2-(4-decyloxyphenyl)-5-octylpyrimidine (8PP10) were obtained from Wako Pure Chemical Industries. A mixture of the phenylpyrimidine-type smectic liquid crystalline compounds 8PP6, 8PP8, and 8PP10 (at a mass ratio of 2:1:1) was used as the host liquid crystal (base-LC). The chiral photoconductive dopant was subsequently mixed with this host liquid crystal mixture along with the electron trap reagent 2,4,7-trinitrofluorenone (TNF, Tokyo Chemical Industries Co.). The concentrations of the photoconductive chiral dopants (C8-4T-2MB and 2EH-4T-2MB) were varied from 2 to 10 wt %. The concentration of TNF was set to 0.1 wt %. The photoconductive B


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Figure 3. Synthetic route to 2EH-4T-2MB.

the solvent, the product was purified by column chromatography on silica gel (eluent: 1:1 mixture of chloroform and hexane). A dark yellow oil was obtained (1.96 g, yield 44.6%). 1 H NMR (CDCl3) δ 0.89 (t, 6H, CH3), 1.20−1.42 (m, 9H, −(CH)− and −(CH2)−), 2.73 (d, 2H, Ar−CH2−), 6.66 (d, 1H, Ar−H), 6.99 (d, 2H, Ar−H), 7.00 (d, 1H, Ar−H), 7.04 (d, 1H, Ar−H), 7.14 (d, 1H, Ar−H), 7.19 (d, 1H, Ar−H). 2.1.1.2. 2-(5″-(2-Ethylhexyl)-[2,2′:5′,2″-terthiophen]-5-yl)4,4,5,5-tetramethyl-1,3,2-dioxaborolane. A quantity of 5-(2ethylhexyl)-2,2′:5′,2″-terthiophene (1.96 g, 5.34 mmol) was dissolved in 30 mL of dry THF in a 300 mL three-necked flask filled with N2. The solution was cooled to −78 °C and n-butyl lithium in n-hexane (2.72 mL, 7.07 mmol) was added dropwise, after which the mixture was heated to 0 °C and stirred for 10 min. The solution was then cooled back to −78 °C and 2-isopropoxy4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.44 mL, 7.07 mmol) was added dropwise. The solution was heated to room temperature and stirred for 12 h, following which an aqueous solution of HCl (1 N, 20 mL) was added and the product was extracted with chloroform. The chloroform was washed with HCl (1 N) and dried over Na2SO4. After removal of the solvent, the product was purified by column chromatography on silica gel (eluent: 2:1 mixture of chloroform and hexane). Dark green crystals were obtained (0.91 g, yield 35.0%). 1 H NMR (CDCl3) δ 0.88 (t, 6H, −CH3), 1.34 (m, 21H, −(CH)−, −(CH2)−, and −CH3), 2.72 (d, 2H, Ar−CH2−), 6.66 (d, 1H, Ar−H), 6.98 (d, 2H, Ar−H), 7.10 (d, 1H, Ar−H), 7.20 (d, 1H, Ar−H), 7.51 (d, 1H, Ar−H). 2.1.1.3. (S)-2-Methylbutyl-5-bromothiophene-2-carboxylate. A combination of 5-bromothiophene-2-carboxylic acid (3.13 g, 15.1 mmol), (S)-2-methyl-1-butanol (1.62 mL, 15.1 mmol), 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC, 4.29 g, 22.4 mmol) and N,N-dimethyl-4-aminopyridine (DMAP, 2.86 g, 23.4 mmol) was dissolved in 70 mL of dry chloroform and the mixture was stirred under N2 for 3 days. The solution was subsequently washed with HCl (1 N) and dried over Na2SO4. After removal of the solvent, the product was purified by column chromatography on silica gel (eluent: chloroform). The product was obtained as a slightly yellow liquid (3.88 g, yield 92.7%). 1 H NMR (CDCl3) δ 0.96 (t, 3H, CH3), 1.00 (d, 3H, CH3), 1.27−1.44 (m, 2H, −(CH2)−), 1.82 (m, 1H, −COO−CH− (CH2)−), 4.10 (dd, 1H, −COO−CH2−), 7.06 (d, 1H, Ar−H), 7.54 (d, 1H, Ar−H).

Figure 4. (a) UV−vis absorption spectra of chiral photoconductive dopants C8-4T-2MB, 2EH-4T-2MB, and 3T-2MB in chloroform solution, (b) UV−vis absorption spectra of a mixture of C8-4T-2MB and TNF in chloroform solution, and (c) comparison of the absorption spectra of C84T-2MB and a mixture of C84T-2MB and TNF in chloroform solution.

product was extracted with chloroform. The chloroform was washed with HCl (1 N) and dried over Na2SO4. After removal of C


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Figure 5. Textures of FLC blends containing C8-4T-2MB in a 10 μm gap LC cell as observed using polarizing microscopy.

Figure 7. Textures of FLC blends containing 2EH-4T-2MB in a 10 μm gap LC cell as observed using polarizing microscopy.

(0.83 g) were placed in a three-necked flask, after which the flask was filled with N2 and 100 mL of dry THF was added. A solution of (S)-2-methylbutyl-5-bromothiophene-2-carboxylate (3.00 g, 10.3 mmol) in 20 mL of dry THF was added dropwise, and the solution was refluxed for 24 h. The mixture was then cooled to room temperature and 100 mL of pure water was added. The product was extracted with chloroform and washed with HCl (1 N). The chloroform solution was dried over Na2SO4. After removal of the solvent, the product was purified by column chromatography on silica gel (eluent: 1:1 mixture of

Figure 6. Phase diagrams of the base-LC and photoconductive chiral dopants (a) 2EH-4T-2MB and (b) C8-3T-2MB.

2.1.1.4. (S)-2-Methylbutyl 5‴-(2-Ethylhexyl)[2,2′:5′,2″:5″,2‴-quaterthiophene]-5-carboxylate (2EH-4T2MB). Quantities of 2-(5″-(2-ethylhexyl)-[2,2′:5′,2″-terthiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.84 g, 1.73 mmol), Pd(PPh3) (0.21 g, 0.182 mmol), and K2CO3 D


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continuous wave) was divided in two by a beam splitter, and these two beams underwent interference with one another in the sample. The laser intensity was 250 mW/cm2 for each beam. The beam angles incident to the glass plane were 30 and 50°, and the resulting interference fringe interval was 1.87 μm. An electric field ranging from 0 to 10 V/μm was applied to the sample from a regulated dc power supply (Kenwood DW36-1), while the change in the transmitted beam intensity was monitored by photodiodes (ET-2040, Electro-Optics Technology, Inc.). The refractive index grating formation time in the FLC blends was determined based on the simplest single-carrier model of photorefractivity. The rising signal of the diffracted beam was fitted using the single exponential function Figure 8. Transmitted beam intensities through FLC samples as functions of concentrations of photoconductive chiral dopants, using a 633 nm laser. Legend: ●, 2EH-4T-2MB; ■, C8-3T-2MB.

γ(t ) − 1 = (γ − 1)[1 − exp( −t /τ )]2

(1)

where γ(t) represents the transmitted beam intensity at time t divided by the initial intensity (γ(t) = I(t)/I0), γ is the transmitted beam intensity at t = ∞ divided by the initial intensity (γ = I(∞)/I0), and τ is the formation time. The two-beam coupling gain coefficient Γ was calculated assuming Bragg diffraction and using the equation7−11

hexane and chloroform) followed by recrystallization from methanol. The product was obtained as yellow crystals (1.52 g, yield: 62.8%). 1 H NMR (CDCl3) δ 0.99−0.85 (m, 12H, −CH3), 1.40−1.20 (m, 8H, −CH2−), 1.61−1.49 (m, 3H, −CH−), 1.85 (m, 1H, −CH−), 2.73 (d, 2H, Ar−CH2−), 4.12 (m, 2H, COO−CH2−), 2.80 (t, 2H, Ar−CH2−), 6.67 (d, 1H, Ar−H), 6.99 (d, 2H, Ar− H), 7.01 (d, 2H, Ar−H), 7.10 (d, 1H, Ar−H), 7.19 (d, 1H, Ar− H), 7.69 (d, 1H, Ar−H). Anal. Calcd for C30H36O2S4: C, 64.71; H, 6.52. Found: C, 64.62; H, 6.10. 2.2. Measurements. The photorefractive effect was investigated via two-beam coupling experimental trials. A linearly polarized (p-polarized) beam from a DPSS laser (532 nm,

Γ = (1/D) ln(gm /(1 + m − g ))

(2)

where D (=L/cos(θ)) is the interaction path for the signal beam (L = sample thickness, θ = propagation angle of the signal beam in the sample), g is the ratio of the signal beam intensities behind the sample with and without a pump beam, and m is the ratio of the beam intensities (pump/signal) in front of the sample.

Figure 9. Light and dark currents obtained from FLC blends containing chiral photoconductive dopants as functions of applied electric field, at a dopant concentration of 6 wt % in all samples. A 488 nm laser beam (10 mW/cm2, 1 mm diameter) and a 532 nm laser beam (10 mW/cm2, 1 mm diameter) were used as irradiation sources. E


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Figure 10. Typical example of asymmetric energy exchange observed in two-beam coupling experiments for a ternary mixture base-LC, 2EH4T-2MB, and TNF at 30 °C. The pump beam was incident at 2 s. The beam incidence conditions are inserted in the figure.

Figure 12. Two-beam coupling gain coefficients (a) and refractive index grating formation times (b) of FLC blends as functions of concentrations of photoconductive chiral dopants, employing a 532 nm laser wavelength at 30 °C. Legend: ●, 2EH-4T-2MB; ■, C8-3T-2MB.

Figure 11. Temperature dependence of the two-wave mixing gain coefficient, employing a concentration of 2EH-4T-2MB in base-LC of 6 wt %.

The branched tail is expected to prevent the crystallization of the quarter-thiophene moiety. The melting point of C8-4T-2MB (179 °C) was lowered by the introduction of the branched tail (2EH-4T-2MB, 93 °C). The phase diagrams of the FLC blends are shown in Figure 6. The temperature range of the SmC* phase was widened in the case of the blend containing the branched tail quarter-thiophene chiral dopant. Figure 7 presents photographic images of the textures of samples containing 2EH-4T-2MB, acquired with a polarizing microscope. No dopant crystallization was observed at dopant concentrations of 4 and 6 wt %. Thus, the miscibility of the quarter-thiophene dopant was improved by the introduction of the branched tail structure. The transmittance of a 633 nm laser through the FLC samples was subsequently investigated. The transmitted intensities of the laser beam through the samples as functions of the concentration of the quarter-thiophene dopants are plotted in Figure 8, from which it is evident that the FLC sample containing 2EH-4T-2MB exhibited greater transparency. 3.2. Photorefractive Effect of FLC Blends Containing Quarter-Thiophene Photoconductive Chiral Dopants. The photoconductivities of the FLC samples were also assessed, providing the data shown in Figure 9. These blends were found to be good insulators under dark conditions, while irradiation with a 488 nm laser beam clearly generated photocurrents.

3. RESULTS AND DISCUSSION 3.1. Miscibilities of Quarter-Thiophene Photoconductive Chiral Dopants with Base-LC. Figure 4a shows the UV− visible absorption spectra of the chiral photoconductive dopants used in this study. The quarter-thiophene moiety absorbs at longer wavelengths compared to the terthiophene because of its expanded π-conjugation. The absorption wavelength of the quarter-thiophene was also extended to longer wavelengths when mixed with the TNF (Figure 4b,c) because of the appearance of a charge transfer absorption band. Thus, the quarter-thiophene dopants were confirmed to be appropriate for use with a writing beam wavelength of 532 nm. Figure 5 presents the textures of the FLC blends containing C8-4T-2MB in a 10 μm gap LC cell observed under a polarizing microscope. C8-4T-2MB was soluble in the base-LC at a concentration of 4 wt %, while precipitation of C8-4T-2MB crystals was observed at concentrations above 6 wt %. The quarter-thiophene moiety is larger than the terthiophene, and so the quarter-thiophene is more likely to crystallize. In order to prevent the quarter-thiophene dopants from crystallizing in the LC medium, the molecular structure was modified. Specifically, a branched structure was introduced to a flexible alkyl chain (or tail unit) attached to the quarter-thiophene moiety (Figure 2, 2EH-4T-2MB). F


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Figure 13. Structures of photoconductive compounds and host FLC mixture used in this study. The concentration of the chiral dopant was 10 wt % relative to the 2:1:1 ternary mixture base-LC.

time was in the range 10−40 ms at the shortest. The value is much longer than the formation time reported for the FLC blends with terthiophene photoconductive chiral dopants. The slower response of the quarter-thiophene samples is considered to result from the heterogeneous dispersion of the quarterthiophene dopants in the LC medium. The ionic conduction of the photogenerated species (quarter-thiophene cation and TNF anion) as well as the hopping conduction occurred in the quarterthiophene sample. The contribution of the ionic conduction leads to a slower response. The grating formation time of LC blends with 2EH-4T-2MB was found to be longer than that with C8-4T-2MB. The difference is considered to come from the difference of the dispersion of the dopants in LC medium. The structure of 2EH-4T-2MB prevents approaching of the molecules so that the hopping conduction is also prevented. C8-4T-2MB molecules can be approached closer to another C8-4T-2MB molecule than that between 2EH-4T-2MB molecules. The hopping conduction is difficult in FLC blends with 2EH-4T-2MB. C8-4T-2MB is difficult to mix with the baseLC mixture without making many defects; however, the distance between the C8-4T-2MB molecules can be closer than that between 2EH-4T-2MB molecules. Thus, the formation of spacecharge field, i.e., the grating formation, was considered to be fastened in FLC blends with C8-4T-2MB. 3.3. Photorefractive Effect of FLC Blends Containing Extended π-Conjugated Compounds. Photoconductive compounds that absorb at longer wavelengths were also examined in this study, as shown in Figure 13. These compounds were combined with an FLC mixture composed of the base-LC and a chiral dopant. Unfortunately, the majority of the compounds in Figure 13 were found to be insoluble in the base-LC, although the SM was soluble at concentrations below 1 wt %. Two-beam coupling experiments were therefore conducted with an FLC blend containing 1 wt % SM (Figure 14), applying

A photocurrent was observed even when a 532 nm beam was employed, although the magnitude of the photocurrent at 532 nm was less than that observed with 488 nm irradiation because of the smaller absorption coefficient. No difference in photoconductivity was observed between FLC samples made with C8-4T-2MB and 2EH-4T-2MB. The photorefractive effects in these FLC blends were examined using two-beam coupling experiments. Figure 10 presents a typical asymmetric energy exchange observed in an FLC sample containing 2EH-4T-2MB, demonstrating a clear asymmetric exchange when applying a writing beam wavelength of 532 nm. The effect of temperature on the magnitude of the gain coefficient of the FLC sample containing 2EH-4T-2MB is summarized in Figure 11. A photorefractive effect was only observed in the case of the SmC* phase (that is, the ferroelectric phase). This result indicates that the photorefractive effect in this material results from spontaneous polarization (Figure 1) and not from a response of molecular polarity. If the photorefractive effect of the 2EH-4T-2MB/TNF/base-LC mixture came from the response of the molecular polarity of 2EH-4T-2MB or baseLC, the magnitude of the gain coefficient could not be dropped at the phase transition temperature. The magnitudes of the gain coefficients at 532 nm are plotted as functions of the concentrations of the quarter-thiophene dopants in Figure 12a. Here it can be seen that, in both cases, the maximum gain coefficients were obtained at a dopant concentration of 6 wt %. It was observed that the samples remained transparent while exhibiting significant photoconductivity at this concentration. In addition, the gain coefficient was larger in the FLC blend containing 2EH-4T-2MB than that in the sample made with C8-4T-2MB because of the greater transparency of the 2EH-4T2MB sample. The refractive index grating formation times at 532 nm are plotted as functions of the concentrations of the quarter-thiophene dopants in Figure 12b. The grating formation G


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The Journal of Physical Chemistry C ORCID

Takeo Sasaki: 0000-0002-7643-3203 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology.

■

Figure 14. Asymmetric energy exchanges observed in an FLC blend containing the photoconductive compound SM at a concentration of less than 1 wt %.

wavelengths of 488, 532, and 638 nm from a continuous wave laser. Although the gain was small and the response was slow, a clear asymmetric energy exchange was observed even at 638 nm. The slow response indicates that the formation of the spacecharge field at the interference fringe in this sample was based on an ionic conduction process.

4. CONCLUSIONS This work investigated the photorefractive effect in FLC blends containing quarter-thiophene chiral photoconductive dopants. The low solubility of the quarter-thiophene moiety in the liquid crystalline medium was improved by introducing a branched chain into the tail unit of the dopant. The quarter-thiophene compound exhibited higher photoconductivity and a longer absorption wavelength compared to terthiophene compounds. Two-beam coupling experiments were conducted using a 532 nm laser and significant photorefractivity was observed in the case of the FLC blend containing the quarter-thiophene chiral photoconductive dopant. It was found that the sensitivity of the photorefractive FLC blends was extended to 638 nm by incorporating larger photoconductive compounds.

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REFERENCES

(1) Tabiryan, N. V.; Khoo, I. C. Recent Advances in Nematic Liquid Crystal Nonlinear Optics. Handbook of Liquid Crystals; Wiley-VCH: 2014; Vol. 8, Part 2, pp 1−21. (2) Sasaki, T.; Naka, Y. Photorefractive Effect in Ferroelectric Liquid Crystals. Opt. Rev. 2014, 21, 99−109. (3) Termine, R.; Golemme, A. Photorefractive Smectic Mesophases. Springer Ser. Mater. Sci. 2016, 240, 187−222. (4) Skarp, K.; Handschy, M. A. Ferroelectric Liquid Crystals. Material Properties and Applications. Mol. Cryst. Liq. Cryst. 1988, 165, 439−509. (5) Oswald, P.; Pieranski, P. Smectic and Columnar Liquid Crystals; Taylor & Francis: New York, 2006. (6) Sasaki, T.; Ikegami, M.; Abe, T.; Miyazaki, D.; Kajikawa, S.; Naka, Y. Real-Time Dynamic Hologram in Photorefractive Ferroelectric Liquid Crystal with Two-Beam Coupling Gain Coefficient of Over 800 cm−1 and Response Time of 8 ms. Appl. Phys. Lett. 2013, 102, 063306. (7) Sasaki, T.; Kajikawa, S.; Naka, Y. Dynamic Amplification of Light Signals in Photorefractive Ferroelectric Liquid Crystalline Mixtures. Faraday Discuss. 2014, 174, 203−208. (8) Lynn, B.; Blanche, P. A.; Peyghambarian, N. Photorefractive Polymers for Holography. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 193−231. (9) Kober, S.; Salvador, M.; Meerholz, K. Organic Photorefractive Materials and Applications. Adv. Mater. 2011, 23, 4725−4763. (10) Ostroverkhova, O.; Moerner, W. E. Organic Photorefractives: Mechanisms, Materials, and Applications. Chem. Rev. 2004, 104, 3267− 3314. (11) Goonesekera, A.; Wright, D.; Moerner, W. E. Image Amplification and Novelty Filtering with a Photorefractive Polymer. Appl. Phys. Lett. 2000, 76, 3358−3360. (12) Koukourakis, N.; Abdelwahab, T.; Li, M. Y.; Höpfner, H.; Lai, Y. W.; Darakis, E.; Brenner, C.; Gerhardt, N. C.; Hofmann, M. R. Photorefractive Two-Wave Mixing for Image Amplification in Digital Holography. Opt. Express 2011, 19, 22004−22023. (13) Sasaki, T.; Miyazaki, D.; Akaike, A.; Ikegami, M.; Naka, Y. Photorefractive Effect of Photoconductive Liquid Crystalline Mixtures Composed of Photoconductive Chiral Compounds and Liquid Crystal. J. Mater. Chem. 2011, 21, 8678−8686. (14) Sasaki, T.; Yoshino, M.; Naka, Y.; Le, K. V.; Sassa, T. Laser Irradiation Durability of Photorefractive Ferroelectric Liquid Crystal Blends Containing Terthiophene Photoconductive Chiral Dopants. RSC Adv. 2016, 6, 70573−70580. (15) Sakurai, T.; Mikami, N.; Higuchi, R.; Honma, M.; Ozaki, M.; Yoshino, K. Synthesis of New Ferroelectric Liquid Crystals and Their Novel Ferroelectricity. J. Chem. Soc., Chem. Commun. 1986, 978−979.

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