*Article* **Development of Hydrogen-Bonded Dimer-Type Photoluminescent Liquid Crystals of Fluorinated Tolanecarboxylic Acid**

**Shigeyuki Yamada 1,\*,†, Mitsuki Kataoka 1,†, Keigo Yoshida 1,†, Masakazu Nagata 2,‡, Tomohiro Agou 2, Hiroki Fukumoto <sup>2</sup> and Tsutomu Konno <sup>1</sup>**


**Abstract:** Functional molecules possessing photoluminescence (PL) and liquid-crystalline (LC) behaviors, known as photoluminescent liquid crystals, along with a small molecular structure, have attracted significant attention. Fluorinated tolane skeletons are small π-conjugated structures, which are promising candidates for such functional molecules. These structures were revealed to exhibit strong PL in solid state but no LC behavior. Based on a report on hydrogen-bonded dimer-type LC molecules of carboxylic acid, in this study, we designed and synthesized a series of fluorinated tolanecarboxylic acids (2,3,5,6-tetrafluoro-4-[2-(4-alkoxyphenyl)ethyn-1-yl]benzoic acids) as promising PLLC molecules. Evaluation of the LC behavior revealed that fluorinated tolanecarboxylic acids with a longer alkoxy chain than a butoxy chain exhibited nematic LC behavior. Additionally, fluorinated tolanecarboxylic acids showed intense PL in the solution and crystalline states. Notably, fluorinated tolanecarboxylic acid with an aggregated structure in the nematic LC phase also exhibited PL with a slight blue shift in PL maximum wavelength compared to the crystalline state. The present fluorinated tolanecarboxylic acid exhibiting PL and LC characteristics in a single molecule can be applied to thermoresponsive PL materials, such as a PL thermosensor.

**Keywords:** diphenylacetylene; fluorinated tolanecarboxylic acid; fluorine; photoluminescence; liquid crystals; nematic phase; phase transition

### **1. Introduction**

Photoluminescent liquid crystals, which possess photoluminescence (PL) and liquidcrystalline (LC) characteristics in a single molecule, have gained recognition as essential organic functional molecules owing to their extensive applicability in PL thermometers and thermoresponsive PL sensors [1–3]. To date, many PLLC molecules have been developed [4,5], which consist of large molecular structures with a π-conjugated structure, mesogenic core, and flexible unit that result in PL and LC behaviors. Therefore, developing PLLC molecules with a small molecular structure is necessary for practical applications considering the manufacturing costs and processes. An effective approach to searching for PLLC molecules with a small molecular structure is designing a common π-conjugated structure that functions as the core structure of PL and LC molecules.

Over the past few years, our group has focused on developing fluorine-containing organic functional molecules with a PL and an LC characteristic [6–14]. Our recent study revealed that fluorinated bistolane-based PLLC molecules (**A**) exhibit PL and LC behaviors

**Citation:** Yamada, S.; Kataoka, M.; Yoshida, K.; Nagata, M.; Agou, T.; Fukumoto, H.; Konno, T. Development of Hydrogen-Bonded Dimer-Type Photoluminescent Liquid Crystals of Fluorinated Tolanecarboxylic Acid. *Crystals* **2023**, *13*, 25. https://doi.org/10.3390/ cryst13010025

Academic Editors: Ingo Dierking, Charles Rosenblatt, Kyosuke Isoda, Takahiro Ichikawa, Kosuke Kaneko, Mizuho Kondo, Tsuneaki Sakurai, Atsushi Seki, Mitsuo Hara and Go Watanabe

Received: 24 November 2022 Revised: 19 December 2022 Accepted: 21 December 2022 Published: 23 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in a single molecule. The PL behavior is switched depending on the structural changes in the molecular aggregates through phase transition between the crystalline (Cry) and LC phases (Figure 1a) [6,7]. However, several issues were to be resolved, thus requiring multiple reaction steps to synthesize bistolane-based PLLC molecules. Because fluorinated tolane derivatives exhibit intense PL in the Cry phase through intermolecular H··· F hydrogen bonds [8–11], we suggested that a fluorinated tolane skeleton, which contains a small and common π-conjugated structure, is effective as the core structure of the PL and LC molecules. Several attempts revealed that alkoxy-substituted fluorinated tolanes with a cyano (CN) [8], a trifluoromethyl (CF3) group [8], and a fluorine (F) atom [9] show intense PL in the Cry phase but no LC phase, whereas fluorinated tolanecarboxylates **B** with a long flexible alkoxy chain, such as C7H15O and C8H17O, reportedly exhibit intense PL in the Cry phase and the nematic (N) LC phase after the cooling process (Figure 1b) [12]. Additionally, fluorinated tolane dimer **C**, which is composed of two fluorinated tolane skeletons connected by a flexible chain, successfully exhibits the PL and LC phases (Figure 1c) [13,14].

**Figure 1.** Chemical structure, phase transition behavior, and crystalline-state photoluminescence (PL) behavior of (**a**) fluorinated bistolane-based PL liquid crystals (PLLCs) (**A**), (**b**) fluorinated tolane-based PLLCs (**B**), and (**c**) fluorinated tolane dimer-type PLLCs (**C**).

Arakawa et al. reported that aromatic carboxylic acids, including tolanecarboxylic acid, show broad LC behavior due to formation of dimer via hydrogen bonds [15,16]. Wen et al. examined fluorinated LC molecules and reported that fluorinated tolanes with an ester structure [17] or fluorinated biphenyls with a carboxy unit exhibit LC behavior [18]. Based on the molecular design of hydrogen-bonded dimer-type LC molecules, we focused on the hydrogen-bonded dimer-type LC molecules of carboxylic acid. In this study, we designed and synthesized a series of fluorinated tolanecarboxylic acids **1**, such as 2,3,5,6 tetrafluoro-4-[2-(4-alkoxyphenyl)ethyn-1-yl]benzoic acids (Figure 2), and evaluated their LC and PL characteristics in detail.

**Figure 2.** Chemical structure of the fluorinated tolanecarboxylic acid **1** used in this study and plausible aggregated structure in crystalline and LC phases through hydrogen bond.

### **2. Materials and Methods**

### *2.1. General*

Column chromatography was performed for purification using Wakogel® 60N (38–100 μm), and thin layer chromatography (TLC) analysis was performed on silica gel TLC plates (silica gel 60F254, Merck). The melting temperature (*T*m) and clearing temperature (*T*c) were determined using polarized optical microscopy (POM). 1H and 13C nuclear magnetic resonance (NMR) spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer (1H: 400 MHz and 13C: 100 MHz) in chloroform-*d* (CDCl3) or dimethyl sulfoxide-*d*<sup>6</sup> or acetone-*d*6, and chemical shifts were reported in parts per million (ppm) using the residual proton in the NMR solvent. 19F NMR (376 MHz) spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer in CDCl3; CFCl3 (*δ*<sup>F</sup> = 0.0 ppm) and hexafluorobenzene (*δ*<sup>F</sup> = −163 ppm) were used as internal standards. Infrared (IR) spectra were recorded using the KBr method with a JASCO FT/IR-4100 type A spectrometer; all spectra were reported in wavenumber (cm−1) unit. High-resolution mass spectrometry (HRMS) was performed on a JEOL JMS-700MS spectrometer using the fast atom bombardment (FAB) method. Synthetic precursor ethyl 4-[2-(4-alkoxyphenyl)ethyn-1-yl]- 2,3,5,6-tetrafluorobenzoate (**2**) was stated in a previous study and synthesized according to the reported procedure [12].

### *2.2. Typical Synthetic Procedure of 2,3,5,6-Tetrafluoro-2-[4-(methoxyphenyl)ethyn-1-yl]benzoic acid (***1a***)*

Ethyl 2,3,5,6-tetrafluoro-4-[2-(4-methoxyphenyl)ethyn-1-yl]benzoate (**2a**, 2.0 g, 5.7 mmol), tetrahydrofuran (THF, 28 mL), and H2O (12 mL) were placed in a two-necked roundbottomed flask, followed by addition of LiOH·H2O (0.6 g, 14 mmol). The mixture was stirred at room temperature for 20 h and then acidified by adding an aqueous solution of HCl until the pH of the solution was below 1. The crude product was extracted with Et2O (10 mL, three times), while the organic layer was washed with brine (20 mL, once). The collected organic layer was dried over anhydrous Na2SO4 and separated from the drying agent by atmospheric filtration. The filtrate was evaporated using a rotary evaporator under reduced pressure and subjected to column chromatography using hexane, ethyl acetate, and acetic acid (*v*/*v*/*v* = 50/50/1) as an eluent, followed by recrystallization from chloroform, generating the title molecule **1a** as a white solid in a 74% isolated yield (1.37 g, 4.2 mmol).

### 2.2.1. 2,3,5,6-Tetrafluoro-4-{2-(4-methoxyphenyl)ethyn-1-yl}benzoic acid (**1a**)

Yield: 74% (white solid); *T*m: 223 ◦C (determined by POM); 1H NMR (DMSO-*d*6): *δ* 3.82 (s, 3H), 7.05 (d, *J* = 8.8 Hz, 2H), 7.59 (d, *J* = 8.8 Hz, 2H), 14.55 (brs, 1H); 13C NMR (DMSO-*d*6): *δ* 55.4, 72.8 (t, *J* = 4.4 Hz), 103.5 (t, *J* = 3.6 Hz), 105.9 (t, *J* = 17.6 Hz), 111.9, 113.6 (t, *J* = 17.6 Hz), 114.7, 133.6, 143.7 (dm, *J* = 253.1 Hz), 146.0 (dm, *J* = 250.8 Hz), 160.0, 160.8; 19F NMR (DMSO-*d*6, CFCl3): *<sup>δ</sup>* −136.3 to −136.6 (m, 2F), −138.9 to −139.1 (m, 2F); IR (KBr): *ν* 3730, 2844, 2221, 1698, 1601, 1475, 1247, 1174, 990, 835 cm−1; HRMS (FAB): [M+] calcd C16H8F4O3: 324.0410, found: 324.0413.

### 2.2.2. 2-{(4-Ethoxyphenyl)ethyn-1-yl}-2,3,5,6-tetrafluorobenzoic acid (**1b**)

Yield: 89% (white solid); *T*m: 224 ◦C (determined by POM); 1H NMR (acetone-*d*6): *δ* 1.39 (t, *J* = 7.2 Hz, 3H), 4.12 (q, *J* = 7.2 Hz, 2H), 7.02 (d, *J* = 8.8 Hz, 2H), 7.58 (d, *J* = 8.8 Hz, 2H), 6.0–8.0 (brs, 1H); 13C NMR (acetone-*d*6): *δ* 14.9, 64.5, 73.4 (t, *J* = 3.6 Hz), 105.0 (t, *J* = 3.7 Hz), 108.1 (t, *J* = 17.6 Hz), 113.5, 113.6 (t, *J* = 17.6 Hz), 115.9, 134.5, 145.5 (ddt, *J* = 253.8, 13.2, 5.9 Hz), 147.4 (ddt, *J* = 250.9, 14.7, 3.6 Hz), 160.2, 161.6; 19F NMR (acetone*d*6, C6F6): *δ* −136.92 (dd, *J* = 20.7, 10.9 Hz, 2F), −140.31 (dd, *J* = 20.7, 10.9 Hz, 2F); IR (KBr): *ν* 3750, 2984, 2212, 1706, 1601, 1479, 1178, 994, 844 cm−1; HRMS (FAB): [M+] calcd C17H10F4O3: 338.0566, found: 338.0563.

### 2.2.3. 2,3,5,6-Tetrafluoro-4-{2-(4-propyloxy)ethyn-1-yl}benzoic acid (**1c**)

Yield: 83% (white solid); *T*m: 220 ◦C (determined by POM); 1H NMR (acetone-*d*6): *δ* 1.03 (t, *J* = 7.2 Hz, 3H), 1.81 (sext., *J* = 7.2 Hz, 2H), 4.031 (t, *J* = 6.8 Hz, 2H), 7.04 (d, *J* = 8.8 Hz, 2H), 7.58 (d, *J* = 8.8 Hz, 2H), 5.0–10 (brs, 1H); 13C NMR (acetone-*d*6): *δ* 10.7, 23.1, 70.4, 73.4 (t, *J* = 4.4 Hz), 105.0 (t, *J* = 3.6 Hz), 108.1 (t, *J* = 17.6 Hz), 113.5, 113.8 (t, *J* = 16.9 Hz), 115.9, 134.5, 145.4 (ddt, *J* = 253.8, 13.9, 5.1 Hz), 147.4 (ddt, *J* = 250.9, 15.4, 3.6 Hz), 160.3, 161.8; 19F NMR (acetone-*d*6, C6F6): *δ* −136.91 (dd, *J* = 20.7, 10.5 Hz, 2F), −140.3 (dd, *J* = 20.7, 10.5 Hz, 2F); IR (KBr): *ν* 3650, 2966, 2211, 1705, 1601, 1476, 1331, 1253, 993 cm<sup>−</sup>1; HRMS (FAB): [M+] calcd C18H12F4O3: 352.0723, found: 352.0733.

### 2.2.4. 2-{(4-Butoxyphenyl)ethyn-1-yl}-2,3,5,6-tetrafluorobenzoic acid (**1d**)

Yield: 78% (white solid); *T*m: 178 ◦C (determined by POM); 1H NMR (acetone-*d*6): *δ* 0.97 (t, *J* = 7.2 Hz, 3H), 1.50 (sext., *J* = 7.2 Hz, 2H), 1.76 (quin, *J* = 7.2 Hz, 2H), 4.03 (t, *J* = 6.8 Hz, 2H), 6.98 (d, *J* = 8.8 Hz, 2H), 7.52 (d, *J* = 8.8 Hz, 2H), 10.0 (brs, 1H); 13C NMR (CDCl3): *δ* 14.1, 19.8, 31.9, 68.6, 73.4 (t, *J* = 5.1 Hz), 105.0 (t, *J* = 3.7 Hz), 108.1 (t, *J* = 16.1 Hz), 113.5, 113.5 (t, *J* = 16.2 Hz), 115.8, 134.4, 145.5 (ddt, *J* = 252.3, 13.2, 5.8 Hz), 147.3 (ddt, *<sup>J</sup>* = 253.0, 13.9, 3.6 Hz), 160.3, 161.7; 19F NMR (acetone-*d*6, C6F6): *<sup>δ</sup>* −136.9 to −137.1 (m, 2F), −140.2 to −140.4 (m, 2F); IR (KBr): *ν* 3743, 2950, 2209, 1707, 1600, 1477, 1252, 1177, 993 cm<sup>−</sup>1; HRMS (FAB): [M+] calcd C19H14F4O3: 366.0879, found: 366.0893.

### 2.2.5. 2,3,5,6-Tetrafluoro-4-{2-(4-pentyloxy)ethyn-1-yl}benzoic acid (**1e**)

Yield: 44% (white solid); *T*m: 175 ◦C (determined by POM); 1H NMR (acetone-*d*6): *δ* 0.94 (t, *J* = 7.2 Hz, 3H), 1.35–1.52 (m, 4H), 1.80 (quin, *J* = 6.8 Hz, 2H), 4.07 (t, *J* = 6.8 Hz, 2H), 7.01 (d, *J* = 8.8 Hz, 2H), 7.55 (d, *J* = 8.8 Hz, 2H), 9.07 (brs, 1H); 13C NMR (acetone-*d*6): *δ* 14.3, 23.1, 29.0, 29.7, 69.2, 73.5 (t, *J* = 4.4 Hz), 105.2 (t, *J* = 3.6 Hz), 108.4 (t, *J* = 18.3 Hz), 113.8, 113.9 (t, *J* = 17.6 Hz), 116.1, 134.6, 145.6 (ddt, *J* = 255.2, 15.3, 4.4 Hz), 147.6 (ddt, *J* = 250.9, 14.7, 3.6 Hz), 160.3, 162.0; 19F NMR (acetone-*d*6, C6F6): *<sup>δ</sup>* −136.92 (dd, *<sup>J</sup>* = 20.7, 10.9 Hz, 2F), −140.28 (dd, *J* = 20.7, 10.9 Hz, 2F); IR (KBr): *ν* 3485, 2948, 2212, 1707, 1600, 1481, 1253, 1176, 996, 837 cm<sup>−</sup>1; HRMS (FAB): [M+] calcd C20H16F4O3: 380.1036, found: 380.1027.

### 2.2.6. 2,3,5,6-Tetrafluoro-4-{2-(4-hexyloxy)ethyn-1-yl}benzoic acid (**1f**)

Yield: 65% (white solid); *T*m: 185 ◦C (determined by POM); 1H NMR (acetone-*d*6): *δ* 0.90 (t, *J* = 6.8 Hz, 3H), 1.30–1.38 (m, 4H), 1.48 (quin, *J* = 6.8 Hz, 2H), 1.79 (quin, *J* = 6.8 Hz, 2H), 4.06 (t, *J* = 6.8 Hz, 2H), 7.02 (d, *J* = 8.8 Hz, 2H), 7.57 (d, *J* = 8.8 Hz, 2H), 8.94 (brs, 1H); 13C NMR (acetone-*d*6): *δ* 14.3, 23.3, 26.4, 32.3, 68.9, 73.4 (t, *J* = 3.7 Hz), 105.0 (t, *J* = 3.6 Hz), 108.1 (t, *J* = 18.4 Hz), 113.5, 113.6 (t, *J* = 16.9 Hz), 115.9, 134.5, 145.5 (ddt, *J* = 252.3, 14.0, 5.1 Hz), 147.4 (ddt, *J* = 251.5, 14.7, 3.7 Hz), 160.2, 161.8; 19F NMR (acetone-*d*6, C6F6): *δ* −136.97 (dd, *J* = 20.3, 12.4 Hz, 2F), −140.29 (dd, *J* = 20.3, 12.4 Hz, 2F); IR (KBr): *ν* 3450, 2946, 2211, 1705, 1602, 1476, 1329, 1172, 993, 834 cm−1; HRMS (FAB): [M+] calcd C21H18F4O3: 394.1192, found: 394.1202.

### 2.2.7. 2,3,5,6-Tetrafluoro-4-{2-(4-heptyloxy)ethyn-1-yl}benzoic acid (**1g**)

Yield: 75% (white solid); *T*m: 172 ◦C (determined by POM); 1H NMR (acetone-*d*6): *δ* 0.89 (t, *J* = 6.8 Hz, 3H), 1.26–1.42 (m, 6H), 1.48 (quin, *J* = 6.8 Hz, 2H), 1.80 (quin, *J* = 6.8 Hz, 2H), 4.07 (t, *J* = 6.8 Hz, 2H), 7.03 (d, *J* = 8.8 Hz, 2H), 7.58 (d, *J* = 8.8 Hz, 2H), 6.0–10 (brs, 1H); 13C NMR (acetone-*d*6): *δ* 14.3, 23.3, 26.7, 29.8, 29.9, 32.6, 68.9, 73.4 (t, *J* = 4.4 Hz), 105.0 (t, *J* = 3.7 Hz), 108.1 (t, *J* = 17.6 Hz), 113.5, 113.7 (t, *J* = 16.9 Hz), 115.9, 134.5, 145.5 (ddt, *J* = 253.1, 13.9, 5.8 Hz), 147.4 (ddt, *J* = 250.8, 14.6, 3.7 Hz), 160.3, 161.8; 19F NMR (acetone-*d*6, C6F6): *δ* −136.98 (dd, *J* = 20.4, 12.0 Hz, 2F), −140.31 (dd, *J* = 20.4, 12.0 Hz, 2F); IR (KBr): *ν* 3680, 2948, 2211, 1704, 1601, 1479, 1330, 1254, 1171, 995, 836 cm<sup>−</sup>1; HRMS (FAB): [M+] calcd C22H20F4O3: 408.1349, found: 408.1343.

### 2.2.8. 2,3,5,6-Tetrafluoro-4-{2-(4-octyloxy)ethyn-1-yl}benzoic acid (**1h**)

Yield: 41% (white solid); *T*m: 167 ◦C (determined by POM); 1H NMR (DMSO-*d*6): *δ* 0.86 (t, *J* = 6.8 Hz, 3H), 1.23–1.34 (m, 8H), 1.40 (quin, *J* = 6.8 Hz, 2H), 1.72 (quin, *J* = 6.8 Hz, 2H), 4.02 (t, *J* = 6.8 Hz, 2H), 7.02 (d, *J* = 8.8 Hz, 2H), 7.56 (d, *J* = 8.8 Hz, 2H), 14.6 (brs, 1H); 13C NMR (DMSO-*d*6): *δ* 13.9, 22.0, 25.4, 24.5, 28.6, 28.7, 31.2, 67.8, 72.8 (t, *J* = 4.4 Hz), 103.6 (t, *J* = 3.6 Hz), 106.0 (t, *J* = 16.9 Hz), 111.8, 113.6 (t, *J* = 17.6 Hz), 115.2, 133.6, 142.3–145.2 (m, 1C), 144.5–147.4 (m, 1C), 160.0, 160.3; 19F NMR (DMSO-*d*6, CFCl3): *<sup>δ</sup>* −136.49 (dd, *<sup>J</sup>* = 23.3, 10.9 Hz, 2F), −140.50 (dd, *J* = 23.3, 10.9 Hz, 2F); IR (KBr): *ν* 3673, 2946, 2210, 1704, 1600, 1477, 1329, 1253, 1170, 995, 836 cm−1; HRMS (FAB): [M+] calcd C23H22F4O3: 422.1505, found: 422.1510.

### *2.3. Single-Crystal X-ray Diffraction*

Single-crystal X-ray diffraction (XRD) spectra were recorded using an XtaLAB AFC11 diffractometer (Rigaku, Tokyo, Japan). The reflection data were integrated, scaled, and averaged using the CrysAlisPro program (ver. 1.171.39.43a; Rigaku Corporation, Akishima, Japan), while empirical absorption corrections were applied using the SCALE 3 AB-SPACK scaling algorithm (CrysAlisPro). The structures were identified by a direct method (SHELXT-2018/2 [19]) and refined using the full matrix least-squares method (SHELXL-2018/3 [20]) visualized by Olex2 [21]. Crystallographic data were deposited in the Cambridge Crystallographic Data Centre (CCDC) database (CCDC 2193549 for **1a** and 2193550 for **1e**), which were obtained free of charge from the CCDC at www.ccdc.cam.ac.uk/data\_ request/cif (accessed on 23 November 2022).

### *2.4. Phase Transition Behavior*

The phase transition behaviors were observed by POM using an Olympus BX53 microscope (Tokyo, Japan) equipped with a cooling and heating stage (10002L, Linkam Scientific Instruments, Surrey, UK). Thermodynamic characterization was performed by differential scanning calorimetry (DSC; DSC-60 Plus, Shimadzu, Kyoto, Japan) at heating and cooling rates of 5.0 ◦C min−<sup>1</sup> under N2.

### *2.5. Photophysical Properties*

Ultraviolet–visible (UV–vis) absorption spectra were recorded using a JASCO V-750 absorption spectrometer (JASCO, Tokyo, Japan). The PL spectra of the solutions were measured using an FP-6600 fluorescence spectrometer (JASCO, Tokyo, Japan). The PL quantum yields were measured using a Quantaurus-QY C11347-01 instrument (Hamamatsu Photonics, Hamamatsu, Japan).

### *2.6. Theoretical Calculations*

All computations were performed using Gaussian 16 program set [22] with the density functional theory (DFT) at the M06-2X hybrid functional [23] and 6-31+G(d,p) (for all atoms) basis set with a conductor-like polarizable continuum model (CPCM) [24] for CH2Cl2. Theoretical vertical transitions were also calculated using the time-dependent DFT (TD-DFT) method at the same theory level using the same solvation model.

### **3. Results and Discussion**

### *3.1. Synthesis and Crystal Structure*

We first synthesized the fluorinated tolanecarboxylic acid **1** from the corresponding ester **2** via hydrolysis under basic conditions; synthesis of **2** was previously accomplished (Figure 3) [12].

**Figure 3.** Synthetic procedure of **1a**–**h** from the corresponding ester **2**.

Treatment of ester **2a** with 2.5 equivalent of LiOH·H2O in a mixed solvent of THF and H2O (*v*/*v* = 7/3) at room temperature for 3 h underwent a hydrolysis reaction, which proceeded smoothly. Subsequently, treatment with an aqueous solution of concentrated HCl produced corresponding fluorinated tolanecarboxylic acid **1a**. The product was purified by column chromatography and recrystallization, and the resulting **1a** was generated as a white solid in a 74% isolated yield. Using a similar synthetic procedure, other analogs **1b**–**h** bearing various alkoxy chains were also produced in 41–89% isolated yields. The molecular structures of **1a**–**h** were assessed by 1H, 13C, and 19F-NMR, along with IR and HRMS. All structures were completely identified and sufficiently pure to evaluate their phase transition and photophysical behaviors.

Among the fluorinated carboxylic acids **1a**–**h**, methoxy-substituted **1a** and pentyloxysubstituted **1e** afforded single crystals that were appropriate for X-ray crystallographic analysis. Figure 4 shows the crystal structures of **1a** and **1e** and their packing structures.

**Figure 4.** (**a**) Crystal structure of **1a** with an ORTEP drawing and (**b**,**c**) packing structures. (**d**) Crystal structure of **1e** with an ORTEP drawing and (**e**–**h**) packing structures.

Methoxy-substituted **1a** crystalized with a triclinic system in the *P*–1 space group and two molecular units were contained in the Cry lattice. The dihedral angle between two aromatic rings in the tolane scaffold was only 4.7◦, almost coplanar to each other (Figure 4a). The dihedral angle between the fluorinated aromatic ring and the carbonyl plane was reported 34◦ for the ester precursor **2a** [12]. However, the dihedral angle of the carboxylic acid **1a** was only 3.7◦, resulting in an almost coplanar structure. With respect to the packing structures, the two planar tolane scaffolds were arranged in a layer structure with an antiparallel direction. This phenomenon is caused by the electrostatic weak π–π interaction (short contact of Cπ··· Cπ: 353 pm) between the electron-rich methoxy-substituted aromatic ring and the electron-deficient fluorinated aromatic ring (Figure 4b). Additionally, the fluorinated tolanecarboxylic acid **1a** formed plural intermolecular interactions (Figure 4c), such as O···H hydrogen bond (short contact of O··· H: 179 pm), H··· F hydrogen bond (short contact of H··· F: 242 and 261 pm), and F··· F interaction (short contact of F··· F: 286 pm), wherein the short contacts mentioned above were almost identical or below the sum of van der Waals radii [25].

In contrast, pentyloxy-substituted **1e** furnished single crystals with a monoclinic system in the *C* 2/*c* space group, and eight molecular units were contained in the Cry lattice. The electron-rich aromatic ring and the electron-deficient fluorinated aromatic ring were nearly coplanar, with a deviation of 3.0◦. The dihedral angle between the fluorinated aromatic ring and the carbonyl plane was 11◦, being almost coplanar (Figure 4d). However, unlike the π–π stacking of the antiparallel orientation in **1a**, **1e** formed a slipped π–π stacking (short contact of Cπ··· Cπ: 344 pm) with a synparallel orientation induced by the electrostatic interaction between the electron-rich pentyloxy aromatic ring and the electrondeficient fluorinated aromatic ring (Figure 4e). As shown in Figure 4f,g, the carboxyl units in **1e** also formed an intermolecular O··· H hydrogen bond with a short contact of 184 pm, leading to formation of layer structures. For construction of the crystal structure of **1e**, more intermolecular interactions, such as additional O··· H and H··· F hydrogen bonds (Figure 4h), were also observed. The interatomic distance was 254 and 261 pm for O··· H and H··· F, respectively, which was also almost identical or below the sum of van der Waals radii [25].

### *3.2. Phase Transition Behavior*

With the fluorinated tolanecarboxylic acids, **1a**–**h**, in hand, we evaluated their phase transition behavior using DSC and POM. Table 1 summarizes the phase sequence and phase transition temperature for **1a**–**h** during the first heating and cooling process determined by POM. Subsequent phase transition behavior is listed in Table S2 (ESI). Figure 5 shows the POM texture images of **1d**–**h** observed in the mesophase.

**Table 1.** The phase transition behavior of the fluorinated tolanecarboxylic acids, **1a**–**h**, during the first heating and cooling process observed by POM.


<sup>1</sup> Determined by POM. Abbreviations: Cry: crystalline; G: Glassy; N: nematic; and Iso: isotropic phases.

**Figure 5.** Optical microphotographs of (**a**) **1d**, (**b**) **1e**, (**c**) **1f**, (**d**) **1g**, and (**e**) **1h** in the mesophase phase.

The DSC measurement of methoxy-substituted **1a** showed a large endothermic peak with an onset temperature of 223 ◦C during the first heating process. No sharp exothermic peak due to the Iso → Cry phase transition was observed during the subsequent cooling process. As a result of POM observation, however, a phase transition from the Iso phase to a glassy amorphous solid (G) phase was observed; **1a** did not show any mesophase (Figure S25). The POM observation also proved that **1a** showed no mesophase between the Cry and isotropic (Iso) phases. Additionally, no mesophase was observed for ethoxysubstituted **1b** and propoxy-substituted **1c** by POM and DSC measurements. In contrast, butoxy-substituted **1d** showed an endothermic phase transition between the Cry and Iso phases in the first heating process of the DSC measurement and a bright-viewing field with fluidity during the heating and cooling processes of the POM observation. Thus, the phase transition behavior of butoxy-substituted **1d** possessed the LC characteristic. A four-brushed Schlieren texture was observed as the optical image (Figure 5a), which is a typical texture for the N LC phase. During the subsequent cooling process, however, only the Iso → G phase transition was observed. The phase transition behavior was also supported by temperature-varying powder X-ray diffraction (VT-PXRD) measurements (Figure S26). Further POM observation was found to show similar phase transition between G and Iso phases during the second cycles. Similar to **1d**, molecules **1e** and **1f** also exhibited an N-phase during the first heating process (Figure 5b,c), while, after the first cooling process, the LC phase disappeared, showing only a phase transition between the G and Iso phases. The other analogs, viz., **1g** and **1h**, with a relatively long alkoxy chain, were found to show a mesophase during both heating and cooling processes due to increasing stabilization of the mesophase. Thus, both **1g** with a C7H15 chain and **1h** with a C8H15 chain exhibited an N-phase with a four-brush Schlieren texture through POM measurements during both heating and cooling cycles (Figure 5d,e), in which the observed mesophase can be assigned as an N-phase by the VT-PXRD measurements (Figure S26). Focusing on the melting temperature (*T*m), which is defined as the phase transition temperature from Cry to Iso or LC phases, the *T*<sup>m</sup> of **1a**–**h** was in the range of 167–224 ◦C for the heating process, which was much higher than that of the corresponding ester derivatives **2a**–**h** (34–109 ◦C) [12]. Unlike the ester derivatives, the carboxylic acids exhibited LC phases even with relatively short alkoxy chains, particularly C4H9O, due to the increased aspect ratio of the mesogenic core induced by formation of a dimeric structure through hydrogen bonds.

### *3.3. Photophysical Behavior in Solution Phase*

A solution sample was prepared to investigate the photophysical behavior of the fluorinated carboxylic acids, **1a**–**h**, in the solution phase by individually dissolving **1a**–**h** in CH2Cl2; the concentration was adjusted to 1.0 × <sup>10</sup>−<sup>5</sup> mol L−1. Figure 6 illustrates the photophysical behavior in the solution, and the photophysical data are summarized in Table 2.

**Figure 6.** (**a**) Ultraviolet (UV)–visible absorption spectrum of **1a**–**h** in the CH2Cl2 solution (concentration: 1.0 <sup>×</sup> <sup>10</sup>−<sup>5</sup> mol L<sup>−</sup>1). (**b**) PL spectrum of **1a**–**<sup>h</sup>** in the CH2Cl2 solution (concentration: 1.0 <sup>×</sup> <sup>10</sup>−<sup>5</sup> mol L<sup>−</sup>1) and a photograph of the PL behavior of **1a** solution under UV light (*λ*ex = 365 nm). Inset: Commission Internationale de l'Eclairage (CIE) diagram for PL color of **1a**–**h** solutions.



<sup>1</sup> Concentration: 1.0 × <sup>10</sup>−<sup>5</sup> mol L−1. <sup>2</sup> Measured using an integrating sphere.

Methoxy-substituted **1a** in CH2Cl2 absorbed UV light with a maximum absorption wavelength (*λ*abs) near 259 nm and 317 nm (Figure 6a). Other analogs, particularly **1b**–**h**, also showed two absorption bands: a high-energy absorption band near 255–259 nm of *λ*abs and a low-energy absorption band near 319–329 nm of *λ*abs (Figure 6a). Quantum chemical calculations were performed by the TD-DFT method using **1a** as a representative, and two allowed transitions with theoretical absorption wavelengths of 319 and 262 nm were calculated as theoretical vertical transitions (Figure S31). The calculated absorption wavelengths were close to the experimentally obtained *λ*abs. Thus, the result confirms that the long-wavelength absorption band of **1a** in CH2Cl2 is the ππ\* transition with an intramolecular charge transfer (ICT) character involving the highest occupied molecular orbital to the lowest unoccupied molecular orbital (HOMO → LUMO) transition, while the short-wavelength band is the ππ\* transition with a local excitation character involving a HOMO–1 → LUMO transition.

With the *λ*abs as the excitation wavelength, the methoxy-substituted **1a** in the solution state was observed to emit blue PL, with a maximum PL wavelength (*λ*PL) of approximately 435 nm (Figure 6b). In addition, **1b**–**h** with varying lengths of alkoxy group were found to have a PL band with *λ*PL in the range of 437–441 nm, leading to the blue PL. Considering the observed PL colors using the Commission Internationale de l'Eclailage (CIE) diagrams

(Figure 6b, inset), the CIE coordinates for the PL colors of **1a**–**h** were similar to each other. The PL color of the fluorinated tolanecarboxylic acids in CH2Cl2 showed a uniform blue PL in the solution state without affecting the alkoxy-chain length. PL quantum yields (*Φ*PL) of **1a**–**h** in CH2Cl2 solutions were in the range of 0.27–0.38, which is higher than that of the unsubstituted tolane [26,27]. This phenomenon is observed because the donor–π–acceptor structure of the fluorinated tolanecarboxylic acid suppresses the internal conversion from the ππ\* excited state to the dark πσ\* excited state. In addition, we investigated the effect of solvent polarity on photophysical properties using **1a** as a representative [28]. We found that, although the solvent polarity did not affect the absorption properties significantly, the PL properties shifted to longer wavelengths as the polarity increased, which is attributed to stabilization of solute–solvent interactions (Figure S28d). Considering the Lippert–Mataga plot [29,30], which is created from the orientational polarizability (Δ*f*) and Stokes shift (Δ*ν*) on the horizontal and vertical axes, respectively, a linear relationship was obtained (Figure S28e). The dipole moment difference (*μ*e–*μ*g) between the excited and ground states was 14.1 D, which was calculated from the slope of the straight line. The large difference in the dipole moment proves that the radiative deactivation from the ICT excited state resulted in the PL of **1a**–**h**.

### *3.4. Photophysical Behavior in Aggregated Phases*

We next examined the PL behavior of fluorinated tolanecarboxylic acids, **1a**–**h**, in the aggregated phases. Figure 7 shows the PL spectrum, photographs of the PL behavior under UV irradiation, and a CIE diagram for the PL colors. The photophysical data of **1a**–**h** in the aggregated phases are summarized in Table 3.

**Figure 7.** (**a**) PL spectra of **1a**–**h** in crystalline state. Excitation wavelength (*λ*ex): 300 nm. Inset: Photographs of the PL behavior of the **1a**–**h** crystals under UV light (*λ*ex = 365 nm). (**b**) CIE color diagram of PL colors for **1a**–**h** crystals.


**Table 3.** Photophysical data of **1a**–**h** in aggregated phases.

<sup>1</sup> Unless mentioned otherwise, the crystalline sample prepared by column chromatography and recrystallization was used. Measured at 25 ◦C. <sup>2</sup> Measured using an integrating sphere. <sup>3</sup> Samples with mesophase aggregate structures were prepared by quenching and immersing mesophase (170 ◦C) during the 1st heating process in liquid nitrogen to maintain the mesophase molecular aggregates at room temperature.

When methoxy-substituted **1a** in the Cry phase was excited by irradiation with incident light of 300 nm, which is the maximum excitation wavelength (*λ*ex), a single PL band was observed with a *λ*PL of approximately 481 nm (Figure 7a). As shown in Figure 8b,c, the CIE coordinate (*x, y*) of the PL color was (0.185, 0.296), indicating that the PL color was light blue. Notably, a CH2Cl2 solution of **1a** had a *Φ*PL of 0.33, whereas the **1a** in the Cry phase dramatically increased the *Φ*PL to up to 0.99. Although **1b**–**h** with varying lengths of the alkoxy chain had almost identical *λ*PL in dilute solutions, they exhibited various *λ*PL in the Cry phase, ranging from 428 to 511 nm (Figure 7a). The alteration in *λ*PL indicated a change in the PL color. Thus, various PL colors ranging from blue to light green were obtained by changing the length of the alkoxy chain, which is evident from the photographs and the CIE diagram demonstrating the PL colors (Figure 7b). The *Φ*PL values of **1b**–**h** in the Cry phase were in the range of 0.49–0.71, which were higher than those in dilute solutions (up to 0.38). Considering the crystal packing structures of **1a** and **1e** shown in Figure 4, the change in the length of the alkoxy chain considerably altered the molecular arrangements in molecular aggregated phases; **1a**–**h** exhibited various PL behaviors in the Cry phase. Furthermore, O··· H and H··· F hydrogen bonds and intermolecular interactions, such as F··· F interactions and weak π··· π interactions, function in the Cry phase, which possibly restricts the molecular motion to suppress non-radiative deactivation, resulting in a significant increase in the *Φ*PL in the Cry state.

The PL behavior in the aggregated structures of the N-phase was evaluated using octyloxy-substituted **1h** with an N LC phase. The measurement sample was prepared by quenching the sample with the N LC phase at 170 ◦C, which was developed during the 1st heating process, with liquid nitrogen. Figure 8 shows the PL spectra and CIE diagrams for **1h** with the Cry- and N-phase molecular aggregated structures.

**Figure 8.** (**a**) PL spectra of **1h** with the Cry- and N-phase molecular aggregated structures. Inset: CIE diagram of the PL colors for **1h** with the Cry- and N-phase aggregated structures. (**b**,**c**) Schematic illustration of plausible structural alteration from the Cry- to N-phase molecular aggregated structures.

The PL spectrum of **1h** with N-phase aggregated structures was also obtained by irradiation with incident light of 300 nm, in which the *λ*PL was approximately 454 and 479 nm, along with a shoulder peak of approximately 428 nm. Compared to the Cry phase, the PL spectra of the N-phase aggregated structure yielded a slight short-wavelength shift with weakened long-wavelength shoulder peaks and increased short-wavelength peaks. In the Cry phase, the dimer mesogens formed a dense packing structure due to the weak π··· π interactions (Figure 8b), as shown in Figure 4g. Conversely, the phase transition to the N-phase increased the molecular fluidity, allowing the increase in the two molecular distances (Figure 8c). The increased spacing between the dimer mesogens in the N-phase aggregated structure and the promotion of the molecular motion drastically reduced the *Φ*PL compared to that in the Cry phase.

### **4. Conclusions**

In conclusion, we designed and synthesized a series of fluorinated tolanecarboxylic acids bearing various lengths of alkoxy chains and investigated their phase transition and photophysical behaviors. The fluorinated tolanecarboxylic acids exhibited the N LC phase due to formation of the dimer-type mesogen of the carboxylic acid moiety via O···H hydrogen bonds. Furthermore, regarding photophysical measurements, the fluorinated tolanecarboxylic acids emitted blue PL in the solution phase. The PL quantum yield (*Φ*PL) was approximately 0.33, which was higher than that of the unsubstituted tolane. The fluorinated tolanecarboxylic acid exhibited remarkably strong PL even in the Cry phase, and its *Φ*PL was much higher than that in the dilute-solution state, which could be attributed to the O··· H and H··· F hydrogen bonds and the weak π··· π and F··· F intermolecular interactions. Investigation of the PL behavior in the N-phase molecular aggregated structure revealed a slight short-wavelength shift and a significant decrease in *Φ*PL, which is attributable to the wider spacing between the dimer-type mesogens caused by increasing the molecular fluidity in the N-phase. These findings will offer a new molecular design for PLLC molecules effectively using intermolecular interactions and pave the way for developing new thermo-responsive luminescent materials in the future.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/cryst13010025/s1, Figures S1–S24: NMR spectra of **1a**–**h**; Figure S25: DSC thermograms of **1a**–**h**; Figure S26: TG thermograms of **1a**–**h**; Figure S27: PXRD patterns of **1d**–**h** on the mesophase; Figure S28: UV–vis absorption and PL spectra of **1a**–**h** in CH2Cl2 solution; Figure S29: UV–vis absorption and PL spectra of **1a** in different solvents; Figure S30: Excitation and PL spectra of **1a**–**h** in the Cry phase; Figure S31: Excitation and PL spectra of **1h** in the aggregated structure of the nematic phase; Figures S32 and S33: Optimized structure of **1a** and **1e** and their orbital distributions; Table S1: Crystallographic data of **1a** and **1e**; Table S2: Phase transition behaviors of **1a**–**h** observed by DSC measurements; Table S3: Solvent effect on the photophysical behavior; Tables S4 and S5: Cartesian coordinate for **1a** and **1e**.

**Author Contributions:** Conceptualization, S.Y.; methodology, S.Y.; validation, S.Y., M.K., K.Y. and T.K.; investigation, S.Y., M.K. and K.Y.; resources, S.Y. and T.K.; data curation, S.Y.; writing—original draft preparation, S.Y. and M.K.; writing—review and editing, S.Y., M.K., K.Y., M.N., T.A., H.F. and T.K.; visualization, S.Y.; supervision, S.Y.; project administration, S.Y.; funding acquisition, S.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Murata Science Foundation and Shorai Foundation for Science and Technology.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We are deeply grateful to Sakurai and Shimizu (Kyoto Inst. Tech.) for their valuable cooperation in the PXRD measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

**Mitsuo Hara 1,\*, Ayaka Masuda 1, Shusaku Nagano <sup>2</sup> and Takahiro Seki 1,\***


**Abstract:** Photoalignment technology enables macroscopic alignment of liquid crystalline molecules and their aggregates in a non-contact process by irradiating photo-responsive liquid crystalline compounds with linearly polarized light. Because photoalignment techniques prevent dust generation and uneven stretching, and accomplish fine and complex patterning, they are involved in the practical process of fabricating display panels, and continue to be applied in the research and creation of various anisotropic materials. Brilliant yellow (BY), a chromonic liquid crystal, has attracted considerable attention as the photoalignment sublayer in recent years, because of its ability to induce a high dichroic nature among many photo-responsive liquid crystalline materials. However, its dichroism is not maintained after prolonged exposure to a humid environment because of its intrinsic strong hygroscopicity of ionic BY molecules. In this study, to overcome this drawback, the photoalignment and successive photo-fixation of the BY columnar phase is proposed using UV-curable ionic polysiloxane as a matrix. Visible light was used for the photoalignment of the BY columnar phase, and UV light for photo-fixation. Consequently, the columnar chromonic phase is found to retain its orientation even after 4 h of exposure to a highly humid environment.

**Keywords:** chromonic liquid crystal; polysiloxane; photoalignment; UV curing

### **1. Introduction**

Liquid crystals (LCs) self-assemble in response to temperature, electric fields, concentration, light irradiation, and other environmental conditions to form nano-periodic structures. Such soft materials are suitable for templates of nanostructures [1]. The dynamic cooperativity of LCs also facilitates control of the arrangement of molecules and their aggregates over a macroscopic scale, and enables their use in a variety of applications such as biosensors, optical devices, separators, and actuators [2–5]. Among them, lyotropic LCs mostly self-assemble in aqueous solvents, and they can be applied to various environmentally friendly processes. Some lyotropic LCs are classified as chromonic LCs [6–11]. Chromonic LCs are soft materials containing dye groups such as mesogens, which selfassemble to form columnar nanostructures via ππ interaction in solvents. The self-assembly structures that involve absorption anisotropy of chromonic LCs can be applied to optically functional films when the structures are align overlarge areas [12–15]. In particular, the optical manipulation of chromonic LC phases using photoalignment techniques [16–18] has the advantage of ready achievement of fine patterning, which is difficult to be accomplish by conventional mechanical rubbing or film stretching techniques [19–22].

Among chromonic LCs, brilliant yellow (BY) (as shown in Scheme 1) has attracted significant attention in recent years, and research using BY films as LC alignment sublayers has been extensively undertaken [23–38]. This is because the columnar LC phase of BY is well photoaligned using linearly polarized UV or visible light, creating highly dichroic optical films [39–42]. Such photoaligned BY films function well as alignment sublayers

**Citation:** Hara, M.; Masuda, A.; Nagano, S.; Seki, T. Photoalignment and Photofixation of Chromonic Mesophase in Ionic Linear Polysiloxanes Using a Dual Irradiation System. *Crystals* **2023**, *13*, 326. https://doi.org/10.3390/ cryst13020326

Academic Editor: Borislav Angelov

Received: 4 February 2023 Revised: 10 February 2023 Accepted: 12 February 2023 Published: 15 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

for low-molecular-mass nematic LCs. However, BY exhibits a hygroscopic nature, which means that it cannot maintain dichroism for a long time in a highly humid environment. If the photoaligned BY columnar phase can be stabilized, it is anticipated that the high dichroic properties of BY could be used in more applications in wider variety situations; however, such a methodology has not yet been proposed.

**Scheme 1.** Chemical structure of chromonic liquid crystal used in this study.

Matrix fixation by sol-gel reaction is often used in the stabilization of lyotropic LC phases [43,44]. However, chromonic LCs generally have low solubility in metallic-alkoxide sol (such as silica sol), and the addition of a compatibilizer is necessary to improve compatibility [14,45]. In addition, the chromonic LCs are solidified by the silica matrix through film formation, which makes photoalignment of BY after film formation difficult to achieve using conventional stabilization methods for lyotropic LCs.

Some recently developed ionic linear polysiloxanes are compatible with the hydrophilic region of lyotropic LC phases [46]. By introducing functional groups that can be cross-linked by ultraviolet light into polysiloxane, it is also possible to fix the LC phase in the matrix at targeted times and locations after film formation [47]. Conversion of the ionic groups of polysiloxanes is relatively easy, and a variety of ionic polysiloxanes can be prepared [48]. The ease of design of the ionic group of polysiloxanes means that development of an ionic polysiloxanes with high compatibility with chromonic LCs can be easy. In this study, the design for an ionic linear polysiloxane that is compatible with BY and has photo-crosslinking groups is proposed. By preparing a mixed thin film of the polysiloxane and BY, the photo-orientation of the BY columnar phase by visible light is demonstrated in the polysiloxane matrix. Subsequent fixation by UV light irradiation is also achieved. By photo-fixing the BY columnar phase, the dichroism ratio may be maintained even after extended exposure to a humid environment.

### **2. Materials and Methods**

### *2.1. Materials*

Scheme 2 shows the chemical structures used in this paper. BY chromonic dye and Irgacure® 2959 (I2959) photoinitiator were purchased from Sigma-Aldrich and Tokyo Chemical Industry (TCI), respectively. An anionic linear polysiloxane containing vinyl groups (PSSV) was synthesized via polycondensation of silane coupling agents. *N*, *N*-Dimethylformamide (DMF) was purchased from Kanto Chemical. The I2959 and DMF used were of commercial purity. Water was obtained through a Direct-Q® 3UV purification system (Millipore Corp., Burlington, MA, USA, *ρ* (resistivity) > 18 MΩ·cm at 25 ◦C).

**Scheme 2.** Chemical structures of hygroscopic siloxane copolymer and photoinitiator used in this study.

### *2.2. Synthesis of PSSV*

PSSV was synthesized according to the scheme shown in Figure 3a [49]. The detailed procedure is follows.

1.8 g (1.0 × <sup>10</sup>−<sup>2</sup> mol) of 3-mercaptopropyl(dimethoxy)methylsilane (TCI) and 3.5 × <sup>10</sup>−<sup>2</sup> <sup>g</sup> (2.6 × <sup>10</sup>−<sup>4</sup> mol) of dimethoxymethylvinylsilane (TCI) were added to 54 g of 2 mol L−<sup>1</sup> of sodium hydroxide (TCI). The mixture solution was stirred at 25 ◦C for 2 h before the addition of polysiloxane containing mercapto groups and vinyl groups. To oxidize the mercapto groups, 6 g of 30% hydrogen peroxide water (Kanto Chemical) was added to the mixture solution, and the resulting solution was stirred at 25 ◦C for 12 h. The solvent was dried using a smart evaporator C1 (BioChromato) and a white powder was obtained. The powder was dissolved in 300 mL of water, and ion exchange occurred using an ion-exchange resin Amberlite® IR120, in hydrogen form (Fluka). After the solution was treated using an ion-exchange resin IR120B Na (Organo) and dried using an evaporator, 2.1 g of white powder was obtained.

1H NMR spectra of the product were recorded using a 400 MHz FT-NMR spectrometer JNM-A400 (JEOL). The spectrum is shown in Figure 3b. The molar ratio of each monomer unit in the copolymer was 90:1, which was calculated from the peak-integration ratio of peak d and f in the 1H NMR spectrum.

### *2.3. Preparation of Pure BY Spin-Coated Films*

BY was added to DMF (BY concentration: 1.5% by weight). The solution was heated in an oil bath at 150 ◦C for 4 h and allowed to cool before the undissolved portion was removed by filtration. Pure BY films were prepared by spin-coating the filtered solution onto the UV-O3 treated quartz. The spin-coated films were prepared at 1500 rpm for 30 s. The relative humidity at the time of spin-coating was 14% (RH = 14%), as measured using an RTR-503 temperature and humidity recorder (T&D Corp.). The resulting films were then annealed at 120 ◦C for 10 min.

### *2.4. Preparation of BY-PSSV Spin-Coated Films*

BY, PSSV, and I2959 were added to mixture of DMF and water. The weight ratio of the components was BY:PSSV:I2959:DMF:water = 1.2 × 101:3.0:6.0 × <sup>10</sup>−2:7.4 × 102: 2.2 × <sup>10</sup>1. The solution was filtered to separate the undissolved portion. The BY-PSSV films were prepared using the filtered solution in the same way as the pure BY film. The resulting films were annealed at 120 ◦C for 10 min.

### *2.5. Film Thickness Measurement*

Surface roughness was characterized by a white light interferometric BW-S507-N microscope (Nikon Corp., Tokyo, Japan). Bridgelements® was used for the software modules. To measure film thickness, the substrate was exposed by scratching the film with a micro spatula. Subsequently, the height difference between the top and bottom layer of the film was measured by a BW-S507-N microscope. The top-to-bottom height was taken as the film thickness.

### *2.6. Water Absorption Measurements*

The hygroscopicity of pure BY and PSSV was evaluated by the quartz crystal microbalance (QCM) method. The QCM measurements were performed based on the methodology of previous research [50]. As PSSV exhibits high hygroscopicity, the QCM measurements could not guarantee accuracy when RH > 50%. Therefore, the water absorption of PSSV in the high humidity range was evaluated by the following method. Approximately 30 mg of PSSV was exposed to various humidity-controlled environments for several days. Once it reached an equilibrium moisture absorption state, its weight was measured. The relative humidity was controlled using a saturated aqueous solution of inorganic salts such as magnesium nitrate (RH ~50%), sodium chloride (RH ~65%), potassium chloride (RH ~80%), and potassium nitrate (RH ~88%). Experimental values of relative humidity realized in the saturated aqueous solutions of each inorganic salts are given in parentheses. All salts were purchased from Kishida Chemical and used as purchased.

### *2.7. Photoalignment of the BY Columnar Phase*

The spin-coated films were placed on a homemade quartz chamber attached to the RTR-503 humidity sensor. The humidity in the chamber was controlled at approximately 15% using a me-40DP series precise dew-point generator (Micro Equipment). The spincoated films in the chamber were exposed to linearly polarized visible (LPVis) light (436 nm) passed through a band pass filter and a polarizer using a mercury lamp REX-250 (Asahi Spectra) at room temperature. The light intensity at the sample position was 10 mW cm<sup>−</sup>2.

Polarized UV-vis absorption spectra were taken on an Agilent 8453 spectrophotometer (Agilent Technologies). The orientation order parameter (*S*) of BY molecules is defined as (*A*<sup>⊥</sup> − *A*||)/(*A*large + 2*A*small), where *A*<sup>⊥</sup> and *A*|| are absorbances taken with perpendicular and parallel polarized probing beams, respectively. *A*large and *A*small represent the larger and smaller absorbances of the two measurements, respectively.

### *2.8. Evaluation of the Photoaligned BY Columnar Phase by X-ray Scattering Measurements*

Grazing-incidence small-angle X-ray scattering (GI-SAXS) measurements were taken by an FR-E X-ray diffractometer equipped with a two-dimensional detector R-axis IV (Rigaku) involving an imaging plate (Fujifilm). An X-ray beam (Cu Kα = 0.154 nm, 0.3 mm collimated) was used, and the camera length was set at 300 mm. The spin-coated films were placed onto a pulse motor stage composed of oblique pulse (ATS-C310-EM, Chuo Precision Industrial) and Z-pulse (ALV-3005-HM, Chuo Precision Industrial) motors. The incident angle of the X-ray beam was adjusted between 0.18 and 0.22◦ to the substrate surface using the pulse motors.

### *2.9. UV-Curing of BY Columnar Phase*

For UV-curing of the BY columnar phase, non-polarized UV light (365 nm, 5 mW cm<sup>−</sup>2) passed through band pass filter from a REX-250 was irradiated to the spin-coated films for 5 min.

### **3. Results and Discussion**

### *3.1. Photoalignment and Humidity Resistance of Pure BY Film*

White interference microscopy images of BY films are shown in Figure 1a. The groove in the center of the image was formed when the film was scraped with a micro spatula. A smooth surface morphology with a film thickness of approximately 23 nm was obtained. Figure 1b shows the polarized UV-vis absorption spectrum of the BY film irradiated with LPVis light. The dichroism was induced upon LPVis irradiation. The order parameter (*S*) for the irradiation dose reached approximately 0.5 at 3 J cm<sup>−</sup>2, and subsequent irradiation provided *S* = 0.7 after further dose (Figure 1c), indicating that BY is highly photoaligned by LPVis irradiation.

**Figure 1.** (**a**) Surface topographical morphology of pure BY film. The cross-section of the average height profile along the A-B line in the box is shown below. (**b**) Polarized UV-vis absorption spectra change of pure BY films associated with exposure to LPVis light. (**c**) Order parameter change associated with dose of visible light. (**d**) GI-SAXS images of pure BY films irradiated with LPVis light of 27 J cm−<sup>2</sup> dose. (**e**) In-plane intensity profiles of d. (**f**) Photoaligned BY columnar phase.

Scattering images and in-plane intensity profiles obtained by GI-SAXS measurements of a BY film irradiated with 27 J cm−<sup>2</sup> of LPVis light are shown in Figure 1d,e, respectively. When the X-rays were incident parallel to the direction of LPVis light, scattering with an in-plane spacing of 1.59 nm was observed in the in-plane direction, suggesting that the average distance between the columnar aggregates of BY was observed as the scattering peak [39]. Thus, the BY aggregates are uniformly oriented with the column axis parallel to the LPVis light, as shown in Figure 1f.

Figure 2 shows UV-visible spectral changes and *S* when the photoaligned the BY columnar phase was exposed to a high RH at 90%. A red shift occurred after 1 h of the humidification, and the *S* value decreased with further humidification. This can be ascribed to the fact that BY exhibits a hygroscopic nature, as shown in Figure 2c. The adsorbed water can cause an orientational relaxation of the hydrated the aggregates, leading to the deterioration of photoalignment.

**Figure 2.** (**a**) Polarized UV-vis absorption spectra change with humidification at RH = 90% for photoaligned pure BY film. Black, blue, brown, and green lines indicate spectra after humidification of 0, 1, 1.5, and 2 h, respectively. (**b**) Order parameter change with humidification for photoaligned pure BY film. (**c**) Humidity-responsive weight change ratios of BY film prepared on a QCM electrode.

### *3.2. Hygroscopicity of Ionic Linear Polysiloxane Containing Sodium Sulfonate Groups*

The ionic linear polysiloxane PSSV was used for the fixation of the BY columnar phase. Spin-coated films of PSSV were prepared on a QCM electrode substrate, and the weight change of the films was monitored upon humidification (Figure 3c). The weight of the films increased as the relative humidity in the chamber increased. Within two minutes of the humidity jump operation, the film reached an equilibrium state. This indicates that the PSSV rapidly absorbs moisture. The humidity dependence of the relative weight on the PSSV in the dry state (RH = 0%) is shown in Figure 3d. Here, the data in the high humidity range at RH > 60% were calculated using the saturated salt method for a bulk PSSV. The weight of PSSV increased continuously with increasing humidity, reaching a factor of approximately 2.2 at RH = 90%. The hygroscopic behavior of PSSV was similar to that of cationic linear polysiloxanes with ammonium salts, which has previously been reported [51].

**Figure 3.** (**a**) Synthetic scheme to prepare anionic linear polysiloxane PSSV. (**b**) 1H NMR spectrum of PSSV. (**c**) Time course profiles of the changes in relative humidity and weight change of PSSV film on QCM electrode. (**d**) Humidity-responsive weight change ratios of film-state PSSV (circle) and bulk-state PSSV (square).

### *3.3. Photoalignment of the BY Columnar Phase in PSSV Matrix Using LPVis Light*

Figure 4a shows the polarized UV-vis absorption spectra of thin films composed of BY and PSSV when irradiated with LPVis light. The maximum absorption wavelength of the spectrum was blue-shifted by 5 nm from 410 nm by LPVis irradiation, and dichroism was also observed. It is likely that the presence of PSSV prevented BY from forming aggregates before LPVis irradiation, because no blue-shift phenomenon was observed when the pure BY film was irradiated with LPVis light. The S of the films irradiated with polarized visible light above 18 J cm−<sup>2</sup> was 0.65–0.70 (Figure 4b), indicating that the BY columnar phase was highly photoaligned even in the PSSV matrix. GI-SAXS measurements also yielded data suggesting photoalignment of the BY columnar phase (Figure 4c,d). The periodic structure size after LPVis irradiation was also identical to that of the pure BY film, suggesting that the PSSV does not encompass each column, but bundles the domains of the BY columnar structure as shown in Figure 4e.

**Figure 4.** (**a**) Polarized UV-vis absorption spectra change of BY-PSSV films associated with exposure to LPVis light. (**b**) Order parameter change associated with dose of visible light. (**c**) GI-SAXS images of BY-PSSV films irradiated with LPVis light of 36 J cm−<sup>2</sup> dose. X-ray beams were aligned to two directions parallel and perpendicular to the irradiated light. (**d**) In-plane intensity profiles of c. (**e**) Photoaligned BY columnar phase in PSSV matrix. PSSV is present in the blue region that bundles the BY domain.

### *3.4. UV Curing of the BY Columnar Phase in the PSSV Matrix*

When the photoaligned BY columnar phase within the PSSV matrix was exposed to a RH level of 90% (show in route 1, Figure 5a), a red shift in the ππ\* absorption band occurred (Figure 5b). As in the case of the pure BY film, it was considered that the BY columnar phase was hydrated by the absorbed water, leading to the collapse in the photoalignment. The increase in time until S decreases compared to that in the pure BY film is considered to be because the PSSV matrix encapsulates the domains of the BY columnar phase and stabilizes the BY columnar phase. However, the decrease in S was more profound than that of the pure BY film. This may be due to the high hygroscopicity of PSSV, which enables a large amount of water to be incorporated into the film. Once the BY columnar phase begins to collapse, the orientation order is suddenly reduced after two hours. The GI-SAXS profiles of the film exposed to humidification also showed a peak (d = 1.91 nm) when the X-ray was incident perpendicular to the LPVis, suggesting a randomization of the photoaligned BY columnar phase. The peak position differs from that before humidification (Figure 4d) because the BY aggregates' transition takes place from the nematic columnar phase to the rectangular columnar phase due to humidification [39].

**Figure 5.** (**a**) Schematic procedure for BY-PSSV film. (**b**) Polarized UV-vis absorption spectra change (**left**), order parameter change (**center**), and in-plane profiles obtained by GI-SAXS measurements (**right**) with humidification at RH = 90% for BY-PSSV film passing through route 1. In the left figure of (**b**), black, blue, and brown lines indicate spectra after humidification of 0, 2, and 4 h, respectively. The right side profiles in (**b**) were obtained by GI-SAXS measurements after humidification of 2 h. (**c**) Data of BY-PSSV film passing through route 2, corresponding to (**b**).

Conversely, S was retained under the same humidity conditions when the UV-irradiated BY-PSSV film was exposed to a humid environment (route 2 in Figure 5a). Additionally, GI-SAXS measurements indicated that the photoalignment of the BY columnar phase was retained (Figure 5c). Thus, it was found that the photoaligned BY columnar phase could be firmly fixed by the crosslinking of PSSV. Figure 6a shows a visual photograph of the BY-PSSV film prepared on a quartz substrate after irradiation with LPVis and non-polarized UV lights, followed by further humidification at RH = 90% for 4 h. Figure 6b displays a photograph of the same film in Figure 6a, taken through a polarizer set above the film. The

color contrast of the film was significantly different when the direction of the transmitted light was perpendicular or parallel to direction of the LPVis light. In Figure 6c, the surface morphology of the film is shown. Some dotted protrusions were observed when compared to one of the pure BY film (see Figure 1a); however, overall, the surface of the BY-PSSV film was flat. These results indicate that it was possible to form the films on centimeter-scale substrates and to prepare large-area dichroic films even after humidification. Combined with pattern exposure through a photomask, various absorption anisotropic films can be developed. The film defects observed in Figure 6b were caused by repeatedly grabbing the film using tweezers during each humidification procedure and spectroscopic measurements.

**Figure 6.** (**a**) Snapshot of BY-PSSV film on quartz. (**b**) Observation of BY-PSSV film through polarizer. The film was irradiated with an LPVis light of 36 J cm−<sup>2</sup> dose and a UV light of 1.5 J cm−<sup>2</sup> dose, then exposed to a humid atmosphere of RH = 90% for 4 h. Brown and black arrows indicate the directions of exposed LPVis light and polarizer, respectively. (**c**) Surface topographical morphology of the BY-PSSV film by white light interference microscopy. The cross-section of the average height profile along the A-B line in the top view image is shown below.

### **4. Conclusions**

In this study, photoalignment and photo-fixation of the BY columnar phase were achieved using anionic linear polysiloxane PSSV as a matrix. Using light of two different wavelengths, the photoalignment and photo-fixation were independently achieved. PSSV bundles the domain of the BY columnar phase; thus, after the UV curing, the film dichroism remained as high as that of pure BY films, even after long-term exposure to the high-humidity environment. As the BY columnar phase is often used as photoalignment sublayers for low-molecular-mass nematic liquid crystals, this method, which improves the moisture resistance, is expected to expand the applications of BY. The BY alignment layer is also expected to be used as the alignment sublayer for water-soluble compounds. This proposal is expected to expand the possibilities of anisotropic materials for the alignment of water-soluble polymers, and for inducing molecular orientation of molecular aggregates in water media.

**Author Contributions:** M.H. and T.S. conceived and designed the project. A.M. conducted most of the experiments. S.N. provided experimental technical assistance and discussions on the data. M.H. and T.S. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by JSPS KAKENHI Grant Numbers JP18K14283, JP20H05217, JP22H02142, and JP22H04536 for MH, and JP21H01983 and JP21K19000 for TS. MH is also thankful for financial support from the Toshiaki Ogasawara Memorial Foundation, the Asahi Glass Foundation, and the Toukai Foundation for Technology.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to thank Tatsuo Hikage of Nagoya University for their assistance with GI-SAXS measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
