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

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.

1. Introduction

Photoluminescent liquid crystals, which possess photoluminescence (PL) and liquid-crystalline (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 [1,2,3]. 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 [6,7,8,9,10,11,12,13,14]. Our recent study revealed that fluorinated bistolane-based PLLC molecules (A) exhibit PL and LC behaviors 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 [8,9,10,11], 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].

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.

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-d₆ or acetone-d₆, 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_F = 0.0 ppm) and hexafluorobenzene (δ_F = −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 round-bottomed 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-d₆): 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-d₆): 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-d₆, CFCl₃): δ −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-d₆): 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-d₆): 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-d₆, 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-d₆): 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-d₆): 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-d₆, 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⁻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-d₆): 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, J = 253.0, 13.9, 3.6 Hz), 160.3, 161.7; ¹⁹F NMR (acetone-d₆, C₆F₆): δ −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⁻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-d₆): 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-d₆): 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-d₆, C₆F₆): δ −136.92 (dd, J = 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⁻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-d₆): 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-d₆): 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-d₆, 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-d₆): 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-d₆): 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-d₆, 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⁻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-d₆): 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-d₆): 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-d₆, CFCl₃): δ −136.49 (dd, J = 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 ABSPACK 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⁻¹ 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].

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 pentyloxy-substituted 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.

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

Molecule	Phase Sequence and Phase Transition Temperature [°C] 1
	Heating Process	Cooling Process
1a	Cry 224 Iso	Iso 165 G
1b	Cry 207 Iso	Iso 140 G
1c	Cry 211 Iso	Iso 158 G
1d	Cry 176 N 198 Iso	Iso 124 G
1e	Cry 1 160 Cry 2 170 N 191 Iso	Iso 132 G
1f	Cry 181 N 190 Iso	Iso 141 G
1g	Cry 169 N 184 Iso	Iso 150 G
1h	Cry 161 N 186 Iso	Iso 146 N 118 G

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

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.

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 ethoxy-substituted 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_m 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 × 10⁻⁵ mol L⁻¹. Figure 6 illustrates the photophysical behavior in the solution, and the photophysical data are summarized in

Table 2. Photophysical data of 1a–h in solution state.

Molecule	Solvent (ET30)	λabs [nm] 1 (ε, 103 [L mol−1 cm−1])	λPL [nm] 1 (ΦPL) 2	CIE Coordinate (x, y)
1a	CH2Cl2 (40.7)	259 (8.69), 317 (12.9)	435 (0.33)	(0.150, 0.085)
	Toluene (33.9)	324 (27.3)	403 (0.35)	(0.160, 0.029)
	CHCl3 (39.1)	274 (36.0), 284 (36.2) 317 (27.7)	420 (0.33)	(0.155, 0.051)
	MeCN (45.6)	256 (14.2), 299sh (26.3), 314 (30.2)	463 (0.10)	(0.194, 0.238)
1b	CH2Cl2 (40.7)	256 (19.8), 323 (21.6)	439 (0.33)	(0.152, 0.088)
1c	CH2Cl2 (40.7)	258 (12.0), 327 (18.5)	441 (0.35)	(0.158, 0.103)
1d	CH2Cl2 (40.7)	258 (8.98), 323 (18.9)	439 (0.37)	(0.161, 0.108)
1e	CH2Cl2 (40.7)	257 (12.6), 319 (24.0)	440 (0.27)	(0.160, 0.107)
1f	CH2Cl2 (40.7)	259 (14.2), 320 (20.3)	437 (0.33)	(0.160, 0.101)
1g	CH2Cl2 (40.7)	255 (12.8), 329 (16.9)	440 (0.38)	(0.167, 0.125)
1h	CH2Cl2 (40.7)	256 (12.9), 327 (16.2)	438 (0.37)	(0.163, 0.105)

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

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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 (Δf) and Stokes shift (Δν) 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. Photophysical data of 1a–h in aggregated phases.

Molecule	Phase 1	λPL [nm] (ΦPL) 2	CIE Coordinate (x, y)
1a	Crystalline (Cry)	481 (0.99)	(0.185, 0.296)
1b	Cry	440 (0.69)	(0.166, 0.127)
1c	Cry	434 (0.71)	(0.162, 0.099)
1d	Cry	428sh, 446(0.63)	(0.167, 0.153)
1e	Cry	454sh, 478 (0.57)	(0.184, 0.288)
1f	Cry	430sh, 458, 480sh (0.60)	(0.178, 0.239)
1g	Cry	465sh, 482, 511sh (0.49)	(0.198, 0.351)
1h	Cry	455, 483, 511sh (0.70)	(0.200, 0.320)
1h	– 3	428sh, 454, 479 (0.12)	(0.189, 0.240)

¹ Unless mentioned otherwise, the crystalline sample prepared by column chromatography and recrystallization was used. Measured at 25 °C. ² Measured using an integrating sphere. ³ 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.

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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.

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.