Effect of Fluoroalkyl-Substituent in Bistolane-Based Photoluminescent Liquid Crystals on Their Physical Behavior

Abstract

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior can switch with the phase transition between the crystalline and LC phases. Although slight modulation of the flexible unit structure dramatically alters the LC and PL behaviors, few studies into the modification of the flexible units have been conducted. With the aim of achieving dynamic changes in their physical behaviors, we developed a family of bistolane derivatives containing a simple alkyl or a fluoroalkyl flexible chain and carried out a detailed systematic evaluation of their physical behaviors. Bistolanes containing a simple alkyl chain showed a nematic LC phase, whereas switching the flexible chain in the bistolane to a fluoroalkyl moiety significantly altered the LC phase to generate a smectic phase. The fluoroalkyl-containing bistolanes displayed a stronger deep blue PL than their corresponding non-fluorinated counterparts, even in the crystalline phase, which was attributed to the construction of rigid molecular aggregates through intermolecular F···H and F···F interactions to suppress non-radiative deactivation.

1. Introduction

The fluorine atom possesses numerous unique characteristics [1,2,3,4], including the largest electronegativity of all elements (4.0 on the Pauling scale), the second smallest atomic size (van der Waals radius, r_vdw = 147 pm) next to hydrogen (r_vdw = 120 pm), and the dissociation energy of the C–F bond (105.3 kJ mol⁻¹) is higher than those of the C–H (98.8 kJ mol⁻¹) and C–C (83.1 kJ mol⁻¹) bonds. Owing to these unique characteristics, the incorporation of fluorine atoms into organic molecules results in dramatic augmentations or alterations in their physical properties, in addition to introducing new functionalities [1,2,3,4]. Over the past few decades, significant attention has been paid to the development of fluorinated functional materials, worldwide. Consequently, several fluorinated materials have been developed for application in the pharmaceutical [5,6,7], agrochemical [8,9,10], and industrial fields, including organic dyes, liquid crystals, and photoluminescence molecules (Figure 1a) [11,12,13].

Previously, our group developed an efficient synthetic methodology for a wide variety of organofluorine compounds using readily available fluorinated substances [14,15] and carried out detailed explorations of fluorine-containing organic materials, such as negative dielectric liquid-crystalline (LC) molecules (A) [16] and efficient solid-state photoluminescence (PL) molecules (B) (Figure 1b) [17,18]. From the latter studies on fluorinated PL molecules, an interesting finding was that polyfluorinated bistolanes (C) exhibited both LC and PL properties [19,20,21,22,23] and were, thus, promising stimulus-responsive PL switching molecules [24,25]. As a result of intensive investigations into fluorinated PL molecules, it was revealed that fluorine atoms in these structures play an essential role in determining the electron density distribution and in the construction of rigid molecular aggregates via F···H hydrogen bonding interactions.

To understand the effect of the fluoroalkyl-type flexible unit on the LC and PL characteristics of bistolane-based photoluminescent LCs (PLLCs) (see Figure 1c), we designed a family of bistolane derivatives 1 bearing an alkyl chain (e.g., methyl or cyclohexyl) and bistolane derivatives 2 bearing a fluoroalkyl unit (e.g., trifluoromethyl, 2,3,5,6-tetrafluorocyclohexyl or undecafluorocyclohexyl). In this article, we discuss the results of our evaluation of the phase transitions and photophysical behaviors of these derivatives and describe in detail the effects of the fluoroalkyl substituents on the physical properties of the bistolane derivatives.

2. Materials and Methods

2.1. General

All chemicals were of reagent grade and were purified in the usual manner prior to use when necessary. Column chromatography was conducted using silica gel (FUJIFILM Wako Pure Chemical Corporation, Wako-gel^® 60N, 38–100 µm; Osaka, Japan), while thin-layer chromatography (TLC) was performed on silica gel TLC plates (Merck, Silica gel 60F₂₅₄; New Jersey, NJ, USA).

High-resolution mass spectra (HRMS) were recorded on a JMS700MS spectrometer (JEOL, Tokyo, Japan) using the fast atom bombardment (FAB) method. Infrared (IR) spectra were recorded using the KBr method with an FTIR-4100 type A spectrometer (JASCO, Tokyo, Japan). All the optical spectra are reported in terms of the wavenumber (cm⁻¹). ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were obtained using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in chloroform-d (CDCl₃) solution, and the chemical shifts are reported in parts per million (ppm) based on the residual protons in the NMR solvent. ¹⁹F NMR (376 MHz) spectra were obtained using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in CDCl₃ solution with CFCl₃ (δ_F = 0 ppm) as an internal standard.

2.2. Materials

Bistolanes 1a and 1b bearing an alkyl chain and 2a–2c bearing a fluoroalkyl unit were synthesized via a Sonogashira cross-coupling reaction using 4-substituted halobenzene and 4-[2-(4-n-hexyloxyphenyl)ethyn-1-yl]phenylacetylene, both of which are readily accessible from the corresponding commercially-available chemicals (Figure 2).

2.3. Typical Synthetic Procedure for Preparation of the Bistolanes (as an Example, 2c)

In a two-necked round-bottomed flask, equipped with a magnetic stirrer bar, were placed Cl₂Pd(PPh₃)₂ (Sigma-Aldrich; 22 mg, 0.031 mmol), PPh₃ (Fujifilm-Wako Pure Chemicals; 10 mg, 0.038 mmol), 4-[2-(4-n-hexyloxyphenyl)ethyn-1-yl]phenylacetylene (76 mg, 0.33 mmol), 1-(undecafluorocyclohexyl)-4-iodobenzene (191 mg, 0.39 mmol), CuI (Fujifilm-Wako Pure Chemicals; 6.0 mg, 0.032 mmol), and Et₃N (Fujifilm-Wako Pure Chemicals; 1.5 mL). The mixture was heated to 60 °C and continuously stirred for 19 h at the same temperature. After 19 h, the precipitate formed in the reaction mixture was removed by atmospheric filtration, the filtrate was added to an aqueous saturated NH₄Cl solution (Fujifilm-Wako Pure Chemicals; 10 mL), and the organic component was extracted with ethyl acetate (EtOAc, 3 × 15 mL). The combined organic layer was then dried over anhydrous Na₂SO₄ and separated by filtration. The filtrate was evaporated under reduced pressure using a rotary evaporator and the residue was subjected to silica-gel column chromatography (eluent: hexane/EtOAc = 30/1), followed by recrystallization from CH₂Cl₂/MeOH (v/v = 1/1) to afford 1-undecafluorocyclohexyl-4-[2-[4-[2-(4-n-hexyloxyphenyl)ethyn-1-yl]phenyl]ethyn-1-yl]benzene (2c) as colorless crystals in 40% yield (87 mg, 0.13 mmol).

2.3.1. 1-Undecafluorocyclohexyl-4-[2-[4-[2-(4-n-hexyloxyphenyl)ethyn-1-yl]phenyl]ethyn-1-yl]benzene (2c)

Yield: 40%; m.p.: 163 °C determined by DSC; IR (KBr) ν_max 2960, 2933, 2861, 2213, 1597, 1521, 1249, 1195, 1025, 968, 866 cm⁻¹; ¹H NMR (CDCl₃) δ 0.91 (3H, t, J = 6.8 Hz), 1.31–1.39 (4H, m), 1.42–1.53 (2H, m), 1.79 (2H, quin., J = 6.4 Hz), 3.98 (2H, t, J = 6.4 Hz), 6.88 (2H, d, J = 8.8 Hz), 7.46 (2H, d, J = 8.8 Hz), 7.51 (3H, s), 7.67 (4H, s); ¹³C NMR (CDCl₃) δ 14.0, 22.6, 25.7, 29.1, 29.7, 31.6, 68.1, 87.6, 89.3, 91.9, 104.2–114.0 (m, 2C), 114.6, 114.7, 121.8, 123.7, 123.9, 124.2, 126.8, 127.0 (t, J = 4.4 Hz), 127.2 (t, J = 3.6 Hz), 131.4, 131.6, 131.9, 133.1, 159.5; ¹⁹F NMR (CDCl₃): δ −118.4 to −119.5 (dm, J = 295.5 Hz, 2F), −121.9 to −123.0 (dm, J = 275.5 Hz, 2F), −123.0 to −124.1 (dm, J = 286.1 Hz, 1F), −132.7 to −133.8 (dm, J = 298.5 Hz, 2F), −138.4 to −139.6 (dt, J = 282.0, 13.6 Hz, 2F), −141.6 to −142.7 (dm, J = 286.1 Hz, 1F), −181.0 to −181.4 (m, 1F); HRMS (FAB+) m/z 658.1720 (calcd for C₃₄H₂₅F₁₁O, 658.1730).

2.3.2. 4-[2-[4-[2-(4-n-Hexyloxyphenyl)ethyn-1-yl]phenyl]ethyn-1-yl]toluene (1a)

Yield: 33%; m.p.: 154 °C determined by DSC; IR (KBr) ν_max 2952, 2921, 2848, 1604, 1519, 1466, 1284, 1242, 1180, 839, 813 cm⁻¹; ¹H NMR (CDCl₃) δ 0.91 (3H, t, J = 7.2 Hz), 1.31–1.39 (4H, m), 1.40–1.52 (2H, m), 1.75–1.83 (2H, m), 2.37 (3H, s), 3.97 (2H, t, J = 6.4 Hz), 6.87 (2H, d, J = 8.8 Hz), 7.16 (2H, d, J = 8.4 Hz), 7.42 (2H, d, J = 8.4 Hz), 7.45 (2H, d, J = 8.8 Hz), 7.475 (4H, s); ¹³C NMR (CDCl₃) δ 14.0, 21.5, 22.6, 25.7, 29.2, 31.5, 68.1, 87.8, 88.6, 91.26, 91.34, 114.6, 114.9, 120.0, 122.9, 123.3, 129.1, 131.3, 131.4, 131.5, 133.0, 138.5, 159.4; HRMS (FAB+) m/z 392.2148 (calcd for C₂₉H₂₈O, 392.2140).

2.3.3. 1-Cyclohexyl-4-[2-[4-[2-(4-n-hexyloxyphenyl)ethyn-1-yl]phenyl]ethyn-1-yl]benzene (1b)

Yield: 53%; m.p.: 164 °C determined by DSC; IR (KBr) ν_max 2926, 2852, 2215, 1604, 1596, 1518, 1246, 1103, 827 cm⁻¹; ¹H NMR (CDCl₃) δ 0.91 (3H, t, J = 7.2 Hz), 1.21–1.30 (1H, m), 1.30–1.52 (10H, m)‚ 1.70–1.92 (8H, m), 3.97 (2H, t, J = 6.4 Hz), 6.87 (2H, d, J = 8.8 Hz), 7.19 (2H, d, J = 8.4 Hz), 7.449 (2H, d, J = 8.4 Hz), 7.454 (2H, d, J = 8.8 Hz), 7.47 (4H, s); ¹³C NMR (CDCl₃) δ 14.0, 22.6, 25.7, 26.1, 26.8, 29.1, 31.5, 34.2, 44.5, 68.1, 87.8, 88.5, 91.3, 114.5, 114.8, 120.3, 122.9, 123.2, 126.9, 129.6, 131.3, 131.4, 131.6, 133.0, 148.6, 159.3; HRMS (FAB+) m/z 460.2760 (calcd for C₃₄H₃₆O, 460.2766).

2.3.4. 4-[2-[4-[2-(4-n-Hexyloxyphenyl)ethyn-1-yl]phenyl]ethyn-1-yl]benzotrifluoride (2a)

Yield: 43%; m.p.: 130 °C determined by DSC; IR (KBr) ν_max 2962, 2937, 2859, 1607, 1594, 1510, 1332, 1286, 1251, 1127, 1067, 1024, 837 cm⁻¹; ¹H NMR (CDCl₃) δ 0.91 (3H, t, J = 6.8 Hz), 1.32–1.38 (4H, m), 1.40–1.52 (2H, m), 1.73–1.86 (2H, m), 3.98 (2H, t, J = 6.8 Hz), 6.88 (2H, d, J = 8.8 Hz), 7.46 (2H, d, J = 8.8 Hz), 7.50 (4H, s), 7.62 (4H, d, J = 1.6 Hz); ¹³C NMR (CDCl₃) δ 14.0, 22.6, 25.7, 29.2, 31.6, 68.1, 87.6, 89.5, 91.5, 91.9, 114.6, 114.7, 121.2 (q, J = 271.3 Hz), 121.8, 124.2, 125.3 (q, J = 3.7 Hz), 126.9, 130.0 (q, J = 33.0 Hz), 131.4, 131.6, 131.8, 133.1, 159.5; ¹⁹F NMR (CDCl₃): δ −63.30 (s, 3F); HRMS (FAB+) m/z 446.1861 (calcd for C₂₉H₂₅F₃O, 446.1858).

2.3.5. 1-(2,3,5,6-Tetrafluorocyclohexyl)-4-[2-[4-[2-(4-n-hexyloxyphenyl)ethyn-1-yl]phenyl]ethyn-1-yl]benzene (2b)

Yield: 64%; m.p.: 175 °C determined by DSC; IR (KBr) ν_max 2934, 2868, 2213, 1604, 1519, 1247, 1107, 1016, 973, 837 cm⁻¹; ¹H NMR (CDCl₃) δ 0.91 (3H, t, J = 6.8 Hz), 1.31–1.39 (4H, m), 1.41–1.52 (2H, m), 1.79 (2H, quin., J = 6.8 Hz), 2.43–2.56 (1H, m), 2.62 (1H, t, J = 36.4 Hz), 2.70–2.86 (1H, m), 3.97 (2H, t, J = 6.4 Hz), 4.51–4.77 (2H, m), 4.87–5.18 (2H, m), 6.87 (2H, d, J = 8.8 Hz), 7.46 (2H, d, J = 8.4 Hz), 7.491 (6H, brs), 7.54 (2H, d, J = 8.4 Hz); ¹³C NMR (CDCl₃) δ 14.0, 22.6, 25.7, 29.1, 31.6, 43.6–44.2 (m, 1C), 68.1, 85.6–90.0 (2C), 87.7, 89.9, 90.4, 114.6, 114.8, 122.4, 123.1, 123.7, 129.3, 131.3, 132.0, 133.1, 135.7, 159.4, three carbons cannot be detected due to low solubility in NMR solvent; ¹⁹F NMR (CDCl₃): δ −190.6 to −190.9 (m, 2F), −210.1 to −210.6 (m, 2F); HRMS (FAB+) m/z 532.2402 (calcd for C₃₄H₃₂F₄O, 532.2389).

2.4. X-ray Crystallographic Analysis

Single crystals of 2a and 2c were obtained by recrystallization from a mixed solvent system of CH₂Cl₂/MeOH (v/v = 1/1). Each single crystal was mounted on a glass fiber and X-ray diffraction patterns were recorded on a XtaLabMini diffractometer equipped with a VariMax Mo optical system (λ = 0.71073 Å) and a Pilatus P200 detector (Rigaku, Tokyo, Japan). The reflection data were processed using the CryslAlisPro (ver. 1.171.38.46; Rigaku Oxford Diffraction, 2015). The structures were solved by a directed method (SHELXT-2014/5) and SHELXL-2014/7 programs [26]. The crystallographic data were deposited in the Cambridge Crystallographic Data Centre (CCDC) database (CCDC 2,065,068 for 2a and 2,065,069 for 2c). These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif (accessed on 19 April 2021).

2.5. Powder X-ray Diffraction (PXRD) Measurements

The liquid crystal structures were evaluated using an FR-E X-ray diffractometer attached to an R-axis IV two-dimensional (2D) detector (Rigaku, Tokyo, Japan). For the purpose of these measurements, 0.3-mm collimated Cu Kα radiation (λ = 1.54187 Å) was used as the X-ray beam, and the camera length was set to 300 mm. The powder sample was loaded into a thin-walled glass capillary tube for XRD analysis (ϕ 1.0–2.5 mm, Hilgenberg GmbH), and the sample was annealed up to an isotropic temperature under vacuum. The glass capillary was placed on a ceramic heater attached to the FR-E sample holder. The exposure time of the X-ray beam was 5–60 min.

2.6. Computations

All computations were performed using the Gaussian 16 software package (Revision B.01) [27]. Geometry optimizations were carried out using the hybrid meta-GGA functional (M06-2X) [28,29] and the 6-31+G(d) basis set with an implicit solvation model (conductor-like polarizable continuum model; CPCM [30,31,32]) for CH₂Cl₂. The vertical excitation energies and dipole moments of the optimized structures were calculated using the time-dependent self-consistent field approximation [33,34] at the same level of theory.

2.7. Thermal Measurements

The phase transition behavior was observed by polarizing optical microscopy using a BX53 microscope (Olympus Corporation, Tokyo, Japan), equipped with a heating and cooling stage (Linkam Scientific Instruments, 10,002 L, Surrey, UK). The thermodynamic behavior was determined using differential scanning calorimetry (DSC, SHIMADZU DSC-60 Plus, Kyoto, Japan) at heating and cooling rates of 5.0 °C min⁻¹ under a N₂ atmosphere.

2.8. Photophysical Measurements

The UV–Vis absorption spectra were recorded using a V-500 absorption spectrometer (JASCO, Tokyo, Japan). The PL spectra of the solution and crystal forms were acquired using an FP-6600 fluorescence spectrometer (JASCO, Tokyo, Japan). The absolute quantum yields in both the solution and crystalline phases were measured using the Quantaurus-QY measurement system C11347-01 (Hamamatsu Photonics, Hamamatsu, Japan).

3. Results and Discussion

3.1. Theoretical Assessment

Our study was initiated with a theoretical assessment based on quantum chemical calculations using the Gaussian 16 software to discuss the effects of the fluoroalkyl units on the physical properties of the various compounds.

Table 1. Theoretical data obtained from quantum chemical calculations ¹.

Compound	µ||/µ⊥ (D) 2	HOMO/LUMO (eV)	Theoretical Transition Probability (%)	λcalcd (nm)
1a	0.96/1.79	−6.88/−1.27	HOMO→LUMO (92%)	330
1b	0.96/1.79	−6.87/−1.27	HOMO→LUMO (92%)	331
2a	6.19/1.49	−6.99/−1.50	HOMO→LUMO (87%)	333
2b	2.44/4.17	−6.93/−1.35	HOMO→LUMO (91%)	331
2c	6.57/1.54	−7.00/−1.52	HOMO→LUMO (87%)	334

¹ The ground state geometry and the TD-SCF approximation were obtained at the M06-2X/6-31+G(d) level of theory with CPCM for CH₂Cl₂. ² µ_|| and µ_⊥ are the dipole moments along the major and minor molecular axes, respectively.

lists the molecular dipole moments (i.e., μ_|| along the major molecular axis and μ_⊥ along the minor molecular axis), the highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO) energies, the theoretical transition probability and distribution, and the theoretical absorption wavelength (λ^calcd) for each compound. Figure 3 illustrates the MOs alongside the corresponding orbital energies.

Bistolanes 1a and 1b, bearing an alkyl chain, were found to possess small dipole moments for both the major and minor molecular axes, µ_|| and µ_⊥, respectively, in both the major and minor molecular axes in the ground state. Comparing the µ_|| and µ_⊥ values for 1a and 1b, the µ_|| values for perfluoroalkyl-containing bistolanes 2a and 2c increased approximately six-fold along the major molecular axis. Interestingly, in the case of bistolane 2b, bearing an all-cis-2,3,5,6-tetrafluorocyclohexyl unit, ~2.5-fold higher values of both µ_|| and μ_⊥ were obtained because of the 1,3-diaxial arrangement of the C–F bonds stemming from the all-cis configuration [35,36]. The dramatic changes in the dipole moments may, therefore, induce a significant alteration in the molecular aggregates, associated with changes in the intermolecular interactions; a significant change in the phase transition behavior during the crystal ⇄ LC and LC ⇄ isotropic liquid phase transformations would also be expected.

Based on our calculations, 1a and 1b, both of which bear alkyl chains, exhibited broad HOMOs and LUMOs that covered the entire π-conjugated structure. In contrast, the partially or fully fluoroalkyl-containing bistolanes 2a–2c showed a molecular orbital distribution wherein the orbital lobes were primarily localized over the electron-rich aromatic ring attached to the n-hexyloxy chain in the HOMO, while in the LUMO, the orbital lobes were primarily localized over the electron-deficient aromatic ring bearing a fluoroalkyl unit. 1a and 1b exhibited high energy levels of −6.88 eV and −6.87 eV for the HOMO, respectively, and −1.27 eV for both LUMOs of 1a and 1b (ΔE = 5.60–5.61 eV). Meanwhile, 2a and 2c with the perfluoroalkyl substituent exhibited reduced HOMO and LUMO energy levels, i.e., −7.00 eV and −1.52 eV, respectively, together with a smaller ΔE value (5.48 eV). Notably, 2b, with a partially fluorinated alkyl fragment, had intermediate HOMO and LUMO energy levels between 1b and 2c: −6.93 eV for the HOMO and −1.35 eV for the LUMO. Consequently, it can be anticipated that switching the flexible unit from an alkyl chain to the corresponding fluoroalkyl unit could also result in significant changes in the photophysical behavior, such as the ultraviolet-visible (UV–Vis) absorption and the PL behavior.

3.2. Synthesis and Crystal Structure Determination

Based on the theoretical assessments, bistolanes 1 and 2 were synthesized via a Pd(0)-catalyzed Sonogashira cross-coupling reaction of 4-[2-(4-n-hexyloxyphneyl)ethyn-1-yl]phenylacetylene with the appropriate iodobenzene or bromobenzene bearing an alkyl or fluoroalkyl substituent at the para-position (Figure 2). All compounds were successfully obtained in 33–64% isolated yields after purification by column chromatography and subsequent recrystallization. Identification of the crystalline compounds was achieved by spectroscopic analyses, and the obtained products were found to be sufficiently pure to allow evaluation of their phase transitions and PL behaviors.

Among the crystalline compounds obtained, trifluoromethyl (CF₃)-containing 2a and undecafluorocyclohexyl (cyclo-C₆F₁₁)-substituted 2c furnished single crystals suitable for X-ray crystallographic analysis. Figure 4 shows the molecular structures and packing structures of these two crystals.

As indicated, the CF₃-containing 2a formed colorless rod-shaped single crystals of a monoclinic crystal system in the Cc space group, and the unit cell contained four molecular units. For 2a, a rigid packing structure was constructed that included several intermolecular interactions (Figure 4a, left). Considering the short interatomic distances, as shown in Figure 4a (right), C–F···H hydrogen bonding (259.5 pm), O···H hydrogen bonding (268.4 pm), C–F···π interactions (316.4 pm) [37,38,39], C–H···π interactions (282.4 and 284.0 pm), and van der Waals interactions (C_alkyl–H···H–C_alkyl = 235.3 pm and C_aryl–H···H–C_alkyl = 237.7 pm) all appeared to be present; in each of these cases, the interatomic distances were less than the sum of r_vdW, for the two atoms (i.e., C: 170 pm, H: 120 pm, and F: 149 pm) [40]. In contrast, the cyclo-C₆F₁₁-substituted 2c formed colorless block-shaped crystals of a monoclinic crystal system in the P2₁ space group, and four molecular units of 2c were contained in a unit cell (Figure 4b, left). As shown in Figure 4b (right), rigid packing structures were formed through several intermolecular interactions, including C–F···H hydrogen bonding (242.8 and 254.7 pm), C–F···F interactions (289.4, 291.1, 293.9, and 294.0 pm) [41,42], C_aryl–H···π interactions (282.2, 283.0, and 289.4 pm), C_alkyl–H···π interactions (283.8 pm), and van der Waals interactions (C_aryl–H···H—C_alkyl = 238.8 pm). It is notable that 2a produced layered structures with a head-to-tail molecular alignment through C–F···H hydrogen bonding interactions. Although 2c also formed layered structures with a head-to-head molecular alignment, in this case, these were attributed to C–F···F interactions resulting from the fluorophilic effect of the highly fluorinated alkyl moieties [42,43].

3.3. Phase Transition Behaviors

The phase transition behaviors of crystalline bistolane derivatives 1a and 1b and 2a–2c were then evaluated using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and variable temperature powder X-ray diffraction (VT-PXRD). Figure 5 shows the phase sequences and POM images observed for the LC phase during the 2nd heating and cooling processes.

During the POM observations, a bright-viewing field with good fluidity was found for all the bistolanes (i.e., 1a, 1b, and 2a–2c) over both the heating and cooling processes. The POM images of 1a and 1b in the LC phase display the four-brush Schlieren texture, which is a typical image of a nematic (N) LC phase. In addition, the VT-PXRD measurements of 1a and 1b at 180 °C revealed a hollow pattern at ~20° (2θ) with no sharp diffraction signals. Based on the POM and VT-PXRD measurements, the observed LC phases for 1a and 1b were identified as the nematic (N) phase with an orientational order. In contrast to the alkyl-substituted 1a and 1b, CF₃-substituted 2a and cyclo-C₆F₁₁-substituted 2c displayed distinct POM textures, namely fan-shaped arrangements, which are typical textures for smectic (Sm) phases with both orientational order and positional order in the LC phase. In the case of 2a, which bears a CF₃-unit, VT-PXRD measurements at 150° showed three sharp peaks, identified as the (110), (200), and (210) mirror indexes, in the wide-angle region. This indicates that 2a forms an in-plane herringbone structure (Figure S23, Supporting Information). Consequently, the LC phase observed between 130 and 202 °C was identified as a smectic E (SmE) phase [44]. Furthermore, the PXRD pattern at 210 °C showed only one sharp diffraction peak, assigned to the (110) plane, which corresponds to a hexagonal structure; the LC phase observed in the range of 202–214 °C was identified as the smectic B (SmB) phase. Additionally, the LC phase displayed between 214 and 236 °C was identified as the smectic A (SmA) phase, which produced a single sharp diffraction peak in the small-angle region. In contrast, for 2c, which bears a cyclo-C₆H₁₁ unit, a single sharp signal was detected in the small-angle region at 180 °C, which indicates that 2c displayed only the SmA phase in the LC form (Figure S25 in SI). Although 2b, which bears an all-cis-2,3,5,6-tetrafluorocyclohexyl (cis-cyclo-C₆H₇F₄) unit, contained four fluorine atoms in its flexible unit, only the N phase was observed as an LC phase, analogous to the case of 1, which bears an alkyl chain. Based on these observations, the dipole moment µ_|| along the major molecular axis was considered to be important for controlling the LC phase. More specifically, the incorporation of fluorine atoms to increase µ_|| induces the formation of a higher-order LC phase with both orientational and positional order, similar to the Sm phase, whereas bistolanes with smaller µ_|| values form an LC phase with only an orientational order, such as the N phase.

3.4. Photophysical Behavior

Subsequently, we focused on the photophysical behaviors of bistolanes 1a and 1b, which bear an alkyl chain, and 2a–2c, which bear a fluoroalkyl unit. Figure 6a shows the UV–Vis absorption (1.5 × 10⁻⁵ mol L⁻¹) and PL spectra (1.0 × 10⁻⁶ mol L⁻¹) in CH₂Cl₂, and the obtained photophysical data are summarized in

Table 2. Photophysical data for 1a, 1b, and 2a–2c recorded in various solvents ¹.

Compound	Solvent [ET(30)] 2	λabs (nm) [ε, 103 L mol−1 cm−1]	λex (nm)	λPL (nm)	ΦPL 3
1a	CH2Cl2 [40.7]	328 [80.4], 348 [sh, 55.6]	330	377	1.00
	Hexane [31.0]	324 [20.4], 344 [sh, 12.9]	324	353, 368, 380	0.30
	CHCl3 [39.1]	328 [57.2], 348 [sh, 37.3]	330	366, 380	0.66
	DMF [43.2]	328 [33.7], 348 [sh, 23.2]	330	388	0.78
	MeCN [45.6]	324 [38.1], 344 [sh, 26.8]	324	388	0.68
1b	CH2Cl2 [40.7]	330 [63.0], 350 [sh, 43.0]	330	386	1.00
2a	CH2Cl2 [40.7]	331 [50.9], 350 [sh, 37.0]	330	400	1.00
	Hexane [31.0]	330 [2.6], 349 [sh, 1.4]	330	361, 377	0.20
	CHCl3 [39.1]	333 [23.4], 350 [sh, 19.0]	330	388	0.86
	DMF [43.2]	333 [23.4], 350 [sh, 19.0]	330	420	0.83
	MeCN [45.6]	325 [5.3], 346 [sh, 3.7]	325	420	0.71
2b	CH2Cl2 [40.7]	329 [61.0], 350 [sh, 42.4]	330	386	1.00
2c	CH2Cl2 [40.7]	332 [56.7], 350 [sh, 41.7]	330	405	1.00

¹ Concentration: 1.0 × 10⁻⁵ mol L⁻¹ for the UV–Vis absorption measurements and 1.0 × 10⁻⁶ mol L⁻¹ for the PL measurements. ² The E_T(30) value is a quantitative indicator of the solvent polarity. ³ Measured using an integrating sphere; sh: shoulder peak.

.

For the UV–Vis absorption measurements carried out in CH₂Cl₂, a single UV–Vis absorption band (λ_abs) was observed for all bistolanes with a maximum absorption wavelength at ~328–332 nm, together with a shoulder peak at ~348–350 nm (Figure 6a, Table 2). Based on the theoretical assessment (λ^calcd = 330–334 nm, Table 1), for all compounds, the major absorption band was concluded to originate from the HOMO→LUMO transition. The slight red-shift of the λ_abs values of 2a–2c, compared to those of 1a and 1b, were likely caused by the narrower HOMO–LUMO energy gap (ΔE) of the former, which is induced by the electron-withdrawing character of the fluoroalkyl unit.

Upon irradiation of the CH₂Cl₂ solutions of the bistolanes with UV light of an energy equal to λ_abs, all compounds were found to exhibit strong PL (Φ_PL = 1.00), with a single PL band of maximum PL wavelength (λ_PL) at ~377–405 nm (Figure 6a, Table 2). In this case, the incorporation of fluorine atoms into the flexible unit caused a gradual red-shift in λ_PL by 20–23 nm for 2a and 2c, with respect to the values for 1a and 1b, although 2b, which contained four fluorine atoms, did not exhibit any such shift in λ_PL. Using theoretical assessments, it was concluded that the red-shift in λ_PL is associated with the orbital gap (ΔE) between the HOMO and LUMO; full substitution of the flexible unit with fluorine atoms, as in the cases of 2a and 2c, caused a 0.12 eV decrease in ΔE compared to that of their non-fluorinated counterparts, i.e., 1a and 1b, resulting in a slight red-shift in λ_PL. However, the partial incorporation of fluorine atoms into the flexible unit, as in the case of 2b, had no significant impact, and this was attributed to the less effective electron withdrawing capability of the flexible unit along the major molecular axis. As shown by the Commission Internationale de l’Eclailage (CIE) color diagram in Figure 6b, all the bistolane analogues produced a deep blue PL color in CH₂Cl₂.

To gain additional insight into the effect of fluorine atoms on the absorption and PL behaviors, the influence of the solvent was examined for CH₃-substituted 1a and CF₃-substituted 2a as a representative comparison (Figure 6c). It was found that the absorption behaviors of 1a and 2a were consistent, without showing any dependence on the solvent polarity, i.e., λ_abs = 324–328 nm for 1a and 325–333 nm for 2a (Figures S28 and S29 in SI). By contrast, the PL behavior was found to change upon varying the solvent polarity. More specifically, a solution of 1a in the less polar hexane (E_T(30) = 31.0) [45] exhibited sharp PL signals at ~353 nm, along with a vibrational structure at ~368 nm. Upon increasing the solvent polarity, the PL band was gradually shifted to the long-wavelength region, reaching up to 388 nm when DMF (E_T(30) = 43.2) or MeCN (E_T(30) = 45.6) was employed as the solvent. CF₃-substituted 2a also exhibited a sharp band at ~361 nm in hexane, together with vibrational structure at ~377 nm. In the case of 2a, the λ_PL reached up to 420 nm when the polar solvents DMF (E_T(30) = 45.6) or MeCN (E_T(30) = 45.6) were used. Upon comparison of the PL behaviors of 1a and 2a in various solvents, a longer-wavelength shift in the PL was observed in the case of CF₃-substituted 2a because 2a is likely to be susceptible to stabilization via solvation in polar solvents due to its large dipole moment, µ_||. To elucidate the correlation between solvent polarity and PL behavior, Lippert–Mataga plots were created with a solvent polarity parameter (Δf) on the horizontal axis and the Stokes’ shift (Δν = ν_abs − ν_PL) on the vertical axis (Figure 6d) [46,47]. Linear relationships between Δν and Δf were found for 1a and 2a, and these are expressed in Equations (1) and (2), respectively:

Δν = 8404.5 Δf + 2342,

(1)

Δν = 16,559 Δf + 1859,

(2)

It is known that the slope of these equations is strongly correlated with the difference in the dipole moments between the excited and ground states [44]: Δμ = µ_e − µ_g, where μ_e and μ_g represent the dipole moments in the excited and ground states, respectively. As a result, the value of Δμ for 2a was larger than that for 1a (i.e., 7.44 D for 2a and 6.33 D for 1a), and this was attributed to the fact that compared to 1a, the CF₃-substituted 2a is likely to exhibit a larger polarization in the excited state. Therefore, the incorporation of a fluoroalkyl substituent resulted in a strong intermolecular charge transfer transition-based PL due to the strong electron-withdrawing character of the fluoroalkyl unit.

It is of significant interest that bistolanes 1a and 1b (bearing an alkyl chain) and 2a–2c (bearing a fluoroalkyl unit) exhibited PL even in the crystalline state, despite the fact that the majority of π-conjugated luminophores have been reported to undergo immediate quenching when molecular aggregates are formed [48,49]. Figure 7 shows the PL spectra and photographic images depicting the PL behaviors of 1a, 1b, and 2a–2c in the crystalline state, along with the corresponding CIE color diagram. The corresponding photophysical data are listed in

Table 3. Photophysical data for 1a, 1b, and 2a–2c in the crystalline form.

Compound	λPL1	ΦPL 2
1a	404, 422	0.57
1b	405, 421	0.79
2a	418, 434(sh)	1.00
2b	401(sh), 417	0.71
2c	421, 434(sh)	0.82

¹ Excitation wavelength of incident light: 300 nm; sh: shoulder peak. ² Measured using an integrating sphere.

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As indicated in Figure 7a, compounds 1a and 1b exhibited two PL bands with λ_PL values of ~404 and 422 nm, while bands at ~418, 417, and 421 nm, were observed for 2a–2c, respectively, in addition to shoulder peaks at 434 nm for 2a and 2c, and at 401 nm for 2b. In comparison with the results obtained for the alkyl-containing 1a and 1b, the λ_PL values for 2a–2c were slightly red-shifted by 10–15 nm. In addition, λ_PL was significantly red-shifted by ~30 nm, in comparison with the λ_PL obtained in the solution states, which is attributable to the stabilization of the excited states via intermolecular interactions in the aggregated states.

It is noteworthy that bistolanes 1a and 1b exhibited high PL efficiencies (Φ_PL) of 0.57 and 0.79, respectively, even in the crystalline state, although the values of Φ_PL in the aggregated states were drastically reduced compared with those in the solution states, due to the acceleration of non-radiative deactivation via intermolecular interactions [48,49]. As mentioned above, 2b, which contains a partially fluorinated alkyl unit, afforded a similar result for the PL measurements, even in the crystalline state, whereas bistolanes 2a and 2c, which bear fully fluorinated alkyl units, exhibited enhanced Φ_PL values. As discussed above, 2a and 2c formed rigid molecular aggregates through multiple intermolecular interactions, which effectively weakened the molecular motions (e.g., vibration and rotation). This, in turn, suppressed non-radiative deactivation, and produced a significant enhancement in Φ_PL. In the other compounds, it is anticipated that non-radiative deactivation cannot be sufficiently suppressed because there are fewer intermolecular interactions, although the aggregated structures of non-fluorinated 1a and 1b and partially fluorinated 2b remain unclear. Accordingly, the incorporation of a fluoroalkyl unit into the bistolane scaffold can achieve rigid packing in the molecular aggregates, leading to a significant enhancement in the Φ_PL, even in the crystalline state.

Notably, the bistolane derivatives bearing a non-fluorinated or fluorinated alkyl chain were also found to be emissive in their LC phase that was formed after a thermal transition from the crystalline to the LC phase. The PL intensity, however, was gradually retarded because of the increasing non-radiative deactivation via micro-Brownian motion under the thermal conditions. The photoluminescent liquid crystal (PLLC) characteristics could be significantly improved if alignment of the LC on a film or substance could be controlled using a photoalignment technique, such as a mechanical rubbing process [50] or a rubbing-free process using photo-cross-linkable polymers [51,52] and photoreactive polyimide polymers [53,54].

4. Conclusions

To elucidate the effects of fluorine atoms in bistolane-based photoluminescent liquid crystals (PLLCs) on their liquid-crystalline (LC) and photoluminescent (PL) behaviors, bistolanes bearing an alkyl chain or a fluoroalkyl unit were synthesized, and their physical behaviors were systematically evaluated. All bistolanes exhibited LC behavior; the non-fluorinated alkyl-containing bistolanes mainly displayed a nematic LC phase, whereas their fluoroalkyl-containing counterparts exhibited smectic LC phases that possessed both an orientational order and a positional order. The large difference in the LC behavior can be attributed to two factors, namely the dipole moment along a major molecular axis, which is induced by the electron-withdrawing fluoroalkyl units, and the construction of head-to-tail or head-to-head ordered structures in molecular aggregates through F···H or F···F intermolecular interactions. In the photophysical measurements, interestingly, all bistolanes exhibited an intense PL behavior both in the dilute solution and in the crystalline phases. The incorporation of fluorine atoms into the flexible unit reduced not only the HOMO and LUMO energy levels, but also the energy gap between the two, thereby generating a slight red-shift of the maximum PL wavelength. Although the PL efficiency in the crystalline state normally decreases via non-radiative deactivation induced by intermolecular energy transfer, it was notable that the bistolanes containing fluoroalkyl units exhibited enhanced PL efficiencies due to the effective suppression of non-radiative deactivation. This was attributed to the formation of rigid molecular packing structures through multiple intermolecular interactions involving F···H and F···F interactions, among others. Consequently, the incorporation of fluorine atoms into the flexible units of the bistolane-based PLLCs likely contributed to the generation of a higher-ordered smectic LC phase in addition to blue PL in both the solution and aggregated states, with a high PL efficiency. These results will pave the way for the development of high-performance functional materials by incorporating fluorine atoms into organic molecules. Further fluorine-containing PLLCs will be reported by our research group in the near future.