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

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.


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.

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 60F 254 ; 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 ). 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were obtained using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in chloroform-d (CDCl 3 ) solution, and the chemical shifts are reported in parts per million (ppm) based on the residual protons in the NMR solvent. 19 F NMR (376 MHz) spectra were obtained using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in CDCl 3 solution with CFCl 3 (δ F = 0 ppm) as an internal standard.

X-ray Crystallographic Analysis
Single crystals of 2a and 2c were obtained by recrystallization from a mixed solvent system of CH 2 Cl 2 /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).

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.

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 N 2 atmosphere.

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).

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  Table 1 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.

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-1yl]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 (CF 3 )-containing 2a and undecafluorocyclohexyl (cyclo-C 6 F 11 )-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 CF 3 -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 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-C 6 F 11 -substituted 2c formed colorless block-shaped crystals of a monoclinic crystal system in the P2 1 space group, and four molecular units of 2c were contained in a unit cell (Figure 4b, left). As shown in Figure 4b 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].

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, CF 3 -substituted 2a and cyclo-C 6 F 11 -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 CF 3 -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-C 6 H 11 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-C 6 H 7 F 4 ) 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.
For the UV-Vis absorption measurements carried out in CH 2 Cl 2 , 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 (∆E) of the former, which is induced by the electron-withdrawing character of the fluoroalkyl unit.

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.   Upon irradiation of the CH 2 Cl 2 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 (∆E) 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 ∆E 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 CH 2 Cl 2 .
To gain additional insight into the effect of fluorine atoms on the absorption and PL behaviors, the influence of the solvent was examined for CH 3 -substituted 1a and CF 3substituted 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. CF 3 -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 CF 3 -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 (∆f ) on the horizontal axis and the Stokes' shift (∆ν = ν abs − ν PL ) on the vertical axis ( Figure 6d) [46,47]. Linear relationships between ∆ν and ∆f were found for 1a and 2a, and these are expressed in Equations (1) and (2), respectively: ∆ν = 8404.5 ∆f + 2342, 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]: ∆µ = 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 ∆µ 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 CF 3 -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. 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.   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 rubbingfree process using photo-cross-linkable polymers [51,52] and photoreactive polyimide polymers [53,54].

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 nonfluorinated 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.

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