Donor-π-Acceptor-Type Fluorinated Tolane Containing a Semifluoroalkoxy Chain as a Condensed-Phase Luminophore

Photoluminescent liquid-crystalline (PLLC) molecules, which can easily tune the PL behavior through the crystal (Cry)–LC phase transition, have attracted significant attention. Previously, we have demonstrated that the incorporation of a semifluoroalkoxy chain into π-conjugated mesogen is a promising approach for developing PLLC molecules with PL and SmA LC characteristics. We focused on the LC and PL characteristics of the molecules induced by the semifluoroalkoxy chain and fluorinated tolanes in the condensed phase. In this study, we developed cyano- or ethoxycarbonyl-terminated donor-π-acceptor-type fluorinated tolanes containing a semifluoroalkoxy flexible chain. The cyano-terminated fluorinated tolanes exhibited intense light-blue photoluminescence in the crystalline phase and did not exhibit any LC phase. In contrast, blue photoluminescence in the ethoxycarbonyl-terminated analogs was slightly weak; however, they exhibited Cry–SmA phase transition during the heating and cooling processes. The PL intensity of the ethoxycarbonyl-terminated fluorinated tolanes significantly decreased in the SmA phase; however, their PL colors changed during the Cry–SmA phase transition. This indicates that the developed tolanes are promising temperature-dependent PL materials, such as PL thermosensors or PL thermometers.


Introduction
Precise control of molecular arrangements in the condensed phase is of great importance for the development of functional molecules because their physical properties, such as luminescence, electrical conductivity, and magnetic properties, vary significantly with the molecular arrangement [1][2][3][4]. Crystal-crystal (Cry-Cry) phase transition using polymorphs is one of the approaches to control the aggregated structures. It has been reported that luminescent molecules significantly switch through phase transitions of polymorphs [5,6]. As an alternative approach, Cry-liquid crystal (LC) or LC-LC phase transition is effective in reversibly ordering molecular arrangements because the LC phase is a mesophase between the Cry and isotropic (Iso) phases and can form different molecular arrangements depending on the LC phase [7,8]. Accordingly, molecules with both fluorescence and LC characteristics are promising candidates for developing functional materials such as fluorescence sensors and fluorescence thermometers, which can reversibly alter their fluorescence behavior through a temperature-and concentration-dependent control of their aggregated structures [9,10].
In 2008, Kato et al. reported that the color of photoluminescence of pyrene derivatives I with dendritic flexible moieties changes from yellow to blue-green owing to their phase transition from the cubic phase to the shear-induced columnar phase [11]. Since then, transition from the cubic phase to the shear-induced columnar phase [11]. Since then, various photoluminescent (PL) LC molecules, such as II and III, have been developed and investigated in detail by Tang et al. [12] and Tsutsumi et al. [13], respectively ( Figure 1a).
Our research group has successfully developed a family of donor--acceptor (D--A)-type fluorinated bistolane derivatives IV to VI; however, their synthetic procedures were tedious and inefficient ( Figure 1b) [14,15]. Furthermore, our group has also developed PLLC molecules VII and VIII (Figure 1c) using fluorinated tolanes as mesogens, which are easy to synthesize [16,17]. These molecules exhibit a nematic (N) phase with the LC phase as the only orientational order, and their Cry-N phase transition significantly reduces the fluorescence quantum yields (PL). Therefore, the combination of higher-order LC phases, such as smectic (Sm) phases, with the PL properties of the condensed phase can produce high photoluminescence even in the LC phase.
It has been shown that incorporating a semifluoroalkyl flexible chain unit into a mesogenic core is an effective molecular design approach to effectively express the Sm LC phase [18,19]. In fact, our recent study has reported that incorporating a semifluoroalkoxy fragment into D--A-type nonfluorinated tolane exhibits a smectic A (SmA) phase as well as weak photoluminescence (Figure 2a) [20]. Based on the knowledge of fluorinated PL molecules in our group, we conceived that a D--A-type fluorinated diphenylacetylene tolane containing a semifluoroalkoxy chain provides efficient PLLC molecules that exhibit efficient PL in the condensed phase ( Figure 2b). In this study, we investigate the photophysical and phase transition behavior of two semifluoroalkoxy-containing D--A-type fluorinated tolanes, (1) 1 with a 2,3,5,6-tetrafluorobenzonitrile moiety and (2) 2 with a 2,3,5,6-tetrafluorobenzoate partial structure.  [11][12][13] and (b) fluorinated bistolane-type PLLCs IV-VI [14,15] and (c) fluorinated tolane-type PLLCs VII and VIII developed by our group [16,17].
Our research group has successfully developed a family of donor-π-acceptor (D-π-A)-type fluorinated bistolane derivatives IV to VI; however, their synthetic procedures were tedious and inefficient ( Figure 1b) [14,15]. Furthermore, our group has also developed PLLC molecules VII and VIII ( Figure 1c) using fluorinated tolanes as mesogens, which are easy to synthesize [16,17]. These molecules exhibit a nematic (N) phase with the LC phase as the only orientational order, and their Cry-N phase transition significantly reduces the fluorescence quantum yields (Φ PL ). Therefore, the combination of higher-order LC phases, such as smectic (Sm) phases, with the PL properties of the condensed phase can produce high photoluminescence even in the LC phase.
It has been shown that incorporating a semifluoroalkyl flexible chain unit into a mesogenic core is an effective molecular design approach to effectively express the Sm LC phase [18,19]. In fact, our recent study has reported that incorporating a semifluoroalkoxy fragment into D-π-A-type nonfluorinated tolane exhibits a smectic A (SmA) phase as well as weak photoluminescence (Figure 2a) [20]. Based on the knowledge of fluorinated PL molecules in our group, we conceived that a D-π-A-type fluorinated diphenylacetylene tolane containing a semifluoroalkoxy chain provides efficient PLLC molecules that exhibit efficient PL in the condensed phase ( Figure 2b). In this study, we investigate the photophysical and phase transition behavior of two semifluoroalkoxy-containing D-π-A-type fluorinated tolanes, (1) 1 with a 2,3,5,6-tetrafluorobenzonitrile moiety and (2) 2 with a 2,3,5,6-tetrafluorobenzoate partial structure.

Synthesis
Syntheses of semifluoroalkoxy-containing D--A type fluorinated tolanes 1 and 2 were investigated according to the scheme shown in Scheme 1. Syntheses of CN-terminated 1 and CO2Et-terminated 2 were performed using the common synthetic intermediate 4, which was prepared by a Pd(0)-catalyzed Sonogashira

Synthesis
Syntheses of semifluoroalkoxy-containing D-π-A type fluorinated tolanes 1 and 2 were investigated according to the scheme shown in Scheme 1.

Synthesis
Syntheses of semifluoroalkoxy-containing D--A type fluorinated tolanes 1 and 2 were investigated according to the scheme shown in Scheme 1. Syntheses of CN-terminated 1 and CO2Et-terminated 2 were performed using the common synthetic intermediate 4, which was prepared by a Pd(0)-catalyzed Sonogashira  Syntheses of CN-terminated 1 and CO 2 Et-terminated 2 were performed using the common synthetic intermediate 4, which was prepared by a Pd(0)-catalyzed Sonogashira cross- coupling reaction according to our previous report [16,20]. The CN-terminated molecules 1a and 1b were synthesized in 26% and 66% yield, respectively, via an addition-elimination reaction between pentafluorobenzonitrile and 4-semifluoroalkoxy-substituted phenylethynyllithium, which was readily prepared from 4. The CO 2 Et-terminated molecules 2a, 2c, and 2d were obtained in 56-67% yield through a Pd(0)-catalyzed Sonogashira cross-coupling reaction between 4 and ethyl 2,3,5,6-tetrafluoro-4-iodobenzoate. The resulting fluorinated tolanes were purified by silica gel column chromatography followed by recrystallization. Nuclear magnetic resonance (NMR) spectroscopic measurements showed that the purities of the resulting tolanes were suitable for evaluating their photophysical and phase transition behavior.

Photophysical Behavior
With the semifluoroalkoxy-containing D-π-A-type fluorinated tolanes 1 and 2 in hand, we initially evaluated the solution-phase photophysical behavior of the semifluoroalkoxycontaining D-π-A-type fluorinated tolanes 1 and 2. The solution sample was prepared by dissolving the crystalline sample with dichloromethane, and its concentration was adjusted to 1.0 × 10 −5 mol L −1 for ultraviolet-visible (UV-vis) light absorption measurements and 1.0 × 10 −6 mol L −1 for photoluminescence measurements. Figure 3 shows the UV-vis absorption and PL spectra of 1 and 2 in a CH 2 Cl 2 solution and the Commission Internationale de l'Eclailage (CIE) plot to quantitatively assess their PL colors. Photophysical data obtained are summarized in Table 1.
The resulting fluorinated tolanes were purified by silica gel column chromatography followed by recrystallization. Nuclear magnetic resonance (NMR) spectroscopic measurements showed that the purities of the resulting tolanes were suitable for evaluating their photophysical and phase transition behavior.

Photophysical Behavior
With the semifluoroalkoxy-containing D--A-type fluorinated tolanes 1 and 2 in hand, we initially evaluated the solution-phase photophysical behavior of the semifluoroalkoxy-containing D--A-type fluorinated tolanes 1 and 2. The solution sample was prepared by dissolving the crystalline sample with dichloromethane, and its concentration was adjusted to 1.0 × 10 −5 mol L −1 for ultraviolet-visible (UV-vis) light absorption measurements and 1.0 × 10 −6 mol L −1 for photoluminescence measurements. Figure 3 shows the UV-vis absorption and PL spectra of 1 and 2 in a CH2Cl2 solution and the Commission Internationale de l'Eclailage (CIE) plot to quantitatively assess their PL colors. Photophysical data obtained are summarized in Table 1.  Both CN-terminated D--A-type fluorinated tolanes (1a and 1b) and CO2Et-terminated analogues (2a, 2c, and 2d) exhibited two main absorption bands, that is, short-and  long-wavelength bands below and above 300 nm of the maximum absorption wavelength (λ abs ), respectively. The absorption bands in the short wavelength region were almost the same, regardless of the terminal substituents to which the fluorinated aromatic rings were attached. In contrast, the λ abs of 1a and 1b with a CN group shifted to the longer wavelength region compared to those of 2a, 2c, and 2d with a CO 2 Et group owing to the changes in the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 1 with a CN group and 2 with a CO 2 Et group ( Figures S22-S25). To understand vertical electronic transitions in 1 and 2, theoretical evaluation of 1a and 2a was conducted using time-dependent density functional theory (TD-DFT) calculations with the M06-2X/6-31+G(d,p) level of theory. As shown in Figures S22 and S24, the two following allowed transitions were calculated: HOMO-2 → LUMO and HOMO → LUMO in 1a, and HOMO-1 → LUMO and HOMO → LUMO in 2a. HOMO-2 in 1a and HOMO-1 in 2a are the molecular orbitals localized on the fluorinated aromatic rings, while HOMO and LUMO are the orbitals covering the entire π-conjugated core. Considering the molecular orbital distributions of 1a and 2a, the short-and long-wavelength absorption bands originate from the ππ* transitions involving local excitation and intermolecular charge transfer, respectively. Next, when a 1.0 × 10 −6 mol L −1 CH 2 Cl 2 solution of 1a was irradiated with longwavelength light out of the λ abs , a single PL band around the PL maximum wavelength (λ PL ) of 437 nm was observed emitting blue fluorescence at (x, y) = (0.155, 0.090) CIE coordinates. The λ PL of 1b with a CN group was nearly identical to that of 1a, while 2a with the CO 2 Et group exhibited a slight blue-shift (428 nm) of the λ PL , in which the CIE coordinates were (x, y) = (0.164, 0.115). The blue-shift of the λ PL is also attributed to the widening of the HOMO-LUMO energy gap with change from the CN group to the CO 2 Et group. It was found that the photophysical behavior of the D-π-A-type fluorinated tolanes significantly affects only the electron-density distribution on the π-conjugated framework and not that on the semifluoroalkoxy flexible unit. The Φ PL of the CN-terminated molecules 1a and 1b was observed to be up to 0.20, which was due to quick internal conversion from the linearly shaped radiative ππ* excited state to the trans-bend-shaped non-radiative πσ* excited state [21,22]. On the other hand, a slightly better Φ PL (up to 0.42) was observed for the CO 2 Et-terminated analogues compared to that of the CN-terminated counterparts, with suppressed fluorescence self-quenching due to the overlap of absorption and PL spectra [23].
Moreover, CN-terminated 1 and CO 2 Et-terminated 2 exhibited PL even in the crystalline (Cry) phase. The PL spectra and CIE plot are shown in Figure 4, and the obtained photophysical data are summarized in Table 1. The PL of 1b with a CN group was nearly identical to that of 1a, while 2a with the CO2Et group exhibited a slight blue-shift (428 nm) of the PL, in which the CIE coordinates were (x, y) = (0.164, 0.115). The blue-shift of the PL is also attributed to the widening of the HOMO-LUMO energy gap with change from the CN group to the CO2Et group. It was found that the photophysical behavior of the D--A-type fluorinated tolanes significantly affects only the electron-density distribution on the -conjugated framework and not that on the semifluoroalkoxy flexible unit. The PL of the CN-terminated molecules 1a and 1b was observed to be up to 0.20, which was due to quick internal conversion from the linearly shaped radiative * excited state to the trans-bend-shaped non-radiative * excited state [21,22]. On the other hand, a slightly better PL (up to 0.42) was observed for the CO2Et-terminated analogues compared to that of the CN-terminated counterparts, with suppressed fluorescence self-quenching due to the overlap of absorption and PL spectra [23]. Moreover, CN-terminated 1 and CO2Et-terminated 2 exhibited PL even in the crystalline (Cry) phase. The PL spectra and CIE plot are shown in Figure 4, and the obtained photophysical data are summarized in Table 1.   flexible unit from a nonafluorodecyloxy of 1a to a tridecafluorodecyloxy chain of 1b formed loose aggregates due to higher fluorine contents, leading to blue PL at (x, y) = (0.163, 0.200) CIE coordinates with a blue-shifted λ PL band around 467 nm and a high Φ PL of 0.71. Considering similar PL spectra in solution, the change in λ PL between 1a and 1b seems to be caused by differences in aggregated structure in the crystalline lattice, supported by PXRD measurements at 25 • C ( Figure S21). The Φ PL s of the CN-terminated molecules 1a and 1b in Cry phase were significantly higher than those of the dilute solution of 1a and 1b because of the significantly suppressed non-radiative deactivation due to the formation of intermolecular H···F hydrogen bonds. The CO 2 Et-terminated molecule 2a exhibited blue PL at (x, y) = (0.160, 0.147) CIE coordinates with a single PL band around 453 nm of λ PL with a relatively high Φ PL (0.48). On the other hand, 2c and 2d exhibited two PL bands around 392-394 nm and 408-409 nm of λ PL to show deep blue PL at (x, y) = (0.169, 0.102) and (x, y) = (0.169, 0.076) CIE coordinates, respectively, with low PL efficiency. According to the results of PXRD measurements at 25 • C in 2a, 2c, and 2d (Figure S21), the three molecules showed different diffraction peaks, which suggests that they formed different aggregated structures in the crystalline phase through a variety of intermolecular interactions. It is considered that 2a, 2c, and 2d showed different PL behavior due to different aggregate structures resulting from their various intermolecular interactions. Fluorinated tolanes, such as 2a, 2c, and 2d, have a sterically bulky CO 2 Et group compared to the CN units of 1a and 1b, leading to a significant decrease in their Φ PL due to the formation of loose aggregates that facilitate non-radioactive deactivation through molecular motion.

Phase Transition Behavior
We also focused on the phase transition behavior of the semifluoroalkoxy-containing D-π-A-type fluorinated tolanes 1 and 2 with condensed-phase PL characteristics. Phase transition behavior of 1 and 2 was evaluated with differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The phase transition sequences and temperatures of 1a, 1b, 2a, 2c, and 2d are summarized in Table 2. Although semifluoroalkoxy-containing D-π-A-type nonfluorinated tolanes were reported to exhibit an SmA LC phase [20], the LC phase disappeared in the case of fluorinated tolane scaffolds. The phase transition enthalpies between the Cry and isotropic (Iso) phases of 1a and 1b were very large, which is attributed to the remarkable stabilization of the Cry phase by the introduction of fluorine atoms on the tolane backbone. In contrast to CN-terminated 1a and 1b, the CO 2 Et-terminated 2 exhibited a mesophase between the Cry and Iso phases ( Figure 5).
During the second heating process, 2a with a nonafluorodecyloxy chain (F content in the flexible chain: 54%) transitioned from Cry to mesophase at 80 • C and to Iso phase at 94 • C. The POM measurements revealed that the focal conic fan texture was in the mesophase; therefore, the observed mesophase was determined to be the SmA phase.
Furthermore, 2c with a tridecafluorododecyloxy chain (F content in the flexible chain: 59%) also exhibited an SmA phase with a focal conic fan texture at a temperature range of 89-117 • C during the second heating process. Increasing the fluorine atom contents within the flexible chain structure broadened the LC temperature range. During the heating and cooling processes, 2d with a heptadecafluorotetradecyloxy chain (F content in the flexible chain: 62%) also exhibited an SmA phase, and the LC phase was observed to be in a range of 101-135 • C, which was wider than the others. To reliably identify the SmA phases observed in 2a, 2c, and 2d, temperature-variable powder X-ray diffraction (VT-PXRD) measurements were performed at the mesophase temperature. In 2a, which has a short fluoroalkyl moiety, no diffraction peak corresponding to the (001) plane index was observed ( Figure S18a). It is considered that the fluorine atoms in the flexible chain formed an interdigitate structures, which formed many defects in the layer structure and reduced the domain size [24,25]. In contrast, 2b and 2c, which has an elongated fluoroalkyl structure, exhibited diffraction peaks corresponding to the (001) plane at 2θ = 2.46 • and 2.22 • , respectively (Figure 5d and Figure S18e). Using Bragg's equation, the interlayer distances (d-spacing) in 2c and 2d were calculated as 3.59 nm and 3.97 nm, respectively. Considering the molecular lengths of 2c (3.02 nm) and 2d (3.28 nm) estimated by quantum chemical calculation (Figures 5e and S26), it is anticipated that the structure in which the fluoroalkyl moieties are interdigitated and the π-conjugated mesogen is intercalated is the aggregated structure in the SmA phase (Figure 5e). During the second heating process, 2a with a nonafluorodecyloxy chain (F content in the flexible chain: 54%) transitioned from Cry to mesophase at 80 °C and to Iso phase at 94 °C. The POM measurements revealed that the focal conic fan texture was in the mesophase; therefore, the observed mesophase was determined to be the SmA phase. Furthermore, 2c with a tridecafluorododecyloxy chain (F content in the flexible chain: 59%) also exhibited an SmA phase with a focal conic fan texture at a temperature range of 89-117 °C during the second heating process. Increasing the fluorine atom contents within the flexible chain structure broadened the LC temperature range. During the heating and cooling processes, 2d with a heptadecafluorotetradecyloxy chain (F content in the flexible chain: 62%) also exhibited an SmA phase, and the LC phase was observed to be in a range

PL Behavior in the SmA Phase
We evaluated the photophysical behavior of 2a, 2c, and 2d, which have both crystallinestate PL and LC properties, in the SmA phase ( Figure 6, Table 3). Using a fluorescence spectrophotometer equipped with a hand-made heating unit containing a ceramic heater and a temperature control device, PL spectra of 2a, 2c, 2d were measured under thermal conditions. We evaluated the photophysical behavior of 2a, 2c, and 2d, which have both crystalline-state PL and LC properties, in the SmA phase ( Figure 6, Table 3). Using a fluorescence spectrophotometer equipped with a hand-made heating unit containing a ceramic heater and a temperature control device, PL spectra of 2a, 2c, 2d were measured under thermal conditions.  Focusing on the PL in the SmA phase under thermal conditions, as shown in Figure  6 and Table 3, a blue shift of approximately 20 nm was observed for 2a. In contrast, the PL bands of 2c and 2d red-shifted by up to 51 nm during the Cry-SmA phase transition, which is due to the formation of - stacking induced by interdigitation of fluoroalkoxy moieties. Furthermore, the fluorescence intensity of the SmA phase under heating was significantly decreased compared to that of the Cry phase at 25 °C. The ratio of PL intensities in the SmA and Cry phases, ISmA/ICry, was 0.22 for 2a, 0.32 for 2c, and 0.20 for 2d. This is due to acceleration of non-radiative deactivation caused by the micro-Brownian motion in the appearance of the SmA phase.  Focusing on the λ PL in the SmA phase under thermal conditions, as shown in Figure 6 and Table 3, a blue shift of approximately 20 nm was observed for 2a. In contrast, the λ PL bands of 2c and 2d red-shifted by up to 51 nm during the Cry-SmA phase transition, which is due to the formation of π-π stacking induced by interdigitation of fluoroalkoxy moieties. Furthermore, the fluorescence intensity of the SmA phase under heating was significantly decreased compared to that of the Cry phase at 25 • C. The ratio of PL intensities in the SmA and Cry phases, I SmA /I Cry , was 0.22 for 2a, 0.32 for 2c, and 0.20 for 2d. This is due to acceleration of non-radiative deactivation caused by the micro-Brownian motion in the appearance of the SmA phase.

General
Semifluoroalkoxy-substituted fluorinated tolanes, 1 and 2, were synthesized according to the scheme shown in Figure 3. The reaction progress was confirmed using thin-layer chromatography (TLC), which was performed on silica gel TLC plates (silica gel 60254, Merck, Rahway, NJ, USA). The tolanes were purified by column chromatography using Wakogel ® 60N (38-100 mm). Melting and clearing temperatures of the molecules 1 and 2 were determined using DSC. 1 H and 13 C-NMR spectra for 1 and 2 were recorded using a Bruker AVANCE III 400 NMR spectrometer ( 1 H: 400 MHz and 13 C: 100 MHz) in chloroformd (CDCl 3 ), and the chemical shifts were reported in parts per million (ppm) using the residual proton in the NMR solvent. 19 F-NMR (376 MHz) spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer in CDCl 3 , and trichlorofluoromethane (CFCl 3 , d F = 0 ppm) or hexafluorobenzene (C 6 F 6 , d F = −163 ppm) was used as an internal standard. Infrared (IR) spectra were recorded using the KBr method with a JASCO FT/IR-4100 type A spectrometer, and all spectra were reported in wavenumber (cm −1 ). High-resolution mass spectrometry (HRMS) was performed on a JEOL JMS-700MS spectrometer using the fast atom bombardment (FAB) method.

Theoretical Assessment
All DFT calculations were performed using the Gaussian 16 program set [26] with the M06-2X hybrid functional [27] and 6-31+G(d,p) (for all atoms) basis set. A conductor-like polarizable continuum model (CPCM) [28] was used for CH 2 Cl 2 . Theoretical vertical transitions were also calculated by TD-DFT at the same theory level using the same solvation model.

Phase Transition Behavior
The phase transition behaviors were observed by POM using an Olympus BX53 mi-croscope (Tokyo, Japan) equipped with a cooling and heating stage (10002L, Linkam Sci-entific Instruments, Surrey, UK). Thermodynamic characterization was performed using DSC (DSC-60 Plus, Shimadzu, Kyoto, Japan) at heating and cooling rates of 5.0 • C min −1 under an N 2 atmosphere. Liquid crystalline structures were evaluated by an FR-E X-ray diffractometer attached with an R-axis IV two-dimensional (2D) detector (Rigaku, Tokyo, Japan). 0.3 mm collimated CuKa radiation (λ = 1.54187 Å) was used as an X-ray beam, and the camera length was set at 300 mm. The powder sample was loaded into a thin wall glass capillary tube for XRD analysis (ϕ 1.0-2.5 mm, Hilgenberg GmbH), and the annealed up to isotropic temperature under vacuum. The glass capillary was set onto a ceramic heater attached to the FR-E sample holder. Exposure time of the X-ray beam was 5 min.

Conclusions
We designed PLLCs, which exhibited both aggregation-state PL and SmA-type LC properties, and developed two types of semifluoroalkoxy-containing D-π-A-type fluorinated tolanes 1 with a tetrafluorobenzonitrile and 2 with a tetrafluorobenzoate moiety. CN-terminated 1 exhibited blue photoluminescence with a low PL quantum yield (Φ PL ; up to 0.20) in a dilute solution phase; however, replacing the CN substituent with an CO 2 Et group increased the Φ PL to up to 0.42. In contrast, CN-terminated 1 exhibited intense lightblue PL with a high Φ PL of up to 0.71 in the crystalline phase, whereas CO 2 Et-terminated 2 exhibited blue PL with lower PL efficiency (Φ PL = up to 0.48). In the phase transition characteristics, CN-terminated 1 did not exhibit any LC behavior upon the heating and cooling processes owing to the formation of tight molecular aggregates through H···F hydrogen bonds. Therefore, CO 2 Et-terminated 2 exhibited an enantiotropic SmA LC phase. Among 2a, 2c, and 2d, the LC temperature range increased with increasing fluorine atom contents in the flexible chain. Using 2a, 2c, and 2d, which have both SmA LC and aggregation state-PL characteristics, we investigated the PL behavior in the SmA phase. As a result, during the Cry-SmA phase transition, a significant decrease in fluorescence intensity was observed, whereas the PL color was altered during the phase transition. The molecular design and PLLC molecules discussed in this study provide novel temperature-dependent PL materials, which would become highly efficient thermal-stimulus responsive luminescent materials in the future.