Luminescence Tuning of Fluorinated Bistolanes via Electronic or Aggregated-Structure Control

: Organic luminescent materials have a wide range of practical applications, but the understanding of the relationship between molecular structure and luminescent behavior is lacking. Herein, we synthesized ﬂuorinated bistolanes with an electron-donating alkoxy substituent at one terminal and an electron-withdrawing substituent at the other to realize systematic control of the electron-density distribution. Evaluation of the phase transition behavior revealed that most of the ﬂuorinated bistolanes showed liquid-crystalline (LC) behavior, with the phase transition temperature depending on the terminal substituents. Additionally, the ﬂuorinated luminophores displayed intense photoluminescence (PL) in solution and in their crystal phases. Remarkably, the PL color shifted dramatically depending on the dipole moment ( µ || ) along the long molecular axis; thus, PL tuning can be achieved through electronic modulation by precise control of the µ || of the luminophore. Interestingly, in the LC phases under thermal conditions, the maximum PL band shifted by 0.210 eV upon phase transition from the crystal to smectic A LC phases, indicating that PL tuning can also be achieved by controlling the aggregated structure. These results o ﬀ er a new molecular design for easily tunable PL materials using the molecular properties or external stimuli for promising applications, including light-emitting displays and PL sensing materials.


Introduction
Organic luminescent materials can be broadly classified into two groups: solution-state and solid-state luminophores. Solution-state luminophores, such as rhodamine and fluorescein, are applicable in biomedical imaging and optical probes [1][2][3][4], whereas solid-state luminophores are typically employed in lighting devices, light-emitting diodes, and electroluminescence devices [5][6][7][8][9]. To realize practical applications, significant efforts have been devoted to the discovery of novel luminescent materials, and as a result, various kinds of luminescent molecules have been developed [10,11]. However, there have been few systematic studies on the relationship between molecular structure and luminescent behavior, even though luminescence tuning is a key feature for practical light-emitting applications.
Over the past few decades, numerous fluorine-containing organic materials have been developed because chemical reactivity and material properties are dramatically influenced by the unique characteristics of fluorine, e.g., the largest electronegativity of all elements, the second smallest atom

General
1 H and 13 C NMR spectra were recorded using a Bruker AVANCE III 400 NMR spectrometer ( 1 H: 400 MHz and 13 C: 100 MHz, Bruker Corporation, Rheinstetten, Germany) in chloroform-d (CDCl3) solution, and the chemical shifts are reported in parts per million (ppm) using the residual proton in the NMR solvent (δH = 7.26 ppm for CHCl3 and δC = 77 ppm for CDCl3). 19 F NMR (376 MHz) spectra were recorded using a Bruker AVANCE III 400 NMR spectrometer in CDCl3 solution with CFCl3 (δF = 0 ppm) as an internal standard. Infrared (IR) spectra were acquired via the KBr method using a JASCO FT/IR-4100 type A spectrometer (JASCO Corporation, Tokyo, Japan); all spectra are reported in wavenumbers (cm −1 ). High-resolution mass spectroscopy (HRMS) was performed using a JEOL JMS-700MS spectrometer (JEOL Ltd., Tokyo, Japan) via fast atom bombardment (FAB). All reactions were performed using dried glassware with a magnetic stirrer bar. All chemicals were of reagent grade and, if necessary, were purified in the usual manner prior to use. Column chromatography was conducted on silica gel (Wako-gel® 60N, 38-100 μm, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) and thin layer chromatography (TLC) was performed on silica gel TLC plates (Merck, Silica gel 60F254, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan). Previous studies by our research group on fluorinated bistolane-type luminophores have demonstrated that the PL behavior depends on the electron-density distribution or aggregated structure. To gain deeper insights into the PL behavior, in this study, we designed six fluorinated bistolanes, viz., 1aB-D and 1bB-D, by systematically modulating the electron-density distribution. Herein, we describe the molecular design and synthesis of these fluorinated bistolane-type luminophores as well as their LC and PL behaviors. Additionally, the effect of precisely modulating the electron-density distribution or aggregated structure on the PL behavior is discussed.

General
1 H and 13 C NMR spectra were recorded using a Bruker AVANCE III 400 NMR spectrometer ( 1 H: 400 MHz and 13 C: 100 MHz, Bruker Corporation, Rheinstetten, Germany) in chloroform-d (CDCl 3 ) solution, and the chemical shifts are reported in parts per million (ppm) using the residual proton in the NMR solvent (δ H = 7.26 ppm for CHCl 3 and δ C = 77 ppm for CDCl 3 ). 19 F NMR (376 MHz) spectra were recorded using a Bruker AVANCE III 400 NMR spectrometer in CDCl 3 solution with CFCl 3 (δ F = 0 ppm) as an internal standard. Infrared (IR) spectra were acquired via the KBr method using a JASCO FT/IR-4100 type A spectrometer (JASCO Corporation, Tokyo, Japan); all spectra are reported in wavenumbers (cm −1 ). High-resolution mass spectroscopy (HRMS) was performed using a JEOL JMS-700MS spectrometer (JEOL Ltd., Tokyo, Japan) via fast atom bombardment (FAB). All reactions were performed using dried glassware with a magnetic stirrer bar. All chemicals were of reagent grade and, if necessary, were purified in the usual manner prior to use. Column chromatography was conducted on silica gel (Wako-gel®60N, 38-100 µm, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) and thin layer chromatography (TLC) was performed on silica gel TLC plates (Merck, Silica gel 60F 254 , FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan).

Synthesis
Fluorinated bistolanes 1aB-D and 1bB-D were synthesized according to the synthetic procedure shown in Scheme 1.

Computations
All density functional theory (DFT) computations were performed using the Gaussian 09 (Revision C.01) package [25]. Geometry optimizations were executed using the CAM-B3LYP hybrid functional [26,27] and the 6-31+G(d) basis set with an implicit solvation model, namely, the conductor-like polarizable continuum model (CPCM) [28][29][30], for CH 2 Cl 2 . The vertical excitation energies and dipole moments of the optimized structures were calculated using the time-dependent (TD)-DFT method at the same level as in the theory. To obtain the optimized structures in the excited (S 1 ) state, geometry optimizations were carried out using the TD-DFT method.

Phase Transition Behavior
The phase transition behavior of the fluorinated bistolanes was observed by polarizing optical microscopy (POM) using an Olympus BX53 microscope (Olympus Corporation, Tokyo, Japan) equipped with a cooling and heating stage (Linkam Scientific Instruments, 10002L, Surrey, UK). The thermodynamic behavior was determined using differential scanning calorimeter (DSC, Shimadzu, DSC-60 Plus, Kyoto, Japan) at heating and cooling rates of 5.0 • C min −1 under a N 2 atmosphere.

Photophysical Behavior
Ultraviolet-visible (UV-vis) absorption spectra were recorded using a JASCO V-500 (JASCO Corporation, Tokyo, Japan) absorption spectrometer. Steady-state PL spectra were obtained using a JASCO FP-8500 (JASCO Corporation, Tokyo, Japan) or a Hitachi F-7000 fluorometer (Hitachi Ltd., Tokyo, Japan). The absolute PL quantum yields were recorded using a calibrated integrating sphere with a Hamamatsu Photonics C11347-01 Absolute PL Quantum Yield Measurement System (Hamamatsu Photonics KK, Hamamatsu, Japan).

Molecular Design
Using DFT and TD-DFT calculations in the Gaussian 09 suite of programs, we examined the effect of the terminal substituent in each designed molecule on the electron-density distribution over the entire molecule. The geometries of 1aB-D in the ground (S 0 ) and excited (S 1 ) states were optimized at the CAM-B3LYP/6-31+G(d) level of theory with the implicit solvation CPCM model for CH 2 Cl 2 . Figure 2 shows the structures of 1aB-D and their calculated electrostatic potential maps, dipole moments along the long molecular axis (µ || ), isodensity surfaces, and orbital energies for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 16 thermodynamic behavior was determined using differential scanning calorimeter (DSC, Shimadzu, DSC-60 Plus, Kyoto, Japan) at heating and cooling rates of 5.0 °C min −1 under a N2 atmosphere.

Photophysical Behavior
Ultraviolet-visible (UV-vis) absorption spectra were recorded using a JASCO V-500 (JASCO Corporation, Tokyo, Japan) absorption spectrometer. Steady-state PL spectra were obtained using a JASCO FP-8500 (JASCO Corporation, Tokyo, Japan) or a Hitachi F-7000 fluorometer (Hitachi Ltd., Tokyo, Japan). The absolute PL quantum yields were recorded using a calibrated integrating sphere with a Hamamatsu Photonics C11347-01 Absolute PL Quantum Yield Measurement System (Hamamatsu Photonics KK, Hamamatsu, Japan).

Molecular Design
Using DFT and TD-DFT calculations in the Gaussian 09 suite of programs, we examined the effect of the terminal substituent in each designed molecule on the electron-density distribution over the entire molecule. The geometries of 1aB-D in the ground (S0) and excited (S1) states were optimized at the CAM-B3LYP/6-31+G(d) level of theory with the implicit solvation CPCM model for CH2Cl2. Figure 2 shows the structures of 1aB-D and their calculated electrostatic potential maps, dipole moments along the long molecular axis (μ||), isodensity surfaces, and orbital energies for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The electrostatic potential maps of 1aB-D reveal that the electron density at the fluorinated aromatic ring decreases in the following order: 1aB > 1aC > 1aD. The dipole moment (μ||) along the long molecular axis can be used to assess the electron-density distribution over the entire molecule quantitatively. AcO-substituted 1aB had the smallest μ|| value (3.48 D), and this value increased when the AcO group was replaced by a CF3 or CN group (μ|| = 9.63 D for 1aB and 12.2 D for 1aC). These results clearly confirm that the incorporation of AcO, CF3, and CN groups into the fluorinated The electrostatic potential maps of 1aB-D reveal that the electron density at the fluorinated aromatic ring decreases in the following order: 1aB > 1aC > 1aD. The dipole moment (µ || ) along the long molecular axis can be used to assess the electron-density distribution over the entire molecule quantitatively. AcO-substituted 1aB had the smallest µ || value (3.48 D), and this value increased when the AcO group was replaced by a CF 3 or CN group (µ || = 9.63 D for 1aB and 12.2 D for 1aC). These results clearly confirm that the incorporation of AcO, CF 3 , and CN groups into the fluorinated bistolane scaffold can be used to systematically change the electron-density distribution over the entire molecule. The electron-density distribution also affected the molecular orbitals, with the orbital lobes in the HOMO largely localized on the relatively electron-rich aromatic rings, whereas the LUMO lobes were relatively localized on the electron-deficient aromatic rings. The orbital separation between the HOMO and LUMO in the S 1 state may contributable to unique PL behavior through intramolecular charge transfer [31]. The energy differences (∆E H-L ) between HOMO and LUMO levels were 5.08 eV for 1aB, 4.89 eV for 1aC, and 4.67 eV for 1aD. Thus, 1aD is expected to show a PL band in the long-wavelength region, with the PL wavelength shifting towards shorter wavelengths as the transition energy increases, i.e., 1aB > 1aC. Theoretical calculations for 1bB-D, bearing long alkoxy substituents, also provided similar µ || and HOMO/LUMO distributions (see Supplementary Materials), assuming that the alkoxy-substituent does not affect in the electron density distribution. Consequently, systematic control of the electron-density distribution induced by modulating the electronic character of the terminal substituent should realize luminescence tuning for the fluorinated luminophores. According to our previous study, modulation of terminal substituents also influences the LC behavior [21,23], not only through changes in the electron-density distribution but also in the steric bulkiness of the substituent. Thus, systematic modulation of the terminal substituent in 1aB-D may also allow control of the LC behavior.

Synthesis and Crystal Structure
Based on the aforementioned molecular design, fluorinated bistolanes 1aB-C with a CH 3 O group and 1bB-D with a hexyloxy (C 6 H 13 O) moiety were synthesized according to the procedure shown in Scheme 1. AcO-substituted 1aB was obtained in 48% yield by the Pd(0)-catalyzed Sonogashira cross-coupling reaction of 3a, readily accessible from 2a [21,23], with B. In contrast, CF 3 -substituted 1aC and CN-substituted 1aD were obtained in 36% or 32% yield, respectively, by nucleophilic aromatic substitution of the in situ generated lithium acetylide derived from 3a with perfluorotoluene and perfluorobenzonitrile, respectively [32]. In a similar manner, C 6 H 13 O-substituted analogue 1bB was prepared in 54% yield through a Sonogashira cross-coupling reaction and 1bC and 1bD were prepared in 32% and 17% yields, respectively, through nucleophilic substitution reactions. Each fluorinated bistolane was purified by column chromatography and recrystallization from a 1:1 solution of CH 2 Cl 2 (good solvent) and MeOH or hexane (poor solvent). The structure of each compound was confirmed by 1 H, 13 C, and 19 F NMR, IR, and HRMS and the spectra indicated that the purity was sufficient to evaluate the LC and PL behaviors.
Among the fluorinated bistolanes, AcO-substituted 1aB and CF 3 -substituted 1aC furnished single crystals appropriate for X-ray crystallographic analysis. 1aB crystalized with a monoclinic crystal structure in the P 2 1 space group with two molecular units in the unit cell. As shown in Figure 3a, the three aromatic rings were found to be almost coplanar, although there may be a rapid equilibrium between the coplanar and twisted conformations through free rotation of the C sp -C sp2 single bonds [33]. The packing structure (Figure 3b) revealed several close contacts that were shorter than the sums of the van der Waals radii (carbon: 170 pm, hydrogen: 120 pm, and fluorine: 148 pm) [34]: H alkyl ···C sp2 (288 pm), H aryl ···C sp (284 pm), H aryl ···F (253 pm), H alkyl ···F (246 pm), H aryl ···O (257 pm). Thus, the crystal lattice of 1aB contains two CH-π interactions and three hydrogen bonding interactions. In contrast, 1aC crystalized with a monoclinic crystal structure in the P 2 1 /c space group with four molecular units in the unit cell. As shown in Figure 3c, each 1aC molecule was also found to be coplanar with several short contacts (Figure 3d), such as C sp2 ···C sp2 (332 pm), H aryl ···F (251 pm), C sp2 ···F (314 pm), and F···F (265 pm). Thus, 1aC also formed a tight packing structure with one π-π interaction, one hydrogen bonding interaction, and one lone-pair-π interaction [35,36].

Phase Transition Behavior
The phase transition behavior of fluorinated bistolanes 1aB-D and 1bB-D was investigated using POM and DSC. Figure 4 shows the phase transition behavior observed during the 2nd heating cooling processes and the microphotographic images of the LC phases.

Phase Transition Behavior
The phase transition behavior of fluorinated bistolanes 1aB-D and 1bB-D was investigated using POM and DSC. Figure 4 shows the phase transition behavior observed during the 2nd heating cooling processes and the microphotographic images of the LC phases.

Phase Transition Behavior
The phase transition behavior of fluorinated bistolanes 1aB-D and 1bB-D was investigated using POM and DSC. Figure 4 shows the phase transition behavior observed during the 2nd heating cooling processes and the microphotographic images of the LC phases. Among 1aB-D with a methoxy group at one molecular terminal and an electron-withdrawing substituent at the other, AcO-substituted 1aB exhibited no LC behavior, as evidenced by the dramatic change in the POM images from nonfluidic bright-viewing to fluidic dark-viewing fields during both the heating and cooling processes. The steric hindrance of the AcO group and the dipole moment (µ ⊥ ) along the short molecular axis induced by the carbonyl moiety in the AcO group likely make it difficult to order structures in the condensed phase, resulting in a direct phase transition to the isotropic (Iso) phase. In contrast, the POM images of 1aC with a CF 3 group and 1aD with a CN group displayed fluidic bright-viewing fields between the crystal (Cr) and Iso phases; POM and DSC measurements only revealed a single LC phase for 1aC and 1aD. As the microphotographic images of 1aC and 1aD display a four-brushed Schlieren texture, which is typical of nematic (N) LC phases, the single LC phases of 1aC and 1aD are assigned as N phases. The melting temperature (T m , defined as the phase transition temperature between the Cr and LC phases) and the clearing temperature (T c , defined as the phase transition temperature between the LC an Iso phases) are much higher for 1aD than for 1aC (Figure 3a). This behavior is consistent with the linear and rigid structure of the CN group, which induces tight packing in an antiparallel direction owing to the large µ || value [37][38][39].
Changing the length of the flexible chain at the molecular terminal by replacing CH 3 O with C 6 H 13 O affected the LC behavior. 1bB-D all showed LC phases, and the T m and T c values for 1bB-D were dramatically decreased compared with the corresponding values for 1aB-D. Typically, such stabilization of LC phases can be attributed to the introduction of a longer flexible unit [23]. The POM and DSC measurements revealed that AcO-substituted 1bB and CN-substituted 1bD displayed only a N LC phase owing to the steric bulkiness of AcO and the rigid linear rigid structure of CN with a large µ || value. However, CF 3 -substituted 1bC exhibited both SmA and N LC phases, analogous to those previously reported for 1bA. Thus, a suitable choice of substituent at the molecular terminal is important for achieving precise control of the LC behavior, including the LC phase, phase transition temperature, and LC temperature range.

PL Behavior in the Solution Phase
Next, we examined the photophysical behavior of the fluorinated bistolanes. First, we investigated the absorption and PL behavior in CH 2 Cl 2 solution (~1.0 × 10 −5 mol L −1 for absorption and 1.0 × 10 −6 mol L −1 for PL). Figure 5a,b shows the absorption and PL spectra, respectively, of 1aB-D (solid lines) and 1bB-D (dashed lines). Figure 5c shows photographs of the PL colors and the corresponding Commission Internationale d'Eclairage (CIE) chromaticity diagram. The photophysical data are listed in Table 1. In CH 2 Cl 2 solution, CH 3 O-Substituted 1aB-D each exhibited a single absorption band and the maximum absorption wavelength (λ abs ) varied significantly depending on the electron-withdrawing substituent: 333 nm (AcO-substituted 1aB), 341 nm (CF 3 -substituted 1aC), and 356 nm (CN-substituted 1aD) (Figure 5a), in which the λ max in the calculated UV-Vis spectra obtained from TD-DFT calculation resulted in a similar tendency: 338 nm (AcO-substituted 1aB), 347 nm (CF 3 -substituted 1aC), and 360 nm (CN-substituted 1aD) ( Figure S21). As listed in Table 1, the µ || values in the S 0 state were calculated to be 1aB (1.91 D) < 1aC (7.22 D) < 1aD (9.67 D), indicating a strong correlation between the λ abs and µ || values. Upon excitation at λ abs , 1aB-D in CH 2 Cl 2 each displayed a single PL band at a maximum PL wavelength (λ PL ) of 410-488 nm with a high PL efficiency (Φ PL = 0.85-0.91) (Figure 5b). As shown in Figure 5c, 1aB-D emitted deep-blue, blue PL, light-green PL, respectively, and the PL colors can also be quantitatively evaluated using the CIE chromaticity diagram. Similar to λ abs , λ PL also exhibited a shift towards longer wavelengths depending on the µ || value. Replacing the CH 3 O substituent with the longer C 6 H 13 O unit did not change the absorption or PL spectra significantly owing to the similar µ || values. These results clearly demonstrate that the electron-density distribution induced by the electron-withdrawing substituent is an important parameter for controlling the photophysical properties in the solution phase.   TD-DFT calculations revealed that the HOMO→LUMO transition corresponded with the lowest-energy excitation with a large oscillator strength. Considering that the energy gap between the HOMO and LUMO (∆E H-L ) in the S 0 state decreases in the order 1aB (5.69 eV) > 1aC (5.55 eV) > 1aD (5.28 eV), increasing the µ || value results in a narrower ∆E H-L and a shift of λ abs towards longer wavelengths: 1aB (338 nm) < 1aC (347 nm) < 1aD (360 nm). ∆E H-L in the S 1 state exhibited a similar tendency (1aB > 1aC > 1aD), corresponding to the order of the µ || values. Thus, these results clearly indicate that the electronic transitions for absorption and PL can be controlled by tuning µ || .
The PL colors of luminophores with large µ || values have often been reported to show obvious variations depending on the solvent polarity [40]. Therefore, we tested the PL sensing properties of CF 3 -substituted 1bC as a representative fluorinated bistolane in various solvents. Figure 6 shows the observed PL behavior, and the photophysical data are summarized in Table 2. TD-DFT calculations revealed that the HOMO → LUMO transition corresponded with the lowest-energy excitation with a large oscillator strength. Considering that the energy gap between the HOMO and LUMO (ΔEH-L) in the S0 state decreases in the order 1aB (5.69 eV) > 1aC (5.55 eV) > 1aD (5.28 eV), increasing the μ|| value results in a narrower ΔEH-L and a shift of λabs towards longer wavelengths: 1aB (338 nm) < 1aC (347 nm) < 1aD (360 nm). ΔEH-L in the S1 state exhibited a similar tendency (1aB > 1aC > 1aD), corresponding to the order of the μ|| values. Thus, these results clearly indicate that the electronic transitions for absorption and PL can be controlled by tuning μ||.
The PL colors of luminophores with large μ|| values have often been reported to show obvious variations depending on the solvent polarity [40]. Therefore, we tested the PL sensing properties of CF3-substituted 1bC as a representative fluorinated bistolane in various solvents. Figure 6 shows the observed PL behavior, and the photophysical data are summarized in Table 2.  Irradiating 1bC at 343 nm in less polar toluene (ε = 2.38, where ε is a dielectric constant at 25 °C and an indicator of solvent polarity [40]) produced deep-blue PL with a single PL band at λPL = 409 nm, which was shifted towards shorter wavelengths by 0.282 eV compared with the λPL in CH2Cl2 (ε = 8.93). In contrast, dramatic shifts towards longer wavelengths were observed when more polar solvents, e.g., MeCN (ε = 35.9) and DMF (ε = 36.7), were employed; based on the CIE diagram, greenish-yellow PL was observed in both cases. Although THF (ε = 7.58) is less polar than CH2Cl2, the PL band (λPL = 482 nm) was shifted towards longer wavelengths by 0.177 eV. The unique PL  Irradiating 1bC at 343 nm in less polar toluene (ε = 2.38, where ε is a dielectric constant at 25 • C and an indicator of solvent polarity [40]) produced deep-blue PL with a single PL band at λ PL = 409 nm, which was shifted towards shorter wavelengths by 0.282 eV compared with the λ PL in CH 2 Cl 2 (ε = 8.93). In contrast, dramatic shifts towards longer wavelengths were observed when more polar solvents, e.g., MeCN (ε = 35.9) and DMF (ε = 36.7), were employed; based on the CIE diagram, greenish-yellow PL was observed in both cases. Although THF (ε = 7.58) is less polar than CH 2 Cl 2 , the PL band (λ PL = 482 nm) was shifted towards longer wavelengths by 0.177 eV. The unique PL behavior in THF is likely due to the oxygen atom, which allows THF to easily solvate 1bC with its large µ || value through intermolecular interactions, such as hydrogen bonding, and may promote the nonradiative deactivation from the S 1 to the S 0 state [41][42][43]. Consequently, it was successfully demonstrated that the fluorinated bistolanes can be applied as PL sensing materials for solvent polarity.

PL Behavior in the Crystal Phase
To our delight, we found that 1aB-D and 1bB-D crystals prepared by recrystallization via a slow evaporation technique exhibited PL. Figure 7 shows the obtained PL spectra acquired, and the photophysical data are listed in Table 3. polarity.

PL Behavior in the Crystal Phase
To our delight, we found that 1aB-D and 1bB-D crystals prepared by recrystallization via a slow evaporation technique exhibited PL. Figure 7 shows the obtained PL spectra acquired, and the photophysical data are listed in Table 3.  As shown in Figure 7a,b, λPL changed dramatically depending on the electron-withdrawing substituent, even in crystals: 414-420 nm (deep-blue PL) for AcO-substituted 1aB and 1bB, 459-479  As shown in Figure 7a,b, λ PL changed dramatically depending on the electron-withdrawing substituent, even in crystals: 414-420 nm (deep-blue PL) for AcO-substituted 1aB and 1bB, 459-479 nm (blue to light-blue PL) for CF 3 -substituted 1aC and 1bC, and 500-526 nm (yellowish-green PL) for CN-substituted 1aD and 1bD. Compared with the λ PL values in CH 2 Cl 2 solution, those observed for the crystals were slightly shifted towards longer wavelengths, which may be due to quick deactivation via nonradiative processes owing to intermolecular interactions in the condensed phases. As discussed above, exchanging the CH 3 O group for a longer C 6 H 13 O moiety did not have a significant effect on the electronic structure; however, an intriguing band shift of 0.113 eV for 1aC vs. 1bC and 0.123 eV for 1aD vs. 1bD was observed. According to our previous reports [21,23], this change in the PL behavior likely originates from a dramatic change in the aggregated structure induced by the difference in alkoxy chain length. Similar to the trend observed for the PL in CH 2 Cl 2 solution, a relatively strong correlation was observed between µ || and λ PL in the crystals. These results demonstrate that the PL behavior of the fluorinated bistolanes can also be modulated in crystal by controlling µ || via the electron-withdrawing character of the terminal substituent.

PL Behavior in the LC Phase
Finally, owing to the fluorinated bistolanes possessing LC phases and unique PL characteristics, we examined the PL behavior in the LC phase under thermal conditions. As representative examples, frozen LC samples of 1bB, 1bC, and 1bD were obtained by rapid cooling in a dry-ice bath after a suitable LC phase was observed. The obtained PL spectra are shown in Figure 8. deactivation via nonradiative processes owing to intermolecular interactions in the condensed phases. As discussed above, exchanging the CH3O group for a longer C6H13O moiety did not have a significant effect on the electronic structure; however, an intriguing band shift of 0.113 eV for 1aC vs 1bC and 0.123 eV for 1aD vs 1bD was observed. According to our previous reports [21,23], this change in the PL behavior likely originates from a dramatic change in the aggregated structure induced by the difference in alkoxy chain length. Similar to the trend observed for the PL in CH2Cl2 solution, a relatively strong correlation was observed between μ|| and λPL in the crystals. These results demonstrate that the PL behavior of the fluorinated bistolanes can also be modulated in crystal by controlling μ|| via the electron-withdrawing character of the terminal substituent.

PL Behavior in the LC Phase
Finally, owing to the fluorinated bistolanes possessing LC phases and unique PL characteristics, we examined the PL behavior in the LC phase under thermal conditions. As representative examples, frozen LC samples of 1bB, 1bC, and 1bD were obtained by rapid cooling in a dry-ice bath after a suitable LC phase was observed. The obtained PL spectra are shown in Figure 8. When the N LC samples of AcO-substituted 1bB and CN-substituted 1bD were irradiated at 320 nm light, single PL bands were observed at λPL = 418 and 527 nm, respectively, which were almost identical to those in the Cr phase (Figure 8a,c). Interestingly, as shown in Figure 8b, the initial Cr sample of CF3-substituted 1bC showed light-blue PL (λPL = 479 mm), but a significant shift to shorter wavelengths was observed after the phase transition to the SmA phase, leading to deep-blue PL (λPL = 443 nm), which is almost identical to that in CH2Cl2 solution. It is anticipated that rearrangement of the molecular aggregates during the Cr → SmA phase transition results in loose molecular packing owing to thermally activated molecular motion and monomeric emission characteristics. Accordingly, it can be concluded that the PL of fluorinated bistolanes can be controlled not only by tuning the When the N LC samples of AcO-substituted 1bB and CN-substituted 1bD were irradiated at 320 nm light, single PL bands were observed at λ PL = 418 and 527 nm, respectively, which were almost identical to those in the Cr phase (Figure 8a,c). Interestingly, as shown in Figure 8b, the initial Cr sample of CF 3 -substituted 1bC showed light-blue PL (λ PL = 479 mm), but a significant shift to shorter wavelengths was observed after the phase transition to the SmA phase, leading to deep-blue PL (λ PL = 443 nm), which is almost identical to that in CH 2 Cl 2 solution. It is anticipated that rearrangement of the molecular aggregates during the Cr → SmA phase transition results in loose molecular packing owing to thermally activated molecular motion and monomeric emission characteristics. Accordingly, it can be concluded that the PL of fluorinated bistolanes can be controlled not only by tuning the electron-density distribution but also by modulating molecular aggregation through thermal phase transitions, which is a promising approach for realizing functional PL sensing materials, such as PL thermometers.

Conclusions
In conclusion, we designed and synthesized fluorinated bistolanes with an alkoxy substituent at one molecular terminal and various electron-withdrawing substituent at the other terminal via a four-step protocol. The phase transition temperatures of the fluorinated bistolanes were found to depend significantly on the terminal substituent, with the LC behavior affected by both the steric and electronic nature of the substituent. The PL behavior in solution clearly revealed that both λ PL and the PL color changed dramatically depending on the µ || value. The present fluorinated bistolanes also showed relatively strong PL in the crystal phase, with the PL behavior modulated not only by µ || but also by molecular aggregation. Notably, the PL emission of a CF 3 -substituted fluorinated bistolane was significantly shifted towards shorter wavelengths by 0.210 eV via a phase transition from the Cr to the SmA LC phase. As the PL behavior of the fluorinated bistolanes can be tuned by the electron-density distribution as well as the aggregated structure in condensed phases, this molecular design approach offers a useful method for fabricating PL materials that can be easily tuned by using molecular properties or external stimuli.