Unsymmetrical Hexaﬂuorocyclopentene-Linked Twisted π -Conjugated Molecules as Dual-State Emissive Luminophores †

: Dual-state emissive (DSE) luminophores, which can luminesce both in solution and in solid states, have recently attracted signiﬁcant attention because of their broad applications. However, their development is difﬁcult due to the difference in molecular design between solution- and solid-state luminophores. In this study, DSE luminophores based on unsymmetrical hexaﬂuorocyclopentene-linked twisted π -conjugated structures carrying various substituents to tune the electron-density were designed and synthesized in a single-step reaction from heptaﬂuorocyclopentene or perﬂuoro-1,2-diphenylcyclopentene derivatives. The twisted π -conjugated luminophores exhibited absorption in the UV region at approximately 330 nm, along with several signals in the high-energy region. Upon irradiating the luminophore solution (wavelength 330 nm), light-green to yellow photoluminescence (PL) was observed in the range of 422–471 nm with high PL efﬁciency. Theoretical calculations revealed that excitation from ground to excited states altered the structural shape of the luminophores from twisted to planar, leading to red-shifted PL and high PL efﬁciency ( Φ PL ). The intense blue PL exhibited by the luminophores in the crystalline state was attributed to their twisted molecular structures that suppressed non-radiative deactivation via the effective blocking of π / π stacking interactions. results theoretical molecular and photophysical measurements discusses the photophysical characteristics of the developed luminophores based on theoretical calculations and molecular packing structures in the crystalline state.


Introduction
Numerous luminescent substances exist that can be used in various applications, such as bioimaging probes [1,2], biosensors [2,3], light-emitting diodes [4][5][6], and lighting devices [4][5][6]. Luminescent materials can be divided into two classes according to the physical state of the luminophores: solution-state luminophores and solid-state luminophores. The former luminophores can be typically designed as extended π-conjugated planar structures, and their luminescence diminishes with the formation of molecular aggregates due to concentration quenching [7] or aggregation-caused quenching effects [8][9][10]. Meanwhile, the latter luminophores require special molecular design to enable the suppression of non-radiative deactivation by restricting molecular rotations or motions [11][12][13]. Despite difficulties in the design of versatile luminescent molecules, luminophores that luminesce in both solutions and solid states, called dual-state emissive (DSE) luminophores [14][15][16][17], have attracted considerable interest because of their broad applications in medicinal, biological, and technological fields [18,19]. However, the development of DSE luminophores is difficult due to the distinct molecular design parameters. To date, reports on the molecular design of DSE luminophores have incorporated the following elements: (i) π-conjugation with a zero-twist donor-acceptor moiety to facilitate emission in solution state [20], (ii) twisted molecular structure to avoid fluorescence quenching in the solid state induced by π/π stacking, and (iii) a V-shaped configuration to enhance intramolecular charge transfer (ICT) and potentially suppress short-range π/π intermolecular interactions [21].
Our group has devoted intensive efforts to the development of efficient DSE luminophores based on the molecular design principles that facilitate the expression of DSE characteristics. It was reported that hexafluorocyclopentene derivatives bearing two donor-acceptor (D-A)-type fluorinated tolane arms in a symmetrical manner exhibited intense yellow fluorescence at a maximum photoluminescence (PL) wavelength (λ PL ) of 559 nm and PL efficiency (Φ PL ) of 0.75 when dissolved in CH 2 Cl 2 . These luminophores also showed light-blue fluorescence (λ PL = 486 nm, Φ PL = 0.43) in their crystalline states (Figure 1a) [22]. In addition to the above molecular design, we envisioned that an unsymmetrical structural motif characterized by broken bilateral symmetry would not only exhibit increased solubility in common organic solvents and enhanced solution-phase PL intensity, but also show excellent crystalline-state PL properties owing to the suppression of ordered structure formation involving π/π stacking interactions. To demonstrate our molecular design for improved DSE luminophores, we designed and synthesized unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated molecules, 1 and 2, bearing both D-A type fluorinated tolane and benzene/multi-fluorinated benzene ring arms, and evaluated their photophysical behavior (Figure 1b). This paper presents the results of the theoretical molecular design and photophysical measurements in detail and discusses the photophysical characteristics of the developed luminophores based on theoretical calculations and molecular packing structures in the crystalline state.

General
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 using an FTIR-4100 type A spectrometer (JASCO, Tokyo, Japan). All IR spectra are reported in wavenumber (cm −1 ) units. 1 H nuclear magnetic resonance (NMR) (400 MHz) and 13 C NMR (100 MHz) spectra were acquired using an AVANCE III 400 NMR spectrometer (Bruker, Rheinstetten, Germany) in chloroform-d (CDCl 3 ) solution, and the chemical shifts were reported in parts per million (ppm) based on the residual protons or carbon in the NMR solvent. 19 F NMR (376 MHz) spectra were acquired 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. All chemicals were of reagent grade and were purified by standard methods prior to use when necessary. The progress of the reactions was monitored via thin-layer chromatography (TLC), which was performed on silica gel TLC plates (Merck, Silica gel, 60F 254 ; Kenilworth, NJ, USA). Column chromatography was performed using silica gel (FUJIFILM Wako Pure Chemical Corporation, Wako-gel ® 60N, 38-100 µm; Osaka, Japan).

Materials
The unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated molecules 1a-c with a benzene ring and 2a-c with a multi-fluorinated benzene ring in one arm, as shown in Figure 1, were synthesized from heptafluorocyclopentene derivatives or perfluoro-1,2-diphenylcyclopentene in a single-step reaction. The synthetic procedures and characterization of the compounds are described in the Supplementary Materials. The 1 H, 13 C, and 19 F-NMR spectra of 1a-c and 2a-c are demonstrated in Figures S2-S29, and their spectra proved that the compounds were of sufficient purity to enable evaluation of their photophysical behavior.

DFT
Computations used in this study were carried out using the Gaussian 16 software package (Revision B.01) [27]. The optimized geometry was obtained from the calculation using the CAM-B3LYP hybrid functional [28,29] and the 6-31G(d) basis set with a conductor-like polarizable continuum solvation model (CPCM [30][31][32]) for CH 2 Cl 2 . The theoretical excitation energies and dipole moments were calculated using time-dependent self-consistent field approximation [33,34] at the same level of theory.

Photophysical Property
Ultraviolet-visible (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 a Quantaurus-QY measurement system C11347-01 (Hamamatsu Photonics, Hamamatsu, Japan). PL lifetime measurement was carried out using a Quantaurus-Tau fluorescence lifetime spectrometer C11367-34 (Hamamatsu Photonics, Hamamatsu, Japan).

Theoretical Assessment
Our study was initiated with a theoretical assessment of the designed unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated molecules, using the Gaussian16 software. The optimized geometries of 1a-c and 2a-c were calculated using density functional theory (DFT) at the CAM-B3LYP/6-311++G(d,p)//CAM-B3LYP/6-31G(d) level of theory with CPCM for CH 2 Cl 2 . The theoretical vertical transitions from the ground (S 0 ) to the excited (S n ) states were assessed using time-dependent (TD)-DFT calculations using the same functional and basis set. Figure 2 presents the calculated results, viz., optimized geometries, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) distributions at both the S 0 and S 1 states of 1a with a CF 3 substituent in the aromatic ring opposite to the D-A type fluorinated tolane scaffold. The results for the other compounds, 1b, 1c and 2a-c, are summarized in the Supplementary Materials ( Figures S30 and S31 and Tables S1-S16). According to the geometry optimization results of 1a in the S 0 state, it formed a twisted molecular configuration between the central hexafluorocyclopentene core and both aromatic arms, in which the dihedral angles made by C 1 -C 2 -C 3 -C 4 and C 3 -C 4 -C 5 -C 6 , viz., φ(C 1 -C 2 -C 3 -C 4 ) and φ(C 3 -C 4 -C 5 -C 6 ), were 122 and −39 • , respectively ( Figure 2a). According to the electronic structure of 1a in the S 0 state, the HOMO (−7.55 eV) lobe existed over the D-A type fluorinated tolane-arm (Figure 2b), whereas the LUMO (−1.72 eV) lobe was primarily localized over the twisted diarylethene moiety (Figure 2c). Interestingly, the optimized geometry at the S 1 state was altered to reduce the twisted structure to a planar structure with increasing C3···C4 double bond length: φ(C 1 -C 2 -C 3 -C 4 ) and φ(C 3 -C 4 -C 5 -C 6 ) were 154 • and −15 • , respectively ( Figure 2d). The increasing structural planarity in the S 1 state slightly spread the HOMO and LUMO lobes throughout the π-conjugated unit in the molecule, which led to an increase in the HOMO energy level of −7.27 eV and a decrease in the LUMO energy level of −2.61 eV. This resulted in a small HOMO-LUMO energy gap (∆E H-L ) of 4.66 eV. The electronic orbital distributions between the HOMO and LUMO in the S 1 state promote ICT to radiatively deactivate excitons via the ICT excited state. Considering the structural and electronic alterations between the S 0 and S 1 states, the unsymmetrical twisted π-conjugated molecules can be anticipated to exhibit luminescence both in solution and solid states. Therefore, the proposed molecular design is suitable for the rational development of efficient DSE luminophores.

Crystal Structures of 1c and 2bB
After validation of the proposed molecular design for DSE luminophores using theoretical studies, the unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated compounds 1a-c with a benzene ring arm and 2a-c with a multi-fluorinated benzene ring-arm were synthesized, and the synthetic protocols are illustrated in Figure S1. The structurally pure compounds were obtained via double isolation techniques of column chromatography (eluent: hexane/EtOAc = 15/1) and recrystallization from CH 2 Cl 2 /MeOH (v/v = 1/1). Suitable structural assessments, such as 1 H, 19 F, and 13 C NMR, IR, and HRMS, proved that the synthesized compounds were of sufficient purity to enable evaluation of their photophysical characteristics. Among the synthesized compounds, 1c with a 4-methoxyphenyl arm and 2bB with a pentafluorophenyl arm furnished single crystals suitable for X-ray crystallographic analysis. Figure 3 shows the crystal structures of 1c and 2bB. Furthermore, 1c belongs to the monoclinic crystalline system in the P 1 2 1 /c 1 space group (Z = 4). The actual dihedral angles, that is, φ(C 1 -C 2 -C 3 -C 4 ) and φ(C 3 -C 4 -C 5 -C 6 ), were observed to be 111 • and −51 • , respectively, indicating a twisted structure, which was in parity with the theoretically optimized geometry. The packing structures were formed with several intermolecular interactions, such as H···F hydrogen bonding (short-distance atomic contact: 237.4 pm for H Ar ···F ArF , 256.6 pm for H Ar ···F CF2 , 262.4 pm for H CH3 ···F CF2 ), F···F interactions (290.9 pm for F Ar ···F Ar ), and CH···π interactions (278.9 pm for C Ar ···H CH3 ), in which the short-distance atomic contacts were less than the sum of the van der Waals radii of two atoms (H: 120 pm, C: 170 pm and F: 147 pm) [35]. However, 2bB crystallized in an orthorhombic crystalline system belonging to the P 2 1 2 1 2 1 space group, and four molecular units of 2bB were included in a unit cell. The central hexafluorocyclopentene core was sharply twisted with respect to both the fluorinated tolane and multi-fluorinated benzene ring arms, in which the dihedral angles φ(C 1 -C 2 -C 3 -C 4 ) and φ(C 3 -C 4 -C 5 -C 6 ), were 66 • and −117 • , respectively. The packing structures constituted several intermolecular interactions. Short-distance contacts, such as H···F hydrogen bonding (247.5 pm for F CF2 ···H alkyl ), F···F interactions (269.6 pm, 274.5 pm, and 290.8 pm for F CF2 ···F Ar ; 292.3 pm for F Ar ···F Ar ), and electrostatic π···π interactions (337.8 pm and 336.3 pm for C Ar ···C Ar-F ), were observed. The unsymmetrical twisted structural shape can be attributed to the avoidance of intermolecular π/π stacking via orbital overlap throughout the π-conjugated unit in the molecule. Therefore, the unsymmetrical twisted molecules were expected to exhibit PL in crystalline states.

Photophysical Measurements in Solution State
We first investigated the photophysical behavior of the hexafluorocyclopentene-linked twisted π-conjugated molecules 1a-c and 2a-c, in a dilute solution state. The sample was prepared by dissolving in CH 2 Cl 2 , and the concentration was adjusted to 1 × 10 −5 mol L −1 for UV-vis absorption measurements and 1 × 10 −6 mol L −1 for PL measurements. Figure 4 shows the UV-vis absorption and PL spectra, and the chromaticity diagram for PL color as defined by the Commission Internationale de l'Eclairage (CIE). Table 1 summarizes the obtained photophysical data.
Hexafluorocyclopentene-linked twisted π-conjugated molecules 1a-c bearing a benzene ring arm absorbed UV light and showed similar absorption spectra with several absorption bands (Figure 4a). 1a with a 4-(trifluoromethyl)phenyl arm when dissolved in CH 2 Cl 2 displayed two absorption bands with maximum absorption wavelengths (λ abs ) of approximately 331 and 257 nm, along with a shoulder signal at approximately 315 nm. Through TD-DFT calculations, the two intense absorption bands observed were assigned to two π-π* transitions involving HOMO → LUMO for the low-energy absorption band and HOMO → LUMO+1 for the high-energy absorption band ( Figure S31 and Table S3). Further, 1b with a 4-fluorophenyl arm showed similar absorption behavior to 1a, while 1c with a 4-methoxyphenyl arm displayed not only the two intense absorption bands with λ abs at approximately 328 and 261 nm, but also an absorption band with λ abs at approximately 300 nm. The corresponding transitions were π-π* transitions involving HOMO → LUMO and HOMO-1 → LUMO for the long-and short-wavelength absorption bands, respectively.  The PL behaviors of 1a-c in CH 2 Cl 2 solution were examined at an excitation wavelength (λ ex ) of 330 nm. 1a, which had a 4-(trifluoromethyl)phenyl arm, showed a single PL band with a maximum PL wavelength (λ PL ) of approximately 522 nm and high PL efficiency (Φ PL = 0.99). 1a dissolved in CH 2 Cl 2 was observed to emit yellowish-green PL with coordinates (x, y) = (0.3, 0.47) according to the PL color and CIE chromaticity diagram (Figure 4b). When the luminescence decay profiles were measured to reveal the luminescence process, the luminescence lifetime (τ) was found to be 3.6 nano-second (ns), indicating that PL of 1a is fluorescence ( Figure S33 and Table S18). 1b with a 4-fluorophenyl arm and 1c with a 4-methoxyphenyl arm exhibited single PL bands at λ PL of approximately 493 and 465 nm, respectively, with a moderate to high Φ PL , upon excitation at 330 nm. Their luminescence process was also concluded to be fluorescence because of the nanosecond order of τ (2.9 ns for 1b and 1.9 ns for 1c). The PL color was bluish-green with coordinates of (0. 22 This indicates that the CF 3 group at the 4-position of the benzene ring arm in 1a causes a decrease in ∆E H-L through a significant reduction in the LUMO energy level, resulting in the emission of long-wavelength PL. Meanwhile, the electron-donating substituents in 1b and 1c increase the HOMO energy level via the resonance effect, which increases the ∆E H-L and results in high-energy emission with a short λ PL . Figure 4c presents the UV-vis absorption and PL spectra of 1a in various solvents, such as hexane, chloroform (CHCl 3 ), and ethyl acetate (EtOAc). The solvent polarity can be determined from the relative dielectric constant at 25 • C: hexane (1.88) < CHCl 3 (4.81) < EtOAc (6.02) < CH 2 Cl 2 (8.93). Table 1 summarizes the photophysical data. The absorption spectra of 1a in different solvents were almost consistent and independent of the solvent polarity, whereas the PL band shifted upon changing the solvent polarity. Thus, 1a in the less polar hexane solution emitted deep blue PL with coordinates of (0.16, 0.07) with a λ PL at approximately 422 nm. Upon increasing the solvent polarity based on the relative dielectric constant, the λ PL was found to be red-shifted, resulting in light-blue PL with coordinates of (0.20, 0.34) in CHCl 3 , yellowish-green PL (0.30, 0.47) in CH 2 Cl 2 , and yellow PL in EtOAc (0.36, 0.50). Based on the effect of solvents on the photophysical properties, we created a Lippert-Mataga plot [36,37] with a solvent polarity parameter (∆f ) on the horizontal axis and the Stokes shift (∆ν = ν abs − ν PL ) on the vertical axis (Figure 4e), which was used to determine the dipole moments of the molecules in the excited states. With a least-squares approximation, a linear relationship between ∆ν and ∆f was found, as expressed in Equation (1): The slope of this equation correlates strongly with the difference in the dipole moments between the excited and ground states: ∆µ = µ e − µ g , where µ e and µ g represents the dipole moments in the excited and ground states, respectively. A large ∆µ value of 24.8 D was obtained for 1a, which clearly indicated that the PL of 1a was due to radiative deactivation of the CT excited state via ICT transition [37]. The major PL bands of 1b and 1c also result from radiative deactivation via ICT transition. Subsequently, we examined the solution state photophysical behavior of 2a-c with a multi-fluorinated benzene ring arm. Figure 5 shows the corresponding UV-vis absorption and PL spectra and the CIE color diagram for PL, and Table 2 summarizes the photophysical data.  In UV-vis absorption measurements, two absorption bands were observed in the UV region at λ abs 332-336 nm and 259-261 nm (Figure 5a, left axis). TD-DFT calculations revealed that the low-energy absorption band originated from the π-π* transition involving HOMO → LUMO, whereas it was calculated that the higher absorption bands were involved in the vertical transitions from HOMO to LUMO+1 for 2a and 2b and HOMO-1 to LUMO for 2c. Upon irradiation of the CH 2 Cl 2 solutions of 2a-c with UV light (λ ex = 330 nm), PL was observed with a Φ PL value in a range of 0.26-0.88. The PL of 2a-c was also assigned as fluorescence resulting from the nano-second order lifetime ( Figure S33). In contrast to the PL characteristics of 1a-c, 2a and 2bA exhibited two PL bands at approximately 409-410 nm and a longer-wavelength region (λ PL = 571 nm for 2a and 533 nm for 2bA). However, the other two analogues, viz., 2bB and 2c, exhibited single PL bands at approximately 535 nm (2bB) and 518 nm (2c) (Figure 5a, right axis), respectively. According to the above discussions and our previous report [22], the major PL band in the longer-wavelength region can be assigned to radiative deactivation via ICT transition. Additionally, the minor PL band observed in the cases of 2a and 2bA in the range of 409-410 nm can be assigned to radiative deactivation from locally excited (LE) states. The lifetime measurement in the high-energy PL band for 2a and 2bA was found to be fitted with the second-order fluorescence decay curve including short-period lifetime ( Figure S33), which can support the assignment of the high-energy PL as radiative deactivation from the LE states. The PL color of 2a-c varied between yellow and green ( Figure 5b). Similar to the PL behavior of 1a-c, λ PL of 2a-c underwent a gradual blue-shift with the alteration of the terminal substituent in the order: CF 3 → F → OCH 3 due to increase in the ∆E H-L of the S 1 state (4.44, 4.58, and 4.63 eV for 2a, 2b, and 2c, respectively) (Table S1). Accordingly, the photophysical properties of the synthesized unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated molecules were similar to those of symmetrical molecules in solution. However, a slight decrease in Φ PL was observed due to the shortened π-conjugation length.

Crystalline-State Photoluminescence Behavior
Next, we tested the crystalline-state PL behavior of the unsymmetrical hexafluorocyclopentenelinked twisted π-conjugated molecules, 1a-c and 2a-c. Figure 6 illustrates the PL spectra of 1a-c and 2a-c in the crystalline state upon irradiation with light of wavelength 300 nm (λ ex ), photographs under UV irradiation (λ ex = 365 nm), and CIE color diagram of the PL. The photophysical data obtained are summarized in Table 3.  A single PL band at approximately 449 nm was observed for 1a, and the corresponding PL color was blue with coordinates of (0.16, 0.16). It was revealed that the PL was assigned as fluorescence from the lifetime measurement ( Figure S36) and, to our delight, the Φ PL was as high as 0.98 in the crystalline state. A similar blue PL with a single PL band at approximately 443 nm was observed for 1b and 1c with high Φ PL (up to 1.0). Single crystal X-ray studies performed on 1c (Figure 3) indicated the formation of rigid structures owing to the contribution of H···F hydrogen bonds and CH/π interactions, and not π/π stacking, which is likely responsible for the significant retardation of the non-radiative rate constant owing to suppression of molecular motions, like rotation, leading to high Φ PL (up to 1.0). In addition, the compounds 2a-c exhibited blue or light-blue PL with a λ PL at approximately 432-471 nm and a high Φ PL in the range 0.52-1.0 in the crystalline state. Compared with the PL behavior in dilute solution, the λ PL in the crystalline state was significantly blue shifted by 20-100 nm. The high-energy PL in the crystalline state is attributable to radiative deactivation through locally excited (LE) states due to the twisted molecular structure, which shortens the effective π-conjugation length. Therefore, the twisted structural shape effectively avoids non-radiative deactivation due to the absence of π/π stacking interactions, resulting in highly efficient PL behavior in the crystalline state.

Conclusions
We designed and synthesized unsymmetrical hexafluorocyclopentene-linked twisted π-conjugated molecules bearing both D-A type fluorinated tolane and aromatic ring arms as efficient DSE luminophores. The prepared DSE luminophores were emissive in the solution state, and their electronic properties were strongly influenced by terminal molecular substituents. Theoretical investigations using TD-DFT calculations indicated that the twisted structures were transformed to planar molecular shapes in the excited states. It is postulated that this structural change promoted ICT, resulting in yellow PL from ICT excited states. The unsymmetrical twisted π-conjugated molecules also exhibited high PL efficiency (Φ PL up to 1.0) with blue PL color in the crystalline state due to the effective inhibition of π/π stacking interactions by the twisted molecular shapes. These observations indicate that the prepared luminophores represent promising molecular designs that can potentially benefit the development of improved DSE luminophores. Further investigation is ongoing in our research group.

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