Systematic Studies on the Effect of Fluorine Atoms in Fluorinated Tolanes on Their Photophysical Properties

In this study, we synthesized a series of fluorinated and non-fluorinated tolanes, in which one or more fluorine atoms were systematically introduced into one aromatic ring of a tolane scaffold, and systematically evaluated their photophysical properties. All the tolanes with or without fluorine substituents were found to have poor photoluminescence (PL) in tetrahydrofuran (THF) solutions. On the other hand, in the crystalline state, non-fluorinated and fluorinated tolanes with one or four fluorine atoms were less emissive, whereas fluorinated tolanes with three or five fluorine atoms exhibited high PL efficiencies (ФPL) up to 0.51. X-ray crystallographic analyses of the emissive fluorinated tolanes revealed that the position of the fluorine substituent played a key role in achieving a high ФPL. Fluorine substituents at the ortho (2/6) and para (4) positions led to tight and rigid packing due to plural π–π stacking and/or hydrogen bonding interactions, resulting in enhanced ФPL caused by the suppression of non-radiative deactivation. Additionally, fluorinated tolanes with three fluorine atoms exhibited notable aggregation-induced PL emission enhancement in THF/water mixed solvents. This demonstrates that the PL characteristics of small PL materials can be tuned depending on the usage requirements.

As a result of extensive photophysical investigations of tolane and the derivatives in recent years, significant advances have been achieved in developing tolane-based compounds that exhibit unique luminescence phenomena. Examples of such innovations include the tethering of two benzene rings of tolane, which induces the emission of phosphorescence in organic glass or solution states (Figure 1a) [15,16], crystallization or the formation of molecular aggregates of tolane derivatives (Figure 1b), which emit fluorescence, and enhancement of the fluorescence of tolanes by suppressing internal conversion from ππ* to the "dark" trans-bent πσ excited states [17,18]. Additionally, tolanes bearing fluoropyrrole groups exhibit dual-state fluorescence, emitted both in the solution and powder states (Figure 1c) [19].
Over the last couple of years, our group has explored the synthesis and properties of fluorinated PL molecules because fluorine atoms incorporated into organic structures play a crucial role in the formation of ordered molecular aggregates. The unique characteristics of fluorine atom that contribute to this effect include its (i) large electronegativity, higher than that of all the other elements; and (ii) small atomic size, second only to the size of the hydrogen atom, etc. [20,21]. Our extensive efforts have led to the successful development of fluorinated tolane-based PL molecules [22][23][24]. From our previous results, fluorinated tolanes were found to possess both crystallization-induced emission enhancement (CIEE) as well as aggregation-induced emission enhancement (AIEE) characteristics ( Figure 1d) [23,24]. A deep investigation using X-ray crystallographic analyses has revealed that fluorinated tolane B in the crystalline state exhibits enhanced PL properties, while the non-fluorinated tolane A counterpart does not, owing to the tight and rigid molecular packing through several intermolecular hydrogen bonds, which are needed to suppress the non-radiative deactivation process. To further elucidate the effect of factors such as the number of fluorine atoms and positions of the fluorine substituents on the PL characteristics, we systematically synthesized tolane derivatives 0F-4F with a systematic arrangement of the number and position of fluorine substituents, as shown in Figure 2. In this paper, we demonstrate and discuss the photophysical behavior of 0F-4F. In addition, the photophysical behavior and molecular aggregated structures in the crystalline state of 5F, which contains five fluorine atoms, are discussed and compared with other such materials in detail, although some photophysical behaviors of 5F in solution and amorphous states have been reported previously [24].

Results and Discussion
Based on the synthesis protocol reported previously for 5F [24], tolane 0F (without fluorine substituents) and 1F-4F (with fluorine substituents) were prepared via a Pd(0)catalyzed Sonogashira cross-coupling reaction using commercially available 4-ethynylanisole and various non-fluorinated or fluorinated aromatic halides. Yields in the range of 46-94% were achieved ( Figure S1). In order to assess the photophysical behavior of the compounds in the crystalline state, tolanes 0F-5F were crystallized through double purification by column chromatography, followed by recrystallization. Based on several spectroscopic studies, the tolane 0F-5F crystals were determined to be adequately pure to evaluate photophysical properties such as ultraviolet-visible light (UV-vis) absorption and PL behavior, both in dilute solutions and the crystalline state ( Figures S2-S15).
To investigate the effect of fluorine substituents introduced into the tolane scaffold on the photophysical behavior, we initially attempted to measure UV-vis absorption and PL for dilute solution samples, which were prepared by dissolving tolane crystalline powder in tetrahydrofuran (THF) to achieve a 1.0 × 10 −5 mol L -1 concentration. The UV-vis and PL spectra obtained for these samples are shown in Figure 3 and Figures S16-S27, while the photophysical data are summarized in Table 1 and Table S1. (a) UV-vis spectra of 0F-5F; (b) photoluminescence (PL) spectra of 0F-5F excited at (λ abs ) 290 nm for 0F, 288 nm for 1F, 291 nm for 3Fa, 290 nm for 3Fb, 293 nm for 3Fc, 300 nm for 4F, and 297 nm for 5F; (c) PL spectra of 0F at various excitation wavelengths; and (d) excitation spectra of 0F at 328 and 447 nm. As shown in Figure 3a, non-fluorinated tolane 0F and its fluorinated counterparts 1F and 3Fa-c containing three or fewer fluorine atoms exhibited absorption bands with two maxima (λ abs ) at approximately 290 and 307 nm. With increasing the number of fluorine substituents, for example, 4F and 5F substituted with four and five fluorine atoms, respectively, in one of the aromatic rings of the tolane scaffold, a slight long-wavelength shift in λ abs of approximately 10 nm was observed. Furthermore, the energy gap (∆E) between the highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO), which was experimentally obtained via cyclic voltammetry (CV) measurements ( Figure S35 and Table S3), decreased with the increasing number of fluorine substituents. Therefore, the red-shift in λ abs may be attributable to the decreased HOMO-LUMO energy gap, ∆E. Additionally, the calculated data involving dipole moment in long molecular axis, HOMO and LUMO energies obtained from DFT calculations are also summarized in Table S6.
As shown in Figure 3b, in all the compounds, the maximum PL wavelength (λ PL ) was 328-406 nm. In addition to the intense PL band observed in the short-wavelength region, interestingly, 0F and 1F exhibited a weak PL band around 450 nm. The two diphenylacetylene emission bands have been reported to originate from radiative deactivation via a ππ* state for the short-wavelength band and a dark πσ* state for the long-wavelength band [14,25]. Furthermore, 3F and 4F exhibited a major PL band with λ PL between 340 and 370 nm, accompanied by a shoulder peak with λ PL at approximately 430 nm. In contrast, 5F containing five fluorine substituents was found to exhibit a single PL band with λ PL at 406 nm. To gain more insights into these two PL bands, the excitation wavelength (λ ex )-dependent PL behavior was investigated using 0F as an example ( Figure 3c). Excitation by higher energy light caused a gradual increase in the PL intensity in the long-wavelength region compared with the PL intensity observed upon excitation by lower energy light. In addition, as shown in Figure 3d, the excitation spectra observed at λ PL at 447 nm were slightly blue-shifted in comparison with the excitation spectra observed at 328 nm. This PL behavior likely originates from increased internal conversion from a higher-order excited state to a dark trans-bent excited state [14]. As a result of this major internal conversion process, the PL efficiency (φ PL ) for 0F-3F appears to be extremely low (<0.01). In the case of 4F and 5F, a slight increase in φ PL (0.04 and 0.08 for 4F and 5F, respectively) was observed, owing to major contributions of the fast radiative process from the charge-transfer state, with the exception of a minor non-radiative process from the dark trans-bent excited state. In order to assess the radiative process for the tolane derivatives, we tested their PL lifetimes (τ PL ).
The τ values are depicted in Figure S33 and the data are also listed in Tables 1 and S2. The τ PL values of the THF solution containing 0F at λ PL of 328 nm were approximately 0.82 and 5.08 ns. The observed τ PL values were found to be different from those reported [25,26], which is likely due to the change of solvent polarity [27,28]. Compound 0F in THF solution exhibited fluorescence due to light emission from two components at the singlet S 1 excited state. Similarly, the 1F-5F compounds also exhibited fluorescence, which stemmed from two luminescent components at the S 1 excited states.
We previously reported that tolane 5F containing a pentafluorobenzene scaffold exhibited weak PL (φ PL = 0.14) in the amorphous state [24]. After extensive trials, we were ultimately successful in producing single crystals of 5F. To our delight, we found that the PL efficiency of crystalline 5F was four-fold higher (φ PL = 0.51) compared to that of amorphous 5F. Based on these results, we focused on the PL characteristics of a series of tolane compounds 0F-5F in the crystalline-state. Figure 4 and Figure S32 shows the PL spectra and photographs obtained under both daylight and UV light conditions (λ ex = 365 nm). The photophysical data obtained are summarized in Table 2.  Crystalline 5F exhibited a single band showing light-blue PL at λ PL of around 465 nm, whereas crystalline samples of 0F-4F, which were prepared by recrystallization from CH 2 Cl 2 /MeOH (v/v = 1/1), were found to exhibit deep-blue PL with λ PL in the 359-381 nm range. As mentioned above, the φ PL of 5F was as high as 0.51, whereas the corresponding values for tolanes 0F, 1F, and 4F were observed to be quite low (0.04, 0.10, and 0.04 for 0F, 1F, and 4F, respectively). Considering the φ PL of a series of 3F containing three fluorine atoms at different substitution positions on the benzene ring, it is interesting that the φ PL was significantly affected by the position of the fluorine substituents. For example, 3Fa with three fluorine atoms at the 2, 3, and 4 positions and 3Fb with three fluorine atoms at the 2, 4, and 6 positions exhibited relatively high φ PL (up to 0.37), whereas 3Fc with three fluorine atoms at the 3, 4, and 5 positions exhibited low φ PL (0.14).
To understand why the φ PL values for 3Fa-c and 5F were higher than those for 0F, 1F, and 4F, we performed X-ray crystallographic analyses for crystalline 3Fa-c, 4F, and 5F, which were successfully obtained by recrystallization from a mixed solvent system containing CH 2 Cl 2 /MeOH. Figure 5 shows the crystal packing structures of 3Fa-c and the crystallographic data are summarized in Table S4.
Tolanes 3Fa-c containing three fluorine substituents were found to possess similar packing structures with four molecular units in a unit cell, in which three molecules, A to C, existed in-plane (Figure 5a-f). Considering the intermolecular π-π stacking interactions of 3Fa-c, 3Fa exhibited two π-π stacking interactions between molecules π C ···π D with 334.1 pm of interlayer distance on a one-to-one basis ( Figure 5g). As shown in Figure 5h,i, in contrast, 3Fb and 3Fc possessed two π-π stacking interactions between two molecules π C ···π D /π C ···π E with interlayer distances of 355.9 and 347.2 pm for 3Fb and 350.7 and 344.0 pm for 3Fc, respectively [29,30]. In addition, in 3Fa and 3Fb, three hydrogen bonding interactions were observed among the three molecules in the plane: (Figure 5j,k). In contrast, 3Fc possessed only one F A ···H B hydrogen bond without any other hydrogen bonding interactions (Figure 5l). Furthermore, the nonradiative rate constant (k nr ) values, which were calculated from τ PL in crystal ( Figure  S34), for 3Fa and 3Fb were approximately one-half or one-fifth of the corresponding value for 3Fc. These results clearly indicate that the intermolecular π-π stacking and hydrogen bonding interactions resulted in the formation of tight and rigid packing structures in the crystalline state, which likely suppresses nonradiative deactivation through molecular motions and results in strong PL in the crystal. Furthermore, 3Fa and 3Fb with three hydrogen bonding interactions exhibited higher φ PL compared to 3Fc, which had one hydrogen bond.  Figure 6 also shows the results of X-ray crystallographic analyses for 4F and 5F, in which the crystallographic data are summarized in Table S5. Tolane 4F containing four fluorine atoms at the 2, 3, 5, and 6 positions was found to have a twisted structure with a dihedral angle of 65.6 • between the two aromatic rings connected to the alkyne moiety (Figure 6a). In the packing structures, two molecules were present in the unit cell ( Figure 6c). One π-π intermolecular interaction was observed between the A···B molecular units, with the closest interatomic distance (π A ···π B ) being 367.5 pm (Figure 6e). Similarly, the crystal structure of 5F containing five fluorine atoms was also found to be twisted with a dihedral angle of 80.5 • between the two aromatic rings connected with the alkyne moiety; four molecular units were present in a unit cell (Figure 6b,d). Two molecular units A···B that existed in the central position were tightly held in place via a π-π stacking interaction (π A ···π B = 339.3 pm) and two hydrogen bonding interactions (F Ar ···H Ar = 248.4 pm). The H···F distances in the hydrogen bonds were observed to be much shorter than those in 3Fa-c, resulting in tighter and more rigid structures and the independent formation of dimer units. Owing to the tight dimer formation through multiple intermolecular interactions, the k nr of 5F (2.07 × 10 8 s −1 ) was found to be one-sixth of the k nr of 0F or 4F. It can be concluded that the PL emission of 5F in the crystal is likely to shift to the long-wavelength region and exhibit higher PL efficiency, compared to the other analogs. Judging from the relationship between the crystal structure and PL efficiency, the incorporation of fluorine substituents at either 2/6 (ortho) and/or 4 (para) positions appears to be essential for efficient PL emission in the crystalline states.
Considering the φ PL change for 3Fa-c in the solution and crystalline states, we tested their AIEE characteristics [31][32][33]. The PL spectra are shown in Figures 7 and S28-S32. Figure 7. PL spectra of (a) 3Fa, (c) 3Fb, and (e) 3Fc in THF/water mixed solvent (λ ex = 290 nm). Differences in the PL spectra between the molecular aggregates obtained in the THF/water system and crystalline states for (b) 3Fa, (d) 3Fb, and (f) 3Fc. (g) Relationship between PL efficiency and additional water ratio (inset: relationship between PL intensity and additional water ratio). (h) Excitation spectra of the molecular aggregates for 3Fa-c obtained after the addition of 90% water. Figure 7a,c,e, the PL intensities of 3Fa-c did not change at all with up to 80% water addition, whereas the PL intensities significantly increased upon the addition of 90% water. In 3Fb, φ PL markedly improved to 0.14 when the water amount reached 90%, whereas φ PL only slightly improved to 0.04 and 0.03 in 3Fa and 3Fc (Figure 7g), respectively, which suggests that the molecular aggregates formed in the solution upon water addition depend on the substitution position of the fluorine atoms. Furthermore, as shown in Figure 7h, this is also supported by the fact that only 3Fb exhibited a different excitation spectrum compared to 3Fa and 3Fc at 90% water content. Comparing the PL spectral shapes for the molecular aggregates formed in THF/water mixtures with those formed in the crystalline state, the λ PL values for 3Fa and 3Fc were slightly red-shifted in the THF/water mixtures, whereas in the crystalline state, only 3Fb exhibited a similar spectral shape (Figure 7b,d,f). The change in the spectral shape likely originates from the changes in the molecular aggregated structures, which clearly indicate that the PL characteristics can be modulated by altering the molecular aggregates.

Materials
The 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) using the residual protons in the NMR solvent. The 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. Infrared (IR) spectra were recorded using the KBr method with an FTIR-4100 type A spectrometer (JASCO, Tokyo, Japan). All the spectra are reported in terms of wavenumber (cm -1 ). High-resolution mass spectra (HRMS) were recorded on a JMS700MS spectrometer (JEOL, Tokyo, Japan) using the fast atom bombardment (FAB) method. All the chemicals, including solvents, were of reagent grade and were purified in the usual manner prior to use. Column chromatography was carried out on silica gel (FUJIFILM Wako Pure Chemical Corporation, Wakogel ® 60 N, 38-100 µm) and thin-layer chromatography (TLC) was performed on silica gel TLC plates (Merck, Silica gel 60F 254 ; Kenilworth, NJ, USA).

General Synthesis Procedure for the Pd(0)-Catalyzed Sonogashira Cross-Coupling Reaction
In a flask, an aromatic halide, 4-ethynylanisole, dichlorobis(triphenylphosphine)palladium(II), triphenylphosphine, copper(I) iodide, and triethylamine, and the suspended solution were stirred at 60 • C overnight. After the reaction times indicated, the precipitate formed during the reaction was separated by atmospheric filtration, while the filtrate was poured into a saturated aqueous ammonium chloride solution. The crude product was extracted with ethyl acetate (EtOAc) three times, and the combined organic layer was washed once with brine. The collected organic layer was dried over anhydrous Na 2 SO 4 , which was separated by filtration. The filtrate was evaporated in vacuo and subjected to silica gel column chromatography (eluent: hexane/EtOAc = 20/1), followed by recrystallization from CH 2 Cl 2 /MeOH (v/v = 1/1), to obtain the desired product in a 46-94% yield.

Photophysical Measurements
UV/vis absorption spectra were recorded on a V-530 absorption spectrometer (JASCO, Tokyo, Japan). PL spectra in the solution and crystal forms were acquired using an FP-6600 fluorescence spectrometer (JASCO, Tokyo, Japan). The absolute quantum yields in solution and crystal forms were measured using the Quantaurus-QY measurement system C11347-01 (Hamamatsu Photonics, Hamamatsu, Japan). The PL lifetime was obtained using a Quantaurus-Tau fluorescence lifetime spectrometer C11367-34 (Hamamatsu Photonics, Hamamatsu, Japan).

Cyclic Voltammetry
Cyclic voltammetry (CV) measurements were carried out using an ECstat-101 potentiostat (EC frontier, Kyoto, Japan) with glassy carbon, Pt, and Ag/AgCl as the working, counter, and reference electrodes, respectively. Ferrocene (Fc)/ferrocenium (Fc + ) was used as an external reference, while tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) was used as the supporting electrolyte (0.1 mol L −1 ). All the measurements were performed after argon bubbling for 30 min in 1 × 10 −3 mol L −1 acetonitrile solution, with a scan rate of 50 mV s −1 . HOMO and LUMO energy levels were estimated from the onset potentials of the oxidation (E Ox ) and reduction (E Red ) waves (versus Fc/Fc + ) using the following equation:

Conclusions
To gain insights into the structure-property relationships of fluorinated tolanes, we synthesized various tolanes with and without fluorine substituents (0F-5F). We evaluated the photophysical properties and crystal structures of these compounds in detail. During extensive investigations, all the derivatives were found to be non-emissive in dilute THF solution, with tight molecular packing structures formed via π-π stacking as well as hydrogen bonding interactions. Interestingly, tight molecular aggregates were formed when the fluorine substituents were incorporated at the ortho and para positions. The fluorinated tolanes containing fluorine substituents at these positions were found to emit PL efficiently, resulting in φ PL values in the 0.31-0.51 range. The range of PL behavior exhibited in solution (non-emissive) and in the crystalline state (emissive) piqued our interest to study the aggregation-induced emission enhancement characteristics. From the PL investigations of 3F (containing three fluorine substituents) in THF/water mixtures, we found a significant enhancement in the PL intensity upon adding 90% water, although the PL intensity was low, at approximately 0.01, when the amount of water was below 80%. The PL spectral shape was different from that in the crystalline state, obtained by recrystallization. It was found that the PL characteristics of fluorinated tolane 3F could be tuned by altering the molecular aggregates, which is promising for fabricating materials with tunable PL properties.  Figure S31: photographs of the PL behavior for 3Fa-3Fc in the mixed solution; Figure S32: PL spectra of 0F-5F in crystal; Figures S33 and S34: PL lifetime decay curve; Figure S35: CV curve of 0F-5F; Table S1: photophysical data of 0F-5F; Table S2: PL lifetime of 0F-5F in THF solution; Table S3: Electrical properties obtained by CV measurement; Tables S4 and S5: crystallographic data of 3Fa-3Fc, 4F and 5F;