Development of Yellow-to-Orange Photoluminescence Molecules Based on Alterations in the Donor Units of Fluorinated Tolanes

Abstract

Since the aggregation-induced emission (AIE) phenomenon was first reported by Tang et al., much effort has been devoted to the development of solid-state luminescent molecules by chemists worldwide. Our group successfully developed fluorinated tolanes as novel compact π-conjugated luminophores with blue photoluminescence (PL) in the crystalline state. Moreover, we reported the yellow-green PL molecules based on their electron-density distributions. In the present study, we designed and synthesized fluorinated tolanes with various amine-based donors and evaluated their photophysical properties. The carbazole-substituted fluorinated tolane exhibited strong PL in the solution state, whereas piperidine- or phenothiazine-substituted fluorinated tolanes showed a dramatic decrease in PL efficiency. Notably, fluorinated tolanes with piperidine or phenothiazine substituents displayed yellow-to-orange PL in the crystalline state; this may have occurred because these tolanes exhibited tightly packed structures formed by intermolecular interactions, such as H···F hydrogen bonds, which suppressed the non-radiative deactivation process. Moreover, fluorinated tolanes with amine-based donors exhibited AIE characteristics. We believe that these yellow-to-orange solid PL molecules can contribute to the development of new solid luminescent materials.

1. Introduction

Solid-state luminescent materials used in lighting and luminescent devices are recognized as essential materials in our lives. Although many luminescent molecules have been reported so far [1,2,3,4], the development of solid-state luminescent molecules has been considerably delayed because of the special molecular design required [5,6,7]. Since Tang et al. first reported the aggregation-induced emission (AIE) [8,9,10] and crystallization-induced emission (CIE) phenomena [11,12,13], solid-state luminescent molecules have gradually appeared, and their further development is still ongoing.

Over the last few years, our group has focused on the development of fluorinated organic luminophores [14,15,16,17] because the incorporation of fluorine atoms into organic molecules often provides them with unique characteristics, owing to the special properties of the fluorine atom. For example, methoxy-substituted donor-π-acceptor (D-π-A)-type fluorinated tolane exhibited intense blue photoluminescence (PL) in the crystalline state; however, its non-fluorinated counterpart displayed very weak PL, which was caused by its immediate internal conversion from the emissive ππ* excited state to the non-emissive πσ* excited state at ambient temperature (Figure 1a) [14]. Furthermore, replacing the methoxy group with a diphenylamino group in the fluorinated tolane molecule led to a significant change in the PL color from blue to yellow-green, which was due to its high electron density distribution (Figure 1a) [17].

Among solid-state luminescent molecules, luminophores showing orange-to-red PL have recently attracted significant attention because of their application in full-color light-emitting diodes [18,19]. However, it is well known that PL wavelength (λ_PL) and PL efficiency (Φ_PL) are inversely proportional to each other; many strategies, such as molecular recognition [20], molecular assembly [21,22], and co-crystallization [23], have been utilized to increase the PL wavelength. Owing to these molecular designs, many luminophores with solid-state yellow-to-red fluorescence and phosphorescence have been reported to date [24,25,26,27].

To develop solid-state PL molecules exhibiting long-wavelength PL with a compact π-conjugated structure, we designed novel compact π-conjugated molecules with a long-wavelength PL based by appropriately controlling their electron density and molecular orbital distributions [14,17]. Therefore, in this study, we synthesized a series of D-π-A-type fluorinated tolanes with amine-based donors, viz., CBZ-F with a carbazole unit, PIP-F with a piperidine unit, and PTZ-F with a phenothiazine unit, and evaluated their photophysical behavior (Figure 1b). The presence of the amine-based donors can control the molecular orbital and electron-density distributions of the tolanes. To determine the effect of the fluorine atoms in the tolane scaffold, non-fluorinated compounds, CBZ-H, PIP-H, and PTZ-H, were synthesized, and their photophysical behavior was studied in detail. In this article, the photophysical behavior of the aforementioned six analogs is described and discussed based on their molecular aggregated structures.

2. Results and Discussion

2.1. Synthesis

Initially, we synthesized a series of fluorinated tolanes, CBZ-F, PIP-F, and PTZ-F, and their non-fluorinated counterparts, CBZ-H, PIP-H, and PTZ-H, according to a previously reported synthetic procedure [17]. The synthetic procedure is illustrated in Figure 2.

Lithium 4-(carbazol-9-yl)phenylacetylide, prepared from 4-(carbazol-9-yl)phenylacetylene and n-butyllithium, reacted with pentafluorobenzonitrile in THF at room temperature for 18 h; the resulting nucleophilic aromatic substitution (S_NAr) reaction afforded CBZ-F in 14% isolated yield. The other analogs, PIP-F and PTZ-F, were synthesized in 45% and 18% isolated yields, respectively, in a similar manner. The non-fluorinated compounds, CBZ-H, PIP-H, and PTZ-H, were obtained in 8–38% yield via the palladium(0)-catalyzed Sonogashira cross-coupling reaction of 4-bromobenzonitrile with 4-amino substituted phenylacetylene. All synthesized compounds were purified by column chromatography and subsequently recrystallized. The structures of these molecules were confirmed by ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and ¹⁹F NMR spectroscopy. Moreover, their infrared spectra and high-resolution mass spectra (HRMS) were obtained. These spectra proved that the target molecules were sufficient to assess the photophysical behavior (Figures S1–S15 in the Supplementary Information).

2.2. Photophysical Behavior in Solution State

The ultraviolet–visible (UV–vis) and PL spectra of CBZ-F, PIP-F, and PTZ-F and their non-fluorinated counterparts, CBZ-H, PIP-H, and PTZ-H, were obtained using their solutions in dichloromethane (CH₂Cl₂). Figure 3 shows the UV–vis and PL spectra of the tolanes, and their related photophysical data are summarized in

Table 1. Photophysical data of the D-π-A-type tolanes obtained in CH₂Cl₂ solution and their calculated data.

	λabs [nm] 1	HOMO 2	LUMO 2	ΔEH−L	λPL [nm] 1,3	τAVE	τ1	τ2	kr5	knr6
	(ε × 10−3 L mol−1 cm−1)	[eV]	[eV]	[eV]	(ΦPL) 4	[ns]	[ns]	[ns]	[107 s−1]	[107 s−1]
CBZ-F	238 (52), 291 (39), 375 (20)	–7.03	–2.09	4.94	542 (0.65)	5.38	–	–	12.1	6.5
CBZ-H	238 (52), 292 (39), 342 (31)	–6.97	–1.58	5.39	447 (1.0)	3.51	–	–	28.5	0.0
PIP-F	277 (30), 408 (44)	–6.94	–1.93	5.01	603 (0.02)	1.18	1.02	4.40	1.7	83.1
PIP-H	266 (27), 363 (35)	–6.60	–1.34	5.26	496 (0.26)	2.50	–	–	10.4	29.6
PTZ-F	258 (45), 307 (26), 326 (23), 393 (21)	–6.75	–2.12	4.63	501 (0.01)	2.38	<1.0	3.39	0.42	41.6
PTZ-H	258 (49), 299 (32), 317 (33), 356 (13)	–6.72	–1.60	5.12	447 (0.02)	1.03	<1.0	5.14	1.9	95.2

¹ Concentration: 1.0 × 10⁻⁵ mol L⁻¹. ² Calculated with density functional theory using the M06-2X/6-31 + G(d) level of theory and a conductor-like polarizable continuum model for CH₂Cl₂. ³ Excited at the longest absorption maximum wavelength. ⁴ Determined using an integrating sphere. ⁵ Radiation rate constant: k_r = Φ_PL/τ_AVE. ⁶ Non-radiation rate constant: k_nr = (1 − Φ_PL)/τ_AVE.

.

In the absorption spectra of CBZ-F and CBZ-H (Figure 3a), the absorption bands in the short-wavelength region below 300 nm were found to be almost identical. However, a slight absorption band shift was observed in the long-wavelength region above 300 nm. The maximum absorption wavelength (λ_abs) for CBZ-F was 375 nm, which was red-shifted by 33 nm compared to that of CBZ-H. The PIP and PTZ analogs also exhibited similar absorption behavior. Both fluorinated and non-fluorinated compounds showed an identical spectrum in the short-wavelength region below 300 nm. However, the fluorinated compounds (PIP-F and PTZ-F) exhibited a red-shifted λ_abs (by 37–45 nm) for the absorption bands observed in the long-wavelength region (Figure 3b,c).

To investigate the theoretical electronic transitions of the tolanes, theoretical calculations were performed using the Gaussian 16 program set [28]. Herein, the calculations were conducted using time-dependent density functional theory (TD-DFT) at the M06-2X/6-31 + G(d,p) [29] level of theory with a conductor-like polarizable continuum model (CPCM) [30] for CH₂Cl₂, which was used as an implicit solvation model. CBZ-F, PIP-F, CBZ-H, and PIP-H showed two allowed electronic transitions (Figure 4,

Table 2. Theoretical values of vertical transition behavior calculated using TD-DFT ¹.

Molecule	Theoretical Transition	Theoretical Transition Energy [eV]	Theoretical λabs [nm]	Oscillator Strength
CBZ-F	HOMO → LUMO	3.4871	355.55	1.3880
	HOMO–1 → LUMO+1	4.7119	263.13	0.4074
CBZ-H	HOMO → LUMO	3.7348	331.97	1.6078
	HOMO−1 → LUMO+1	4.7075	263.37	0.5154
PIP-F	HOMO → LUMO	3.3143	374.09	1.6677
	HOMO → LUMO+2	5.1695	239.84	0.2601
PIP-H	HOMO → LUMO	3.5015	354.09	1.7485
	HOMO → LUMO+3	4.4592	278.04	0.0599
PTZ-F	HOMO → LUMO	3.4666	357.65	0.0000
	HOMO−2 → LUMO	3.9363	314.98	1.7773
PTZ-H	HOMO → LUMO	3.7800	328.00	0.0003
	HOMO−1 → LUMO	4.0564	305.65	1.8205

¹ Calculated with density functional theory using the M06-2X/6-31 + G(d) level of theory and a conductor-like polarizable continuum model for CH₂Cl₂.

).

In the case of CBZ-F and CBZ-H, the electronic transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) was assigned to the long-wavelength absorption band, which was calculated as the electronic transition from HOMO−1 to LUMO+1. The HOMO lobe is mainly localized over the electron-rich aromatic ring substituted with a carbazole moiety, while the LUMO is localized over the electron-deficient fluorinated aromatic ring. The HOMO → LUMO transition with a low transition energy is considered as a charge transfer (CT) transition, which probably causes the red-shift of λ_abs for CBZ-F. In contrast, the HOMO−1 and LUMO+1 orbital lobes were localized over the carbazole moiety. This result indicates that the HOMO−1 → LUMO+1 transition with a high transition energy is a local excitation (LE) transition. Similarly, in the case of PIP-F and PIP-H, the CT transition involved in the HOMO → LUMO transition had a low transition energy, and the LE transition involved in the HOMO → LUMO+2 transition had a high transition energy (Figure 4). In contrast, the PTZ analogs displayed an orthogonal molecular geometry between the tolane scaffold and phenothiazine moiety, which resulted in a large orbital separation between the HOMO and LUMO. Owing to the HOMO-LUMO orbital separation, the HOMO → LUMO transition is now a forbidden transition, and only one transition is allowed; HOMO−2 → LUMO in PTZ-F and HOMO−1 → LUMO in PTZ-H can be considered as LE transitions (Figure 4).

Subsequently, the PL behavior of the CH₂Cl₂ solutions of D-π-A-type tolanes was investigated. When the solution of CBZ-F in CH₂Cl₂ was irradiated with light (λ_abs in the long-wavelength absorption region), green PL with a single band at the maximum PL wavelength (λ_PL) of approximately 542 nm was observed with a high PL efficiency (Φ_PL = 0.65) (Figure 3d). Although CBZ-H was found to emit intense blue PL, its PL behavior (λ_PL = 447 nm, Φ_PL = 1.0) was remarkably different from that of fluorinated CBZ-F. The PL bands for CBZ-H and CBZ-F can be attributed to the fluorescence involving the intramolecular charge transfer (ICT) from the donor to acceptor for the HOMO-LUMO transition, which can be reasonably explained by the solvent effect of absorption and PL behavior (Figure S19) [31,32]. The PL lifetimes (τ) of the tolanes were monitored at their corresponding λ_PL. The τ was 5.38 ns for CBZ-F and 3.51 ns for CBZ-H (Figure S18); considering the nanosecond order of τ, the PL of the CBZ analogues can be considered as fluorescence. The radiation rate constant (k_r) and non-radiation rate constant (k_nr) were determined from the Φ_PL and τ values. The k_nr value of CBZ-F was larger than that of CBZ-H. Owing to the presence of fluorine atoms, the HOMO-LUMO energy gap (ΔE) in CBZ-F was lower than that in CBZ-H. This likely facilitates the non-radiative process, resulting in low Φ_PL based on the energy-gap law.

When the PL behavior of the PIP and PTZ analogs was measured in CH₂Cl₂ solution, the same PL tendency as that of the CBZ analogs was observed (Figure 3e,f). The PL band of the fluorinated analogs appeared in the longer-wavelength region than that of the corresponding non-fluorinated counterpart. Based on the Lippert–Mataga relationship, this PL behavior of the tolane analogs was probably caused by the ICT character resulting from the large electron-density distribution. PIP-F exhibited a long-wavelength PL because the excited state was stabilized owing to the large ICT characteristics. In contrast, PTZ-F resulted in a short-wavelength PL. PIP and PTZ analogs exhibited a significantly reduced Φ_PL (less than 0.26) with a large difference from that of the CBZ analog. This may have occurred for the PIP analogs because the strong amine-based donor therein rotated in the excited state to form a stable charge-separation state, called the twisted intramolecular charge transfer (TICT) state, resulting in a weak PL intensity and a rapid decay in PL (τ = 1.18 ns for PIP-F) due to non-radiative deactivation through dramatic structural relaxation of the donor moiety [33]. In the case of PTZ analogs, the orthogonal molecular geometry significantly reduces the electron-donating character of phenothiazine to the tolane moiety, causing fast PL decay (τ = 2.38 ns) on account of structural relaxation from the emissive ππ* excited state to the non-emissive πσ* excited state, resulting in low PL due to non-radiative deactivation [34,35].

2.3. Photophysical Properties in Crystalline State

Next, we focused on the photophysical behavior of the synthesized D-π-A-type tolanes in their crystalline states. Figure 5 shows the measured PL spectra and the CIE color diagram, and the photophysical data obtained are summarized in

Table 3. Photophysical data of D-π-A-type tolane with an amine-based donor in crystal.

	λPL [nm] 1	τAVE	τ1	τ2	kr3	knr 4
	(ΦPL) 2	[ns]	[ns]	[ns]	[107 s−1]	[107 s−1]
CBZ-F	507 (0.42)	52.8	19.4	62.1	0.8	1.1
CBZ-H	442 (0.48)	5.32	2.67	5.27	9.0	9.8
PIP-F	551 (0.18)	8.82	7.41	13.1	2.0	9.3
PIP-H	485 (0.30)	3.18	1.95	5.75	9.4	22.0
PTZ-F	575 (0.16)	93.1	32.0	276	0.2	0.9
PTZ-H	478 (0.06)	2.31	–	–	2.6	40.7

¹ Excited at 300 nm for CBZ-F, CBZ-H, PIP-H, PTZ-F, and at 310 nm for PIP-F and PTZ-H. ² Determined using an integrating sphere. ³ Radiation rate constant: k_r = Φ_PL/τ_AVE. ⁴ Non-radiation rate constant k_nr = (1 − Φ_PL)/τ_AVE.

.

Crystalline CBZ-F was found to exhibit relatively intense PL (Φ_PL = 0.42) with a single PL band at approximately 507 nm of λ_PL (Figure 4a). In contrast, CBZ-H emitted PL (Φ_PL = 0.48) with a λ_PL at approximately 442 nm in its crystalline state. Although PIP-F and PTZ-F exhibited a weak PL in solution, their crystalline samples were found to exhibit PL in the range of 0.16–0.18 Φ_PL with an λ_PL at approximately 551 nm for PIP-F and 575 nm for PTZ-F (Figure 4b,c). However, no significant improvement was observed in the PL behavior of their non-fluorinated counterparts. PIP-H showed similar PL in the crystalline (λ_PL = 485 nm, Φ_PL = 0.30) and solution states (λ_PL = 496 nm, Φ_PL = 0.26), and PTZ-H displayed a weak PL both in the crystalline (λ_PL = 478 nm, Φ_PL = 0.06) and solution states (λ_PL = 447 nm, Φ_PL = 0.02). Considering the Comission Internationale de l’Eclailage (CIE) color diagram, the PL color of CBZ-F, PIP-F, and PTZ-F was observed to be green, yellow, and orange, respectively, although the corresponding hydrogenated counterparts, viz., CBZ-H, PIP-H, and PTZ-H, were in a range of the blue to yellow region (Figure 5d).

To understand the PL characteristics of the synthesized tolanes in the crystalline state, their molecular aggregated structures were investigated. After several recrystallization processes, the single crystals of CBZ-H, PIP-F, PTZ-F, and PTZ-H were used for X-ray crystallography. Figure 6 shows the crystal structures from the top view and the packing structures with an interatomic short contact for CBZ- H, PIP-F, PTZ-F, and PTZ-H.

CBZ-H, which contained the carbazole unit, crystallized in the monoclinic P12₁/c space group, and four molecular units were contained in a unit cell. The tolane scaffold of CBZ-H has a planar structure, while the carbazole unit is twisted by 46.1° with respect to the tolane plane (Figure 6a). In the packing structures, several intermolecular π/π stacking interactions (π···π: 329 pm) between the electron-deficient aromatic ring and electron-rich carbazole unit and hydrogen bonds (N···H: 263 pm) between the hydrogen atom in the carbazolyl group and the nitrogen atom in the cyano group were observed. The interatomic distances for these interactions are less than the sum of their van der Waals radii (C: 170 pm, H: 120 pm, and N: 155 pm) [36].

PIP-F with a piperidine substituent crystallizes in the triclinic P–1 space group, and the unit cell contains two molecular units. The fluorinated tolane scaffold exhibits an almost planar structure with a 5.1° dihedral angle composed of two aromatic rings (Figure 6b). The dihedral angle formed by the piperidine ring and tolane plane was only 4.3°, which was found to be a flat conformation of the entire molecule. In the packing structures, as shown in Figure 5b, two fluorinated tolanes were packed in an antiparallel arrangement with π/π stacking interactions (π···π: 340 pm). In addition, the H···F hydrogen bonds between the fluorine and hydrogen atoms and the F/F interaction between fluorine atoms were observed (H···F: 262 pm, F···F: 284 pm) in the packing structures. The observed interatomic distances are also shorter than the sum of their van der Waals radii (F: 147 pm) [36].

PTZ-F with a phenothiazine unit belongs to the monoclinic C12/c1 space group and contains six molecules in a unit cell. With a large difference from the above crystal structures, that is, CBZ-H and PIP-F, two aromatic rings in the tolane scaffold were twisted at 79.4° (Figure 6c). Additionally, the phenothiazine unit was twisted at 87.9° with respect to the aromatic ring in the tolane scaffold. As shown in Figure 5c, because of the twisted tolane scaffold, the tolane molecules were tightly packed with a short interfacial distance (C···C: 336 pm) between the electron-deficient fluorinated aromatic ring and electron-rich phenothiazine unit. Additional intermolecular interactions involve S···F interactions (short contact: 327 pm) between sulfur in the phenothiazine unit and fluorine atoms and π···F interactions (short contact: 344 pm) between the phenothiazine unit attached to the electron-rich aromatic ring and fluorine atoms (S: 180 pm) [36].

PTZ-H forms crystal structures in the triclinic P–1 space group, and the unit cell contains four molecular units. In contrast to CBZ- H, PIP-F, and PTZ-F, PTZ-H exhibited two conformational structures: one of the two aromatic rings in the tolane scaffold of conformer A possessed a twisted structure of 20.4°, whereas conformer B was twisted at 60.7°. In both conformers, the phenothiazine unit was twisted at 63.8° for A and 81.0° for B with respect to the aromatic ring in the tolane scaffold (Figure 6d). As shown in Figure 5d, the more twisted B intermolecularly formed loose π/π stacking with a short contact of 346 pm. Additional π/π stacking (C···C 338 pm) between the two phenothiazine units is also observed in the packing structures. Moreover, CH···π interactions (interatomic distance: 283 and 299 pm) between the carbon and hydrogen atoms between the two phenothiazine units were confirmed intermolecularly.

From the above crystal structure, the following points can be reasonably explained:

(i)

The Φ_PL of CBZ-H in the crystalline state is lower than that in the solution state because of the promotion of non-radiative deactivation through intermolecular energy transfer via tight π/π stacking.

(ii)

The increase in the Φ_PL of PIP-F and PTZ-F in the crystalline state compared to that in the solution state is due to the effective suppression of non-radiative deactivation and the formation of rigid molecular aggregated structures. This is supported by the significant reduction in the nonradiative rate constant (k_nr) in the crystalline state, rather than in the solution state.

(iii)

Comparing PL decay among the three fluorinated tolanes, viz., CBZ-F, PIP-F, and PTZ-F, however, irregular PL decay profiles were observed depending on the donor moiety. This can be attributed to the significant alteration of molecular aggregated structures in the crystalline state. Thus, the retardation of PL in case of CBZ-F and PTZ-F crystals is due to the dimer-like molecular packing induced by strong CT interactions between the electron-donating aromatic ring-substituted amine-donor moieties and the electron-withdrawing fluorinated aromatic rings. In contrast, PIP-F crystals demonstrated rapid PL decay due to structural relaxation of the donor moiety, which is similar to solution-state PL decay.

(iv)

The red-shift of the PL band of PTZ-F in the crystalline state compared to that in the solution state is due to the formation of dimer-like molecular packing structures through tight π/π stacking caused by the orthogonal molecular structure characteristic of PTZ-F [15]. This is supported by the significantly delayed τ in PTZ-F compared with that in CBZ-F and PIP-F (Figure S19).

2.4. Aggregation-Induced Emission Behavior

The phenomenon in which the Φ_PL of CBZ-F decreases in polar solvents, as shown in Table S9, can be applied to the aggregation-induced emission (AIE) phenomenon of CBZ-F. In addition, the Φ_PL of PIP-F is enhanced in the crystalline state and not in its solution state. Therefore, we assumed that CBZ-F and PIP-F show aggregation-induced emission (AIE) behavior as a selected example. Therefore, we evaluated the AIE behavior of CBZ-F and PIP-F by dissolving their crystalline samples in THF. Subsequently, their PL intensity and Φ_PL were assessed in a mixed solution with varying proportions of THF and water. Figure 7 shows the AIE behavior of CBZ-F and PIP-F.

CBZ-F showed an intense PL with a high Φ_PL (0.64) in the THF solution. When the addition ratio of the water fraction was gradually increased, the PL intensity of CBZ-F gradually decreased with decreasing Φ_PL (Figure 7a,b), which was due to the significant acceleration of the non-radiative deactivation process induced by the increasing solvent polarity. When 60% water was added, the PL intensity was reduced to approximately one fifty-third of that of 100% THF (Figure 7a). Molecular aggregation was initiated by the addition of water (Figure S22), and a gradual increase in the PL intensity was observed, resulting in a yellow PL. When the water fraction reached 85%, the PL intensity and Φ_PL recovered to almost the same value (0.59) as that of 100% THF (Figure 7b). The addition of more than 85% water reduced the PL efficiency owing to the polar effect of the solvent and the quenching effect caused by aggregation. In contrast to CBZ-F, PIP-F displayed very little PL in the water fraction range of 0–60 %. However, when the water ratio reached 70%, the PL intensity dramatically increased after forming molecular aggregates (Figure S22), and the maximum PL intensity with the highest Φ_PL (0.08) was achieved when the water fraction reached 80% (Figure 7c,d). Accordingly, both CBZ-F and PIP-F showed AIE characteristics, which could pave the way for the development of promising solid-state yellow-luminescent materials. Analyzing the PL behavior in THF and polyethylene glycol (PEG) mixed solvent, increasing the PEG fraction, the PL intensity and Φ_PL monotonically decreased and a slight long-wavelength shift was observed for CBZ-F (Figure S21a), which is due to the solute–solvent interaction induced by increasing solvent polarity. On the other hand, the PL intensity and Φ_PL of PIP-F decreased monotonically below 50% PEG fraction, but increased gradually when the PEG fraction exceeded 60% (Figure S21b), which is due to restriction of intramolecular motion (RIM) caused by increasing solvent viscosity. Judging from the above results, the AIE phenomenon of CBZ-F is considered to be mainly caused by the intermolecular H···F hydrogen bonds, not the dynamic structural relaxation effect, whereas the AIE phenomenon of PIP-F is considered to be caused by the RIM mechanism, in which the rotational motion of the piperidyl group is suppressed.

3. Materials and Methods

3.1. General Method

All reactions were performed using dried glassware and magnetic stirrer bars. All chemicals were of reagent grade and purified in the usual manner prior to use. Wakogel^® 60 N, 38–100 μm), and TLC analysis was performed on silica gel TLC plates (Merck, Silica gel 60F₂₅₄). ¹H and ¹³C NMR spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer (¹H: 400 MHz and ¹³C: 100 MHz) in chloroform-d (CDCl₃), and chemical shifts were reported in parts per million (ppm) using the residual proton in the NMR solvent. ¹⁹F NMR (376 MHz) spectra were obtained using a Bruker AVANCE III 400 NMR spectrometer in CDCl₃, and CFCl₃ (δ_F = 0.0 ppm) or hexafluorobenzene (δ_F = −163 ppm) were used as internal standards. Infrared spectra (IR) spectra were recorded using the KBr method with a JASCO FT/IR-4100 type A spectrometer; all spectra were reported in wavenumber (cm⁻¹). High-resolution mass spectra (HRMS) were recorded on a JEOL JMS-700MS spectrometer using the fast atom bombardment (FAB) method.

3.2. Synthesis

3.2.1. Typical Procedure for the Nucleophilic Aromatic Substitution Reaction of Lithium 4-Carbazol-9-ylphenylacetylide with Pentafluorobenzonitrile

In a two-necked round-bottomed flask, 4-carbazol-9-ylphenylacetylene (1.1 g, 4.2 mmol) was added to THF (40 mL), and the mixture was cooled to 0 °C. n-butyllithium (1.6 mol L⁻¹ hexane solution, 3.9 mL, 6.2 mmol) was added dropwise to this mixture, and the resulting solution was continuously stirred at 0 °C for 0.5 h. Pentafluorobenzonitrile (0.80 mL, 6.2 mmol) was then added dropwise to the solution. After the addition was complete, the reaction mixture was warmed to 25 °C and stirred for an additional 18 h. The reaction mixture was then poured into a saturated aqueous NH₄Cl solution, the crude product was extracted with ethyl acetate (EtOAc) three times, and the combined organic layer was washed with brine. The organic layer was dried over anhydrous sodium sulfate (Na₂SO₄), which was then separated by filtration. The filtrate was evaporated in vacuo and purified by silica-gel column chromatography (eluent: hexane/EtOAc = 10/1), followed by recrystallization from a mixed solvent system (hexane/CHCl₃ = 1/1), which afforded the corresponding fluorinated tolane, CBZ-F, in 15% isolated yield (0.27 g, 0.61 mmol) as green crystals.

4-(2-(4-Carbazol-9-ylphenyl)ethynyl)-2,3,5,6-Tetrafluorobenzonitrile (CBZ-F)

Yield: 15% (green crystal); M.p.: 210.9–213.1 °C; ¹H NMR (CDCl₃): δ 7.33 (t, J = 7.9 Hz, 2H), 7.49–7.42 (m, 4H), 7.68 (d, J = 8.6 Hz, 2H), 7.86 (d, J = 8.6 Hz, 2H), 8.15 (d, J = 7.7 Hz, 2H); ¹³C NMR (CDCl₃): δ 74.5, 93.8, 106.1, 107.5, 109.8, 111.1, 119.4, 120.1, 120.7, 123.9, 126.4, 127.0, 134.0, 140.0, 140.3, 146.7 (dm, J = 265.6 Hz), 147.2 (dm, J = 266.8 Hz); ¹⁹F NMR (CDCl₃, CFCl₃); δ −134.6 (m, 2F), −133.9 (m, 2F); IR (KBr) ν 3056, 2222, 1647, 1602, 1559, 1520, 1489, 1450, 1356, 1335, 1242, 1188, 1077, 984 cm⁻¹; HRMS: (FAB+) m/z [M]⁺ calcd for C₂₇H₁₂F₄N₂: 440.0937, Found: 440.0934.

4-(2-(4-Piperidylphenyl)ethynyl)-2,3,5,6-Tetrafluorobenzonitrile (PIP-F)

Yield: 45% (yellow crystal); M.p.: 182.4–184.1 °C; ¹H NMR (CDCl₃): δ 1.55–1.66 (m, 6H), 3.32 (t, J = 4.8 Hz, 4H), 6.86 (d, J = 8.8 Hz, 2H), 7.46 (d, J = 8.8 Hz, 2H); ¹³C NMR (CDCl₃): δ 24.4, 25.5, 48.9, 73.2, 92.0, 107.9, 108.6, 109.6, 112.3, 114.4, 133.8, 146.1 (dm, J = 251.2 Hz), 147.1 (dm, J = 258.4 Hz), 152.7; ¹⁹F NMR (CDCl₃): δ −136.05 (m, 2F), −134.90 (m, 2F); IR (KBr): δ 3286, 3044, 2947, 2845, 2567, 2480, 2242, 2211, 1896, 1597, 1542, 5121, 1492, 1458, 1426, 1335, 1243, 1168, 1001, 930 cm⁻¹; HRMS: (FAB+) m/z [M]⁺ calcd for C₂₇H₁₄F₄N₂: 358.1093; found: 358.1099.

4-(2-(4-Phenothiazin-10-ylphenyl)ethynyl)-2,3,5,6-Tetrafluorobenzonitrile (PTZ-F)

Yield: 18% (orange crystal); M.p.: 201.3–201.9 °C; ¹H NMR (CDCl₃): δ 6.84 (d, J = 8.0 Hz, 2H), 7.04 (t, J = 7.7 Hz, 2H), 7.12 (t, J = 7.7 Hz, 2H), 7.24 (d, J = 8.8 Hz, 2H), 7.27 (d, J = 8.8 Hz, 2H), 7.62 (d, J = 8.8 Hz, 2H); ¹³C NMR (CDCl₃): δ 74.1, 93.3, 107.1, 116.4, 121.1, 121.9, 123.0, 124.2, 124.5, 124.8, 127.2, 127.9, 128.1, 134.2, 142.4, 146.52 (dm, J = 259.6 Hz), 145.8, 147.2 (dm, J = 259.3 Hz); ¹⁹F NMR (CDCl₃, C₆F₆): δ −134.90 (m, 2F), −135.08 (m, 2F); IR (KBr): δ 3066, 3008, 2260, 2246, 2230, 2027, 1932, 1605, 1559, 1512, 1457, 1443, 1431, 1412, 1405, 1310, 1164, 1017 cm⁻¹; HRMS: (FAB+) m/z [M]⁺ calcd for C₂₇H₁₂F₄N₂S: 472.0657; found: 472.0651.

3.2.2. Typical Procedure for the Pd(0)-Catalyzed Sonogashira Cross-Couplilng Reaction of 4-Carbazol-9-ylphenylacetylene with 4-Bromobenzonitrile

In a two-necked round-bottomed flask were added 4-carbazol-9-ylphenylacetylene (1.3 g, 5.0 mmol), dichlorobis(triphenylphosphine)palladium(0) (0.18 g, 0.25 mmol), triphenylphosphine (0.066 g, 0.25 mmol), copper(I) iodide (0.095 g, 0.50 mmol), 4-bromobenzonitrile (1.1 g, 6.0 mmol), and triethylamine (15 mL). The resultant mixture was stirred at 80 °C for 20 h. The precipitate formed during the reaction was separated by atmospheric filtration, and the filtrate was poured into a saturated aqueous NH₄Cl solution. The crude product was extracted three times with ethyl acetate (EtOAc), and the combined organic layer was washed once with brine. The organic layer was dried over anhydrous sodium sulfate (Na₂SO₄), which was separated by filtration. The filtrate was evaporated in vacuo and purified by silica-gel column chromatography (eluent: hexane/EtOAc = 20/1), followed by recrystallization from a mixed solvent system (hexane/CH₂Cl₂ = 1/1) to afford the corresponding fluorinated tolane, CBZ-H, in 38% isolated yield (0.71 g, 1.9 mmol) as colorless crystals.

4-(2-(4-Carbazol-9-ylphenyl)ethynyl)benzonitrile (CBZ-H)

Yield: 38% (colorless crystal); M.p.: 221.1–222.0 °C; ¹H NMR (CDCl₃): δ 7.33 (t, J = 3.2 Hz, 2H), 7.41–7.47 (m, 4H), 7.63 (d, J = 8.8 Hz, 2H), 7.65–7.69 (m, 4H), 7.78 (d, J = 8.8 Hz, 2H), 8.16 (d, J = 8.8 Hz, 2H); ¹³C NMR (CDCl₃): δ 88.7, 93.1, 109.8, 111.9, 118.6, 120.5, 120.6, 121.2, 123.8, 126.3, 127.0, 128.1, 132.3, 133.5, 135.6, 149.6; IR (KBr) ν 3067, 3042, 2537, 2230, 2215, 1911, 1541, 1479, 1405, 1366, 1271, 1150, 1003 cm⁻¹; HRMS (FAB) m/z [M]⁺ calcd for (M⁺) C₂₇H₁₆N₂: 368.1313, Found: 368.1321.

4-(2-(4-Piperidylphenyl)ethynyl)-2,3,5,6-Tetrafluorobenzonitrile (PIP-H)

Yield: 8% (colorless crystal); M.p.: 188.0–190.6 °C; ¹H NMR (CDCl₃): δ 1.62–1.69 (m, 6H), 3.26 (t, J = 2.8 Hz, 4H), 6.86 (d, J = 8.8 Hz, 2H), 7.40 (d, J = 8.8 Hz, 2H), 7.55–7.60 (m, 4H); ¹³C NMR (CDCl₃): δ 24.5, 25.6, 49.4, 86.6, 95.5, 110.7, 111.0, 115.1, 118.8, 129.3, 131.8, 132.1, 133.1, 152.1 ; IR (KBr): ν 3399, 3091, 3042, 2942, 2855, 2222, 2209, 2167, 1891, 1685, 1606, 1543, 1466, 1451, 1445, 1425, 1407, 1262, 1197, 1026 cm⁻¹; HRMS: (FAB+) m/z [M]⁺ calcd for C₂₇H₁₄F₄N₂: 268.1470; found: 268.1473.

4-(2-(4-Phenothiazin-10-ylphenyl)ethynyl)benzonitrile (PTZ-H)

Yield: 38% (white crystal); M.p.: 174.3–175.2 °C; ¹H NMR (CDCl₃): δ 6.53 (d, J = 8.0 Hz, 2H), 6.94 (t, J = 7.4 Hz, 2H), 7.00 (t, J = 8.4 Hz, 2H), 7.15 (d, J = 8.8 Hz, 2H), 7.32 (d, J = 8.4 Hz, 2H), 7.61–7.69 (m, 6H); ¹³C NMR (CDCl₃): δ 88.6, 93.3, 111.7, 118.6, 118.9, 120.3, 123.7, 124.1, 127.1, 127.2, 127.5, 128.2, 132.2, 132.2, 134.0, 143.1, 143.3 ; IR (KBr): ν 3085, 3075, 3069, 3060, 3042, 3021, 3016, 3007, 2229, 2176, 1921, 1793, 1457, 1405, 1379, 1270, 1188, 1156, 1064 cm⁻¹; HRMS: (FAB+) m/z [M]⁺ calcd for C₂₇H₁₆N₂S: 400.1034; found: 400.1038.

3.3. Single Crystal X-ray Diffraction

Single-crystal X-ray diffraction spectra were recorded using an XtaLAB AFC11 diffractometer (Rigaku, Tokyo, Japan). The reflection data were integrated, scaled, and averaged using the CrysAlisPro program (ver. 1.171.39.43a; Rigaku Corporation, Akishima, Japan). Empirical absorption corrections were applied using the SCALE 3 ABSPACK scaling algorithm (CrysAlisPro). The structures were identified by a direct method (SHELXT-2018/2 [37]) and refined using the full matrix least-squares method (SHELXL-2018/3 [38]) visualized by Olex2 [39]. Crystallographic data were deposited in the Cambridge Crystallographic Data Centre (CCDC) database (CCDC 2151065 for PIP-F, 2151066 for CBZ-H, 2151067 for PTZ-H, and 2151068 for PTZ-F). These data can be obtained free of charge from the CCDC at www.ccdc.cam.ac.uk/data_request/cif (accessed on 4 September 2022).

3.4. Photophysical Properties

UV–vis absorption spectra were recorded using a JASCO V-750 absorption spectrometer (JASCO, Tokyo, Japan). The PL spectra of the solutions were measured using a FP-6600 fluorescence spectrometer (JASCO, Tokyo, Japan). Photoluminescence quantum yields were measured using a Quantaurus-QY C11347-01 instrument (Hamamatsu Photonics, Hamamatsu, Japan). The PL lifetime was measured using a Quantaurus-Tau fluorescence lifetime spectrometer (C11367-34, Hamamatsu Photonics, Japan).

3.5. Theoretical Calculation

All computations were performed using density functional theory (DFT) at the M06-2X hybrid functional [29] and 6-31 + G(d,p) (for all atoms) basis set with a conductor-like polarizable continuum model (CPCM) [30] for CH₂Cl₂ using the Gaussian 16 program package [28]. Theoretical vertical transitions were also calculated using a time-dependent DFT (TD-DFT) method at the same level of theory using the same solvation model.

4. Conclusions

We synthesized three types of fluorinated tolanes with amine-based donors and evaluated their photophysical behavior in detail. The carbazole-substituted fluorinated tolane showed intense yellow PL in its dilute solution state; however, piperidine or a phenothizine-substituted tolanes exhibited a significant decrease in PL efficiency (Φ_PL), which was caused by the twisted intramolecular charge transfer (TICT) process or an immediate internal conversion of the tolane scaffold from ππ* to dark πσ* excited states. Interestingly, fluorinated tolanes exhibited PL in the crystalline state. Moreover, fluorinated tolanes with piperidine or phenothiazine substituents showing weak PL properties in solution were found to show improved PL efficiencies in their crystalline states, leading to yellow-to-orange PL. Crystal structure analysis shows that the tight packing structures formed in the crystal are attributable to the significant suppression of non-radiative deactivation, leading to a dramatic improvement in Φ_PL. Furthermore, the Φ_PL of the non-fluorinated tolanes did not increase in their crystalline state; this suggests that the introduction of fluorine atoms into the tolane scaffold strongly influences the formation of aggregated structures via H···F hydrogen bonds. In addition, we confirmed that the photophysical behavior of fluorinated tolanes showed aggregation-induced emission behavior. These findings provide guidelines for the molecular design of new solid-state luminescent materials, and we believe that functional materials based on this molecular design approach will emerge in the near future.