Tuning of Thermally Activated Delayed Fluorescence Properties in the N,N-Diphenylaminophenyl–Phenylene–Quinoxaline D–π–A System

Abstract

Thermally activated delayed fluorescence (TADF) often achieves high device efficiencies in organic light-emitting diodes. Here we develop TADF dyes, 1-H and 1-Me, based on an N,N-diphenylaminophenyl–phenylene–quinoxaline donor–π–acceptor system, which contains an unsubstituted 1,4-phenylene and a 2,5-dimethyl-1,4-phenylene π-spacer, respectively. In UV–vis absorption spectra in toluene at room temperature, 1-H showed a relatively intense shoulder band at 400 nm, whereas 1-Me had a weak, blue-shifted shoulder at 386 nm, indicating 1-Me adopts a more twisted π-conjugation system. On the other hand, the photoluminescence (PL) wavelength of 1-Me (λ_PL; 558 nm) under the same conditions was slightly red-shifted in comparison with that of 1-H (λ_PL; 552 nm), due to larger structural relaxation of 1-Me. From PL lifetime measurements, both the dyes showed TADF in 10 wt%-doped poly(methyl methacrylate) film, and their PL quantum yields were moderate (Φ_PL; ca. 0.5 at 300 K). As for the photokinetics, 1-Me exhibited larger rate constants for intersystem crossing and reverse intersystem crossing than 1-H due to the small excited-state singlet–triplet energy gap (ΔE_ST) of 1-Me. Furthermore, theoretical calculations indicated the triplet state of 1-Me is destabilized by localization of the spin density, resulting in the reduced ΔE_ST to facilitate TADF.

1. Introduction

Organic light-emitting diodes (OLEDs) [1,2] have been applied to color elements in flat panel displays of television sets and smartphones due to their technical advantages such as flexibility, thinness, light weight, high device efficiency, surface emission, and self-emission [3,4,5,6,7,8,9,10]. One of the important constituents determining the characteristics of OLEDs is the emitter used in the emission layer. In the case of OLEDs, holes and electrons are injected from the anode and the cathode, respectively, into the emission layer and recombine to generate singlet and triplet excitons of the emitting material in a ratio of 1:3 according to the spin statistics theorem [11]. The devices with conventional fluorescent emitters do not utilize the triplet excitons, and hence the internal quantum efficiency (IQE) is limited to a theoretical maximum of 25%. To overcome the limitation for fluorescent emitters, the utilization of thermally activated delayed fluorescence (TADF) has been suggested since 2012 by Adachi and coworkers as a promising photophysical process to harvest the triplet excitons, achieving high IQEs up to 100% [12,13,14,15,16,17,18,19,20,21,22,23]. In the case of TADF emitters, the upconversion of the triplet exciton to the singlet excited state via the reverse intersystem crossing (RISC) is a crucial process. It is well known that an extremely small excited-state singlet–triplet energy gap (ΔE_ST) leads to efficient RISC as expressed in Equation (1) [24]:

$$k_{\mathrm{R}\mathrm{I}\mathrm{S}\mathrm{C}}\propto {\xi }_{\mathrm{S}\mathrm{T}}^2\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{-\Delta E_{\mathrm{S}\mathrm{T}}}{k_{\mathrm{B}}T}}\right)$$

(1)

where k_RISC, ξ_ST, k_B, and T denote the RISC rate constant, the spin–orbit coupling constant, the Boltzmann constant, and absolute temperature, respectively.

To design fluorescent emitters with small ΔE_ST, twisted donor (D)–acceptor (A) dyes have been developed because the spatial separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) effectively reduces ΔE_ST as expressed in Equation (2) [25,26,27]:

$$\Delta E_{\mathrm{S}\mathrm{T}}=E_{\mathrm{S}}-E_{\mathrm{T}}=2J=2\langle {\phi }_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}\left(1\right){\phi }_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}\left(2\right)\left|{\displaystyle \frac{e^2}{r_{12}}}\right|{\phi }_{\mathrm{H}\mathrm{O}\mathrm{M}\mathrm{O}}\left(2\right){\phi }_{\mathrm{L}\mathrm{U}\mathrm{M}\mathrm{O}}\left(1\right)\rangle$$

(2)

where E_S and E_T are the energy levels of the lowest excited singlet (S₁) and triplet (T₁) states, respectively, J is the singlet–triplet electron exchange energy, φ_HOMOφ_LUMO denotes the HOMO–LUMO overlap, e is the elementary charge, and r₁₂ is the distance between the positive and negative charge centers. In general, it is empirically known that efficient TADF requires a ΔE_ST of less than 0.2 eV [28,29].

We recently reported that an intramolecular charge transfer (ICT) dye (2, Figure 1), consisting of an N,N-diphenylaminophenyl–6,7-difluoro-3-(trifluoromethyl)quinoxaline donor–acceptor system, showed TADF behavior in poly(methyl methacrylate) (PMMA) film under nitrogen atmosphere (photoluminescence (PL) wavelength λ_PL; 539 nm, PL quantum yield Φ_PL; 0.77) [30]. However, the contribution of TADF to the overall emission was limited to ca. 10% due to a relatively large ΔE_ST of 0.20 eV. To achieve more efficient TADF, further reduction in the ΔE_ST is required. In this work, we designed and synthesized a new D–π–A ICT dye 1-H, in which a 1,4-phenylene spacer was inserted between the N,N-diphenylaminophenyl donor unit and the quinoxaline acceptor unit of 2 (Figure 1). In addition, 1-Me, incorporating a 2,5-dimethyl-1,4-phenylene spacer in place of the 1,4-phenylene spacer in 1-H, was synthesized to adopt a more twisted conformation due to enhanced steric hindrance between the trifluoromethyl and methyl substituents. Here we report the synthesis and PL properties of 1-H and 1-Me, with a particular focus on the influence of the π–spacer on the TADF behavior.

2. Materials and Methods

2.1. General Information

For the synthesis of 1-H and 1-Me, the starting materials, catalysts, and solvents were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan), Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), Kanto Chemical Co. (Tokyo, Japan), Inc., Sigma-Aldrich Co. (St. Louis, MO, USA), or BLD Pharmatech Ltd (Shanghai, China). and used without further purification. Precursor 5-H was prepared according to the reported method [31]. Precursor 7 was also prepared according to the literature [30]. ¹H NMR (400 MHz), ¹³C NMR (101 MHz), and ¹⁹F NMR (376 MHz) spectra were obtained on a JEOL (Akishima, Japan) ECX-400 spectrometer, using tetramethylsilane (0.00 ppm for ¹H NMR), residual CHCl₃ (77.16 ppm for ¹³C NMR), and hexafluorobenzene (−162.9 ppm for ¹⁹F NMR) as internal standards. Electrospray ionization time-of-flight mass spectra (ESI-TOF MS) were measured on a JEOL (Akishima, Japan) JMS-T100LP mass spectrometer using dichloromethane/methanol (1:1, v/v) as solvent. Elemental analyses were carried out on a J-Science Lab (Kyoto, Japan) MICRO CORDER JM10 analyzer.

2.2. Photophysical Measurements

UV–vis absorption spectra were recorded on a Shimadzu (Kyoto, Japan) UV-3600 spectrophotometer. PL spectra were recorded on a Horiba Jobin Yvon (Longjumeau, France) SPEX Fluorolog-3 spectrofluorometer. The PL quantum yields (Φ_PLs) were measured on a Hamamatsu Photonics (Hamamatsu, Japan) C13534-01 absolute PL quantum yield measurement system. PL lifetimes (τs) were obtained on a Horiba Jobin Yvon (Longjumeau, France) FluoroCube spectroanalyzer using a 390 nm nanosecond-order LED light source. The sample solutions for spectral data acquisition were prepared using spectroscopic-grade solvents. PMMA films doped with 1-H and 1-Me were prepared by spin-coating from chloroform solutions onto a quartz plate and dried at 60 °C for 15 min. PL spectra and PL lifetimes at various temperatures were obtained by using a Unisoku (Hirakata, Japan) CoolSpek cryostat.

2.3. Synthesis

• 4’-Bromo-2′,5′-dimethyl-N,N-diphenyl-[1,1′-biphenyl]-4-amine (5-Me)

A mixture of 3 (1.6 g, 6.1 mmol), 4 (1.0 g, 3.5 mmol), 1,4-dioxane (56 mL), potassium carbonate (2.4 g, 17 mmol), and water (14 mL) was saturated with nitrogen through the gas bubbling. Then, under nitrogen atmosphere, Pd(PPh₃)₄ (0.40 g, 3.5 mmol) was added, and the mixture was stirred at 110 °C for 40 h. After cooling to room temperature, the solvent was removed, and water (60 mL) was added to the residue, and extracted with chloroform (60 mL × 3). The combined organic layer was washed with brine and dried over anhydrous magnesium sulfate. After the solvent was removed, the crude product was purified by silica gel column chromatography (chloroform/hexane = 1:7, v/v) to give the titled compound as a colorless powder (1.1 g, 77%); ¹H NMR (400 MHz, CDCl₃) δ 7.43 (s, 1H), 7.27 (t, ³J = 7.3 Hz, 4H), 7.07–7.16 (m, 9H), 7.03 (t, ³J = 7.3 Hz, 2H), 2.38 (s, 3H), 2.26 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ 147.66, 146.73, 140.65, 134.91, 134.74, 134.67, 133.71, 132.02, 129.77, 129.26, 124.43, 123.15, 123.08, 122.91, 22.26, 19.85; ESI-TOF MS (m/z) calcd for C₂₆H₂₂BrN⁺ ([M]⁺): 427.0936; Found: 427.0921. Anal. Calcd for C₂₆H₂₂BrN: C, 72.90; H, 5.18; N, 3.27. Found: C, 72.95; H, 5.33; N, 3.34.

• N,N-Diphenyl-4′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-[1,1′-biphenyl]-4-amine (6-H)

A mixture of 5-H (0.33 g, 0.81 mmol), bis(pinacolato)diboron ((Bpin)₂, 0.21 g, 0.82 mmol), potassium acetate (0.24 g, 2.5 mmol), and dry 1,4-dioxane (15 mL) was saturated with nitrogen through the gas bubbling. Then, under nitrogen atmosphere, Pd(dppf)Cl₂∙CH₂Cl₂ (0.034 g, 0.042 mmol) was added, and the mixture was stirred at 100 °C for 6 h. After cooling to room temperature, the mixture was poured into water (50 mL) and extracted with ethyl acetate (50 mL × 3). The combined organic layer was washed with brine and dried over anhydrous magnesium sulfate. After the solvent was removed, the crude product was purified by silica gel column chromatography (chloroform/hexane = 1:1, v/v) to give the titled compound as a colorless powder (0.16 g, 45%); ¹H NMR (400 MHz, CDCl₃) δ 7.87 (d, ³J = 8.2 Hz, 2H), 7.59 (d, ³J = 8.2 Hz, 2H), 7.51 (d, ³J = 8.7 Hz, 2H), 7.27 (t, ³J = 7.3 Hz, 4H), 7.14 (m, 6H), 7.04 (t, ³J = 7.3 Hz, 2H), 1.37 (s, 12H); ¹³C NMR (101 MHz, CDCl₃) δ 147.58, 147.44, 143.27, 135.23, 134.71, 129.26, 127.83, 125.87, 124.45, 123.72, 122.97, 83.75, 24.87; ESI-TOF MS (m/z) calcd for C₃₀H₃₁BNO₂⁺ ([M+H]⁺): 448.2453; Found: 448.2409. Anal. Calcd for C₃₀H₃₀BNO₂: C, 80.54; H, 6.76; N, 3.13. Found: C, 80.63; H, 7.04; N, 3.17.

• 2′,5′-Dimethyl-N,N-diphenyl-4′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-[1,1′-biphenyl]-4-amine (6-Me)

Precursor 6-Me was prepared as a colorless powder (0.25 g, 57%) according to a similar method for the preparation of 6-H, where 5-Me (0.40 g, 0.93 mmol) was used as a starting material in place of 5-H; ¹H NMR (400 MHz, CDCl₃) δ 7.67 (s, 1H), 7.27 (t, ³J = 7.3 Hz, 4H), 7.19 (d, ³J = 8.2 Hz, 2H), 7.14 (d, ³J = 7.3 Hz, 4H), 7.09 (d, ³J = 8.2 Hz, 2H), 7.08 (s, 1H), 7.02 (t, ³J = 7.3 Hz, 2H), 2.53 (s, 3H), 2.29 (s, 3H), 1.35 (s, 12H); ¹³C NMR (101 MHz, CDCl₃) δ 147.73, 146.53, 143.84, 142.23, 138.01, 135.90, 131.45, 131.35, 129.82, 129.22, 126.76, 124.34, 123.14, 122.79, 83.33, 24.87, 21.66, 19.84; ESI-TOF MS (m/z) calcd for C₃₂H₃₅BNO₂⁺ ([M+H]⁺): 476.2766; Found: 476.2729. Anal. Calcd for C₃₂H₃₄BNO₂: C, 80.84; H, 7.21; N, 2.95. Found: C, 80.85; H, 7.19; N, 3.12.

• 4′-(6,7-Difluoro-3-(trifluoromethyl)quinoxalin-2-yl)-N,N-diphenyl-[1,1′-biphenyl]-4-amine (1-H)

A mixture of 6-H (0.12 g, 0.26 mmol), 7 (0.065 g, 0.24 mmol), 1,2-dimethoxyethane (4.8 mL), tripotassium phosphate (0.13 g, 0.60 mmol), and water (2.4 mL) was saturated with nitrogen through the gas bubbling. Then, under nitrogen atmosphere, Pd(PPh₃)₄ (0.014 g, 0.012 mmol) was added, and the mixture was stirred at 100 °C for 2 h. After cooling to room temperature, the mixture was poured into water (10 mL) and extracted with dichloromethane (20 mL × 3). The combined organic layer was washed with brine and dried over anhydrous sodium sulfate. After the solvent was removed, the crude product was purified by silica gel column chromatography (chloroform/hexane = 1:1, v/v) to give the titled compound as a yellow powder (0.13 g, 99%); ¹H NMR (400 MHz, CDCl₃) δ 8.02 (dd, ³J_H–F = 9.9 Hz, ⁴J_H–F = 8.0 Hz, 1H), 7.97 (dd, ³J_H–F = 10.3 Hz, ⁴J_H–F = 8.0 Hz, 1H), 7.74 (d, ³J_H–H = 8.2 Hz, 2H), 7.69 (d, ³J_H–H = 8.2 Hz, 2H), 7.55 (d, ³J_H–H = 8.7 Hz, 2H), 7.29 (t, ³J_H–H = 7.3 Hz, 4H), 7.13–7.19 (m, 6H), 7.06 (t, ³J_H–H = 7.3 Hz, 2H); ¹³C NMR (101 MHz, CDCl₃) δ 154.26 (dd, ¹J_C–F = 261.18 Hz, ²J_C–F = 16.29 Hz), 153.16 (dd, ¹J_C–F = 259.74 Hz, ²J_C–F = 16.29 Hz), 152.63 (d, ³J_C–F = 2.88 Hz), 147.74, 147.46, 142.05, 141.34 (q, ²J_C–F = 34.98 Hz), 140.33 (d, ³J_C–F = 11.50 Hz), 136.63 (d, ³J_C–F = 11.50 Hz), 134.90, 133.56, 129.29, 128.35, 127.80, 126.48, 124.58, 123.52, 123.12, 121.19 (q, ¹J_C–F = 276.04 Hz), 115.25 (d, ²J_C–F = 17.25 Hz), 114.88 (d, ²J_C–F = 17.25 Hz); ¹⁹F NMR (376 MHz, CDCl₃) δ −62.87, −125.56, −127.84; ESI-TOF MS (m/z) calcd for C₃₃H₂₀F₅N₃⁺ ([M]⁺): 553.1577; Found: 553.1609. Anal. Calcd for C₃₃H₂₀F₅N₃: C, 71.61; H, 3.64; N, 7.59. Found: C, 71.27; H, 3.76; N, 7.59.

• 4′-(6,7-Difluoro-3-(trifluoromethyl)quinoxalin-2-yl)-2′,5′-Dimethyl-N,N-diphenyl-[1,1′-biphenyl]-4-amine (1-Me)

Dye 1-Me was prepared as a yellow powder (0.051 g, 47%) according to a similar method to the preparation of 1-H, where 6-Me (0.089g, 0.19 mmol) was used as a starting material in place of 5. After silica gel column chromatography, 1-Me was further purified by removing residual 6-Me under vacuum; ¹H NMR (400 MHz, CDCl₃) 8.04 (dd, ³J_H–F = 9.9 Hz, ⁴J_H–F = 8.2 Hz, 1H), 7.95 (dd, ³J_H–F = 10.1 Hz, ⁴J_H–F = 8.2 Hz, 1H), 7.28 (t, ³J_H–H = 7.3 Hz, 4H), 7.22–7.26 (m, 3H), 7.11–7.17 (m, 7H), 7.04 (t, ³J_H–H = 7.3 Hz, 2H), 2.32 (s, 3H), 2.07 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ 154.29 (dd, ¹J_C–F = 261.18 Hz, ²J_C–F = 16.29 Hz), 153.45 (d, ³J_C–F = 2.88 Hz), 153.37 (dd, ¹J_C–H = 259.74 Hz, ²J_C–H = 16.29 Hz), 147.72, 146.78, 142.73, 142.10 (q, ²J_C–H = 37.38 Hz), 140.37 (d, ³J_C–F = 11.50 Hz), 136.93 (d, ³J_C–F = 10.54 Hz), 135.21, 134.29, 133.32, 132.57, 131.81, 130.47, 129.94, 129.27, 124.44, 123.11, 122.91, 121.03 (q, ¹J_C–F = 276.04 Hz), 115.43 (d, ²J_C–F = 17.25 Hz), 114.98 (d, ²J_C–F = 17.25 Hz), 20.13, 19.14; ¹⁹F NMR (376 MHz, CDCl₃) δ −64.90, −125.50, −127.75; ESI-TOF MS (m/z) calcd for C₃₅H₂₄F₅N₃⁺ ([M]⁺): 581.1890; Found: 581.1896. Anal. Calcd for C₃₅H₂₄F₅N₃: C, 72.28; H, 4.16; N, 7.23. Found: C, 72.53; H, 4.39; N, 7.32.

3. Results and Discussion

3.1. Synthesis of 1-H and 1-Me

The synthesis of 1-H and 1-Me is shown in Scheme 1. Precursors 5-H [31] and 7 [30] were prepared according to the reported methods. Precursor 5-Me was obtained by the Suzuki–Miyaura cross-coupling reaction of 3 and 4. Precursors 5-H and 5-Me were then borylated by the Miyaura–Ishiyama borylation using (Bpin)₂ to obtain 6-H and 6-Me, respectively. Then, 1-H and 1-Me were obtained by the Suzuki–Miyaura cross-coupling reaction of 7 with 6-H and 6-Me, respectively. The characterization of 1-H and 1-Me was carried out by ¹H, ¹³C, and ¹⁹F NMR spectroscopy, ESI-TOF MS spectrometry, and elemental analysis. The ¹H, ¹³C, and ¹⁹F NMR spectra of 5-Me, 6-H, 6-Me, 1-H, and 1-Me are shown in Figures S1–S5.

3.2. Photophysical Properties in Toluene

We firstly evaluated UV–vis absorption properties of 1-H and 1-Me in toluene at room temperature. The spectra are shown in Figure 2a, and the spectral data such as absorption peaks (λ_abs) and molar absorption coefficients (ε_abs) are summarized in

Table 1. UV–vis absorption and PL properties of 1-H and 1-Me in toluene under various conditions.

Compd.	λabs (nm) [εabs (mol−1 L cm−1)]	λPL (nm)	ΦPL [Air/N2]	CIE 2 (x, y)	τ1 (ns) [A1 (%)]/τ2 (μs) [A2 (%)]
					300 K, Air	300 K, N2	360 K, N2
1-H	334 [26,900], 400 [9000] 1	552	0.24/0.55	(0.41, 0.54)	7.5 [100]/-	10 [100]/-	6.1 [100]/-
1-Me	313 [32,0], 386 [1500] 1	558	0.06/0.21	(0.42, 0.50)	8.0 [96.7]/0.15 [3.3]	24 [88.7]/3.7 [11.3]	17 [87.5]/1.9 [12.5]

¹ Obtained by peak deconvolution using Gaussian functions. ² Commission internationale de l’éclairage (CIE) chromaticity coordinate.

. Dyes 1-H and 1-Me had strong absorption bands at 334 and 313 nm with ε_abss of 26,900 and 32,000 mol⁻¹ L cm⁻¹, respectively, which were assigned to moderate ICT transitions from the donor unit to the π-spacer and acceptor units as shown in Figure S6. In addition, both the dyes showed relatively weak shoulder absorption bands at 400 and 386 nm with ε_abss of 9000 and 1500 mol⁻¹ L cm⁻¹, respectively, attributed to a long-range ICT transition from the donor unit to the acceptor unit. The poor and slightly blue-shifted ICT absorption of 1-Me in comparison with the corresponding ICT absorption of 1-H implied the HOMO–LUMO separation by the largely twisted D–π–A structure of 1-Me as discussed in the section of theoretical calculations.

Contrary to the results for the UV–vis absorption properties, the λ_PL of 1-Me (λ_PL; 558 nm) was red-shifted in comparison with that of 1-H (λ_PL; 552 nm), corresponding to CIE chromaticity coordinates of (0.41, 0.54) for 1-H and (0.42, 0.50) for 1-Me. From the theoretical calculations (vide infra), this result was attributed to the enhancement of ICT by the electron-donating methyl groups on the π-spacer as well as the larger structural relaxation of 1-Me than that of 1-H. The Φ_PL of 1-H was 0.24 in toluene at room temperature under air, while that of 1-Me (Φ_PL; 0.06) was quite low. Under nitrogen atmosphere, the Φ_PLs increased to 0.55 and 0.21 for 1-H and 1-Me, respectively, indicating that the PL decay involves intersystem crossing to and from the triplet states, thus showing characteristics of typical TADF behavior [32,33].

To investigate the TADF properties of 1-H and 1-Me, we conducted PL lifetime measurements in toluene, as shown in Figure 2b, and the data are summarized in Table 1. Dye 1-H showed only a single component PL lifetime in the nanosecond order (τ₁; 7.5 ns) at 300 K under air, assignable to prompt fluorescence. A similar result was obtained under nitrogen atmosphere, although the lifetime was a little bit elongated (τ₁; 10 ns). On the other hand, 1-Me exhibited biexponential PL decay profiles at 300 K under air, consisting of a nanosecond-order lifetime component (τ₁; 8.0 ns) and a sub-microsecond-order one (τ₂; 150 ns), with contributions of A₁ = 96.7% and A₂ = 3.3%, respectively. In contrast, at 300 K under nitrogen atmosphere, τ₂ increased to 3.7 μs, with the enlarged contribution of A₂ = 11.3%. Furthermore, upon raising the temperature from 300 to 360 K, the contribution of τ₂ further increased to A₂ = 12.5%, indicating that 1-Me showed TADF characteristics.

3.3. Photophysical Properties in PMMA Film

We also evaluated the PL properties of 1-H and 1-Me in PMMA film. In 1 wt%-doped PMMA film, 1-H and 1-Me exhibited blue-shifted emission at 527 and 539 nm, respectively. Under air, their Φ_PLs (0.41 and 0.19, respectively) were higher than those in toluene, owing to reduced structural relaxation in the rigid polymer matrix (Figure S7 and Table S1). Furthermore, the Φ_PLs increased under nitrogen, particularly for 1-Me, indicating the involvement of TADF in the observed emission. We attempted to measure the PL lifetime. However, due to the weak emission intensity, reliable measurements could not be conducted. Therefore, the doping concentration was increased to 10 wt%.

In Figure 3a is shown the excitation and PL spectra, and in Figure 3b is shown the PL decay profiles. The PL data including the PL lifetime data are also summarized in

Table 2. Excitation and PL properties of 1-H and 1-Me in 10 wt%-doped PMMA film under various conditions.

Compd.	Condition	λEX (nm)	λPL (nm)	ΦPL	CIE 3 (x, y)	τ1 (ns) [A1 (%)]	τ2 (ns) [A2 (%)]	τ3 (μs) [A3 (%)]	τ4 (μs) [A4 (%)]	τp (ns) 4 [Ap%] 5	τd (μs) 6 [Ad%] 7
1-H	300 K, Air	326, 403 1	547	0.38	(0.39, 0.50)	16 [79.5]	180 [8.0]	2.0 [1.0]	21 [11.5]	-	-
	300 K, N2	-	555	0.55	-	16 [68.3]	200 [7.7]	2.3 [2.2]	32 [21.9]	35 [76.0]	29 [24.0]
	360 K, N2	-	551	0.61 2	-	13 [53.9]	150 [8.0]	2.2 [3.1]	29 [34.9]	26 [61.9]	27 [38.1]
1-Me	300 K, Air	313, 394 1	550	0.23	(0.38, 0.49)	3.0 [69.7]	60 [13.9]	1.8 [5.0]	10 [11.4]	-	-
	300 K, N2	-	554	0.46	-	3.0 [51.0]	70 [19.0]	3.4 [12.7]	17 [17.3]	21 [70.0]	11 [30.0]
	360 K, N2	-	554	0.49 2	-	2.0 [41.3]	60 [19.8]	3.1 [21.7]	16 [17.2]	21 [61.1]	8.8 [38.9]

¹ Obtained by peak deconvolution using Gaussian functions. ² Estimated from the integrated PL intensity relative to that at 300 K. ³ Commission internationale de l’éclairage (CIE) chromaticity coordinate. ⁴ τ_p = (A₁τ₁ + A₂τ₂)/(A₁ + A₂). ⁵ A_p = A₁ + A₂. ⁶ τ_d = (A₃τ₃ + A₄τ₄)/(A₃ + A₄). ⁷ A_d = A₃ + A₄.

. As shown in Figure 3a, the excitation and PL spectra in 10 wt%-doped PMMA film at 300 K under air were slightly red-shifted for both the dyes, suggesting contributions from aggregate formation. In the case of 1-Me, the red shift of λ_PL by 11 nm was smaller than that of 1-H (20 nm), which can be attributed to steric hindrance from the methyl substituents on the π-spacer, preventing close molecular packing. In 10 wt%-doped PMMA film at 300 K under air, 1-H and 1-Me showed Φ_PLs of 0.38 and 0.23, respectively. As observed in toluene and 1 wt%-doped PMMA film, the Φ_PLs significantly increased to 0.55 and 0.46, respectively, under nitrogen atmosphere. PL lifetimes were successfully obtained in 10 wt%-doped PMMA film, providing further insight into the TADF properties. Interestingly, 1-H had delayed fluorescence components (τ₃; 2.0 μs and τ₄; 21 μs) as well as prompt fluorescent components (τ₁; 16 ns and τ₂; 180 ns) at 300 K under air, although it exhibited only prompt fluorescence in toluene. The delayed fluorescence components of 1-H in 10 wt%-doped PMMA film increased in the contribution from A₃ = 1.0% and A₄ = 11.5% under air to A₃ = 2.2% and A₄ = 21.9% under nitrogen atmosphere. The contribution of the delayed fluorescence components further increased to A₃ = 3.1% for τ₃ and A₄ = 34.9% for τ₄ at 360 K under nitrogen atmosphere. Thus, it was demonstrated that 1-H shows TADF in 10 wt%-doped PMMA film, albeit involving aggregate-based emission to some extent. Dye 1-Me also showed two prompt fluorescent components in the nanosecond order and two microsecond-order delayed fluorescence components under air. The delayed fluorescence components were enlarged under a nitrogen atmosphere and at higher temperatures, indicating that TADF is involved in its PL. To simplify the discussion, PL lifetimes were averaged using weights based on their relative contributions, and the averaged prompt and delayed fluorescence lifetimes were represented as τ_p and τ_d, respectively (Table 2) [34]. As a result, the contribution of the delayed fluorescence component (A_d) increased from 24.0 to 38.1% for 1-H and from 30.0 to 38.9% for 1-Me, when the temperature was elevated to 360 K under nitrogen atmosphere.

3.4. Estimation of ΔE_ST

For further discussion about the TADF properties of 1-H and 1-Me, ΔE_STs were estimated from the fluorescence and phosphorescence spectra in glassy 2-methyltetrahydrofuran at 77 K under nitrogen atmosphere. The obtained spectra are shown in Figure 4, and the E_S, E_T, and ΔE_ST are listed in

Table 3. Onset wavelengths of fluorescence and phosphorescence spectra (λ_{flu, onset} and λ_{phos, onset}, respectively), energy levels of the lowest excited singlet and triplet states (E_S and E_T, respectively) of 1-H and 1-Me, and the energy gap between the excited singlet and triplet states (ΔE_ST).

Compd.	λflu, onset (nm)	λphos, onset (nm)	ES (eV) 1	ET (eV) 2	ΔEST (eV) 3
1-H	444	486	2.79	2.55	0.24
1-Me	463	475	2.68	2.61	0.07

¹ E_S (eV) = 1240/λ_{flu, onset}. ² E_T (eV) = 1240/λ_{phos, onset}. ³ ΔE_ST (eV) = E_S (eV) − E_T (eV).

. The fluorescence spectra were recorded immediately after flash excitation without any delay, whereas the phosphorescence spectra were measured with delays of 0.10 ms for 1-H and 0.20 ms for 1-Me to eliminate fluorescence. The values of E_S and E_T were determined from the onsets of fluorescence and phosphorescence spectra, respectively. The onset wavelengths were obtained from the x-intercept of the best-fit line to the rising edge of each spectrum. The E_S of 1-H (E_S; 2.79 eV) was higher than that of 1-Me (E_S; 2.68 eV), while the E_T of 1-H (E_T; 2.55 eV) was more stabilized than that of 1-Me (E_T; 2.61 eV). As a result, it was found that 1-Me possesses a sufficiently small ΔE_ST of 0.07 eV to allow an efficient RISC process. In contrast, 1-H exhibited a moderately large ΔE_ST of 0.24 eV, which is somewhat high for efficient TADF.

3.5. Photophysical Parameters

Based on the Φ_PLs and PL lifetimes of 1-H and 1-Me in 10 wt%-doped PMMA film at 300 K under nitrogen atmosphere, we estimated the photophysical parameters according to Equations (3)–(9) [35,36]:

$$k_{\mathrm{p}}={\Phi }_{\mathrm{p}}/{\tau }_{\mathrm{p}}$$

(3)

$${\Phi }_{\mathrm{P}\mathrm{L},\mathrm{N}2}=k_{\mathrm{p}}/\left(k_{\mathrm{p}}+k_{\mathrm{I}\mathrm{C}}\right)$$

(4)

$${\Phi }_{\mathrm{p}}=k_{\mathrm{p}}/\left(k_{\mathrm{p}}+k_{\mathrm{I}\mathrm{C}}+k_{\mathrm{I}\mathrm{S}\mathrm{C}}\right)$$

(5)

$${\Phi }_{\mathrm{I}\mathrm{C}}=k_{\mathrm{I}\mathrm{C}}/\left(k_{\mathrm{p}}+k_{\mathrm{I}\mathrm{C}}+k_{\mathrm{I}\mathrm{S}\mathrm{C}}\right)$$

(6)

$${\Phi }_{\mathrm{I}\mathrm{S}\mathrm{C}}=1-{\Phi }_{\mathrm{p}}-{\Phi }_{\mathrm{I}\mathrm{C}}$$

(7)

$$k_{\mathrm{d}}={\Phi }_{\mathrm{d}}/{\Phi }_{\mathrm{I}\mathrm{S}\mathrm{C}}{\tau }_{\mathrm{d}}$$

(8)

$$k_{\mathrm{R}\mathrm{I}\mathrm{S}\mathrm{C}}=k_{\mathrm{p}}{k}_{\mathrm{d}}{\Phi }_{\mathrm{d}}/k_{\mathrm{I}\mathrm{S}\mathrm{C}}{\Phi }_{\mathrm{p}}$$

(9)

where the parameters Φ, τ, and k represent the quantum yield, the PL lifetime, and the rate constant, respectively. The subscripts p, d, IC, ISC, and RISC denote prompt fluorescence, delayed fluorescence, internal conversion from S₁ to the ground state (S₀), intersystem crossing from S₁ to T₁, and reverse intersystem crossing from T₁ to S₁, respectively.

The photophysical parameters are summarized in

Table 4. Photophysical parameters of 1-H and 1-Me in 10 wt%-doped PMMA film at 300 K under nitrogen atmosphere.

Compd.	Φp	Φd	kp (107 s−1)	kd (104 s−1)	ΦIC	ΦISC	kIC (106 s−1)	kISC (106 s−1)	kRISC (104 s−1)
1-H	0.42	0.13	1.2	1.9	0.34	0.24	9.8	6.9	1.0
1-Me	0.32	0.14	1.5	4.2	0.38	0.30	18	14	1.9

. Although no significant difference was found for the λ_PL among 2 (λ_PL; 539 nm [30]), 1-H, and 1-Me, the Φ_IC increased in the order of 2 (Φ_IC; 0.21 [30]) < 1-H < 1-Me, implying that the introduction of the π-spacer enhanced molecular flexibility. Thus, 1-Me showed the smaller Φ_p than 1-H due to its larger Φ_IC. In contrast, the Φ_d of 1-Me was the same as that of 1-H. Since the k_ps of both the dyes were comparable (k_p; 1.2–1.5 × 10⁷ s⁻¹), the higher k_d of 1-Me (k_d; 4.2 × 10⁴ s⁻¹) relative to that of 1-H (k_d; 1.9 × 10⁴ s⁻¹) indicated more efficient TADF for 1-Me. The values of k_ISC and k_RISC were 6.9 × 10⁶ and 1.0 × 10⁴ s⁻¹ for 1-H, and 1.4 × 10⁷ and 1.9 × 10⁴ s⁻¹ for 1-Me. Compared to 1-H, the larger k_ISC and k_RISC of 1-Me indicated that the ISC and RISC processes of 1-Me are promoted due to the small ΔE_ST.

3.6. Theoretical Calculations

To discuss the influence of introduction of the π-spacer on the TADF properties, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted on the Gaussian 09 program package [37]. Optimized geometries at the S₀, S₁, and T₁ states and the spin density at the T₁ state were obtained at PBE0/6-31G(d,p) level. The natural transition orbital (NTO) analysis was conducted on the Multiwfn program [38,39]. Cartesian coordinates of 1-H and 1-Me at S₀, S₁, and T₁ geometries are shown in Tables S2–S7.

To support the TADF behavior of 1-H and 1-Me, energy levels of the S₁ and T₁ states were estimated. The calculated energy level of the S₁ state (E_{S, calcd}) of 1-Me (E_{S, calcd}; 2.26 eV) was slightly lower than that of 1-H (E_{S, calcd}; 2.30 eV). In contrast, the calculated energy level of the T₁ state (E_{T, calcd}) of 1-Me (E_{T, calcd}; 2.20 eV) was significantly higher than that of 1-H (E_{T, calcd}; 2.03 eV). As a result, the calculated ΔE_ST (ΔE_{ST, calcd}) of 1-Me (ΔE_{ST, calcd}; 0.06 eV) was smaller than that of 1-H (ΔE_{ST, calcd}; 0.27 eV). The ΔE_{ST, calcd}s were consistent with the experimental values and suggested that the narrow ΔE_{ST, calcd} of 1-Me facilitates efficient RISC. The difference in the energy level was considered in terms of the molecular geometry. As shown in Supplementary Materials (Figure S8), the dihedral angle between the phenylene spacer and the N,N-diphenylaminophenyl donor unit is defined as φ₁ (°), and that between the phenylene spacer and the quinoxaline unit is defined as φ₂ (°). As shown in Figure 5, 1-H at the S₀ geometry showed a φ₁ of −34.9° and a φ₂ of 39.1°, indicating that the D–π–A system at the S₀ state is largely distorted on the donor and acceptor sides of the π-spacer. Compared to 1-H, 1-Me showed larger distortion angles at the 2,5-dimethyl-1,4-phenylene spacer; φ₁ and φ₂ of −50.7 and 61.4°, respectively, consistent with its blue-shifted ICT absorption with the lower ε_abs. As shown in Figure S8, the energetic barriers around φ₁ and φ₂ in 1-H are relatively low (less than 0.14 eV), whereas those in 1-Me are obviously high when the D–π–A conjugation system is less distorted. Thus, the large steric hindrance imposed by the methyl groups on the N,N-diphenylaminophenyl donor, the trifluoromethyl group, and the quinoxaline unit contributed to the large dihedral angles of 1-Me. Upon transition from the S₀ to the S₁ state, 1-H and 1-Me showed further twisted geometries with φ₂ of 44.7 and 69.0°, respectively, although both the dyes adopted slightly planar geometries between the donor and the π-spacer; φ₁ of −34.8 and −44.6°, respectively. Especially, 1-Me exhibited greater increases in the dihedral angles, φ₁ and φ₂ at the S₁ state, than 1-H. To quantify the geometric change between the S₀ and S₁ states, the molecular root mean square deviation (RMSD) was calculated for 1-H and 1-Me (Figure S9), according to Equation (10) [40]:

$$\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{D}=\sqrt{{\displaystyle \frac{1}{n}}{\sum }_i^{n_{atom}}\left[{\left({\mathrm{x}}_{\mathrm{i}}-{{\mathrm{x}}^{\prime }}_{\mathrm{i}}\right)}^2+{\left({\mathrm{y}}_{\mathrm{i}}-{{\mathrm{y}}^{\prime }}_{\mathrm{i}}\right)}^2+{\left({\mathrm{z}}_{\mathrm{i}}-{{\mathrm{z}}^{\prime }}_{\mathrm{i}}\right)}^2\right]}$$

(10)

where x_i, y_i, z_i and x′_i, y′_i, z′_i refer to the x-, y-, z-coordinates of the i-th atom of the S₁ and S₀ geometries, respectively. From the result of the RMSD calculations, 1-Me had larger RMSD (0.1962 Å) than 1-H (0.1139 Å), indicating that 1-Me undergoes larger substantial structural relaxation than 1-H. From the result of the NTO analysis for the S₀–S₁ transition of 1-H and 1-Me, the hole and particle at the S₁ geometry were mainly distributed over the donor and acceptor units, respectively, with a slight contribution of the π-spacer on the hole. The slight stabilization of E_{S, calcd} for 1-Me in comparison with E_{S, calcd} for 1-H should be attributed to the enhanced ICT character due to the electron-donating methyl groups on the π-spacer. On the other hand, the NTOs at the T₁ optimized geometry on the S₀–T₁ transition were quite different between 1-H and 1-Me. Dye 1-H exhibited a π–π* character in the S₀–T₁ transition, whereas 1-Me showed an ICT transition from the donor and the π-spacer to the acceptor unit. To explain the difference in the E_T,scalcd between 1-H and 1-Me, spin density distribution at the T₁ state was calculated, as it is known that localization of the spin density tends to destabilize the triplet state [41]. Compared to the delocalized spin density of 1-H, the spin density of 1-Me was found to be localized on the quinoxaline unit. Thus, the T₁ energy level of 1-Me was relatively destabilized, thereby resulting in the small ΔE_ST.

4. Conclusions

We developed new ICT-type TADF dyes 1-H and 1-Me, consisting of an N,N-diphenylaminophenyl–phenylene–quinoxaline D–π–A system. Introduction of the π-spacer led to a blue shift in the λ_abs accompanied by a decrease in ε_abs, in comparison with the D–A dye 2. This indicated spatial separation of the frontier orbitals was enhanced in the present D–π–A system due to the highly twisted molecular geometry. On the other hand, both the dyes exhibited the PL with the λ_PL at ca. 550 nm in toluene, and the π-spacer hardly affected the λ_PL. In toluene, 1-Me showed TADF behavior due to the narrow ΔE_ST of 0.07 eV, while 1-H with a relatively large ΔE_ST of 0.24 eV did not show any TADF characteristics. In contrast, in 10 wt%-doped PMMA film, TADF behavior was observed for both the dyes. Notably, 1-Me showed a greater contribution of TADF to the overall PL than 1-H, indicating that the TADF process was more effectively promoted in 1-Me. The relatively large k_ISC and k_RISC of 1-Me in comparison with 1-H upon the transition between the S₁ and T₁ states should be attributed to the small ΔE_ST of 1-Me. Theoretical calculations revealed that, at the S₁ state, the donor–π-spacer and π-spacer–acceptor dihedral angles of 1-Me are larger than those of 1-H, indicating that 1-Me shows larger structural relaxation than 1-H. Based on the NTO analyses, the S₁ energy level of 1-Me is slightly stabilized in comparison with that of 1-H by the enhanced ICT through electron donation from the methyl groups on the π-spacer as well as the larger structural relaxation upon the S₀–S₁ transition. Contrary to the S₁ energy level, the T₁ state of 1-Me is destabilized in comparison with 1-H. This is because, in the T₁ state, the planar structure of 1-H facilitates delocalization of spin density, whereas the twisted geometry of 1-Me results in spin density localization on the acceptor unit. As a result, the stabilized S₁ and destabilized T₁ states of 1-Me lead to a smaller ΔE_ST than that of 1-H. Therefore, we conclude that we have succeeded in managing the ΔE_ST in the present D–π–A system by introducing the π-spacer that induces the molecular distortion of the D–π–A structure. We believe that the present study should provide valuable insight into the development of ICT-type TADF emitters with narrow ΔE_STs.