The 3 TADF molecules, BF-oTCz, BF-oPCz, and BF-oTMCz, are designed with dimesitylborane-dibenzofuran (BF) electron-acceptors and carbazole-derived electron-donors (Fig. 1A). Bulky 1,3,5-trimethylbenzene is introduced to protect the tricoordinate boron center from being attacked by nucleophiles. The donor type is finely tuned to manipulate the components of low-lying excited states. Synthesis of each designed TADF molecule was carried out by the 2-step reaction with an overall yield surpassing 40%, as depicted in the Supplementary Materials (Scheme S1). The obtained compounds were purified by column chromatography and temperature-gradient vacuum sublimation and then were fully characterized using NMR spectroscopy and elemental analysis.

The single-crystal structures obtained from x-ray diffraction analysis are presented in Fig. 1B. For each molecule, the carbazole unit and triarylboron unit are ortho-linked on an electron-withdrawing dibenzofuran unit. The steric hindrance leads to a large dihedral angle (74.5° to 86.4°) between the carbazole and dibenzofuran units and a face-to-face D–A alignment in close proximity. The carbazole donors are nearly parallel to one of the aryl rings of the triarylboron acceptor with small distances ranging from 3.42 to 4.28 Å, indicating strong intramolecular interactions between each other, which are expected not only to confine the molecular configuration, but also to provide a through-space charge transfer (TSCT) pathway in the stacking space, in addition to through-bond charge transfer (TBCT) [49–56]. These inferences were further validated by reduced density gradient (RDG) analysis of their crystal structures. The large and nearly continuous region in RDG isosurface maps (Fig. 1D) together with the obvious spikes in the region that Sign (λ₂) ρ is near zero in the scatter diagrams (Fig. S2) reveal the presence of significant intramolecular interactions and TSCT transitions between the close-range face-to-face D and A moieties for all these compounds. The significant D–A interactions could suppress the molecular rotational and vibrational relaxations and thus are conducive to realizing high PLQYs.

To investigate the electronic properties of these emitters, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed at the PBE0/6-311g (d, p) level based on the single-crystal structures [57–59]. Due to the large torsion angles between the carbazole and dibenzofuran units, FMO distributions of these molecules are effectively separated. As shown in Fig. 1C, the highest occupied molecular orbitals (HOMOs) are mainly localized on the carbazole donor moieties and slightly extended to the dibenzofuran units, and the lowest unoccupied molecular orbitals (LUMOs) are entirely distributed over the dimesitylboron and dibenzofuran units. A small overlap of FMOs always gives rise to S₁ with CT character and also leads to small ¹CT–³CT splitting, which is a necessary but not sufficient condition for realizing small ΔE_ST and efficient RISC. The TD-DFT calculations (Fig. S3) reveal that the S₁ states of these 3 molecules show predominant CT contributions ranging from 86.3% to 90.1%. For the T₁ states of BF-oTCz and BF-oPCz, significant LE components appear (36.6% to 41.5%), while the LE character in the T₁ state of BF-oTMCz remains very limited (9.9%), in accordance with its S₁ counterpart. The lowest-lying ³CT state and the lowest-lying ³LE state are close in energy for all 3 compounds. Thus, ³CT and ³LE will inevitably mix with each other, to form T₁, which are in accordance with the theoretical predictions (vide supra, Fig. S3) [60]. The obviously different orbital nature of T₁ states originate from the different energy level alignments of the ³CT and ³LE states, namely, ΔE(³CT–³LE). The lowest ³LE state depends on the longest π-conjugation length in the whole molecule. All the 3 molecules adopt very similar configurations and consist of the same π-extended dibenzofuran subunit on which the lowest LE transitions probably occur. Therefore, the lowest ³LE states of these 3 molecules should be very close in energy (~2.88 eV, vide infra). However, their lowest CT energies are different due to the different donor strengths. The ΔE_ST values were theoretically calculated to be 0.14, 0.14, and 0.03 eV for BF-oTCz, BF-oPCz, and BF-oTMCz, respectively. Obviously, the much smaller ΔE_ST value of BF-oTMCz compared with those of the other 2 compounds can be attributed to the much less ³LE contribution in the T₁ state.

The UV-Vis absorption spectra were recorded in dilute toluene solutions of these materials (10⁻⁵ M) (Fig. S4). The strong absorption bands ranging from 290 to 380 nm can be attributed to the n–π* and π–π* transitions localized on dibenzofuran and carbazole derivatives, and the weak absorptions in the wavelength region of 380 to 470 nm are assigned to the D–A intramolecular charge transfer (ICT) transitions from carbazole units to dimesitylboron–dibenzofuran units. The solvent effects on PL behaviors were studied by comparing PL spectra of these emitters in various solvents. As shown in Fig. S4, the PL spectra of all 3 emitters display obvious bathochromic shifts and broadened spectrum profiles by increasing the solvent polarity, indicating the CT characteristics of their emissive states. The compound BF-oTMCz shows the most significant solvent effect because of its more CT components.

To gain insight into the excited state nature, the time-resolved PL (fluorescence and phosphorescence) spectra of these emitters were recorded in 20 wt%-doped BCPO films (BCPO = bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide) at 77 K (Fig. 2A to C) [61]. The fluorescence spectra of all 3 compounds are broad and structureless, confirming that the S₁ states possess predominant CT nature in approximate energy levels, which are consistent with the solvatochromism phenomena of these compounds (Fig. S4).

The phosphorescence spectra of BF-oTCz and BF-oPCz are strongly redshifted relative to the fluorescence spectra and are both asymmetrical, each showing a shoulder band in addition to the main CT band, which suggests that the T₁ states of these 2 emitters are hybrid local-charge transfer excited state (³HLCT). In contrast, the T₁ state of BF-oTMCz should be a typical CT state according to its nearly symmetric and slightly broader phosphorescence spectrum. These assignments are in line with theoretical predictions (vide supra, Fig. S3). From these time-resolved PL spectra (Fig. 2A to C), S₁ and T₁ energies and ΔE_ST are estimated to be 2.66, 2.51, and 0.15 eV for BF-oTCz, 2.68, 2.53, and 0.15 eV for BF-oPCz, and 2.65, 2.61, and 0.04 eV for BF-oTMCz, respectively. The photophysical properties of these emitters were further investigated in the 20 wt%-doped BCPO films. The film samples of BF-oTCz, BF-oPCz, and BF-oTMCz exhibit bluish-green to green emission with CT-type steady-state PL spectra peaking at 508, 502, and 515 nm, respectively (Fig. S5a). The transient PL decay characteristics of these doped films at 300 K (Fig. 2D) reveal that each compound shows a typical 2-component TADF decay behavior consisting of a prompt fluorescence (PF) component and a delayed fluorescence (DF) component. The lifetimes of the PF (τ_PF) and the DF (τ_DF) are 24.2 ns and 332.8 μs for BF-oTCz, 20.4 ns and 297.1 μs for BF-oPCz, and 25.6 ns and 6.0 μs for BF-oTMCz, respectively. Impressively, the DF lifetime of BF-oTMCz is only ~1/55 and ~1/50 of those of BF-oTCz and BF-oPCz, respectively. Moreover, BF-oTMCz has a much stronger DF contribution to the entire emission, as compared with the other 2 materials.

As listed in Table 1, the overall PLQYs (Φ_PL) of BF-oTCz, BF-oPCz, and BF-oTMCz in doped BCPO films reach 96.3%, 89.5%, and 94.8% at room temperature, which include quantum efficiencies of 79.7%, 74.8%, and 7.1% for PF (Φ_PF) and 16.6%, 14.7%, and 87.7% for DF (Φ_DF), respectively. The much higher TADF contribution and much shorter TADF lifetime generally suggest a much more efficient RISC process for BF-oTMCz. Based on the PLQYs and lifetimes at 300 K, the ISC rate constants (k_ISC) and the RISC rate constants (k_RISC) are estimated to be approximately 8.37 × 10⁶ s⁻¹ and 0.03 × 10⁶ s⁻¹ for BF-oTCz, 1.24 × 10⁷ s⁻¹ and 0.03 × 10⁶ s⁻¹ for BF-oPCz, and 3.63 × 10⁷ s⁻¹ and 2.26 × 10⁶ s⁻¹ for BF-oTMCz, respectively, using a previously reported method (Table 1, see the Supplementary Materials for details) [62]. Notably, although having similar molecular structures, the k_RISC of BF-oTMCz is approximately 75 times higher than those of BF-oTCz and BF-oPCz. Chou and coworkers have recently reported that the host–guest interaction plays a critical role in boosting TADF emission of multiple-resonance TADF emitters [63]. They have demonstrated that the host–guest interaction that boosts TADF depends on the alignment of orbital energy between the host and guest. Here, in order to investigate the effect of host–guest interaction on the RISC, we selected bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) as a control host to prepare doped films of the TADF emitters. Compared with the host BCPO (T₁ = 3.0 eV), DPEPO has a rather high triplet state energy (3.3 eV), which would prevent significant host–guest interaction [63]. The photophysical properties of the doped DPEPO films were measured and summarized in Fig. S8 and Table S6. The k_RISC of BF-oTCz, BF-oPCz, and BF-oTMCz in 20 wt%-doped DPEPO films was calculated to be 0.06 × 10⁶, 0.05 × 10⁶, and 2.27 × 10⁶ s⁻¹, respectively, which are comparable to those of the 20 wt%-doped BCPO films (0.03 × 10⁶, 0.03 × 10⁶, and 2.26 × 10⁶ s⁻¹). These results suggest that the spin-flipping RISC of these TADF emitters is not significantly dependent on host–guest interactions.

Most previous studies attributed the fast RISC process to small ΔE_ST, and especially to the enhanced SOC between the lowest ¹CT states and the close-lying ³LE states. As mentioned above, the TD-DFT calculations revealed that the lowest ³LE states mainly reside on the BF acceptor moieties in our TADF molecules. To further confirm the lowest ³LE energies of these TADF molecules, the phosphorescence spectra of all the donor units (TCz for 3,6-Di-tert-butyl-carbazole, PCz for 3,6-Biphenyl-carbazole, and TMCz for 1,3,6,8-Tetramethyl-carbazole) and the BF acceptor unit were recorded (Fig. S6). Based on these phosphorescence spectra, the T₁ energy levels of TCz, PCz, TMCz, and BF were estimated to be 3.06, 2.91, 3.03, and 2.91 eV, respectively. The acceptor moiety (BF) exhibits the lowest T₁ energy among all the donor and acceptor moieties used for constructing these TADF molecules, suggesting that the lowest ³LE of all these TADF molecules probably reside on the BF unit. Accordingly, based on the combination of these theoretical and experimental results, the T₂ (³LE) energy levels of the TADF emitters could be estimated from the phosphorescence spectrum of BF with a value of 2.91 eV, which agree well with the theoretically predicted T₂ energy levels (2.85 to 2.88 eV, Table S2). In order to figure out the origin of the distinct RISC processes of these materials, we investigated the temperature dependence of k_ISC and k_RISC. As depicted in Fig. 2e, the k_RISC values of all the 3 TADF materials decreased with temperature. Based on the Arrhenius equation \left[k_{\mathrm{RISC}}=A\cdot \text{exp}\left(-\frac{E_{\mathrm{a}}}{k_{\mathrm{B}}T}\right)\right], where A is the prefactor, k_B is the Boltzmann constant, and T is temperature, the activation energies of RISC (E_a^RISC) for BF-oTCz, BF-oPCz, and BF-oTMCz were estimated to be 0.15, 0.15, and 0.04 eV, respectively, which are equal to the corresponding ΔE_ST values determined from the fluorescence and phosphorescence spectra. In contrast to k_RISC, the k_ISC of these materials does not show any temperature dependence (Fig. 2E); that is, the activation energies (E_a^ISC) are zero. The combination of these experimental results suggests that, for all these materials, the upper-lying states, e.g., T₂ (³LE), are not directly involved in both the ISC and RISC processes between S₁ and T₁ (Fig. 3A and B). In addition, the energy of ΔE_T1-_T2 is large (around 0.3 to 0.4 eV); thus, the reverse IC between T₁ and T₂ is assumed to be very slow, inhibiting the T₁–T₂ (³LE)–S₁ (¹CT) process. In this case, it seems uncommon that the RISC process (between ³CT and ¹CT states with the same transition character) of BF-oTMCz is much higher than those (RISC between³HLCT and ¹CT) of BF-oTCz and BF-oPCz, because the ³CT–¹CT transition is usually neglected according to the El-Sayed rules. We could rationalize this observation with the ΔE_ST, the RISC reorganization energies, and the non-zero ³CT–¹CT SOC in inhomogeneous media, based on the Marcus–Levich equation (Eq. 1). First, the much smaller ΔE_ST of BF-oTMCz compared with those of BF-oTCz and BF-oPCz contributes to its high RISC rate. Second, the reorganization energies of RISC and ISC processes of these materials were theoretically calculated. As shown in Table 1, both the RISC reorganization energy (λ_3CT–1CT = 0.004 eV) and the ISC reorganization energy (λ_1CT–3CT = 0.005 eV) of BF-oTMCz are much lower than those of BF-oTCz (λ_3HLCT–1CT = 0.210 eV, λ_1CT–3HLCT = 0.172 eV) and BF-oPCz (λ_3HLCT–1CT = 0.310 eV, λ_1CT–3HLCT = 0.277 eV). According to Eq. 1, a small RISC reorganization energy is conducive to realizing fast RISC. Third, as demonstrated by Park et al. [42], the statistically weighted SOC between the ³CT and ¹CT states that have the same transition character is non-negligible by taking into account the contribution of rotamers, especially in inhomogeneous media [42].

Based on the Marcus–Levich equation (Eq. 1) and the theoretically obtained values of ΔE_ST, RISC reorganization energy and SOC, the k_RISC values of BF-oTCz, BF-oPCz, and BF-oTMCz were calculated to be 7.30 × 10⁴, 4.32 × 10⁴, and 1.43 × 10⁶ s⁻¹, respectively (Table S3). These values are in good agreement with the experimentally obtained k_RISC values (Table 1). In order to distinguish the contributions of ΔE_ST, RISC reorganization energy, and SOC to k_RISC, we summarized the theoretically obtained values of , \sqrt{\frac{\pi }{{\lambda }_{\mathrm{RISC}}{k}_{\mathrm{B}}T}}\exp \left[-\frac{{\left(\mathrm{\Delta }{E}_{\mathrm{ST}}+{\lambda }_{\mathrm{RISC}}\right)}^2}{4{\lambda }_{\mathrm{RISC}}{k}_{\mathrm{B}}T}\right] (herein, the contributions of ΔE_ST and λ_RISC are considered as a whole), and k_RISC in Table S3. It can be found that the value of BF-oTMCz is about one order of magnitude smaller, while the \sqrt{\frac{\pi }{{\lambda }_{\mathrm{RISC}}{k}_{\mathrm{B}}T}}\exp \left[-\frac{{\left(\mathrm{\Delta }{E}_{\mathrm{ST}}+{\lambda }_{\mathrm{RISC}}\right)}^2}{4{\lambda }_{\mathrm{RISC}}{k}_{\mathrm{B}}T}\right]value of BF-oTMCz is about 125 to 296 times larger than those of the other 2 TADF molecules. Therefore, we can conclude that the much higher k_RISC observed for BF-oTMCz as compared to BF-oTCz and BF-oPCz can be primarily attributed to the contributions of ΔE_ST and RISC reorganization energy, rather than the SOC between S₁ and T₁.

To investigate the electroluminescence (EL) performances of these TADF compounds, we fabricated and tested OLEDs using BF-oTCz, BF-oPCz, and BF-oTMCz as the emitters. Figure 4A depicts the optimized device configuration: ITO/HATCN (10 nm)/TAPC (30 nm)/TCTA (10 nm)/mCBP (8 nm)/BCPO: 20 wt% emitters (20 nm)/m4PO (7.5 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (100 nm). The related molecular structures are shown in Fig. S12. The graphic and numerical data of device performances are shown in Fig. 4B to D, Fig. S13, and Table 2, respectively. The OLEDs employing all these TADF emitters turned on (luminance = 1 cd/m²) at a low voltage of around 2.9 V, indicating the effective charge injection and transport with the well-matched HOMO levels and LUMO levels of the functional layers. These devices showed strong bluish-green to green EL with excellent color stability over a wide range of operating voltages from 3 to 10 V (Fig. S14). The unstructured EL spectra of BF-oTCz-, BF-oPCz-, and BF-oTMCz-based OLEDs peaked at 505, 499, and 515 nm, respectively, which are in accordance with the PL counterparts. No additional emission from the host material or other functional layers indicates that the electrogenerated excitons can be well confined on the dopants.

The OLED based on BF-oTMCz achieved a very high maximum external quantum efficiency (EQE_max) of 27.1% and a maximum luminance (L_max) of 28,150 cd/m². In comparison, much smaller EQE_max values of 17.0% and 18.9% and much lower L_max values of 6,616 cd/m² and 8,478 cd/m² were obtained for the BF-oTCz- and BF-oPCz-based OLEDs, respectively. Considering the approximately equal PLQYs of the dopants and the same device configuration, the obvious difference in the above device performances should be mainly attributed to the notably different RISC rates and exciton lifetimes of TADF emitters. The long TADF lifetimes of BF-oTCz (332.8 μs) and BF-oPCz (297.1 μs) would lead to severe exciton annihilation in the emitting layer and thus reduce efficiency and luminance. In particular, the exciton annihilation process would be dramatically exacerbated at high excitation densities. For instance, at a high luminance of 1,000 cd m⁻², the BF-oTMCz-based OLED maintains an EQE value of 26.0% with an ideal efficiency roll-off of 4.1%. In contrast, the EQEs of the BF-oTCz- and BF-oPCz-based devices dramatically decreased by nearly 60% to 7.7% and 7.8% at 1,000 cd/m², respectively. The sharp contrast between these OLEDs reveals that the RISC rates and exciton lifetimes of the TADF dopants play a critical role in determining device performance.

The BF-oTMCz-based OLED achieved a very high maximum external quantum efficiency (EQE_max) of 27.1% and a maximum luminance (L_max) of 28,150 cd/m². In comparison, much smaller EQE_max values of 17.0% and 18.9% and much lower L_max values of 6,616 cd/m² and 8,478 cd/m² were obtained for the BF-oTCz- and BF-oPCz-based OLEDs, respectively. Considering the approximately equal PLQYs of the dopants and the same device configuration, the obvious difference in the above device performances should be mainly attributed to the markedly different RISC rates and exciton lifetimes of TADF emitters, which are attributed to the fast spin-flip ³CT–¹CT transition. The long TADF lifetimes of BF-oTCz (332.8 μs) and BF-oPCz (297.1 μs) would lead to severe exciton annihilation in the emitting layer and thus reduce efficiency and luminance. In particular, the exciton annihilation process would be dramatically exacerbated at high excitation densities. For instance, at a high luminance of 1,000 cd m⁻², the BF-oTMCz-based OLED maintains an EQE value of 26.0% with an ideal efficiency roll-off of 4.1%. In contrast, the EQEs of the BF-oTCz- and BF-oPCz-based devices dramatically decreased by nearly 60% to 7.7% and 7.8% at 1,000 cd/m², respectively. The sharp contrast between these OLEDs reveals that the RISC rates and exciton lifetimes of the TADF dopants, which originate from the fast spin-flipping between ³CT and ¹CT without involvement of the upper-lying LE states, play a critical role in determining device performance.