SeBSe was synthesized in two steps using commercially available 2-bromo-1,3-diiodobenzene (1) as the starting material (Scheme 1). Precursor 2 was prepared by reacting 1 with diphenyl diselenide (Ph₂Se₂) in acetonitrile under reflux in the presence of a catalytic amount of copper(I) iodide and a large excess of cesium carbonate Zhang et al. (2021). The latter step is the so-called one-pot borylation (Hatakeyama et al., 2016), which consists of lithiation/substitution followed by tandem bora-Friedel-Crafts reactions. The final target, SeBSe, was fully characterized using ¹H and ¹³C NMR, mass spectrometry, and single-crystal X-ray crystallography. The detailed synthesis procedures and characterization data are provided in the Supplementary Material (Supplementary Figures S1, S2).

Single crystals suitable for X-ray analysis were obtained by the slow diffusion of ethanol into a chloroform solution of SeBSe at room temperature. Interestingly, SeBSe self-organized into a crystal structure with the Sohncke space group P2₁2₁2₁, which contained no mirror nor inversion symmetry operations (Supplementary Table S1). The fact that SeBSe without a chiral center crystallizes in such a non-centrosymmetric space group can be related to its molecular helicity. Crystallographic analysis revealed that SeBSe adopts a highly distorted nonplanar structure, forming a pair of helical enantiomers (Figures 2A, B) because of the bond length mismatch caused by the rather long C–Se bonds and intramolecular steric repulsion between the adjacent benzene rings. As indicated by the dihedral angles (φ = ∠C6–C7–C8–C9) around the helicity, the right-handed helicene (P-SeBSe, φ = +53.2°) seems to be slightly more distorted compared to the left-handed one (M-SeBSe, φ = −52.7°). The bond angles ∠C1–Se1–C4 and ∠C3–Se2–C5 are 100.3° and 99.9° for M-SeBSe, while they are 99.6° and 103° for P-SeBSe. For M-SeBSe, all four C–Se bonds fall in the range of 1.88–1.94 Å, suggesting single bond character. In the case of the more distorted P-SeBSe, the outer C4–Se1 and C5–Se2 bonds (2.00–2.03 Å) are elongated, whereas the inner C1–Se1 and C3–Se2 bonds (1.71–1.80 Å) are contracted. Therefore, SeBSe inevitably prefers to form helicene structures rather than planar structures because these C–Se bonds are substantially longer than the C–B bonds (1.54–1.57 Å). As illustrated in Figure 2C, M-SeBSe (green) and P-SeBSe (pink) enantiopairs are alternately arranged and closely packed via noncovalent C–H···π _(C) and C–H···n _(Se) interactions in the crystals.

The ground-state (S ₀) geometries of SeBSe and its congeners (SBS and OBO) were optimized using density functional theory (DFT) calculations at the B3LYP/6-31G(d) level (Supplementary Figure S3). In contrast to the fully planar OBO, the computed S ₀ state of SeBSe adopted a helically distorted structure, as expected, which is in good agreement with the single-crystal structure. The dihedral angle around the helicity (φ) was estimated to be 50.8°, which is comparable to that observed in the single-crystal structure (∼53°). To gain insight into the dynamic helicity inversion (i.e., M-SeBSe ⇄ P-SeBSe), potential energy surface scans were performed for SeBSe by varying the degree of φ using the DFT method (Supplementary Figure S4). A nearly planar conformer of SeBSe has the highest energy, with an energy barrier of ∼69 kJ mol⁻¹ for helicity inversion. Hence, we attempted optical resolution using preparative HPLC equipped with a chiral column but were unable to separate each helical enantiomer under multiple sets of conditions. Unlike in the solid states, helicity interconversion (or racemization) of SeBSe may occur rapidly in solution at room temperature.

To understand the nature of each ring comprising SeBSe, we calculated the nucleus-independent chemical shifts (NICS) (Chen et al., 2005) at the geometrical center of the ring (NICS(0)) (Figure 3A). The large negative NICS(0) values of the three peripheral benzene rings (−5.9 to −6.0) indicate the presence of induced diatropic ring currents owing to aromaticity, whereas the two selenaborin rings exhibit small positive NICS(0) values (+1.6 and +1.1) attributable to non-aromaticity.

As with common MR-TADF molecules, the Frontier molecular orbitals (FMOs) of SeBSe were characterized by significant localization on the constituent atoms (Figure 3B). Despite the helically distorted structure, the HOMO and LUMO of SeBSe are extended throughout the molecule with complementary spatial distributions, where the Se and B atoms intensively contribute to inducing the MR effects. Although similar FMO patterns were observed in OBO and SBS, the electron density distribution on the two chalcogen atoms increased significantly when changing from O to S and Se (Figure 3B and Supplementary Figure S5). The calculated HOMO energy levels gradually increased in the order of OBO, SBS, and SeBSe (−5.62, −5.44, and −5.34 eV, respectively), whereas the LUMO energy levels decreased in the same order (−1.67, −1.90, and −1.95 eV, respectively). Accordingly, the HOMO–LUMO gap of SeBSe (3.39 eV) is considerably smaller than those of SBS (3.54 eV) and OBO (3.95 eV) and therefore, SeBSe is expected to emit at a longer wavelength (or lower energy).

We further computed and analyzed the energy landscape and natural transition orbitals (NTOs) of the excited singlet and triplet states. For SeBSe, the two lowest excited singlet states (S ₁ and S ₂) were nearly degenerate and energetically close to the higher-order triplet state (T ₂) (Figure 3C). This configuration is suitable for effectively harvesting the radiative S ₁ state through various channels from T ₁, T ₂, and S ₂. The NTO analysis of SeBSe (Figure 3D) indicates that the highest occupied NTOs (HONTOs) for S ₁ and T ₁ are very similar to the HOMO, while the HONTOs for S ₂ and T ₂ correspond to HOMO−1. The lowest unoccupied NTOs (LUNTOs) for these excited states are predominantly characterized by the LUMO. We also calculated the SOC matrix elements ( S Ĥ SOC T ) for SeBSe and its congeners (Figure 3C and Supplementary Figure S6). For SeBSe, the calculated S 1 Ĥ SOC T 1 value was not zero but relatively small (∼0.3 cm⁻¹), reflecting the minimal change in orbital angular momentum between T ₁ and S ₁. However, the SOC matrix elements between T ₁ and S ₂ ( S 2 Ĥ SOC T 1 ∼7.6 cm⁻¹) as well as T ₂ and S ₁ ( S 1 Ĥ SOC T 2 ∼18.0 cm⁻¹) were significantly enhanced, presumably because of the synergistic effect of the heavy Se atoms and large orbital angular momentum changes. As a result, SOC enhancement should promote exciton spin interconversion and hence increase the RISC rate constant (k _RISC), as rationalized by the following relationship: k RISC ∝ S Ĥ SOC T 2 / Δ E ST .

Figure 4 shows the basic photophysical properties of SeBSe in a dilute toluene solution, and Table 1 summarizes the relevant data. SeBSe exhibited an intense absorption peak (λ _abs) at 448 nm, which can be attributed to the HOMO→LUMO electronic transition. This main absorption band is considerably red-shifted compared to those reported for OBO (λ _abs = 378 nm) and SBS (λ _abs = 431 nm) (Chen et al., 2021), which is in agreement with the results of their computationally simulated absorption spectra (Supplementary Figure S7). The deoxygenated solution of SeBSe exhibited strong sky-blue photoluminescence (PL) with an emission peak (λ _PL) at 477 nm and absolute quantum yield (Φ _PL) of 71%. Similar to the absorption profile, the emission band of SeBSe was red-shifted with respect to those of OBO and SBS (λ _PL = 396 and 457 nm, respectively) but maintained a narrow spectral FWHM of 34 nm (0.18 eV). The PL emission of SeBSe was completely quenched in an aerated solution (Figure 4C), suggesting that the ISC (S ₁→T ₁) and subsequent exciton quenching by triplet oxygen is much faster than the fluorescence radiative process (S ₁→S ₀). The S ₁ and T ₁ excitation energies (E _S and E _T) of SeBSe were estimated to be 2.60 and 2.47 eV, respectively, from the fluorescence and phosphorescence peaks (Figure 4B), resulting in a ΔE _ST of 0.13 eV. This value is smaller than those of OBO (ΔE _ST = 0.18 eV) and SBS (ΔE _ST = 0.15 eV) (Chen et al., 2021), suggesting that heavier chalcogen doping reduces the ΔE _ST.

The transient PL properties highlight the unique exciton kinetics of SeBSe, which are somewhat different from typical TADF (Figures 4D, E). The emission component in the nanosecond regime (typically attributed to prompt fluorescence) was almost negligible, as characterized by an extremely small fractional quantum yield (<0.1%) and short picosecond-order lifetime (τ _p ∼200 ps). In contrast, the emission component in the microsecond regime (usually regarded as delayed fluorescence) dominated the overall Φ _PL. SeBSe thus demonstrated a quasi-single-component transient PL decay with a lifetime (τ _d) of 27.8 μs, which is markedly different from the two-component behavior commonly observed in TADF. This implies that the spin-flip ISC/RISC cycles were drastically accelerated by Se doping, making them much faster than the competing fluorescence radiative process. Following a recent method for exciton kinetic analysis (Park et al., 2022a), we estimated the photophysical rate constants for the fluorescence radiative decay, ISC, and RISC (k _r, k _ISC, and k _RISC, respectively; Table 1). As expected, the k _RISC of SeBSe reached 1.2 × 10⁸ s⁻¹, which is nearly three orders of magnitude higher than that of SBS (Chen et al., 2021), as a consequence of enhanced SOC and reduced ΔE _ST. Furthermore, the k _RISC of SeBSe was approximately two orders of magnitude higher than its k _r (1.1 × 10⁶ s⁻¹), indicating that the rate-limiting process was no longer RISC but the fluorescence radiative process. This observation is reminiscent of metal-TADF emitters, where the k _ISC (typically 10⁹–10¹¹ s^–1) is much faster than k _r owing to the large SOC imparted by the metal ions. To et al. (2020); Li et al. (2022) The fast ICT characteristics in metal-TADF emitters allow for equilibration of the lowest energy singlet and triplet excited states. Consequently, the emission decays of metal-TADF emitters typically show only a delayed component as a single exponential signal in the microsecond range, similar to the emission observed in phosphorescent organometallic complexes.

We also measured the steady-state and transient PL characteristics of SeBSe in doped thin films using 3,3′-di(carbazole-9-yl)-1,1′-biphenyl (mCBP) as the host matrix (Table 1 and Supplementary Figure S8). The photophysical properties of the SeBSe doped films agreed with those measured as solutions, still retaining high Φ _PL and k _RISC values as well as the narrowband emission.

To evaluate the electroluminescence (EL) performance of SeBSe, we fabricated OLEDs with the following device structure: indium tin oxide (ITO, 50 nm)/2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN, 10 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 40 nm)/1,3-bis(1,8-dimethylcarbazol-9-yl)benzene (mMCP, 5 nm)/1 wt%-SeBSe doped in 3,3′-di(carbazole-9-yl)-1,1′-biphenyl (mCBP) (30 nm)/2,8-bis(diphenylphosphinyl)dibenzo[b,d]furan (PPF, 5 nm)/1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene (B3PyPB, 40 nm)/8-quinolinolato lithium (Liq, 1 nm)/Al (100 nm). As depicted in Figure 5A, the SeBSe-based OLED exhibited narrowband sky-blue EL, with an emission peak (λ _EL) at 481 nm and the CIE chromaticity coordinates of (0.106, 0.241). The EL spectrum coincided well with the corresponding PL spectrum and retained a narrowband feature. The SeBSe-based device exhibited a maximum external quantum efficiency (EQE_max) of 9.3% (Figure 5C), maximum current efficiency (CE_max) of 13.3 cd A^–1, and maximum power efficiency (PE_max) of 9.5 lm W⁻¹ (Supplementary Figure S9).