Photophysics of heteroatom-doped B,N multiple-resonance emitters: A computational investigation.

Boron/nitrogen multiple-resonance thermally activated delayed fluorescence (B/N MR-TADF) emitters offer high color purity, but accelerating reverse intersystem crossing (RISC) without sacrificing narrowband emission remains challenging. Here, 16 C-, O-, S-, and Se-containing B/N MR-TADF emitters with varied framework symmetries are investigated using density functional theory (DFT), time-dependent density functional theory (TD-DFT), and the thermal vibration correlation function (TVCF) formalism. We find that the conventional T1 → S1 pathway cannot explain the accelerated RISC because the S1-T1 gap remains thermally unfavorable. Instead, a near-resonant high-lying T2 state provides an intrinsically fast spin-conversion pathway, although its productive contribution is governed by the competition between T2 → S1 RISC and T2 → T1 internal conversion. Chalcogen embedding and framework symmetry breaking cooperatively tune the T2 orbital character, enhance S1-T2 spin-orbit coupling (SOC), and yield direct T2 → S1 RISC rate constants from 8.65 × 107 to 8.77 × 109 s-1 for S- and Se-containing emitters with numerically stable T2-state vibronic parameters. Spectral simulations further show that the same heavy-atom perturbation can increase S1/S0 relaxation, reorganization energy, and vibronic coupling, thereby broadening emission. The emission bandwidth correlates more closely with S1 → S0 reorganization energy than with global structural displacement, identifying vibronic reorganization as the key color-purity descriptor. Thus, S1-T2 energy alignment and SOC govern the intrinsic spin-conversion capability, whereas framework rigidity, symmetry, and reorganization energy determine narrowband emission. Symmetric double locking with localized S or Se embedding, therefore, provides a practical strategy for balancing intrinsic spin-conversion capability and color purity.