Static vs Herzberg–Teller spin–orbit coupling under symmetry-reducing distortion in multi-resonance TADF emitters

Multi-resonance thermally activated delayed fluorescence emitters typically rely on rigid, symmetry-defined molecular frameworks to achieve narrowband emission and small singlet-triplet energy gaps (ΔEST), but such symmetry often suppresses static spin-orbit coupling (SOC), making reverse intersystem crossing (RISC) dependent on vibronic assistance. Here, the recently reported emitter 2S-BN was adopted as a model system to examine how molecular-symmetry reduction, accompanied by non-planar structural distortion, affects spin-conversion pathways. Two symmetry-reduced derivatives, A and B, were computationally designed through peripheral substituent removal and asymmetric framework modification. Density functional theory, SOC calculations, and thermal vibration correlation function simulations reveal that the C2v-symmetric parent 2S-BN exhibits nearly vanishing Franck-Condon SOC, whereas the symmetry-reducing structural modifications make finite static FC-SOC contributions accessible in A and B, with values of ∼0.20 and 0.18 cm-1, respectively. The absorption and emission energies remain comparable across the series, suggesting preservation of the essential multi-resonance character. Under the same computational protocol, the calculated kRISC values of A and B are only ∼3.3 and 2.2 times larger than those of 2S-BN. This modest increase is not caused by a larger total effective SOC; the effective SOC of 2S-BN exceeds those of A and B. In the present rate model, the kISC values remain nearly unchanged, because a large FCWD-related factor offsets the smaller |SOCeff|2 of A and B. The faster kRISC then follows mainly from their smaller ΔESTadi through the Boltzmann relation. Within this molecular series, symmetry reduction, inseparably accompanied by non-planar distortion, therefore provides a handle on the accessibility of static vs vibronic SOC contributions rather than a standalone control of the RISC rate.