Regulation of heteroatom positioning in multiple resonance thermally activated delayed fluorescence materials:...

Strategically incorporating Se into B/N-heteropolycyclic frameworks is key to modulating multiple resonance (MR) systems and developing high-performance TADF materials. Combining density functional theory and spin-component scaled coupled cluster calculations, this study reveals the TADF mechanism regulated by the heavy-atom effect in model molecule BNSSe, and designs two types of high-performance MR-TADF materials based on BNSSe. (1) The blue-emission molecule 1 (469 nm) achieves complementary short-range charge transfer/long-range charge transfer advantages via three para-B-π-N units. Its reverse intersystem crossing rate (k _RISC) reaches 10⁸ s^-1 (three orders of magnitude higher than BNSSe), benefiting from the synergy of a reduced singlet-triplet energy gap, moderately enhanced spin-orbit coupling (SOC), and suppressed structural relaxation. (2) The red-emission molecules 2-7 (≈630 nm) were further developed via a delicately designed π-bonding/non-bonding molecular orbital hybridization strategy. This series of molecules not only maintain a high radiative decay rate but also effectively inhibit the non-radiative decay process, and their design concept is expected to break through the constraints of the traditional energy gap law. Theoretical studies reveal that the spatial position of heteroatoms can modulate molecular geometry and induce axial redistribution of frontier molecular orbital electron density, thereby selectively enhancing the contribution of Se atoms to the excited-state transition orbitals and enabling precise regulation of SOC strength. This work not only deepens the theoretical understanding of the heavy-atom effect mechanism but also provides a new strategy for developing high-performance MR-TADF materials through the targeted regulation of SOC and k _RISC via atomic site engineering.