Synergistic Heavy-Atom and Vibronic-Coupling Effects for High-Performance Ionic TADF Emitters: A Theoretical Study

Ionic thermally activated delayed fluorescence (iTADF) materials are promising for organic optoelectronics due to their solution processability and structural tunability, yet the synergistic regulation of reverse intersystem crossing (RISC) by heteroatom anchoring, heavy-atom effects, and vibronic coupling remains underexplored. Four molecules (AC-TPPO⁺, AC-TPPS⁺, AC-TPPO[Br], and AC-TPPS[Br]) are systematically investigated via density functional theory (DFT), TD-DFT, and excited-state dynamics. All molecules feature twisted donor-acceptor conformations, enabling spatial separation of frontier molecular orbitals and a small singlet-triplet energy gap (ΔE_ST) for RISC. The bromide counterion (Br^-) induces a heavy-atom effect, enhancing spin-orbit coupling strength and accelerating RISC rates by two orders of magnitude. Low-to-medium-frequency vibrations further facilitate RISC by driving S₁ and T₁ potential energy surfaces to near-degeneracy. The optimal AC-TPPO[Br] achieves a high delayed fluorescence quantum yield (Φ_DF = 74.88%) with nearly 100% room-temperature TADF contribution. This work provides a synergistic design paradigm and theoretical guidance for high-performance OLED emitters.