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

Area of Science:

• Materials Science
• Organic Electronics
• Quantum Chemistry

Background:

• Ionic thermally activated delayed fluorescence (iTADF) materials are promising for organic optoelectronics due to their solution processability and structural tunability.
• Synergistic regulation of reverse intersystem crossing (RISC) by heteroatom anchoring, heavy-atom effects, and vibronic coupling is underexplored.

Purpose of the Study:

• To systematically investigate the synergistic regulation of RISC in ionic TADF materials.
• To provide a synergistic design paradigm and theoretical guidance for high-performance organic light-emitting diode (OLED) emitters.

Main Methods:

• Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.
• Excited-state dynamics simulations.
• Investigation of four ionic TADF molecules (AC-TPPO+, AC-TPPS+, AC-TPPO[Br], and AC-TPPS[Br]).

Main Results:

• Twisted donor-acceptor conformations facilitate spatial separation of frontier molecular orbitals and a small singlet-triplet energy gap (ΔE_ST) for RISC.
• Bromide counterion induces a heavy-atom effect, enhancing spin-orbit coupling and accelerating RISC rates by two orders of magnitude.
• Low-to-medium-frequency vibrations 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.

Conclusions:

• Synergistic regulation of RISC through heteroatom anchoring, heavy-atom effects, and vibronic coupling is crucial for high-performance iTADF materials.
• The bromide counterion and specific vibrational modes significantly enhance RISC efficiency.
• This work offers a design strategy for developing efficient room-temperature TADF emitters for OLED applications.