Spin-vibronic interaction induced reverse intersystem crossing: A case study with TXO-TPA and TXO-PhCz molecules

Area of Science:

• Photophysics and photochemistry
• Organic electronics
• Quantum chemistry

Background:

• Reverse intersystem crossing (RISC) is crucial for efficient thermally activated delayed fluorescence (TADF) emitters.
• Understanding the factors governing RISC rates is essential for designing high-performance organic optoelectronic devices.

Purpose of the Study:

• To investigate the contributions of direct spin-orbit (DSO) coupling and spin-vibronic (SV) coupling to the RISC mechanism in TXO-TPA and TXO-PhCz molecules.
• To elucidate the role of solvent dielectric properties on the RISC process.
• To correlate RISC rates with the efficiency of TADF.

Main Methods:

• Calculation of the RISC rate constant (k_RISC) using a time-dependent correlation function-based method.
• Application of second-order perturbation theory.
• Investigation in two different solvents: toluene and chloroform.

Main Results:

• The RISC rate constant (k_RISC) is influenced by both DSO and SV coupling, with the latter being solvent-dependent.
• For TXO-TPA, k_RISC is primarily governed by DSO coupling, irrespective of the solvent.
• For TXO-PhCz, the SV mechanism contributes over 30% to k_RISC in chloroform, while being less significant in toluene.
• Higher nonadiabatic coupling (NAC) values between T₂ and T₁ states were observed for TXO-PhCz, particularly in chloroform.

Conclusions:

• The solvent-assisted SV mechanism plays a significant role in enhancing the net RISC rate constant.
• The interplay between DSO, SV coupling, and solvent dielectric properties dictates RISC efficiency.
• Optimizing these factors can maximize the efficiency of TADF emitters.