Effect of Perturbative Vibronic Correction for Spin-Orbit Coupling in Hot Exciton OLED Systems.

Hot exciton materials have recently attracted attention for their potential to improve the efficiency of fluorescent organic light-emitting diodes (OLEDs) by harvesting triplet excitons through high-lying reverse intersystem crossing (hRISC). As internal conversion (IC) typically proceeds quite fast, an efficient hRISC pathway will be essential for the device performance. Vibronic coupling is one of the key mechanisms known to enhance RISC in conventional donor-acceptor type thermally activated delayed fluorescence (TADF) molecules. Motivated by this, we evaluate the vibronic correction of spin-orbit coupling (SOC) in 10'-diphenyl-9,9'-bianthracene (PPBA) and its derivatives using a first-order perturbative approach based on time-dependent density functional theory (TDDFT). Although the target states T3 and S1 are close in energy, their similar charge-transfer (CT) character results in negligibly small zeroth-order SOC, leading to intrinsically slow hRISC rates in all considered molecules. However, vibronic corrections significantly enhance SOC and increase the hRISC rate by nearly 4 orders of magnitude. At vibrationally distorted geometries, we observe partial mixing of CT and locally excited (LE) character in a nonidentical fashion between the two electronic states, thereby dynamically enhancing SOC. These findings demonstrate the importance of vibronic effects in hot exciton systems with flexible multiple-ring backbones and highlight the useful correction schemes in photophysical analyses of OLED materials.