Vibronic coupling effect on intersystem crossing rates of TADF emitters

Thermally activated delayed fluorescence (TADF) emitters have become a hot topic in organic light-emitting diode materials due to their advantages of high efficiency, long lifespan and low cost. Many investigations have shown that TADF efficiency is determined by the reverse intersystem crossing (ISC) rate from the triplet to singlet excited state where the direct spin-obit coupling (SOC) is a pivoted factor. This study demonstrates that nonadiabatic coupling (NAC) is critical in enhancing TADF efficiency, as elucidated through second-order perturbation theory. We start from the time-dependent correlation function approach for calculating ISC rate to incorporate SOC and NAC as well as the Herzberg-Teller-like vibronic coupling. From three typological molecular model simulations, we find that the NAC can even reach to 32.6% contribution to ISC rate, and these contributions are sensitive to the motions of vibrational modes. This finding may provide important reference and guidance for further optimizing material properties and designing new TADF emitters.