Nonradiative Spin-Forbidden Decay Processes in π- and Multiresonance-Acceptor-Based TADF Molecules

Abstract

We elucidate the mechanisms behind the nonradiative decay processes, such as intersystem crossing (ISC) and reverse intersystem crossing (rISC) in thermally activated delayed fluorescence (TADF) emitters having π- and MR-acceptor moieties with a common donor fragment. Experimental observations suggest that replacing a π-acceptor with an MR-acceptor enhances the rISC rate constant (k rISC values are 1.12 × 106 for MR-type and 1.1 × 105 s–1 for π-acceptor molecules) while maintaining a high radiative decay rate and thus enabling efficient light emission from the lowest excited singlet to the ground state. The present theoretical calculations using the time-dependent correlation function method indicate that ISC and rISC in MR-type acceptor molecules are assisted by the Herzberg–Teller (HT) contribution toward spin–orbit coupling (SOC), unlike π-acceptor molecule, where ISC is HT-driven and rISC is direct SOC-driven. Rotational motion around the donor–acceptor (D–A) σ bond reinforces the central role of HT interactions in reproducing the experimental k ISC and k rISC of the MR-type acceptor molecule. It has also been noted that the second-order spin-vibronic effect hardly has any impact on the ISC/rISC mechanisms in these molecules, contrary to that observed in the traditional- and MR-TADF molecules.