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

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.