Nonradiative Spin-Forbidden\nDecay Processes in π-\nand Multiresonance-Acceptor-Based TADF Molecules

We elucidate the mechanisms behind\nthe nonradiative decay\nprocesses,\nsuch as intersystem crossing (ISC) and reverse intersystem crossing\n(rISC) in thermally activated delayed fluorescence (TADF) emitters\nhaving π- and MR-acceptor moieties with a common donor fragment.\nExperimental observations suggest that replacing a π-acceptor\nwith an MR-acceptor enhances the rISC rate constant (<i>k</i><sub>rISC</sub> values are 1.12 × 10<sup>6</sup> for MR-type\nand 1.1 × 10<sup>5</sup> s<sup>–1</sup> for π-acceptor\nmolecules) while maintaining a high radiative decay rate and thus\nenabling efficient light emission from the lowest excited singlet\nto the ground state. The present theoretical calculations using the\ntime-dependent correlation function method indicate that ISC and rISC\nin MR-type acceptor molecules are assisted by the Herzberg–Teller\n(HT) contribution toward spin–orbit coupling (SOC), unlike\nπ-acceptor molecule, where ISC is HT-driven and rISC is direct\nSOC-driven. Rotational motion around the donor–acceptor (D–A)\nσ bond reinforces the central role of HT interactions in reproducing\nthe experimental <i>k</i><sub>ISC</sub> and <i>k</i><sub>rISC</sub> of the MR-type acceptor molecule. It has also been\nnoted that the second-order spin-vibronic effect hardly has any impact\non the ISC/rISC mechanisms in these molecules, contrary to that observed\nin the traditional- and MR-TADF molecules.