Investigating Vibronic Coupling Effects in Multiple-Resonance Thermally Activated Delayed Fluorescence Molecules

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials demonstrate superior color purity compared to conventional TADF emitters based on twisted intermolecular charge transfer excited states. However, MR-TADF materials typically exhibit slow rates of reverse intersystem crossing (rISC), limiting their practical applications. Herein, we investigate the role of vibronic coupling in the rISC mechanism by introducing heavy adamantyl substituents to an MR-TADF molecule to dampen the vibrational modes required for rISC. Our investigation reveals that, while the addition of adamantyl groups does reduce vibronic coupling in a key excited-state transition, it also facilitates rISC through an alternative mechanism, ultimately leading to an unexpected increase in the overall rISC rate in the vibrationally damped molecule. These findings highlight the complex interplay among various excited-state pathways in MR-TADF emitters and indicate a potentially reduced vibrational requirement for efficient rISC compared to donor-acceptor systems.