Elucidating Nonradiative Relaxation of Charge-Transfer-Type Thermally Activated Delayed Fluorescence Emitters: The Vibrational Effect

Abstract

The S1/S0 nonradiative decay rate (kNRS) plays a key role in determining emission efficiency of thermally activated delayed fluorescence (TADF) emitters, yet the involvement of numerous vibrational modes often makes it challenging to elucidate vibrational effect on kNRS. Here, we investigate S1/S0 nonradiative decay in four prototypical charge-transfer (CT) TADF emitters, PXZ-TRZ, DMAC-TRZ, PXZ-DPS, and DMAC-DPS, using time-resolved spectroscopy, theoretical calculations, and vibrational analysis. Despite comparable CT character, emitters exhibit fluorescence quantum yields differing by up to two-orders-of-magnitude, originating from largely differed kNRS. By applying the Englman–Jortner energy-gap law and the Jortner–Bixon model, we quantitatively reveal the vibrational effect on kNRS: (1) kNRS of D–A and D-A-D emitters is dominated by high- (∼900 cm–1) and low-frequency (∼60 cm–1) modes, respectively; (2) low-frequency modes promote kNRS far less effectively than high-frequency modes, even though they couple to S1 → S0 transition equally; and (3) the more rigid DMAC donor reduces the reorganization energy, thereby suppressing the vibrational promotion of kNRS. These results establish a general framework for analyzing vibrational effects in CT-TADF emitters lacking a dominant promoting mode and suggest rational molecular design strategies for high-performance emitters.