Role of Bridging Unit in Down-Converted Reverse Intersystem Crossing Mechanism

Abstract Efficiency roll-off and limited operational lifetime remain major limitations in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs), primarily due to inefficient triplet-to-singlet conversion. In this study, we establish a structure–property relationship that enables modulation of reverse intersystem crossing (rISC) mechanism via the nature of the bridging unit. Temperature-dependent time-resolved photoluminescence reveals that the en-one-bridged compound (TCON) exhibits an exceptionally shorter delayed fluorescence (DF) lifetime of 185 ns, compared to 292 and 456 μs for its C≡C-bridged (TyneN) and nonbridged (TBrN) analogs, respectively. TCON also displays temperature-independent rISC, indicative of a barrierless, down-converted rISC mechanism, whereas the other compounds follow a conventional thermally activated process. Ultrafast transient absorption spectroscopy further supports rapid intersystem crossing dynamics in TCON and the participation of higher-lying triplet states in the down-converted rISC process. Although TCON shows a relatively low photoluminescence quantum yield (∼12%) and weak DF intensity, these findings provide valuable insights into the fundamental spin-flipping dynamics and establish a structure–property relationship for developing next-generation OLED emitters.