Identity of T* matters! improved krisc by modulating locally excited triplet state in TADF emitters

We have presented findings on TADF emitters consisting of a strong D-A structure, exhibiting hybridized electronic excited states that encompass a primary donor-to-acceptor long-range (LR) interaction and an ancillary short-range (SR) charge-transfer characteristic. This configuration successfully balances a small ΔEST and a large f value. Our thorough theoretical and experimental analysis has uncovered that the addition of peripheral donor units to the core D-A backbone leads to the formation of multiple triplet excited states between the S1 and T1 states, exhibiting locally excited or hybridized characteristics. The close alignment of the 1CT and 3LE states accelerates the spin-flip process, resulting in a sizable radiative rate nearly equivalent to the intersystem crossing (ISC) rate. Additionally, a decent reverse ISC rate (107 s-1) is simultaneously obtained in one emitter, leading to a short (ns) delayed lifetime and effectively suppressing efficiency roll-off.