Thermally Activated Delayed Fluorescence from Locally Excited State: Theoretical Prediction and Experimental Validation

As is well known, the thermally activated delayed fluorescence (TADF) is always generated from charge-transfer (CT) excited states in electron-donor (D) – electron-acceptor (A) systems. Here, a novel design strategy is proposed for realizing TADF from a locally excited (LE) state through controlling the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes between the LE singlet and higher triplet CT states. Based on the strategy, a boron difluoride derivative is theoretically predicted to emit TADF from the LE state, whose radiative decay rate constant is much larger kr (S1 →S0 )=1.12 * 108 s -1 , two orders of magnitude larger than those of common TADF systems. And its lifetimes of the prompt and delayed fluorescence are experimentally validated to be 0.44 ns and 0.7 μs, respectively. This work is a breakthrough in the understanding of TADF and opens a new avenue for extending the TADF materials.