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A Novel Strategy toward Thermally Activated Delayed Fluorescence from a Locally Excited State.

It is well-known that thermally activated delayed fluorescence (TADF) is always generated from charge-transfer (CT) excited states in donor-acceptor (D-A) systems, which limits its application owing to a slow radiative process and a small stimulated emission cross section. Herein, a design strategy is proposed for realizing TADF from a locally excited (LE) state without a typical donor-acceptor type structure through controlling the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes between the lowest excited singlet with LE character and higher triplet states. Using this strategy, a boron difluoride derivative is theoretically predicted and experimentally synthesized to exhibit locally excited TADF (LE-TADF) with a fairly large radiative rate of 1.12 × 108 s-1, extremely fast RISC rate of 5.09 × 1010 s-1, and a large stimulated emission cross section of 4.35 × 10-17 cm2, making this a promising organic amplified spontaneous emission (ASE) material. This work might open a new avenue to extend TADF materials, especially TADF laser emitters.