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crossing (RISC), originally considered forbidden in purely organic materials,
has been recently enabled by minimizing the energy gap between the lowest
singlet excited state (S1) and lowest triplet state (T1) in
thermally activated delayed
fluorescence (TADF) systems.
However, direct spin-inversion between S1 and T1 is still
inefficient when both states are of the same charge transfer (CT) nature (i.e. 1CT
and 3CT, respectively). Intervention of locally excited triplet
states (3LE) between 1CT and 3CT is expected
to trigger fast spin-flip. Here, we report on the systematic-design of the
ideal TADF molecules
with near-degenerate 1CT, 3CT and 3LE states
by controlling the through-space distance between the donor and acceptor
segments in a molecule with tilted intersegment angles. The new system realizes
very fast RISC with a rate constant (kRISC) of 1.2×107 s−1. The large kRISC
of the emitter resulted in great device performance in the applications to blue
TADF assisted fluorescence organic light-emitting diodes (OLEDs) as well as