Molecular Design Realizing Very Fast Reverse Intersystem Crossing in Purely Organic Emitter

Reverse intersystem 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 (k_RISC) of 1.2×10⁷ s⁻¹. The large k_RISC of the emitter resulted in great device performance in the applications to blue TADF assisted fluorescence organic light-emitting diodes (OLEDs) as well as TADF-emitter OLEDs.