Co-facial through space charge transfer enables fast reverse intersystem crossing in sky-blue TADF emitters

Abstract

Through-space charge transfer (TSCT) engineered emitters featuring thermally activated delayed fluorescence (TADF) have garnered significant interest, yet their device performance remains constrained by slow reverse intersystem crossing (RISC) rates. We present a novel molecular design strategy for TSCT-TADF emitters, featuring spatially confined donor-acceptor configurations to achieve high efficiency and accelerated RISC. Two novel emitters, DPCZ-PTRZ and BLCZ-PTRZ, were designed by integrating carbazole-based donor units and triazine acceptors within a rigid spiro-configured TSCT framework. In DPCZ-PTRZ, the donor unit promotes a close, face-to-face alignment with the acceptor at a distance of 3.477 Å, facilitating efficient TADF while minimizing structural reorganization during the RISC process. In contrast, BLCZ-PTRZ exhibits low efficiency and slow RISC, attributed to its sterically locked donor unit, which hinders close, face-to-face alignment with the acceptor. DPCZ-PTRZ demonstrated a remarkable RISC rate of 2.13 × 107 s-1, resulting in good device performance with a maximum external quantum efficiency (EQE) of 18.4% and a current efficiency (CE) of 30 cd/A.