Multichannel charge transfer enhanced radiative decay and RISC in TADF materials containing multiple donors and acceptors

Abstract

Coexistence of through-bond charge transfer (TBCT) and through-space charge transfer (TSCT) is observed to strongly enhance the performance of thermally activated delayed fluorescence (TADF) materials that contain multiple donors and acceptors. A group of TADF emitters were developed with carbazole as the donor, benzophenone as the acceptor, and phenylene as the linking bridge. TBCT was responsible for the TADF feature of the para-linked analogue p-tCz-BP, while both TBCT and TSCT were observed in the ortho-linked isomer o-tCz-BP. The multiple-donor–acceptor analogue D-tCz-D-BP was proved to exhibit multi-channel TBCT and TSCT processes with more near-degenerate excited states and more TSCT (81.0%) contribution than o-tCz-BP (66.3%). Thus D-tCz-D-BP combines the merits of both the high oscillator strength of p-tCz-BP and the tiny energy splitting (ΔE_ST) between the lowest singlet and triplet excited states caused by the twisted conformation of o-tCz-BP, leading to a high rate constant of radiation (k_r) of 1.01 × 10⁷ s⁻¹, of reverse intersystem crossing (k_risc) of 0.56 × 10⁶ s⁻¹ and a high photoluminescence quantum yield of 96.8%. The sky-blue organic light-emitting diode (OLED) of D-tCz-D-BP exhibited an external quantum efficiency (EQE) of 24.9%, much higher than those of p-tCz-BP (6.3%) and o-tCz-BP (11.1%). D-tCz-D-BP was also capable of hosting an orange-red iridium phosphor to form a two-emitting-component white OLED that realized an EQE of 18.8% and a high CRI of 80.