Asymmetric intramolecular charge transfer enables highly efficient red thermally activated delayed fluorescent emitters

Abstract

A feasible strategy of asymmetric donor–acceptor-second acceptor (D-A-A) structure is proposed for improving the photoluminecense (PL) and electroluminescence (EL) performance of red thermally activated delayed fluorescence (TADF) emitters. As a concept, four newly TADF emitters are designed and synthesized, D-A-A structure oTPAPO-DTPZ and pTPAPO-DTPZ and D-A-D type oDTPA-DTPZ and pDTPA-DTPZ, by using triphenylamine (TPA), dithieno[2,3-a:3′,2′-c]phenazine (DTPZ) and diphenylphosphine oxide (DPPO) groups as the donor, acceptor and second acceptor, respectively. It is demonstrated that abound intra- and inter-molecular CH···O hydrogen bonds are forming between TPA donor and DPPO acceptor for the asymmetric compounds, which effectively promote the intra- and inter-molecular charge transfer. Moreover, the steric hindrance of DPPO group availably suppresses the concentration quenching. As a consequence, the asymmetric D-A-A structured oTPAPO-DTPZ and pTPAPO-DTPZ exhibit relatively higher PL quantum yields and lower nonradiation rates, and thus better EL performance. In particular, device based on pTPAPO-DTPZ achieves the maximum luminance of 19360 cd m−2 and external quantum efficiency of 11.4 % at 632 nm, which are 2.3 and 1.5 times the values of pDTPA-DTPZ based device.