Intrinsically-ionic donor-acceptor molecules featuring thermally-activated delayed fluorescence for high-performance host-guest blue light-emitting electrochemical cells

Abstract

High-performance blue light-emitting electrochemical cells (LECs) have remained a formidable challenge. Ionic donor–acceptor molecules, especially those featuring thermally-activated delayed fluorescence (TADF), are promising hosts for developing high-performance blue LECs, but such hosts have remained undeveloped. Here, we report two intrinsically-ionic donor–acceptor molecules, 1-(3-(4-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H1) and 1-(3-(4-(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H2), and their use as hosts for high-performance blue LECs. 3,6-di-tert-butylcabazole and imidazolium-substituted 2,4-diphenylpyrimidine are used as the donor and acceptor, respectively, which are located para (for H1) or ortho (for H2) to each other on the central phenyl linker. While H1 shows a low triplet energy (<2.6 eV), a large singlet–triplet energy gap (ΔEST) (>0.4 eV) and no TADF, H2 exhibits a high triplet energy (>2.8 eV), a small ΔEST (∼0.2 eV) and distinct TADF, due to its twisted donor–acceptor conformation. Both H1 and H2 show excellent electrochemical stability. Host-guest blue LECs using H2 as the host and a phosphorescent cationic Ir(III) complex as the guest afford sky-blue electroluminescence with high blue-color stability and peak brightness/peak external quantum efficiency/half-lifetime at 1078 cd m−2/8.6%/66 min under a constant-current driving. Upon adding LiPF6 into the active layer, a half-lifetime of ∼ 2 h is achieved at 918 cd m−2. These performances are the highest for host–guest blue LECs reported so far. The efficiencies and operational stability achieved at the high brightness are also the highest for blue LECs with inert-metal cathodes reported so far.