Excited-state intramolecular proton transfer emitter for efficient violet-blue organic light-emitting diodes with hybridized local/charge transfer channel

Abstract

Excited-state intramolecular proton transfer (ESIPT) materials have attracted much interest, attributed to their metastable excited states which provide suitable platforms for efficient intersystem crossing (ISC) and reverse intersystem crossing (rISC). However, the studies on the utilization of ESIPT to achieve effective hybridized local/charge transfer (HLCT) emitters are rarely reported, due to their ubiquitous issue of low photoluminescence quantum yields (ΦPLs). In this research, a novel ESIPT molecule with HLCT characteristic, 4′-(diphenylamino)-4-(1,4,5- triphenyl-1H-imidazol-2-yl)-[1,1′-biphenyl]-3-ol (HIT-TPA), is designed and synthesized. The hydrogen atom in this molecule can transfer on the way of an intramolecular hydrogen bond from a proton-donating part (–OH/–NH–) to the accepting one (CO/CN–). The HLCT and /or color-tuned fluorescence are observed by the irradiation of specific wavelength, which followed the guidance of ESIPT. It is consistent with the results of transient absorption and time-resolved photoluminescent spectroscopy. By the synergistic interaction of ESIPT and HLCT, the film based on the target molecule presents a high ΦPL value of ∼75.5%. After employing HIT-TPA as the emitter for doped/undoped organic light-emitting diodes (OLEDs), a maximum external quantum efficiency of ∼10.08% is achieved, which proves the efficient triplet harvesting in the ESIPT-based HLCT material. Noticeably, the device exhibits a violet-blue emission with the CIE(x,y) of (0.152, 0.051). It is among the highest reported EQE values on the ESIPT-HLCT materials for violet-blue devices. These results not only open a novel pathway to design effective ESIPT attributed HLCT materials, but also provide a strategy to efficiently utilize ESIPT emitters in achieving high-performance OLEDs.