Intramolecular hydrogen bond – enhanced electroluminescence performance of hybridized local and charge transfer (HLCT) excited-state blue-emissive materials

Abstract

The hybridized local and charge transfer (HLCT) excited state is a successful strategy to produce both high external and internal quantum efficiencies. Based on the HLCT scheme, two isomeric donor–acceptor (D–A)–type excited-state intramolecular proton transfer (ESIPT) chromophores of o-hydroxyphenyl phenanthroimidazole (HPI)-based emissive molecules (mTAHPI and pTAHPI) and their OH-protected derivatives (pTAPI) were designed and explored for organic light–emitting diodes (OLEDs). The photophysical study and density functional theory (DFT) calculations revealed that all molecules possessed the HLCT excited-state characters without exhibiting ESIPT photophysical properties, whereas the nuclear magnetic resonance spectroscopy, single-crystal and physical property analyses discovered the existence of strong intramolecular H bonds and intermolecular interactions in both mTAHPI and pTAHPI. Consequently, their OLEDs displayed blue emissions with a narrow full width at half maximum (65–68 nm) and achieved excellent electroluminescence (EL) performance with a low turn-on voltage of 2.8 V. Particularly, pTAHPI-based devices showed the highest maximum external quantum effciency (EQE) of 8.13% with an ultra-high brightness of 18 100 cd m⁻². The maximum singlet exciton utilization efficiency (η_s) of the device was estimated to be as high as 94%, which is among the best results of blue electroluminescence to our knowledge.