Indazole-based deep-blue-emitting hot exciton material: conjugated polycyclic aromaticity molecular design

Abstract

Efficiently capturing electroluminescent triplet excitons is crucial for the fabrication of high-performance organic light-emitting diodes (OLEDs). In the past decade, hot exciton materials have emerged as up-and-coming luminogens to utilize abundant triplet excitons via rapid high-lying excited-state reverse intersystem crossing (hRISC) processes. Although great progress has been made in promoting the photophysical mechanism and in device fabrication, the scope of hot exciton materials remains still limited to only a few of chromophores, namely, phenanthroimidazole, benzothiazole, and their derivatives. This could be ascribed to the scarcity of a feasible material design strategy and basic guidelines, which means the current molecular screening has to resort to lengthy trial-and-error methods. Thus, developing a feasible material design strategy for constructing new hot exciton molecular systems is of great importance. In this work, we propose a new hot exciton material design based on conjugated polycyclic aromaticity. The different aromaticity of heterocycle systems in the ground state and excited state originates from the distinctive π electron delocalization in the two states, which could lead to suitable molecular excited-state energetics, including the desired singlet–triplet energy gaps to facilitate an efficient hRISC process and the low-lying triplet state and ensuing a large triplet–triplet energy gap to restrain triplet energy internal conversion loss. On the strength of such design guidelines, a novel indazole-based deep-blue-emitting hot exciton material was successfully developed and subsequently applied to OLED devices, obtaining a maximum external quantum efficiency of 5.71%. The study not only provides a series of new electroluminescent triplet-harvesting materials but also offers a practicable design guideline for screening and developing a new generation of luminogens for OLED applications.