Isomerization effect of triphenylamine-acridine derivatives on excited-state modification, photophysical property and electroluminescence performance

Abstract

Charge-transfer (CT) state is becoming a useful excited state to design next-generation luminescence materials for high-performance organic light-emitting diode (OLED). However, strong CT state usually causes serious decrease in photoluminescence (PL) efficiency due to the small overlap between HOMO and LUMO. In order to harvest both high exciton utilization and high PL efficiency in fluorescent OLEDs, figuring out the strategy of fine modification of CT component in excited state through structural modulation is important. In this work, four donor–acceptor structure triphenylamine (TPA) - acridine (AC) isomers (TPA-9AC, TPA-1AC, TPA-2AC and TPA-3AC) were designed and synthesized to investigate structure-property relationship between isomerization effect and excited state properties. Density functional theory (DFT) calculations and solvatochromic absorption, emission and photoluminescence decay spectra are carried out for the deep understanding of their emissive state character. The four isomers exhibit gradually increased PL efficiency from low-polarity hexane to medium-polarity tetrahydrofuran (THF), which could be assigned to the formation of hybridized local and charge-transfer (HLCT) states and effectively suppressed non-radiative transition arised from acridine in medium-polarity solvents. Among four isomers, TPA-3AC achieved the best EL performance, due to the proper LE and CT compositions in the emissive state, demonstrating that isomerization of donor and acceptor functional moieties is an effective approach for structural modification for high-efficiency fluorescent OLED emitters.