Referential tuning strategy for high-lying triplet energy level setting in OLED emitter with hot-exciton characteristics

Abstract

“Hot-exciton” emitters featuring high-lying reverse intersystem crossing (hRISC) have served as promising candidates due to their improved exciton utilization efficiency and efficiency roll-off in organic light-emitting diodes (OLEDs). Generally, optimal and expected hRISC processes should take place from T₂ to S₁, which could be achieved by fabricating a larger energy gap between T₂ and T₁ (ΔE_T2T1) and a small energy difference between T₂ and S₁ (ΔE_T2S1). The accurate and controlled regulation of the distribution of excited-state energy levels showed the special importance of activating hRISC but, regrettably, work has rarely been reported on how to regulate the position of high-lying triplet levels. In this work, two similar emitters (2NpNMZ and 2AnNMZ) were designed to attempt to tune the T₂ state based on a new “hot-exciton” core of naphtho[2,3-d][1,2,3]triazole (NMZ). An interesting discovery was that the T₂ levels of the two emitters were contributed by the T₁ levels of the corresponding substituent group, which was ascribed to the orbital electrons on the T₂ state being concentrated in substituent units (naphthalene and anthracene). Their T₁ energy levels were similar because the orbital electrons of the T₁ state were both contributed by the NMZ core. 2NpNMZ possessed larger ΔE_T2T1 than 2AnNMZ because of the higher T₁ level of Np than that of an An unit, which might suppress the internal conversion of a T₂ exciton to a T₁ platform (T₂ → T₁). In addition, the higher PLQY in neat film and smaller ΔE_T2S1 mean 2NpNMZ enjoys better efficiency than 2AnNMZ when they are used as the emitting layers of non-doped OLEDs.