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

Journal of Materials Chemistry C Pub Date: 2022-07-04 DOI: 10.1039/D2TC01036D

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 (ΔET2T1) and a small energy difference between T2 and S1 (ΔET2S1). 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 ΔET2T1 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 ΔET2S1 mean 2NpNMZ enjoys better efficiency than 2AnNMZ when they are used as the emitting layers of non-doped OLEDs.

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