Excited State Intramolecular Proton Transfer-Mediated Thermally Activated Delayed Fluorescence Behavior of Hexapole Hydrogen Bonding Molecule

Abstract

Increasing the emission quantum efficiency of organic compounds composed of ubiquitous elements will lead to the production of bright organic light-emitting diodes. One solution to this issue is to utilize the triplet exciton, and excited-state intramolecular proton transfer has recently attracted attention. In this study, we investigated the photophysical properties of compound 1, which has three alternating benzimidazole and hydroxy groups substituted on the benzene ring and can form hexapole hydrogen bonds. 1 showed an emission maximum at 440 nm, and solvent polarity had a small influence. The emission intensity increased under an argon atmosphere, and the degassed sample solution showed a long-lived component with a time constant of 15 μs. The emission spectrum at 80 K, which had a maximum at 440 nm and a shoulder around 460 nm, implies the simultaneous fluorescence and phosphorescence emissions. Variable temperature lifetime measurements suggested that phosphorescence disappears at 160 K, and thermally activated delayed fluorescence is observed at elevated temperatures. Theoretical calculations revealed that all-keto form 1A is photoexcited, generating 1B* after one proton transfer, and thermally activated delayed fluorescence is observed via reverse intersystem crossing from higher-lying triplet states (T₂ and T₃) to the singlet state (S₁).