Synthesis and Optical Properties of Hydroxyphenyl-Benzimidazole Derivatives with Triple Intramolecular Hydrogen Bonds

Abstract We synthesized compounds 1 and 2 that form three intramolecular hydrogen bonds with different patterns. DFT calculations demonstrated that the all-enol form is the most stable ground-state structure. The THF solution of 1 exhibited ESIPT emission with a maximum at 457 nm, whereas the DMF solution showed fluorescence with maxima at 438 and 516 nm attributable to emission from an anionic species. The THF solution of 2 also exhibited ESIPT emission at 489 nm with a fluorescence quantum yield of 0.87. In contrast, the DMF solution exhibited excitation-wavelength-dependent fluorescence spectra. Excitation at 311 nm resulted in typical ESIPT emission at 484 nm, whereas excitation at 400 nm gave fluorescence at 437 nm characterized by a small Stokes shift, attributed to an anionic species. Room-temperature luminescence lifetime measurements indicated that only fluorescence on the nanosecond timescale is observed for both compounds. When variable-temperature measurements were performed on 1, phosphorescence was observed at 500 nm at 80 K. Upon increasing the temperature, this peak disappeared and no delayed emission was observed. Theoretical calculations revealed that, even after ESIPT, the energy difference between the singlet and triplet excited states remains large, and TADF via RISC does not occur.