Investigation of a Series of 2-(2'-Hydroxyaryl)benzazole Derivatives: Photophysical Properties, Excited-State Intramolecular Proton-Transfer Reactions, and Observation of Long-Lived Triplet Excited States.

Excited state intramolecular proton transfer (ESIPT) has drawn much attention for its important applications in a variety of areas. Here, the steady-state and time-resolved absorption spectroscopic experiments as well as DFT/TD-DFT calculations are employed to study the photophysical properties and photochemical reaction mechanisms of 2-(2'-hydroxyphenyl) benzoxazole (HBO) and selected derivatives (compounds 1-3). Because of their larger π-conjugation framework, compounds 1-3 display red-shifted absorbance but blue-shifted fluorescence compared with HBO. A fast ESIPT process is observed directly for HBO while compound 3 has an enol/keto equilibrium type of ESIPT that exhibits dual emission. Interestingly, only the emission of the enol form is observed for compounds 1 and 2 which suggests that the ESIPT process is strongly inhibited. These results indicate the decoration with electron-withdrawing groups such as thiadiazol and pyrazine on the hydroxyphenyl ring (compounds 1 and 2) apparently suppresses the proton-transfer processes in their excited states. Whereas the ESIPT process is rarely increased for compound 3 that modified with the phenanthrol ring, because the effective conjugation is reduced for compound 3 compared with HBO. The work here provides fundamental insights that may be useful for designing novel ESIPT molecules in the future.