The role of tautomeric and rotameric species in the photophysics of 2-(2′-hydroxyphenyl)benzoxazole

The complex photophysical behaviour of 2-(2′-hydroxyphenyl)benzoxazole (HBO) has been investigated using both steady-state absorption and fluorescence and time-resolved (picosecond) emission spectroscopy, and found to be consistent with the existence of various tautomeric and rotameric species in equilibrium in the ground state. The natures of these species are discussed. Molecular-orbital calculations (PPP), performed for all species, gave results in good agreement with experiment. Excited-state intramolecular proton transfer was found to occur with a rate constant exceeding 1 × 10¹¹ s^?1.