TRIPLET-TRIPLET ABSORPTION OF 2-(2'-HYDROXYPHENYL) BENZOXAZOLE (HBO) IN POLAR-SOLVENTS

The transient T-T absorption spectra of 2-(2'-hydroxyphenyl) benzoxazo le (HBO) have been measured in liquid polar solvents and in polymethyl mathacrylate. In non-protonic polar solvents the main T-T absorption i s from the keto form of the compound (K-3). With a triplet sensitizer the compound just gives a T-T absorption of its enol form ((3)E) aft er triplet energy transfer from the sensitizer to HBO. The excited sta te intramolecular proton transfer (ESIPT) from the enol form to keto f orm occurs just in the singlet state, The protonic solvent can break t he intramolecular hydrogen bond of HBO and decrease the ESIPT process, In solid solution both. (3)E and K-3* can be formed and the T-T abso rptions were detected because of the inhibition of the rotational conf ormation transfer.