Triplet-triplet absorption of 2-(2′-hydroxyphenyl) benzoxazole (HBO) in polar solvents

The transient TT absorption spectra of 2-(2′-hydroxyphenyl) benzoxazole (HBO) have been measured in liquid polar solvents and in polymethylthacrylate. In non-protonic polar solvents the main TT absorption is from the keto form of the compound (3K∗). With a triplet sensitizer the compound just gives a TT absorption of its enol from (3E∗) after triplet energy transfer from the sensitizer to HBO. The excited state intramolecular proton transfer (ESIPT) from the enol form to keto form occurs just in the singlet state. The protonic solvent can break the intramolecular hydrogen bond of HBO and decrease the ESIPT process. In solid solution both 3E∗ and 3K∗ can be formed and the TT absorptions were detected because of the inhibition of the rotational conformation transfer.