Abstract
The fluorescence of 2-(2'-hydroxyphenyl)benzoxazole (HBO) is strongly Stokes-shifted due to intramolecular proton transfer in the excited singlet state by which the enol form ('E*) of HBO is converted into the keto form ('K*). At room temperature the decay of 'K* is dominated by a thermally activated radiationless transition. At lower temperatures fluorescence and intersystem crossing are the main deactivation processes. In the triplet state of the keto tautomer proton back-transfer to the enol triplet state, 3E*, takes place. At high triplet-state concentrations the transient decay is mainly governed by triplet-triplet annihilation. This process was used to determine the extinction coefficients of the triplet-triplet absorption. In contrast to other work the singlet ground-state absorption of the keto tautomer, 'K, was not observed.
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