Solvent Effect on the Excited State Dynamics of Indazole

The dynamics of the excited state of indazole in various protic solvents has been studied by fluorescence and phosphorescence measurements and picosecond spectroscopy. It is shown that the fluorescenct properties of indazole are remarkably dependent on the nature of the solvent. In a nonacidic solvent only fluorescence of 1H indazole is observed. In a carboxylic acid with pKa～4.5 such as acetic acid, 1H is converted into 2H indazole in the excited state by double proton transfer. The rate of conversion and its activation energy are determined. In an acid with a smaller pKa value indazole is protonated in the ground state and provides a broad Stokesshifted fluorescence spectrum. It is shown that a structural change takes place in the excited singlet state of the protonated species in the time period of 10～100 ps depending on the solvent. It is suggested that indazole is protonated in a benzoic acid host and the triplet states of both 1H and 2H indazoles are produced from the excited state of protonated indazole