Fluorescence on–off switching mechanism of benzofurazans

Abstract

Many fluorescent reagents with a benzofurazan (2,1,3-benzoxadiazole) skeleton have been developed and widely used in bio-analyses. In this study, we try to elucidate the fluorescence on–off switching mechanism of three fluorogenic reagents and their derivatives. Ten 4,7-disubstituted benzofurazans were used for this purpose and the measurements of their fluorescence, phosphorescence, photolysis, and time-resolved thermal lensing signal in acetonitrile were obtained in order to understand the relaxation processes of these compounds. These results indicate that the competition of fluorescence with a fast intersystem crossing or fast photoreaction plays a key role in the fluorescence on–off switching. Semi-empirical molecular orbital calculations show that the existence of the triplet nπ* state is responsible for the fast intersystem crossing while the proximity of the reactive second singlet ππ* state to the first singlet ππ* state contributes to the fast photoreaction in the excited states.