title: 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 n pi* state is responsible for the fast intersystem crossing while the proximity of the reactive second single pi pi* state to the first singlet pi pi* state contributes to the fast photoreaction in the excited states.
authors.affiliation: Graduate School of Pharmaceutical Sciences, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Seiichi
authors.initials: S
authors.lastname: Uchiyama
authors.forename: Kazuyuki
authors.initials: K
authors.lastname: Takehira
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kohtani
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Imai
authors.forename: Ryoichi
authors.initials: R
authors.lastname: Nakagaki
authors.forename: Seiji
authors.initials: S
authors.lastname: Tobita
authors.forename: Tomofumi
authors.initials: T
authors.lastname: Santa