Photolysis of a dansylamide moiety of N-dansylcyclen (L5) in the presence of zinc(II) ion in aqueous solution is reported (cyclen = 1,4,7,10-tetraazzacyclododecane). By potentiometric pH titrations, the complexation constant for the 1:1 complex of L5 and zinc(II), log Ks(ZnL5), in aqueous solution at 25 oC with I = 0.1 (NaNO3) was determined to be 6.5 +0.1. Upon complexation with zinc(II) at pH 7.4, fluorescent emission of L5 at 555 nm decreased (excitation at 308 nm). However, we accidentally found the considerable enhancement in fluorescence emission of ZnL5 upon UV irradiation, while UV and emission spectra of the metal-free L5 exhibited negligible change upon photoirradiation. We have revealed that photoinduced hydrolysis (by UV irradiation at 338 nm) of a sulfonylamide bond of ZnL5 promoted by Zn2+-bound HO- in the excited states afforded Zn2+-cyclen and dansyl acid, whose emission is stronger than that of ZnL5.
