Water Soluble Molecular Switches of Fluorescence Based on the NiIII/NiII Redox Change

The water soluble NiII complexes of the cyclam derivatives 2 and 3 display the fluorescent emission typical of the covalently linked fluorophores, 1,3 benzodioxole and 1,2,3-trimethoxybenzene, which results from a charge transfer excited state. On oxidation to NiIII, the fluorescence is completely quenched due to the occurrence of an electron transfer (eT) process from the excited fluorogenic fragment Fl* to the oxidized metal. Thus, fluorescence can be switched ON/OFF at will, for several cycles, by consecutively oxidizing and reducing the metal center, in controlled potential electrolysis experiments both in acetonitrile and in aqueous 0.1M HClO4. Occurrence of an eT process from Fl* to NiIII ultimately depends upon the easy oxidation of Fl to Fl+, whereas failure of the occurrence of an eT process from NiII to Fl* has to be ascribed to the particular resistance of Fl fragments to the reduction.