Selective Fluorescence Sensing of Copper(II) and Water via Competing Imine Hydrolysis and Alcohol Oxidation Pathways Sensitive to Water Content in Aqueous Acetonitrile Mixtures

Addition of hydrazines to a 1,8-disubstituted anthraquinone macrocycle containing a polyether ring produces site-selective imination, where hydrazone formation produces the more sterically hindered adduct. Reduction of the remaining carbonyl group to a secondary alcohol followed by addition of copper­(II) ion causes intense yellow fluorescence to occur, which is selective for this metal cation and allows this system to be used as a fluorescence sensor. In the presence of water, a green-fluorescent intermediate appears, which slowly decomposes to produce the original starting anthraquinone. The addition of a large amount of water radically changes the reaction pathway. In this case, oxidation of the secondary alcohol is kinetically faster than hydrolysis of the hydrazone, although the same anthraquinone product is ultimately produced. Stern–Volmer data suggest that dioxygen quenches the green emission through both dynamic and static mechanisms; the static ground-state effect is most likely due to association of oxygen with the copper-bound fluorescent intermediate