‘On–off–on’ fluorescent indicators of pH windows based on three separated components

‘On–off–on’ fluorescent indicators for pH windows are obtained with ternary systems in which the three separated components are Cu2+, a tetraaza ligand and the fluorophore Coumarin 343: protonation of Coumarin 343, its coordination to Cu2+ and its displacement from Cu2+ by OH− give an inverse bell-shaped variation of emission with pH