An 'off-on-off' fluorescent sensor for pH based on ligand-proton and ligand-metal-proton interactions

A new tripodal tetradentate ligand (L) containing two pyridines and two amines as donor groups, and bearing an appended anthracene fragment, has been prepared. In a water-acetonitrile mixture, it displays a characteristic 'off-on-off' window behaviour in the I-f vs. pH profile. By means of potentiometric titrations, four protonation constants have been determined for L and a distribution diagram (% of species vs. pH) drawn. Superimposition of the I-f us. pH profile on the distribution diagram suggested that the species responsible for full anthracene emission is LH22+, which contains two free pyridines and two protonated amines, whereas the fluorescence quenching is to be associated with the formation of LH33+ in the acidic zone and LH+ in basic solutions. Moreover, spectrofluorimetric titrations in the presence of Zn2+ (1:1 molar ratio) evidenced also in this case an 'off-on-off' behaviour in the I-f, vs. pH profile (with a maximum centred at a lower pH than in the absence of added metal) due to a partial quenching of the fluorescence emission, also in basic conditions. This latter effect has been ascribed to the formation of [Zn(L)(OH)(n)]((2-n)+) (n = 1,2) species.