Anion detection by fluorescent Zn(II) complexes of functionalized polyamine ligands

The Zn²⁺ coordination chemistry and luminescent behavior of two ligands constituted by an open 1,4,7-triazaheptane chain functionalized at both ends with 2-picolyl units and either a methylnaphthyl (L1) or a dansyl (L2) fluorescent unit attached to the central amino nitrogen are reported. The fluorescent properties of the ZnL1²⁺ and ZnLi²⁺ complexes are then exploited toward detection of anions. L1 in the pH range of study has four protonation constants. The fluorescence emission from the naphthalene fluorophore is quenched either at low or at high pH values leading to an emissive pH window centered around pH = 5. In contrast, in L2 the fluorescence emission from the dansyl unit occurs only at basic pH values. In the case of L1, a red-shifted band appearing in the visible region was assigned to an exciplex emission involving the naphthalene and the tertiary amine of the polyamine chain. L1 forms Zn²⁺ mononuclear complexes of ZnH_pL1 ^(p+2)+ stoichiometry with p = 1, 0, -1. Formation of the ZnL1 ²⁺species produces a strong enhancement of the L1 luminescence leading to an extended emissive pH window from pH = 5 to pH = 9. Addition of several anions to this last complex leads to a partial quenching effect. On the contrary, the fluorescence emission of L2 is partially quenched upon complexation with Zn²⁺ in the same pH window (5 < pH < 9). The lower stability of ZnL2²⁺ with respect to ZnL1²⁺ suggests a lack of involvement of the sulfonamide group in the first coordination sphere. However, there is spectral evidence for an interesting photoinduced binding of the sulfonamide nitrogen to Zn²⁺. While addition of diphosphate, triphospate, citrate, and D,L-isocitrate to a solution of ZnL2²⁺ restores the fluorescence emission of the system (λ_max ca. 600 nm), addition of phosphate, chloride, iodide, and cyanurate do not produce any significant change in fluorescence. Moreover, this system would permit one to differentiate diphosphate and triphosphate over citrate and D,L-isocitrate because the fluorescence enhancement observed upon addition of the first anions is much sharper. The ZnL2²⁺ complex and its mixed complexes with diphosphate, triphosphate, citrate, and D,L-isocitrate have been characterized by ¹H, ³¹P NMR, and Electrospray Mass Spectrometry.