Two polyaminophenolic fluorescent chemosensors for H+ and Zn(ii). Spectroscopic behaviour of free ligands and of their dinuclear Zn(ii) complexes

Abstract

The UV-Vis and fluorescence optical properties of the two polyamino-phenolic ligands 3,3′-bis[N,N-bis(2-aminoethyl)aminomethyl]-2,2′-dihydroxybiphenyl (L1) and 2,6-bis{[bis(2-aminoethyl)amino]methyl}phenol (L2) were investigated in aqueous solution at different pH values as well as in the presence of Zn(II) metal ion. Both ligands show two diethylenetriamine units separated by the 1,1′-bis(2-phenol) (BPH) or the phenol (PH) for L1 and L2, respectively. Both ligands are fluorescence-emitting systems in all fields of pH examined, with L1 showing a higher fluorescence emission than L2. In particular, the emission of fluorescence mainly depends on the protonation state of the phenolic functions and thus on pH. The highest emitting species is H₃L³⁺ for both systems, where the BPH is monodeprotonated (in L1) and the PH is in the phenolate form (in L2). On the contrary, when BPH and PH are in their neutral form both ligands show the lowest fluorescence, since H-bonds occurring between the phenol and the closest tertiary amine functions decrease fluorescence. The Zn(II)-dinuclear species are also fluorescent in the pH range where they exist; the highest emitting species being [Zn₂(H₋₂L1)]²⁺ and [Zn₂(H₋₁L2)]³⁺ which are present in a wide range of pH including the physiological one. Fluorescence experiments carried out at physiological pH highlighted that, in the case of L1, the presence of Zn(II) ion in solution produces a simultaneous change in λ_em with a drop in fluorescence due to the formation of the [Zn₂(H₋₂L1)]²⁺ species, while, in the case of L2, it gives rise to a strong CHEF effect (a twenty-fold enhancement was observed) due to the formation of the [Zn₂(H₋₁L2)]³⁺ species. These results, supported by potentiometric, ¹H and ¹³C NMR experiments, are of value for the design of new efficient fluorescent chemosensors for both H⁺ and Zn(II) ions.