Tuning the Selectivity/Specificity of Fluorescent Metal Ion Sensors Based on N2S2 Pyridine-Containing Macrocyclic Ligands by Changing the Fluorogenic Subunit: Spectrofluorimetric and Metal Ion Binding Studies

Two new fluorescent chemosensors for metal ions have been synthesized and characterized, and their photophysical properties have been explored; they are the macrocycles 5-(2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane (L5) and 5-(5-chloro-8-hydroxyquinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane (L6). Both systems have a pyridylthioether- containing 12-membered macrocycle as a binding site. The coordination properties of these two ligands toward Cu(II), Zn(II), Cd(II), Hg(II), and Pb(II) have been studied in MeCN/H2O (1:1 v/v) and MeCN solutions and in the solid state. The stoichiometry of the species formed at 25 °C have been determined from absorption, fluorescence, and potentiometric titrations. The complexes [CuL5](ClO4)2 . 1/2MeCN, [ZnL5(H2O)](ClO4)2, [HgL5(MeCN)](ClO4)2, [PbL5(ClO4)2], [Cu3(5-Cl-8-HDQH-1)(L6H-1)2](ClO4)3 . 7.5H2O (HDQ ) hydroxyquinoline), and [Cu(L6)2](BF4)2 . 2MeNO2 have also been characterized by X-ray crystallography. A specific CHEF-type response of L5 and L6 to the presence of Zn(II) and Cd(II), respectively, has been observed at about pH 7.0 in MeCN/H2O (1:1 v/v) solutions