Zn²⁺/Cd²⁺ optical discrimination by fluorescent chemosensors based on 8-hydroxyquinoline derivatives and sulfur-containing macrocyclic units

Four new fluorescent chemosensors for metal ions based on 8-hydroxyquinoline (8-HDQ) derivatives and sulfur-containing macrocyclic units were synthesized and characterized, namely 1-(5-chloro-8-hydroxy-7- quinolinylmethyl)-1-aza-4,7,10-trithiacyclododecane (L1), 1-(5-chloro-8-hydroxy- 7-quinolinylmethyl)-1-aza-4,13-dithia-7,10-dioxacyclopentadecane (L2), 1-(8-hydroxy-2-quinolinylmethyl)-1-aza-4,7,10-trithiacyclododecane (L3), and 1-(8-hydroxy-2-quinolinylmethyl)-1-aza-4,13-dithia-7,10-dioxacyclopentadecane (L4). Preliminary fluorimetric titrations indicated L1 as the only member of the family of ligands to give a selective CHEF-type response to the presence of Zn²⁺ in MeCN-H₂O (1:1, v/v) solutions, which allowed imaging of this metal ion in Cos-7 cells in vitro. The other ligands either did not show any fluorescence response (L3, L4) to any of the metal ions considered (Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb ²⁺) or gave (L2) a CHEF-type response also to the presence of Cd ²⁺. The coordination properties of L1 towards Zn²⁺ were, therefore, fully investigated by potentiometric measurements and absorption and emission spectroscopy at different pH values, which indicated that the formation of 2:1 L1/Zn²⁺ complexes is responsible for the CHEF-type effect observed. The complexes [Zn(L1)₂H₂O](BF₄) ₂ and [Zn(L3)](ClO₄)₂ were characterized in the solid state by X-ray crystallography, and DFT calculations were performed to understand the origin of the Zn²⁺/Cd²⁺ optical discrimination of the 8-HDQ-based "conjugate" fluorescent chemosensors reported.