TQOPEN (N,N,N',N'-Tetrakis(2-quinolylmethyl)-3-oxa-1,5-pentanediamine) Family as Heptadentate Fluorescent Cd2+ Sensors

Abstract

A quinoline-based heptadentate ligand, N,N,N',N'-tetrakis(2-quinolylmethyl)-3-oxa-1,5-pentanediamine (TQOPEN), exhibits a fluorescence increase (I_Cd/I₀ = 25, ϕ_Cd = 0.017) at 428 nm upon addition of 1 equiv of Cd²⁺. In contrast, 1 equiv of Zn²⁺ induces a negligible fluorescence change due to weak interaction (I_Zn/I₀ = 2.5, I_Zn/I_Cd = 10%). In comparison with TQOPEN, the thia and aza derivatives TQSPEN and TQNPEN exhibit improved Cd²⁺/Zn²⁺ selectivity and higher Cd²⁺-binding affinity, respectively. The solid-state structures of mononuclear Cd²⁺ and hydroxide-bridged dinuclear Zn²⁺ complexes of TQOPEN were elucidated by X-ray crystallography. Although the crystal structure of the TQOPEN-Cd²⁺ complex exhibits a six-coordinate metal center, in which one quinoline weakly interacts with the Cd center (Cd···N_quinoline = 3.303(3) Å), a ¹H NMR study at 233 K suggests that all quinolines interact with the Cd center to form a symmetrical seven-coordinate structure in solution. Theoretical calculations (TDDFT) support the flexible coordination environment around the Cd center, leading to intramolecular excimer formation with two quinoline moieties in the excited state. The importance of a heptadentate structure was further demonstrated by the lack of Cd²⁺ specificity with hexadentate ligands TriQOPEN (N,N,N'-tris(2-quinolylmethyl)-3-oxa-1,5-pentanediamine) and TQCPEN (N,N,N',N'-tetrakis(2-quinolylmethyl)-1,5-pentanediamine).