A Preparative and Spectroscopic Study of Fluorophores for Zinc(II) Detection

Abstract

Zinc(II) specific fluorophores are of substantial importance in the study of intracellular Zn ²⁺ . Two such widely used fluorophores are 2-methyl-8-(4-toluenesulfonamido)-6-quinolyloxyacetic acid, Zinquin (1), and its ethyl ester, Zinquin ester (2), which fluoresce strongly when bound by Zn ²⁺ . To gain insight into the factors affecting the fluorescence of such fluorophores the closely related 4-methyl-N-(6-methoxy-2-methyl-8-quinolyl)-benzenesulfonamide (3), and nine analogues (4–12), substituted at sulfur by nine different substituents have been prepared and their fluorescing characteristics and those of their Zn ²⁺ complexes have been examined. The nine substituents are: 2,2,2-trifluoroethyl (4), 4-methoxyphenyl (5), 4-acetamidophenyl (6), 4-bromophenyl (7), 4-nitrophenyl (8), 3-trifluoromethylphenyl (9), naphth-1-yl (10), naphth-2-yl (11) and 5-dimethylaminonaphth-1-yl (12). Under neutral conditions in 75/25% v/v ethanol/water solutions, (3)–(7) and (9)–(11) fluoresce weakly in the free state, (8) does not fluoresce and (12) fluoresces strongly. Ultraviolet (UV)-visible spectroscopy shows that (3)–(12) complex to Zn ²⁺ , but unlike the remainder, the complexes of (8) and (12) do not fluoresce, with those possessing electron-withdrawing substituents, (4) and (9), being the most fluorescent. On this basis ethyl-2-(2-methyl-quinolyloxy-8-(2,2,2-trifluoroethylsulfonamido))acetate (19) was prepared and its Zn ²⁺ complex was found to be substantially more fluorescent than that of (2).