A Preparative and Preliminary Spectroscopic Study of Analogues of a Zinquin-Related Fluorophore

Abstract

The syntheses of the 4- and 5-methoxy isomers of 4-N-(6-methoxy-2-methyl-8-quinolyl)-4-methylbenzenesulfon-amide and of N-(2-methoxy-8-quinolyl)-4-methylbenzenesulfonamide are described. The 6-methoxy compound is a precursor of Zinquin ester, a specific fluorophore for Zn(II). The 2-methoxy analogue was synthesized by nitration of 2-chloroquinoline and subsequent functional group manipulation. The 4-methoxy isomer was synthesized from a 4-quinolone derivative, and the 5-methoxy isomer was synthesized by a standard Skraup quinoline synthesis. The structures of the 4- and 5-methoxy isomers were determined by single-crystal X-ray analysis. All of these compounds showed a bathochromic shift in their ultraviolet/visible spectra upon addition of Zn(II) to the solution. These compounds are all weakly or non-fluorescent in solution. All form fluorescent complexes with Zn(II) except the 5-methoxy compound. The 4-methoxy compound forms a significantly more fluorescent complex than those of the 6-methoxy compound and Zinquin ester and has a higher quantum yield than the others.