Herein we report the preparation of novel boronic acid-substituted 4,7-phenanthrolinium viologens and demonstrate their ability to quench the fluorescence of 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (pyranine). In an aqueous buffer solution at pH 7.4, the quenching is suppressed upon binding to monosaccharides, as was determined by Stern-Volmer analysis. Thus, the combination of the viologen. and pyranine is shown to act as a saccharide sensor. A spectroscopic study reveals a ground-state complexation between the viologens and pyranine. In the presence of monosaccharide, the complex dissociates, resulting in a relative increase in the fluorescence. In comparison to the dipyridyl-derived viologen, the phenanthroline-derived viologen shows a greater fluorescence quenching as well as sensitivity to and an unusual selectivity for glucose.