Fluorescence quenching of carmoisine by viologens in neat methanol: Observation of inversion in quenching

Fluorescence quenching is an important technique having chemical and biochemical applications and it follows the well-known Stern-Volmer theory. It is interesting to explore anomalously behaving systems, especially when two structurally related quenchers are used. We have observed for the first time, a unique dependence of fluorescence quenching of an azo-dye, carmoisine, on diheptyl viologen (HV) concentration in neat methanol. It shows recovery of fluorescence at higher quencher concentrations. Initially, quenching increases followed by a maximum and at still higher concentrations fluorescence is recovered. But structurally related methyl viologen (MV) instead shows only saturation with the same dye. Finally, fluorescence quenching so reported here has been found to follow static mechanism.