Hydrogen bonding effects in the photophysics of a drug, Piroxicam, in homogeneous media and dioxane–water mixtures

The fluorescence emission of Piroxicam, an anti-inflammatory drug exhibiting excited-state intramolecular proton transfer (ESIPT), was studied in pure solvents and in dioxane–water mixtures. The solvent proticity was seen to play a key role in the ground-state promoting the existence of different conformers (open and closed form), and in the excited-state where it is responsible for a pronounced quenching of the tautomer emission in the aforementioned mixtures. A derived Stern–Volmer relation shows the diffusive control of the process (kq≈4.3×109 M-1 s-1). The values of polarity (π*), proticity (α) and ET(30) of dioxane–water mixtures assessed by the solvatochromism of this molecule, are in agreement with a “critical’' concentration ≈22 M above which water presents the characteristics of the free solvent. Below that concentration, Piroxicam adopts a closed conformation through an intramolecular hydrogen bond in a six-membered ring, which is very stable and thus insensitive to microenvironmental changes.