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 (k(q)approximate to 4.3x10(9) M-1 s(-1)). The values of polarity (pi*), proticity (alpha) and E-T(30) of dioxane-water mixtures assessed by the solvatochromism of this molecule, are in agreement with a "critical'' concentration approximate to 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.