Fluorescence and reactivity of p-aminosalicylic acid: an example of proton transfer in the solid state

Abstract

The emission spectrum of p-aminosalicylic acid in ethanol solutions exhibits maxima in the visible (445 nm) and u.v. (ca. 350 nm) spectral regions. These maxima are attributed to the protonated and unprotonated forms of this molecule. Protonation occurs in the excited state as a result of intramolecular transfer of the phenolic proton to the oxygen of the carbonyl group and involves an ordered transition state. A similar process is identified in the solid state. By following the fluorescence emission attributed to this protonated species as a function of time and temperature we have evaluated the activation parameters for the deprotonation occurring in the excited state. Decarboxylation of p-aminosalicylic acid together with the dehydration and decarboxylation of the sodium salt have also been investigated. The mechanism of the solid state decomposition of these materials involves the initial step of intramolecular proton transfer from the phenolic hydroxy to the carbonyl oxygen with the creation of a disordered transition state leading to the final products.