Spectral and time domain fluorescence spectroscopy of gentisic acid molecule in protic and aprotic polymer matrix

In the present work, the effect of polymer microenvironment on the photophysics of gentisic acid molecule [2,5-dihydroxybenzoic acid] (GA), steady-state and time-domain fluorescence measurements at different pH conditions were carried out in protic [polyvinyl alcohol PVA] and aprotic [polymethyl methacrylate (PMMA)] polymer matrices. Change in the proticity of the microenvironment of the polymer traps different ionic species along with the neutral form of rotamer P and R conformers of GA molecule, are found to be responsible for the change in the spectral, multi-exponential decay behaviour. In protic polymer, the appearance of a single emission band indicates, dissociation of the GA molecule is very high, and it present as a monoanion along with hydrogen-bonded P and R rotamers. However, in the basic polymer film, most of the conformers of R converted to the anion. In contrast, protonation slows down the dissociation of both P and R forms in the acidic film. Unlike PVA matrix, in PMMA, dual emission band appears due to slow dissociation of GA molecule and hydrogen-bonded rotamer P, and R form exists with monoanion species. The magnitude of large stokes shifted red emission due to excited-state intramolecular proton transfers (ESIPT) found grater in rotamer P compared to its anionic species (green emission) and a blue emission corresponds to rotamer R.