Theoretical study of excited state proton transfer in pyrrole-2-carboxylic acid

Pyrrole 2-carboxylic acid (PCA) shows dual emission (310 nm and 430 nm) in water on photo-excitation, which indicates that more than one species is in the excited state. This paper reports on the quantum chemical analysis of pyrrole 2-carboxylic acid (PCA) in the light of a possible excited state proton transfer. Dipole moment, excited state energy and findings in molecular orbital calculations (HOMO, LUMO) establish that PCA is a likely candidate for transfer of a proton from the pyrrole moiety to the C=O of carboxylic moiety (possible zwitterionic form) in the excited state. Overall, the computed predicted results of intramolecular and intermolecular excited state proton transfer corroborates the experimental results.