This is a computational study of 2-(2'-pyridyl)benzimidazole (2PBI), which has experimentally been found to undergo an excited state proton transfer (ESPT) in the protonated form. In this Letter, the structures of the different forms of this fluorophore have been optimized and CIS calculations have been performed in order to investigate the corresponding excited states. The results help explain some of the experimental observations made earlier by our group as well as by others. (c) 2006 Elsevier B.V. All rights reserved.