Excited state behavior of pyrrole 2-carboxyldehyde: theoretical and experimental study.

Photophysical and photochemical dynamics of excited state proton transfer reaction have been reported for Pyrrole 2-carboxyldehyde (PCL). Experimental and theoretical observations yield all possible signatures of intramolecular and intermolecular proton transfer in an excited state. Dual emission (~325 nm, ~375 nm) on photo excitation indicates the existence of more than one species in an excited state. Computed reaction pathway and two-dimensional potential energy profile in the ground state reveals a single minimum corresponding to normal form (E). Dual minima in excited state energy profile shows the existence of two species, one normal and other zwitterionic (Z*) species. A large Stokes shifted emission at ~375 nm in hydrocarbon medium reveals the existence of zwitterionic species due to Excited state intramolecular proton transfer (ESI(ra)PT). Excited State Intermolecular proton transfer (ESI(er)PT) is observed in a hydroxylic environment around 430-490 nm. pH variation in hydroxylic medium suggests the formation of anion (A((-))) from Z*.