Nature of stokes shifted dual fluorescence in 2-acetyl-pyrrole: Tuning between intramolecular hydrogen bonding and ESIPT pathways

Possibility of ground/excited state intramolecular proton transfer (GSIPT/ESIPT) have been studied in 2-acetyl-pyrrole (2AP) by theoretical (TDDFT, CASSCF) calculations and its experimental (spectral) verifications. Appearances of different conformers (C, T) and rotamers (Z, Z*) have been investigated by single absorption (284 nm) and stokes shifted dual emission (375 nm and 520 nm) originating from ESIPT. Dual minima in PES scan show lowest energy state for C at S0 state. Small potential energy gap between two minima predicts the equal stability of C* and Z* in S1 state. Lower band gap (C*–Z*) validates barrier less ESIPT from C* → Z*.