Photophysical properties of 2,3,6,7-tetrahydro-8-hydroxy-1H, 5H-benz[i, j] quinolizine-9-carboxaldehyde: Evidence of excited state intramolecular proton transfer but not of intramolecular charge transfer process

The photophysical behaviours of 2,3,6,7-tetrahydro-8-hydroxy-1H, 5H-benzo[i, j] quinolizine-9-carboxaldehyde (THBQC), a molecule having both the intramolecular excited state donor acceptor charge transfer and six member intramolecular hydrogen bonded proton transfer sites, have been investigated by steady state and time resolved spectroscopy in combination with quantum chemical calculations. The observed spectral characteristics of THBQC with variation of solvent properties, pH and temperature of the medium confirm the existence of different neutral and ionic species in the ground and excited states. Comparatively less solvent polarity dependent red shifted emission band of THBQC in all solvents is attributed to excited state intramolecular proton transfer of the closed conformer leading to keto-enol tautomerism but not of intramolecular charge transfer process. In polar solvents, apart from the proton transfer emission band, another band at higher energy region is attributed to the emission from the open solvated form. Evaluation of the potential energy surfaces by quantum chemical calculations using Density functional theory (DFT) and Hartree–Fock (HF) levels point towards the possibility of proton transfer reaction in the first excited state and correlate well with the experimental findings.