The electronic origin of the ground state spectral features and excited state deactivation in cycloalkanones: the role of intermolecular H-bonding in neat and binary mixtures of solvents.

In this study, a D-A cycloalkanone (K1) has been investigated by steady state absorption and fluorescence in neat solvents and in three binary mixtures of nonpolar aprotic/polar protic, polar aprotic/polar protic, and polar protic/polar protic solvents. The experimental findings were complemented by density functional theory (DFT), time-dependent density functional theory (TD-DFT), and NBO quantum-mechanical calculations. Experimentally, effective changes in absorption and fluorescence were observed by solute-solvent interaction. The binary K1-solvent-solv configuration, modeled at the B3LYP-DFT level, confirms involvement of inter-molecular H-bonding with the carbonyl C=O in the fluorescence deactivation process (quenching). This is supported by considerable electron delocalization from C=O to the solvent's hydroxyl (n → σ*). This type of hyperconjugation was found to be the main driver for solute-solvent stabilization.