Hydrogen bonded dimers of ketocoumarin in solid state and alcohol: water binary solvent by fluorescence spectroscopy, crystal structure and DFT investigation.
The photophysical properties of 3,3’ Carbonylbis (7-diethylamino)-2H-chromen-2-one (Ketocoumarin) was investigated in solid state , different solvents and alcohol: water binary mixture using steady state, time-resolved fluorescence spectroscopy, XRD studies and DFT calculations. In neat solvents ketocoumarin (KC) shows small Stokes shift as compared to other 7-diethylaminocoumarin dyes (7-DEC) and is due to the π-π* electronic transitions involving the bridge keto group. In aqueous alcoholic solution, the fluorescence maximum of KC depends on the percentage of water and a new emission at 550 nm with large Stokes shift is observed at 90% water content. The solid state fluorescence and XRD crystal structure analysis revels that the new decay component with lifetime 2.71 ns and new emission maximum in alcohol: water mixture is due to the intermolecular hydrogen bonding [HB] interaction between KC molecules in alcohol: water binary mixture. XRD structure reveals that the two modes of intermolecular CH…O=C hydrogen bonding (HB) which lead to two dimeric forms of KC, D1 and D2. The emission wavelength dependent excitation spectrum in solid state and alcohol: water binary mixture confirms the H-type nature of dimer D1 and J-type nature of dimer D2. The time-resolved fluorescence studies indicates the new decay component with lifetime 2.71 ns is due the dimer D1 and new emission with maximum at 550 nm is assigned to dimer D2. Density functional theory (DFT) analysis confirms that the KC dimers are stabilized by hydrogen bonding and other non-covalent interactions. The hydrogen bonded KC dimers are responsible for the anomalous fluorescence behavior of KC in alcohol: water binary mixture and bright yellow fluorescence from solid state.