Photo-physical study of coumarins in aqueous organic solvents: An experimental and theoretical approach

In the present work, we have studied the spectroscopic behavior of four structurally similar coumarin dyes and ground (mu(g)) and excited state (mu(e)) dipole moments in aqueous DMSO as well as aqueous DMA using absorption and fluorescence emission spectroscopy. The mu(g) and mu(e) have been determined using the Bilot-Kawski method. The experimental results show that the dipole moments of solutes are influenced by solute-solvent interaction. The increase in solvent polarity leads to an increase in Stokes shift in both the aqueous mixtures while a decrease in Stokes shift has been observed for C519 and C523 in aqueous DMA. The mu(e) is observed to be higher than that of the mu(g) in all the solvents indicating the intramolecular charge transfer (ICT). To support experimental results, the electronic structure and spectroscopic properties of Coumarins have been investigated in gas and different solvent phases by DFT/ TD-DFT calculations. The HOMO-LUMO, molecular electrostatic potential map (MEP), and Natural Bond Orbital (NBO) analysis have been carried out to study the intermolecular charge transfer process.