Influence of solvent polarity and hydrogen bonding on the electronic transition of coumarin 120: a TDDFT study

The characteristics of the electronic transition energy of Coumarin 120 (C120) and its H-bonded complexes in various solvents have been examined by time-dependent density functional theory (TDDFT) in combination with a polarizable continuum solvent model (PCM). Molecular structures of C120 and its H-bonded complexes are optimized with the B3LYP method in PCM solution, and the dihedral angle H14-N13-C7-H15 is dependent on solvent polarity and the type of H-bond. A linear correlation of the absorption maximum of C120 with the solvent polarity function is revealed with the PCM model for all solvents except DMSO. The experimental absorption maximum of C120 in nine solvents is well described by a PCM-TDDFT scheme augmented with explicit inclusion of a few H-bonded solvent molecules, and quantitative agreement between our calculated results and experimental measurements is obtained with an average error of less than 2 nm. H-bonding at three different sites shifts the absorption wavelength of C120 either to the blue or to the red, that is, a significant role is played by solvent molecules in the first solvation shell in determining the electronic transition energy of C120. The dependence on the H-bonding site and solvent polarity is examined by using the Kamlet-Taft equation for solvatochromism.