Effect of H‐Bonding on the Photophysical Behaviour of Coumarin Dyes

Abstract

The Lippert-Mataga [1, 2] expression is the most generalized equation used to describe the solvent-dependent spectral shifts of many chromophoric molecules. The Lippert-Mataga equation considers mainly two characteristic factors of the solvents, namely the refractive index (n) and the static dielectric constant («). According to the Lippert-Mataga representation, a chromophore should show bathochromic shifts in the absorption and emission spectra and a linear increase in the Stokes shift with increasing solvent polarity parameter, Df (Df ¼ [(« À 1)/(2« þ 1) À (n 2 À 1)/(2n 2 þ 1)]). This general solvent effect predicted by the Lippert-Mataga equation is expected to be independent of the chemical properties of the chromophore and the solvent used.Besides its extensive use, the Lippert-Mataga equation fails to predict the spectral properties of several chromophores in different solvent environments [3][4][5][6][7][8]. This failure is mainly due to the presence of specific interactions of the chromophores with their surrounding solvent molecules. These specific interactions can arise owing to different mechanisms such as preferential solvation [9-11], charge transfer interaction [12], hydrogen bonding interaction [13][14][15], etc. The changes in spectral characteristics experienced by a chromophore owing to its specific interaction with the surrounding solvent can sometimes be much more prominent than those expected from the general solvent effect as predicted by the Lippert-Mataga equation. For example, N-phenyl-2-aminonaphthalene shows a structured emission spectrum in a non-polar solvent such as cyclohexane. However, the structures in the emission spectrum disappear, and the emission maxima undergo a large shift on the addition of just $0.2% of ethanol in the cyclohexane solution of the dye [16]. Further, it is seen that the spectrum of N-phenyl-2-aminonaphthalene in 3% ethanolic solution is only marginally different from that observed in the neat ethanol solution. Such a small amount of ethanol in