Solvatochromic shift studies in LD-425 and LD-423: Estimation of ground and excited state dipole moments

The absorption and fluorescence spectra of LD-425 and LD-423 have been recorded at room temperature in different solvents and dioxane-acetonitrile solvent mixtures. The UV-Vis absorption spectra are less sensitive to solvent polarity than the corresponding fluorescence spectra in both the dyes. The effects of solvents upon the spectral properties are analyzed using Lippert-Mataga polarity function, Richardts microscopic solvent polarity parameter and Kamlet's multiple linear regression approach. Both general solute-solvent interactions and hydrogen bonding interactions are operative in these systems. The solvatochromic correlations are used to estimate excited state dipole moments using experimentally determined ground state dipole moments. The excited state dipole moments for both the dyes are found to be larger than their corresponding ground state dipole moment. The solvation studies in 1,4-dioxane-acetonitrile solvent mixtures indicates that these dyes are preferentially solvated by 1,4-dioxane.