Solvent dependent absorption and fluorescence of a ketocyanine dye in neat and binary mixed solvents

Abstract

The environmental effect on the ground and excited state properties of a ketocyanine dye has been studied by monitoring its absorption and fluorescence characteristics in various pure and binary mixed solvents. The spectroscopic transition leading to the longest wavelength absorption or fluorescence has been found to involve considerable intramolecular charge transfer as evidenced by semiempirical MO calculations at the AM1 level. Ground state complexation is indicated in protic solvents, aromatic hydrocarbons and cyclic ethers. The fluorescence maximum, <i>E </i>(F), shows a correlation with <i>E</i>_T (30) scale of solvent polarity. A multiple linear correlation of <i>E(</i>F<i>) </i>with Kamlet-Taft parameter representing solvent dipolarity <i>(p*) </i>and hydrogen bonding ability (α and <i>β</i>) suggests that apart from dipolar interactions specific solute-solvent interactions are important in determining fluorescence maxima. Study of <i>E </i>(F) in mixed binary solvents points to a preferential solvation of the solute in the S₁ state.