Solvent and temperature effects on the electronic transitions of 3-H-indolo-2-dimethinehemicyanine dyes

Solvent effects on the UV/vis absorption spectra of five hemicyanine dyes 1, obtained by the condensation of (1,3,3-trimethylindolin-2-ylidene)acetaldehyde 2 with different substituted anilines 3, have been investigated. Analysis of the observed hypsochromic shifts using Reichardt's single parameter solvent polarity scale (ETN) revealed that the dye obtained with 4-aminophenol is the most sensitive to solvent effects. Multiple regression analysis based on the Kamlet–Taft equation indicated that (i) the hydrogen bond donor properties and the dipolarity/polarizability of the solvent have an equal contribution to the solvatochromic behavior of the dye obtained with aniline, and (ii) the hydrogen bond accepting properties of the solvent have a significant contribution to the solvatochromic behavior of the 3-hydroxy-substituted dye. UV/vis absorption spectra were also recorded in mixtures of water and 2-propanol. A preferential solvation in 2-propanol was observed and quantified for all dyes. Finally, the effects of the temperature on the solvatochromism in binary mixtures were also investigated in order to reveal the susceptibilities of dye–water–propanol interactions. Increasing temperatures produced a depletion of the 2-propanol in the solvation shell of the hemicyanines.