Solvent effect on indocyanine dyes: A computational approach

he solvatochromic behaviour of a series of indocyanine dyes (Dyes I-VIII) was investigated by quantum chemical calculations. The effect of the polymethine chain length and of the indolenine structure has been satisfactorily reproduced by semiempirical Pariser-Parr-Pople (PPP) calculations. The solvatochromism of 3,3,3',3'-tetramethyl-NN'-diethylindocarbocyanine iodide (Dye I) has been deeply investigated within the ab initio time-dependent density functional theory (TD-DFT) approach. Dye I undergoes non polar solvation and a linear correlation has been individuated between absorption shifts and refractive index. Computed absorption lambda(max) and oscillator strengths obtained by TD-DFT are in good agreement with the experimental data.