Computational study of steric and spectroscopic characteristics of bi-chromophoric cyanine dyes: Comparison with experimental data

The spectral and energetic characteristics of four bi-chromophoric cyanine dyes (BCDs) which possess angles between chromophores 180°, 150°, 120°and 90°, were studied using quantum chemical calculations in comparison with experimental data. It was demonstrated that for BCD with 180°, 150° and 90° trans–trans isomers possess the lowest energy, while for BCD with 120° the trans–trans and cis–trans isomers have comparable energies and in the temperature range from 273 K up to 373 K both isomers of this dye are present. It was also demonstrated that the splitting of the spectra of cyanine dyes with two chromophores (BCD) was determined by two effects: the dipole–dipole chromophore interaction and the electron tunneling through the central heterocycle. Both effects depend on the central heterocycle structure, which on the one hand determines the distance between the chromophores, thus determining the value of the dipole–dipole interaction, and on the other hand the degree of π-conjugation in the central heterocycle determines the probability of electron tunneling. The central heterocycle structure determines relative orientation of the chromophore dipoles, as well, thus determining the intensities of the short-wavelength and long-wavelength bands in the BCD absorption spectra.