TDDFT study of the optical properties of Cy5 and its derivatives.

Cyanine dyes which show a very strong pi-pi* type electronic absorption have attracted attention as active components for optoelectronic applications. In the present study, the optical properties of Cy5 and Cy5-X (X = F, Cl, Br, and I) have been theoretically explored by applying the time-dependent density functional theory (TDDFT) method. The geometries of the ground state of the studied models have been fully optimized at the B3LYP/LanL2DZ and B3LYP/SDD levels of theory. Based upon the optimized structures of the ground state, the TDDFT calculations were performed to compute the vertical singlet excitation energies and the absorption spectra of the studied species. Compared with the optical properties of Cy5, halogen ions lower the vertical excitation energies and cause the maximum absorption wavelength to be red-shifted for the Cy5-X derivatives. By employing the polarizable continuum model (PCM) approach for all the gas-phase-optimized structures, the influence of solvent effects on the spectra of Cy5 and Cy5-X was considered in three solvents (ethanol, DMSO, and water). The obtained results suggest that the solvents may lower the excitation energy of Cy5. However, for the Cy5-X species the excitation energies are higher in solvents than in the gas phase.