Prediction of Vibronic Coupling and Absorption Spectra of Dimers from Time-Dependent Density Functional Theory: The Case of a Stacked Streptocyanine.

Methods based on density functional theory calculations have been used to simulate the absorption spectra of a streptocyanine and of its covalently bonded dimer. Two approaches, based on multimode Franck-Condon overlap integrals, have been employed. In the first approach the monomer and the dimer are treated as single molecules, and the Franck-Condon factors are determined for both systems. The second approach is based on the diagonalization of the dimer Hamiltonian which is constructed from the monomer Franck-Condon overlap integrals and quantities describing the intermonomer electronic coupling. Both approaches succeed in reproducing the hypsochromic shift of the maximum of absorption occurring upon dimerization with an accuracy of 0.05 eV. The vibronic structure of the monomer is also in good agreement with experiment and depends little on the inclusion of Duschinsky rotation effects. The shape and relative intensity of the dimer spectrum is qualitatively reproduced by the two methods, each of them being able to describe most of the vibronic features. Moreover, accounting for the solvent effects in the calculation of the intermonomer electronic coupling improves the agreement with experiment by reducing the intensity of the maximum and by enlarging the spectrum at longer wavelengths.