COMPUTER-SIMULATION OF BAND SHAPES IN ELECTRONIC ABSORPTION-SPECTRA OF DIMERS OF ORGANIC-DYES

The band shapes in the absorption spectra of dimers of cyanine dyes we re simulated using a combination of an empirical molecular force field for the ground stale with quantum-chemical calculations of the electr on excitation energy as a function of normal nuclear coordinates. The shape and the width of an absorption band strongly depend on the mutua l arrangement of the monomers. If the monomers are located one directl y above the other, the sublevels arising from intramolecular vibration s disappear in the spectrum, and a large hypsochromic shift of the 0-0 -transition band is observed, which results mainly from through-space interaction of monomer orbitals. If the monomers are strongly shifted relative to each other, the sublevels mentioned are also absent in the spectrum, but the bathochromic shift of the 0-0-transition band is sm all and results from interaction of dipole moments of electron transit ions. A rather broad region of intermediate structures is found betwee n these dimer forms, where the interaction of dipole moments of electr on transitions in monomers is low, and the shapes of absorption bands are similar to those of the monomers.