Computer simulation of the shape of absorption bands in electronic spectra ofJ-aggregates

Abstract

The shape of absorption bands of aggregates formed by two, four, and nine molecules of a polymethine dye was calculated by the Monte-Carlo method. The energy of interaction of the molecules in the ground state was simulated using atom-atom potentials, and the energies of interaction between dipole moments of electronic transitions of the monomers were estimated by quantum-chemical methods. In the dimer aggregate the dipole moments of the electronic transitions in the monomers interact weakly; therefore, the electron absorption spectrum should be similar to that of the monomer. On going from the dimer to the aggregates consisting of four and nine monomers, the relative positions of monomers change and this, in turn, increases the energy of interaction between the dipole moments of their electronic transitions, resulting in a red shift characteristic ofJ-aggregates and narrowing of the absorption bands.