Extended analytical model for the description of light absorption spectra of linear molecular aggregates

We study optical properties of linear dye aggregates in which the transition dipole matrix elements of two monomer molecules forming their unit cell are not coplanar with the aggregate axis, and the Frenkel exciton is delocalized along this axis. An analytical model has been developed for the description of polarization effects in the light absorption spectra of such aggregates. It is shown that the nature of their optical spectra differs drastically from previously studied linear aggregates with a single molecule per unit cell. The developed theory contains simple formulas of the well-known Davydov–McRae–Kasha model for conventional linear aggregates as a particular case. A quantitative explanation of the experimental data is given for the absorption spectra of the pseudoisocyanine bromide dye aggregate.