Polymethine dyes have recently demonstrated promise for all-optical switching applications at telecommunications wavelengths as they can combine large refractive optical nonlinearities with low single-photon and two-photon optical losses. Here, we use density functional theory and symmetry-adapted cluster configuration interaction calculations to characterize model streptocyanine molecules. We first consider the isolated, closed-shell cationic molecules and then complexes formed by the molecules with chloride counter-ions and a series of aggregates. Our goal is to examine the influence of: (i) the presence of counter-ions and (ii) aggregation on the electronic structure and nonlinear optical properties. We find that the counter-ions increase the degree of bond-length alternation along the cyanine backbone, while aggregation significantly reduces the energy window between the lowest one-photon and two-photon excited states. Our results provide insight toward the design of new polymethine derivatives that could maintain large figures-of-merit for all-optical switching applications in the solid state.
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