Modeling of nonlinear optical properties of two-dimensional molecular aggregates

A general method for calculation of all components of first hyperpolarizability tensor for two-dimensional molecular aggregates is applied to the case of push–pull molecules, modeled as a four-site acceptor–bridge–donor structure. Coupling of electrons to intra-site molecular vibrations is shown to influence strongly the nonlinear response. The key model parameters include donor and acceptor abilities of the side groups, bridge length, intrasite small-polaron binding energies, intersite transfer integrals, and molecular geometry. These parameters have a clear physical meaning, can be estimated independently, and may be modified in a controlled way by an appropriate molecular engineering. Such an approach is useful for optimization of a search of push–pull molecules with desired values of specific components of the hyperpolarizability tensors.