We present a model for molecular materials (MM) made up of polar and polarizable molecular units. A simple two-state model is adopted for each molecular site and only classical intermolecular interactions are accounted for, neglecting any intermolecular overlap. The complex and interesting physics driven by interactions among polar and polarizable molecules becomes fairly transparent in the adopted model. Collective effects are recognized in the large variation of the molecular polarity with supramolecular interactions, and cooperative behavior shows up with the appearance, in attractive lattices, of discontinuous charge crossovers. The mean-field approximation proves fairly accurate in the description of the ground-state properties of MM, including static linear and nonlinear optical susceptibilities, apart from the region in the close proximity of the discontinuous charge crossover. Sizable deviations from the excitonic description are recognized both in the excitation spectrum and in linear and nonlinear optical responses. Interesting phenomena are recognized near the discontinuous charge crossover for noncentrosymmetric clusters, where the primary photoexcitation event corresponds to a multielectron transfer.

• Received 24 June 2003

DOI:https://doi.org/10.1103/PhysRevB.68.165405