Collective and cooperative phenomena in molecular functional materials

We discuss cooperative and collective behavior resulting from classical electrostatic intermolecular interactions in molecular materials with negligible intermolecular overlap. With reference to materials based on push-pull chromophores, we discuss the merits of several approximation schemes for the calculation of linear and non-linear optical susceptibilities. Collective and cooperative behavior is recognized in important deviations of the material properties from the oriented gas approximation scheme, and/or from the exciton model. Extreme collective and cooperative behavior in attractive clusters is discussed, where bistable behavior and the phenomenon of multielectron-transfer appear.