A two-state description of (hyper) polarizabilities of push-pull molecules based on a two-form model

We have established analytical structure/(hyper)polarizability relationships for donor-acceptor conjugated molecules, within the framework of the two-state approximation. In this two-form model, a molecule is described as a mixture between neutral and charge-separated resonance forms. We defined a parameter MIX, characterizing the mixing between the limiting-resonance forms, that is measurable in solution. MIX is proportional to the change in dipole between the ground and excited states. We studied analytically the variation of (hyper)polarizabilities with MIX. Finally, we showed how this model can account for external perturbation, such as the solvent electric reaction field.