Valence-Bond Charge-Transfer Model for Nonlinear Optical Properties of Charge-Transfer Organic Molecules

The nonlinear optical properties of charge-transfer organic materials are discussed in the framework of a simple valence-bond charge-transfer model. This model leads to analytic formulas for the absorption frequency, hyperpolarizabilities, and bond length alternation, all of which are described in terms of three parameters, V, t, and Q related to the band gap, bandwidth, and dipole moment of the charge-transfer state. These parameters are derivable from experiment or from theory. The valence-bond charge-transfer model provides a clear physical picture for the dependence of the hyperpolarizabilities on the structure of charge-transfer molecules and leads to good agreement with the trends predicted by the AMI calculations.