Design of chromophores for nonlinear optical applications

In designing chromophores for second harmonic generation applications one would like to have a large hyperpolarizability β and an absorption well away from the second harmonic frequency. Unfortunately, there appears to be a connection between the experimentally observed absorption maximum energy and the observed zero-frequency hyperpolarizability β0, when comparing different molecules within a class, such as donor/acceptor substituted benzenes and stilbenes, that makes this difficult to achieve. In this paper, the origins of this relationship are investigated by comparing results from semi-empirical calculations with experimental values. A significant contribution to the observed relationship between absorption maximum and β0 is seen to arise from frequency dependences of the ground and excited state dipole moments. Since the two-level model for β is used both as an extrapolation technique to obtain β0 and as a tool for analyzing the data, a discussion of the validity of this model is also presented.