Correlation between substituent constants and hyperpolarizabilities for di-substituted trans-azobenzenes.

Nonlinear optical properties of a series of disubstituted trans-azobenzenes were studied. The structures were fully optimized by B3LYP/6-31+G* and both static polarizabilities and hyperpolarizabilities were then calculated by the derivative method. In order to show the relationships between dipole moments, (hyper)polarizabilities and the structures, three kinds of substituent constants were applied to correlate with both ground state dipole moment and hyperpolarizabilities. Both physical properties have a satisfactory correlation with substituent constants Σσ(+/-) and bond length alternation. Overall, the electronic excitation contribution to the hyperpolarizabilities is rationalized in terms of the two-level model.