Post-saturation nonlinear optical properties of polymers

The additivity law of properties for weakly coupled supermolecules was proposed to predict post-saturation NLO properties of polymers. For the first time, the linear chain length dependence of the first- and the third-order polarizabilities was directly observed in the finite-field computation of polyphenylenes and polypyridines containing up to 17 units (102 π electrons). Geometries and polarizabilities are in good agreement with the available experimental results. The similarity of saturated values of these polarizabilities for both kinds of iso-π-electronic polymers suggests that the π electron response is probably the principal contribution to polarizabilities. The another contribution from the CT effect of end groups will become a mere perturbation after the saturation. The onset of the additivity was found to occur in longer polymers for higher order of polarizability. Another important factor for the linear post-saturation chain length dependence besides the weak coupling is the regular structure which makes each unit in the middle part of a polymer have the same contribution to a given property. Other factors that affect the onset point of the additivity law in different laboratory work were also discussed. This additivity law of properties was proved to be quite helpful for the multi-component molecular material design.