Environmental Effects on the Nonlinearities of Organic Molecules

We have studied the influence of different environmental conditions on the optical nonlinearities of selected organics in Langmuir-Blodgett type monolayers. Changes in the environmental conditions, such as protonation or de-protonation or addition of nonlinear inactive, however highly polarizing molecules to the species under consideration may influence the nonlinearities in quite different ways. Protonation, as the first example, can enhance or decrease the nonlinearity by producing a new chemical species upon inclusion of a proton in the original structure. An increasing rate of protonation will result in the production of more and more molecules of this new subspecies in the monolayer, an effect that can be conveniently monitored by the concomitant changes in linear optical absorption. Alternatively, we would like to show that changes in the environmental symmetry of a monolayer induce a strong nonlinear polarization which is not present in the isolated organic molecule.