Theoretical Predictions of Nonlinear Optical Properties of Molecules and Polymers

Abstract

ABSTRACTThe prediction of nonlinear optical properties of molecules and polymers is an important goal toward which a considerable effort has been recently applied. It entails the calculation of the polarizability and first and second hyperpolarizability tensor, for the systems under study, using their quantum wavefunctions. In principle several approaches are possible. The wavefunctions themselves can be calculated at the ab initio or semi-empirical levels of approximation and then the calculations can proceed via the sum over states (SOS) or the finite field (FF) formalisms.Since we intend to calculate large systems we decided on the semi-empirical MNDO/AMI approaches for the calculation of the wavefunction. We also used the FF approach and numerical methods for finding the necessar) derivatives. The comparison between calculated and available experimental results is very encouraging. We also calculated the tensors for considerably large molecules, providing the first theoretical prediction of these values. The effect of different substituents on a given structure and the result of doping, were also studied. The polarizabilities of some oligomers as functions of the number of repeat units were calculated to assess the convergence properties. Because of the required periodicity of the perturbations, calculations cannot be made for infinite polymers in the same fashion as for small molecules. This problem is currently being addressed.