CHARGE-DISTRIBUTION AND 2ND-ORDER NONLINEAR-OPTICAL RESPONSE OF CHARGED CENTROSYMMETRIC CHROMOPHORE AGGREGATES - AN AB-INITIO ELECTRONIC-STRUCTURE STUDY OF P-NITROANILINE DIMERS

This contribution presents a quantum chemical investigation of the non linear optical response of charged centrosymmetric aggregates of the a rchetypical chromophore p-nitroaniline using an ab initio supermolecul ar approach. Localized hole states, responsible for aggregate nonlinea rity, are predicted to be more stable than delocalized states. The cal culated aggregate hyperpolarizabilities are consistently larger than i n the isolated chromophore, and the mechanism giving rise to the nonli nearity is essentially ''intramolecular'' in nature. The aggregate for mation is predicted to stabilize the positive charge and to enhance th e nonlinearity. Moreover, the calculations reveal that a (formally) ce ntrosymmetric charged aggregate can possess a substantial dipole momen t the principal axis of which is oriented perpendicular to the princip al hyperpolarizability tensor; the dipole moment arises because electr on localization breaks the charge distribution symmetry. These results explain a number curious features of chromophoric polymer films which are electric field poled with an ''in-plane'' electrode geometry.