Charge distribution and second-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 nonlinear optical response of charged centrosymmetric aggregates of the archetypical chromophore p-nitroaniline using an ab initio supermolecular approach. Localized hole states, responsible for aggregate nonlinearity, are predicted to be more stable than delocalized states. The calculated aggregate hyperpolarizabilities are consistently larger than in the isolated chromophore, and the mechanism giving rise to the nonlinearly is essentially 'intramolecular' in nature. The aggregate formation is predicted to stabilize the positive charge and to enhance the nonlinearity. Moreover, the calculations reveal that a (formally) centrosymmetric charged aggregate can possess a substantial dipole moment the principal axis of which is oriented perpendicular to the principal hyperpolarizability tensor; the dipole moment arises because electron 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.