Nonlinear optical properties of stacked conjugated systems

Abstract

We study linear and nonlinear optical properties of two push-pull polyenes stacked in head to head (HtH) and head to tail (HtT) configurations, at different stacking angles within the Pariser-Parr-Pople model using exact diagonalization method. By varying the stacking angle between the polyenes, we find that the optical gap varies marginally, but transition dipoles show large variations. We find that the dominant first-order hyperpolarizability component β_XXX for HtH arrangement and β_YYY for HtT arrangement strongly depend on the distance of separation between molecules, while the other smaller component (β_XYY for HtH arrangement and β_XXY for HtT arrangement) does not show this variation with distance. We find that the β_XXX for HtH configuration shows a maximum at an angle away from 0, in contrast with the oriented gas model. This angle varies with distance between the polyenes, and at large distance it falls to 0. The ratio of all components of β of a dimer to monomer is less than two for HtH configuration for all angles. But for HtT configurations the ratio of the dominant β component is greater than two at large angles. Our ZINDO study on two monomers (4-hydroxy-4'-nitroazobenzene) connected in a nonconjugative fashion shows a linear increase in |β→av| without much red shift in optical gap. There is a linear increase in |β→av| with increase in number of monomers connected nonconjugatively without resulting in a red shift in optical gap.