Rotationally Invariant Representation of the Static Hyperpolarizability Calculations in Push-Pull Molecules

Abstract

Push-pull molecules, where donor and acceptor groups are linked by a π-conjugated bridge, attract a lot of interest due to their potential applications in nonlinear optics [1]. Various families of such molecules with modifications of size, chemical properties, and molecular geometry have been synthesized by several groups (see e.g. [2]). Some of these molecules have very high nonlinear response. Recently, a general method for calculation of all components of the polarizability and hyperpolarizabilities tensors for two-dimensional molecular aggregates has been developed and applied to push-pull molecules modeled as a four-site acceptor-bridge-donor aggregate with two sites representing the bridge [3]. The key model parameters include donor and acceptor abilities of the side groups, bridge length, intrasite small polaron binding energies, intersite transfer integrals, and molecular geometry. These parameters have a clear physical meaning, can be estimated independently, and may be modified in a controlled way by appropriate molecular engineering. However, the Cartesian tensorial coefficients found in [3] may not be the best way for comparison of different geometries. In this communication, we report the results of calculations of rotationally invariant quantities following the formalism developed by J. Zyss et al. [4, 5]. Such approach is useful for optimization of a search of push-pull molecules with desired nonlinear properties.