Towards an Understanding of Structure-Nonlinearity Relationships in Triarylamine-based Push-Pull Electro-Optic Chromophores: The Influence of Substituent and Molecular Conformation on Molecular Hyperpolarizabilities

We calculated the second-order hyperpolarizability (beta) of a series of triarylamine (TAA) based donor-bridge-acceptor (D-π-A) push-pull type nonlinear optical (NLO) chromophores with different electron donor moieties and the same thiophene π-bridge and dicyanovinyl electron acceptor using a time-dependent Hartree-Fock (TDHF) approach within the software package MOPAC 2012. NLO chromophores with various quantities and positions of methoxy groups in the TAA donor moiety were investigated. The relationship between NLO properties and the electronic or geometric structures of the TAA donor subunit is discussed through the calculation results. Both substituent and conformational effects affect the delocalization of the nitrogen lone pair into the aryl rings, leading to a dramatic influence on the nonlinear optical properties. Introduction of methoxy groups at the ortho positions of the TAA moiety has a larger influence on the molecular hyperpolarizability and dipole moment than the introduction of methoxy group at the para or meta positions. Our calculation results demonstrate how to improve the NLO properties of TAA based chromophores while meeting practical device requirements.