Ab initio calculations of first hyperpolarizabilities for donor-acceptor azobenzenes and their dimers

To understand the large nonlinear optical activities of azobenzene dendrons, which are dendritic macromolecules comprising donor-acceptor azobenzenes as branching units, we carried out ab initio calculations for azobenzene monomers, i.e., 4-dimethylamino-4′-carboxyazobenzene and 4-dimethylamino-2′-nitro-4′-carboxyazobenzene, and their dimers. We determined the stable structures and calculated the first hyperpolarizabilities, using the coupled-perturbed Hartree-Fock method. We found that two azobenzenes in the stable structure of each of the dimers have correlated orientations that produce a large overall dipole. The hyperpolarizabilities for the dimers were larger than those for the corresponding monomers. From these findings, we conclude that the poling of branching units by the interaction between azobenzene moieties within the azobenzene dendrons results in cone-shape structures that cause the large nonlinear optical activities.