Molecular modeling of a polar rod-like aromatic polyester forming nematic liquid crystal. Part 1. Ab initio study of origin of strong SHG-activity

The aromatic copolyester of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-nap htoic acid (HNA) shows strong SHG-activity in the nematic phase as well as the crystalline phase in spite of no absorption in the visible region. Ab i nitio quantum chemistry calculations of the copolyester have been carried o ut in order to clarify the origin of this strong SHG-activity. The second-o rder hyperpolarizability of the HBA dimer is comparable to that of p-nitroa niline and it increases with the degree of polymerization. The large hyperp olarizability of the aromatic polyester is ascribed to the large electronic transition moment.