Molecular structures and non-linear optical properties of donor-acceptor quinoid-type molecules

AM1 calculations are performed to study the molecular structures and the linear and non-linear polarizabilities of the donor-acceptor quinoid-type molecules under the influence of an external electric field defined by point charges (Sparkles). We investigate and compare the relationships between the molecular structures and the molecular polarizabilities of the substituted quinoid-type molecules. The results show that the relative sizes of the molecular polarizability values for the three molecules depend on the selection of the donor-acceptor pair. We also systematically examine the influence of molecular architecture on the hyperpolarizabilities. It is found that the substituted quinoid-type molecules have considerably larger hyperpolarizabilities than the substituted aromatic heteroring molecules and the corresponding polyenes with chain extension, and yet the hyperpolarizabilities of the substituted polyenes are larger than that of the substituted aromatic heteroring molecules with chain extension. In addition, the increasing extents of the hyperpolarizabilities for the substituted quinoid-type molecules are far larger than that for the substituted aromatic heteroring molecules and the corresponding polyenes. These properties make the quinoid-type molecules interesting candidates for applications in the non-linear optics, particularly in the area of poled polymer films.