An ab initio study on nonlinear optical properties of squarates

The linear polarizabilities, first and second hyperpolarizabilities of four squarates have been studied at ab initio/4 - 31G + pd level by couple - perturbed Hartree - Fork (CPHF) method. A discussion of the relationships between their structures and properties has been done in terms of charge distributions, transition dipole moments, frontier orbitals, etc.. The calculations show that the squarates consist of the donor - acceptor - donor structures, where the four - membered rings act as electron donating groups and the substituents as electron - withdrawing groups. Their molecular nonlinear optical susceptibilities strongly depend on the substituted effects.