Self-assembly and third-order optical properties of HgI₂ with linear and angular bipyridine ligands

Two bis(pyridyl) ligands: N,N′-bis(4-pyridyl)-1,4- benzenedicarboxamide (bpba) and N,N′-bis(3-pyridylformyl)imidazolidine-2- thione (bpit) have been designed and synthesized. Self-assembly of the linear ligand bpba and angular ligand bpit with HgI₂ results in a one-dimensional zigzag polymer {[HgI₂(bpba)] • 1.5CH ₃OH}_n (1) and a binuclear metallamacrocycle [HgI ₂(bpit)]₂ • 3H₂O (2), respectively. Single crystal X-ray diffraction analyses reveal that in the crystal structure of 1 the linear ligand bpba is in transoid conformation, while in the crystal structure of 2 the angular ligand bpit coordinates with HgI₂ in cisoid conformation, the different geometries of the two ligands attribute to forming the dissimilar frameworks of two HgI₂ adducts. The determination of third-order nonlinear optical (NLO) properties of 1 and 2 in DMF solution shows that adducts 1 and 2 possess strong NLO self-focusing effect. The hyperpolarizability γ values of 1 and 2 are calculated to be 2.10 × 10^-29 and 3.24 × 10^-29 esu, respectively, which are comparable to those of some NLO materials. And the result indicates that the heavier atoms Hg and I together with the polymeric aggregation play an important role in determining their NLO properties.