New five-coordinated mercury (II) dyes based on a novel 2,2′:6′,2″-terpyridine ligand: Structures, photophysical properties and DFT calculations to evaluate the halogen effect on the two-photon absorption

A simple D-π-A type ligand ( L : 4′-(4-[4-(1H-[1,2,4]-triazolyl)styryl]phenyl)-2,2′:6′,2″-terpyridine) was designed and synthesized, which reacted with HgX 2 (X   =   Cl, Br, I, SCN) yielding a series of uncommon five-coordinated mercury (II) complexes ( Dyes 1–4 ). The ligand and Dye 4 were characterized by single crystal X-ray diffraction determination. Linear and nonlinear optical properties of the ligand and dyes were exhibited. Experimental results revealed that the two-photon absorption (TPA) cross-sections of the four dyes are considerably larger than that of the ligand. Density function theory (DFT) calculations performed on Dyes 1–4 showed that the halogens affect the accepting capability of the complexed metal due to different electron inductive effects , which finally influence the TPA cross-section values.