New zinc(II) dyes with enhanced two-photon absorption cross sections based on the imidazolyl ligand

A series of new zinc(II) complexes (Dyes 1–3) were obtained by self-assembly of the donor-bridge-donor (D–π–D) type ligand L with ZnX2 (X = Br, I, SCN). The single crystal of Dye 3 was obtained by slow evaporation of a mixture solution of methanol and dichloromethane. The molecular structure has been determined by X-ray crystallography. Hydrogen bonds (C–H⋯N, C–H⋯S) play significant roles in the final topological structures. Linear and nonlinear optical properties of the ligand and complexes were investigated in details. Experimental results revealed that effective two-photon absorption (TPA) cross sections of these complexes are larger than that of ligand L (18.7 GM) with the maximum values of 88.7 GM (Dye 1), 41.9 GM (Dye 2) and 46.5 GM (Dye 3) in DMF, respectively. The biological imaging result shows that L and Dyes 1–3 had excellent penetrability in living cells.