Halide-Directed Synthesis of Coordination Polymers [Hg2X4(ppt)]n(X=I and Br, ppt=1-(4-Pyridyl)-pyridinium-4-thiolate) with Different One-Dimensional Chain Structures and Third-Order Nonlinear Optical Properties

Abstract

Solvothermal reactions of HgX2 (X=I, Br) with 4,4'-dipyridyl disulfide (dpds) in acetonitrile gave rise to two one-dimensional polymers (Hg2X4(ppt))n (ppt=1-(4-pyridyl)-pyridinium-4-thiolate; X=I (1), Br (2)). In the structures of 1 and 2, the ppt ligand was in situ generated from the Hg(II)-engaged cleavage of both S—S and S—C bonds of dpds and the subsequent rearrangement reactions under solvothermal conditions. Although compounds 1 and 2 have similar chemical for- mula, 1 crystallizes in the orthorhombic space group Ima2 and holds a 1D non-centrosymmetric zigzag chain while 2 crystal- lizes in the orthorhombic space group Pnma and has a 1D centrosymmetric zigzag chain. Such a difference in structural symmetry between 1 and 2 may be due to the fact that the radius of iodide is larger than that of bromide. The third-order nonlinear optical (NLO) behaviours of 1 and 2 in DMF were investigated by using femtosecond degenerate four-wave mixing technique. Compound 1 exhibited relatively strong NLO responses while compound 2 showed nothing. Such a remarkable difference in the NLO property may be due to the fact that iodide is a better electron donor than bromide, which allows more efficient spin-orbital coupling, and facilitates intersystem crossing and more efficient usage of triplet excited state absorption. Keywords halide-directed; coordination polymer; solvothermal synthesis; structure; nonlinear optical properties