Polymeric complexes with "piperazine-pyridine" building blocks: Synthesis, network structures, and third-order nonlinear optical properties

A new bidentate "building block", N,N′-bis(3-pyridylformyl)piperazine (bpfp), was synthesized. X-ray diffraction studies reveal that the piperazine ring in bpfp forms a chairlike structure conformation, bpfp self-assembled into two-dimensional layered rhombohedral grid polymers {[Mn(H₂O)₂(SO₄)(bpfp)](H₂O)₃ (CH₃OH)}_n (1) and {[Zn(NCS)₂(bpfp)₂]·2H₂O}_n (2) with Mn(II) and Zn(II), a three-dimensional polymer [Cd(N₃)₂(bpfp)]_n (3) with Cd(II), and a one-dimensional zigzag polymer [HgI₂(bpfp)]_n (4) with Hg(II). Each rhombohedral grid in polymer 1 is composed of four Mn, two bpfp, and two SO₄^2-; the dimensions of the grid are 6.537 × 12.843 Å. In polymer 2, each rhombohedral grid consists of 60-membered rings Zn₄(bpfp)₄ showing the dimensions of 14.231 × 15.586 Å. Zn₄(bpfp)₄ grids are bridged by Zn ions and all pyridyl-N atoms of bpfp ligands into the 2-D network structure along a and c directions. Polymer 3 exhibits a 3-D layered structure with tetragonal prism channel viewing from a direction. The bpfp ligands occupy the four edges of tetragonal prism channel, which is cut off by layers consisting of rhombohedral grids Cd₄(N₃)₄. Polymer 4 is quite different from polymers 1-3; it exhibits a 1-D zigzag framework extending along the c axis. Polymers 1-3 possess a very strong NLO absorption and self-focusing effect. Their third-order NLO absorptive coefficients α₂ are 9.2 × 10^-9, 6.9 × 10^-9, and 7.1 × 10^-9 m W^-1. The third-order NLO hyperpolarizabilities γ are 1.79 × 10^-28, 9.10 × 10^-29, and 9.66 × 10^-29 esu, respectively. The γ values are comparable to those of the best NLO materials and coordination polymers. We found that the valence shell structures of central metal ions can influence the NLO properties of coordination polymers.