Designs, Syntheses And Molecular Self-assenblies Of Carboxyl-viologen Ligands

Viologens or viologen derivatives have been widely applied in photoelectric conversion, photochromic, eletrochromic material fields due to their excellent oxidation-reduction reversibility, unusual light and electrochemical performance. It is well known that the carboxylate ligands may provide various coordination modes, so the carboxylic acid groups are introduced to viologen derivatives. Our project is to design and synthesize the viologen derivatives ligands with carboxylic acid groups, and to focus on the coordination assemblies with specific electronic function. Based on the coordination effect between the metal ions and ligands, the photoresponsive and electrochemical properties can be controlled. It will be significant for the development of novel photoelectric response materials.We have designed and synthesized four caeboxybenzyl-substituted viologen ligands:1,1’-bis(4-carboxybenzyl)-4,4’-bipyridinium dichloride (H2BpybcCl3) (L1),1-(4-carboxybenzyl)-4,4’-bipyridinium chloride (HBCbpyCl) (L2),1,1’-bis(3,4-dicarboxybenzyl)-4,4’-bipyridinium dibromide (L3),1,1’-bis{[4’-carboxy(1,1’-biphenyl)]-4- methyl}-4,4’-bipyridinium dibromide (L4). Six charge-transfer complexes or coordination polymers were prepared by solution method and liquid diffusion method. These compounds have been characterized with the aid of IR, UV, electrochemistry, NMR, TGA, luminescent spectroscopy and X-ray single-crystal diffraction. At the same time the 2D COS-IR is applied with thermal perturbation to the compound 1.Compound{[(H2Bpybc)(SCN)2]}n(1) is a carboxybenzyl-substituted viologen charge-transfer salt with SCN-. The intermolecular hydrogen bonds link the molecules into 2-D supermolecular network. Compounds{[Cu(SCN)2(Bpybc)]}n(2) and {(Bpybc)3[Cu2(H2O)6](OH)4·15H2O}n(3) were obtained by the self-assembly with metal salts and L1 ligand. Complex 2 is built up by [Cu(SCN)2] units bridged sequentially by ladder-shaped Bpybc ligands to form one-dimensional zigzag chains. The chains are held together by π-π interactions, thus yielding a 3-D extended supramolecular network. Compound 3 possesses 2-D non-interpenetrating layer network with large honeycomb-like hexagonal cavities. Complex {(BCbpy)2[Zn2(N3)2(OH)2]·3H2O}n(4) was prepared by the self-assembly with zinc salts and L2 ligand.Complex 4 is built up by Zn(II) centers bridged sequentially by V-shaped BCbpy ligand to form a zigzag chain, the N3- and μ2-O link the chains into 2-D layer structure. Two 1-D coordination polymers,{[Cu(SCN)2(4,4’-bipy)]}n(5) and {[Co(SCN)2(4,4’-bipy)2(H2O)2]}n (6) are synthesized by using 4,4’-bipyridine ligand and metal thiocyanate. The studies of the influencing factors (temperature, solvent, pH and mixing time) on the coordination modes of the ligand have been carried out. And the best reaction conditions have also been found. Complexes 1,2,3 and 4 show excellent reversible thermochromic and photochromic behavior. Complexes 1 and 2 change their color at around 120℃, compound 3 changes its color at around 70℃, compound 4 is more sensitive with color-change and has two different temperature at 50℃ or 70℃. Especially, it is also sensitive to ultraviolet light. And ion exchange property of compound 3 has been carried out. This work will be beneficial for the development of novel structural sensitive materials.