Influence of the Molecular Framework on the Photoconductivity Responses of Five Cu/Ni and Fe Heterometallic Materials Built with Cyanide and Ethylenediamine

The contribution of weak interactions in the photoconductive property of hybrid materials is clearly shown in this work with five materials having the same units but different connection modes: [Ni(en)₃]₃[Fe(CN)₆]₂ ·5H₂O (1N), {[Ni(en)₂]₃[Fe(CN)₆]₂}_n·3H₂O (2N), [Cu(en)₂(H₂O)_1.935]₂ [Fe(CN)₆]· 4H₂O (3N), {[Cu(en)₂][KFe(CN)₆]}_n (4N), and {[Cu(en)₂]₂[Fe(CN)₆]· 4.5H₂O}_n (5N); en = ethylenediamine (C₂H₈N₂); 1N and 3N are supramolecular compounds while 2N, 4N, and 5N are coordination polymers. 1N is a new material while 2N, 3N, 4N, and 5N were already reported with different synthesis methods. In the structure of 1N, two [Fe(CN)₆]^3– anionic units over three crystallographic sites (noted A^–, B^–, and C^–) and three cationic [Ni(C₂H₈N₂)₃]²⁺ units (noted A⁺, B⁺, and C⁺) are involved. They build infinite chains with the packing of the B⁺----A^–--- A⁺----C^–--- A⁺---- A^–----B⁺ units connected with the C⁺---B^–---C⁺ short units. This packing results in zigzag 2D arrays running along the [010] direction and stacked along the [100] direction with the water molecules located between them. 4N exhibits a one-electron quasi-reversible redox process while 1N, 2N, 3N, and 5N show multielectron processes in which the redox couples Cu²⁺/Cu³⁺ or Ni²⁺/Ni³⁺, Fe²⁺/Fe³⁺, and Fe³⁺/Fe⁴⁺ are involved. All these compounds are photoactive; 1N and 3N, which are built from hydrogen bonds, produced higher faradaic current with illumination than the materials 2N, 4N, and 5N, which are built with coordinative bonds. The solid-state photoconductivity of 3N shows a current increase as the voltage increases in the presence of light, while no real response was observed without light.