Charge-transfer metal–organic frameworks based on CuCN architecture units: crystal structures, luminescence properties and theoretical investigations

Abstract

Four CuCN complexes, namely Cu₄(CN)₄(bix)₂ (1), Cu₂(CN)₂(bmimb) (2), Cu₂(CN)₂(bmimb) (3) and Cu₃(CN)₃(bimb) (4), have been prepared via the synchronous redox and self-assembly reaction of Cu(NO₃)₂, K₄[Fe(CN)₆] and three structurally related flexible bis(imidazole) ligands, 1,4-bis(imidazol-1-ylmethyl)benzene (bix), 1,4-bis(2-methylimidazol-1-ylmethyl)benzene (bmimb) and 4,4′-bis(1-imidazolyl-1-ylmethyl)biphenyl (bimb) under solvothermal conditions. Although all prepared complexes contain one-dimensional CuCN subchains, they have different structures of 2₁ helical chain, meso-helical chain, 2₁-helical chain and zigzag chain for 1, 2, 3 and 4, respectively. Complex 1 presents a three-dimensional framework with (10, 3)-d (utp) topology and exhibits an interesting five-fold interpenetration structure attributed as Class Ia type. Moreover, the five-fold entangled network is turned into an unprecedented three-dimensional binodal (3,6)-connected self-penetrated network via the Cu⋯Cu bond interactions. Most interestingly, 2 and 3 are a pair of isomers, and also generate a three-dimensional uninodal (10, 3) network with ThSi₂ and utq topology, respectively, which all display an interesting three-fold interpenetration, and also belong to Class Ia type. Complex 4 displays a distorted two-dimensional (6, 3) topology layer, and further forms a three-dimensional supramolecular structure by weak π⋯π interactions. It is indicated that the organic ligands play a crucial role in the final product structures as well as the solvents. Meanwhile, the complexes present strong green (λ_max = 553 (1), 565 (2), 565 (3) and 563 nm (4)) photoluminescence in the solid state at room temperature. The theoretical calculations show that the intense green experimental band can be assigned to a combination of the cyanide group to copper(I) center and cyanide group to cyanide group charge transfer transitions. Additionally, the thermal analyses show that complexes 1–4 possess high thermal stabilities.