Syntheses, structures, and photoluminescent properties of ten metal–organic frameworks constructed by a flexible tetracarboxylate ligand

Abstract

Reactions of zinc ions with the flexible tetracarboxylate ligand, 5,5′-(1,4-phenylenebis(methylene))bis(oxy)diisophthalic acid (H₄L₁) gave rise to ten complexes, namely {[Zn₃(L₁)(OH)₂(H₂O)₂]3H₂O}_n (1), {[Zn(H₂L₁)(H₂O)₂]H₂O}_n (2), [Zn₂(L₁)(H₂O)₄]_n (3), {[Zn₂(H₃L₁)₂(H₂L₁)(H₂O)₂(MeCN)₂]}_n (4), [Zn₂(H₃L₁)₂(H₂L₁)(2,2′-bipy)₂]_n (5), [Zn₂(H₃L₁)₂(H₂L₁)(1,10-phen)₂]_n (6), {[Zn₂(L₁)(4,4′-bipy)₂]4H₂O}_n (7), {[Zn₂(L₁)(bpe)₂]4H₂O}_n (bpe = 1,2-bi(4-pyridyl)ethene) (8), {[Zn₂(L₁)(pdp)₂]4H₂O}_n (pdp = 4-[(E)-4-Pyridinylazo]pyridine) (9) and {[Zn₂(L₁)(bpmp)₂]6H₂O}_n (bpmp = N,N′-bis-(4-pyridyl-methyl) piperazine) (10). The H₄L₁ ligand not only displayed different deprotonated forms, but also diverse coordination modes and conformations. Compounds 1–4 are constructed of only zinc atoms with H₄L₁ ligands while compounds 5–10 are constructed of zinc atoms and H₄L₁ ligands with various N-donor ligands (Scheme 1). Compound 1 features a 3D architecture constructed of a linear Zn₃ cluster with H₄L₁ ligands. Compound 2 is a 2D waved layer structure with (4,4) grid as sql topology. Compound 3 displays a three-dimensional (3D) network simplified by a (3,6)-connected 2-nodal net with ant topology. Compounds 4, 5 and 6 all exhibit 1D infinite chains coordination structure, in which the H₄L₁ ligand is displayed as two kinds of deprotonated forms. Compounds 7, 8, 9 and 10 are similar and possess (4,4)-connected 3D frameworks with bbf topology, while also exhibiting an intriguing three-fold interpenetrated structure. These results indicate that subtle environmental factors, such as solvent, pH value and neutral auxiliary ligands play important roles in the formation of the structures of the final framework. At the same time, the photochemical properties of compounds 1–10 were tested in the solid state at room temperature, and the luminescent properties of compound 9 and 10 when dispersed in different solvents were investigated. These showed solvent-dependent luminescent spectra with emission intensities significantly quenched towards nitrobenzene. The quenching effects are observed at a low nitrobenzene concentration of 200 ppm, which indicates high sensitivities of these compounds towards nitrobenzene.