Microwave-assisted large scale synthesis of lanthanide metal–organic frameworks (Ln-MOFs), having a preferred conformation and photoluminescence properties

Abstract

Preparation of MOF on a large scale is a great challenge due to difficulties in reproducibility. A microwave synthesis procedure plays a major role in solving this problem. Moreover, achievement of the preferred conformation in the case of the flexible ligand is also an important factor as it affects the stability of the MOF. In this regard, lanthanides are suitable candidates due to their large size and coordination capabilities. A series of isostructural microporous lanthanide metal–organic frameworks (Ln-MOFs), formulated as [Ln (TTTPC)(NO₂)₂(Cl)]·(H₂O)₁₀ {Ln = La (1), Ce (2), Pr (3), Nd (4), Eu (5), Tb (6), Dy (7), Ho (8), Yb (9); H₃TTTPC = 1,1′,1′′-tris(2,4,6-trimethylbenzene-1,3,5-triyl)-tris(methylene)-tris(pyridine-4-carboxylic acid)}, has been synthesized on a large scale via a microwave-assisted solvothermal reaction over 5 min. Otherwise, if a conventional solvothermal reaction is carried out at the same temperature, a much longer reaction time (2 days) and slow evaporation (5 days) are needed to produce the same compound in similar yield. Moreover, in these circumstances, conventional methods are useful only for small scale (10 mg) syntheses, but on using microwave-assisted synthesis, up to 2 g was obtained. Structural analysis reveals that the framework of the as-synthesized MOFs is a 6-connected network with point symbol (4¹¹·6⁴), which is a subnet of a uninodal net having a new topology, sqc885. Thermal gravimetric analyses performed on as-synthesized MOFs reveal that the frameworks have moderate thermal stability. Gas sorption properties of 1 and 8 were studied by experimentally measuring nitrogen and hydrogen sorption isotherms. The luminescent properties of 5 and 6 were investigated and show characteristic emissions for Eu³⁺ and Tb³⁺ at room temperature.