Syntheses, structures, photoluminescence, photocatalysis, and photoelectronic effects of 3D mixed high-connected metal–organic frameworks based on octanuclear and dodecanuclear secondary building units

Abstract

Four novel metal–organic frameworks (MOFs) based on high-nuclear metal carboxylate clusters, namely, [Zn₄(L)₂(μ₃-OH)₂(H₂O)_1.5]·2H₂O (1), [Zn₅Na(L)₂(μ₃-OH)₄(CH₃CH₂O)(H₂O)₂] (2), [Cd₄(L)₂(BIME)_0.5(μ₃-OH)₂(H₂O)_1.5]·2H₂O (3), and [Zn₄(L)₂(BET)_0.5(μ₃-OH)₂(H₂O)] (4), where H₃L = (3,5-dicarboxyl-phenyl)-(4-(2′-carboxyl-phenyl)-benzyl)ether, BET = 1,1′-(2′-oxybis(ethane-2,1-diyl))bis(1,2,4-triazol-1-yl), BIME = 1,2-bis(imidazol-1-yl)ethane, have been synthesized under hydrothermal conditions. In 1, each planar octanuclear zinc cluster [Zn₈(μ₃-OH)₈(CO₂)₁₂(H₂O)₃] is linked by twelve L anions to give an unusual (3,12)-connected framework with (4³)₂(4²⁰·6²⁸·8¹⁸) topology. In 2, dodecanuclear heterometallic clusters [Zn₁₀Na₂(μ₃-OH)₈(CO₂)₁₂(CH₃CH₂O)₂(H₂O)₄] are surrounded by twelve L anions to form a (3,12)-connected net with the same topology as 1. However, 3 exhibits a rare (3,14)-connected (4³)(3²4)(3⁴·4²⁰·5¹⁴·6³⁴·7¹¹·8⁸) net based on octanuclear cadmium cluster [Cd₈(BIME)₂(μ₃-OH)₄(CO₂)₁₂(H₂O)₄]·2H₂O as a 14-connected node and H₃L as a 3-connected node. In 4, each octanuclear zinc cluster [Zn₈(BET)₂(μ₃-OH)₄(CO₂)₁₂(H₂O)₂] is linked by twelve L anions to furnish a (3,14)-connected net with the same topology as 3. The optical energy gaps and photoluminescence were investigated for the compounds. Moreover, compounds 1–4 exhibit good photocatalytic activities for degradation of methylene blue (MB) under UV irradiation and show good stability towards UV photocatalysis. In addition, 1 and 4 display a photocurrent density under several on–off cycles of illumination.