Luminescent uranium–thiophene dicarboxylate frameworks supported by transition-metal–organic cations: dimensionality and luminescence control in uranyl speciation

Abstract

Studies on the recycling of uranium, such as its usage as new materials, can promote the safe treatment of nuclear waste. In this work, with the direction of transition-metal complex cations, three uranyl–organic frameworks (UOFs) with different dimensions have been synthesized via a hydrothermal reaction, i.e. [Cu(phen)₂(SO₄)(UO₂)(TDC)·H₂O]_n (1), {[Zn(phen)₃][(UO₂)₂(TDC)₃(TDC)]·6H₂O}_n (2), and {[Cd(phen)₃][(UO₂)₂(TDC)₂(OX)·2H₂O]}_n (3), (TDC = thiophene-2,5-dicarboxylic acid, OX = oxalate, and phen = 1,10-phenanthroline). With the presence of SO₄²⁻, 1 presents a rare neutral 1-D Cu/U heterometallic chain. Under the similar [Zn(phen)₃]²⁺ and [Cd(phen)₃]²⁺ templates, 2 is a 2-D [(UO₂)₂(TDC)₃]_n planar non-interpenetrated honeycomb layer, but 3 presents an interesting twofold interpenetrated (6,3) sheet structure, which is led by the larger cavity of the hexagonal motif with the presence of the second ligand OX. This trend could be helpful for the oriented construction of UOFs. Intra- and inter-layer hydrogen bonds between the honeycomb structure and guest cations and π–π stacking interactions contribute to the stabilization of the overall framework. Complete quenching of uranyl luminescence via nonradiative decay can be observed in 1, and 2 and 3 emit yellow fluorescence with well-resolved vibronic fine structures. The interpenetrated arrangements could give rise to weakened uranyl emission.