High photoluminescence quantum yield of a uranyl ion complex with the hemi-zwitterion 1,3-bis(carboxylatomethyl)-1H-benzimidazol-3-ium

[UO2(L)2] (1) and [(UO2)2(L)2(C2O4)] (2), where L– is 1,3-bis(carboxylatomethyl)-1H-benzimidazol-3-ium, are two uranyl ion complexes with a hemi-zwitterionic dicarboxylate ligand which have been synthesized under solvo-hydrothermal conditions and characterized by their crystal structure and luminescence properties. In both cases, the uranium atom is in a pentagonal-bipyramidal environment. Although diperiodic networks are formed in both cases, differences in the coordination mode of L– and the presence of bis-chelating oxalate anions generated in situ in 2 result in distinct topologies, hcb (with double edges) in 1 and V2O5 in 2. Parallel-displaced π–π interactions occur in both cases but are particularly prominent in 2. While 2 is non-emissive under excitation at 420 nm, 1 gives a well-resolved spectrum with the maxima locations in agreement with those usual for O5 equatorial uranyl environments. A photoluminescence quantum yield of ∼ 43 % has been measured for 1 in the solid state, this being among the highest values known for a uranyl carboxylate complex.