Self-assembly of a helical zinc-europium complex: speciation in aqueous solution and luminescence

Two new tridentate(NNO)-bidentate(NN) compartmental ligands, HL(5) and HL(6), are synthesized from pyridine and benzimidazole synthons. They react in aqueous solution under physiological conditions with Zn(II), Ln(III), or a mixture thereof, to yield complexes of different stoichiometries, 1:3, 2:2, 2:3, 1:1:3, the speciation of which is established by UV-visible titrations and ESI mass spectrometry. Photophysical studies of the Eu(III)-containing solutions in Tris-HCl 0.1 M (pH = 7.4) show that lanthanide luminescence arises from a unique N(6)O(3) coordination site with pseudo D(3) symmetry. Relevant parameters such as crystal field splitting, lifetime, radiative lifetime, and intrinsic quantum yield perfectly match those reported for dinuclear 4f-4f helicates in which the Eu(III) ion has the same coordination environment.