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

Emmanuel Deiters,Jean-Claude G Bünzli,Svetlana V Eliseeva

doi:10.3389/fchem.2013.00015

Emmanuel Deiters, Jean-Claude G Bünzli + Show 1 more

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https://doi.org/10.3389/fchem.2013.00015

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Abstract

Two new tridentate(NNO)-bidentate(NN) compartmental ligands, HL5 and HL6, are synthesized from pyridine and benzimidazole synthons. They react in aqueous solution under physiological conditions with ZnII, LnIII, 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 EuIII-containing solutions in Tris-HCl 0.1 M (pH = 7.4) show that lanthanide luminescence arises from a unique N6O3 coordination site with pseudo D3 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 EuIII ion has the same coordination environment.

Highlights

Helical structures have attracted chemists’ attention when Linus Pauling published a seminal series of papers at the beginning of the 1950’s dealing with the secondary structures of proteins induced by three-dimensional helical patterns (Pauling et al, 1951)
A few years later, Claude Piguet applied the same concept to trivalent lanthanide ions and successfully self-assembled the first LnIII dinuclear triple-stranded helicate, [Eu2(LA)3]6+ (Scheme 1), the crystal structure of which evidences a stabilization of the molecular architecture by π-π stacking interactions between the ligand strands (Bernardinelli et al, 1992)
SPECIATION IN SOLUTION Conditional stability constants of both homometallic (M = ZnII, LaIII, EuIII, LuIII) and heterometallic (M1 = ZnII, M2 = LaIII, EuIII, LuIII) complexes have been determined in Tris-HCl 0.1 M

Summary

Introduction

Helical structures have attracted chemists’ attention when Linus Pauling published a seminal series of papers at the beginning of the 1950’s dealing with the secondary structures of proteins induced by three-dimensional helical patterns (Pauling et al, 1951). The two 9-coordinate metal ions lie on a pseudo-C3 axis of symmetry (Figure 1); in solution the average symmetry of the edifice is D3 on NMR time scale (Piguet et al, 1993). This initial work paved the way for the development of several series of lanthanide polynuclear and polymetallic complexes including heterobimetallic nd-4f (Piguet et al, 1995a) and 4f-4f (André et al, 2004) chelates, as well as tri- and tetranuclear homometallic and heterometallic entities (Piguet et al, 2000; Piguet and Bünzli, 2010). Subsequent molecular engineering led to the series of the more water-soluble [Ln2(LC2)3] helicates and their bioconjugates which proved to be adequate luminescent bioprobes for live cell staining (Song et al, 2008; Chauvin et al, 2013) and for specific detection of biomarkers expressed by cancerous cells (Fernandez-Moreira et al, 2010)

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