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© Research
Publication : J. Chem. Soc., Dalton Trans.

Effect of a halogenide substituent on the stability and photophysical properties of lanthanide triple-stranded helicates with ditopic ligands derived from bis(benzimidazolyl)pyridine

Scientific Fields
Diseases
Organisms
Applications
Technique

Published in J. Chem. Soc., Dalton Trans. - 17 May 2000

Carlos Platas Iglesias, Mourad Elhabiri, Marcel Hollenstein, Jean-Claude G. Bünzli and Claude Piguet

J. Chem. Soc., Dalton Trans. 2000, 2031-2043

Bis{1-ethyl-2-[6-(N,N-diethylcarbamoyl)-4-halogenopyridin-2-yl]benzimidazol-5-yl}methane (halogeno = chloro,
LE; bromo, LF) have been synthesized as ditopic receptors for the development of lanthanide-containing helicates
able to couple with biological material and to test the influence of the halogeno substituent on the wrapping process,
the structure of the resulting dimetallic edifices, and the photophysical properties of the encapsulated ions. The
stability of the [Eu2(L)3]6 helicates, as determined by NMR competitive titrations, decreases by respectively one
(LF) and three (LE) orders of magnitude compared to the value found for the unsubstituted ligand (LB) although it
remains large, log β23 = 23.8 (LF) and 21.8 (LE) in acetonitrile. The [Ln2(LE)3]6 helicates are shown to be isostructural
in acetonitrile over the lanthanide series (Pr to Yb) and the crystal structure of [Tb2(LB)3]6 appears to be a good
model for their solution structure, as demonstrated by paramagnetic NMR measurements (lanthanide induced shift
method) and relaxation time determination. Ligand LE appears to be a fair sensitiser of EuIII, the quantum yield of
[Eu2(LE)3]6 being 25% larger than that found for [Eu2(LB)3]6, but the ligand 3ππ* state and Tb(5D4) excited level
are in resonance, which limits the sensitisation of TbIII. High resolution luminescence spectra of [Eu2(LE)3]6, both
in solution and in the solid state, are presented and discussed in terms of site symmetry and vibronic coupling
mechanisms.