Helvetica Chimica Acta p. 1915 - 1929 (2002)
Update date:2022-09-26
Topics:
Tripier, Raphael
Hollenstein, Marcel
Elhabiri, Mourad
Chauvin, Anne-Sophie
Zucchi, Gael
Piguet, Claude
Buenzli, Jean-Claude G.
Bis(2-{6-(diethylcarbamoyl)-4-[(4-isothiocyanatophenyl) ethynyl]pyridin-2′-yl}-]-1-ethylbenzimidazol-5-yl)-methane (LG) reacts with trivalent lanthanide ions in acetonitrile to yield triple-stranded dimetallic helicates [Ln2(LG)3]6+. 1H-NMR Data point to the helicates being the only species formed under stoichiometric conditions and having a time-averaged D3 symmetry on the NMR time scale. The photophysical properties of LG and its helicates are discussed with respect to the closely related ligands LB, LE, and their complexes, two ligands devoid of the isothiocyanatophenylethynyl substituent. The quantum yield of the ligand fluorescence is three times smaller compared to LE, while that of the EuIII-centered luminescence (1.1%) is three times larger. On the other hand, the luminescence of TbIII is not sensitized by LG. This is explained in terms of energy differences between the singlet and triplet states on one hand, and between the 0-phonon transition of the triplet state and the excited metal ion states on the other. This work demonstrates that bulky substituents in the 4-position of the pyridine ring do not prevent the formation of triple-stranded helicates, opening the way for luminescent probes that can easily be coupled to biological materials.
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