Jean-Claude Bunzli - Self-assembled triple-stranded lanthanide dimetallic helicates with a ditopic ligand derived from bis(benzimidazole)pyridine and featuring an (4-isothiocyanatophenyl)ethynyl substituent

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      Tripier,R., Hollenstein,M., Elhabiri,M., Chauvin,A.S., Zucchi,G.,   Piguet,C., Bunzli,J.C.G. Helvetica Chimica Acta  2002 85 (7) 1915-1929

      Abstract:  Bis(2-{6-(diethylcarbamoyl)-4-[(4-isothiocyanatophenyl)ethynyl]pyridin-2'-yl}-1 -ethylbenzimidazol-5-yl)methane   (L-G) reacts with trivalent lanthanide ions in acetonitrile to   yield triple-stranded dimetallic helicates [Ln(2)(L-G)(3)](6+).   H-1-NMR Data point to the helicates being the only species formed   under stoichiometric conditions and having a time-averaged D-3   symmetry on the NMR time scale. The photophysical properties of   L-G and its helicates are discussed with respect to the closely   related ligands L-B, L-E, and their complexes, two ligands devoid   of the isothiocyanatophenylethynyl substituent. ne quantum yield   of the ligand fluorescence is three times smaller compared to   L-E, while that of the Eu(III)centered luminescence (1.1%) is   three times larger. On the other hand, the luminescence of Tb-III   is not sensitized by L-G. 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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