Helv. Chim. Acta 2002, 85, 1915-1929
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-averagedD3 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.