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

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 [Ln₂(L^G)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 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. The 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.