Self-Assembled Triple-Stranded Lanthanide Dimetallic Helicates with a Ditopic Ligand Derived from Bis(benzimidazole)pyridine and Featuring an (4-Isothiocyanatophenyl)ethynyl Substituent
摘要:
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.