Effect of Ethynyl and 2-Thienyl Substituents on the Complexation of 4'- Substituted 2,2':6',2''-Terpyridines with Zn2+ and Fe2+ Ions, and the Spectroscopic Properties of the Ligands and Formed Complex Species
Abstract
The complexation of terpyridine (tpy) and its 4'-ethynyl (E) and 4'-(2-thienyl) (T) derivatives, as promising chelate end groups for oligomeric components of conjugated metallosupramolecular polymers, with Zn2+ and Fe2+ ions as ion couplers was investigated in acetonitrile using NMR and UV/ Vis spectroscopy. The binding of these ligands with Zn2+ ions shows two distinct steps with comparable binding constants in the range of 106 dm3mol-1, whereas their binding with Fe2+ ions shows a strong tendency to form [FeL2]2+ species, which almost do not dissociate to monoligand species [FeL]2+ even at a twofold excess of Fe2+ ions.
The ligand exchangedynamics between [ZnL2]2+ species in solution was studied by NMR spectroscopy and the related parameters were determined. UV/Vis spectra taken during complexation show an apparent isosbestic point at 310+-10 nm, and those of the systems with Fe2+ ions also show a metal-to-ligand chargetransfer band at 550-580 nm. 4'-Substituents strongly enhance the luminescence of tpy (ca. 400 times).
Additional enhancement is provided by binding the ligands to Zn2+ ions, whereas their binding to Fe2+ ions quenches the fluorescence.