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

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 dm3?mol1, 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 exchange dynamics 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 charge-transfer band at 550580 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.