Structures, metal ion affinities, and fluorescence properties of soluble derivatives of tris((6-phenyl-2-pyridyl)methyl)amine

Metal complexes of tris((6-phenyl-2-pyridyl)methyl)amine (2) have hydrophobic cavities that potentially accommodate small molecules. However, the utility of this attractive motif has been hampered by the poor solubility of such complexes in many common solvents. In this study, two tripodal ligands (3, tris-[6-(3,4,5-trimethoxy-phenyl)-pyridin-2-ylmethyl]-amine, and 4, tris((6-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)pyridin-2-yl) methyl)amine) derived from 2 were prepared with enhanced solubility in organic and aqueous solvents. The X-ray crystallographic analyses of selected ligands and complexes revealed that the hydrophobic cavities inside the zinc complexes were retained after derivatization. Fluorescence, nuclear magnetic resonance (NMR), and potentiometrie titration studies, which were enabled by the improved solubility, were performed to investigate the binding properties of the soluble ligands (3 and 4) with metal ions such as Zn²⁺ and Cu²⁺. When saturating quantities of Zn²⁺ ions are added to ligand 3 in acetonitrile, the fluorescence emission maximum exhibits a pronounced red shift of ∼80 nm (from 376 to 457 nm) and is enhanced by a factor of >100 when measured at 520 nm. The fluorescence properties of the Zn²⁺ ion-coordinated ligands in the Zn(3) complex are consistent with a charge-transfer character in the excited state, with possible contributions from a penalization of the pyridyl-trimethoxyphenyl groups in the excited state, and from excitonic interactions.