Dinuclear chloride-binding foldamers based on fluorescent oligoureas

摘要

A series of dinuclear chloride-binding foldamers - (TEA)_2[L ~1Cl_2]·Et_2O (1), (TEA)_2[L ~2Cl_2] (2), (TBA)_2[L~3Cl_2] (3), (TBA)_2[L~4Cl_2] (4), [K(18-crown-6)] _2[L~4Cl_2]·H_2O (5), and (TBA)_2[L~5Cl_2]·H_2O (6) - have been synthesized from naphthyl- and anthracenyl-decorated oligoureas (tetrakisurea to hexakisurea, L~1-L~5). In the crystal structures of 1-6, the oligoureas adopt helical conformations and two chloride anions are bound inside by the urea groups. The solution binding properties of L~1-L~5 toward chloride ion were investigated by ~1H NMR and fluorescence spectroscopy. Upon binding of the anions, characteristic downfield shifts of the NH signals and enhanced fluorescent emission relative to the free ligands were observed. In DMSO solution, the ligands each display a 1:1 binding ratio to chloride, with association constants of K = 1.16-7.58 × 10~2 M~(-1), which contrasts with the 1:_2 (host to guest) binding mode in the crystal structures. However, the formation of dinuclear foldamers in the less polar solvent CHCl_3 was indicated by the HRMS (ESI) spectra.