Highly Selective Fluorescent Recognition of Sulfate in Water by Two Rigid Tetrakisimidazolium Macrocycles with Peripheral Chains

The
reinforced molecular recognition of two rigid tetrakisimidazolium
macrocycles resulted in highly selective fluorescent recognition of
sulfate dianion in water with an unprecedentedly high association
constant of 8.6 × 109 M–2. Besides
the electrostatic interaction, the single crystal X-ray analysis revealed
that sulfate was encapsulated in a pseudohexahedral cavity of a sandwich
structure by two orthogonally packed macrocycles via eight hydrogen
bonds between the C2 hydrogen atoms of the imidazolium units and the
oxygen atoms of sulfate. This sandwich structure was reinforced by
the π–π stacking between the phenyl and the triazinonide
rings and multiple charge-assisted hydrogen bonds between the peripheral
chains and the rigid backbones. Notably, these peripheral-backbone
hydrogen bonds rendered the flexible peripheral chains to coil around
the sandwich structure to shield sulfate inaccessible to water. This
binding process was visible by fluorescence enhancement, which was
attributed to a restrained rotation and better conjugation of the
macrocycle backbone upon binding to sulfate.