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.