Interdigitated Hydrogen Bonds: Electrophile Activation for Covalent Capture and Fluorescence Turn-On Detection of Cyanide

Hydrogen-bonding promoted covalent modifications are finding useful applications in small-molecule chemical synthesis and detection. We have designed a xanthene-based fluorescent probe 1, in which tightly held acylguanidine and aldehyde groups engage in multiple intramolecular hydrogen bonds within the concave side of the molecule. Such an interdigitated hydrogen bond donor-acceptor (HBD-HBA) array imposes significant energy barriers (Delta G(double dagger) = 10-16 kcal mol(-1)) for internal bond rotations to assist structural preorganization and effectively polarizes the electrophilic carbonyl group toward a nucleophilic attack by CN- in aqueous environment. This covalent modification redirects the de-excitation pathways of the cyanohydrin adduct 2 to elicit a large (>7-fold) enhancement in the fluorescence intensity at lambda(max) = 440 nm. A remarkably faster (> 60-fold) response kinetics of 1, relative to its N-substituted (and therefore "loosely held") analogue 9, provided compelling experimental evidence for the functional role of HBD-HBA interactions in the "remote" control of chemical reactivity, the electronic and steric origins of which were investigated by DFT computational and X-ray crystallographic studies.