New Chromogenic and Fluorescent Probes for Anion Detection: Formation of a [2 + 2] Supramolecular Complex on Addition of Fluoride with Positive Homotropic Cooperativity

Two new chromogenic and fluorescent probes for anions have been designed, synthesized, and
characterized. These probes contain multiple hydrogen bonding donors including hydrazine, hydrazone,
and hydroxyl functional groups for potential anion interacting sites. Despite the possible flexible structural
framework due to the presence of sp3 carbon linkage, X-ray structure analysis of probe 2 displayed an
essentially planar conformation in the solid state owing to strong crystal packing interactions comprising
a combination of favorable π−π stacking effect and hydrogen bonding to cocrystallized CH3OH molecules.
Both probes 1 and 2 display orange color in DMSO solution and show fairly weak fluorescence at room
temperature. Binding studies revealed that both probes 1 and 2 show noticeable colorimetric and fluorescent
responses only to F-, OAc-, and H2PO4- among the nine anions tested (F-, Cl-, Br-, I-, OAc-, H2PO4-,
HSO4-, ClO4-, and NO3-). The general trend of the sensitivity to anions follows the order of F- >
OAc- > H2PO4- > Cl- > Br- ≈ I- ≈ HSO4- ≈ ClO4- ≈ NO3-. A 1:2 (probe to anion) binding
stoichiometry was found for probe 1 with OAc- and H2PO4- and probe 2 with F-, OAc-, and H2PO4-.
The binding isotherm of probe 1 to F- was found to be complicated with apparent multiple equilibria
occurring in solution. The formation of an aggregated supramolecular complex upon addition of fluoride
is proposed to rationalize the observed optical responses and is supported by ESI mass spectrometry and
pulsed-field gradient NMR spectroscopy. Data analysis suggests that the binding of probe 1 to F- shows
positive homotropic cooperativity.