Synthesis, Structure, Anion Binding, and Sensing by Calix[4]pyrrole Isomers

The synthesis, structure, and anion binding properties of chromogenic octamethylcalix[4]pyrroles
(OMCPs) and their N-confused octamethylcalix[4]pyrrole isomers (NC-OMCPs) containing an inverted pyrrole
ring connected via α‘- and β-positions are described. X-ray diffraction analyses proved the structures of
two synthesized isomeric pairs of OMCPs and NC-OMCPs. The addition of anions to solutions of
chromogenic OMCPs and NC-OMCPs resulted in different colors suggesting different anion-binding
behaviors. The chromogenic NC-OMCPs showed significantly stronger anion-induced color changes
compared to the corresponding chromogenic OMCP, and the absorption spectroscopy titrations indicated
that chromogenic OMCPs and NC-OMCPs also possess different anion binding selectivity. Detailed NMR
studies revealed that this rather unusual feature stems from a different anion-binding mode in OMCPs and
NC-OMCPs, one where the β-pyrrole C−H of the inverted pyrrole moiety participates in the hydrogen-bonded anion−NC-OMCP complex. Preliminary colorimetric microassays using synthesized chromogenic
calixpyrroles embedded in partially hydrophilic polyurethane matrices allow for observation of analyte-specific changes in color when the anions are administered in the form of their aqueous solutions and in
the presence of weakly competing anions.