Probing receptor-anion interactions by ratiometric chemosensors containing pyrrolecarboxamide interacting sites

We have designed and synthesized a series of molecular probes integrating both an amide and a pyrrole functionality for anion-recognition and -sensing. The interactions between these probe molecules and various anions have been investigated to elucidate the influence of electronic effects on the anion-recognition site. Changes in the UV/Vis and fluorescence spectra in the presence of anions reveal that probes 15 typically display a strong response to cyanide. Moreover, the appearance of the ratiometric phenomenon upon interaction with anions further enhances spectral differentiation for anion-sensing. The mechanism for the reaction between the probe molecules and the anions has been further explored by H-1 NMR titration experiments. Anions strongly interacting with probes and producing large changes in the UV/ Vis and fluorescence spectra during the titration usually result in deprotonation of the amide group, whereas weakly interacting anions initially form hydrogen bonds with amide and pyrrole NH groups followed by deprotonation at higher anion concentrations. The weakest anions, however, form only two or four hydrogen bonds with the amide and pyrrole N-H groups during the titrations. Moreover, both probes 2 and 3 are able to recognize cyanide in a semi-aqueous environment with extremely high selectivity. The formation of covalently bonded cyanohydrin derivatives from cyanide addition to an electron-deficient amide carbonyl center is attributed to the effectiveness of probes 2 and 3 in a semi-aqueous environment. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).