1-Arylazo-5,5-dimethyl dipyrromethanes: Versatile chromogenic probes for anions

1-Arylazo-5,5-dimethyl dipyrromethanes (azo-DPM's) have been synthesized by electrophilic substitution reactions of 5,5-dimethyl dipyrromethanes in aqueous solution in 23–68% yields. The solution phase binding studies of azo-DPM's have been investigated by naked eye, absorption and proton NMR spectroscopy. The receptors, appended with electron-withdrawing groups display enhanced affinity and selectivity for F−, H2PO4− and AcO− than electron-donating groups in different solvents. Furthermore, the solvatochromic responses of 2,5-bis(4′-nitrophenyl)azo-pyrrole have been examined by visual and UV–vis spectroscopy with or without structurally different anions in a variety of solvents. The present studies of anion receptors may be used in environmental sensors, molecular electronics and development of molecular switches.