Role of positional isomers on receptor–anion binding and evidence for resonance energy transfer

New urea-based sensors show a strong affinity for F−, CH3COO−, and H2PO4− with an appreciable color change in the presence of excess F−. The position of the nitro group in the urea derivative influences the relative affinity toward anionic analytes. Spectral and ab initio studies showed the difference in the deprotonation sites for the ortho- and meta/para-isomers in these cases. Photophysical studies confirmed the resonance energy transfer in the case of the ortho-isomer. The ortho-isomer can act as a dual emission probe for F−.