A new ruthenium(II) complex [Ru(bpy)(2)(1-(6-nitro-[1,10]phenanthrolin-5-yl)-3-(4-nitrophenyl)-urea) ] (bpy = 2,2'-bipyridyl) was synthesized and characterized using standard analytical and spectroscopic techniques. Detailed absorption, emission, and H-1 NMR spectral studies revealed that this receptor molecule acts as a sensor. for F-, CH3COO-, and H2PO4- in acetonitrile solution. Binding of these anions caused an appreciable change in the color of the acetonitrile solution, which could be detected with the naked eye. At relatively lower concentration of anions, 1:1 H-bonded adduct was formed; however, at higher concentration, classical Bronsted acid-base-type reaction prevailed. The relative binding affinity of different anions toward this receptor was evaluated and was rationalized with quantum chemical calculations. Narrowing of the gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels on deprotonation of the receptor molecule caused a faster decay of the luminescence lifetime for the Ru-d pi --> L-pi center dot/bpy(pi center dot)-based triplet excited state.