The quenching compounds of the invention are weakly luminescent cyanines that are substituted by one or more heteroaromatic quenching moieties. The quenching compounds of the invention exhibit little or no observable luminescence and efficiently quench a broad spectrum of luminescent compounds. The chemically reactive quenching compounds possess utility for labeling a wide variety of substances, including biomolecules. These labeled substances are highly useful for a variety of energy-transfer assays and applications.