BHQ-2 NHS is a non-fluorescent dark quencher featuring a polyaromatic azo framework and a terminal NHS ester. It exhibits a broad and intense quenching range between 560-670 nm, making it suitable for use as an acceptor in fluorescence resonance energy transfer (FRET) applications with orange to far-red emitting dyes. The NHS ester allows labeling of proteins, amine-modified oligonucleotides, and other primary amine (-NH2) containing molecules.