The reversible modulation of the emission of CdSe/ZnS semiconductor nanocrystals (quantum dots) was achieved by binding photochromic diheteroarylethenes and switchable acceptors for fluorescence resonance energy transfer. A biotinylated diheteroarylethene derivative was bound to quantum dots bearing conjugated streptavidin, leading to an intensity decrease as a consequence of energy transfer to the closed form of the acceptor. Interconversion between the open and closed forms by irradiation with 365 and 546 nm light enabled deactivation and activation, respectively, of the FRET process with a corresponding modulation of quantum dot emission, observed both in solution and by sequential wide-field imaging.