Fluorescence photoswitching based on a photochromic pKa change in an aqueous solution.

Reversible fluorescence photoswitching of RSA-AZO dyad 1 was clearly demonstrated in an acidic aqueous solution. The fluorescence photoswitching mechanism is based on the reversible ring opening/closing reactions of the RSA unit induced by a photochromic pK(a) change along with the photoisomerization of the AZO unit.