Controlling reactivity by remote protonation of a basic side group in a bifunctional photoacid

6-Hydroxy-2-naphthoic acid and its sulfonate derivatives belong to a family of bifunctional photoacids where the -OH group acts as a proton donor and the -COO(-) group acts as a proton acceptor. Upon electronic excitation, the -OH group becomes more acidic and the -COO(-) group turns more basic. Change in the ionization state of one functional group causes a change (switch) in the reactivity of the other functional group. Using picosecond time-resolved and steady state spectroscopy, we find clear evidence for an ultrafast reactivity switch caused by a diffusional proton transfer through the water solvent between the two functional groups with no evidence of a concerted proton transfer.