Room-temperature absorption and fluorescence spectra of 7-hydroxyquinoline in hydroxylic and carboxylic matrices show the coexistence of free, monobonded (bridged and unbridged), dibonded (bridged and unbridged), and keto forms of this dye in these media. In both matrices, the ground-state keto tautomer is stable. The bridged enol forms (lambda(em) = 380-400 nm) can undergo an excited-state (double or triple) proton-switching reaction, leading to the keto tautomer (lambda(em) = 500-520 nm) through an energy barrier. The reverse process does not occur upon excitation of the keto tautomer due to the greater stabilization (79 kJ mol(-1)) of this tautomer relative to that of the enol species in the excited state. Irradiation of the enol forms leads to stable keto tautomers in the ground state, and this opens the way for storing information at a molecular level.