Switching a pH indicator by a reversible photoacid: A quantitative analysis of a new two-component photochromic system

Abstract A UV/visible and pH-metric quantitative analysis of the spiropyran reversible photoacid 1 -MEH allowed us to determine the pK a 's of the open and closed forms, the rate constants of relaxation, the quantum yield of decolouration and the UV/visible spectra of the protonated and deprotonated open and closed isomers. The coupling between 1 -MEH and a selected pH-indicator acidochromic dye via proton exchange has also been analysed. The initial pH, stoichiometric ratio between the photoacid and the pH-indicator, light intensity and pK a range of the pH-indicator have been established to maximize the proton transfer from the photoacid donor to the pH-indicator acidochromic dye acceptor. When the pK a 's are matched, the two-component system behaves like a single photochromic dye. The proposed model explains why 1 -MEH photo-switches BromoCresol Green but not Methyl-Orange.