Photoinduced proton transfer of 6-hydroxynaphthalene-1-sulfonic acid in n-propanol/water mixtures

► The proton transfer rate is non-linearly related to the concentration of water. ► A water dimer is found to be an effective proton acceptor. ► The size of water cluster depends on the acidity and the lifetime of photoacid. ► The deprotonation at room temperature requires water concentration of 4 M. ► Proton transfer takes place to water dimer above the critical water concentration.