Evaluation of the role that photoacid excited-state acidity has on photovoltage and photocurrent of dye-sensitized ion-exchange membranes

Light-driven ion pumps can be fabricated from ion-exchange membranes infiltrated with water as the protonic semiconductor. Absorption of visible light and generation of mobile charge carrier protons are accomplished using photoacids that are covalently bonded to the membranes. Prior results from our work suggest that the photoacid excited-state acidity is not large enough to result in significant yields for conversion of light into mobile protons. Herein we compare a series of photoacid-bearing membranes that are even stronger acids in their excited states, and we determine that excited-state acidity does not correlate with photovoltage. By assessing the photoresponse of a series of bipolar membranes fabricated by laminating a photoacid-bearing cation-exchange membrane to an anionexchange membrane, no clear trend was observed between net built-in electric potential and photovoltaic performance. This suggests that other properties dictate the effectiveness of these light-driven proton pumps.