Reversing the Photovoltage of a Photoacid in Aqueous Glycerol and Ethylene Glycol Solutions

Pyranine is a photoacid, which upon illumination in water, deprotonates. It has been well studied and can therefore be used as a model system for developing sensors and energy-harvesting devices based on photoacids. Under normal circumstances, the release of protons results in an open-circuit voltage of a fixed polarity. Here, it is demonstrated that the polarity of the photovoltage in glycerol and ethylene glycol is reversed compared to that in water. The polarity and kinetics of the photovoltage are found to depend on the ratio of glycerol or ethylene glycol to water. A simple model for the observed photovoltage kinetics is proposed, based on the assumption that both positive and negative charges are released and exhibit different diffusion constants.