Identification of Photoacidic Behavior Using AC and Open-Circuit Photoelectrochemical Techniques

Photoacids are molecules whose acidity increases through absorption of light. When the excited-state lifetime of a photoacid is sufficiently long, proton transfer from its thermally equilibrated electronic excited state results in a transient change in pH and/or pOH, which is commonly detected using spectroscopic techniques. Herein we expand this measurement toolkit by introducing alternating AC and open-circuit photoelectrochemical techniques that characterize photoacidic behavior from a model photoacid, the sodium salt of 8-hydroxypyrene-1,3,6-trisulfonate, dissolved in aqueous solutions in a thin-pathlength two-electrode cell. Continuous illumination of protonated photoacids in their electronic ground state results in significant and reproducible changes in low-frequency impedance and open-circuit potential. When these molecules are made to be non-acidic, via deprotonation using more alkaline pH conditions or methoxylation via synthesis, electrochemical data measured in the dark and under illumination are nearly identical. Best fits of AC electrochemical data to a simplified equivalent circuit support that photoelectrochemical responses are likely due to changes in local proton concentration at the electrode divide electrolyte interface, and not changes in proton flux due to mass transfer, as previously suggested. Collectively, our results provide further insight into the utility of these photoelectrochemical techniques to probe photoacidic behavior.