A novel metastable-state photoacid for reversible protonation of strong bases

Metastable-state photoacid (mPAH) has become a common tool for controlling and driving chemical processes with light. However, previously developed mPAHs could not be used for reversible protonation of strong bases, including many common amines and heterocycles. In this work, we developed a novel mPAH using benzimidazole as the structural moiety with the active proton. The very low dark acidity of the photoacid avoids the protonation of strong bases in the absence of irradiation. Absorption spectroscopy and NMR studies showed that the photoacid could be paired with NEt₃ and DMAP and underwent reversible photoreaction and proton transfer. This new type of photoacid allows the photocontrol of functional materials with strong proton acceptors.