A Reversible Photoacid Functioning in PBS Buffer under Visible Light.

A metastable-state photoacid that can reversibly release a proton in PBS buffer (pH = 7.4) under visible light is reported. The design is based on the dual acid–base property and tautomerization of indazole. The quantum yield was as high as 0.73, and moderate light intensity (102 μmol·m2·s–1) is sufficient for the photoreaction. Reversible pH change of 1.7 units was demonstrated using a 0.1 mM aqueous solution. This type of photoacid is promising for control of proton-transfer processes in physiological conditions and may find applications in biomedical areas.