Photoacids are molecules containing Brønsted acidic moieties that become (more) acidic upon photo-excitation. This interesting feature has led to the development of numerous lightcontrolled applications using photoacids. For instance, metastable merocyanine-based photoacids are capable to modulate the pH of the medium and initiate polymerization reactions. Polymerization reactions can also be initiated by photoacid generators, which are species that deprotonate irreversibly in the excited state, thereby distinguishing them from true photoacids. More recently, photoacids have further been used for interesting applications in organic synthesis and biochemistry. The capacity of a photoacid depends on its 𝑝𝐾 𝑎 and 𝑝𝐾 𝑎 ℎ𝑣 , which measure its acidity in the electronic ground state (S0) and the electronically excited state (S1 or T1), respectively.