Photoacid

Photoacids are molecules which become more acidic upon absorption of light. This is due either to the formation of strong acids upon photodissociation, or to the dissociation of protons upon photoassociation (e.g. ring-closing). There are two main types of molecules that release protons upon illumination: photoacid generators (PAGs) and photoacids (PAHs). PAGs undergo proton photodissociation irreversibly, while PAHs are molecules that undergo proton photodissociation and thermal reassociation. In this case the excited state is strongly acidic, but reversible.

An example due to photodissociation is triphenylsulfonium triflate with the formula [(C₆H₅)₃S⁺][CF₃SO−
3]. This colourless salt consists of a sulfonium cation and the triflate anion. Many related salts are known including those with other noncoordinating anions and those with diverse substituents on the phenyl rings.

The triphenylsulfonium salts absorb at 233 nm, which induces the series of reactions shown below. As this series of reactions is irreversible, triphenylsulfonium triflate is a photoacid generator.

[(C₆H₅)₃S⁺][CF₃SO−
3] + hν → [(C₆H₅)₂S^+.][CF₃SO−
3] + C₆H.
5 [(C₆H₅)₂S^+.][CF₃SO−
3] + C₆H.
5 → (C₆H₅C₆H₄)(C₆H₅)S + [CF₃SO−
3][H⁺]

Applications of these photoacids include photolithography and catalysis of the polymerization of epoxides.

An example of a photoacid which undergoes excited-state proton transfer without prior photolysis is the fluorescent dye pyranine (8-hydroxy-1,3,6-pyrenetrisulfonate or HPTS).