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