Photoacid

Photoacids are

) that leads to an increased acidity and dissociation of a proton.

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.[1] In this latter case, the excited state is strongly acidic, but reversible.

Photoacid generators

An example due to photodissociation is triphenylsulfonium triflate. This colourless salt consists of a

and those with diverse substituents on the phenyl rings.

The triphenylsulfonium salts absorb at a wavelength of 233 nm, which induces a dissociation of one of the three

.

[(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^[3] and catalysis of the polymerization of epoxides.

Photoacids

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).^[4]

The Förster cycle was proposed by Theodor Förster^[5] and combines knowledge of the ground state acid dissociation constant (pK_a), absorption, and fluorescence spectra to predict the pK_a in the excited state of a photoacid.

The name photoacid can be abbreviated PAH, where the H does not stand for a word starting with H, but rather for a hydrogen atom which is lost when the molecule reacts as a Brønsted acid. This use of PAH should not be confused with other meanings of PAH in chemistry and in medicine.

References