Photoacids aremolecules that become moreacidic upon absorption of light. Either the light causes aphotodissociation to produce astrong acid, or the light causesphotoassociation (such as aring forming reaction) 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) andphotoacids (PAHs). PAGs undergo proton photodissociation irreversibly, while PAHs are molecules that undergo proton photodissociation and thermal reassociation.[1] In this latter case, theexcited state is strongly acidic, but reversible.
An example due to photodissociation is triphenylsulfonium triflate. This colourless salt consists of asulfonium cation and thetriflate anion. Many related salts are known including those with othernoncoordinating anions 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 threephenyl rings. This dissociated phenyl radical then re-combines with remaining diphenylsulfonium to liberate an H+ ion.[2] The second reaction is irreversible, and therefore the entire process is irreversible, so triphenylsulfonium triflate is a photoacid generator. The ultimate products are thus a neutralorganic sulfide and the strong acidtriflic acid.
Applications of these photoacids includephotolithography[3] and catalysis of thepolymerization ofepoxides.
An example of a photoacid which undergoes excited-state proton transfer without prior photolysis is the fluorescent dyepyranine (8-hydroxy-1,3,6-pyrenetrisulfonate or HPTS).[4]
The Förster cycle was proposed byTheodor Förster[5] and combines knowledge of the ground stateacid dissociation constant (pKa), absorption, and fluorescence spectra to predict the pKa 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 aBrønsted acid. This use of PAH should not be confused with othermeanings of PAH in chemistry and in medicine.