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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.[1] 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
]. 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.[2] As this series of reactions is irreversible, triphenylsulfonium triflate is a photoacid generator.

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

Applications of these photoacids include photolithography[3] 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).[4]

Förster cycle[edit]

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


  1. ^ V. K. Johns, P. K. Patel, S. Hassett, P. Calvo-Marzal, Y. Qin and K. Y. Chumbimuni-Torres, Visible Light Activated Ion Sensing Using a Photoacid Polymer for Calcium Detection, Anal. Chem. 2014, 86, 6184−6187. (Published online: 3 June 2014) doi:10.1021/ac500956j
  2. ^ W. D. Hinsberg, G. M. Wallraff, Lithographic Resists, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-VCH, Weinheim, 2005. (Published online: 17 June 2005) doi:10.1002/0471238961.1209200808091419.a01.pub2
  3. ^ J. V. Crivello The Discovery and Development of Onium Salt Cationic Photoinitiators, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 4241−4254. doi:10.1002/(SICI)1099-0518(19991201)37:23<4241::AID-POLA1>3.0.CO;2-R
  4. ^ N. Amdursky, R. Simkovitch and D. Huppert, Excited-state proton transfer of photoacids adsorbed on biomaterials, J. Phys. Chem. B., 1979, 118, 13859−13869. doi:10.1021/jp509153r
  5. ^ Kramer, Horst E. A.; Fischer, Peter (9 November 2010). "The Scientific Work of Theodor Förster: A Brief Sketch of his Life and Personality". ChemPhysChem. 12 (3): 555–558. doi:10.1002/cphc.201000733. PMID 21344592.