Model Studies on the Photochemistry of Phenolic Sulfonate Photoacid Generators

Abstract

The mechanism of photodissociation and acid generation for three phenolic sulfonate esters, ranging from alkyl, to benzyl, to aromatic, was investigated by laser flash photolysis and product studies. All the sulfonate esters studied showed the presence of phenoxyl and other complex radicals in the transient spectra. The formation of these complex transients indicates that the radical pair formed upon excitation of the sulfonate can escape the solvent cage, and undergo further chemical transformations. It was observed that all of the sulfonate esters investigated resulted in the formation of acidic species. Photoproduct studies indicate that phenyl methanesulfonate and phenyl toluene-p-sulfonate undergo a photo-Fries type rearrangement and also produce a large excess of phenol with the corresponding sulfonic acid. Upon excitation, phenyl toluene-α-sulfonate undergoes near quantitative SO2 extrusion, with the formation of no Fries rearrangement photoproducts; instead it was observed that the benzyl radicals, generated by SO2 loss, undergo a “pseudo” Fries rearrangement to form the ortho and para phenylmethane isomers. Further, the SO2 photogenerated undergoes oxidative and hydrolytic processes to form sulfuric and sulfurous acids.