Bronsted photogeneration

In the following sections the properties of photogenerators of strong Bronsted acids and their use in microlithography are summarized. 

Timpe classified the onium salts as bivalent initiators , that is, capable of initiating both radical and cationic cures simultaneously, hence capable of so-called dual cures of mixtures of radical and cationically polymerizable monomers [18]. This theme has been developed in the more recent review by Rabek [19]. More recently, use of photogenerated Bronsted or Lewis acids to effect decomposition of acid-labile polymers has found utility in photoimaging and been reviewed [20],... 

Catalysis of cationic polymerization requires formation of Bronsted acid, while the product of interest in connection with free radical cures is an aryl radical. Early results [69] suggested that the photogeneration of acid, followed by an indicator technique, might be more efficient than formation of the radical products, estimated as Phi and Ph2S, for photolysis of Ph2I+ and... 

Quantum yield values measured in solution may not necessarily apply to polymer films, the usual environment for practical application of this photochemistry. McKean et al. have adapted the indicator dye method to the measurement of quantum yields for Bronsted acid photogeneration in poly-(4-tert-butoxycarbonyloxystyrene) [20], As with the solution photochemistry of diphenyliodonium salts [71], an inverse dependence of quantum yield on exposure intensity was observed absolute quantum yields from 0.26 to 0.40 were measured at 254 nm, which extrapolate to approximately 0.45 at zero intensity, comparable to the value estimated by Dektar and Hacker [82b] in solution. McKean et al. [20b] note that similar quantum yields in solution and polymer films below Tg have also been reported for photo-Fries rearrangements [84] and photodissociation of diacyl peroxides [85]. 

For a review, see Wan, P. and Yates, K., Photogenerated carbonium ions and vinyl cations in aqueous solution. Rev. Chem. Intermed., 5, 157, 1984 See also McEwen, J. and Yates, K., Photohydration of styrenes and phenylacetylenes. General acid catalysis and Bronsted relationships, /. Am. Chem. Soc., 109, 5800, 1987 3-Nitrostyrenes undergo anti-Markovnikov addition Wan, R, Davis, M.J. and Teo, M.-A., Photoaddition of water and alcohols to 3-nitrostyrenes. Structure-reactivity and solvent effects, /. Org. Chem., 54,1354,1989. 

---