Mechanism of photoacid generation for an arylcycloalkylsulfonium salt by ring opening and aryl cleavage

Abstract

The photochemistry of 1-(4-tert-butylphenyl)-tetrahydro-thiopyranium triflate (1), an arylcycloalkylsulfonium salt, was investigated in acetonitrile and methanol at low conversion in order to understand the reaction mechanism and its efficiency as photoacid generator. Both types of C–S bond in 1 are cleaved from the excited state. The heterolytic cleavage of the methylene C–S bond produces 4-t-BuC₆H₄S(CH₂)₄CH₂⁺ by ring opening. The carbocation generates acid and arylalkenylsufides by elimination or 1,2 hydride shift and elimination. The predominantly homolytic cleavage of the aryl C–S bond yields 4-t-BuC₆H₄˙ and c-C₅H₁₀S⁺˙ as the fragmentation products. The radicals react with the solvent forming acid, pentamethylene sulfide and tert-butylbenzene. In methanol, the formation of 4-tert-butylanisole indicates a contribution of solvolysis in the excited state of 1 or a competing formation of free aryl cation by heterolytic fragmentation. The acid generation efficiency of 1 in solution (acetonitrile or methanol) is lower than that corresponding to triphenylsulfonium triflate (TPS OTf) under the same conditions. This suggests a pathway for the regeneration of 1 after photocleavage. The photochemistry of 1 is discussed in terms of the contribution of fragmentation and ring opening reaction paths to its overall acid generation efficiency, a key property in terms of its applications in resist formulations.