Photoacid Generators Activated Through Sequential Two-Photon Excitation: 1-Sulfonatoxy-2-alkoxyanthraquinone Derivatives

Two sulfonate ester derivatives of anthraquinone, 1-tosyloxy-2-methoxy-9,10-anthraquinone (1a) and 1-trifluoromethylsulfonoxy-2-methoxy-9,10-anthraquinone (1b) were prepared and their ability to produce strong acids upon photoexcitation was examined. It is shown that these compounds generate acid with a yield that increases with light intensity when the applied photon dose is held constant. Additional experiments show that the rate of acid generation increases 4 fold when visible light (532 nm) laser pulses are combined with ultraviolet (355 nm) compared with ultraviolet alone. Continuous wave diode laser photolysis also effects acid generation with a rate that depends quadratically on the light intensity. Density functional theory calculations, laser flash photolysis, and chemical trapping experiments support a mechanism whereby an initially formed triplet state (T₁) is excited to a higher triplet state which in turn undergoes homolysis of the RS(O₂)–OAr bond. Secondary reactions of the initially formed sulfonyl radicals produce strong acids. It is demonstrated that high intensity photolysis of either 1a or 1b can initiate cationic polymerization of ethyl vinyl ether.