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 (<b>1a</b>) and 1-trifluoromethylsulfonoxy-2-methoxy-9,10-anthraquinone (<b>1b</b>) 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 (T<sub>1</sub>) is excited to a higher triplet state which in turn undergoes homolysis of the RS(O<sub>2</sub>)–OAr bond. Secondary reactions of the initially formed sulfonyl radicals produce strong acids. It is demonstrated that high intensity photolysis of either <b>1a </b>or <b>1b</b> can initiate cationic polymerization of ethyl vinyl ether.
