Photolysis of thiophene vapor

Abstract

Thiophene vapor was photoexcited with 2139 and 2288 Å radiation as well as by mercury sensitization at 2537 Å. Experiments were done at 25, 200, and 305 °C at various pressures and light intensities as well as in the presence of CO2, C2H4, butenes, and O2. In all cases the products were C2H2, CH2CCH2, CH3CCH, CS2, CH2CHCCH, and polymer. When O2 was present CO2, COS, SO2, and CO were also produced. The product quantum yields dropped as the pressure was raised. A mechanism was deduced and is given by the lettered reactions in the text. The initially formed excited state, Th* (where Th stands for thiophene), as well as the excited intermediate leading to CH2CCH2 production, I*, decomposed at a rate dependent only on the total energy content, regardless of whether thermal or photolytic. The efficiencies for energy removal from both Th* and I* decrease along the series: thiophene, C2H4, CO2, O2. Appropriate rate constant ratios for all the gases were obtained and are listed in Table IX. Experiments in the presence of olefins showed that sulfur atoms were not intermediates. The experiments with O2 further eliminated a number of possibilities. The hydrocarbones are all produced in primary processes with additional CH2CHCCH and C2H2 coming respectively from the postulated intermediates •CHCHCHCHS• and C2H2S. Along with the C3 hydrocarbons, excited CS species are produced. They can either react with the above intermediates to produce CS2 or be deactivated to eventually form polymer (or oxidation products in the presence of O2).