Pyrolysis transformed the title diene (1) into a 9: l mixture of perfluorobicyclo [3.1. llheptane (2) and cis-perfluorobicyclo [3.2. 0] heptane (3) at 300" C and at> 450" C principally into perfluorocyclopentene (11). Mercury-sensitized vapor phase photolysis of 1 also yielded 2 and 3, but the latter now dominated and its trans isomer 13 was formed as well. In the presence of nitrogen as a bath gas, the product was further enriched in the r3. 2.01 ...