PHOTOCHEMISTRY OF [2.2]PARACYCLOPHAN-ENES - A MATRIX-ISOLATION STUDY

The UV photolysis of [2.2]paracyclophan-ene (3) and benzo[2.2]paracycl ophan-ene (4), matrix-isolated in argon at 10 K, results in the cleava ge of the ethano bridge and formation of compounds 5 and 8, respective ly. The IR and UV/Vis spectra are compared with p-xylylene and the ben zyl radical as models for a quinoid or diradicaloid structure. The abs ence of an ESR signal reveals a singlet ground state for these compoun ds. Remarkable is a sharp absorption in the red part of the visible sp ectrum which is not observed in the case of p-xylylene or the benzyl r adical. The electronic structure can be described as quinoid with a su bstantial amount of diradicaloid character.