Photochemistry of [2.2]Paracyclophan‐enes — a Matrix‐Isolation Study

The UV photolysis of [2.2]paracyclophan‐ene ( 3 ) and benzo‐[2.2]paracyclophan‐ene ( 4 ), matrix‐isolated in argon at 10 K, results in the cleavage of the ethano bridge and formation of compounds 5 and 8 , respectively. The IR and UV/Vis spectra are compared with p ‐xylylene and the benzyl radical as models for a quinoid or diradicaloid structure. The absence of an ESR signal reveals a singlet ground state for these compounds. Remarkable is a sharp absorption in the red part of the visible spectrum which is not observed in the case of p ‐xylylene or the benzyl radical. The electronic structure can be described as quinoid with a substantial amount of diradicaloid character.