γ-Irradiation of 7,7a-dihydro-6bH-cycloprop[a]acenaphthylene (1) and its 7,7-dimethyl derivative (2) in haloalkane glasses at 77 K yields persistent parent radical cations of these compounds. On visible irradiation, 1˙⁺+++ is transformed into the radical cation of phenalene, 3˙⁺++, which was also generated independently from neutral 3. The transformation 1˙⁺+++→3˙⁺++ presumably proceeds by opening of the central cyclopropane bond and subsequent H-shift in the resulting perinaphthadiyl radical cation which is, however, not observed. Surprisingly, the dimethyl derivative, 2˙⁺+, is also transformed into a phenalene radical cation by visible photolysis. In this case the mechanism is probably a different one, involving a distonic diyl cation obtained by cleavage of a lateral cyclopropane bond. In contrast to the above, the ring opening of the dicyclopropa[a,g]pyracyclene 4 radical cation proceeds stepwise, and the perinaphthadiyl cation intermediate can be observed. A second photochemical ring opening yields the radical cation of 2,7-dihydro-2,2,7,7-tetramethylpyrene that was generated independently from the neutral precursor.

The electronic absorption spectra of all observed radical cations are discussed in conjunction with the corresponding photoelectron spectra (where available) and with quantum chemical calculations.