Generation and identification of 1,4-perinaphthadiyl radical cation - first observation of the electronic absorption spectrum of an ionized diradical

gamma-Irradiation of 6b,7,8,8a-tetrahydrocyclobut[a]acenaphthylene (CAN) in haloalkane glasses at 77 K yields a persistent radical cation, CAN(.+), whose electronic absorption spectrum has been obtained. Irradiation at >640 nm converts this into an isomer which is identical with that formed by ionization of 1,4-dihydro-1,4-ethanonaphtho[1,8-de][1,2]diazepine (END) and subsequent irradiation of the radical cation at lambda > 540 nm. In both cases, further irradiation at 450 nn yields the radical cation of 1,8-divinylnaphthalene (DVN.+) which was identified by ionization of DVN generated in situ by photolysis of CAN. These related observations identify the photoisomer of CAN(.+) as 1,4-perinaphthadiyl radical cation (7,8,9,10-tetrahydrocyclohepta[de] naphthalene-7,10-diylium, CND.+). [GRAPHICS] The molecular and electronic structures of all observed radical cations, as well as their electronic absorption spectra, are discussed on the basis of B3LYP and CASPT2 quantum chemical calculations and, in the case of CAN(.+) and DVN.+, also by reference to the photoelectron spectra of the neutral hydrocarbons. The CAN(.+)-->PND.+-->DVN.+ sequence of photoreactions is discussed on the basis of orbital and state correlation diagrams.