The behavior of the singlet and triplet spin states of methylene-bridged 1,8-naphthoquinodimethane with O2

It has been shown that thermolysis of naphthocyclopropane 8 yields the singlet 1,8-naphthoquinodimethane 7 by disrotatory ring opening. In fluid solution the singlet biradical prefers to ring close to regenerate the starting material rather than under a 1,2-hydrogen shift to yield phenalene (9). It has been demonstrated that the singlet 7 does not react with O₂ or undergo intersystem crossing to ground state triplet 7. It has also been shown by an intricate set of experiments that triplet 7 is produced on photolysis of 8 and that it reacts with O₂. Unfortunately, it was not possible to elucidate unequivocally the mechanism by which triplet 7 reacts with O₂.