The Photorearrangements of a Naphthobarrelene‐like System. Dependence on excited‐state spin multiplicity and electronic configuration, and evidence for biradical intermediates. Preliminary communication

8‐Benzoyl‐9‐deuterio‐naphtho [ de ‐2.3.4]bicyclo [3.2.2]nona‐2,6,8‐triene ( 1 ) rearranged quantitatively in a photochemical di‐π‐methane‐type process to 2‐, 6‐, and 9‐deuteriated 1‐benzoyl‐naphtho [ de ‐2.3.4]tricyclo [4.3.0.0 2,9 ]nona‐2, 6‐diene ( 8a–c ). The phenylhydroxymethyl analogue 2 underwent a similar regioselective rearrangement to 9a–c . The rearrangement 1 → 8a–c is proposed to proceed along three reaction paths evolving from two primary photochemical processes of naphthylvinyl and vinyl‐vinyl bonding ( 1 → 3 + 6 ). Evidence for a competition between several paths and involvement of biradical intermediates derives from changes in the isotopomeric composition with temperature, and from laser flash detection (λ exc 353 nm) of a transient. The dependence of the quantum yield for product formation from 1 on excitation wavelength and sensitizer triplet energy leads to the conclusion that reaction to the primary biradicals occurs directly from the S 1 ( n , π*) and T 2 ( n , π* ) states, and that reaction from T 1 (π, π*) and from S 2 (π, π*) proceed either directly or via T 2 .