Biphenalenylidene: Isolation and Characterization of the Reactive Intermediate on the Decomposition Pathway of Phenalenyl Radical

First isolation and characterization
of biphenalenylidenes, which
have long been unidentified reactive intermediates on the decomposition
pathway of phenalenyl radical, were accomplished. Photoinduced electrocyclic
ring-opening reaction of anti-dihydroperopyrene resulted
in a successful conversion to E-biphenalenylidene,
which enabled a detailed investigation of the electronic structure
of E-biphenalenylidene by means of spectroscopic
techniques. A stereoisomer, Z-biphenalenylidene,
was also observed by suppressing a facile E–Z isomerization to E-biphenalenylidene
in a rigid matrix. Furthermore, Z-biphenalenylidene
demonstrated a thermal ring-closure in conrotatory process, which
is not conforming to the Woodward–Hoffmann rule. These unusual
reactivities of biphenalenylidene are ascribed to the ground states
destabilized by its singlet biradical character, which was fully supported
by theoretical calculations. The presence of E-biphenalenylidene
on the decomposition pathway of phenalenyl was confirmed experimentally,
leading to the full understanding of the decomposition mechanism of
phenalenyl.