Biradicaloid Behavior of a Twisted Double Bond

A compound
with a highly twisted CC bond in the intermediate
bond dissociation region was isolated, and its biradicaloid nature
was experimentally demonstrated. A bianthrone derivative substituted
with long-chain alkoxy groups was synthesized, and its twisted conformer
was assembled into crystals through hydrophobic intermolecular interactions
of the introduced alkoxy chains. X-ray crystallography revealed that
the CC bond connecting the two anthrone rings has a long bond
length (1.429 Å) and a large torsional angle (57.33°). In
addition, ESR studies revealed that the twisted bianthrone has biradicaloid
character with a small singlet–triplet energy gap of 23.8 kJ
mol–1. The thermally excited triplet species were
observed in the ESR spectrum as distinct fine structures along with
a ΔMS = ±2 forbidden transition.