A Chichibabin’s Hydrocarbon-Based Molecular Cage: The Impact of Structural Rigidity on Dynamics, Stability, and Electronic Properties

A three-dimensional
π-conjugated polyradicaloid molecular
cage c-Ph14, consisting
of three Chichibabin’s hydrocarbon motifs connected by two
benzene-1,3,5-triyl bridgeheads, was synthesized. Compared with its
linear model compound l-Ph4, the prism-like c-Ph14 has a more rigid structure, which shows significant impact on the
molecular dynamics, stability, and electronic properties. A higher
rotation energy barrier for the quinoidal biphenyl units was determined
in c-Ph14 (15.64 kcal/mol)
than that of l-Ph4 (11.40
kcal/mol) according to variable-temperature NMR measurements, leading
to improved stability, a smaller diradical character, and an increased
singlet–triplet energy gap. The pressure-dependent Raman spectroscopic
studies on the rigid cage c-Ph14 revealed a quinoidal-to-aromatic transformation along the biphenyl
bridges. In addition, the ellipsoidal cavity in the cage allowed selective
encapsulation of fullerene C70 over C60, with
an associate constant of about 1.43 × 104 M–1. Moreover, c-Ph14 and l-Ph4 exhibited similar redox
behavior and their cationic species (c-Ph146+ and l-Ph42+) were obtained
by chemical oxidation, and the structures were identified by X-ray
crystallographic analysis. The biphenyl unit showed a twisted conformation
in l-Ph42+ and remained coplanarity in c-Ph146+. Notably, molecules
of c-Ph146+ form a one-dimensional columnar structure via close π–π
stacking between the bridgeheads.