Solid-State Characterization and Photoinduced Intramolecular Electron Transfer in a Nanoconfined Octacationic Homo[2]Catenane

An octacationic homo[2]­catenane comprised
of two mechanically interlocked
cyclobis­(paraquat-p-phenylene) rings has been obtained
from the oxidation of the septacationic monoradical with nitrosonium
hexafluoroantimonate. The nanoconfinement of normally repulsive bipyridinium
units results in the enforced π-overlap of eight positively
charged pyridinium rings in a volume of <1.25 nm3. In
the solid state, the torsional angles around the C–C bonds
between the four pairs of pyridinium rings range between 16 and 30°,
while the π–π stacking distances between the bipyridinium
units are extended for the inside pair and contracted for the pairs
on the outsidea consequence of Coulombic repulsion between
the inner bipyridinium subunits. In solution, irradiation of the [2]­catenane
at 275 nm results in electron transfer from one of the paraphenylene
rings to the inner bipyridinium dimer, leading to the generation of
a temporary mixed-valence state within the rigid and robust homo[2]­catenane.