Mechanical Bond-Induced
Radical Stabilization
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Abstract
A homologous series of [2]rotaxanes, in which cyclobis(paraquat-<i>p</i>-phenylene) (CBPQT<sup>4+</sup>) serves as the ring component,
while the dumbbell components all contain single 4,4′-bipyridinium
(BIPY<sup>2+</sup>) units centrally located in the midst of oligomethylene
chains of varying lengths, have been synthesized by taking advantage
of radical templation and copper-free azide–alkyne 1,3-dipolar
cycloadditions in the formation of their stoppers. Cyclic voltammetry,
UV/vis spectroscopy, and mass spectrometry reveal that the BIPY<sup>•+</sup> radical cations in this series of [2]rotaxanes are
stabilized against oxidation, both electrochemically and by atmospheric
oxygen. The enforced proximity between the BIPY<sup>2+</sup> units
in the ring and dumbbell components gives rise to enhanced Coulombic
repulsion, destabilizing the ground-state co-conformations of the
fully oxidized forms of these [2]rotaxanes. The smallest [2]rotaxane,
with only three methylene groups on each side of its dumbbell component,
is found to exist under ambient conditions in a monoradical state,
a situation which does not persist in acetonitrile solution, at least
in the case of its longer analogues. <sup>1</sup>H NMR spectroscopy
reveals that the activation energy barriers to the shuttling of the
CBPQT<sup>4+</sup> rings over the BIPY<sup>2+</sup> units in the dumbbells
increase linearly with increasing oligomethylene chain lengths across
the series of [2]rotaxanes. These findings provide a new way of producing
highly stabilized BIPY<sup>•+</sup> radical cations and open
up more opportunities to use stable organic radicals as building blocks
for the construction of paramagnetic materials and conductive molecular
electronic devices