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<p>Biradical spin probes can provide detailed
information about the distances between molecules/regions of molecules because
the through-space coupling of radical centres, characterised by <i>J</i>, is
strongly distance dependent. However, if the system can adopt multiple
configurations, as is common in supramolecular complexes, the shape of the EPR
spectrum is influenced not only by <i>J</i> but also the rate of exchange
between different states. In practice, it is often hard to separate these
variables and as a result, the effect of the latter is sometimes overlooked. To
demonstrate this challenge unequivocally we synthesised rotaxane biradicals
containing nitronyl nitroxide units at the termini of their axles. The
rotaxanes exchange between the available biradical conformations more slowly
than the corresponding non-interlocked axles but, despite this, in some cases,
the EPR spectra of the axle and rotaxane remain remarkably similar.<i> </i>Detailed
analysis allowed us to demonstrate that the similar EPR spectral shapes result
from different combinations of <i>J</i> and rates of conformational
interconversion, a phenomenon suggested theoretically more than 50 years ago. This
work reinforces the idea that thorough analysis must be performed when interpreting
the spectra of biradicals employed as spin probes in solution.</p></div>