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Issue 24, 2021
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Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

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Abstract

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 J, 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 J 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. Detailed analysis allowed us to demonstrate that the similar EPR spectral shapes result from different combinations of J 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.

Graphical abstract: Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

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Supplementary files

Article information


Submitted
12 Mar 2021
Accepted
14 May 2021
First published
14 May 2021

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2021,12, 8385-8393
Article type
Edge Article

Spin-labelled mechanically interlocked molecules as models for the interpretation of biradical EPR spectra

L. Gualandi, P. Franchi, E. Mezzina, S. M. Goldup and M. Lucarini, Chem. Sci., 2021, 12, 8385
DOI: 10.1039/D1SC01462E

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