Mechanical bond-induced radical stabilization

Hao Li, Zhixue Zhu, Albert C. Fahrenbach, Brett M. Savoie, Chenfeng Ke, Jonathan C. Barnes, Juying Lei, Yan Li Zhao, Laura M. Lilley, Tobin J. Marks, Mark A. Ratner, J. Fraser Stoddart*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

75 Scopus citations


A homologous series of [2]rotaxanes, in which cyclobis(paraquat-p- phenylene) (CBPQT4+) serves as the ring component, while the dumbbell components all contain single 4,4′-bipyridinium (BIPY2+) 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•+ radical cations in this series of [2]rotaxanes are stabilized against oxidation, both electrochemically and by atmospheric oxygen. The enforced proximity between the BIPY2+ 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. 1H NMR spectroscopy reveals that the activation energy barriers to the shuttling of the CBPQT 4+ rings over the BIPY2+ 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•+ 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.

Original languageEnglish (US)
Pages (from-to)456-467
Number of pages12
JournalJournal of the American Chemical Society
Issue number1
StatePublished - Jan 9 2013

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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