Mechanistic studies of isomeric [2]rotaxanes consisting of two different tetrathiafulvalene units reveal that the movement of cyclobis(paraquat-p-phenylene) can be controlled

Sofie K. Jensen, Mathias S. Neumann, Rikke Frederiksen, Mathias L Skavenborg, Mads C. Larsen, Stinne E. Wessel, Jan O. Jeppesen*

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

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Abstract

Controlling the movement in artificial molecular machines is a key challenge that needs to be solved before their full potential can be harnessed. In this study, two isomeric tri-stable [2]rotaxanes 1·4PF6 and 2·4PF6 incorporating both a tetrathiafulvalene (TTF) and a monopyrrolotetrathiafulvalene (MPTTF) unit in the dumbbell component have been synthesised to measure the energy barriers when the tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) ring moves across either a TTF2+ or an MPTTF2+ dication. By strategically exchanging one of the thiomethyl barriers on either the TTF unit or the MPTTF unit with the bulkier thioethyl group, the movement of the CBPQT4+ ring in 14+ and 24+ can be controlled to take place in only one direction upon tetra-oxidation. Cyclic voltammetry and 1H NMR spectroscopy were used to investigate the switching mechanism and it was found that upon tetra-oxidation of 14+ and 24+, the CBPQT4+ ring moves first to a position where it is located between the TTF2+ and MPTTF2+ dications producing high-energy co-conformations which slowly interconvert into thermodynamically more stable co-conformations. The kinetics of the movement occurring in the tetra-oxidised [2]rotaxanes 18+ and 28+ were studied at different temperatures allowing the free energy of the transition state, when CBPQT4+ moves across TTF2+ (21.5 kcal mol−1) and MPTTF2+ (20.3 kcal mol−1) at 298 K, to be determined. These results demonstrate for the first time that the combination of a TTF and an MPTTF unit can be used to induce directional movement of the CBPQT4+ ring in molecular machines with a 90% efficiency.

Original languageEnglish
JournalChemical Science
Volume14
Issue number43
Pages (from-to)12366-12378
ISSN2041-6520
DOIs
Publication statusPublished - 8. Nov 2023

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