Issue 43, 2023

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

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.

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

Supplementary files

Article information

Article type
Edge Article
Submitted
22 Aug 2023
Accepted
23 Oct 2023
First published
23 Oct 2023
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2023,14, 12366-12378

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

S. K. Jensen, M. S. Neumann, R. Frederiksen, Mathias L. Skavenborg, M. C. Larsen, S. E. Wessel and J. O. Jeppesen, Chem. Sci., 2023, 14, 12366 DOI: 10.1039/D3SC04408D

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