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Abstract Complexation between a viologen radical cation (V.+) and cyclobis(paraquat‐p‐phenylene) diradical dication (CBPQT2(.+)) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair ofV.+using radical‐pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+) in a size‐matched bisradical dicationic host — namely, cyclobis(paraquat‐2,6‐naphthalene)2(.+), i.e.,CBPQN2(.+). Central to this dual recognition process was the choice of 2,6‐bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations inCBPQN2(.+)suitable for binding twoMV.+with relatively short (3.05–3.25 Å) radical‐pairing distances. The size‐matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the twoMV.+in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex — namely, [(MV)2⊂CBPQN]4(.+)– in MeCN was confirmed by VT1H NMR, as well as by EPR spectroscopy. The solid‐state superstructure of [(MV)2⊂CBPQN]4(.+)reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three differentV.+, suggesting that localization of the radical‐pairing interactions has a strong influence on the packing of the twoMV.+inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host‐guest radical‐pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox‐switchable recognition motif for the construction of MIMs and AMMs.
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Insight from Electrochemical Analysis in the Radical Cation State of a Monopyrrolotetrathiafulvalene‐Based [2]Rotaxane
Abstract Mechanically interlocked molecules are a class of compounds used for controlling directional movement when barriers can be raised and lowered using external stimuli. Applied voltages can turn on redox states to alter electrostatic barriers but their use for directing motion requires knowledge of their impact on the kinetics. Herein, we make the first measurements on the movement of cyclobis(paraquat‐p‐phenylene) (CBPQT4+) across the radical‐cation state of monopyrrolotetrathiafulvalene (MPTTF) in a [2]rotaxane using variable scan‐rate electrochemistry. The [2]rotaxane is designed in a way that directs CBPQT4+to a high‐energy co‐conformation upon oxidation of MPTTF to either the radical cation (MPTTF⋅+) or the dication (MPTTF2+).1H NMR spectroscopic investigations carried out in acetonitrile at 298 K showed direct interconversion to the thermodynamically more stable ground‐state co‐conformation with CBPQT4+moving across the oxidized MPTTF2+electrostatic barrier. The electrochemical studies revealed that interconversion takes place by movement of CBPQT4+across both the MPTTF•+(19.3 kcal mol−1) and MPTTF2+(18.7 kcal mol−1) barriers. The outcome of our studies shows that MPTTF has three accessible redox states that can be used to kinetically control the movement of the ring component in mechanically interlocked molecules.
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- Award ID(s):
- 2403941
- PAR ID:
- 10642367
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 30
- Issue:
- 55
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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