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Chiral Supramolecular Switches Based on (R)-Binaphthalene–Bipyridinium Guests and Cucurbituril Hosts

Authors

  • Chao Gao,

    1. Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Università degli Studi di Bologna via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456
    2. Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai, 200237 (P.R. China), Fax: (+86) 21-64252288
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  • Dr. Serena Silvi,

    1. Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Università degli Studi di Bologna via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456
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  • Dr. Xiang Ma,

    1. Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai, 200237 (P.R. China), Fax: (+86) 21-64252288
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  • Prof. He Tian,

    Corresponding author
    1. Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai, 200237 (P.R. China), Fax: (+86) 21-64252288
    • Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai, 200237 (P.R. China), Fax: (+86) 21-64252288
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  • Prof. Alberto Credi,

    Corresponding author
    1. Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Università degli Studi di Bologna via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456
    • Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Università degli Studi di Bologna via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456
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  • Prof. Margherita Venturi

    Corresponding author
    1. Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Università degli Studi di Bologna via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456
    • Photochemical Nanosciences Laboratory, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Università degli Studi di Bologna via Selmi 2, 40126 Bologna (Italy), Fax: (+39) 051-2099456
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Abstract

We designed and synthesized the three molecular tweezers 1 ac4+ containing an electron acceptor 4,4-bipyridinium (BPY2+) unit in each of the two arms and an (R)-2,2-dioxy-1,1-binaphthyl (BIN) unit that plays the role of chiral centre and the hinge of the structure. Each BPY2+ unit is connected to the BIN hinge by an alkyl chain formed by two- (1 a4+), four- (1 b4+), or six-CH2 (1 c4+) groups. The behavior of 1 ac4+ upon chemical or photochemical reduction in the absence and in the presence of cucurbit[8]uril (CB[8]) or cucurbit[7]uril (CB[7]) as macrocyclic hosts for the bipyridinium units has been studied in aqueous solution. A detailed analysis of the UV/Vis absorption and circular dichroism (CD) spectra shows that the helicity of the BIN unit can be reversibly modulated by reduction of the BPY2+ units, or by association with cucurbiturils. Upon reduction of 1 ac4+ compounds, the formed BPY+. units undergo intramolecular dimerization with a concomitant change in the BIN dihedral angle, which depends on the length of the alkyl spacers. The alkyl linkers also play an important role in association to cucurbiturils. Compound 1 a4+, because of its short carbon chain, associates to the bulky CB[8] in a 1:1 ratio, whereas in the case of the smaller host compound CB[7] a 1:2 complex is obtained. Compounds 1 b4+ and 1 c4+, which have longer linkers, associate to two cucurbiturils regardless of their sizes. In all cases, association with CB[8] causes an increase of the BIN dihedral angle, whereas the formation of CB[7] complexes causes an angle decrease. Reduction of the CB[8] complexes results in an enhancement of the BPY+. dimerization with respect to free 1 ac4+ and causes a noticeable decrease of the BIN dihedral angle, because the BPY+. units of the two arms have to enter into the same macrocycle. The dimer formation in the CB[8] complexes characterized by a 1:2 ratio implies the release of one macrocycle showing that the binding stoichiometry of these host–guest complexes can be switched from 1:2 to 1:1 by changing the redox state of the guest. When the reduction is performed on the CB[7] complexes, dimer formation is totally inhibited, as expected because the CB[7] cavity cannot host two BPY+. units.

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