We designed and synthesized the three molecular tweezers 1 a–c4+ 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 a–c4+ upon chemical or photochemical reduction in the absence and in the presence of cucurbituril (CB) or cucurbituril (CB) 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 a–c4+ 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 in a 1:1 ratio, whereas in the case of the smaller host compound CB 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 causes an increase of the BIN dihedral angle, whereas the formation of CB complexes causes an angle decrease. Reduction of the CB complexes results in an enhancement of the BPY+. dimerization with respect to free 1 a–c4+ 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 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 complexes, dimer formation is totally inhibited, as expected because the CB cavity cannot host two BPY+. units.
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