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

We designed and synthesized the three molecular tweezers 1a-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- (1a4+), four- (1b4+), or six-CH2 (1c4+) groups. The behavior of 1a-c4+ 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 1a-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 1a4+, 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 1b4+ and 1c4+, 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 1a-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[8] complexes characterized by a 1:2 ratio implies the release of one macrocycle showing that the binding stoichiometry of these hostguest 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.