Chiral supramolecular switches based on (R)-binaphthalene-bipyridinium guests and cucurbituril hosts

We designed and synthesized the three molecular tweezers 1 a-c(4+) containing an electron acceptor 4,4'-bipyridinium (BPY(2+)) 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 BPY(2+) unit is connected to the BIN hinge by an alkyl chain formed by two- (1 a(4+)), four- (1 b(4+)), or six-CH(2) (1 c(4+)) groups. The behavior of 1 a-c(4+) 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 BPY(2+) units, or by association with cucurbiturils. Upon reduction of 1 a-c(4+) 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 a(4+), 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 b(4+) and 1 c(4+), 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 a-c(4+) 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.