Photochemical Nanosciences Laboratory, Dipartimento di Chimica "G. Ciamician", Alma Mater Studiorum, Università degli Studi di Bologna, Italy.
Chemistry. 2012 Dec 21;18(52):16911-21. doi: 10.1002/chem.201202378. Epub 2012 Nov 7.
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.
我们设计并合成了三种分子夹 1a-c(4+)，它们的每个臂中都含有一个电子受体 4,4'-联吡啶（BPY(2+)）单元和一个（R）-2,2'-二氧-1,1'-联萘（BIN）单元，该单元起到手性中心和结构铰链的作用。每个 BPY(2+)单元通过由两个-CH(2)-（1a(4+)）、四个-CH(2)-（1b(4+)）或六个-CH(2)-（1c(4+)）组成的烷基链连接到 BIN 铰链。在不存在和存在瓜环[8]（CB[8]）或瓜环[7]（CB[7]）作为联吡啶单元的大环主体的情况下，研究了 1a-c(4+)化合物在化学或光化学还原时的行为。详细分析紫外/可见吸收和圆二色性（CD）光谱表明，BIN 单元的手性可以通过 BPY(2+)单元的还原或与瓜环的缔合来可逆调节。在 1a-c(4+)化合物还原时，形成的 BPY(+·)单元发生分子内二聚化，同时 BIN 二面角发生变化，这取决于烷基间隔物的长度。烷基连接物在与瓜环的缔合中也起着重要作用。由于其短的碳链，化合物 1a(4+)与庞大的 CB[8]以 1:1 的比例缔合，而对于较小的主体化合物 CB[7]，则得到 1:2 的配合物。具有较长连接物的化合物 1b(4+)和 1c(4+)无论其大小如何，都与两个瓜环缔合。在所有情况下，与 CB[8]的缔合都会导致 BIN 二面角增大，而 CB[7]配合物的形成会导致角度减小。CB[8]配合物的还原会增强 BPY(+·)二聚化相对于游离 1a-c(4+)，并导致 BIN 二面角明显减小，因为两个臂的 BPY(+·)单元必须进入同一个大环。以 1:2 比例表征的 CB[8]配合物的二聚形成表明释放了一个大环，表明通过改变客体的氧化还原状态，可以将这些主客体配合物的结合化学计量从 1:2 切换到 1:1。当在 CB[7]配合物上进行还原时，二聚化完全被抑制，这是意料之中的，因为 CB[7]腔不能容纳两个 BPY(+·)单元。
