Giansante Carlo, Mazzanti Andrea, Baroncini Massimo, Ceroni Paola, Venturi Margherita, Klärner Frank-Gerrit, Vögtle Fritz
Dipartimento di Chimica G. Ciamician, Alma Mater Studiorum, Università di Bologna, Via Selmi 2, 40126 Bologna, Italy.
J Org Chem. 2009 Oct 2;74(19):7335-43. doi: 10.1021/jo9014134.
We have investigated the complex formation between dendritic guests and a molecular tweezer host by NMR, absorption, and emission spectroscopy as well as electrochemical techniques. The dendrimers are constituted by an electron-acceptor 4,4'-bipyridinium core appended with one (DnB(2+)) or two (Dn(2)B(2+)) polyaryl-ether dendrons. Tweezer T comprises a naphthalene and four benzene components bridged by four methylene groups. Medium effects on molecular recognition phenomena are discussed and provide insight into the conformation of dendrimers: change in solvent polarity from pure CH(2)Cl(2) to CH(2)Cl(2)/CH(3)CN mixtures and addition of tetrabutylammonium hexafluorophosphate (NBu(4)PF(6), up to 0.15 M), the supporting electrolyte used in the electrochemical measurements, have been investigated. The association constants measured in different media show the following trend: (i) they decrease upon increasing polarity of the solvent, as expected for host-guest complexes stabilized by electron donor-acceptor interactions; (ii) no effect of generation and number of dendrons (one for the DnB(2+) family and two for the Dn(2)B(2+) family) appended to the core is observed in higher polarity media; and (iii) in a low-polarity solvent, like CH(2)Cl(2), the stability of the inclusion complexes is higher for DnB(2+) dendrimers than for Dn(2)B(2+) ones, while within each dendrimer family it increases by decreasing dendron generation, and upon addition of NBu(4)PF(6). The last result has been ascribed to a partial dendron unfolding. Kinetic investigations performed in lower polarity media evidence that the rate constants of complex formation are slower for symmetric Dn(2)B(2+) dendrimers than for the nonsymmetric DnB(2+) ones, and that within the Dn(2)B(2+) family, they decrease by increasing dendron generation. The dependence of the rate constants for the formation and dissociation of the complexes upon addition of NBu(4)PF(6) has also been investigated and discussed.
我们通过核磁共振、吸收光谱、发射光谱以及电化学技术研究了树枝状客体与分子钳主体之间的复合物形成。树枝状聚合物由一个电子受体4,4'-联吡啶鎓核心组成，该核心连接有一个（DnB(2+)）或两个（Dn(2)B(2+)）聚芳基醚树枝状分支。分子钳T由一个萘和四个通过四个亚甲基桥连的苯组分构成。讨论了介质对分子识别现象的影响，并深入了解了树枝状聚合物的构象：研究了从纯二氯甲烷到二氯甲烷/乙腈混合物的溶剂极性变化，以及添加电化学测量中使用的支持电解质六氟磷酸四丁铵（NBu(4)PF(6)，浓度高达0.15 M）的情况。在不同介质中测得的缔合常数呈现以下趋势：（i）随着溶剂极性增加，缔合常数降低，这对于通过电子供体-受体相互作用稳定的主客体复合物来说是预期的；（ii）在较高极性介质中，未观察到连接在核心上的树枝状分支的代数和数量（DnB(2+)家族为一个，Dn(2)B(2+)家族为两个）的影响；（iii）在低极性溶剂如二氯甲烷中，DnB(2+)树枝状聚合物的包合物稳定性高于Dn(2)B(2+)树枝状聚合物，而在每个树枝状聚合物家族中，随着树枝状分支代数的减少以及添加NBu(4)PF(6)，稳定性会增加。最后一个结果归因于树枝状分支的部分展开。在较低极性介质中进行的动力学研究表明，对称的Dn(2)B(2+)树枝状聚合物形成复合物的速率常数比不对称的DnB(2+)树枝状聚合物慢，并且在Dn(2)B(2+)家族中，随着树枝状分支代数的增加，速率常数降低。还研究并讨论了添加NBu(4)PF(6)后复合物形成和解离速率常数的依赖性。
