Host−Guest Complexes between an Aromatic Molecular Tweezer and Symmetric and Unsymmetric Dendrimers with a 4,4‘-Bipyridinium Core

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4‘-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor−acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4‘-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host−guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and 1H NMR titration plots, (i) are of the order of 104 M-1, (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent 1H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (ΔG⧧ < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4‘-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host−guest formation causes a displacement of the first reduction wave of the 4,4‘-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host−guest complexes between the tweezer and the dendrimers are stabilized by electron donor−acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.