Molecular Meccano. 4. The Self-Assembly of [2]Catenanes Incorporating Photoactive and Electroactive π-Extended Systems

Six new [2]catenanes, which have been constructed by efficient (38–64%) template-directed methods incorporate (a) within their crown-10 components hydroquinone and/or 1,5-dioxynaphthalene residues as π-donors and (b) within their tetracationic cyclophane components, bipyridinium and/or trans-bis(pyridinium)ethylene units as π-acceptors, the latter as potential photochemically-addressable functions. Four of the [2]catenanes exhibit translational isomerism in solution, as evidenced by dynamic ¹ H NMR spectroscopic studies. The preferred isomers have a 1,5-dioxynaphthalene residue inside the tetracationic cyclophane components and a bipyridinium unit inside the crown-10 components. Where the [2]catenanes have crystallized, X-ray crystallography has revealed that the predominant isomer in solution is the one that exists in the crystal—for example, only one amongst four possible translational isomers is detected in solution at low temperatures as well as in the solid state, in the case of the [2]catenane containing two different π-donors [hydroquinone and 1,5-dioxynaphthalene rings] and two different π-acceptors [bipyridinium and trans-bis(pyridinium)ethylene units]. The isomer populations in solution are also indicated by electrochemical experiments. There is evidence in the case of the [2]catenane composed of bisparaphenylene-34-crown-10 and a tetracationic cyclophane incorporating a bipyridinium unit as well as a trans-bis(pyridinium)ethylene unit of being able to switch the translational isomer preference by reducing and oxidizing sequentially the bipyridinium unit electrochemically. These results illustrate the progress that is being made toward the construction of controllable [2]catenanes. A detailed photochemical and photophysical investigation has shown that, although the trans-bis(pyridinium)ethylene units undergo efficient trans → cis photochemical process even when incorporated in a cyclophane structure, the corresponding [2]catenanes cannot be photoaddressed because of the presence of low energy charge-transfer levels.