MOLECULAR MECCANO .4. THE SELF-ASSEMBLY OF [2]CATENANES INCORPORATINGPHOTOACTIVE AND ELECTROACTIVE PI-EXTENDED SYSTEMS

Six new [2]catenanes, which have been constructed by efficient (38-64% ) template-directed methods incorporate (a) within their crown-10 comp onents hydroquinone and/or 1,5-dioxynaphthalene residues as pi-donors and (b) within their tetracationic cyclophane components, bipyridinium and/or trans-bis(pyridinium)ethylene units as pi-acceptors, the latte r as potential photochemically-addressable functions. Four of the [2]c atenanes exhibit translational isomerism in solution, as evidenced by dynamic H-1 NMR spectroscopic studies. The preferred isomers have a 1, 5-dioxynaphthalene residue inside the tetracationic cyclophane compone nts and a bipyridinium unit inside the crown-10 components. Where the [2]catenanes have crystallized, X-ray crystallography has revealed tha t the predominant isomer in solution is the one that exists in the cry stal-for example, only one amongst four possible translational isomers is detected in solution at low temperatures as well as in the solid s tate, in the case of the [2]catenane containing two different pi-donor s [hydroquinone and 1,5-dioxynaphthalene rings] and two different it-a ccepters [bipyridinium and trans-bis(pyridinium)ethylene units]. The i somer populations in solution are also indicated by electrochemical ex periments. There is evidence in the case of the [2]catenane composed o f bisparaphenylene-34-crown-10 and a tetracationic cyclophane incorpor ating a bipyridinium unit as well as a trans-bis(pyridinium)ethylene u nit of being able to switch the translational isomer preference by red ucing and oxidizing sequentially the bipyridinium unit electrochemical ly. These results illustrate the progress that is being made toward th e 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 corresp onding [2]catenanes cannot be photoaddressed because of the presence o f low energy charge-transfer levels.