Molecular Meccano, 56. - Anthracene-containing [2]rotaxanes: Synthesis, spectroscopic, and electrochemical properties

Two dumbbell-shaped compounds (8 and 11), each composed of two polyether chains intercepted by a 1,4-dioxybenzene recognition site, terminated by tetraarylmethane-based stoppers, and emanating from a central 9,10- or a 2,6- dioxyanthracene unit, have been synthesized. Two [2]rotaxanes (9 · 4 PF₆ and 12 · 4 PF₆) have been prepared by interlocking these dumbbell-shaped compounds (8 and 11) with a bipyridinium-based tetracationic cyclophane (15 · 4 PF₆) - namely, cyclobis(paraquat-p-phenylene). A [3]rotaxane (10 · 8 PF₆) incorporating two cyclophane components (15 · 4 PF₆) was also obtained when the 9,10-dioxyanthracene-containing dumbbell-shaped compound (8) incorporating two 1,4-dioxybenzene recognition sites was employed. The ¹H-NMR spectroscopic investigation of the [2]rotaxanes (9 · 4 PF₆ and 12 · 4 PF₆) revealed that the cyclophane component encircles one of the two 1,4-dioxybenzene recognition sites in the 9,10-dioxyanthracene-containing [2]rotaxane (9 · 4 PF₆) and the 2,6-dioxyanthracene unit in the other [2]rotaxane (12 · 4 PF₆). These structures have been confirmed by UV/Vis and electrochemical experiments. Comparison with the spectroscopic properties of simple model compounds shows the presence of electronic interactions which lead to (i) the occurrence of very efficient energy transfer processes in the dumbbell-shaped components and (ii) perturbations in the absorption spectra with appearance of two charge-transfer absorption bands and complete luminescence quenching in the [2]rotaxanes. For the 2,6-dioxyanthracene- containing [2]rotaxane (12 · 4 PF₆), it has been demonstrated that the cyclophane can be displaced from the dioxyanthracene to the 1,4-dioxybenzene station upon electrochemical oxidation.