In the Twilight Zone between [2]Pseudorotaxanes and [2]Rotaxanes

A [2]pseudorotaxane, based on a semi‐dumbbell‐shaped component containing asymmetrically substituted monopyrrolotetrathiafulvalene and 1,5‐dioxynaphthalene recognition sites for encirclement by cyclobis(paraquat‐ p ‐phenylene) and with a “speed bump” in the form of a thiomethyl group situated between the two recognition sites, has been self‐assembled. This supramolecular entity is a mixture in solution of two slowly interconverting [2]pseudorotaxanes, one of which is on the verge of being a [2]rotaxane at room temperature, allowing it to be isolated by employing flash column chromatography. These two [2]pseudorotaxanes were both characterized in solution by UV/Vis and 1 H NMR spectroscopies (1D and 2D) and also by differential pulse voltammetry. The spectroscopic and electrochemical data reveal that one of the complexes behaves wholly as a [2]pseudorotaxane, while the other has some [2]rotaxane character to it. The kinetics of the shuttling of cyclobis(paraquat‐ p ‐phenylene) between the monopyrrolotetrathiafulvalene and the 1,5‐dioxynaphthalene recognition sites have been investigated at different temperatures. The shuttling processes, which are accompanied by detectable color changes, can be monitored using UV/Vis and 1 H NMR spectroscopies; the spectroscopic data have been employed in the determination of the rate constants, free energies of activation, enthalpies of activation, and the entropies of activation for the translation of cyclobis(paraquat‐ p ‐phenylene) between the two recognition sites.