title: In the twilight zone between [2]pseudorotaxanes and [2]rotaxanes.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry,University of California, Los Angeles,405 Hilgard Avenue, Los Angeles, CA 90095-1569, USA. joj@chem.sdu.dk
authors.forename: Jan O
authors.initials: JO
authors.lastname: Jeppesen
authors.forename: Scott A
authors.initials: SA
authors.lastname: Vignon
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart
mesh_terms: D004563:Electrochemistry / Q000379:methods; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D010269:Paraquat / Q000031:analogs & derivatives; D043862:Rotaxanes / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013816:Thermodynamics
chemical_list: D043862:Rotaxanes; D010269:Paraquat