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.

Using Molecular Force to Overcome Steric Barriers in a Springlike Molecular Ouroboros**

“…4) of the self-complexed molecule 1 4+ displays a significant change in the visible to NIR region of the spectrum, where a broad and relatively weak band is observed for the self-complex at 825 nm (Me 2 CO: e = 1100 mol -1 cm -1 ). It is well known [17,25] that such a band is typical of charge-transfer (CT) interactions arising from the encirclement of the MPTTF unit by the tetracationic cyclophane CBPQT 4+ . The effect of the solvent on the localization of the CT band is limited.…”

Toward Chemically Controlled Nanoscale Molecular Machinery

“…Ideally, the p···p stacking interactions [14] between the p-electrondeficient CBPQT 4+ ring and the more p-electron-donating MPTTF site would result in this [2]rotaxane existing as a single co-conformation wherein the CBPQT 4+ ring encircles only the MPTTF site (K a = 1300 m À1 in MeCN). [15] However, both UV/Vis and 1 H NMR spectroscopies have indicated [12] the presence, in Me 2 CO (CD 3 COCD 3 ) solution, of two stable translational isomers [16] in a 1:1 ratio at room temperature. Since the MPTTF site fails to entice the CBPQT 4+ ring to reside exclusively on it, its subsequent oxidation, and removal of its stabilizing interactions with the CBPQT 4+ ring, produces a switch that works no more than 50 % of the time.…”

Toward Chemically Controlled Nanoscale Molecular Machinery

“…[7] A [2]rotaxane consists of two components: a ring and a dumbbell-shaped one. [15] However, both UV/Vis and 1 H NMR spectroscopies have indicated [12] the presence, in Me 2 CO (CD 3 COCD 3 ) solution, of two stable translational isomers [16] in a 1:1 ratio at room temperature. When two constitutionally different recognition sites [10] are incorporated into the rodlike portion of the dumbbell, the [2]rotaxane can exist in two nondegenerate co-conformations [11] as translational isomers whose relative populations reflect the free energy difference for binding at these two sites.…”