Kinetic Studies Reveal that the Secondary Station Impacts the Rate of Motion of Cyclobis(Paraquat-p-Phenylene) in Out-of-Equilibrium [2]Rotaxanes

Abstract

Control of movement in artificial molecular machines relies on the formation of out-of-equilibrium states that can subsequently interconvert to their ground states. However, a detailed description of molecular machines that are out of equilibrium is a challenge because they are often too short-lived to be characterized. Herein, the synthesis of two cyclobis(paraquat-p-phenylene) [2]rotaxanes that incorporate a redox-active monopyrrolotetrathiafulvalene unit as the primary station and either a hydroquinone or a xylyl moiety as the secondary station is described. It is shown that the bistable [2]rotaxanes can be pushed out of equilibrium by an oxidation/reduction cycle and since a steric barrier is located between the two stations, the out-of-equilibrium states of the [2]rotaxanes can be physically isolated as solids. This allows to make detailed spectroscopic and electrochemical investigations of the [2]rotaxanes in both the di-oxidized and un-oxidized states. The outcome of the studies shows that the replacement of the secondary hydroquinone station with a xylyl station has no impact on the thermodynamic properties but has a significant effect on the kinetic properties of the [2]rotaxanes illustrating that the nature of the secondary station can be used to control the speed of [2]rotaxane-based molecular machines.

Keywords: kinetics; molecular machines; out‐of‐equilibrium states; rotaxanes; tetrathiafulvalenes.

Figure 1

a) Molecular structures of the bistable [2]rotaxanes R1•4PF₆ and R2•4PF₆ (only one translational isomer is shown in each case). b) Cartoon representations of the two possible translational isomers (or co‐conformations) of the bistable [2]rotaxanes corresponding to the ground‐state co‐conformations (GSCCs) R1•MPTTF•4PF₆ and R2•MPTTF•4PF₆ in which CBPQT⁴⁺ (blue) encircles the MPTTF station (green) and the metastable co‐conformations (MSCCs) R1•HQ•4PF₆ and R2•XY•4PF₆ in which CBPQT⁴⁺ encircles the HQ or the XY station (red).

Figure 2

A cartoon representation illustrating the processes leading to the formation of the MSCCs R1•HQ⁴⁺ and R2•XY⁴⁺ following an oxidation/reduction cycle and their subsequent relaxation into the GSCCs R1•MPTTF⁴⁺ and R2•MPTTF⁴⁺, respectively, together with simplified free‐energy diagrams (free energy versus ring‐axle location) for the processes.

Scheme 1

Synthesis of a) the [2]rotaxane R1•4PF₆ and b) the [2]rotaxane R2•4PF₆.

Figure 3

UV/Vis/NIR absorption spectra recorded in MeCN (0.8 mM) at 298 K of a) the [2]rotaxane R1 ⁴⁺, the dumbbell D1, and CBPQT⁴⁺ and b) the [2]rotaxane R2 ⁴⁺, the dumbbell D2, and CBPQT⁴⁺.

Figure 4

Partial ¹H NMR spectra recorded in CD₃CN (1.5 mM) at 298 K of a) the [2]rotaxane R1 ⁴⁺ (500 MHz) and b) the [2]rotaxane R2 ⁴⁺ (400 MHz). The signals marked with green and red labels are in a) associated with R1•MPTTF⁴⁺ and R1•HQ⁴⁺, respectively, and in b) they are associated with R2•MPTTF⁴⁺ and R2•XY⁴⁺, respectively, while the signals marked with black labels for both spectra are associated with a mixture of the two co‐conformations.

Figure 5

a) Cartoon representation of the conversion of R1•DIPP⁶⁺ to R1•HQ⁶⁺ because of the movement of CBPQT⁴⁺ across the MPTTF²⁺ barrier, along with the obtained rate constants (k ₁ ^ox and k ₋₁ ^ox) and corresponding Gibbs Free energies of activation (ΔG ^‡ ₁ and ΔG ^‡ ₋₁) for the two processes. b) Partial ¹H NMR spectra (500 MHz, 298 K, CD₃CN, 2.0 mM) showing the increase in the signals corresponding to R1•HQ⁶⁺ and the decrease in the signals corresponding to R1•DIPP⁶⁺, recorded from 6 to 35 min after the addition of ten equiv. of TBPASbCl₆ to the equilibrium mixture of the [2]rotaxane R1 ⁴⁺. c) Plots of normalized I against t for the complete time range and the corresponding numerical solutions affording the k ₁ ^ox and k ₋₁ ^ox values using combinations of increasing and decreasing probes.

Figure 6

a) Partial ¹H NMR spectra recorded of the [2]rotaxane R1 ⁴⁺ (top, 500 MHz, 298 K, CD₃CN, 1.5 mM) containing mainly the GSCC R1•MPTTF⁴⁺ (80%) and the isolated MSCC R1•HQ⁴⁺ (bottom, 400 MHz, 298 K, CD₃CN, 1.5 mM). The signals marked with green and red are associated with R1•MPTTF⁴⁺ and R1•HQ⁴⁺, respectively. b) Pictures of the isolated solids, top: R1•4PF₆ (mainly containing the GSCC R1•MPTTF•4PF₆) and bottom: the MSCC R1•HQ•4PF₆.

Figure 7

a) A cartoon representation illustrating the conversion of the MSCC R1•HQ⁴⁺ into the GSCC R1•MPTTF⁴⁺ because of the movement of the CBPQT⁴⁺ ring across the SMe/SR steric barrier, along with the obtained rate constants (k ₁ and k ₋₁) and corresponding free energies of activation (ΔG ^‡ ₁ and ΔG ^‡ ₋₁) for the two processes. b) Photos illustrating the change in color taking place from 0 to 90 min after dissolving the isolated R1•HQ•4PF₆ in MeCN at 298 K. c) Partial ¹H NMR spectra (400 MHz, 298 K, CD₃CN, 1.5 mM) showing the increase in the signals originating from the GSCC R1•MPTTF⁴⁺ (green), and the decrease in the signals originating from the MSCC R1•HQ⁴⁺ (red), recorded from 5 to 90 min after dissolving the isolated R1•HQ•4PF₆ in CD₃CN. d) Plot of normalized I against t for the complete time range and the corresponding numerical solutions affording the k ₁ and k ₋₁ values using a combination of an increasing and a decreasing probe.

Figure 8

a) A cartoon representation illustrating the conversion of the MSCC R2•XY⁴⁺ into the GSCC R2•MPTTF⁴⁺ because of the movement of the CBPQT⁴⁺ ring across the SMe/SR steric barrier, along with the obtained rate constants (k ₁ and k ₋₁) and corresponding free energies of activation (ΔG ^‡ ₁ and ΔG ^‡ ₋₁) for the two processes. b) Partial ¹H NMR spectra (500 MHz, 298 K, CD₃CN, 4.0 mM) showing the increase in the signals originating from the GSCC R2•MPTTF⁴⁺ (green), and the decrease in the signals originating from the MSCC R2•XY⁴⁺ (red), recorded from 4 to 67 min after dissolving the isolated R2•XY•4PF₆ in CD₃CN. The range from 1.0 to 1.2 ppm is scaled down in intensity. c) Plot of normalized I against t for the complete time range and the corresponding numerical solutions affording the k ₁ and k ₋₁ values using a combination of an increasing and a decreasing probe.

Figure 9

a) Cartoon representations and b) free‐energy diagrams (free energy versus ring‐axle location) for the movement of CBPQT⁴⁺ across the MPTTF²⁺ barrier in the dioxidized [2]rotaxanes R1 ⁶⁺ and R2 ⁶⁺ (left) and for the movement of CBPQT⁴⁺ across the SMe/SR barrier in the un‐oxidized [2]rotaxanes R1 ⁴⁺ and R2 ⁴⁺ (right).