Abstract
In donor-acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers--present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor-acceptor vintage--can be elucidated by slow scan rate cyclic voltammetry. The practice of transitioning from a fast scan rate regime to a slow one permits the measurement of an intermediate redox couple that is a function of the equilibrium that exists between the two translational isomers in the case of all three mechanically interlocked molecules investigated. These intermediate redox potentials can be used to calculate the ground-state distribution constants, K. Whereas, (i) in the case of the bistable [2]rotaxane, composed of a dumbbell component containing π-electron-rich tetrathiafulvalene and dioxynaphthalene recognition sites for the ring component (namely, a tetracationic cyclophane, containing two π-electron-deficient bipyridinium units), a value for K of 10 ± 2 is calculated, (ii) in the case of the two bistable [2]catenanes--one containing a crown ether with tetrathiafulvalene and dioxynaphthalene recognition sites for the tetracationic cyclophane, and the other, tetrathiafulvalene and butadiyne recognition sites--the values for K are orders (one and three, respectively) of magnitude greater. This observation, which has also been probed by theoretical calculations, supports the hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor-acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether.
Highlights
In donor–acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers— present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor–acceptor vintage—can be elucidated by slow scan rate cyclic voltammetry
In bistable mechanically interlocked molecules (MIMs) involving TTF and another π-electron-rich recognition site, two co-conformations can be defined as one (i) where the CBPQT4þ ring resides around the TTF unit and the other (ii) where the CBPQT4þ ring resides around the other π-electron-rich unit
We focus on the results obtained for the bistable [2] rotaxane R4þ (Fig. 3A). It has been reported (33, 34, 38) that the ground-state distribution for [2]rotaxanes of this type, which contain DNP and TTF stations in its dumbbell component and are encircled by the CBPQT4þ ring, exhibit a 9∶1 distribution (K ≈ 10) favoring the encirclement of the TTF unit. This determination is made possible as a consequence of the fact that at relatively fast scan rates a modest oxidation process is observed, generally around þ400 mV, corresponding to the oxidation of the metastable-state co-conformation (MSCC) to generate the TTF radical cation, and in a 1∶9 proportion with respect to the oxidation observed for the ground-state co-conformation (GSCC) generally observed around þ800 mV
Summary
In donor–acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers— present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor–acceptor vintage—can be elucidated by slow scan rate cyclic voltammetry. It has been reported (33, 34, 38) that the ground-state distribution for [2]rotaxanes of this type, which contain DNP and TTF stations in its dumbbell component and are encircled by the CBPQT4þ ring, exhibit a 9∶1 distribution (K ≈ 10) favoring the encirclement of the TTF unit This determination is made possible as a consequence of the fact that at relatively fast scan rates a modest oxidation process is observed, generally around þ400 mV, corresponding to the oxidation of the MSCC to generate the TTF radical cation, and in a 1∶9 proportion with respect to the oxidation observed for the GSCC generally observed around þ800 mV. Scanning at slower and slower scan rates, a new oxidation peak becomes increasingly apparent, until, upon using a scan rate of 10 mV s−1, a new reversible redox process is observed with a redox potential of þ0.49 V
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