Translational isomers

More recently, the second-generation molecular shuttle 374+ (Fig. 13.32) was designed and constructed.38 The system is composed of two devices a bistable redox-driven molecular shuttle and a module for photoinduced charge separation. In the stable translational isomer, the electron-accepting cyclophane 124+, which is confined in the region of the dumbbell delimited by the two stoppers Tj and T2, encircles the better electron donor tetrathiafulvalene (TTF) station. 

Figure 17. The degenerate forms A and B of the [2]catenanes 32-4PF6-38-4PF6 and the translational isomers A and B of the [2]catenanes 394PF6-48-4PF6.

A polyrotaxane 29 possesses two electron-donating sites (TTF and hydro-quinone moieties) as stations in the polymer backbone, hence, the incorporated cyclic acceptor 28 moves by external stimuli and possibly two translational isomers (29a and b) would exist (Scheme 10) [106, 107]. The ratio between two isomers was reported to be very solvent dependent (Table 2), with a preference however for the hydroquinone moiety. In the CV measurement, it was also observed that the cyclic acceptor 28 moved from TTF to hydroquinone moiety along the chain of 29 upon oxidation of the TTF unit. 

The intramolecular mechanism, illustrated on the left-hand side of Figure 6.8, is based on four separate operations [52]. (a) Destabilization of the stable translational isomer light excitation of the photoactive unit P (step 1) is followed by the transfer of an electron from the excited state to the Al station, which is encircled by the macrocycle (step 2) with the consequent deactivation of this station such a photoinduced electron-transfer process has to compete with the intrinsic decay of P (step 3). (b) Ring displacement the ring moves from the reduced station Ah to A2 (step 4), a step that has to compete with the back electron-transfer process from Ah (still encircled by the macrocycle) to the oxidized photoactive unit P+ (step 5). This is the most difficult requirement to meet in the intramolecular mechanism, (c) Electronic reset a back electron-transfer process from the free reduced station Ah to P+ (step 6) restores the electron-acceptor power to the Al station, (d) Nuclear reset as a consequence of the electronic reset, back movement of the ring from A2 to Al takes place (step 7). 

Scheme 9 Chemically and electrochemically driven translational isomer switching of [2] catenane 17 [56]...

In a similar manner donor-acceptor [2]rotaxanes such as the derivative 213 can be assembled. This example has been described as a molecular switch. The CBPQT4+ ring can occupy two positions as shown in the translational isomers 213 and 214 (Scheme 18). At equilibrium in acetonitrile solution the CBPQT4+ ring mainly occupies the benzidine site (84%) 213. Protonation (or electrochemical oxidation) eliminates the favorable CBPQT4+/benzidine donor-acceptor interaction and results in preferential occupation of the biphenol site 214 <2008T8231>. 

The stable translational isomer is the one in which the ring component encircles the EAi2+ unit, as expected because this station is a better electron acceptor than the other. The photoinduced ring shuttling between the two station occurs with an intramolecular working mechanism (Fig. 16) based on the following four operations [76] ... 

Destabilization of the stable translational isomer. Light excitation of the photoactive unit P2+ is followed by transfer of an electron from the excited state to the EA22+ station, which is encircled by the ring (step 1), with the consequent deactivation of this station such a photoinduced electron-transfer process must compete with the intrinsic decay of the excited state of P2+. 

The [2]catenanes 185 and 186 incorporated different 7t-electron-rich macrocyclic components. As a result, their H NMR spectra showed the existence of two translational isomers in solution as shown in Scheme 28. The ratios between the two translational isomers A and B of [2]catenanes 185 (by 181 -[cyclobis(para( uat-/)-xylylene)][PF,3]4) and 186 (by 182-[cyclobis(paraquat-p-xylylene)][PF6]4) are 60 40 and 70 30 at — 30°C, respectively. Increasing the temperature up to +30 °C resulted in an increase of the population of the translational isomers B in 185 and 186 to 55 45 and 30 70, respectively. A temperature dependence of the equilibrium between the translational isomers associated with 185 and 186 was observed. These [2]catenanes can be regarded as temperature-responsive molecular switches. 

Scheme 28 Translational isomers associated with the [2]catenanes 185 and 186.

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