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IR Spectroscopy on Jet-Cooled Isolated Two-Station Rotaxanes Anouk M. Rijs,*,† Euan R. Kay,§ David A. Leigh,*,§ and Wybren Jan Buma*,‡ †
FOM Institute for Plasma Physics Rijnhuizen, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands § School of Chemistry, University of Edinburgh, The King’s Building, West Mains Road, Edinburgh EH9 3JJ, United Kingdom ‡
ABSTRACT: High-resolution IR spectroscopy has been employed to study isolated, switchable [2]rotaxanes. IR absorption spectra of two-station rotaxanes, their separate thread, and macrocycle components, as well as those of the individual stations incorporated into the thread, have been measured in the 18001000 cm1 region. These spectra have been fully analyzed, aided by quantum chemical predictions of the IR spectra. From these analyses, a comprehensive picture emerges of the conformational structure and binding interactions between the mechanically interlocked components of the rotaxane.
’ INTRODUCTION Molecular motors are used in nature to drive a large number of fundamental processes by converting chemical energy into directed molecular motion.1 These biomolecular motors have been the source of inspiration for the development of artificial molecular systems in which large-amplitude molecular motion is induced by applying an external stimulus.2 Examples of such molecular machinery include switches,3 rotors,4 valves,5 elevators,6 and walkers.7 One class of molecular architectures that have attracted considerable attention are hydrogen-bonded amide [2]rotaxanes.8 [2]Rotaxanes are molecular systems composed of a macrocycle that is mechanically locked onto a linear thread by bulky stoppers. In general, the thread contains one or more recognition sites (“stations”) with which the macrocycle can form hydrogen bonds and therefore prefers to reside on. Changing the relative affinities of these stations by manipulating the hydrogen bond interactions between the thread and macrocycle affects the thermodynamic equilibrium between the accessible co-conformers. The return to equilibrium then in effect translates into an overall shuttling of the macrocycle between the various stations. This controlled translocation of one component with respect to the other makes rotaxanes promising candidates for applications as molecular level devices. Solution-phase measurements have been used to characterize the co-conformational structures and have shown that the station’s affinity can be modified efficiently and reversibly by applying (electronic, photonic, chemical, etc.) stimuli, leading to translational and/or rotational motion of the macrocycle with respect to the thread.2 To fully exploit the potential of these molecules and for the design of molecules with improved functionality, a full elucidation and characterization of their intrinsic structural properties and intercomponent interactions is required. Such information can be obtained by studying the spectroscopic properties of molecular systems under isolated conditions, i.e., in the gas phase where they are not influenced by environmental perturbations. r 2011 American Chemical Society
IR spectroscopy is in this respect particularly appealing as it provides a direct view on the relevant hydrogen bond interactions and thus shows how the thread and macrocycle are connected in the interlocked structure.911 However, for [2]rotaxanes such studies are far from trivial since from a high-resolution spectroscopic point of view their size and conformational complexity form quite a challenge. Recently, we pioneered these kinds of gas-phase IR absorption experiments on prototypical single-station hydrogenbonded rotaxanes as well as their associated threads and macrocycles. IR spectra were shown to provide a spectroscopic map of the intercomponent hydrogen bond interactions and to offer a direct means to study structural and dynamical properties of rotaxanes.12 Moreover, we demonstrated that under isolated-molecule conditions it is still possible to gain specific control over intercomponent interactions by discrete solvation. By adding one solvent molecule at the time to the bare isolated molecular assembly, we induced conformational changes that uncoupled the separate components from each other.13 In the present study, we go an important step further and apply IR spectroscopy to characterize isolated switchable rotaxanes with two different recognition sites. The rotaxane studied consists of an adamantylic amide macrocycle that is mechanically locked onto a thread with two potential binding sites for the macrocycle, a succinamide, and a naphthalimide station (Chart 1). The macrocycle is held in position by intramolecular hydrogen bonds, while the bulky end groups prevent the macrocycle from slipping off the thread. Similar systems, albeit with a slightly different macrocycle, have been employed in the past to study in real time shuttling under solution conditions with Special Issue: David W. Pratt Festschrift Received: January 27, 2011 Revised: April 4, 2011 Published: April 27, 2011 9669
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Chart 1. Chemical Structure of Rotaxane 1, the Two-Station SuccinamideNaphthalimide Thread 2, the Macrocycle 3, the Mimic 4 for the Naphthalimide Station, and the Mimic 5 for the Succinamide Station
time-resolved electronic absorption.14,15 Under normal conditions, the succinamide station is known to be a better hydrogenbond acceptor than the naphthalimide station. Hence, the most stable conformation is where the macrocycle rests at the succinamide station.14 The succinamide binding site is from the same family as in the single-station rotaxanes studied previously in the gas phase.12 The naphthalimide station, on the other hand, has not been studied before under isolated-molecule conditions. This station opens up new possibilities to induce and study molecular motion of the macrocycle with respect to the thread in the gas phase. Apart from discrete solvation, the affinity of the naphthalimide station can be enhanced by reduction via, for example, electron bombardment. Here, we report on gas-phase IR spectroscopic studies of the two-station rotaxane 1 and its constituting parts, thread 2 and macrocycle 3, as well as the model compounds for the two stations, the naphthalimide compound 4 and the succinamide compound 5 (Chart 1). We will show that the studies on compounds 25, in combination with quantum chemical calculations of conformational structure and predictions of IR spectra, enable us to identify vibrational anchor points for further analysis of rotaxane 1. We will show that despite the structural and conformational complexity of rotaxane 1 it is still possible to trace all these vibrational markers and thereby acquire fundamental insights into intercomponent binding interactions.
’ EXPERIMENTAL SECTION To obtain IR spectra of the isolated [2]rotaxane and its building blocks, a pulsed molecular beam setup equipped with a laser desorption source and a time-of-flight mass spectrometer was used in combination with the free electron laser FELIX.16,17 Each sample was mixed with an equal amount of graphite powder and applied on a graphite sample bar. To provide a fresh sample every laser shot, this solid graphite sample bar (50 15 1 mm) was placed on a translation stage directly below a pulsed valve (Jordan Co.) equipped with a 0.5 mm diameter nozzle. The valve with an opening time of about 60 μs operated at 10 Hz. The fundamental output at 1064 nm (1 mJ per pulse) of a pulsed
Nd:YAG laser (Polaris II, New Wave Research) was used to desorb the sample molecules from the graphite matrix. The neutral gas phase molecules were directly cooled in a supersonic expansion of argon with a backing pressure of 3 bar. About 10 cm downstream, the neutral molecular beam was skimmed and entered the differentially pumped reflector-based time-of-flight mass spectrometer (Jordan Co.), where the isolated molecules interacted with the frequency-doubled (BBO) output of a Nd:YAG (Innolas GmbH, Spitlight 1200) pumped dye laser (Radiant Dye, Narrowscan) using Coumarin 153 and Coumarin 120. Ions created by photoionization were accelerated into the reflector time-of-flight tube and detected with a dual microchannel plate detector (Jordan Co.), yielding mass spectra with a resolution of M/ΔM of about 2000. IR absorption spectra were recorded using IRUV ion dip spectroscopy (IRIDS).17,18 In this approach, ions are produced constantly from ground-state molecules using a two-photon resonant ionization scheme. About 500 ns prior to the UV laser beam responsible for ionization, the IR laser interacts with the molecular beam. If the IR laser is resonant with a vibrational transition, population is transferred from the ground state to a vibrational level, leading to depletion of the ground state population. As a result, the number of produced ions is reduced, and a dip occurs in the ion yield. By measuring the ion yield of the mass of interest while varying the wavelength of the IR laser, a mass-selected IR ion-dip spectrum is obtained. IR absorption was studied in the frequency range from 1800 to 1000 cm1. IR laser radiation at these wavelengths was produced by the free electron laser FELIX. FELIX produces pulses with a pulse duration of about 5 μs and pulse energies of about 100 mJ. The spectral line width is typically 0.5% of the IR frequency which implies a bandwidth of 35 cm1 in the low-energy region and 9 cm1 around 1800 cm1. The IR beam was aligned perpendicular with respect to the molecular beam and was counter-propagating the UV beam. Both the molecular beam and the UV laser beam were running at 10 Hz, while FELIX was running at 5 Hz. To minimize signal fluctuations due to longtime drifts in the UV laser power or in source conditions, a normalized ion-dip spectrum was obtained by recording 9670
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Figure 2. Comparison of the IR spectrum of the naphthalimide station 4 (red) with the predicted spectrum of the naphthalimide station mimic (gray).
Figure 1. IR spectra of rotaxane 1 and its building blocks 25.
separately the alternating IR-off and IR-on signals. Additionally, the IR spectra were corrected for the intensity variations of the IR power over the complete wavelength range. Optimized structures of mimics of the two stations and of the macrocycle, as well as their predicted IR spectra, were generated using density functional theory at the B3LYP/6-31G* level. To compare predicted with experimentally obtained spectra, calculated frequencies were scaled by 0.95 and convoluted with a Gaussian line shape with a fwhm of 10 cm1.
’ RESULTS AND DISCUSSION Figure 1 reports the IR spectra of rotaxane 1 and its components 25. In the presented region (18001000 cm1), the IR spectra show a wealth of well-resolved absorption bands. Here, we focus in the first instance on the amide I (CdO stretch) and the amide II (NH bend) region between 1800 and 1400 cm1. Comparison of the frequencies and intensities of these bands in rotaxane 1 with the frequencies and intensities of the analogous bands in the separate building blocks of this rotaxane offers a direct probe for the intercomponent interactions in the rotaxane. The IR spectrum of 4, the model for the naphthalimide station, shows numerous features. To assign the various bands in this spectrum, quantum chemical calculations have been performed on a slightly simplified mimic of the naphthalimide station in which the t-butyl groups and the alkyl chain have been replaced by methyl groups. The spectrum predicted for this mimic is depicted as the gray trace in Figure 2 where it is compared with the IR spectrum of 4. Both the experimental and predicted spectra show two peaks in the CdO stretching region. Inspection of the modes shows that the bands at 1706 and 1671 cm1 in the experimental
spectrum should be assigned to the symmetric and antisymmetric CdO stretching modes, respectively. The bands at lower frequencies originate either from aromatic ring vibrations or from modes of the various alkyl groups. The peak at 1264 cm1 results from the CNC bending vibration. In general, we observe that the predicted spectrum is in good agreement with the experimental spectrum, although for some bands a much smaller intensity is predicted than actually observed in the experimental spectrum. The reason for this apparent discrepancy lies in the fact that these bands are associated with the methyl groups that have been taken to replace the t-butyl groups. A similar two-station rotaxane has been studied in solution.9 However, in that work only the 17201580 cm1 region was considered. In this region, the solution-phase spectrum of the naphthalimide station showed four peaks, two of which are associated with the CdO stretching modes and two with aromatic ring vibrations. For the symmetric and antisymmetric CdO stretching modes, the frequency is 10 cm1 lower in solution than what is observed here for the naphthalimide binding site under gas-phase conditions. The shift of 10 cm1 is small but relevant as shifts of similar magnitude occur when the carbonyl group is hydrogen-bonded. Since the CdO stretching modes are sensitive probes for the double-bond character and thus charge distributions on the carbonyl groups, we conclude that the solvent has a marked influence on these charge distributions. The same conclusion was reached in the solution study in which it was found that the IR frequencies in 4 are subject to substantial red shifts in the solvent range of THF, PrCN, ClCH2CH2Cl, CH2Cl2, and CHCl3. In that study, frequency shifts were fitted to a linear solvation energy relationship. It is highly gratifying to observe that the extrapolated gas-phase frequency for the antisymmetric CdO stretching mode from that analysis (1705.8 ( 0.5 cm1) reproduces exactly the value observed here. The IR spectrum of the succinamide model 5 depicted in Figure 3 shows two dominant bands at 1698 and 1510 cm1. These bands can be assigned to the CdO stretching and NH bending modes, respectively, using the assignment from ref 12 since the presented succinamide model has the same binding motif as the thread we have studied previously.12 The only difference is in the number of phenyl groups attached to the 9671
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Figure 3. Top panel: Comparison of the IR spectrum of the succinamide station 5 (black) with the predicted spectrum for the succinamide station mimic. The spectrum for the cis configuration of the mimic is given in red and the spectrum for the trans configuration of the mimic in blue. Bottom panel: Analysis of the amide I and II region in the IR spectrum of the succinamide station by a fit of two Gaussians to the CdO stretch band and a single Gaussian to the NH bend.
bulky end groups. It is therefore not surprising that both IR spectra are essentially identical. Both the amide I peak and the amide II peak are broader than the spectral line width determined by FELIX. The rather broad band around 1700 cm1 shows a shoulder on the low-frequency side and has been examined in further detail by fitting it to a sum of two Gaussian distributions. From this fit it becomes clear that the band is in fact a superposition of two bands with maxima at 1699.5 and 1687.8 cm1 (see Figure 3, bottom panel). DFT calculations on a mimic of the succinamide station 5 in which a methyl group has been taken to replace the bulky stopper attached to the nitrogen atom show that this station can adopt two configurations, a cis- and a trans-configuration (Figure 3). The calculations predict that the cis-configuration is 4.8 kcal/mol more stable than the trans-configuration. One would thus expect that under molecular beam conditions the cis-configuration is dominantly present as will be confirmed in the following by our experimental results. In the cis-configuration, an intramolecular hydrogen bond is formed between the CdO and the NH groups,
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Figure 4. Top panel: Comparison of the IR spectrum of the naphthalimide station 4 (red) and the succinamide station 5 (gray) with the IR spectrum of the entire thread 2 (black). Lower panel: Analysis of the CdO band of thread 2 by a fit to the sum of four Gaussian bands. The magenta Gaussian bands are associated with the naphthalimide station and the blue bands with the succinamide station.
leading to a seven-membered ring conformation. The IR spectrum predicted for this conformation (Figure 3, red trace) shows a double-peak pattern with one band at 1696 cm1 that is associated with the free CdO stretching mode and a second band at 1670 cm1 that originates from the hydrogen-bonded CdO stretch mode. In contrast, the trans-conformation (Figure 3, blue trace) only gives rise to one single band located at the same frequency as that of the free CdO stretching mode in the cis form (1699 cm1). This band is associated with the asymmetric stretching vibration of the two CdO groups. The symmetric combination of the two CdO stretching modes has a negligible intensity because of cancellation of the CdO transition dipole moments. Although the agreement between the fitted and the calculated frequencies is not perfect, the comparison between experiment and theory strongly supports the presence of two modes with different frequencies. Apart from the CdO stretching region, also the amide II and the fingerprint region show good agreement with the IR spectrum predicted for the cis 9672
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Figure 5. Comparison of the IR spectrum of the macrocycle 3 (blue) with the predicted spectrum of the macrocycle mimic (gray).
configuration. We thus conclude that under the employed experimental conditions the succinamide model 5 is indeed predominantly present in the seven-membered ring, internal hydrogen bonded, structure. The previously discussed compounds 4 and 5 are connected by an ethyl group to form thread 2 of the switchable hydrogen bonded rotaxane 1. A priori, one does not expect significant interactions between the succinamide and naphthalimide stations. If that is indeed the case, the IR spectrum of 2—and especially in the CdO stretching region—is expected to be very similar to the sum of the spectra of 4 and 5. This is indeed confirmed by the experimental results (Figure 4, top panel). The bands in the CdO region can be fitted by the sum of four Gaussian distributions (Figure 4, lower panel). These four bands represent the different CdO vibrations of both stations: at 1704 cm1 the symmetric CdO vibration of the naphthalimide station, at 1698 and 1687 cm1 the free CdO and the internally hydrogen-bonded CdO, respectively, of the succinamide station, and at 1669 cm1 the antisymmetric CdO stretching mode of the naphthalimide station. However, some bands that originate from the succinamide station differ in intensity or are completely absent. For example, the band associated with the NH bending mode at 1516 cm1 has significantly less intensity and is blue-shifted in thread 2 with respect to 5. If thread 2 would only be present in the cis configuration, this band is expected to have the same intensity in 2 and 5. On the other hand, the calculations for the trans-configuration of 5 predict that in this configuration the NH bending mode has a negligible intensity (Figure 3, top panel). The difference in intensity of the NH bending mode in 2 and 5 thus strongly suggests that in our experiments on thread 2 both the cis and trans conformations of the succinamide station are present in the molecular beam. Further support for this conclusion is found in the shifts and the reduced intensity or disappearance of several other peaks associated with 5 in the IR spectrum of 2. Macrocycle 3 is an analogue of the macrocycle that normally is employed in the Leigh hydrogen-bond-assembled [2]rotaxanes.2 From a spectroscopic point of view, it is therefore rather frustrating that this compound is for all practical purposes not soluble in common solvents. In fact, the only way to obtain the IR spectrum of macrocycle 3 is under isolated conditions as is done in the present study. This makes the employed molecular beam
Figure 6. Top panel: Comparison of the IR spectrum of thread 2 (black) and macrocycle 3 (blue) with the IR spectrum of rotaxane 1 (magenta). Lower panel: Analysis of the CdO band by a fit to the sum of six Gaussians; in magenta the bands resulting from the naphthalimide station and in blue the CdO bands from the succinamide station and macrocycle. Similarly, the NH bend band has been analyzed in terms of two Gaussian bands.
technique the ideal method to study all of the components of rotaxane 1 separately. Figure 5 depicts the IR spectrum recorded for macrocycle 3. This spectrum shows prominent bands at 1689 and 1502 cm1, which are assigned to the CdO stretch and the NH bend vibrations, respectively. Figure 5 compares the experimental spectrum of 3 with the predicted spectrum for the lowenergy conformer of the benzylic amide macrocycle.19 It is observed immediately that the computed spectrum matches the experimental spectrum very well, despite the fact that to reduce computing time a slightly different benzylic amide macrocycle was used in which phenyl groups replaced the adamantane groups. In the predicted spectrum amide I and II modes dominate the spectrum, each band containing contributions from IR-active combinations of local CdO and NH modes of the four amide groups. The experimental spectrum does not show splittings of the CdO and NH bands, indicating only minor interactions between these local modes. This is indeed confirmed by the calculations which predict nonresolvable splittings with the resolution that can be achieved in the present study. Figure 6 shows that key aspects of the IR spectrum of rotaxane 1 are fundamentally different from the spectra of the stations 4 9673
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The Journal of Physical Chemistry A and 5, thread 2, and the macrocycle 3. The detailed comparison of the IR spectra of the individual components with those of the rotaxane thus offers a unique and sensitive probe for the intercomponent interactions between the various entities that come into play when they are assembled into the mechanically interlocked rotaxane architecture. The spectrum of rotaxane 1 shows instead of multiple narrow bands two broad structures in the amide I and II region. This is not surprising since the 16001700 cm1 region, for example, has contributions from multiple modes. We notice that in the bare thread 2 we concluded that the succinamide station is present in both the cis and trans configuration. In the rotaxane, however, the succinamide station is enforced into a trans configuration as a consequence of intercomponent binding with the macrocycle. We thus anticipate in the amide I region six modes, i.e., two originating from the now overlapping aromatic vibrations and four different CdO stretching modes. Energy considerations—the succinamide station is a better hydrogen bond acceptor than the naphthalimide station—lead us to expect that under molecular beam conditions the macrocycle will reside at the succinamide station. This suggests that the frequency of the CdO modes associated with the naphthalimide station remain unaltered, while the CdO modes of both the succinamide station and the macrocycle will shift toward lower frequencies. Previous studies on prototypical single-station succinamide [2]rotaxanes have shown that upon rotaxanation the frequency shift of key vibrational modes due to hydrogen bonding is identical for all rotaxanes with this succinamide recognition site.12 The absolute frequencies, of course, depend on the finer details of the rotaxane. The CdO stretching vibration of the thread shifts by 40 cm1 upon hydrogen bonding to the macrocycle, while the NH bending mode of the macrocycle shifts by þ16 cm1. The formation of bifurcated hydrogen bonds between the carbonyl groups in the thread and the NH groups in the macrocycle also leads to a red shift of about 10 cm1 of the frequency of the non-hydrogen-bonded CdO modes in the macrocycle. This “secondary” shift can be explained by a model that treats the hydrogen bonding interaction in a donoracceptor approach.20,21 Taking these shifts into account, the broad band around 1665 cm1 has been analyzed by fitting it with the sum of six Gaussian distributions. In these fits the frequencies of both bands originating from the aromatic ring vibrations (1602 and 1628 cm1), as well as the symmetric (1705 cm1) and asymmetric (1671 cm1) CdO stretching modes of naphthalimide, were fixed since the bands associated with the naphthalimide station are expected to be unaffected by rotaxanation. The frequency of the CdO vibration of the succinamide thread, which is now hydrogen-bonded to the NH groups of the macrocycle, as well as the CdO frequency of the macrocycle were varied. Figure 6 shows the resulting fit of the six Gaussian distributions. The fit has a dominant contribution of the hydrogen-bonded CdO groups of the thread at 1655 cm1. The observed shift (43 cm1) matches perfectly with the predicted red shift of about 40 cm1. The shift of the macrocycle is slightly less than predicted by the results of ref 12. In addition, the spectrum shows also a broad peak in the amide II region associated with the NH bending mode of the hydrogen-bonded NH of the macrocycle and the free NH of the thread. The analysis of this amide II band is less straightforward since both modes lie more or less on top of each other. Nevertheless, the band could well be fitted by the sum of two Gaussians with one
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band at 1514 cm1 and another at 1517 cm1 in a 1:2 intensity ratio. In view of the fact that the macrocycle and thread contain four and two NH groups, respectively, we assign the latter band to the macrocycle. This implies that it has shifted by about 13 cm1 to the blue with respect to the situation in the free macrocycle, in agreement with the expected effects of hydrogen bonding. The former band is assigned to the NH bending vibration of the thread. Its frequency is in excellent agreement with the frequency observed in a single-station rotaxane with the same succinamide binding motif (1512 cm1).12 Recently, it has been suggested that in multistation rotaxanes like rotaxane 1 the thread might fold back in such a way that hydrogen bonds would form between the naphthalimide station and the macrocycle.22,23 In such conformations, the frequencies of the symmetric and asymmetric CdO stretching modes of the naphthalimide station would be significantly redshifted. Our observation that we do not need to invoke additional red-shifted bands when fitting the amide I band in rotaxane 1 indicates that under molecular beam conditions such folded-back conformations are not present in a significant fraction.
’ CONCLUSIONS The IR absorption spectra of a two-station rotaxane and its individual components have been measured under isolated conditions in the gas phase by IRUV ion dip spectroscopy. The IR spectra of the isolated thread stations, as well as those of the isolated thread and macrocycle, contain a large number of sharp lines in the amide I and II regions and in the fingerprint region down to 1000 cm1. These spectra have been fully analyzed with the help of quantum chemical calculations of mimics of these components. This analysis has elucidated the conformational structure of each component and has provided vibrational markers that enable one to map interactions between the various components. The spectrum of the complete rotaxane 1 shows features that are broadened compared to its isolated parts. Nevertheless, it has been shown that a careful comparison with the spectra of the separate components can still reveal detailed spectroscopic—and thereby structural—information. The present study has demonstrated that we can extend the gas-phase IR absorption technique to characterize switchable rotaxanes with two binding sites, thereby allowing us to unravel their conformational properties in the absence of interfering elements such as solvent molecules. In the case of translational motion of the macrocycle in the succinamidenaphthalimide rotaxane, hydrogen bonds of the macrocycle to the succinamide station are broken, and after translation new hydrogen bonds with the naphthalimide station are formed.15 The frequency changes of the CdO stretching and NH bending modes thus enable us to study in detail the shuttling process in the gas phase under isolated conditions and offer us exciting new ways to come to a better understanding of the functioning of molecular machinery. Experiments along these lines in which shuttling is triggered by an external stimulus are presently being performed. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected]; d.a.leigh@ ed.ac.uk. 9674
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’ ACKNOWLEDGMENT This work was carried out with financial support from the EU project Hy3M and The Netherlands Organization for Scientific Research (NWO). AMR acknowledges The Netherlands Organization for Scientific Research (NWO) for a VENI postdoctoral fellowship. We thank Dr. D. C. Jagesar for the synthesis of compound 4. We thank the FELIX-group, in particular, Dr. AFG van der Meer and Dr. B. Redlich, for their assistance with this work. ’ REFERENCES (1) (a) Molecular Motors; Schliwa, M. Ed.; Wiley-VCH: Weinheim, 2003. (b) Vale, R. D. Cell 2003, 112, 467–480. (c) Schliwa, M.; Woehlke, G. Nature 2003, 422, 759–765. (2) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72–191. (3) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001. (4) (a) Kelly, T. R.; De Silva, H.; Silva, R. A. Nature 1999, 401, 150–152. (b) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa, B. L. Nature 1999, 401, 152–155. (c) Leigh, D. A.; Wong, J. K. Y.; Dehez, F.; Zerbetto, F. Nature 2003, 424, 174–179. (d) Hernandez, J. V.; Kay, E. R.; Leigh, D. A. Science 2004, 306, 1532–1537. (e) Fletcher, S. P.; Dumur, F.; Pollard, M. M.; Feringa, B. L. Science 2005, 310, 80–82. (f) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. Rev. 2005, 105, 1281–1376. (5) Nguyen, T. D.; Tseng, H.-R.; Celestre, P. C.; Flood, A. H.; Liu, Y.; Stoddart, J. F.; Zink, J. I. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10029–10034. (6) Badjic, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F. Science 2004, 303, 1845–1849. (7) (a) von Delius, M.; Geertsema, E. M.; Leigh, D. A. Nat. Chem. 2010, 2, 96–101. (b) Otto, S. Nat. Chem. 2010, 2, 75–76. (c) von Delius, M.; Geertsema, E. M.; Leigh, D. A.; Tang, D.-T. D. J. Am. Chem. Soc. 2010, 132, 16134–16145. (d) Barrell, M. J.; Campa~na, A. G.; von Delius, M.; Geertsema, E. M.; Leigh, D. A. Angew. Chem., Int. Ed. 2011, 50, 285–290. (8) (a) Kay, E. R.; Leigh, D. A. Top. Curr. Chem. 2005, 262, 133–177. (b) Berna, J.; Bottari, G.; Leigh, D. A.; Perez, E. M. Pure Appl. Chem. 2007, 79, 39–54. (c) Johnston, A. G.; Leigh, D. A.; Murphy, A.; Smart, J. P.; Deegan, M. D. J. Am. Chem. Soc. 1996, 118, 10662–10663. (d) Leigh, D. A.; Murphy, A.; Smart, J. P.; Slawin, A. M. Z. Angew. Chem., Int. Ed. 1997, 36, 728–732. (e) Lane, A. S.; Leigh, D. A.; Murphy, A. J. Am. Chem. Soc. 1997, 119, 11092–11093. (f) Clegg, W.; GimenezSaiz, C.; Leigh, D. A.; Murphy, A.; Slawin, A. M. Z.; Teat, S. J. J. Am. Chem. Soc. 1999, 121, 4124–4129. (g) Gatti, F. G.; Leigh, D. A.; Nepogodiev, S. A.; Slawin, A. M. Z.; Teat, S. J.; Wong, J. K. Y. J. Am. Chem. Soc. 2001, 123, 5983–5989. (h) Biscarini, F.; Cavallini, M.; Leigh, D. A.; Leon, S.; Teat, S. J.; Wong, J. K. Y.; Zerbetto, F. J. Am. Chem. Soc. 2002, 124, 225–233. (i) Hannam, J. S.; Kidd, T. J.; Leigh, D. A.; Wilson, A. J. Org. Lett. 2003, 5, 1907–1910. (j) Schalley, C. A.; Reckien, W.; Peyerimhoff, S.; Baytekin, B.; V€ogtle, F. Chem.—Eur. J. 2004, 10, 4777–4789. (k) Leigh, D. A.; Venturini, A.; Wilson, A. J.; Wong, J. K. Y.; Zerbetto, F. Chem.—Eur. J. 2004, 10, 4960–4969. (l) Fradera, X.; Marquez, M.; Smith, B. D.; Orozco, M.; Luque, F. J. J. Org. Chem. 2003, 68, 4663–4673. (m) Arunkumar, E.; Forbes, C. C.; Noll, B. C.; Smith, B. D. J. Am. Chem. Soc. 2005, 127, 3288–3289. (n) Arunkumar, E.; Forbes, C. C.; Smith, B. D. Eur. J. Org. Chem. 2005, 4051–4059. (o) Onagi, H.; Rebek, J. Chem. Commun. 2005, 4604–4606. (p) Li, Y.; Li, H.; Li, Y.; Liu, H.; Wang, S.; He, X.; Wang, N.; Zhu, D. Org. Lett. 2005, 7, 4835–4838. (q) Marlin, D. S.; Gonzalez Cabrera, D.; Leigh, D. A.; Slawin, A. M. Z. Angew. Chem., Int. Ed. 2006, 45, 77–83. (r) Marlin, D. S.; Gonzalez Cabrera, D.; Leigh, D. A.; Slawin, A. M. Z. Angew. Chem., Int. Ed. 2006, 45, 1385–1390. (s) Chatterjee, M. N.; Kay, E. R.; Leigh, D. A. J. Am. Chem. Soc. 2006, 128, 4058–4073. (t) Arunkumar, E.; Fu, N.; Smith, B. D. Chem.—Eur. J. 2006, 12, 4684–4690. (u) Huang, Y. L.;
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