Letter pubs.acs.org/OrgLett
Triggering a [2]Rotaxane Molecular Shuttle by a Photochemical Bond-Cleavage Strategy Chuan Gao, Zhou-Lin Luan, Qi Zhang, Shun Yang, Si-Jia Rao, Da-Hui Qu,* and He Tian Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *
ABSTRACT: The successful triggering of ring-shuttling motion between two stations in a [2]rotaxane is demonstrated by employing a photochemical bond-cleavage strategy. A photolabile bulk barrier is covalently introduced into two identical stations of the thread to prevent dynamic shuttling of the macrocycle, resulting in a “gated” state. Irradiation of UV light (λ = 365 nm) results in the complete removal of the bulk barrier and the balanced shuttling motion of the macrocycle, indicating an “open” state of the rotaxane. In addition, the process from the “open” rotaxane to the “gated” rotaxane was executed by a chemical-rebonding method.
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enrich light-controlled rotaxane systems, as the photochemical process would occur precisely. To control the degenerate rotaxanes, many chemists have introduced a photoresponsive azobenzene unit6c or other functional units9 between two identical stations to mediate the shuttling movement of rotaxanes. Herein, we report the successful triggering of a subunit shuttling motion in a degenerate [2]rotaxane via a photochemical bond-cleavage strategy (Figure 1a). In this design, a photolabile bulk barrier is introduced and covalently bonded into and between two recognition sites of the thread to prevent dynamic shuttling of the macrocycle between two identical stations, resulting in a “gated” state that has absolute distribution of the macrocycle on one of the two stations. Upon irradiation of UV light (λ = 365 nm), complete photolysis occurs in a controllable manner, resulting in the removal of the bulk barrier and the subsequent recovery of balanced shuttling motion of the macrocycle, which is referred to as an “open” state that has the same distribution of the macrocycle in the two stations with a fast shuttling motion. The inverse process from the “open” state to “gated” state can be realized via a chemical rebonding of the corresponding phototrigger into the rotaxane. As shown in Figure 1b, the methyl-6-nitroveratryloxycarbonyl group, known as a typical phototrigger that responds to UV light,8b,d,e,10 was selected as the bulky photolabile group. A wellknown Leigh-type11a,d−f hydrogen-bonding-assembled [2]rotaxane was chosen to investigate the phototriggered shuttling motion. The dumbbell-shaped thread 4, which contains two succinamide recognition sites, was treated with phototrigger 5 to yield compound 3, and the subsequent five-component clipping reaction involving p-xylylenediamine, isophthaloyl
ynthetic chemists have created and developed various artificial molecular machines,1 such as rotaxanes,1i,k catenanes, 1h,c and motors, 1j anticipated to mimic the machine-like behaviors in biological systems.2 Rotaxane, wellknown for its unique shuttling motion in interlocked nanoscale systems, has proved to be versatile as a molecular switch and molecular machine.2,3 These functional applications strongly rely on the responsiveness of the stimuli of rotaxane systems.4 The reliable external stimuli modes include pH,4a,b redox,2a,4c light, 2b,4d−f chemical input, 4g and microenvironmental changes4h in which light has been considered the most promising mode to trigger rotaxane-based molecular machines and shuttles.1g,5 In most cases, light-controlled rotaxane molecular shuttles have been incorporated into the backbones of azobenzene, stilbene, or fumaramide moieties.6 The trans/cis or E/Z isomerizations of the above-mentioned photoresponsive functional units can induce macrocyclic ring shuttling between two distinguishable recognition sites in effective modes. Recently, other strategies for light-controlled rotaxanes have been reported,4d,e,7 such as the photoacid strategy7a and the photosensitizer strategy.4d,e,7b These intelligent methodologies provide important potential driving modes to trigger a molecular shuttling motion in bistable or multistable rotaxanes. However, it is still desirable to develop new, reliable, and efficient light-controlled modes for the development of rotaxane-based molecular shuttles. A phototrigger, also known as a type of photolabile or photoremovable group, represents an inherently irreversible photoresponsive mode.8 Although it has been widely developed in biomaterials and photoresponsive supramolecular polymerization,8c−e little effort has been made for their applications in the control of motion in rotaxane molecular shuttle systems. Indeed, the inherent irreversibility of a phototrigger increases the difficulty in constructing a switchable rotaxane system; however, on the other hand, we foresee that the successful introduction of a phototrigger would provide a novel strategy to © XXXX American Chemical Society
Received: February 13, 2017
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DOI: 10.1021/acs.orglett.7b00393 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 2. 1H NMR spectra (400 MHz, DMSO-d6, and 298 K) of (a) dumbbell-shaped thread 3 and (b) [2]rotaxane 1. See Figure 1b for proton assignments.
due to the introduction of the macrocycle. Meanwhile, similar shielding signals from protons H6−8 and H12 to protons H6′−8′ and H12′ are shown in Figure 2b, while the 0.11 ppm downfield shift of H13′, the protons of the phenyl stoppers, was attributed to the deshielding effects of the aromatic rings of the macrocycle. In principle, rotaxane 1 exists as four stereoisomers, as it contains two centers of point chirality: one (C1) arising from the presence of the mechanically interlocked ring and the other (CA) due to four different covalently linked substituents. However, the signals of the stereoisomers were not observed according to 1H NMR (Figure 2b), which is probably because the macrocycle is located far away from the ester moiety, making the mechanical point chirality poorly exhibited. Therefore, all these observed results verified the interlocked structure of the target [2]rotaxane 1, as shown in Figure 1. The photocleavage reaction of [2]rotaxane 1 was detected by 1 H NMR spectroscopy, as shown in Figure 3. A hand-held UV
Figure 1. (a) Schematic representation of the two modes of the [2]rotaxane before and after irradiation with UV light; (b) synthetic routes of the target [2]rotaxane 1, the dumbbell-shaped thread compound 3, and 4, the photolysis reaction8f of [2]rotaxane 1 to yield degenerate [2]rotaxane 2, and the chemical rebonding reaction from [2]rotaxane 2 to [2]rotaxane 1.
chloride, and thread 3 in the presence of triethylamine11 resulted in the formation of [2]rotaxane 1. The interlocked structure of the target [2]rotaxane 1 was confirmed with 1H NMR and 13C NMR spectroscopies and high-resolution electrospray ionization (HR-ESI) mass spectrometry. To confirm the interlocked structure of [2]rotaxane 1, we compared the 1H NMR spectra of dumbbell-shaped thread 3 and [2]rotaxane 1, as shown in Figure 2a,b. The protons of [2]rotaxane 1 (Figure 1b) were properly assigned, and the chemical shifts at δ = 8.02, 7.62, 6.94, 8.31, and 4.17 ppm are attributed to the protons of the macrocycle (Ha−g). Significantly, signals of the methylene protons (H9, H10) on the succinamide station of dumbbell-shaped thread 3 were observed as a single signal in Figure 2a because of the symmetrical structure of thread 3. However, in Figure 2b, part of these protons (H9′, H10′) were shielded (Δδ = −1.22 ppm)
Figure 3. 1H NMR spectra (400 MHz, DMSO-d6, 298 K, and 1.24 × 10−2 M) of the photocleavage process of [2]rotaxane 1 to yield an “open” [2]rotaxane 2.
lamp was used as the source of 365 nm UV light with an intensity of 8W/cm2. The DMSO-d6 solution of [2]rotaxane 1 was sealed in a quartz NMR tube and irradiated continuously by the UV light, which was monitored by 1H NMR spectroscopy every 10 min. Obviously, as shown in Figure 3, the continuous irradiation of UV light led to the disappearance B
DOI: 10.1021/acs.orglett.7b00393 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 3 (spectrum of the photochemical mixture). Obviously, kinetic changes of the proton signals (H8−10 and H12) can be observed in Figure 3, which is completely consistent with the observation in Figure 4, indicating the successful realization of photocontrolled activation of rotaxane 1 via a phototriggered strategy; however, this strategy could result in the formation of byproducts. Interestingly, the observed shuttling motion of [2]rotaxane 2 in DMSO is not consistent with the results reported by Leigh11f in which the macrocycle was localized over the alkyl linker because of a solvophobic effect in DMSO. In this case, the linker between the two recognition sites of [2]rotaxane 2 was composed of a hydroxyl-substituted chain and benzene rings. The shuttling motion between the two recognition sites of [2]rotaxane 2 could be observed in DMSO instead of locating in the middle of the linker, which is probably due to the less hydrophobic hydroxyl-substituted chain and the steric effect of the benzene rings. For further evidence and investigation of the shuttling motion of [2]rotaxane 2, variable-temperature 1H NMR measurements were conducted in DMSO-d6 to elucidate the kinetic parameters (Figure S2). Both the shuttling rate (ks) and the free energy of activation (ΔG⧧) are similar to those in the previous report (Figure S3).11f Finally, the inverse process, converting from the “open” rotaxane to the “gated” rotaxane, was implemented by a chemical rebonding method (Figure 1b and Figure S4), indicating that the whole process can be reversibly driven. Although a chemical “re-stoppering” can be performed, the process is not fully efficient (79% yield in Figure 1b), and it may be difficult to perform a second switching cycle in situ, which limits the reversibility of the approach. In conclusion, a novel strategy for the construction of a photocontrolled molecular shuttling rotaxane was successfully proposed and demonstrated via the introduction of a phototrigger as a removable bulk barrier. Shuttling motion in a typical [2]rotaxane was activated by irradiation with UV light, and the inverse process was driven by an efficient and facile chemical-rebonding method. We envision that this novel phototriggered strategy could serve as an efficient method for the construction of photocontrolled rotaxanes and enrich the driving forces for the operation modes of molecular machines, thus advancing the study of artificial molecular machines.
of the signals (δ = 6.14 ppm) of the protons HA and the signals (δ = 1.62 ppm) of the protons HB as well as the appearance and gradual intensity enhancement of the signals (δ = 2.78 ppm) of the protons HB′ on the photocleavage reaction byproducts and the signals (δ = 5.38 ppm) of the protons Hx on the hydroxyl of [2]rotaxane 2. Slight changes observed in the signals of HE and HF are also related to the minor change of the corresponding chemical environment. Significantly, the signals (δ = 5.19 ppm) of the protons H1 on the thread moiety of [2]rotaxane 1 disappeared completely, and the simultaneous appearance of a new signal (δ = 4.07 ppm) can be explained as a change in bonding from the carbonic ester in [2]rotaxane 1 to the hydroxyl in [2]rotaxane 2. These observations directly show the efficient and complete photocleavage reaction of the methyl-6nitroveratryloxycarbonyl group, leaving a hydroxyl group in the obtained [2]rotaxane 2, in an expected photocontrolled mode, as shown in Figure 1b. Meanwhile, the rate constant k = 0.0686 min−1 for the photocleavage process, which was calculated according to the chemical kinetic eq (Figure S1). To further confirm the final product of the photocleavage process, we purified the “open” [2]rotaxane 2 for further investigation. The 1H NMR spectra of [2]rotaxane 1, 2, and dumbbell-shaped thread 4 are shown in Figure 4. Significantly,
Figure 4. 1H NMR spectra (400 MHz, DMSO-d6, and 298 K) of (a) [2]rotaxane 1, (b) [2]rotaxane 2, and (c) dumbbell-shaped thread 4.
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as shown in Figure 4a,b, the key signals at 2.48, 2.39, 1.34, and 1.18 ppm, i.e., the chemical shifts of H9, H9′, H10, and H10′ in [2]rotaxane 1, respectively, merged into two single peaks (H9, δ = 1.96 ppm; H10, δ = 1.79 ppm) in [2]rotaxane 2, suggesting the two succinamide stations are in identical chemical environments in [2]rotaxane 2. Meanwhile, compared with the signals of the protons H9 and H10 on reference thread 4 without an interlocked macrocycle, a shielding effect still existed (Δδ = −0.59 ppm), proving the macrocycle still included the two succinamide stations in [2]rotaxane 2. Therefore, it can be concluded that the macrocycle included two identical succinamide stations in [2]rotaxane 2 and that the macrocycle was characterized by fast shuttling between the two stations along the thread in [2]rotaxane 2. The corresponding change of the observed deshielding effect of proton H11 and the merger of the peaks of protons H5−7 and H12 also prove the active shuttling. To consider whether a similar shuttling motion of [2]rotaxane 2 occurs in a photocontrolled mode and whether this mode has a mixed nature, further comparison is needed between Figure 4b (spectrum of the pure sample) and
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00393. Details of experimental procedures and characterization data, investigation of shuttling dynamics, and calculation of the rate constant for photocleavage process (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Da-Hui Qu: 0000-0002-2039-3564 He Tian: 0000-0003-3547-7485 Notes
The authors declare no competing financial interest. C
DOI: 10.1021/acs.orglett.7b00393 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21672060, 21421004), the Fundamental Research Funds for the Central Universities, and the Programme of Introducing Talents of Discipline to Universities (B16017).
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DOI: 10.1021/acs.orglett.7b00393 Org. Lett. XXXX, XXX, XXX−XXX
