Molecular motors: new designs and applications
Roke, Gerrit Dirk
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date: 2018
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Roke, G. D. (2018). Molecular motors: new designs and applications. Rijksuniversiteit Groningen.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
foldamers
With the goal of forming photoswitchable foldamers, first generation molecular motors are copolymerized with fluorene and m-phenylene ethylene linkers. Initially, model compounds consisting of a single motor with linkers on both halves are synthesized, showing that appending these linkers does not impede the photoswitching. Unfortunately, the copolymer of motors with fluorene were showing very little photoswitching, but show fluorescence instead. The Sonogashira polymerization to form polymers with m-phenylene ethylene linkers was unsuccessful, as no polymeric material was obtained.
106
6.1
Introduction
Foldamers are oligomers or polymers that are able to adopt a specific compact conformation in solution.[1–6] Inspired by Nature’s broad collection of structurally well-defined macromolecules, the folding of synthetic analogues such as peptoids,[7] -peptides[8,9] and polyamide nucleic acids[10] were initially studied. Soon thereafter, the scope was expanded to aromatic foldamers, such as oligo(meta-phenylene ethylene)s (OmPEs)[11,12] and polyamides.[13] Although many conformations can be adopted, helices are by far the most studied. As such, foldamers have shown promising applications in various fields, such as molecular recognition,[14,15] biomedicine[16–18] and catalysis.[19,20] Incorporation of photoswitches in foldamers offers the possibility to dynamically control folding using light.[21,22] Azobenzenes are mainly used, as the photochemical E-Z isomerization results in a large geometry change. They have been attached as side chains,[23] tethers[22] or incorporated in the backbone.[24] In most cases they have been introduced in aromatic foldamers, such as OmPEs,[25,26] polyamides[27] and aryl-triazole foldamers.[28,29] Studies on OmPEs showed that the photoswitching is cooperative, in which the terminal azobenzene moieties isomerize first, leading to unfolding at the termini.[30] Subsequent isomerization of the internal azobenzenes is then facilitated by unfolding of the helix. This unique feature of foldamers allows for the amplification of the photoswitching event.
The incorporation of overcrowded alkene-based molecular motors as chiroptical switches in foldamers would be highly beneficial, as they possess helical chirality, which can be transferred to the helix of the polymer,[31,32] allowing control over the folding and the helical chirality (Figure 6.1). Second generation molecular motors have been incorporated in polymers by copolymerizing them with xanthone or fluorene with the aim to perform light-driven movement.[33] The motor function was retained in the polymers, but no significant movement was observed. The motors were polymerized at the lower halves, and therefore isomerization does not lead to a large geometry change of the polymer. In our current design, first generation molecular motors[34,35] were chosen to achieve a large geometric change upon photoisomerization (Figure 6.1). Isomerization of the trans isomer to the unstable cis isomer upon irradiation at 312 nm should convert the linear polymer chain to a helix. Unfolding of the helix could be achieved by irradiation at 365 nm to isomerize the unstable cis to the stable trans.
107
Figure 6.1. A light switchable foldamer employing intrinsic overcrowded alkene-based molecular motors.
6.2
Synthesis of linkers and model compounds
Two designs of polymers were considered (Figure 6.2), both bearing a first generation molecular motor in their backbone. In polymer P1 the motor is copolymerized with dioctylfluorene. The linkers in polymer P2 are based on m-phenylene ethynylene (mPE), which have been widely studied as foldamers.[11,12] To investigate the effect of appending these linkers to molecular motors on their switching behavior, model compounds 1 and 2 were first synthesized, bearing a linker at both the upper and lower half (Scheme 6.1).
Figure 6.2: Designs for molecular motor containing polymers
For model compound 1, commercially available fluorene 3 could be converted to the corresponding boronic ester via halogen-lithium exchange and quenching with tributyl borate. Boronic ester 4 was then in situ coupled to motor 5 in a Suzuki cross coupling. Even though motor 5 was used as a mixture of cis and trans, the resulting product was
108
isolated exclusively as the trans isomer as determined by 2D NOESY NMR (Figure 6.3). A correlation was observed between Ha and Hd, as well as between Hb and Hd and only in the
trans isomer these protons are in close proximity to each other. Alongside di-substituted
motor 1, mono-substituted product was isolated after column chromatography, as a mixture of cis and trans. Most likely, the cis isomer is too hindered to undergo a second coupling. The mPE linker was synthesized starting from 3,5-dibromobenzoic acid, which was first esterified using hexanol according to a literature procedure.[36] The alkyne linkers were installed by performing a double Sonogashira coupling with 2-methylbut-3-yn-2-ol, providing 8 in high yield. The acetonide protecting groups can then be removed using NaH, giving a mixture of 9 and 10. Linker 10 could be used for the synthesis of model compound 2, which is synthesized using a double Sonogashira cross-coupling. Essential in this reaction is the use of iodide substituted motor 11,[37] as the bromide substituted motor proved to be unreactive in this cross-coupling. Model compound 2 was, as in the case of 1, isolated exclusively as the trans isomer.
109
Figure 6.3. Partial ROESY spectrum of model compound 1.
6.3
Switching studies of model compounds
The UV/vis spectra of model compounds 1 and 2 show a strong absorption band in the UV region, similar to the unsubstituted parent motor (Figure 6.4).[38] Upon irradiation with max = 312 nm light in both cases a clear bathochromic shift is seen, characteristic for the
formation of the unstable cis state. When these samples are irradiated at 385 nm, the motor isomerizes back to the stable trans isomer and the UV/vis spectra almost completely return to the original state. Both model compounds show clear isosbestic points, indicative of a unimolecular process (Figure 6.5 and Figure 6.6).
110
Figure 6.4: UV/vis spectra of model compounds 1 (left, CH2Cl2, c = 8.1 x 10-6M) and 2 (right,
DMSO, c = 9.7 x 10-6M)
Figure 6.5. UV/vis spectra of model compound 1 (CH2Cl2, c = 8.1 x 10-6 M) upon irradiation
111
Figure 6.6. UV/vis spectra of model compound 2 (DMSO, c = 9.7 x 10-6 M) upon irradiation at 312 nm (left) and 385 nm (right) with isosbestic points (inset).
To confirm that the switching behavior of the motor is retained in these model compounds, its isomerization behavior was followed using 1H-NMR (Figure 6.7). An NMR sample containing motor 1 in THF-d8 was irradiated at 312 nm, showing the disappearance of the stable trans isomer and the emergence of a new isomer: unstable cis (Figure 6.7ii). The most clear changes are seen for Ha, Hb and Hc. The sample was irradiated until no
further changes were observed and at this photostationary state (PSS) the ratio of unstable cis to stable trans was 79:21. When this sample is irradiated at 385 nm, the stable trans isomer is again obtained as is seen in Figure 6.7iii. At the PSS, the ratio of stable trans to unstable cis is 93:7. In the same manner, the isomerization behavior of model compound 2 was followed (Figure 6.8). Upon irradiation at 312 nm clear changes are seen for the aromatic protons on the linker, Hb, Hc and Hd (Figure 6.8ii). A PSS of
unstable cis to stable trans of 85:15 is obtained. Irradiation at 385 nm isomerizes this photogenerated isomer back to stable trans as the original spectrum is reobtained (Figure 6.8iii). Combined, these results show that appending either mPE or fluorene linkers does not impede the switching behavior of this first generation molecular motor, making them promising candidates to be used as switchable polymers.
112
Figure 6.7. Selected regions of 1H-NMR spectrum of model compound 1 in THF-d8 (c = x 10-4
M). i) start ii) PSS 312 nm iii) PSS 385 nm.
Figure 6.8: Aromatic region of 1H-NMR spectrum of model compound 2 in DMSO-d6 (c = 8.5
113 polydispersity (1.3) and a number average molecular weight of 7.5 kDa, which corresponds to 11 units per chain (Table 6.1). End-group analysis with 1H-NMR showed an average of 46 units per chain. The large difference between the GPC and 1H-NMR could suggest that not all polymers are end-capped. Unfortunately, attempts to polymerize (S,S)-cis-5 failed. Most likely, the mono-substituted motor is too hindered to undergo a second cross-coupling, in analogy to model compounds 1 and 2 (vide infra).
Scheme 6.2. Polymerizations towards polymers P1 and P2.
M
n7.5 kDa
M
w9.7 kDa
PI
1.3
n
11 units/chain
n (
1H-NMR)
46 units/chain
Table 6.1. Analysis of polymer P1
For polymer P2, a Sonogashira polymerization was envisioned, using racemic iodo-substituted motor trans 11 and and dialkyne 9. Unfortunately, different polymerization conditions gave a complex mixture and did not yield any high weight polymeric material
114
upon precipitation with MeOH. As the presence of trace amounts of oxygen could cause significant homocoupling of linker 9, the polymerization was carried out under strict oxygen-free conditions using an argon atmosphere, but this did not lead to a significant improvement.
The UV/vis spectrum of P1 in THF is slightly red-shifted compared to model compound 1, most likely due to extended conjugation (Figure 6.9). Upon irradiation at 312 nm only small changes are seen in the UV/vis spectrum, suggesting that only very little photoswitching occurs. The same is seen in the CD spectrum, which shows a small increase in the CD signal (Figure 6.9). On the other hand, fluorescence with a maximum at em = 399 nm was observed when P1 was excited at 312 nm, even in the nanomolar range
(Figure 6.10). It was shown before that molecular motors incorporated in gels show increased fluorescence[41] and that the introduction of rigid arms leads to increased fluorescence.[42] Interestingly, negligible fluorescence was observed in polymers containing second generation molecular motors, in which photoisomerizaiton does not lead to a geometry change in the polymer chain.[33] Incorporation of a motor in a rigid polymer might hamper the photoswitching as the whole chain needs to be reoriented in order to achieve the E-Z isomerization. Instead, fluorescence is observed, similar to the blue fluorescence of polyfluorenes.[43] The small degree of isomerization might be originating from motors in polymers in the low molecular weight fraction, or from the chain ends.
Figure 6.9. UV/vis (left) and CD (right) spectra of P1 in THF (c = 1.5 x 10-6 M) upon irradiation at 312 nm.
115
Figure 6.10. Fluorescence spectrum of P1 in THF (c = 4.5 x 10-9 M) with ex = 312 nm.
6.5
Conclusions
In summary, a polymer containing first generation molecular motors was synthesized using a Suzuki polymerization. Although UV/vis and NMR studies on model compounds showed promising results, photoswitching of the motors embedded in the polymer main chains was very poor. Instead, fluorescence is observed. Possibly, photoswitching is inhibited as the whole polymer chain needs to be reoriented upon E-Z isomerization of the motor unit. To confirm this, low molecular weight oligomers might be synthesized, in which photoswitching should be easier as fewer units have to be rearranged to facilitate isomerization.
6.6
Experimental procedures
For general remarks regarding experimental procedures see Chapter 2.
2-Bromo-9,9-dihexylfluorene (3) was bought from Sigma-Aldrich. Ester 7 was synthesized according to literature procedures.[36] Racemic motors 5[44] and 11[37] were synthesized according to literature procedures. Enantiopure motor 5 was synthesized from enantiopure ketone 12, obtained via an enantioselective protonation following literature procedures (Figure 6.11).[35] Cis and trans isomers were subsequently separated by layered crystallization with CH2Cl2/MeOH.
116
Figure 6.11. Synthesis of enantiopure motor 5.
(E)-2,2'-(2,2',4,4',7,7'-Hexamethyl-2,2',3,3'-tetrahydro-[1,1'-biindenylidene]-6,6'-diyl)bis(9,9-dihexyl-9H-fluorene) (1)
2-Bromo-9,9-dihexylfluorene (3) (109 mg, 0.263 mmol) was dissolved in dry THF (3 mL) under N2 atmosphere and the solution was cooled to -78 °C. n-BuLi (2.5 M in hexanes, 0.12
mL, 0.29 mmol) was added dropwise and the resulting solution was stirred for 30 min. B(OBu)3 (78 L, 0.29 mmol) was added and the mixture was allowed to warm to room
temperature and stirred for an additional 30 min. Dioxane (3 mL) and 1M aq. K2CO3 (5 mL)
were added the mixture was purged with N2 for 30 min. PdCl2dppf (4.3 mg, 0.0053 mmol)
and motor 5 (50 mg, 0.11 mmol) were added the resulting mixture was stirred at 95 °C for 2 d. After cooling to rt., water was added and the mixture was extracted three times with CH2Cl2. The combined organic layers were washed with water and brine, dried over MgSO4
and the volatiles were removed in vacuo. The residue was purified by flash column chromatography (SiO2, pentane/CH2Cl2 0-10%) to yield 1 as a colorless solid. 1 H NMR (400 MHz, CDCl3) δ 7.76 (dd, J = 10.0, 7.3 Hz, 4H), 7.42 – 7.28 (m, 10H), 7.07 (s, 2H), 3.12 (p, J = 6.3 Hz, 2H), 2.84 (dd, J = 14.5, 5.6 Hz, 2H), 2.38 (d, J = 10.8 Hz, 2H), 2.34 (s, , 6H), 2.30 (s, 6H), 2.11 – 1.92 (m, 8H), 1.23 – 0.97 (m, 24H), 0.85 – 0.60 (m, 20H). 13C NMR (101 MHz, CDCl3) δ 151.1, 150.6, 142.4, 142.3, 141.9, 141.8, 141.6, 141.2, 139.7, 131.3, 129.7, 129.1, 128.3, 127.0, 126.9, 124.4, 123.0, 119.7, 119.4, 55.2, 42.7, 40.6, 40.6, 39.2,
117 Ester 7 (5.95 g, 16.3 mmol), PdCl2(PPh3)2 (458 mg, 0.652 mmol) and CuI (124 mg, 0.652
mmol) were dissolved in THF/Et3N 3:1 (160 mL) and the mixture was degassed by purging
with N2for 30 min. 2-methylbut-3-yn-2-ol (4.7 mL, 49 mmol) was added and the resulting
mixture was refluxed o.n. After cooling to rt. the volatiles were removed in vacuo, H2O
was added to the residue and extracted three times with CH2Cl2. The combined organic
layers were washed 2x with 1M aq. HCl, 2x with H2O and brine. The combined organic
layers were dried over MgSO4 and the volatiles were removed in vacuo. The residue was
purified by flash column chromatography (SiO2, pentane/EtOAc 10-40%) to yield 8 (5.81 g,
96%) as an off-yellow oil. 1H NMR (400 MHz, CDCl3) δ 7.98 (d, J = 1.6 Hz, 2H), 7.61 (t, J =
1.6 Hz, 1H), 4.31 (t, J = 6.8 Hz, 2H), 2.08 (bs, 2H) 1.76 (dq, J = 7.9, 6.8 Hz, 2H), 1.61 (s, 12H), 1.48 – 1.29 (m, 6H), 0.95 – 0.86 (m, 3H). 13C NMR (101 MHz, CDCl3) δ 168.0, 141.0, 134.8,
133.6, 126.1, 97.9, 83.1, 68.3, 68.2, 34.1, 34.0, 31.3, 28.3, 25.2, 16.7. HRMS (ESI+, m/z): Calcd for C23H29O3 [M-OH]+: 353.21112, found: 353.21152. Calcd for C23H30O4Na [M+Na]+:
393.20363, found 393.20371.
Hexyl 3-ethynyl-5-(3-hydroxy-3-methylbut-1-yn-1-yl)benzoate (9)
NaH (130 mg, 3.24 mmol, 60% dispersion in mineral oil) was dissolved in dry THF (10 mL) under N2 atmosphere. Ester 8 (1.00 g, 2.70 mmol) was added in THF (2 mL) and the
mixture was heated to 50 °C for 2 h. After cooling to rt., 1 M aq. HCl was added carefully and the layers were separated. The aqueous layer was extracted twice with EtOAc and the combined organic layers were dried over MgSO4. The volatiles were removed in vacuo and
the residue was purified by column chromatography to yield alkyne 9 (30%, ) as a red oil.
1
118
Hz, 1H), 4.31 (t, J = 6.7 Hz, 2H), 3.12 (s, 1H), 1.81 – 1.71 (m, 2H), 1.62 (s, 6H), 1.47 – 1.38 (m, 2H), 1.40 – 1.29 (m, 4H), 0.94 – 0.87 (m, 3H). 13C NMR (101 MHz, CDCl3) δ 165.1, 144.0,
138.8, 132.7, 132.6, 131.1, 123.6, 122.8, 95.4, 81.8, 80.3, 78.6, 65.7, 65.5, 31.4, 31.3, 28.6, 25.6, 22.5, 14.0. HRMS (ESI+, m/z): Calcd for C20H23O2 [M-OH]+: 295.16926, found:
295.16971. Calcd for C23H30O4Na [M+Na]+: 335.16177, found 335.16207.
Dihexyl 5,5'-((2,2',4,4',7,7'-hexamethyl-2,2',3,3'-tetrahydro-[1,1'-biindenylidene]-6,6'-diyl)bis(ethyne-2,1-diyl))(E)-bis(3-(3-hydroxy-3-methylbut-1-yn-1-yl)benzoate) (2)
Motor 11 (57 mg, 0.10 mmol), alkyne 9 (65 mg, 0.21 mmol), CuI (1.5 mg, 0.0078 mmol) and PdCl2(PPh3)2 (5.6 mg, 0.0080 mmol) were dissolved in degassed DMF/iPrNH 3:1 (2 mL)
and stirred at 50 °C on. After cooling to rt., H2O was added and the mixture was extracted
3x with CH2Cl2. The combined organic layers were washed with 2M aq. HCl, H2O and brine
and dried over MgSO4. The volatiles were removed in vacuo and the residue was purified
by flash column chromatography (SiO2, pentane/EtOAc 5-40%) to yield 2 (25 mg, 27%) as a
white solid. 1H NMR (400 MHz, CDCl3) δ 8.13 (t, J = 1.6 Hz, 2H), 8.01 (t, J = 1.6 Hz, 2H), 7.76 (t, J = 1.6 Hz, 2H), 7.27 (s, 2H), 4.34 (t, J = 6.7 Hz, 4H), 2.90 (p, J = 6.4 Hz, 2H), 2.68 (dd, J = 15.0, 5.7 Hz, 2H), 2.61 (s, 6H), 2.27 (d, J = 14.9 Hz, 2H), 2.20 (s, 6H), 1.79 (p, J = 6.8 Hz, 4H), 1.65 (s, 12H), 1.51 – 1.41 (m, 4H), 1.41 – 1.31 (m, 8H), 1.13 (d, J = 6.4 Hz, 6H), 0.95 – 0.89 (m, 6H). 13C NMR (101 MHz, CDCl3) δ 168.1, 146.5, 144.3, 143.9, 140.7, 136.1, 134.7, 134.6, 134.4, 134.2, 133.7, 127.3, 126.1, 123.6, 97.8, 93.6, 93.4, 83.3, 68.3, 68.3, 44.6, 41.9, 34.1, 34.1, 31.3, 28.3, 25.2, 23.5, 21.7, 20.7, 16.7. HRMS (ESI+, m/z): Calcd for C64H72O6 [M+H]+: 936.5323, found: 936.5326.
Suzuki Polymerization
Motor (S,S)-trans-5 (57 mg, 0.12 mmol), fluorene 12 (78 mg, 0.12 mmol) and Aliquat 336 (20 mg) were dissolved in toluene (6 mL) and 2M aq. K2CO3 (1 mL). The mixture was
119
6.7
References
[1] S. Hecht, I. Huc, Foldamers: Structure, Properties, and Applications, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2007.
[2] S. H. Gellman, Acc. Chem. Res. 1998, 31, 173–180.
[3] D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes, J. S. Moore, Chem. Rev. 2001, 101, 3893–4011.
[4] G. Guichard, I. Huc, Chem. Commun. 2011, 47, 5933–5941.
[5] E. Yashima, K. Maeda, H. Iida, Y. Furusho, K. Nagai, Chem. Rev. 2009, 109, 6102–6211. [6] B. A. F. Le Bailly, J. Clayden, Chem. Commun. 2016, 52, 4852–4863.
[7] R. J. Simon, R. S. Kania, R. N. Zuckermann, V. D. Huebner, D. A. Jewell, S. Banville, S. Ng, L. Wang, S. Rosenberg, C. K. Marlowe, et al., Proc. Natl. Acad. Sci. U. S. A. 1992,
89, 9367–71.
[8] D. H. Appella, L. A. Christianson, I. L. Karle, D. R. Powell, S. H. Gellman, J. Am. Chem.
Soc. 1996, 118, 13071–13072.
[9] D. Seebach, J. L. Matthews, A. Meden, T. Wessels, C. Baerlocher, L. B. McCusker, Helv.
Chim. Acta 1997, 80, 173–182.
[10] P. E. Nielsen, M. Egholm, R. H. Berg, O. Buchardt, Science 1991, 254, 1497–1500. [11] J. C. Nelson, J. G. Saven, J. S. Moore, P. G. Wolynes, Science 1997, 277, 1793–1796. [12] L. Brunsveld, E. W. Meijer, R. B. Prince, J. S. Moore, J. Am. Chem. Soc. 2001, 123,
7978–7984.
[13] D.-W. Zhang, X. Zhao, J.-L. Hou, Z.-T. Li, Chem. Rev. 2012, 112, 5271–5316. [14] A. Tanatani, T. S. Hughes, J. S. Moore, Angew. Chem. Int. Ed. 2002, 41, 325–328. [15] H. Juwarker, J. M. Suk, K. S. Jeong, Chem. Soc. Rev. 2009, 38, 3316–3325.
[16] R. Gopalakrishnan, A. I. Frolov, L. Knerr, W. J. Drury, E. Valeur, J. Med. Chem. 2016, 59, 9599–9621.
[17] W. S. Horne, S. H. Gellman, Acc. Chem. Res. 2008, 41, 1399–1408.
120
2016, 55, 11096–11100.
[19] M. M. Müller, M. A. Windsor, W. C. Pomerantz, S. H. Gellman, D. Hilvert, Angew.
Chem. Int. Ed. 2009, 48, 922–925.
[20] Z. C. Girvin, S. H. Gellman, J. Am. Chem. Soc. 2018, jacs.8b05869. [21] Z. Yu, S. Hecht, Chem. Commun. 2016, 52, 6639–6653.
[22] G. A. Woolley, Acc. Chem. Res. 2005, 38, 486–493.
[23] A. Natansohn, P. Rochon, Chem. Rev. 2002, 102, 4139–4175.
[24] M. Irie, Y. Hirano, S. Hashimoto, K. Hayashi, Macromolecules 1981, 14, 262–267. [25] A. Khan, C. Kaiser, S. Hecht, Angew. Chem. Int. Ed. 2006, 45, 1878–1881. [26] Z. Yu, S. Hecht, Angew. Chem. Int. Ed. 2013, 52, 13740–13744.
[27] C. Tie, J. C. Gallucci, J. R. Parquette, J. Am. Chem. Soc. 2006, 128, 1162–1171. [28] Y. Hua, A. H. Flood, Chem. Soc. Rev. 2010, 39, 1262–1271.
[29] Y. Hua, Y. Liu, C.-H. Chen, A. H. Flood, J. Am. Chem. Soc. 2013, 135, 14401–14412. [30] Z. Yu, S. Hecht, Chem. - Eur. J. 2012, 18, 10519–10524.
[31] D. Pijper, B. L. Feringa, Angew. Chem. Int. Ed. 2007, 46, 3693–3696.
[32] T. van Leeuwen, G. H. Heideman, D. Zhao, S. J. Wezenberg, B. L. Feringa, Chem.
Commun. 2017, 53, 6393–6396.
[33] A. Kulago, Nanotechnological Tools Built on Synthetic Light-Driven Molecular Motors, University of Groningen, 2011.
[34] B. L. Feringa, N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada, Nature 1999,
401, 152–155.
[35] T. M. Neubauer, T. van Leeuwen, D. Zhao, A. S. Lubbe, J. C. M. Kistemaker, B. L. Feringa, Org. Lett. 2014, 16, 4220–4223.
[36] K. Suda, K. Akagi, Macromolecules 2011, 44, 9473–9488.
[37] A. S. Lubbe, Q. Liu, S. J. Smith, J. W. de Vries, J. C. M. Kistemaker, A. H. de Vries, I. Faustino, Z. Meng, W. Szymanski, A. Herrmann, et al., J. Am. Chem. Soc. 2018, 140, 5069–5076.
[38] M. M. Pollard, A. Meetsma, B. L. Feringa, Org. Biomol. Chem. 2008, 6, 507–512. [39] A. Yokoyama, H. Suzuki, Y. Kubota, K. Ohuchi, H. Higashimura, T. Yokozawa, J. Am.
Chem. Soc. 2007, 129, 7236–7237.
[40] T. Yokozawa, A. Yokoyama, Chem. Rev. 2009, 109, 5595–5619.
[41] Q. Li, G. Fuks, E. Moulin, M. Maaloum, M. Rawiso, I. Kulic, J. T. Foy, N. Giuseppone,
Nat. Nanotechnol. 2015, 10, 161–165.
[42] J. Conyard, P. Stacko, J. Chen, S. McDonagh, C. R. Hall, S. P. Laptenok, W. R. Browne, B. L. Feringa, S. R. Meech, J. Phys. Chem. A 2017, 121, 2138–2150.
