Photoresponsive Self-Assembled Systems Cheng, Jinling
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Bidirectional Photomodulation of Surface Tension in
Langmuir Films
Switching systems operating in a cooperative manner capable of converting light energy into mechanical motion are of great interest for optical devices, data storage, nanoscale energy converters and molecular sensing. Herein, photoswitchable monolayers were formed at the air-water interface from either a pure bis(thiaxanthylidene)-based photoswitchable amphiphile or from a mixture of the photoswitchable amphiphile with a conventional lipid dipalmitoylphosphatidylcholine (DPPC). Efficient photoisomerization of the anti-folded to
syn-folded geometry of the amphiphile’s central core induces changes in the surface
pressure in either direction, depending on the initial molecular density. Additionally, the switching behaviour can be regulated in the presence of DPPC, which influences the packing of the molecules, thereby controlling the transformation upon irradiation. Bis(thiaxanthylidene)-based photoswitchable monolayers provide a promising system to explore cooperativity and amplification of motion
This chapter was published as: J. Cheng, P. Štacko, P. Rudolf, R. Y. N. Gengler and B. L. Feringa, Angew. Chem. Int. Ed, 2017, 129, 297-302.
2.1 Introduction
In Nature, light-stimulated biochemical transformation is a crucial process by which optical signals are recorded and used to trigger a variety of chemical events including photosynthesis, the process of vision, ion transport and muscle activity.1-3 Particularly interesting is the involvement of photochromic units in biological processes offering major prospects for high precision control of dynamic functions as has been explored in recent years in optogenetics,4-6 photopharmacology,7-8 membrane transport 7,9-11 and drug delivery.12-14 Taking inspiration from integrated complex photoresponsive systems controlling dynamic functions in Nature, the design and exploration of photoswitchable molecules and materials have received major attention in the past decade.15-19 Upon irradiation, the molecules can access different isomeric states with distinct chemical and physical properties, allowing various functions of the system to be modulated with different wavelengths of light. Several applications have emerged from the ability to control the geometry of the molecules by light, such as optical memory20-21, switchable sensors4,22, responsive surfaces,23-24 gels,25-26 liquid crystals27-28 and delivery systems.3,29 The overcrowded alkene, bis(thiaxanthylidene) and its derivatives are particularly attractive photochromic switches due to their fast photoisomerization, featuring an unique geometrical change from anti-folded to syn-folded structure (Figure 2.1a).30-31 The transformation is fully reversible as the syn-folded molecule thermally relaxes back to its more energetically favourable anti-folded configuration.
Figure 2.1. (a) Molecular structure of anti-folded 1 and syn-folded 2. (b) Structural change of
the switch: front view (top) and top view (bottom).
However, investigation of these photoswitchable molecules has been so far carried out mostly in solution where the lack of control over the orientation and position of the photochromic molecules prevents exploring cooperative and amplification effects of an ensemble since the systems are naturally disordered.3, 32 Hence, achieving a supramolecular arrangement of such molecules is a key challenge to fabricate nano-assemblies where the molecules can operate in a cooperative fashion via light-induced geometrical changes.33-40 While efforts have been made to operate switches in a coordinated fashion, an efficient strategy is to incorporate the photoswitches into a self-assembled monolayer.36, 41-44 Among
the various techniques, the Langmuir-Blodgett (LB) method provides an elegant and fully controllable tool to achieve the assembly of responsive materials into a well-ordered monolayer at the air-water interface.45-46
In addition, Langmuir films have attracted great interest due to their potential application in modelling biological membranes in order to investigate drug action,47 permeability48-49 and sensing.50-51 It has been reported that switching a surfactant between different geometries can result in pronounced differences in surface pressure, orientation and molecular packing.44,52-57 Molecular transport across cell membranes employing protein channels, ion pumps or motors proteins and their delicate interplay with the membrane organization (and pressure) are essential to natural activity. In this context artificial membrane systems that include embedded photosensitive molecules are very attractive, although the design of such light-responsive systems remains challenging. 55,58-60 A particularly desirable feature is to control the membrane function with high temporal and spatial precision by light. Studies on photoswitchable artificial membrane systems include the use of azobenzene or spiropyran molecules as photochromic units. 34, 54, 58, 61-63.
2.2 Results and Discussion
2.2.1 Concept
To achieve a better understanding of the cooperative effects between neighbouring amphiphilic molecules, their molecular structure and photoswitching properties, novel photoresponsive amphiphiles have to be designed which combine excellent reversibility and stability when assembled into condensed systems. Here we present a photoresponsive Langmuir film, based on a bis(thiaxanthylidene) amphiphile that shows for the first time a bidirectional photomodulation of surface pressure. To the best of our knowledge, the ability to reversibly increase and decrease pressure (and packing) in a Langmuir film at a single irradiation wavelength is unprecedented.
Figure 2.2 Cryo-TEM images of self-assembled sheets generated by amphiphile 2.1.
The amphiphile 2.1 contains a central bis(thiaxanthylidene) unit bearing hydrophilic tetraethylene glycol and hydrophobic alkyl tails (Figure 2.1a). The structure of 1 is a modified version of a bis(thiaxanthylidene) that was shown to form nanotubes, by extending the hydrophilic moieties each with one ethylene glycol unit (see experimental section for
synthesis and characterization of 2.1). 30-31 By increasing the size of the hydrophilic head group, 2.1 was found to readily self-assemble into sheets in water as demonstrated by
Cryo-monolayer of the amphiphile 2.1 at the air-water interface as well as a mixed Cryo-monolayer containing both 2.1 and DPPC. The Langmuir film surface pressure behaviour was examined systematically in response to UV irradiation, and found to be dramatically influenced by the packing of the molecules. Our investigation suggests that, besides the modification of molecular structure upon irradiation, the packing mode within the Langmuir film can be used to tune the properties of the whole system
2.2.2 Photoresponsive Behaver in Solutions
Figure 2.3. (a) The conversion of 2.1 (blue) to 2.2 (red) in dichloromethane upon irradiation
at 365 nm and the thermal recovery (gray) monitored by UV/vis absorption spectroscopy. Inset: the irradiation (365 nm) of 2.1 for 10 min at -20 ºC followed by thermal relaxation at 5 ºC for 20 min monitored by absorption at 365 nm over 6 cycles. (b) Emission spectra during the irradiation process of 2.1 (λexc=365 nm) at -20 ºC. (c) Key signals in the 1H NMR of the stable anti-folded 2.1 (bottom panel), after irradiation, (middle panel) and after heating to room temperature (top panel).
Because of steric hindrance between the lower and upper halves (Figure 2.1b), the stable isomer 2.1 preferentially adopts an anti-folded conformation with an absorption maximum at 355 nm for the π→π* transition in the UV/vis spectrum. When a dichloromethane solution of 2.1 is exposed to 365 nm light (Figure 2.3a), a decrease of the band at 355 nm with a concomitant increase of the absorption at 320 nm is observed with clear isosbestic points. This corresponds to the disappearance of the anti-folded isomer and the formation of the thermodynamically less stable syn-folded isomer 2.2 in accordance with our previous
studies on bis(thiaxanthylidenes), supported by DFT calculations.64 In addition, the fluorescence emission band at 480 nm disappeared completely in the process (Figure 2.3b). The photoisomerization was also studied by 1H-NMR (-40 °C) and provided additional evidence for the formation of syn-folded 2.2 with a photostationary state containing 80% of the syn-folded isomer (Figure 2.3c). Upon increasing the temperature to 5 °C, syn-folded 2.2 underwent complete thermal conversion back to the anti-folded 2 (Figure 2.3a and b) and a complete recovery of both the UV/vis spectrum and 1H NMR spectrum of 2.1 was observed. The irradiation/thermal recovery cycles could be repeated multiple times without noticeable signs of fatigue (insert in Figure 2.3a).
2.2.3 Photoresponsive Behavior of Langmuir Films
Preparation of Stable Langmuir Films
A stable monolayer of anti-folded 2.1 was obtained by spreading a chloroform solution (0.07 mg/ml) onto the water-air interface. The surface pressure-area isotherm indicates a well-condensed monolayer with a collapse pressure of 45 mN/m for a molecular area of 70 Å/molecule (Figure 2.5a). This collapse area is smaller than the estimated value of our DFT calculations, 91.1 Å2, (Figure 2.1b), suggesting a highly compressed Langmuir film structure, which can be attributed to π−π stacking of the aromatic core. The detailed process is shown in the experimental part.
Photoresponsive Behaver of Langmuir Films
As discussed earlier, the molecular properties, for example the molecular geometry, change as a consequence of the isomerization from anti- to syn-folded. We therefore set out to investigate the behaviour of the monolayer upon exposure to UV light. First, the monolayer was compressed to 23 mN/m (corresponding to 92 Å2/molecule) and the area was kept constant for the duration of the irradiation experiment. The compressed monolayer was sequentially exposed to UV irradiation (365 nm) for 30 sec and kept in the dark for 2 min to allow for thermal backward isomerization. Throughout each cycle, the configuration of 1 varied between an anti-folded and syn-folded geometry, resulting first in an increase of the surface pressure by approximately 3 mN/m upon irradiation, followed by recovery of the original pressure when left standing in the dark (Figure 2.5c). Next, the behaviour of the Langmuir film was investigated at slightly increased initial surface pressure. The monolayer was therefore compressed to 24.0 mN/m, corresponding to a molecular area of 90 Å2. Much to our surprise and contrary to the behaviour at lower pressures, UV irradiation (365 nm) led to a rapid decrease of the surface pressure, to 22.8 mN/m (Figure 2.5d). The pressure returned to the original value upon leaving the sample standing in the dark, with a slight fatigue observed over multiple cycles 21b and the fatigue is neglected at 26 mN/m.
Figure 2.4. (a) Surface pressure versus area isotherm for a monolayer of 2.1 at the air-water
interface. (b) Absorption spectra of a monolayer of 2.1 on quartz deposited at different compressions; 23 mN/m (black) and 25 mN/m (red). (c and d) Kinetics of the anti-syn photoisomerization in a monolayer of 2.1 as monitored by a surface pressure at the air-water interface at an initial molecular area of 92 Å2/molecule (c) and 90 Å2/molecule (d). The surface pressure was recorded while the monolayers of the anti-folded 2.1 were held at a constant area while being exposed to the radiation at 365 nm (30 s) and then left in the dark (120 s). Note: The monolayers of anti-folded isomer 2.1 were compressed to the specific pressure and maintained at a constant area during the following two procedures: 1) under radiation with 365 nm for 30 s and 2) kept in the dark for 2 min.
In order to gain additional understanding of this behaviour, we set out to probe the surface pressure changes upon irradiation at different initial surface pressures ranging from 16 to 29 mN/m (Figure 2.4). At lower starting pressures (16–23 mN/m; 91.2–100 Å2/molecule), the surface pressure increases by 1–3 mN/m upon irradiation with UV light. It was observed that the surface pressure change enhanced with an increasing initial surface pressure, culminating at 22 mN/m (Δ3 mN/m), before declining again for 23 mN/m. Interestingly, in stark contrast, upon further increase above 23 mN/m (~91.2 Å2/molecule) a decrease of the surface pressure (0–1 mN/m) is observed upon irradiation with UV light. This decrease gradually levels off with increasing pressure. At initial pressures higher than 29 mN/m, the monolayer collapsed immediately upon exposure to UV light.
Figure 2.5 Kinetics of the anti-syn photoisomerization on pure monolayer as monitored by
surface pressure at the air-water surface with footprint of 95 Å2 with pressure at ~20 mN/m(a) and 89 Å2 at ~26 mN/m (b) and surface pressure changes at the air-water interface in monolayer of 2.1 upon irradiation with UV-light (365 nm) at different initial surface pressures.
DFT Calculations and Proposed Mechanisms
To the best of our knowledge, this type of dual behaviour has not been observed for Langmuir films before and we propose the following model based on DFT/DFT-D3 calculations. The structures of both anti-folded 2.1 and syn-folded 2.2 have been optimized by DFT/b3lyp6-31G (d, p) and the footprints of individual molecules (considered as spheres) were determined to be 91.1 and 96.5 Å2/molecule, respectively Next, the packing ability of
2.1 and 2.2 as trimers was investigated by DFT-D3 calculations, capable of describing London
(long-range) dispersion interactions.65 The optimized supramolecular structure is shown in Figure 2.6. The π-π stacking between adjacent monomers is obviously observed in the optimized supramolecular structure. The distances between the stacked π-discs are ca. 3.72 Å for anti-folded trimer and 2.65 Å for syn-folded trimer. These calculations confirm that the switches can form supramolecular assemblies by the π-π stacking interactions. As illustrated in Figure 2.6, the anti-folded 2.1 isomer prefers to pack as J-aggregates, with the lower half of the overcrowded alkene stacking with the upper half of the adjacent molecule (head-to-tail). On the other hand, the metastable syn-folded 2.2 assumes H-aggregate packing, with the upper half of the alkene stacking with the upper half of the adjacent (head-to-head) molecule. While the individual molecules of syn-folded 2.2 have a larger footprint than anti-folded 2.1 (vide supra), they are capable of more efficient packing since, based on the DFT-D3 calculations, the three molecules of 2.1 and 2.2 occupy area of 290.3 and 262.0 Å2, respectively.
Figure 2.6. Optimized supramolecular structure of trimers for anti- and syn-folded. For
clarity, only the central overcrowded alkene nuclear are presented. Blue represents sulfur atoms, white represents hydrogen and gray represents carbon atoms. The intermolecular packing is indicated by dashed lines.
At low pressures (≤ 23 mN/m), the molecules are presumably loosely organized in the monolayer (Figure 2.7a) and transition to J-aggregate packing at higher surface pressures (≥ 24 mN/m, Figure 2.7b) to compensate for the increase of pressure. In order to confirm this, the Langmuir films obtained at different pressures (23 and 25 mN/m), were deposited onto quartz slides by immersing the substrate vertically (Figure 2.4c). Comparison of the absorption spectra of the deposited LB films indicates a bathochromic shift of the absorption band from λ= 362 nm to 372 nm for the monolayer at 25 mN/m compared to the one at 23 mN/m, with a concomitant sharpening of the absorption band. The red-shift and sharpening of the absorption band indicates a formation of J-aggregates. 66-68.
Figure 2.7. Proposed model of the packing for supramolecular structures before and after
exposure to UV irradiation based on DFT/DFT-D3 calculations at (a) lower and (b) higher initial pressure. The hydrophilic tetraethylene glycol and hydrophobic alkyl tails are omitted for clarity. Blue represents sulfur, white represents hydrogen and gray represents carbon atoms
In more disorganized Langmuir films at low initial pressures (≤ 23 mN/m), irradiation with UV-light leads to the formation of syn-folded 2.2 which possesses a larger footprint, imposing sterical strain within the monolayer. The strain should translate into an expansion of the monolayer, however since the area containing the molecules is kept constant, the packing density increases, resulting in a higher surface pressure (Figure 2.5). 53, 69-70 Contrary to that, at high initial surface pressure (≥ 24 mN/m), irradiation of anti-folded 2.1 packed in the form of J-aggregates leads to formation of syn-folded 2.2, which is forced to assume more space-efficient H-aggregate packing due to already strained monolayer. As a consequence, a surface pressure drop is observed under these conditions. Based on our experimental data, we cannot exclude that some of the amphiphilic molecules are expelled from the monolayer at high pressures, thus also decreasing the surface pressure. 71
Photoresponsive Behaver of mixed Langmuir Films
It has been recognized that conformational changes in biological molecules embedded in membranes are important in controlling the properties of natural membranes. 53,72 Therefore, the interaction between the amphiphilic photoswitch and artificial membrane components in a two component system is of particular interest for the development of responsive materials. Hence we used 2.1 as a dopant in a mixture with DPPC, a lipid occurring in natural membranes. The mixture of 2.1 and DPPC with different ratios was first investigated by UV-vis absorption spectroscopy in chloroform. No pronounced difference in isomerization behaviour was observed upon introduction of DPPC, indicating negligible interaction between the two species in solution as illustrated in Figure 2.7a.
Figure 2.8. (a) Absorption spectra of pure 2.1, DPPC and different ratio (1:DPPC=1:3, blue;
1:8, green) of mixtures (in mole ratios) in DCM. (b) Surface pressure versus area isotherm for
2.1 (red), DPPC (black), mixture of 2.1:DPPC with different ratios (in mole) of 1:3 (blue), 1:8
Figure 2.9. Kinetics of the anti-syn isomerization of the mixed monolayer monitored by
surface pressure at the air-water interface at low initial pressure (bottom) and high initial pressure (top). (a) The ratio of the mixture is 2.1:DPPC=1:3 and (b) 2.1:DPPC =1:8.
To study the influence of DPPC on the switching behaviour in the Langmuir film, monolayers with two different ratios of the amphiphile 2.1 and DPPC (1:3 and 1:8) at the air-water interface were prepared (Figure 2.8b). The isotherms of both doped lipids are between the isotherms of the two components. Accordingly, they are not a simple sum of the isotherms nor exist of two well-separated phases indicating that DDPC and switches might be miscible without significant attractive interactions. The monolayers were then irradiated in the same manner as described above (Figure 2.9). The mixture with low DPPC content displayed the same behaviour as the monolayer consisting of pure anti-folded 2.1, with the surface pressure changes being dependent on the initial molecular density, albeit with lower magnitude (Figure 2.9a). Conversely, when the proportion of DPPC was raised to 89%, a pressure increase upon irradiation was observed for both low and high molecular density, which is in a sharp contrast with the observation made for both the pure amphiphile 2.1 and the 1:3 mixture of 2.1 and DPPC. These abnormal changes at high DPPC content can be explained in terms of smaller footprint of the lipid molecules. Extensive dilution of the monolayer with DPPC effectively increases the area individual molecules of 2.1 and 2.2 can occupy and thus decreases the strain within the monolayer. This allows for increase of the surface pressure upon photoisomerization even at higher initial surface pressures. At the same time, at high concentrations, DPPC can be expected to perturb efficient stacking of 2.1 and 2.2 in the form of J- and H- aggregates, thus disallowing more favourable packing of the
syn-folded 2.2. Therefore, the use of DPPC can be viewed as an effective way of modulation
of shape of the curve, as well as the position of the inversion point. These findings demonstrate that different packing modes alter the effect of the optical switching and can, under certain circumstances, lead to inversion of the observed behavior in the Langmuir films.
2.3 Conclusion
In conclusion, we have shown that properly designed Langmuir films of a bis(thiaxanthylidene)-based molecule exhibits remarkable bidirectional surface-response behaviour upon anti-syn folded photoisomerization which depends on the interfacial organization determined by the initial molecular density. The photoisomerization results in an increase of the surface pressure at low molecular density, while at higher molecular
density, a decrease of the surface pressure is observed which can be ascribed to adopting a different, more favourable, stacking of syn-folded 2.2 under these conditions. The character of the surface pressure response can be further modulated by mixing the switch with different amounts of DPPC in in the monolayer. Our investigation indicates that not only the external stimuli but also the morphology and ratio of the building blocks influence the responsive properties of the Langmuir film, thus opening new avenues for reversibly changing monolayers and ultimately surface and membrane function in a controlled fashion.
2.4 Acknowledgement
Dr Peter Štacko is gratefully acknowledged for the synthesis of the compounds. Dr. Marc A. Stuart and Derk J. van Dijken are acknowledged for cryo-TEM measurements.
2.5 Experimental Section
2.5.1. General information
All the chemicals applied in the synthesis were purchased from Acros, Aldrich, Fluka or Merck and used without further purification. Solvents were dried and distilled before used. TLC was performed with Merck silica gel 60 F254 plates. 1H NMR and 13C NMR spectra were recorded on Varian VXR-300S, Varian Mercury Plus-400 at 298K. 1H and 13C NMR chemical shifts are reported per million (ppm) with TMS as the standard of the chemical shifts. High Resolution Mass spectra (HRMS) were recorded on an LTQ Orbitrap XL. Irradiation at 365 nm was carried out using a hand-held UV lamp (Spectroline E Series).
2.5.2. UV-Vis and fluorescence spectra
UV-Vis absorption and fluorescence spectra were taken on a Jasco V-630 spectrophotometer and Jasco FP-6200 spectrofluorimeter. Uvasol grade dichloromethane (Merck) was used for absorption and fluorescence spectroscopy with 1 cm pathlength quartz cuvettes (Hellma). The low temperature measurements were carried out by using a polystat CC1 cryostat with a Quantum Northwest temperature controller.
2.5.3 Monolayer measurements
All the measurements of monolayer were conducted by the Wilhelmy plate method in the LB through (Nima Technology Ltd., UK). The subphase consist of ultrapure water (18.0 MΩ/cm-1 resistance) produced by a WaterPro PS HPLC/ HPLC/Ultrafilter Hybrid system (NO. 9000602, Labconco, Kansas City, MO) and the temperature was actively controlled and maintained at 20 ºC.
Isotherm measurements: All the measurements of the surface pressure-molecular area (π-A) isotherm were performed using the Wilhelmy plate method on the LB through. The maximum surface area of the through was 512.0 cm2 and the minimum surface area between the two barriers is 62.0 cm2. The through was first cleaned with chloroform and then water in order to remove any contamination or remaining solvent. A solution of the switch in chloroform with a concentration of 0.02 mg/ml (unless otherwise stated) was spread dropwise onto the water-air surface using 50 µl micro-syringe. After a waiting time of 30 min (to allowed for the solvent evaporation), the two Teflon barriers were compressed at the speed of 0.1 (cm2/min) to the specific pressure. Each isotherm was measured at least three times to ensure reproducibility. In the case of the mixed monolayers, the solution of switches was added dropwise onto the homogeneous surface after spreading a DPPC
LB deposition: The vertical dipping method (Langmuir Blodgett) was adopted for all depositions. The quartz microscope slides (1*2 cm2, UQG Ltd., UK) were employed for UV/Vis absorption spectroscopic measurements. Before deposition, the quartz plate were cleaned using Piranha solution (3:1 mixture of H2SO4 and H2O2) to get rid of the organic
residues on substrates and making them highly hydrophilic. For deposition, the substrate was immersed into water (1-2 cm) with only the top 3~5 mm of the substrates remaining in the air. After spreading the solution and subsequently compressing to different stabilized monolayers, the slides were pulled out of the water with a rate of 5 mm/s.
2.5.4 Cryo-TEM
Samples for cryo-TEM were prepared by depositing typically 3 μL of nanotube solution on holey carbon coated grids (Quantifoil 3.5/1, Quantifoil Micro Tools, Jena, Germany). After blotting the excess liquid at constant humidity of 100%, the grids were vitrified in liquid ethane (Vitrobot, FEI, Eindhoven, The Netherlands) and transferred to either a Philips CM 12 cryo-electron microscope operating at 120 kV, a Philips CM 120 cryo-electron microscope operating at 120 kV or a FEI Tecnai 20 microscope operating at 200kV. Micrographs were recorded under low-dose conditions with a slow-scan CCD camera.
A stock solution of amphiphile 2.1 in chloroform was mixed with the desired amount of a stock solution of DOPC in chloroform and subsequently dried under nitrogen gas resulting in the formation of a thin film. After further solvent removal under high vacuum for 15 min, the sample was hydrated with water and the suspension was subjected to three freeze-thaw cycles. The concentration of amphiphile 2.1 was 1 mg/ml with an additional 1 mg/ml of DOPC.
2.5.5 Calculations
All the calculations were performed using Gaussian09 package.31 For the sake of clarity and brevity, the long hydrophilic and hydrophobic chains are omitted. The stationary geometries for monomers (anti-folded and syn-folded) were optimized by using the density functional theory (DFT) with the hybrid Becke-3−Lee−Yang−Parr (B3LYP) functional with 6-31G(d, p) basis set.2, 74 The optimized structures are evaluated by frequency analysis to make sure that the conformations have the minima energy. using the minima structures as the starting point, geometry optimizations were performed along the C1-C2-C3-C4 dihedral angle (θ) to identify the one-dimensional rotatory potential energy curve and the syn-folded switches structure.
The effect of long-range weak interactions (π-π packing) between the switches using the DFT-D3 package.3 B3LYP functional with 6-31G(d, p) basis set were used for the ground-state geometries of the dimers and trimers for both of the stable anti-folded and metastable syn-folded.
2.5.6 Synthesis 75
4,5-Bis(dodecyloxy)-9H-thioxanthen-9-one (S2).
A mixture of S1 (7.52 g, 31 mmol), 1-bromododecane (19.22 ml, 80 mmol), and K2CO3 (21.27 g, 154 mmol) in DMF (150 ml) was
heated to 100 °C for 36 h. The reaction was quenched with water (350 ml) and dichloromethane was added (400 ml). The organic phase was separated and washed with water (4 × 400 ml). The solvents were evaporated at reduced pressure and the residue was recrystallized from ethanol (100 ml) to give the title product.
Yield: 15.7 g (88 %); yellow solid, mp 78.1‒79.2 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 8.24
(dd, 2H, J1 = 8.1 Hz, J2 = 0.9 Hz), 7.42 (dd, J1 = 8.0 Hz, J2 = 8.0 Hz, 2H), 7.13 (d, J = 7.9 Hz, 2H),
4.19 (t, J = 6.5 Hz, 4H), 1.95 (m, 4H), 1.59 (m, 4H), 1.27 (m, 32H), 0.88 (t, J = 6.8 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ (ppm) 180.7, 154.6, 130.2, 128.7, 126.0, 121.5, 113.3, 69.7, 32.1,
29.9, 29.9, 29.88, 29.87, 29.6, 29.58, 29.3, 26.3, 22.9, 14.3.
4,5-Bis(dodecyloxy)-9H-thioxanthen-9-ylidene)hydrazine (S4). A mixture of S2 (11.6 g, 20.0mmol) and Lawesson's reagent (15.0 g, 37.1 mmol) in toluene (250 ml) was heated at reflux under nitrogen atmosphere for 2 h. The resulting yellow solution was cooled to ambient temperature and filtered through a plug of silica gel. The plug was washed with dichloromethane until the filtrate was colorless. The solvents were evaporated and the residue was purified by flash column chromatography on silica gel (pentane : ethyl acetate - 10 : 1). The thioketone was
redissolved in THF (150 ml). The solution was treated at room temperature with aq. hydrazine monohydrate (40.0 ml, 70.0 mmol). The solution decolorized to an orange solution within several minutes. The solvent and excess hydrazine was removed at reduced pressure. The resulting solid was recrystallized from ethanol (100 ml) to give the title compound. Yield: 15.7 g (88 %); yellow solid, mp 81.2‒82.7 °C. 1H NMR (400 MHz, CDCl3):
(dd, J1 = 8.0 Hz, J2 = 8.0 Hz, 2H), 7.24 (dd, J1 = 8.0 Hz, J2 = 8.0 Hz, 2H), 6.89 (dd, J1 = 8.1 Hz, J2 = 0.8 Hz, 2H), 6.83 (dd, J1 = 8.1 Hz, J2 = 1.0 Hz, 2H), 5.84 (brs, 2H), 4.10 (m, 4H), 1.90 (m, 4H), 1.54 (m, 4H), 1.26 (m, 32H), 0.89 (t, 6H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 155.9, 154.3, 142.0, 135.3, 127.0, 126.7, 125.7, 121.8, 120.1, 118.4, 111.2, 110.2, 69.4, 32.2, 29.9, 29.6, 29.4, 26.3, 22.9, 14.4. 4,5-Bis(2-(2-(2-(2-hydroxyethoxy)ethoxy)ethoxy)ethoxy)-9H-thioxanthen-9-one (S5).
A mixture of S1 (300 mg, 1.23 mmol) and
2-(2-(2-(2-hydroxyethoxy)ethoxy)ethoxy)ethyl
4-methylbenzenesulfonate (1.11 g, 3.19 mmol) and K2CO3 (849 mg, 6.14 mmol) in acetone (30 ml) was heated at reflux for 24 h. The acetone was evaporated under reduced pressure. Water (50 ml) and dichloromethane (100 ml) were added. The separated water phase was
washed with dichloromethane (2 × 50 ml). The solvents were evaporated at reduced pressure and the residue was purified by column chromatography (ethyl acetate : methanol − 20 : 1) to give the compound. Yield: 645 mg (88 %); yellow oil.. 1H NMR (400 MHz, CDCl3): δ (ppm) 8.24 (d, J = 8.3 Hz, 2H), 7.42 (dd, J1 = 8.0 Hz, J2 = 8.0 Hz, 2H), 7.18 (d, J = 7.9 Hz, 2H), 4.37 (m,4H), 4.00 (m, 4H), 3.82 (m, 4H), 3.57-3.83 (m, 20H). 13C NMR (100 MHz, CDCl3): δ (ppm) 180.4, 154.3, 130.0, 128.1, 126.1, 122.0, 113.9, 72.7, 71.3, 70.83, 70.81, 70.5, 69.7, 69.4, 61.8. HRMS (APCI+): calcd for C29H40O11S M + H+ 597.2370 found 597.2364.
4,5-Bis((2,2-dimethyl-3,3-diphenyl-4,7,10,13-tetraoxa-3-silapentadecan-15-yl)oxy)-9H-thioxanthen-9-one (S6).
A mixture of S5 (600 mg, 1.01 mmol),
TBDPSCl (1.19 g, 4.32 mmol) and imidazole (288 mg, 4.22 mmol) in dichloromethane (30 ml) was stirred for 24 h. The reaction mixture was filtered over celite. The solvents were evaporated at reduced pressure and the residue was purified by
column chromatography (pentane : ethyl acetate − 7 : 3) to give compound S6. Yield: 930 mg
(86 %); yellow oil. 1H NMR (400 MHz, CDCl3): δ (ppm) 8.28 (dd, J1 = 8.1 Hz, J2 = 0.9 Hz, 2H),
7.71 (dd, J1 = 7.7 Hz, J2 = 1.8 Hz, 8H), 7.37-7.44 (m, 14H), 7.18 (dd, J1 = 8.0 Hz, J2 = 0.8, 2H),
4.35 (t, J = 4.9 Hz, 4H), 3.99 (t, J = 4.9 Hz, 4H), 3.83 (m, 8H), 3.72 (m, 4H), 3.68 (s, 8H), 3.62 (t,
J = 5.3 Hz, 4H), 1.08 (s, 18H).13C NMR (100 MHz, CDCl3): δ (ppm) 180.3, 154.3, 135.7, 133.8,
130.1, 129.7, 128.4, 127.7, 126.0, 121.9, 113.9, 72.6, 71.2, 70.9, 70.9, 69.7, 69.3, 63.5, 27.0, 19.3. HRMS (APCI+): calcd for C61H77O11SSi2 M + H+ 1073.4725 found 1073.4729.
4,5-Bis((2,2-dimethyl-3,3-diphenyl-4,7,10,13-tetraoxa-3-silapentadecan-15-yl)oxy)-9H-thioxanthene-9-thione (S7). A solution of S6 (530 mg, 0.49 mmol) and Lawesson’s reagent
(300 mg, 0.74 mmol) in toluene (30 ml) was heated to 130 °C for 2 h. The solvents were evaporated at reduced pressure and the residue purified by column chromatography (pentane : ethyl acetate - 1 : 1) to give product S7. Yield: 511 mg (95 %); dark yellow oil. 1H NMR (400 MHz, CDCl3): δ (ppm) 8.67
7.15 (dd, J1 = 7.9 Hz, J2 = 0.8Hz, 2H), 4.35 (t, J = 5.0 Hz, 4H), 3.98 (t, J = 5.0 Hz, 4H), 3.81 (m, 8H), 3.69 (m, 4H), 3.64 (s, 8H), 3.59 (t, J = 5.3 Hz, 4H), 1.05 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) 210.9, 154.4, 138.5, 135.8, 133.8, 129.8, 127.8, 126.5, 125.8, 123.8, 113.0, 72.6, 71.3, 70.9, 70.9, 70.88, 69.7, 69.6, 63.6, 27.0, 19.4. HRMS (APCI+): calcd for C61H77O10S2Si2 M + H+ 1089.4497 found 1089.4499.
Episulfide (S8).
Solid MnO2 (203 mg, 2.34 mmol) was added to
a solution of S7 (167 mg, 0.28 mmol) in THF (40 ml) at 0 °C. The resulting solution was stirred for 1 h at 0 °C and filtered through a plug of silica gel. The silica was washed with a small amount of THF (10 ml). The light green solution was cooled back to 0 °C. A solution of
S4 (255 mg, 0.23 mmol) in THF (2.4 ml) was added dropwise and the resulting mixture was stirred overnight. The solvents were evaporated
at reduced pressure and the residue purified by flash column chromatography (pentane : ethyl acetate − 3 : 1) to give the title product S8. Yield: 315 mg (81%); light yellow oil. 1
H NMR (400 MHz, CDCl3): δ (ppm) 7.72 (dd, J1 = 7.7 Hz, J2 = 1.7 Hz, 8H), 7.33-7.42 (m, 12H), 7.33 (dd, J1 = 7.9 Hz, J2 = 0.8 Hz, 2H), 7.29 (dd, J1 = 7.9 Hz, J2 = 0.8 Hz, 2H), 6.87 (dd, J1 = 8.0 Hz, J2 = 8.0 Hz, 2H), 6.86 (dd, J1 = 8.0 Hz, J2 = 8.0 Hz, 2H), 6.58 (dd, J1 = 8.1 Hz, J2 = 0.8 Hz, 2H), 6.53 (dd, J1 = 8.1 Hz, J2 = 0.7 Hz, 2H), 4.08 (m 2H), 3.91−4.03 (m, 4H), 3.57-3.83 (m, 30H), 1.79 (m, 4H), 1.49 (m, 4H), 1.29 (m, 32H), 1.08 (s, 18H), 0.91 (t, J = 6.8 Hz, 6H). 13 C NMR (100 MHz, CDCl3): δ (ppm) 154.0, 153.8, 135.8, 133.9, 132.5, 132.0, 129.8, 127.8, 125.9, 125.6, 125.1, 125.0, 124.0, 123.3, 111.6, 110.3, 72.6, 71.1, 70.88, 70.87, 70.84, 69.7, 69.5, 69.4, 66.64, 66.61, 63.6, 32.1, 29.9, 29.87, 29.86, 29.7, 29.6, 29.4, 27.0, 26.2, 22.9, 19.4, 14.3. HRMS (APCI+): calcd for C98H132O12S3Si2Na M + Na+ 1676.8351 found 1676.8345
15,15'-((4',5'-Bis(dodecyloxy)- 9,9'-bithioxanthenylidene -4,5-diyl)bis(oxy))bis(2,2-dimethyl-3,3-diphenyl-4,7,10,13-tetraoxa-3-silapentadecane)( S9).
PPh3 (149 mg, 0.57 mmol) was added to a
solution of S8 (313 mg, 0.19 mmol) in toluene
(30 ml). The resulting mixture was heated to 100 °C overnight. The solvents were evaporated at reduced pressure and the residue purified by flash column chromatography (pentane : ethyl acetate – 3 : 1) to give the title product. Yield: 264 mg (86%); light yellow solid, mp 62.8‒63.6 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.68 (dd, J1 = 7.5 Hz, J2 = 1.6 Hz, 8H), 7.33-7.44 (m, 12H), 6.80 (m, 4H), 6.65 (m, 4H), 6.42 (d, J = 7.8 Hz, 2H), 6.39 (d, J = 7.8 Hz, 2H), 4.28 (m, 2H), 4.14 (m, 4H), 3.93−4.02 (m, 6H), 3.81 (m, 8H), 3.71 (m, 4H), 3.65 (m, 8H), 3.60 (t, J = 5.4 Hz, 4H), 1.90 (m, 4H), 1.57 (m, 4H), 1.28 (m, 32H), 1.05 (s, 18H), 0.89 (t, 6H, J = 6.7 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 155.3, 155.0, 137.0, 136.5, 135.7, 133.8, 133.2, 132.8, 129.7, 127.8, 125.9, 125.8, 124.9, 124.8, 122.7, 122.2, 109.5, 109.2, 72.6, 71.2, 70.9, 70.89, 70.86, 69.8, 69.1, 68.8, 63.6, 32.1, 29.9, 29.8, 29.83, 29.82, 29.7, 29.5, 29.4, 27.0, 26.3, 22.8, 19.3, 14.3. HRMS (APCI+): calcd for C98H132O12S2Si2Na M + Na+ 1644.8630
2,2'-((((((((4',5'-Bis(dodecyloxy)- 9,9'-bithioxanthenylidene -4,5-diyl)bis(oxy))bis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1-diyl))bis(oxy))bis(ethane-2,1-diyl))bis(oxy))diethanol (2.1).
A solution of S9 (75 mg, 0.05 mmol) in THF (5 ml) was
treated dropwise with TBAF (102 l, 0.10 mmol, 1 M in THF). After 24 h of stirring, the solvents were evaporated at reduced pressure and the residue purified by flash column chromatography (dichloromethane : methanol − 10 : 1) to give the title product. Yield: 48 mg (91%); white solid, mp 96.3‒ 97.4 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 6.83 (dd, J1 = 8.0 Hz, J2 = 7.9 Hz, 2H), 6.82 (dd, J1 = 7.9 Hz, J2 = 7.9 Hz, 2H), 6.67 (m, 4H), 6.44 (dd, J1 = 7.7 Hz, J2 = 0.6, 2H), 6.39 (dd, J1 = 7.8 Hz, J2 = 0.7, 2H), 4.32 (m 2H), 4.11-4.20 (m, 4H), 3.93-4.05 (m, 6H), 3.89 (t, J = 4.6 Hz, 4H), 3.65-3.78 (m, 16H), 3.60 (t, J = 4.4 Hz, 4H), 3.06 (brs, 2H), 1.91 (m, 4H), 1.59 (m, 4H), 1.28 (m, 32H), 0.89 (t, 6H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 155.4, 155.4, 155.0, 137.1, 136.5, 133.4, 132.8, 126.0, 125.8, 125.0, 124.8, 122.8, 122.2, 109.5, 109.3, 72.8, 71.3, 70.9, 70.8, 70.5, 69.8, 69.2, 68.9, 61.9, 32.1, 29.93, 29.89, 29.88, 29.87, 29.7, 29.6, 29.5, 26.3, 22.9, 14.3. HRMS (APCI+): calcd for C66H96O12S2 M + Na+ 1167.6241 found 1167.6235.
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