Supramolecular Packing and Macroscopic Alignment Controls Actuation Speed in Macroscopic Strings of Molecular Motor Amphiphiles.

Three-dimensional organized unidirectionally aligned and responsive supramolecular structures have much potential in adaptive materials ranging from biomedical components to soft actuator systems. However, to control the supramolecular structure of these stimuli responsive, for example photoactive, materials and control their actuation remains a major challenge. Toward the design of "artificial muscles", herein, we demonstrate an approach that allows hierarchical control of the supramolecular structure, and as a consequence its photoactuation function, by electrostatic interaction between motor amphiphiles (MA) and counterions. Detailed insight into the effect of various ions on structural parameters for self-assembly from nano- to micrometer scale in water including nanofiber formation and nanofiber aggregation as well as the packing structure, degree of alignment, and actuation speed of the macroscopic MA strings prepared from various metal chlorides solution, as determined by electronic microscopy, X-ray diffraction, and actuation speed measurements, is presented. Macroscopic MA strings prepared from calcium and magnesium ions provide a high degree of alignment and fast response photoactuation. By the selection of metal ions and chain length of MAs, the macroscopic MA string structure and function can be controlled, demonstrating the potential of generating multiple photoresponsive supramolecular systems from an identical molecular structure.

Introduction

Supramolecular polymers are found in many living systems, for example, cytoskeleton filaments (F-actin[1] and microtubules[2]), flagellar filaments of bacteria,[3] and polymers of viral proteins and muscles,[4] to serve vital roles in key biological functions. While biological systems provide precise control in supramolecular polymerization,[1−4] synthetic supramolecular polymers[5,6,14] in aqueous media allow tunable features due to the design based on synthetic compounds and bioinspired functionality.[6,7,16,17,8−15] This delicate molecular design strategy allows the construction of hierarchical supramolecular assemblies along multiple-length-scales. At the microscopic length-scale level,[10,11] numerous unimolecular amphiphilic molecules have been shown to assemble into highly ordered one-dimensional (1D) supramolecular systems, through noncovalent interaction, for example, hydrogen bonding,[18−25] arene interaction,[26−33] and electrostatic effects.[32,34−37] At macroscopic length-scales, the obtained 1D supramolecular polymers of unimolecular amphiphiles can further assemble, instead of forming a three-dimensional (3D) randomly entangled network, into 3D unidirectionally aligned hierarchical supramolecular structures, providing exciting opportunities toward applications for instance in regenerative (biomedical) materials,[38−40] actuators, electronics, and optoelectronic materials.[41−43] To further demonstrate the importance of 3D unidirectionally aligned hierarchical supramolecular structures, we recently reported that a photoresponsive hierarchical supramolecular assembled structure derived from an amphiphilic molecular motor with precise control of molecular organization and cooperativity allows energy conversion, accumulation of strain, and amplification of the molecular rotation of motor amphiphile (MA) to macroscopic muscle-like contractive motions.[44] This supramolecular approach provides a complementary method to the existing macroscopic actuators obtained by stimuli-responsive crystals,[45−48] polymeric gels,[49−52] and polymeric liquid crystals.[53−59]

The dynamic nature of the supramolecular polymers provides inherent sensitivity of the assembled structure to the external environment, for example, chemicals, solvents, external shear force, electric and magnetic fields. In addition, the design of molecular amphiphiles allows precise control over the hierarchical structure from microscopic to macroscopic length-scale and their intrinsic functions.[34,44] Notably, electrostatic screening of amphiphilic self-assembled structures, pioneered by Stupp et al., by careful choice of counterions, provides a mean to control stiffness of 3D randomly entangled supramolecular structures[60−62] and has enabled to govern important functions, for example, cell proliferation, differentiation, adhesion, and migration.[63−65] However, the control of 3D unidirectionally aligned hierarchical supramolecular structure by a single non-invasive external stimulus, without covalent chemical modification of the molecular amphiphiles structure, at different length-scale and as a consequence its function remains highly challenging.

In our recently reported artificial muscle, the electrostatic interaction between carboxylate groups of MA and Ca2+ allows the MA nanofiber stabilization and the formation of a MA macroscopic string using a shear flow method to provide unidirectionally aligned MA strings for photoactuation. We envisioned that by manipulating the electrostatic interaction of the carboxylate groups of MA and its counter-cations (M) allows further control of the hierarchical assembled structure of the motor amphiphile and elucidates key parameters for supramolecular aggregation (Figure ). The nature of the cationic counterion effect on the organization of MA might enable the control of induction of nanofibers formation, aggregation of nanofibers, structural order parameters of the unidirectionally aligned structures, and speed of photoactuation of the string of unidirectionally aligned nanofibers. Additionally, the side chains of MA are modified to provide insight in the effect of the chain length effect on the structure and functioning of MA macroscopic strings. By elucidating the key design of supramolecular muscles, ultimately, this could open up new prospects toward the development of controllable stimuli-responsive materials and future soft robotic systems.

Figure 1

Molecular structures of molecular motor amphiphiles and the hierarchical organization and photoactuation process of their assembled structures in the macroscopic string.

Results and Discussion

Molecular Design and Synthesis

The motor amphiphile was designed with a second-generation molecular motor core, and a dodecyl chain was attached to the upper half, and two carboxyl groups connected with alkyl-linkers to the lower half (Figure ). In addition to the countercation effect, we envision that various chain lengths of the alkyl-linker, which connected the two carboxyl groups to the lower half of the motor unit, allow for systematic modification of the packing in the resultant MA string and its actuation function. Motor amphiphiles with shorter chain lengths, MA and MA, as well as longer chain lengths, MA, were designed (Figure ). The general synthesis is summarized in Scheme . Compounds 1 and 4 were prepared by our reported procedures[44] (Scheme ). The key step in the synthesis of MA, MA, and MA is the formation of the central overcrowded olefinic bond by diazo-thioketone coupling (Scheme ). The precursors 2 were obtained by Williamson ether formation of thioxanthone 1 with the corresponding alkyl bromides in the presence of K2CO3 in DMF, followed by conversion into the corresponding thioketones 3 with Lawesson’s reagents in toluene. Hydrazone 4 was in situ oxidized with diacetoiodobenzene in DMF into the corresponding diazo compound, and subsequent addition of the freshly prepared thioketones 3 provided the corresponding episulfides 5. Desulfurization with triphenylphosphine in toluene gave the corresponding overcrowded alkenes 6. The motor amphiphiles MAs were obtained by hydrolysis of the ester groups into carboxylic acid groups in the presence of LiOH in water and THF. The structures of all new motor amphiphiles were unambiguously determined by 1H, 13C NMR, and high-resolution ESI-TOF mass spectrometry (Figures S15–S37).

Scheme 1

Synthesis of Motor Amphiphiles

Reagents and conditions: (a) BrC6H12COOMe, BrC8H16COOMe, BrC10H20COOMe, or BrC11H22COOMe, K2CO3, DMF, 85 °C, 16 h; (b) Lawesson’s reagent, toluene, 100 °C, 1 h. Yields and detailed procedures are provided in Supporting Information.

Ionic Effect of MA Assembled Structure

Freshly prepared aqueous solutions of MA with 2 equiv of sodium hydroxide were heated at 80 °C for 30 min and cooled down to room temperature to afford a colorless transparent solution, indicating that the deprotonated form is soluble up to 50.0 mM concentration. A Nile Red fluorescence assay (NRFA), which probes the internal hydrophobicity of assemblies,[66] revealed a decrease in blue shift when diluting beyond 0.01 mM and showed a critical aggregation concentration (CAC) of 2.67 μM (Figure a). The MA assemblies formed by the water-soluble carboxylates were imaged using cryogenic transmission electron microscopy (cryo-TEM) to capture their solution-state morphologies. MA assembled into fibers hundreds of nanometers to micrometers in length at 5.0 mM concentration (Figure S1a), while no nanostructure was observed below CAC (Figure S1b). To investigate the countercation effect of LiCl, NaCl, KCl, BeCl2, MgCl2, CaCl2, SrCl2, BaCl2, and ScCl3 on nanofiber formation of the MA, NRFA was employed to probe the change of internal hydrophobicity of the MA assembly. The blue shift of Nile Red was monitored in the MA solutions (1.01 μM; below CAC) with various concentrations of CaCl2 (0.01–15.0 mM) (Figure b). A gradual increase of the blue shift of the MA assembly was observed with increasing concentration of CaCl2 reflecting an increase of the internal hydrophobicity. At 1.0 mM of CaCl2, nanofibers formed hundreds of nanometers to micrometers in length (Figure S1c). The results suggested that the excess amount of counterion (Ca2+) promotes the formation of nanofibers below CAC. An increase of internal hydrophobicity of MA assembly for other metal chloride solutions was only observed above 1.0 mM, indicating that Ca2+ ions induce nanofiber formation of MA more effectively than the other ions (Figure b). Surprisingly, also Mg2+ ions (above 1.0 mM) induced nanofiber formation effectively. However, the high charge density Be2+ and Sc3+ as well as large ionic radii Sr2+ and Ba2+ ions showed no significant effect on nanofibers formation. Similarly, high concentration of LiCl, NaCl, and KCl showed no significant nanofiber formation.

Figure 2

Nile Red fluorescence assay (a) for determination of the critical aggregation concentration of MA (concentration: 5.0 × 10–5 to 2.0 mM) and (b) for determination of countercation effect to MA (1.01 μM; below CAC) nanofiber formation concentration with various metal chlorides (concentration: 0.01–15.0 mM).

The complementary ionic interaction between the carboxylate moieties of MA and its counterions induces the aggregation of nanofibers. Dynamic light scattering (DLS) was then employed to investigate the aggregation of MA nanofibers with its counterions (concentration: 0.01 mM to 15.0 mM).[66] The molar scattering intensity of MA solution (1.01 mM; above CAC) gradually increased with the concentration of CaCl2, indicating that aggregation of nanofibers occurred (Figure ). At 15.0 mM of CaCl2, 145.3 ± 13 M Counts s–1 M–1 molar scattering intensity was observed. A lower molar scattering intensity 45.9 ± 3 M Counts s–1 M–1 was obtained in MgCl2 solutions (15.0 mM). Comparable molar scattering intensities of 26.7 ± 1 M Counts s–1 M–1, 27.1 ± 1 M Counts s–1 M–1, and 26.3 ± 1 M Counts s–1 M–1 were obtained in BeCl2, SrCl2, and BaCl2 solutions (15.0 mM), respectively. Considering the binding affinity between carboxylate and M2+, Ca2+ is expected to provide more significant aggregation.[67] However, the triple-charged Sc3+ (15.0 mM) afforded aggregates with a comparable molar scattering intensity 29.4 ± 2 M Counts s–1 M–1 to that of aggregates based on Be2+, Sr2+, and Ba2+ ions, possibly due to charge mismatch to MA in the nanofiber structure. No significant aggregation was observed with LiCl, NaCl, and KCl adding up to 15.0 mM (Figure ).

Figure 3

Molar scattering intensity of MA nanofibers (1.01 mM; above CAC) in the presence of metal chlorides (concentration: 0.01 mM to 15.0 mM).

To further investigate the counterion effect (metal chlorides) on the interfibrillar interaction and the structure features of MA nanofibers, a macroscopic string of MA was prepared according to our previous reported procedure.[44] Typically, a 50.0 mM solution of MA was manually drawn into an aqueous solution of MgCl2 (150 mM) from a pipet, and a noodle-like string with an arbitrary length was formed. Scanning electronic microscopy (SEM) of the string, prepared from the solution of MgCl2, shows arrays of unidirectionally aligned nanofiber bundles (Figure a), which was essentially identical to that of the string prepared from CaCl2 solution (Figure S2a,b). Notably, the MA strings prepared from the solutions of BeCl2, SrCl2, and ScCl3 also showed similar morphology, that is, arrays of nanofiber bundles with unidirectional alignment, in SEM images (Figures S3a, S4a, and 4b), while no alignments were observed in the MA strings prepared form solutions of BaCl2, LiCl, NaCl, and KCl (Figures S5a, S6a, S7a, and 4c). The freshly prepared MA string from a MgCl2 solution showed uniform birefringence in the direction of the strings long axis in polarized optical microscopy (POM) images (Figures d and S8a), which is essentially identical to POM images of the MA string obtained from CaCl2 solution (Figure S2c). A lower birefringence was observed in the POM images of the MA strings prepared from solutions of BeCl2, SrCl2, and ScCl3 (Figures S3b, S4b,4e, and S8b). In addition, no birefringence was observed in the POM images of the MA strings prepared from solutions of BaCl2, LiCl, NaCl, and KCl (Figures S5b, S6b, S7b, 4f, and S8c). The results indicated that MA nanofibers are aligned unidirectionally in the presence of Mg2+ and Ca2+ ions, while a lower degree of alignment in MA nanofibers is found in the presence of Be2+, Sr2+, and Sc3+ ions. However, no significant alignment of MA nanofibers was observed in the presence of Ba2+, Li+, Na+, and K+ ions.

Figure 4

SEM images of a macroscopic aligned string composed of MA prepared from solutions of (a) MgCl2, (b) ScCl3, and (c) KCl (150 mM). Optical microscopic images of a macroscopic aligned string composed of MA prepared from solutions of (d) MgCl2, (e) ScCl3, and (f) KCl (150 mM) under crossed polarizers. The POM images of the string were tilted at 45°, 135°, 225°, and 315° relative to the transmission axis of the analyzer, the  scale bar applys for all POM images. 2D SAXS images of a macroscopic aligned string composed of MA prepared from solutions of (g) MgCl2, (h) ScCl3, and (i) KCl (150 mM) (inset: enlarged 2D image for q = 0.1–0.45 nm–1 at 25 °C. 1D SAXS patterns (j) MgCl2, (k) ScCl3, and (l) KCl of 2D SAXS images in (g) MgCl2, (h) ScCl3, and (i) KCl, respectively, showing the diffraction pattern in the direction perpendicular to long axis of the string.

To provide the structural parameters and orientational order, that is, degree of alignment, of the MA nanofibers in the macroscopic strings, we carried out through-view small-angle X-ray scattering (SAXS) measurements. In the 2D SAXS image of the MA string prepared from MgCl2 solution on a sapphire substrate at 25 °C (Figure g), a pair of spot-like scatterings is observed in a smaller-angle region (q = 0.1–0.45 nm–1) (Figure g, inset), which is due to scatterings from the unidirectionally aligned nanofiber bundles. The diffraction arcs with d-spacings of 5.23, 2.58, and 1.75 nm (Figure j), arising from the diffractions from the (001), (002), and (003) planes, respectively, of a lamellar structure, which is constructed by the unidirectionally aligned nanofibers of MA with ionic interaction between Mg2+ and carboxylates of MA as interfibrillar interaction. The layer spacing of the lamellar structure (c = 5.23 nm) of the MA string prepared from MgCl2 solution is shorter than that of observed MA string prepared from CaCl2 solution (c = 5.48 nm) (Figure S2d,e), indicating that Mg2+ ions induce a closer packing in the nanofibers of MA. The angular dependency of the peak intensity of the diffraction from the (001) plane converted from the through-view 2D SAXS image of the MA string prepared from MgCl2 solution (Figure g) showed the intensity maxima at 0° and 180° (Figure S9a). The peak intensity of the diffraction from the (001) plane was quantified by full-width half-maximum (fwhm) to obtain an ∼95° azimuthal angle, with a smaller azimuthal angle representing a larger degree of unidirectional alignment (Table ).[68] Indeed, a smaller fwhm (∼65°) was observed in the MA string prepared from CaCl2 solution (Figure S9b and Table ), indicating that a higher degree of alignment of MA nanofibers is obtained in CaCl2 solution.

Table 1

Structural Parameters and Actuation Speed of MA Strings Prepared with Metal Chlorides (M)

aMA/Mn+	d001 (nm)	d002 (nm)	d003 (nm)	fwhm (°)a	actuation speed (°/s)b
MAC10/Be2+	4.99	–	–	124	1.02 ± 0.1
MAC10/Mg2+	5.23	2.58	1.75	95	4.29 ± 0.4
MAC10/Ca2+	5.48	2.70	1.82	65	7.94 ± 0.4
MAC10/Sr2+	5.69	–	–	115	1.77 ± 0.3
MAC10/Ba2+	5.77	–	–	isotropic	0.0
MAC10/Sc3+	5.07	2.53	1.68	150	1.33 ± 0.1
MAC10/Li+	6.72	–	–	isotropic	0.0
MAC10/Na+	6.84	–	–	isotropic	0.0
MAC10/K+	6.88	–	–	isotropic	0.0
MAC8/Ca2+	5.38	–	–	110	1.84 ± 0.2
MAC11/Ca2+	5.71	2.80	1.89	108	2.21 ± 0.1

Full-width half-maximum (fwhm).

MA/M samples prepared and photoactuation speed experiments performed, which in all cases were determined from the bending process of a string with a saturated flexion angle of 90° within a particular time, as described in the Supporting Information.

A pair of spot-like scatterings was observed in a smaller-angle region (q = 0.1–0.45 nm–1) in the 2D SAXS image of the MA string prepared from solutions of ScCl3, BeCl2, and SrCl2 (Figures h, S3c, and S4c, inset). The d-spacings of the diffraction arcs from the (001), (002), and (003) planes are summarized in Table . In accordance with the ionic radii of the cations, smaller layer spacings of a lamellar structure (c = 5.07 nm and c = 4.99 nm) were observed in the MA strings prepared from solutions of ScCl3 and BeCl2, respectively (Figures k and S3d) compared to those observed with Ca2+ and Mg2+(Table ). Meanwhile, a larger layer spacing of a lamellar structure (c = 5.69 nm) was observed in the MA strings prepared from solutions of SrCl2 (Figure S4d, Table ). In good agreement with the POM results, a lower degree of alignment was observed in the MA strings prepared from solutions of ScCl3, BeCl2, and SrCl2 (fwhm >110°, Table , Figure S9).

Consistent with the results obtained in SEM and POM analysis, no spot-like scatterings in a smaller-angle region (q = 0.1–0.45 nm–1) and an isotropic ring of (001) diffraction plane were observed in the 2D SAXS images of the MA strings prepared from solutions of BaCl2, LiCl, NaCl, and KCl (Figures S5c, S6c, S7c, and 4i). The results indicated the lack of alignment of MA nanofibers in the presence of Ba2+, Li+, Na+, and K+ ions. On the basis of monocharged ions, larger layer spacings of lamellar structures (c = 6.72, 6.84, 6.88 nm) were observed in the MA strings prepared from solutions of LiCl, NaCl, and KCl, respectively (Figures S6d, S7d, and 4l, Table ). In accordance with the ionic radius of double-charged Ba2+ ion, a larger layer spacing of the lamellar structure (c = 5.77 nm) was observed in the MA strings prepared from solutions of BaCl2 (Figure S5d, Table ). In summary, the order for degree of unidirectional alignment in the MA strings prepared form metal chloride is Ca2+ > Mg 2+ > Be2+ ≈ Sr2+ ≈ Sc3+ > Ba2+ ≈ Li+ ≈ Na+ ≈ K+. A similar order of binding constants between low molecular weight organic carboxylates and alkali/alkaline earth metals has been described in the literature.[67] For instance, the binding constant of succinate-alkali/alkaline earth metal complexes follows the order Ca2+ > Mg2+ > Sr2+ > Ba2+ > Li+ ≈ Na+ ≈ K+.[67] The results indicated that a higher binding constant of MA and its counterions allows a structurally more ordered macroscopic string formation.

Ionic Effect of MAC10 Actuation Speed

According to the anticipated photoactuation mechanism of MA,[44] the photochemical isomerization of MA from the stable isomer to the unstable isomer induces the actuation of the MA string toward the light source(Figure a). With a comprehensive structural investigation of MA strings prepared from various metal chlorides solutions, the resultant hierarchical supramolecular structure, which seems to be to a large extent governed by the electrostatic interaction of M and carboxylate groups of MA, would be expected to control the actuation speed of a MA string. Next, a freshly prepared MA string was studied in a cuvette containing an aqueous solution of CaCl2 (150 mM). Upon photoirradiation (λ = 365 nm, power output 15.5 mW), the MA string bent toward the light source from an initial angle of 0° to a saturated flexion angle of 90° within 15 s, indicating that the actuation speed is 7.94 ± 0.4°/s (Figure a,b). It should be noted that a higher power output was employed (0.7 A applied current) than in our previous study (0.2 A applied current, actuation speed = 1.5°/s),[44] providing a wider measuring window of actuation speed investigation. The MA string, prepared from MgCl2 solution (150 mM), bent with a saturated flexion angle of 90° within 25 s (4.29 ± 0.2°/s). Based on the degree of alignment of MA strings prepared from solutions of CaCl2 and MgCl2 (Table ), consistently, a higher degree of alignment of MA strings provided a faster actuation toward the light source. Meanwhile, a comparable degree of alignment in the MA strings prepared from solutions of ScCl3, BeCl2, and SrCl2, showed a similar photoactuation speed (1.0–1.8°/s, Figure b and Table ). In addition, the MA strings prepared from solutions of BaCl2, LiCl, NaCl, and KCl revealed no alignment, and no actuation was observed upon photoirradiation (Figure b and Table ). In general, by choosing a particular metal chloride to prepare the MA string, control over the degree of alignment of the MA string and its structural packing was achieved to provide a means to control the actuation speed.

Figure 5

(a) Photoisomerization step of MA and Photoactuation of a string prepared from MA solution (50 mM), after irradiation with 365 nm light source. A single enantiomer is shown. Scale bar, 5.0 mm. (b) The actuation speed (°/s) of the MA string (50.0 mM) prepared from metal chloride solutions (150 mM).

Chain Length Effect of MA Structure and its Actuation

To further elucidate key structural parameters, subsequently self-assembled structures based on motor amphiphiles with different chain length, that is, MA, MA, and MA, and their actuation speed were studied. The photochemical isomerization steps of MA, MA, and MA were examined by 1H NMR and UV–vis spectroscopy (Figures S10 and S11). Essentially identical 1H NMR signal shifts were observed in CD2Cl2 solutions of MA, MA, and MA (Figure S10),[44] and upon extended irradiation time, photostationary states with an unstable/stable isomer ratios of 9:1 were formed in CD2Cl2 solutions of MA, MA, and MA. In UV–vis absorption studies of CH2Cl2 solutions of MA, MA, and MA, an isosbestic point at 327 nm over the course of irradiation indicated that a comparable and selective photoisomerization process occurs (Figure S11). In accordance with the sample preparation method as for MA (vide supra), freshly prepared aqueous solutions of MA, MA, and MA with 2.0 equiv of NaOH were heated at 80 °C for 30 min and cooled down to room temperature to afford colorless transparent solutions, showing that the deprotonated form is soluble up to 50.0 mM concentration. Nanofibers of MA, MA, and MA (1.0 mM) were observed by cryo-TEM with uniform diameter (∼5–6 nm) and several micrometers in length (Figure S12). To provide robust and stable macroscopic MA strings, 50.0 mM solutions of MA or MA were manually drawn into an aqueous solution of CaCl2 (150 mM) from a pipet, and a noodle-like string with an arbitrary length formed, but no aligned string was formed from the solution of MA, possibly due to an unstable macroscopic structure formed upon addition of CaCl2 (Figure S13). SEM images of the MA and MA strings prepared from the solution of CaCl2, showed arrays of unidirectionally aligned nanofiber bundles, (Figure a,b), which are essentially identical to that of the MA string (Figure S2a). The freshly prepared MA and MA strings showed a lower birefringence in the direction of their long axis in POM images (Figures c,d and S14) compared to that of observed in the MA string (Figure S2c). The structural parameters and degree of alignment of the MA and MA nanofibers in the macroscopic string were again analyzed by SAXS measurement. The 2D image of the MA string prepared from CaCl2 solution on a sapphire substrate at 25 °C (Figure e,f) revealed a weak pair of spot-like scatterings in a smaller-angle region (q = 0.1–0.45 nm–1) (Figure e, inset), and the diffraction arc of the (001) plane with d-spacing of 5.38 nm of a lamellar structure was observed (Figure g). Higher order diffraction planes (002) and (003) were found in the MA string prepared from a CaCl2 solution (Figure f,h). The layer spacing of the lamellar structure (c = 5.38 nm) of the MA is shorter than that of the MA string (c = 5.48 nm) (Figure S2e) and MA string (c = 5.71 nm), indicating that the shorter alkyl-linker in MA induces a closer packing of the MA nanofibers in the corresponding macroscopic string. In good agreement with POM results, lower degrees of alignments were observed in the MA (fwhm = 110°) and MA (fwhm = 108°) strings (Table , Figure S9). Upon photoirradiation, the freshly prepared MA and MA strings bent toward the light source from an initial angle of 0° to a saturated flexion angle of 90° with an actuation speed of 1.84 ± 0.2°/s and 2.21 ± 0.1°/s, respectively, which were slower than that seen for the MA string prepared in the aqueous CaCl2 solution (Table ). These results clearly demonstrate that the structure of motor amphiphile is crucial to the macroscopic responsive behavior and there is a distinct effect of chain length going from n = 6 (no actuation) to n = 11 (slower actuation).

Figure 6

SEM images of a macroscopic aligned string composed of (a) MA and (b) MA prepared from an aq. CaCl2 solution (150 mM). Optical microscopic images of a macroscopic aligned string composed of (c) MA and (d) MA prepared from a solution of CaCl2 (150 mM) under crossed polarizers. The POM images of the string are tilted at 45°, 135°, 225°, and 315° relative to the transmission axis of the analyzer, the scale bar applys for all POM images. 2D SAXS images of a macroscopic aligned string composed of (e) MA and (f) MA prepared from a aq. solution of CaCl2 (150 mM) (inset: enlarged 2D image for q = 0.1–0.45 nm–1 at 25 °C. 1D SAXS patterns (g) MA and (h) MA of 2D SAXS images in (e) MA and (f) MA, respectively, showing the diffraction pattern in the direction perpendicular to long axis of the string.

Conclusion

Motor amphiphiles with various chain lengths at the lower half of the motor moiety were synthesized and probed for their self-assembly properties. Nanofibers of MA, MA, MA, and MA in water were observed by cryo-TEM. As shown by NRFA, calcium ions enhanced the formation nanofibers of MA dramatically, while other ions were shown less effective. DLS measurements were consistent with NRFS showing that the calcium-ion-induced nanofiber aggregation of MA is more efficient than with the other ions used in present study. By applying a shear flow method, macroscopic strings of MA prepared in the presence of calcium and magnesium ions provided a higher degree of alignment which facilitated a faster response to light during photoactuation. The current approach demonstrates the potential of generating muscle-like functions with distinct mobility, allowing access to multiple photoresponsive supramolecular actuation systems from identical molecular structure. We envisage that a permanent macroscopic motion powered by light might be feasible by employing a molecular motor with a lower barrier for the thermal helix inversion step, and studies toward such systems are currently in progress.