Dynamic Assemblies of Molecular Motor Amphiphiles Control Macroscopic Foam Properties.

Stimuli-responsive supramolecular assemblies controlling macroscopic transformations with high structural fluidity, i.e., foam properties, have attractive prospects for applications in soft materials ranging from biomedical systems to industrial processes, e.g., textile coloring. However, identifying the key processes for the amplification of molecular motion to a macroscopic level response is of fundamental importance for exerting the full potential of macroscopic structural transformations by external stimuli. Herein, we demonstrate the control of dynamic supramolecular assemblies in aqueous media and as a consequence their macroscopic foam properties, e.g., foamability and foam stability, by large geometrical transformations of dual light/heat stimuli-responsive molecular motor amphiphiles. Detailed insight into the reversible photoisomerization and thermal helix inversion at the molecular level, supramolecular assembly transformations at the microscopic level, and the stimuli-responsive foam properties at the macroscopic level, as determined by UV-vis absorption and NMR spectroscopies, electron microscopy, and foamability and in situ surface tension measurements, is presented. By selective use of external stimuli, e.g., light or heat, multiple states and properties of macroscopic foams can be controlled with very dilute aqueous solutions of the motor amphiphiles (0.2 weight%), demonstrating the potential of multiple stimuli-responsive supramolecular systems based on an identical molecular amphiphile and providing opportunities for future soft materials.

Introduction

Controlled supramolecular assembly of biomolecules into functional structures with low structural fluidity, e.g., cytoskeleton filaments,[1,2] flagellar filaments of bacteria,[3,4] and high structural fluidity, e.g., cell membranes,[5,6] serve key roles in correct functioning of biological processes. Inspired by natural supramolecular polymers,[1−4] the delicate structural tunability and stimuli-responsiveness of synthetic supramolecular polymers[7−16] in aqueous media allow sophisticated bioinspired functionality.[8−19] A molecular design with precise control of molecular organization and cooperativity enables energy conversion and amplification from nanometer to macroscopic length scales and induces a mechanical response[20−30] in hierarchical supramolecular assemblies.[31−38] Stimuli-responsive amphiphilic molecules can assemble from one-dimensional (1D) systems with low structural fluidity at microscopic length scales to various three-dimensional (3D) macroscopic supramolecular structures, to gain full control of macroscopic structural transformations by the cooperative action of molecular motion.[39−41] For instance, using external stimuli, e.g., pH,[38] heat,[42] and light,[43−48] the isomerization of stimuli-responsive molecules can induce a macroscopic rupture of the 3D randomly entangled supramolecular structures from a gel state into a solution state. In addition to gel–sol transformations, we recently reported that a photoresponsive hierarchical supramolecular macroscopic string of a motor amphiphile allows the amplification of molecular rotation to macroscopic muscle-like contraction.[31−33] However, macroscopic structural transformations and collective behavior usually require a high concentration of stimuli-responsive units as well as high structurally ordered and rigid supramolecular assemblies.[31−33,38,42] Alternatively, as inspired by nature, e.g., the cell membrane, a supramolecular assembly with high structural fluidity can potentially be constructed with a minimized amount of stimuli-responsive units that might allow a more effective energy conversion and amplification from nanometer length-scale motion to macroscopic structural transformations. Foams are a particularly attractive class of soft materials with applications ranging from biomedical systems to industrial processes. Particularly important for textile coloring, photoresponsive foams are highly warranted. By purging aqueous solutions of an amphiphilic molecular structure (in general below 1.0 weight%) with air, macroscopic foams with high structural fluidity are generated by dispersing air bubbles in the aqueous phase.[49−51] The foam structure spans over multilength scales from nanometer dimensions to macroscopic levels.[51] To provide control of macroscopic foam properties, e.g., foamability and foam stability, advantage can be taken of the molecular isomerization of stimuli-responsive amphiphilic structures at molecular length scales.[52−63]

Recently, we and others have demonstrated photoresponsive foams of azobenzene amphiphiles[55−63] for controlling foam rupture upon irradiation. In an alternative approach, a coassembled lipid system showed photo- and thermal-responsive transformation based on heat produced upon irradiation.[64,65] The noninvasive and reversible control of molecular interactions and function in water over length scales from molecular motions to assembly transformations at the microscopic level and precise control of multiple-state foam properties at the macroscopic level remains challenging. The amphiphiles used to date typically have two states triggered by light, limiting the development of more subtle and precisely controllable foams with multiple states which is crucial for practical applications. Also, the coupling of several reversible isomerization processes in a single photoresponsive molecular system to dynamic assembly formation at the microscopic scale in order to identify the key processes for energy conversion from nanometer-length-scale motion to macroscopic structural transformations, i.e., macroscopic foam rupture, remains largely unexplored. We envisioned that the development of a robust stimuli-responsive amphiphile with multiple states and intrinsic foam properties would not only allow more sophisticated control over foamability but also reveal how to achieve dynamic regulation of assemblies in water and enable amplification along length scales.

Here we present a robust molecular motor-based light-responsive amphiphile that has the intrinsic propensity to trigger the formation of multiple states and assemblies in water and allows changes at the nanoscale to be translated to macroscopic properties, i.e., foam formation. To the best of our knowledge, this is also the first amphiphile where we have absolute and unprecedented control over the aggregation behavior, i.e., switching from worm-like structures to vesicles and back without helper lipids or effectuators to get out of a situation where the amphiphile is locked in an (kinetically stable) aggregate.

Molecular motors can be controlled with high precision by two external stimuli, light and heat, to sustain a unidirectional rotation involving four distinct isomerization states. We envision that molecular motor amphiphiles hold significant prospects for the development of multiple states macroscopic foams with a minimized amount of stimuli-responsive units as well as the study of essential parameters for amplification from nanometer-length-scale motion to macroscopic foam properties. We previously reported that motor amphiphiles composed of a second generation molecular motor core used as dopants into lipids provided supramolecular assembly transformations, but extra freeze–thaw cycles were required in the transformation processes.[66−68] Furthermore, the nanofibers of motor amphiphiles employed in artificial muscle-like motions showed no significant supramolecular assembly transformations upon photoirradiation.[31−33] To sustain a large geometrical transformation, the change in packing parameters between trans- and cis-isomers[69,70] is of fundamental importance for generating supramolecular assembly transformations from an identical molecular structure upon photoisomerization. In the present design, the molecular motor amphiphile contains a hydrophilic chain with a charged end group and a hydrophobic alkyl chain connected to a first-generation molecular motor core (Figure ). If the motor unit adopts a trans geometry, the two side chains will be remote from each other and we expect a lower packing parameter in the structure (e.g., worm-like micelles, 1/3 < P ≤ 1/2), while the cis form will bring the two side chains in close proximity, allowing for a higher packing parameter (e.g., vesicles, 1/2 < P ≤ 1).[69,70] Large geometrical transformations of motor amphiphiles might therefore enable supramolecular transformations, without coassembly with other lipids, for controlling macroscopic foam properties. By elucidating the key transformation processes and parameters governing assembly, this could open up new prospects toward the development of externally controlled stimuli-responsive materials.

Figure 1

Schematic illustration of (a) the reversible photoisomerization and thermal helix inversion of molecular motor amphiphile (MA) and (b) the change in macroscopic foaming processes due to structural transformations in the supramolecular assembly. The state 4 is obtained by using synthetically purified stable cis-MA.

Results and Discussion

Molecular Design and Synthesis

The motor amphiphile (MA) was designed with a first-generation molecular motor core, functionalized with an alkyl chain as the hydrophobic part and a quaternary ammonium moiety connected via a triethylene glycol-linker as the hydrophilic part. The synthesis is summarized in Scheme . Phenolic motor 1 and 1,2-bis(2-bromoethoxy)-ethane (3) were prepared according to previously reported procedures,[71,72] and the stable trans and stable cis isomers of motor 1 were isolated by flash column chromatography.[71] Motor 2 was obtained by a Williamson ether formation in the presence of 1-bromohexane, tetrabutylammonium iodide, and K2CO3 in acetonitrile. The freshly prepared motor 2 was treated with 1,2-bis(2-bromoethoxy)ethane (3) and Cs2CO3 in DMF to afford stable trans-4 and stable cis-4 in 43% and 50% yields, respectively. Subsequent reaction of motor 4 with trimethylamine gave the corresponding stable trans-MA and stable cis-MA in 90% yield. The detailed procedures of the synthesis are provided in the Supporting Information. The structures of all new molecules were confirmed by 1H, 13C NMR, and high-resolution ESI-MS (see Supporting Information, Figures S11–S18).

Scheme 1

Synthesis of Stable trans-MA and Stable cis-MA

Photoisomerization and Thermal Helix Inversion Steps to Control Packing Parameters of MA

For the first-generation motor core of MA, its 360° unidirectional rotary cycle involves four stages, in which one-half of the molecule rotates with respect to the other half around a central double bond via two photochemical isomerization processes and two thermal helix inversion (THI) steps (Figure S2). Because of the fast thermal isomerization between unstable trans-MA and stable trans-MA (<10 s at rt),[73,74] only the reversible photoisomerization from stable trans-MA to unstable cis-MA as well as the THI between unstable cis-MA and stable cis-MA are employed for the three-stage control of the modulation of molecular geometries and, as a consequence, packing parameter transformations (Figure ). First, the proper functioning of the motor amphiphiles trans-MA and cis-MA was studied in an organic solvent (see Figures S3a–c, S4a–c, S5, and Table S1). A methanol solution of stable trans-MA (30 μM) shows a strong absorption band at 260–340 nm in the UV–vis absorption spectrum (Figure S3a). A new absorption band appears at 340–400 nm with a clear isosbestic point at 330 nm upon 312 nm light irradiation for 6 min at 5 °C (Figure S3a), indicating a selective photoisomerization process from stable trans-MA to unstable cis-MA. The resulting solution, prepared by the photoisomerization from stable trans-MA to unstable cis-MA, showed the reverse switching process to stable trans-MA upon 365 nm light irradiation for 1 min (Figure S3b), or the selective rotation from unstable cis-MA to stable cis-MA by heating at 55 °C for 2 h via the THI step (Figure S3c). A CD3OD solution of stable trans-MA (2.0 mM) shows distinctive proton shifts in the 1H NMR spectra upon isomerization (Figure S4a–c). The proton signals of Ha (δ = 1.10 ppm) and Hb (δ = 2.18 ppm) shift downfield to δ = 1.23 ppm and δ = 2.60 ppm, respectively, while an upfield shift of Hc is observed with an unstable cis-MA/stable trans-MA isomer ratio of 1:1, upon 312 nm light irradiation for 6 min (Figure S4a,b). Notably, the obtained unstable cis-MA in the solution can be fully switched back to stable trans-MA with 365 nm light irradiation, or selectively isomerized to stable cis-MA by heating at 55 °C for 2 h via the THI step with a distinct set of proton shifts (Ha: δ = 1.23 ppm to 1.05 ppm, Hb: δ = 2.60 ppm to 2.41 ppm; Figure S4a–c). The rate constants of the thermal isomerization process from unstable cis-MA to stable cis-MA were studied by means of Eyring analysis (Figure S5), and a standard Gibbs energy of activation (Δ‡G° = 100.0 kJ mol–1) was obtained, which corresponds to a half-life of 10.6 h at 25 °C (Table S1). The results clearly demonstrated that the geometrical transformations could be controlled selectively by external stimuli light and heat in methanol.

Next, the motor amphiphiles were studied in aqueous media (Figures S3d–f, S4d–f, S6, 2, and S7). A Tris-EDTA buffer (pH 7.4) solution of stable trans-MA (30 μM) shows an absorption band at 260–340 nm in the absorption spectrum; however, only a weak absorption band is observed at 340–400 nm upon irradiating with 312 nm light for 6 min at 5 °C (Figure S3d). Consequently, an unstable cis-MA/stable trans-MA isomer ratio of only 1:22 is obtained in a buffer solution of stable trans-MA (2.0 mM) irradiated with 312 nm light for 6 min, as shown by the 1H NMR spectra (Figure S4d,e). As expected, only limited ratiometric changes in the absorption spectra and proton signal shifts in the 1H NMR spectra are observed in a buffer solution, containing the mixture of unstable cis-MA/stable trans-MA in a 1:22 ratio, upon heating at 55 °C for 2 h (Figures S3f and S4e,f). To allow a more efficient photoisomerization in aqueous media, a higher-energy light source was employed (λmax = 254 nm). A higher unstable cis-MA/stable trans-MA isomer ratio of 1:13 was established upon 254 nm light irradiation for 6 min of a buffer solution of stable trans-MA (2.0 mM) at 5 °C by 1H NMR analysis (Figure S6a). Furthermore, the photoisomerization and THI steps of a buffer solution of stable trans-MA (30 μM) were followed by UV–vis spectroscopy at 5 °C (Figures a and S6b–d). As expected, upon increasing the temperature during irradiation at 254 nm from 5 to 25 °C, a higher unstable cis-MA/stable trans-MA isomer ratio of 1:6 is observed based on 1H NMR analysis (Figure b,c), which is higher than that observed in a buffer solution of stable trans-MA upon 312 nm light irradiation at 25 °C for 6 min (unstable cis-MA/stable trans-MA isomer ratio of 1:12; Figure S7a).

Figure 2

(a) UV–vis absorption spectra during the 180° rotation process of MA (30 μM), from stable trans-MA to stable cis-MA in aq. buffer solution (Tris-EDTA, pH 7.4). Aromatic and aliphatic region in the 1H NMR spectra during the isomerization process of MA (2.0 mM, CD3OD, 25 °C, 500 MHz). (b) A buffer solution of stable trans-MA was (c) irradiated with 254 nm light at 25 °C for 6 min, (d) followed by heating at 55 °C for 24 h (see also Figure S4). The buffer solutions after irradiation or THI step were subjected to a freeze-drying process, and the compound was dissolved in CD3OD for 1H NMR studies. For the proton assignment, see Figure .

It should be noted that the ratio of 1:6 was obtained upon irradiation for only 6 min, not reaching the photostationary state, but this was sufficient to induce significant self-assembly transformations and macroscopic foam rupture. Furthermore, similar to isomerization in methanol, the obtained unstable cis-MA in buffer solution, prepared by the photoisomerization of the stable trans-MA solution with 254 nm light irradiation for 6 min, was able to switch back to stable trans-MA by 365 nm light irradiation (Figure S7b), or to convert selectively to stable cis-MA by heating at 55 °C for 24 h via the THI step (Figure c,d). The results indicate that molecular changes can be controlled selectively by external stimuli not only in organic solvents but also in water, allowing for further investigations of molecular geometrical transformations at higher length scales in aqueous media.

Supramolecular Assembly Transformation at Microscopic Length Scales

Freshly prepared Tris-EDTA buffer solutions (pH 7.4) of stable trans-MA (2.0 mM) were diluted into a range of concentrations from 2.0 × 10–5 to 2.0 mM for the determination of the critical aggregation concentration (CAC) by using a Nile Red fluorescence assay (NRFA), which probes the internal hydrophobicity of the assemblies.[32,75,76] A significant decrease in blue shift is observed when the concentration of the stable trans-MA buffer solution is diluted below 0.1 mM (Figure ). The CAC of the stable trans-MA buffer solution was determined as 5 μM (Figure ). It is noted that the concentrations in the buffer solutions of stable trans-MA used in UV–vis absorption (30 μM) and 1H NMR spectroscopic studies (2.0 mM) were higher than its CAC. The freshly prepared buffer solution of stable trans-MA (2.0 mM) was irradiated with 254 nm light for 6 min at 25 °C, whereupon the CAC of the resulting solution of MA increased to 12 μM (Figure ). The results imply that a possible transformation of the supramolecular assembly occurs upon irradiation in the buffer solution of MA.

Figure 3

Nile Red fluorescence assay for the determination of the critical aggregation concentration (CAC) of buffer solutions of stable trans-MA before and after irradiation with 254 nm light for 6 min at 25 °C (concentration: 2.0 × 10–5 to 2.0 mM).

The assemblies of MA were imaged using cryogenic transmission electron microscopy (cryo-TEM) to investigate their solution-state morphologies. Worm-like micelles with hundreds of nanometers to micrometers in length are observed by the cryo-TEM image of the buffer solution of stable trans-MA (concentration: 2.0 mM, Figure a). A mixture of worm-like micelles/vesicles is observed in the cryo-TEM image of the same solution after 254 nm light irradiation for 6 min at 25 °C (Figure b). An identical sample was employed in 1H NMR study (Figure c), indicating that the presence of only ∼15% of unstable cis-MA in the mixture with stable trans-MA was able to induce a drastic transformation of the supramolecular assembly of MA in aqueous media. Notably, the supramolecular assemblies of the mixture of worm-like micelles/vesicles remain stable for 4 h at 25 °C without illumination (Figure S8a–c), but can be reversibly converted back to worm-like micelles after exposure to 365 nm light for 6 min (Figure c). An identical sample was used in 1H NMR study (Figure S7b), which demonstrated that the conversion of the mixture of worm-like micelles/vesicles back to worm-like micelles was attributed to the reversible geometrical transformation from unstable cis-MA to stable trans-MA, upon 365 nm light irradiation for 6 min at 25 °C. Furthermore, the buffer solution of stable trans-MA irradiated with 254 nm light for 6 min and subsequently heated at 55 °C for 24 h shows a mixture of worm-like micelles/vesicles (Figure S8d). Although the obtained morphologies (Figure S8d) are essentially identical to those observed in the sample of stable trans-MA irradiated with 254 nm light for 6 min at 25 °C (Figure b), the obtained vesicle assemblies (Figure S8d) remain stable upon further irradiation with 365 nm light (Figure S8e). Moreover, a freshly prepared buffer solution of stable cis-MA (2.0 mM) shows vesicle assemblies (Figure d), similar to those observed in the sample of stable trans-MA irradiated with 254 nm light and subsequently heated at 55 °C for 24 h (Figure S8d), suggesting that the obtained mixture of worm-like micelles/vesicles (Figure S8d) is possibly composed of stable trans-MA and stable cis-MA. The results showed that the manipulation of molecular geometry with external stimuli can induce the transformations of packing parameters, allowing for the control of supramolecular assemblies from nanometers to hundreds of nanometers length scales systematically. Namely, four states of supramolecular assemblies can be achieved: (1) worm-like micelles from stable trans-MA only, (2) a mixture of worm-like micelles/vesicles from ∼15% of unstable cis-MA in stable trans-MA (fully reversible), (3) a mixture of worm-like micelles/vesicles from a mixture of stable cis-MA/stable trans-MA which cannot be switched back by 365 nm light irradiation, (4) vesicles from stable cis-MA only (Figures and 4).

Figure 4

Cryo-TEM images of a buffer solution of stable trans-MA (2.0 mM) (a) before and (b) after 254 nm light irradiation for 6 min at 25 °C, (c) followed by exposure to 365 nm light for 6 min at 25 °C. (d) A stable cis-MA buffer solution (2.0 mM).

Macroscopic Photoresponsive Foam

Amphiphilic molecules can assemble into supramolecular aggregates in solutions and monolayers at air–water interfaces.[49−51] It is noted that both the supramolecular aggregates and monolayers are highly dependent on the concentration and molecular structure. Therefore, we envisioned that motor amphiphiles in aqueous media and at air–water interfaces might allow the control of macroscopic phenomena by amplification of packing/organization due to geometrical changes. Stable foams were generated from a buffer solution of stable trans-MA (1.5 mM, 0.4 mL) by bubbling with argon gas (6 cm3 min–1) for 8 min in a quartz tube (Figure a and Movie S1). The buffer solution of stable trans-MA showed good foamability with a foaming ratio of ∼8.3. However, no stable foam was formed by the identical preparation method from a buffer solution of stable cis-MA (Figure a and Movie S1). The foaming ratio (R) was calculated from the equation, R = Vfoam/Vliquid, in which Vfoam and Vliquid referred to the foam volume and the original liquid volume to prepare foams, respectively.[63,77] A higher foaming ratio indicates a higher foamability. Drainage[51,78] is one of the main processes resulting in the destabilization of foams, which can be affected by the self-assembled structures in the bulk solutions. Given that long fibrillar assemblies can slow down the drainage process and no significant reduction of drainage process is observed with vesicle assemblies,[64,79,80] the worm-like structures in the solution of stable trans-MA can prove to be a stable macroscopic foam structure. In sharp contrast, no stable foam was observed in the solution of stable cis-MA. These promising results provided a hint for the control of macroscopic foam formation and the related foam parameters by using the four states of supramolecular assemblies of MA (vide infra).

Figure 5

(a) Foamability of stable trans-MA and stable cis-MA buffer solutions (1.5 mM, bubbling with a flow of argon gas of 6 cm3 min–1 for 8 min). (b) Foams, prepared from a solution of stable trans-MA (2.0 mM) by bubbling with a flow of argon gas (10 cm3 min–1 for 8 min), were kept at 25 °C for 20 min and then irradiated with 254 nm light for 6 min. (c) Foamability of stable trans-MA buffer solutions before and after irradiations and THI steps (2.0 mM, bubbling with a flow of argon gas of 10 cm3 min–1 for 8 min). Stable trans-MA buffer solutions (sample 1) were irradiated with 254 nm light for 6 min (sample 2). The resulting solutions were exposed to 365 nm light for 6 min (sample 3) or heated at 55 °C for 24 h (sample 4). Sample 4 was subsequently irradiated with 365 nm light for 6 min to obtain sample 5.

For a buffer solution of stable trans-MA (2.0 mM) by bubbling with a flow of argon gas (10 cm3 min–1) for 8 min (Figure b,c, sample 1), a foaming ratio of 13.5 (±0.3) is observed. It is noted that a very diluted stable trans-MA buffer solution (2.0 mM, containing 99.8 wt % of buffer and 0.2 wt % of MA) shows excellent foamability with a high foaming ratio while the foams remain stable over 20 min at 25 °C without any sign of foam rupture (Figure b). Notably, fast response of foam rupture, i.e., ∼57% of foam rupture, is observed after 254 nm light irradiation at 25 °C for 6 min (Figures b). The obtained solutions after foam rupture were subjected to a freeze-drying process and studied by 1H NMR (Figure S9a,b). An unstable cis-MA/stable trans-MA isomer ratio of 1:6 is obtained, which is essentially identical to that observed in the buffer solution of stable trans-MA irradiated with 254 nm light for 6 min at 25 °C (Figure b,c). The foam rupture is attributed to the supramolecular assembly transformations from worm-like micelles into the mixture of worm-like micelles/vesicles as imaged by cryo-TEM (Figure a,b). The solution obtained after foam rupture was bubbled with a flow of argon gas (10 cm3 min–1) for 8 min, which showed a lower foaming ratio of 8.6 ± 0.3 (Figure c, sample 2). Furthermore, the resulting solution of stable trans-MA irradiated with 254 nm light was subsequently exposed to 365 nm light for 6 min at 25 °C, generating stable foams again with a foaming ratio of 13.7 ± 0.3 (Figure c, sample 3), comparable to that observed in the solution of stable trans-MA (2.0 mM; Figure b,c; sample 1). Additionally, the obtained solution shows a clear reverse switching process from unstable cis-MA to stable trans-MA based on the 1H NMR spectra (Figure S9b,c). The restoration of foamability after sequential irradiation with 254 nm and then 365 nm light is attributed to the transformation from the state (2) a mixture of worm-like micelles/vesicles into the state (1) worm-like micelles, as seen in the cryo-TEM images (Figure b,c). Furthermore, the foams obtained from a solution of stable trans-MA after irradiation with 254 nm light and subsequent heating at 55 °C for 24 h show a foaming ratio of 8.9 ± 0.2 (Figure c, sample 4), which is comparable to that observed in the foams obtained from the solution of stable trans-MA after irradiation with 254 nm light (8.6 ± 0.3, Figure c, sample 2). However, when the 254 nm light irradiated solutions of stable trans-MA were subjected to a THI process (55 °C for 24 h), the foaming ratio of the resulting solution cannot switch back to the value of ∼13 by exposure to 365 nm light (Figure c, sample 5).

The results showed clearly that by alternating molecular geometry with light and heat stimuli, the macroscopic foaming ratio also can be finely controlled at four states, i.e., (1) foaming ratio of ∼13 from stable trans-MA only, (2) a reversible switching of foaming ratio between ∼8 and ∼13 from the photoisomerization of unstable cis-MA and stable trans-MA, (3) foaming ratio of ∼8 from a mixture of stable cis-MA/stable trans-MA which cannot be switched back by 365 nm light irradiation, (4) no stable foam from stable cis-MA only (Figure ). The four states of foaming ratio manipulation were consistent with the four states of supramolecular assembly transformations (Figure ), indicating the control of macroscopic foam properties by fine adjustment of supramolecular assemblies with external stimuli of light (254 and 365 nm) and heat.

Buffer solutions of stable trans-MA at 3.0 mM or 4.0 mM by bubbling with a flow of argon gas (10 cm3 min–1 for 8 min) show a comparable foaming ratio (Figure a,b) to the foams prepared from the stable trans-MA solution at 2.0 mM concentration (Figure b). The obtained foams, prepared from the stable trans-MA solutions (at 3.0 mM or 4.0 mM), remain stable over 20 min at 25 °C, and upon exposure to 254 nm light for 6 min, 31% or 14% of foam rupture is observed, respectively (Figure a,b). Moreover, the foaming properties can be reversibly tuned by alternating 254 nm light and 365 nm light irradiation over 10 cycles (Figure c).

Figure 6

Foams, prepared from solutions of stable trans-MA at (a) 3.0 mM and (b) 4.0 mM concentration by bubbling with a flow of argon gas (10 cm3 min–1 for 8 min), were kept at 25 °C for 20 min and then irradiated with 254 nm light for 6 min. (c) The change of foaming ratio of a MA buffer solution (2.0 mM) after 10 irradiation cycles by alternating 254 and 365 nm light.

To provide insight into the molecular packing of MA at the air–water interface, an in situ surface tension measurement was employed. The surface tension was determined by a drop-shape analysis system, which was based on the fitting of the pendant drop shape of the measured solutions with the Yong–Laplace equation of capillarity.[81−83] The detailed experimental setup and measurement conditions are presented in the Supporting Information (Figure S1). The surface tension of the stable trans-MA buffer solution (2 mM) remains stable at 33.1 mN/m (Figure ). A quick increase of surface tension from 33.1 to 33.9 mN/m was observed after irradiating the droplet of the stable trans-MA buffer solution with 254 nm light for 30 s. Furthermore, when 254 nm light irradiation was prolonged, the surface tension increased continuously to 35.3 mN/m until the droplet fell down at 5 min; thus the measurement was halted (Figure ). However, the surface tension of a Tris-EDTA buffer solution irradiated under identical conditions almost remains stable at ∼69.5 mN/m over 5 min (only 0.5 mN/m increase, Figure S10a). The surface tension of a droplet of the stable trans-MA buffer solution (2 mM) without 254 nm light irradiation also shows limited variation (only 0.5 mN/m increase) over 5 min (Figure S10b). The results clearly indicated that the significant increase of surface tension of the stable trans-MA buffer solution (up to 2.43 ± 0.27 mN/m) upon irradiation with 254 nm light was attributed to the photoisomerization of stable trans-MA to unstable cis-MA.

Figure 7

In situ surface tension measurement of a droplet of a stable trans-MA (2.0 mM) buffer solution irradiated with 254 nm light over 4.6 min until the droplet fell down at 5 min.

Following the photochemical isomerization of stable trans-MA to unstable cis-MA, a large geometrical transformation is observed, which in turn leads to a disturbance of molecular packing at air–water interfaces.[84] Based on the results of in situ surface tension measurements, indeed, a gradual increase of surface tension (up to 2.43 mN/m) was observed, which was attributed to the desorption of unstable cis-MA from the air–water interfaces to the bulk solution, suggesting that a similar desorption of unstable cis-MA from the air–water interfaces to the foam films and plateau borders[85] might occur (Figure ).[81−83] On the basis of supramolecular assembly transformations at the microscopic length scale, long fibrillar structures obtained from the solution of stable trans-MA result in the reduction of liquid drainage in foams, while a mixture of worm-like micelles/vesicles obtained from the solution of unstable cis-MA/stable trans-MA increases the destabilization of foams.[64,79,80] Furthermore, the corresponding transformation from worm-like micelles to the mixture of worm-like micelles/vesicles comes with an increase of CAC (Figure ), which possibly facilitates the desorption of unstable cis-MA from the air–water interface to the foam films and the plateau borders. In combination of the aforementioned variation of macroscopic foam parameters, as the ultimate result, foam rupture is observed.

Figure 8

Schematic illustration of supramolecular assembly transformations from stable trans-MA to unstable cis-MA upon 254 nm light irradiation at air–water interfaces and in plateau borders.

Conclusions

Amphiphilic motors to control foam properties were designed, and the reversible photoisomerization and thermal helix inversion were followed by UV–vis absorption and proton NMR spectroscopies. As shown by NRFA, a clear increase of CAC of stable trans-MA was observed after irradiation. By sequential control with light and heat, worm-like micelles of stable trans-MA, vesicles of stable cis-MA, and a mixture of worm-like micelles/vesicles of stable trans-MA/unstable cis-MA or stable trans-MA/stable cis-MA were obtained and analyzed by cryo-TEM. Our responsive system allows a systematic control of macroscopic foam properties and achieves a fine adjustment of foaming ratio over 10 cycles with a low content of MA in aqueous media (0.2 wt %). The current approach demonstrates the dual control of multiple state macroscopic foam properties by supramolecular assembly transformations based on light/heat responsive motor amphiphiles. We also identified the key processes and parameters of amplification from molecular motion to macroscopic structural transformations. Besides controlling macroscopic foam properties, it might open up new prospects to generate externally controlled stimuli-responsive soft materials.