Dissipative and Autonomous Square-Wave Self-Oscillation of a Macroscopic Hybrid Self-Assembly under Continuous Light Irradiation

Light-Fueled Nonreciprocal Self-Oscillators for Fluidic Transportation and Coupling

Light-fueled self-oscillators based on soft actuating materials have triggered novel designs for small-scale robotic constructs that self-sustain their motion at non-equilibrium states and possess bioinspired autonomy and adaptive functions. However, the motions of most self-oscillators are reciprocal, which hinders their use in sophisticated biomimetic functions such as fluidic transportation. Here, an optically powered soft material strip that can perform nonreciprocal, cilia-like, self-sustained oscillation under water is reported. The actuator is made of planar-aligned liquid crystal elastomer responding to visible light. Two laser beams from orthogonal directions allow for piecewise control over the strip deformation, enabling two self-shadowing effects coupled in one single material to yield nonreciprocal strokes. The nonreciprocity, stroke pattern and handedness are connected to the fluidic pumping efficiency, which can be controlled by the excitation conditions. Autonomous microfluidic pumping in clockwise and anticlockwise directions, translocation of a micro-object by liquid propulsion, and coupling between two oscillating strips through liquid medium interaction are demonstrated. The results offer new concepts for non-equilibrium soft actuators that can perform bio-like functions under water.

Autonomous Non-Equilibrium Self-Assembly and Molecular Movements Powered by Electrical Energy

The ability to exploit energy autonomously is one of the hallmarks of life. Mastering such processes in artificial nanosystems can open technological opportunities. In the last decades, light- and chemically driven autonomous systems have been developed in relation to conformational motion and self-assembly, mostly in relation to molecular motors. In contrast, despite electrical energy being an attractive energy source to power nanosystems, its autonomous harnessing has received little attention. Herein we consider an operation mode that allows the autonomous exploitation of electrical energy by a self-assembling system. Threading and dethreading motions of a pseudorotaxane take place autonomously in solution, powered by the current flowing between the electrodes of a scanning electrochemical microscope. The underlying autonomous energy ratchet mechanism drives the self-assembly steps away from equilibrium with a higher energy efficiency compared to other autonomous systems. The strategy is general and might be extended to other redox-driven systems.

Temporal Stimulus Patterns Drive Differentiation of a Synthetic Dipeptide-Based Coacervate

The fate of living cells often depends on their processing of temporally modulated information, such as the frequency and duration of various signals. Synthetic stimulus-responsive systems have been intensely studied for >50 years, but it is still challenging for chemists to create artificial systems that can decode dynamically oscillating stimuli and alter the systems' properties/functions because of the lack of sophisticated reaction networks that are comparable with biological signal transduction. Here, we report morphological differentiation of synthetic dipeptide-based coacervates in response to temporally distinct patterns of the light pulse. We designed a simple cationic diphenylalanine peptide derivative to enable the formation of coacervates. The coacervates concentrated an anionic methacrylate monomer and a photoinitiator, which provided a unique reaction environment and facilitated light-triggered radical polymerization─even in air. Pulsed light irradiation at 9.0 Hz (but not at 0.5 Hz) afforded anionic polymers. This dependence on the light pulse patterns is attributable to the competition of reactive radical intermediates between the methacrylate monomer and molecular oxygen. The temporal pulse pattern-dependent polymer formation enabled the coacervates to differentiate in terms of morphology and internal viscosity, with an ultrasensitive switch-like mode. Our achievements will facilitate the rational design of smart supramolecular soft materials and are insightful regarding the synthesis of sophisticated chemical cells.

Re-entrant Photoinduced Morphological Transformation and Temperature-Dependent Kinetic Products of a Rectangular Amphiphilic Diarylethene Assembly

An amphiphilic rectangular-shaped photochromic diarylethene bearing two hydrophobic alkyl chains and two hydrophilic tri(ethylene glycol) chains was synthesized, and its photoinduced morphological transformation in water was investigated. Two unexpected phenomena were revealed in the course of the experiments: a re-entrant photoinduced macroscopic morphological transformation and temperature-dependent kinetic products of supramolecular assembly. When the pure closed-ring isomer was dispersed in water, a re-entrant photoinduced morphological transformation, that is, a photoinduced transition from the hydrated phase to the dehydrated phase and then back to the hydrated phase, was observed by optical microscopy upon irradiation with green light at 20 °C; this was interpreted by the V-shaped phase diagram of the LCST transition. The aqueous assembly of the pure closed-ring isomer was controlled by changing the temperature; specifically, rapid cooling to 15 and 5 °C gave J and H aggregates, respectively, as the kinetic products. The thermodynamic product at both temperatures was a mixture of mostly H aggregate with a small amount of J aggregate. This behavior was rationalized by the temperature-dependent potential energy surface of the supramolecular assembly.

Mechanism of periodic field driven self-assembly process

Dissipative self-assembly, a ubiquitous type of self-assembly in biological systems, has attracted a lot of attention in recent years. Inspired by nature, dissipative self-assembly driven by periodic external fields is often adopted to obtain controlled out-of-equilibrium steady structures and materials in experiments. Although the phenomena in dissipative self-assembly have been discovered in the past few decades, fundamental methods to describe dynamical self-assembly processes and responsiveness are still lacking. Here, we develop a theoretical framework based on the equations of motion and Floquet theory to reveal the dynamic behavior changing with frequency in the periodic external field driven self-assembly. Using the dissipative particle dynamics simulation method, we then construct a block copolymer model that can self-assemble in dilute solution to confirm the conclusions from the theory. Our theoretical framework facilitates the understanding of dynamic behavior in a periodically driven process and provides the theoretical guidance for designing the dissipative conditions.

Photo-responsive Helical Motion by Light-Driven Molecular Motors in a Liquid-Crystal Network

Controlling sophisticated motion by molecular motors is a major goal on the road to future actuators and soft robotics. Taking inspiration from biological motility and mechanical functions common to artificial machines, responsive small molecules have been used to achieve macroscopic effects, however, translating molecular movement along length scales to precisely defined linear, twisting and rotary motions remain particularly challenging. Here, we present the design, synthesis and functioning of liquid‐crystal network (LCN) materials with intrinsic rotary motors that allow the conversion of light energy into reversible helical motion. In this responsive system the photochemical‐driven molecular motor has a dual function operating both as chiral dopant and unidirectional rotor amplifying molecular motion into a controlled and reversible left‐ or right‐handed macroscopic twisting movement. By exploiting the dynamic chirality, directionality of motion and shape change of a single motor embedded in an LC‐network, complex mechanical motions including bending, walking and helical motion, in soft polymer materials are achieved which offers fascinating opportunities toward inherently photo‐responsive materials.

Light-Powered Autonomous Flagella-Like Motion of Molecular Crystal Microwires

The ability to exhibit life-like oscillatory motion fueled by light represents a new capability for stimuli-responsive materials. Although this capability has been demonstrated in soft materials like polymers, it has never been observed in molecular crystals, which are not generally regarded as dynamic objects. In this work, it is shown that molecular crystalline microwires composed of (Z)-2-(3-(anthracen-9-yl)allylidene)malononitrile ((Z)-DVAM) can be continuously actuated when exposed to a combination of ultraviolet and visible light. The photo-induced motion mimics the oscillatory behavior of biological flagella and enables propagation of microwires across a surface and through liquids, with translational speeds up to 7 μm s^-1 . This is the first example of molecular crystals that show complex oscillatory behavior under continuous irradiation. A model that relates the rotation of the transition dipole moment between reversible E→Z photoisomerization to the microscopic torque can qualitatively reproduce how the rotational frequency depends on light intensity and polarization.

Light-Driven Flipping of Azobenzene Assemblies-Sparse Crystal Structures and Responsive Behaviour to Polarised Light

For creation of autonomous microrobots, which are able to move under conditions of a constant environment and a constant energy supply, a mechanism for maintenance of mechanical motion with a capacity for self-control is required. This requirement, known as self-organisation, represents the ability of a system to evade equilibrium through formation of a spatio-temporal pattern. Following our previous finding of a self-oscillatory flipping motion of an azobenzene-containing co-crystal, we present here regulation of the flipping motion by a light-receiving sensor molecule in relation to the alignment and role of azobenzene molecules in crystals. In the anisotropic structure, a specific azobenzene molecule acts as a reaction centre for the conversion of light to a mechanical function process, whereas the other molecules act as modulators for spatio-pattern regulation. The present results demonstrate that autonomously drivable molecular materials can exhibit information-responsive, self-sustainable motion by incorporating stimulus-responsive sensors.

Phototransformative Supramolecular Assembly of Amphiphilic Diarylethenes Realized by a Combination of Photochromism and Lower Critical Solution Temperature Behavior

Amphiphilic diarylethenes bearing octyloxycarbonyl and N-octylcarbamoyl groups have been designed and synthesized. These ester- and amide-linked compounds form micrometer-sized supramolecular assemblies in water, and these assemblies exhibit photoinduced macroscopic morphological transformations upon alternate irradiation with UV and visible light. The ester-linked diarylethene showed a transformation between colorless spheres and a red-purple hazy fringe, whereas the microspheres of the amide-linked diarylethene showed changes in color, size, and shape, but the spheres did not show division. TEM images revealed that the spheres of the open-ring isomers have coacervate structures, with bicontinuous aqueous and organic phases. The closed-ring isomers of the ester- and amide-linked compounds were found to form nanofibers and thin layers, respectively. These compounds showed absorption spectral shifts at temperatures corresponding to the lower critical solution temperature (LCST) transition. This morphological transformation can be rationalized as the photoinduced phase transition between the high- and low-temperature phases of the LCST transition. These results open up a new avenue for the design of phototransformative supramolecular assemblies based on a combination of photochromism and LCST behavior.