Light-Responsive Springs from Electropatterned Liquid Crystal Polymer Networks

Future robotic systems will have to adapt their operation to dynamic environments and therefore their development will require the use of active soft components. Bioinspired approaches toward novel actuation materials for active components rely on integrating molecular machines in soft matter, and ensuring that their nanoscale movement is amplified to the macroscale, where mechanically relevant motion is generated. This approach is successfully used in the design of photoresponsive soft springs and other mechanically active materials. Here, this study reports on a new approach where the operation of photoswitches and chiral liquid crystals are combined with an original and mask-free microscopic patterning method to generate helix-based movement at the macroscale, including light-driven winding and unwinding accompanied with inversion of handedness. The microscopic patterning is the result of the unique organization of cholesteric liquid crystals under weak electric field. At a higher level, the pitch and the handedness of the active springs are defined by the imprinted pattern and the angle at which the spring ribbons are cut in the material. These findings are likely to enable soft and responsive robotic systems, and they show how transmission of molecular operation into macroscale functional movement is enabled by materials design across multiple hierarchical levels.</p

Humidity-responsive actuators from integrating liquid crystal networks in an orienting scaffold

Developing shape-shifting materials requires combining the flexibility needed by shape-shifting properties, with the toughness that is demanded to maintain their mechanical performance. Typically, in liquid crystal networks, the amplitude of the shape transformation can be hindered by large cross-linking densities. Here, we argue that a promising strategy to address this limitation consists in integrating liquid crystal networks into an anisotropic and porous material that acts as an orienting scaffold. This strategy shows similarities with the principles of stimuli-responsive deformation in plants, where inflexible elements with specific orientations are integrated into a stimuli-responsive matrix. By aligning liquid crystals in a porous polypropylene orienting scaffold, we demonstrate liquid crystal networks that respond to humidity with a shape change, yet they display high elastic modulus and toughness. Various chiral shapes can be generated in single and double layers of these films, and the complexity of their actuation modes is enhanced, including twisting, curling or winding. We anticipate that these hybrid composites and the strategy they embody can find application to other stimuli-responsive anisotropic soft materials

Knotting a molecular strand can invert macroscopic effects of chirality

Transferring structural information from the nanoscale to the macroscale is a promising strategy for developing adaptive and dynamic materials. Here we demonstrate that the knotting and unknotting of a molecular strand can be used to control, and even invert, the handedness of a helical organization within a liquid crystal. An oligodentate tris(2,6-pyridinedicarboxamide) strand with six point-chiral centres folds into an overhand knot of single handedness upon coordination to lanthanide ions, both in isotropic solutions and in liquid crystals. In achiral liquid crystals, dopant knotted and unknotted strands induce supramolecular helical organizations of opposite handedness, with dynamic switching achievable through in situ knotting and unknotting events. Tying the molecular knot transmits information regarding asymmetry across length scales, from Euclidean point chirality (constitutional chirality) via molecular entanglement (conformation) to liquid-crystal (centimetre-scale) chirality. The magnitude of the effect induced by the tying of the molecular knots is similar to that famously used to rotate a glass rod on the surface of a liquid crystal by synthetic molecular motors. [Figure not available: see fulltext.

Unsupervised inference approach to facial attractiveness

The perception of facial beauty is a complex phenomenon depending on many, detailed and global facial features influencing each other. In the machine learning community this problem is typically tackled as a problem of supervised inference. However, it has been conjectured that this approach does not capture the complexity of the phenomenon. A recent original experiment (Ib\'a\~nez-Berganza et al., Scientific Reports 9, 8364, 2019) allowed different human subjects to navigate the face-space and ``sculpt'' their preferred modification of a reference facial portrait. Here we present an unsupervised inference study of the set of sculpted facial vectors in that experiment. We first infer minimal, interpretable, and faithful probabilistic models (through Maximum Entropy and artificial neural networks) of the preferred facial variations, that capture the origin of the observed inter-subject diversity in the sculpted faces. The application of such generative models to the supervised classification of the gender of the sculpting subjects, reveals an astonishingly high prediction accuracy. This result suggests that much relevant information regarding the subjects may influence (and be elicited from) her/his facial preference criteria, in agreement with the multiple motive theory of attractiveness proposed in previous works.Comment: main article (10 pages, 4 figures) + supplementary information (22 pages, 10 figures). minor typos corrected. Federico Maggiore added as autho

Reversible Charge Trapping in Bis-Carbazole-Diimide Redox Polymers with Complete Luminescence Quenching Enabling Nondestructive Read-Out by Resonance Raman Spectroscopy

The coupling of substituted carbazole compounds through carbon–carbon bond formation upon one-electron oxidation is shown to be a highly versatile approach to the formation of redox polymer films. Although the polymerization of single carbazole units has been proposed earlier, we show that by tethering pairs of carbazoles double sequential dimerization allows for facile formation of redox polymer films with fine control over film thickness. We show that the design of the monomers and in particular the bridging units is key to polymer formation, with the diaminobenzene motif proving advantageous, in terms of the matching to the redox potentials of the monomer and polymer film and thereby avoiding limitations in film thickness (autoinsulation), but introduces unacceptable instability due to the intrinsic redox activity of this moiety. The use of a diimide protecting group both avoids complications due to <i>p</i>-diamino-benzene redox chemistry and provides for a redox polymer in which the photoluminescence of the bis-carbazole moiety can be switched reversibly (on/off) with redox control. The monomer design approach is versatile enabling facile incorporation of additional functional units, such as naphthalene. Here we show that a multicomponent carbazole/naphthalene containing monomer (<b>APCNDI</b>) can form redox polymer films showing both p- and n- conductivity under ambient conditions and allows access to five distinct redox states, and a complex electrochromic response covering the whole of the UV/vis–NIR spectral region. The highly effective quenching of the photoluminescence of both components in poly-<b>APCNDI</b> enables detailed characterization of the redox polymer films. The poly-<b>APCNDI</b> films show extensive charge trapping, which can be read out spectroscopically in the case of films and is characterized as kinetic rather than chemical in origin on the basis of UV/vis–NIR absorption and resonance Raman spectroscopic analyses. The strong resonantly enhanced Raman scattering for the various oxidized and reduced states of <b>APCNDI</b> enables nondestructive “read-out” of the state of the polymer, including that in which charges are trapped kinetically at the surface, making poly-<b>APCNDI</b> highly suitable for application as a component in organic nonvolatile memory devices

Molecular machines for life-like adaptive matter

Biological systems adapt to their environment by using the work of biomolecular machines, whose operation is harnessed by coupling with hierarchically organized assemblies and networks. In this thesis, I show how artificial molecular machines can mediate adaptive responses to light in inanimate matter. The motion of these machines is brought to the functional level by cooperative effects in liquid crystalline systems. As a result, the dynamic functional systems and materials presented in this thesis reach beyond the state of the art, by driving macroscopic motion continuously, purposefully, and effectively

Life-like motion driven by artificial molecular machines

Essentially, all motion in living organisms emerges from the collective action of biological molecular machines transforming chemical energy, originally harvested from light, into ordered activity. As a man-made counterpart to nature’s biomolecular machines, chemists have created artificial molecular machines that display controlled and even directional motion in response to light. However, to be of practical value, the motion of these light-fuelled molecular machines will have to be coupled to the rest of the world. Inspired by the complex functional movement seen in the plant and animal world, chemists have undertaken the challenge to harness molecular motion and, so, they have set artificial molecular motors and switches to work and perform useful mechanical action at the macroscopic level. Here, we review these recent developments. We show how modern research has embraced the full complexity of the molecular world by aiming at the design of autonomous, and sometimes adaptive, molecular systems that work continuously under the effect of illumination. We report evidence that molecular motion can be engineered into highly sophisticated movements and that, from a fundamental point of view, continuous movement can only emerge when man-made molecules cooperate, in space and time. Eventually, unravelling the rules of molecular motion will support the creation of molecular materials that produce work continuously under a constant input of energy

Shape-Persistent Actuators from Hydrazone Photoswitches

Interfacing molecular photoswitches with liquid crystal polymers enables the amplification of their nanoscale motion into macroscopic shape transformations. Typically, the mechanism responsible for actuation involves light-induced molecular disorder. Here, we demonstrate that bistable hydrazones can drive (chiral) shape transformations in liquid crystal polymer networks, with photogenerated polymer shapes displaying a long-term stability that mirrors that of the switches. The mechanism involves a photoinduced buildup of tension in the polymer, with a negligible influence on the liquid crystalline order. Hydrazone-doped liquid crystal systems thus diversify the toolbox available to the field of light-adaptive molecular actuators and hold promise in terms of soft robotics

Motile behaviour of droplets in lipid systems

Motility is the capacity for living organisms to move autonomously and with purpose, and is essential to life. The transition from abiotic chemistry into motile cellular compartments has yet to be understood, but motile behaviour likely followed chemical evolution because primeval cell survival depended on scouting for resources effectively. Minimalistic motile systems provide an experimental framework to delineate the emergence mechanisms of such an evolutionary asset. In this Review, we discuss frontier developments in controlling the movement of droplets in lipid systems, in particular, chemotactic behaviours driven by fluctuations in interfacial tension, because of its simple mechanism and prebiotic relevance. Although most efforts have focused on designing oil droplet motility in lipid-rich aqueous solutions, we highlight that water droplets can also move in lipid-enriched oils. First, we describe how droplets evolve chemotactic motility in lipid systems. Next, we review how these oil droplets can adapt their movement to illumination conditions. Finally, we discuss examples where chemical reactivity brings complexity to motility. This work contributes to systems chemistry, where chemical reactions combined with physicochemical phenomena can yield new functions, such that a limited set of molecules can promote complex movement at larger functional scales by following the rules of molecular chemistry. [Figure not available: see fulltext.