Extraction of mechanical work from stimuli-responsive molecular systems and materials

A wide variety of stimuli-responsive materials has been recently developed. In particular, the integration of thermodynamic or photodynamic molecular switches in appropriate materials can trigger their macroscopic actuation, possibly in combination with oscillating phenomena. Most of these systems, however, cannot progressively perform work on their environment because they lack spatial asymmetry in their actuation trajectory. Consequently, any work done in one direction is cancelled once the systems return to their initial state. To create actuators that can produce continuous work, ratcheting strategies must be implemented to break the spatial symmetry during their operation. Macroscopic ratchets can be designed by coupling the whole material with an engineered asymmetric environment during its actuation. Moreover, ratcheting mechanisms can also be implemented in dissipative systems at nanoscale by breaking the spatial symmetry of molecular switches’ energy profile, thereby accessing molecular motors. Molecular switches and advanced molecular motors, which are the elementary building blocks for the construction of molecular machines, have been recently integrated into soft materials to generate macroscopic actuation under various types of external stimulation. However, to produce continuous work from these materials, and therefore to potentially achieve more advanced tasks, important structural and dynamic aspects should be considered at all length scales. Here, we discuss the implementation of thermodynamic, photodynamic, and dissipative molecular switches and motors in such stimuli-responsive materials. We also highlight the different ratcheting strategies that can be implemented in these actuators to confer on them the capacity of achieving unidirectional cyclic motion and to increase their work output continuously and autonomously. Molecular switches and advanced molecular motors, which are the elementary building blocks for the construction of molecular machines, have been recently integrated into soft materials to generate macroscopic actuation under various types of external stimulation. However, to produce continuous work from these materials, and therefore to potentially achieve more advanced tasks, important structural and dynamic aspects should be considered at all length scales. Here, we discuss the implementation of thermodynamic, photodynamic, and dissipative molecular switches and motors in such stimuli-responsive materials. We also highlight the different ratcheting strategies that can be implemented in these actuators to confer on them the capacity of achieving unidirectional cyclic motion and to increase their work output continuously and autonomously. mechanically interlocked structure comprising two intertwined macrocycles. The term is derived from the Latin for ‘chain’ (catena). mechanically interlocked structure comprising a dumbbell-shaped molecule threaded through a macrocycle. The term is derived from the Latin for ‘wheel’ (rota) and ‘axle’ (axis). mechanically interlocked rotaxane comprising two identical molecules consisting of a ring (macrocycle) covalently linked to an axle. The axle of one molecule is threaded through the macrocycle of the other one, thereby leading to a cyclic topology [c2]. material that can perform a task by energy dissipation from its environment. In particular, when such materials can sense and adapt to what occurs in their environment, when they display motility, or if they reorganize themselves to perform multiple tasks, they are sometimes named ‘life-like’ materials. an oscillating redox reaction occurring in acidic aqueous solution in the presence of bromine derivatives, transition-metal catalysts, and reducing agents and driven by nonlinear thermodynamics. principle stating that, at equilibrium, every process occurs at the same rate as its reverse process. This principle is therefore tightly related to microscopic reversibility. a polymer network comprising mesogenic units. The phase transition from the ordered mesophase to the disordered isotropic phase leads to volume expansion when the material is heated. Cooling the material back to its liquid-crystalline phase allows the recovery of its initial shape. critical temperature below which a polymer is fully miscible in a solvent, for any composition. Conversely, the upper critical solution temperature (UCST) is the critical temperature above which a polymer is fully miscible in a solvent, for any composition. Crossing these critical temperatures can lead to macroscopic shrinkage/expansion of the polymer materials. entanglement in space between two or several molecular components that cannot be undone without breaking a covalent bond. The possible mechanically interlocked structures include, for instance, catenanes and rotaxanes. principle stating that the probability of any trajectory of a microscopic process through phase space equals that of the time-reversed trajectory. It is tightly related to detailed balance for chemical reactions at equilibrium. a molecular assembly that can perform a task through the controlled mechanical actuation of its elementary parts under an appropriate stimulus. molecule capable of repetitive directional motion when fueled with a source of energy. There can be linear molecular motors, like molecular walkers, or rotary molecular motors, like motors based on overcrowded alkenes. The unidirectionality is provided by a ‘ratcheting’ mechanism contained within the molecular motor. phenomenon where a material emits heat after being excited by light irradiation. macroscopically, a ratchet is a device with asymmetric teeth that bias its continuous motion in one preferential direction (because they prevent motion in the opposite direction). By analogy, at the nanoscopic scale in a Brownian environment, ratcheted energy profiles are asymmetric and can therefore drive the net unidirectional motion of a system when pumping energy from their environment. In particular, mechanically active molecular systems can make use of such ratcheting strategies to continuously move a particle up to an energy gradient.