Molecular Motors in Aqueous Environment

Molecular Machines For The Control Of Transmembrane Transport

Nature embeds some of its molecular machinery, including ion pumps, within lipid bilayer membranes. This has inspired chemists to attempt to develop synthetic analogues to exploit membrane confinement and transmembrane potential gradients, much like their biological cousins. In this perspective, we outline the various strategies by which molecular machines─molecular systems in which a nanomechanical motion is exploited for function─have been designed to be incorporated within lipid membranes and utilized to mediate transmembrane ion transport. We survey molecular machines spanning both switches and motors, those that act as mobile carriers or that are anchored within the membrane, mechanically interlocked molecules, and examples that are activated in response to external stimuli.

Excited-State Dynamics Simulations of a Light-Driven Molecular Motor in Solution

Molecular motors, where light can be transformed into motion, are promising in the design of nanomechanical devices. For applications, however, finding relationships between molecular motion and the environment is important. Here, we report the study of excited-state dynamics of an overcrowded alkene in solution using a hybrid quantum mechanics/molecular mechanics (QM/MM) approach combined with excited-state molecular dynamics simulations. Using QM/MM surface-hopping trajectories, we calculated time-resolved emission and transient absorption spectra. These show the rise of a short-lived Franck-Condon state, followed by the formation of a dark state in the first 150 fs before the molecular motor relaxes to the ground state in about 1 ps. From the analysis of radial distribution functions, we infer that the orientation of the solvent with respect to the molecular motor in the electronic excited state is similar to that in the ground state during the photoisomerization.

Synergistic interplay between photoisomerization and photoluminescence in a light-driven rotary molecular motor

Photoactuators and photoluminescent dyes utilize light to perform mechanical motion and undergo spontaneous radiation emission, respectively. Combining these two functionalities in a single molecule would benefit the construction of advanced molecular machines. Due to the possible detrimental interaction between the two light-dependent functional parts, the design of hybrid systems featuring both functions in parallel remains highly challenging. Here, we develop a light-driven rotary molecular motor with an efficient photoluminescent dye chemically attached to the motor, not compromising its motor function. This molecular system shows efficient rotary motion and bright photoluminescence, and these functions can be addressed by a proper choice of excitation wavelengths and solvents. The moderate interaction between the two parts generates synergistic effects, which are beneficial for lower-energy excitation and chirality transfer from the motor to the photoluminescent dye. Our results provide prospects towards photoactive multifunctional systems capable of carrying out molecular rotary motion and tracking its location in a complex environment.

Combining photofunctionalities in a single molecule is challenging due to inherent detrimental interactions. Here, the authors construct a molecular motor that exhibits photoinduced rotary motion together with bright photoluminescence.

Solvation Effects on the Thermal Helix Inversion of Molecular Motors from QM/MM Calculations

Molecular motors convert light and thermal energies into mechanical work, offering good opportunities to design novel molecular devices. Among them, molecular motors alternate a photoisomerization and a thermal helix inversion to achieve unidirectional rotation. The rotational speed is limited by the helix inversion step, which in turn is governed by a barrier in the electronic ground state. In this work, we systematically study the solvation effect on the thermal process of selected molecular motors, comparing reaction barriers obtained from both density functional theory (DFT) in the isolated system and umbrella sampling within a hybrid quantum mechanics/molecular mechanics (QM/MM) model in solution. We find more prominent solvation effects on those molecular motors with larger dipole moments. The results could provide insight into how to functionalize molecular motors to speed up their rotation.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Motorized Macrocycle: A Photo-responsive Host with Switchable and Stereoselective Guest Recognition

Designing photo-responsive host-guest systems can provide versatile supramolecular tools for constructing smart systems and materials. We designed photo-responsive macrocyclic hosts, modulated by light-driven molecular rotary motors enabling switchable chiral guest recognition. The intramolecular cyclization of the two arms of a first-generation molecular motor with flexible oligoethylene glycol chains of different lengths resulted in crown-ether-like macrocycles with intrinsic motor function. The octaethylene glycol linkage enables the successful unidirectional rotation of molecular motors, simultaneously allowing the 1:1 host-guest interaction with ammonium salt guests. The binding affinity and stereoselectivity of the motorized macrocycle can be reversibly modulated, owing to the multi-state light-driven switching of geometry and helicity of the molecular motors. This approach provides an attractive strategy to construct stimuli-responsive host-guest systems and dynamic materials.

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.

Light-Activated Organic Molecular Motors and Their Applications

Molecular motors are at the heart of cellular machinery, and they are involved in converting chemical and light energy inputs into efficient mechanical work. From a synthetic perspective, the most advanced molecular motors are rotators that are activated by light wherein a molecular subcomponent rotates unidirectionally around an axis. The mechanical work produced by arrays of molecular motors can be used to induce a macroscopic effect. Light activation offers advantages over biological chemically activated molecular motors because one can direct precise spatiotemporal inputs while conducting reactions in the gas phase, in solution and in vacuum, while generating no chemical byproducts or waste. In this review, we describe the origins of the first light-activated rotary motors and their modes of function, the structural modifications that led to newer motor designs with optimized rotary properties at variable activation wavelengths. Presented are molecular motor attachments to surfaces, their insertion into supramolecular structures and photomodulating materials, their use in catalysis, and their action in biological environments to produce exciting new prospects for biomedicine.

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Anion-Regulated Molecular Rotor Crystal: The First Case of a Stator-Rotator Double Switch with Relaxation Behavior

Molecular rotational motion is crucial in artificial molecular machines and is expected to be very significant for the development of an electronic information molecular machine as mentioned in the 2016 Nobel Prize. However, controlling multiple motor modes is a huge challenge. Here, we report a case in which the structural phase transition effectively triggers multiple motor modes by regulating the rotational speed of the cation and/or anion. A novel switchable crystalline supramolecular rotor, [(cyclohexylammonium)(18-crown-6)] FSO₃ (1), exhibits prominent temperature-dependent double switching behavior at 157.9 and 389.1 K induced by the variation of the rotational speed of the FSO₃^- anion (which acts as a super miniature rotator) in response to temperature. Moreover, it exhibits significant relaxation behavior and excellent pyroelectric switch characteristics. To the best of our knowledge, this might be the first discovery of the stator-rotator double switch with a relaxation effect, which could be a promising candidate for a slow/fast responsive double switch over a wide temperature range.

Phase transition of spiropyrans: impact of isomerization dynamics at high temperatures

Isomerization behaviors of spiropyran derivatives in neat condensed phase were studied to understand their unusual phase transitions including cold-crystallization after extreme supercooling down to -50 °C. Compounds with different functional groups were compared, and the equilibrium between isomers at high temperatures was found to determine phase transitions.

Origin of the isotropic motion in crystalline molecular rotors with carbazole stators

Herein we report two crystalline molecular rotors 1 and 4 that show extremely narrow signals in deuterium solid-state NMR spectroscopy. Although this line shape is typically associated with fast-moving molecular components, our VT 2H NMR experiments, along with X-ray diffraction analyses and periodic DFT computations show that this spectroscopic feature can also be originated from low-frequency intramolecular rotations of the central phenylene with a cone angle of 54.7° that is attained by the cooperative motion of the entire structure that distorts the molecular axis to rotation. In contrast, two isomeric structures (2 and 3) do not show a noticeable intramolecular rotation, because their crystallographic arrays showed very restricting close contacts. Our findings clearly indicate that the multiple components and phase transitions in crystalline molecular machines can work in concert to achieve the desired motion.

Pumping a Ring-Sliding Molecular Motion by a Light-Powered Molecular Motor

Designing artificial molecular machines to execute complex mechanical tasks, like coupling rotation and translation to accomplish transmission of motion, continues to provide important challenges. Herein, we demonstrated a novel molecular machine comprising a second-generation light-driven molecular motor and a bistable [1]rotaxane unit. The molecular motor can rotate successfully even in an interlocked [1]rotaxane system through a photoinduced cis-to -trans isomerization and a thermal helix inversion, resulting in concomitant transitional motion of the [1]rotaxane. The transmission process was elucidated via ¹H NMR, ¹H-¹H COSY, HMQC, HMBC, and 2D ROESY NMR spectroscopies, UV-visible absorption spectrum, and density functional theory calculations. This is the first demonstration of a molecular motor to rotate against the appreciably noncovalent interactions between dibenzo-24-crown-8 and N-methyltriazolium moieties comprising the rotaxane unit, showing operational capabilities of molecular motors to perform more complex tasks.

Design of photo-activated molecular machines: highlights from the past ten years

Researchers continue to generate ingenious (supra)molecular structures in which light can trigger controlled and directed movements of the components.

A hemithioindigo molecular motor for metal surface attachment

We report on the synthesis of a hemithioindigo molecular motor bearing thioether feet for metal surface attachment and a comprehensive study of its light induced unidirectional motion in solution.