Thermal Helix Inversion Research Articles

Aqueous Supramolecular Transformations of Motor Bola-Amphiphiles at Multiple Length-Scale.

Molecular motor amphiphiles have already been widely attempted for dynamic nanosystems across multiple length-scale for developments of small functional materials, including controlling macroscopic foam properties, amplifying motion as artificial molecular muscles, and serving as extracellular matrix mimicking cell scaffolds. However, limiting examples of bola-type molecular motor amphiphiles are considered for constructing macroscopic biomaterials. Herein, this work presents the designed two second generation molecular motor amphiphiles, motor bola-amphiphiles (MBAs). Aside from the photoinduced motor rotation of MBAs achieved in both organic and aqueous media, the rate of recovering thermal helix inversion step can be controlled by the rotor part with different steric hindrances. Dynamic assembled structures of MBAs are observed under (cryo)-transmission electron microscopy (TEM). This dynamicity assists MBAs in further assembling as macroscopic soft scaffolds by applying a shear-flow method. Upon photoirradiation, the phototropic bending function of MBA scaffolds is observed, demonstrating the amplification of molecular motion into macroscopic phototropic bending functions at the macroscopic length-scale. Since MBAs are confirmed with low cytotoxicity, human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can grow on the surface of MBA scaffolds. These results clearly demonstrate the concept of designing MBAs for developing photoresponsive dynamic functional materials to create new-generation soft robotic systems and cell-material interfaces.

Excited State Dynamics in Unidirectional Photochemical Molecular Motors.

Unidirectional photochemically driven molecular motors (PMMs) convert the energy of absorbed light into continuous rotational motion. As such they are key components in the design of molecular machines. The prototypical and most widely employed class of PMMs is the overcrowded alkenes, where rotational motion is driven by successive photoisomerization and thermal helix inversion steps. The efficiency of such PMMs depends upon the speed of rotation, determined by the rate of ground state thermal helix inversion, and the quantum yield of photoisomerization, which is dependent on the excited state energy landscape. The former has been optimized by synthetic modification across three generations of overcrowded alkene PMMs. These improvements have often been at the expense of photoisomerization yield, where there remains room for improvement. In this perspective we review the application of ultrafast spectroscopy to characterize the excited state dynamics in PMMs. These measurements lead to a general mechanism for all generations of PMMs, involving subpicosecond decay of a Franck-Condon excited state to populate a dark excited state which decays within picoseconds via conical intersections with the electronic ground state. The model is discussed in the context of excited state dynamics calculations. Studies of PMM photochemical dynamics as a function of solvent suggest exploitation of intramolecular charge transfer and solvent polarity as a route to controlling photoisomerization yield. A test of these ideas for a first generation motor reveals a high degree of solvent control over isomerization yield. These results suggest a pathway to fine control over the performance of future PMMs.

Ultrafast Photocontrolled Rotation in a Molecular Motor Investigated by Machine Learning-Based Nonadiabatic Dynamics Simulations.

The thermal helix inversion (THI) of the overcrowded alkene-based molecular motors determines the speed of the unidirectional rotation due to the high reaction barrier in the ground state, in comparison with the ultrafast photoreaction process. Recently, a phosphine-based motor has achieved all-photochemical rotation experimentally, promising to be controlled without a thermal step. However, the mechanism of this photochemical reaction has not yet been fully revealed. The comprehensive computational studies on photoisomerization still resort to nonadiabatic molecular dynamics (NAMD) simulations based on electronic structure calculations, which remains a high computational cost for large systems such as molecular motors. Machine learning (ML) has become an accelerating tool in NAMD simulations recently, where excited-state potential energy surfaces (PESs) are constructed analytically with high accuracy, providing an efficient approach for simulations in photochemistry. Herein the reaction pathway is explored by a spin-flip time-dependent density functional theory (SF-TDDFT) approach in combination with ML-based NAMD simulations. According to our computational simulations, we notice that one of the key factors of fulfilling all-photochemical rotation in the phosphine-based motor is that the excitation energies of four isomers are similar. Additionally, a shortcut photoinduced transformation between unstable isomers replaces the THI step, which shares the conical intersection (CI) with photoisomerization. In this study, we provide a practical approach to speed up the NAMD simulations in photochemical reactions for a large system that could be extended to other complex systems.

Lanthanide‐Functionalized Water‐Soluble Ionic Motors: Synergetically Regulated Rotary Motion by Allostery and Triplet Sensitization

AbstractMolecular motors, based on overcrowded alkenes (L) serving as ligands to chelate lanthanide nitrate, afford the first lanthanide‐functionalized water‐soluble ionic compound [L2‐Ln(NO3)2(H2O)2]·NO3 (LnL2). Benefitting from the “pull–push” feature of L together with 3LMCT from L to LnIII, irradiation with visible light not only drives photochemical isomerization but also induces energy transfer from L to LnIII, decreasing their intensity of low‐energy 3LMCT (ligand‐to‐metal triplet energy transfer) absorptions and sensitizing the Ln‐based luminescence. Thus, the typical EuIII‐based luminescence of EuL2 favors tracing its movement by time‐resolved luminescence, and temperature‐dependent luminescence renders it possibly to be used as potential luminescent thermometers. Very excitingly, besides a contraction of the stator by Ln–L interactions on decreasing the steric hindrance on the “fjord region,” the efficiency of intramolecular 3LMCT sensitization synergetically regulates the rotary motion of lanthanide‐complexed motors in thermal helix inversion steps.

Coupled Rocking Motion in a Light-Driven Rotary Molecular Motor.

Coupled motion is ubiquitous in Nature as it forms the base for the direction, amplification, propagation, and synchronization of movement. Herein, we present experimental proof for the coupling of the rocking motion of a dihydroanthracene stator moiety with the light-induced rotational movement of an overcrowded alkene-based molecular motor. The motor was desymmetrized, introducing two different alkyl substituents to the stator part of the molecular scaffold, resulting in the formation of two diastereomers with opposite axial chirality. The structure of the two isomers is determined with nuclear Overhauser effect spectroscopy NMR and single-crystal X-ray analysis. The desymmetrization enables the study of the coupled motion, that is, rotation and oscillation, by 1H NMR, findings that are further supported by density functional theory calculations. A new handle to regulate the rotational speed of the motor through functionalization in the bottom half was also introduced, as the thermal barrier for thermal helix inversion is found to be largely dependent on the alkyl substituents and its orientation toward the upper half of the motor scaffold. In addition to the commonly observed successive photochemical and thermal steps driving the rotation of the motor, we find that the motor undergoes photochemically driven rotation in three of the four steps of the rotation cycle. Hence, this result extends the scope of molecular motors capable of photon-only rotary behavior.

Rotor orientation direction controls geometric curvature and chirality for assemblies of motor amphiphiles in water

AbstractControl over geometric curvature and chirality of assemblies in pure aqueous media is key to the design of responsive materials and molecular machines. Here we show how aggregate geometric curvature and chirality of motor amphiphiles could be switched from bicontinuous calabashes to nanoribbons or from vesicles to nanoribbons by modulating rotor orientation direction with dual light/heat stimuli to influence spontaneous curvature in assemblies. The photoisomerization and thermal helix inversion processes of molecular motors have been studied at the molecular level, and the transformation of supramolecular assemblies has been investigated at the microscopic level. The morphological evolution of the calabash‐shaped assembly can be kinetically captured, suggesting that the bicontinuous calabash‐shaped structures are different from the bowl‐shaped aggregates based on solvent‐driven assembly upon the addition of non‐solvent or solvent. The investigation of dual optical/thermal control of rotor orientation can provide a new strategy for tuning the geometric curvature and chirality of nanoassemblies at the nanoscale, arriving ultimately the clusteroluminescence through‐space electronic communication at responsive supramolecular nanosystems.

Effect of Temperature on Photoisomerization Dynamics of a Newly Designed Two-Stroke Light-Driven Molecular Rotary Motor.

The working mechanism of conventional light-driven molecular rotary motors, especially Feringa-type motors, contains two photoisomerization steps and two thermal helix inversion steps. Due to the existence of a thermal helix inversion step, both the ability to work at lower temperatures and the rotation speed are limited. In this work, a two-stroke light-driven molecular rotary motor, 2-(1,5-dimethyl-4,5-dihydrocyclopenta[b]pyrrol-6(1H)-ylidene)-1,2-dihydro-3H-pyrrol-3-one (DDPY), is proposed, which is capable of performing unidirectional and repetitive rotation by only two photoisomerization (EP→ZP and ZP→EP) steps. With trajectory surface-hopping simulation at the semi-empirical OM2/MRCI level, the EP→ZP and ZP→EP nonadiabatic dynamics of DDPY were systematically studied at different temperatures. Both EP→ZP and ZP→EP photoisomerizations are on an ultrafast timescale (ca. 200–300 fs). The decay mode of EP→ZP photoisomerization is approximately bi-exponential, while that of ZP→EP photoisomerization is found to be periodic. For EP and ZP isomers of DDPY, after the S→S excitation, the dynamical processes of nonadiabatic decay are both followed by twisting about the central C=C double bond and the pyramidalization of the C atom at the stator-axle linkage. The effect of temperature on the nonadiabatic dynamics of EP→ZP and ZP→EP photoisomerizations of DDPY has been systematically investigated. The average lifetimes of the S excited state and quantum yields for both EP→ZP and ZP→EP photoisomerization are almost temperature-independent, while the corresponding unidirectionality of rotation is significantly increased (e.g., 74% for EP→ZP and 72% for ZP→EP at 300 K vs 100% for EP→ZP and 94% for ZP→EP at 50 K) with lowering the temperature.

The Influence of Strain on the Rotation of an Artificial Molecular Motor

AbstractIn artificial small‐molecule machines, molecular motors can be used to perform work on coupled systems by applying a mechanical load—such as strain—that allows for energy transduction. Here, we report how ring strain influences the rotation of a rotary molecular motor. Bridging the two halves of the motor with alkyl tethers of varying sizes yields macrocycles that constrain the motor's movement. Increasing the ring size by two methylene increments increases the mobility of the motor stepwise and allows for fine‐tuning of strain in the system. Small macrocycles (8–14 methylene units) only undergo a photochemical E/Z isomerization. Larger macrocycles (16–22 methylene units) can perform a full rotational cycle, but thermal helix inversion is strongly dependent on the ring size. This study provides systematic and quantitative insight into the behavior of molecular motors under a mechanical load, paving the way for the development of complex coupled nanomachinery.

The Influence of Strain on the Rotation of an Artificial Molecular Motor.

In artificial small‐molecule machines, molecular motors can be used to perform work on coupled systems by applying a mechanical load—such as strain—that allows for energy transduction. Here, we report how ring strain influences the rotation of a rotary molecular motor. Bridging the two halves of the motor with alkyl tethers of varying sizes yields macrocycles that constrain the motor's movement. Increasing the ring size by two methylene increments increases the mobility of the motor stepwise and allows for fine‐tuning of strain in the system. Small macrocycles (8–14 methylene units) only undergo a photochemical E/Z isomerization. Larger macrocycles (16–22 methylene units) can perform a full rotational cycle, but thermal helix inversion is strongly dependent on the ring size. This study provides systematic and quantitative insight into the behavior of molecular motors under a mechanical load, paving the way for the development of complex coupled nanomachinery.

Design and Nonadiabatic Photoisomerization Dynamics Study of a Three-Stroke Light-Driven Molecular Rotary Motor.

Working cycle of conventional light-driven molecular rotary motors (LDMRMs), especially Feringa-type motors, usually have four steps, two photoisomerization steps, and two thermal helix inversion (THI) steps. THI steps hinder the ability of the motor to operate at lower temperatures and limit the rotation speed of LDMRMs. A three-stroke LDMRM, 2-(2,7-dimethyl-2,3-dihydro-1H-inden-1-ylidene)-1,2-dihydro-3H-pyrrol-3-one (DDIY), is proposed, which is capable of completing an unidirectional rotation by two photoisomerization steps and one thermal helix inversion step at room temperature. On the basis of trajectory surface-hopping simulation at the semi-empirical OM2/MRCI level, the EP→ZP and ZP→EM nonadiabatic photoisomerization dynamics of DDIY were systematically analyzed. Quantum yields of EP→ZP and ZP→EM photoisomerization of DDIY are ca. 34% and 18%, respectively. Both EP→ZP and ZP→EM photoisomerization processes occur on an ultrafast time scale (ca. 100–300 fs). This three-stroke LDMRM may stimulate further research for the development of new families of more efficient LDMRMs.

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.

The role of π-donors/acceptors in molecular rotors towards development of ambient blue light sensors - A density functional theory study

A series of molecular rotors based on cyclopentane are taken into consideration to explore the influence of different π-donors/acceptors substitution on their efficiency in blue light detection. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods are used to investigate the electronic excitations and photophysical properties of the designed rotors. The influence of mono- and di-substitution on photoisomerization and thermal helix inversion reaction barriers are studied. We find that the di-substitution of a π-donor and a π-acceptor have improved the parent rotor's optical properties towards a longer wavelength. Especially, the rotor R2,3(NH2, CHO) exhibits the light response range in the blue region (∼408 nm) due to its narrow bandgap (4.94 eV). Our results show that although the substitution of π-donors/acceptors improved the photophysical properties, it has no significant influence on the overall rate-determining thermal helix inversion step barriers. This theoretical work provides new insight into the role of π-donors/acceptors substitution in designing blue light sensors.

Theoretical research on molecular motors based on 4,5‐diazafluorenyl coordination motifs

AbstractThe properties of naphthalene‐containing molecular motors (L1), pyrene‐containing molecular motor (L2), and two complexes (Zn (CN)2(L1) and Zn (CN)2(L2)) based on 4,5‐diazafluorenyl coordination motifs are compared and analyzed. The influence of structural modification on the thermal helix inversion step is studied through density functional theory (DFT). The analysis of the rotation energy barrier shows that the expansion of the rotor structure of motor increases the energy barrier of the rotation process, but the motors L1 and L2 act as ligands to coordinate with the metal, which is an effective method to reduce the rotation energy barrier value. Through frontier molecular orbital (FMO), it is found that expanding the π‐conjugation system in the rotor part of the motor can increase the chemical reactivity of the motor. Then the ultraviolet‐visible (UV/Vis) absorption spectra of the motors is calculated, indicating the expansion of the π‐conjugation system in the upper half of the molecular motor is a feasible method to move the excitation wavelength to the visible light region. Finally, hole‐electron analysis is carried out. The expansion of the π‐electron system of the motor rotor and the coordination of the rotor and metal can improve the efficiency of hole‐electron separation and facilitate electron transfer.

Mechanistic analysis of light-driven overcrowded alkene-based molecular motors by multiscale molecular simulations.

We analyze light-driven overcrowded alkene-based molecular motors, an intriguing class of small molecules that have the potential to generate MHz-scale rotation rates. The full rotation process is simulated at multiple scales by combining quantum surface-hopping molecular dynamics (MD) simulations for the photoisomerization step with classical MD simulations for the thermal helix inversion step. A Markov state analysis resolves conformational substates, their interconversion kinetics, and their roles in the motor's rotation process. Furthermore, motor performance metrics, including rotation rate and maximal power output, are computed to validate computations against experimental measurements and to inform future designs. Lastly, we find that to correctly model these motors, the force field must be optimized by fitting selected parameters to reference quantum mechanical energy surfaces. Overall, our simulations yield encouraging agreement with experimental observables such as rotation rates, and provide mechanistic insights that may help future designs.

Effect of charge-transfer enhancement on the efficiency and rotary mechanism of an oxindole-based molecular motor.

Harvesting energy and converting it into mechanical motion forms the basis for both natural and artificial molecular motors. Overcrowded alkene-based light-driven rotary motors are powered through sequential photochemical and thermal steps. The thermal helix inversion steps are well characterised and can be manipulated through adjustment of the chemical structure, however, the insights into the photochemical isomerisation steps still remain elusive. Here we report a novel oxindole-based molecular motor featuring pronounced electronic push–pull character and a four-fold increase of the photoisomerization quantum yield in comparison to previous motors of its class. A multidisciplinary approach including synthesis, steady-state and transient absorption spectroscopies, and electronic structure modelling was implemented to elucidate the excited state dynamics and rotary mechanism. We conclude that the charge-transfer character of the excited state diminishes the degree of pyramidalisation at the alkene bond during isomerisation, such that the rotational properties of this oxindole-based motor stand in between the precessional motion of fluorene-based molecular motors and the axial motion of biomimetic photoswitches.

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‐Driven Liquid Crystalline Networks and Soft Actuators with Degree‐of‐Freedom‐Controlled Molecular Motors

AbstractDesign and fabrication of photomechanical soft actuators has attracted intense scientific interest because of their potential in the manufacture of untethered intelligent soft robots and advanced functional devices. Trifunctional and monofunctional polymerizable molecular motors are judiciously designed and synthesized. Novel light‐driven liquid crystalline networks (LCN) are prepared by crosslinking overcrowded‐alkene‐based molecular motors with different degrees of freedom into the anisotropic LCN. The photoisomerization and thermal helix inversion of light‐driven molecular motors are reversible when only the upper part of the molecular motor is linked to the network, endowing the LCN film with remarkable photoactive performance. However, photochemical geometric change of the light‐driven molecular motor does not work after crosslinking both the upper and lower part of the motor by polymer chains. Interestingly, it is found that the fastened motor can transfer the light energy into localized heat instead of performing photoisomerization. The light‐driven molecular‐motor‐based LCN soft actuators are demonstrated to function as a grasping hand, where the continuous motions of grasping, moving, lifting, and releasing an object are successfully achieved. This work may provide inspiration to the preparation of next‐generation photoactive advanced functional materials toward their wide applications in the areas of photonics, optoelectronics, soft robotics, and beyond.

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.

Unidirectional rotary motion in a metal-organic framework.

Overcrowded alkene-based light-driven molecular motors are able to perform large-amplitude repetitive unidirectional rotations. Their behaviour is well understood in solution. However, Brownian motion precludes the precise positioning at the nanoscale needed to harness cooperative action. Here, we demonstrate molecular motors organized in crystalline metal-organic frameworks (MOFs). The motor unit becomes a part of the organic linker (or strut), and its spatial arrangement is elucidated through powder and single-crystal X-ray analyses and polarized optical and Raman microscopies. We confirm that the light-driven unidirectional rotation of the motor units is retained in the MOF framework and that the motors can operate in the solid state with similar rotary speed (rate of thermal helix inversion) to that in solution. These 'moto-MOFs' could in the future be used to control dynamic function in crystalline materials.

Solvent Mixing To Induce Molecular Motor Aggregation into Bowl-Shaped Particles: Underlying Mechanism, Particle Nature, and Application To Control Motor Behavior.

Control over dynamic functions in larger assemblies is key to many molecular systems, ranging from responsive materials to molecular machines. Here we report a molecular motor that forms bowl-shaped particles in water and how confinement of the molecular motor affects rotary motion. Studying the aggregation process in a broader context, we provide evidence that, in the case of bowl-shaped particles, the structures are not the product of self-assembly, but a direct result of the mixing a good solvent and a (partial) non-solvent and highly independent of the molecular design. Under the influence of the non-solvent, droplets are formed, of which the exterior is hardened due to the increase in the glass transition temperature by the external medium, while the interior of the droplets remains plasticized by the solvent, resulting in the formation of stable bowl-shaped particles with a fluid interior, a glass-like exterior, and a very specific shape: dense spheres with a hole in their side. Applying this to a bulky first-generation molecular motor allowed us to change its isomerization behavior. Furthermore, the motor shows in situ photo-switchable aggregation-induced emission. Strong confinement prohibits the thermal helix inversion step while altering the energy barriers that determine the rotary motion, such that it introduces a reverse trans–cis isomerization upon heating. These studies show a remarkable control of forward and backward rotary motion by simply changing solvent ratios and extent of confinement.