Stereodivergent Anion Binding Catalysis with Molecular Motors

A photoresponsive chiral catalyst based on an oligotriazole‐functionalized unidirectional molecular motor has been developed for stereodivergent anion binding catalysis. The motor function controls the helical chirality of supramolecular assemblies with chloride anions, which by means of chirality transfer enables the enantioselective addition of a silyl ketene acetal nucleophile to oxocarbenium cations. Reversal of stereoselectivity (up to 142 % Δee) was achieved through rotation of the motor core induced by photochemical and thermal isomerization steps.     

Computerized ultrasonic tomography for testing solid propellant rocket motors

Certain types of defects, such as inhomogeneities in, for example, a plastic-like propellant, are not easily detected by pulse-echo techniques, so a through transmission technique is required. However, the shape of the test object may not permit transmission through the object in any required direction as is normally necessary for computerized ultrasonic tomography.A solid propellant rocket motor is such an object and in the course of this work a scanning technique is employed together with a suitable reconstruction algorithm to obtain a reconstructed image of the cross-section under investigation, overcoming the problem of not being able to transmit pulses through the object along every line normally required. Both a computer simulation and a real system were realized for the test object and images were produced from both. The time of flight of an ultrasonic beam through the specimen is the parameter used in the reconstruction (sound velocity measurement).     

Quadrupole versus dipole photomotors: Symmetry constraints and operating properties

The operating properties of different-type Brownian photomotors have been compared in relation to the symmetry interplay between their moving (particle/molecule) and immovable (substrate) units. As found, essentially different symmetry constraints and hence different motor behavior are characteristic of molecules if their electron density redistribution on excitation involves only odd-order or only even-order multipole moments. The two respective types of molecules give rise to dipole and quadrupole photomotors. Purely dipole photomotors can operate only on substrates with symmetric charge distribution and their ensemble with random molecular orientations undergoes equidirectional diffusion rather than directed motion. Contrary to this, purely quadrupole photomotors need substrates with antisymmetric charge distribution for directed motion to occur and they can move directionally in an ensemble. Also, quadrupole photomotors provide much smaller velocities of directed motion than their dipole counterparts (as a result of the much weaker interaction of quadrupoles than dipoles with an electric field). The generic distinctions between the two photomotor types are exemplified by donor-acceptor-substituted stilbenoids (dipole molecules) and symmetric squaraines (quadrupole molecules). The model used affords the rational design of photomotors by the selection of promising moving molecules and by the continuous wide-range tuning of charge-distribution symmetry in substrates.     

Computational Insight to Improve the Thermal Isomerisation Performance of Overcrowded Alkene‐Based Molecular Motors through Structural Redesign

Synthetic overcrowded alkene‐based molecular motors achieve 360° unidirectional rotary motion of one motor half (rotator) relative to the other (stator) through sequential photochemical and thermal isomerisation steps. In order to facilitate and expand the use of these motors for various applications, it is important to investigate ways to increase the rates and efficiencies of the reactions governing the rotary motion. Here, we use computational methods to explore whether the thermal isomerisation performance of some of the fastest available motors of this type can be further improved by reducing the sizes of the motor halves. Presenting three new redesigned motors that combine an indanylidene rotator with a cyclohexadiene, pyran or thiopyran stator, we first use multiconfigurational quantum chemical methods to verify that the photoisomerisations of these motors sustain unidirectional rotary motion. Then, by performing density functional calculations, we identify both stepwise and concerted mechanisms for the thermal isomerisations of the motors and show that the rate‐determining free‐energy barriers of these processes are up to 25 kJ mol^?1 smaller than those of the original motors. Furthermore, the thermal isomerisations of the redesigned motors proceed in fewer steps. Altogether, the results suggest that the redesigned motors are useful templates for improving the thermal isomerisation performance of existing overcrowded alkene‐based motors.     

Effects of rebinding rate and asymmetry in unbinding rate on cargo transport by multiple kinesin motors

Many intracellular transports are performed by multiple molecular motors in a cooperative manner.Here,we use stochastic simulations to study the cooperative transport by multiple kinesin motors,focusing mainly on effects of the form of unbinding rate versus force and the rebinding rate of single motors on the cooperative transport.We consider two forms of the unbinding rate.One is the symmetric form with respect to the force direction,which is obtained according to Kramers theory.The other is the asymmetric form,which is obtained from the prior studies for the single kinesin motor.With the asymmetric form the simulated results of both velocity and run length of the cooperative transport by two identical motors and those by a kinesin-1 motor and a kinesin-2 motor are in quantitative agreement with the available experimental data,whereas with the symmetric form the simulated results are inconsistent with the experimental data.For the cooperative transport by a faster motor and a much slower motor,the asymmetric form can give both larger velocity and longer run length than the symmetric form,giving an explanation for why kinesin adopts the asymmetric form of the unbinding rate rather than the symmetric form.For the cooperative transport by two identical motors,while the velocity is nearly independent of the rebinding rate,the run length increases linearly with the rebinding rate.For the cooperative transport by two different motors,the increase of the rebinding rate of one motor also enhances the run length of the cooperative transport.The dynamics of transport by N(N=3,4,5,6,7 and 8)motors is also studied.     

Synthetic Molecular Motors: Thermal N Inversion and Directional Photoinduced CN Bond Rotation of Camphorquinone Imines

The thermal and photochemical E/Z isomerization of camphorquinone‐derived imines was studied by a combination of kinetic, structural, and computational methods. The thermal isomerization proceeds by linear N inversion, whereas the photoinduced process occurs through C?N bond rotation with preferred directionality as a result of diastereoisomerism. Thereby, these imines are arguably the simplest example of synthetic molecular motors. The generality of the orthogonal trajectories of the thermal and photochemical pathways allows for the postulation that every suitable chiral imine qualifies, in principle, as a molecular motor driven by light or heat.     

Are Two‐Station Biased Random Walkers Always Potential Molecular Motors?

The short answer to the title question is no. Despite their tremendous complexity, many nanomachines are simply one‐dimensional systems undergoing a biased, that is, unidirectional, walk on a two‐minima potential energy curve. The initially prepared state, or station, is higher in energy than the final equilibrium state that is reached after overcoming an energy barrier. All chemical reactions comply with this scheme, which does not necessarily imply that a generic chemical reaction is a potential molecular motor. If the barrier is low, the system may walk back and the motion will have a large purely Brownian component. Alternatively, a large distance from the barrier of either of the two stations may introduce a Brownian component. Starting from a general inequality that leverages on the idea that the amount of heat dissipated along the potential energy curve is a good indication of the effectiveness of the biased walk, we provide guidelines for the selection of the features of artificial molecular motors.     

Tuning the Ground and Excited State Dynamics of Hemithioindigo Molecular Motors by Changing Substituents

Efficiency and performance of light triggered molecular motors are crucial features that need to be mechanistically understood to improve the performance and enable conscious property tailoring for specific applications. In this work, three different hemithioindigo-based molecular motors are investigated and all four steps in their complete unidirectional rotation are unraveled fully quantitatively. Transient absorption spectroscopy across twelve orders of magnitude in time is used to probe the fs nuclear motions up to the ms thermal kinetics, covering the timeframe of the whole motor rotation. The newly known full mechanisms allow simulation of the motor systems to scrutinize their performance at realistic illumination conditions. This highlights the importance of photoisomerization quantum yields for the rotation speed. The substitution pattern in close proximity to the rotation axle influences the excited and ground state properties. Reduction of electron donation and concomitant increase of steric hindrance leads to faster photoisomerization reactions with quasi-ballistic behavior, but also to a slight decrease in the quantum efficiency. The expected decelerating effects of increased sterics are primarily manifested in the ground state. A promising approach for next-generation hemithioindigo motors is to elevate electron donation at the rotor fragment followed by an increase of steric hindrance.     

Effects of diversity and coupling on transport properties of globally coupled active Brownian particles

Based on recent work [B. Lindner and E. M., Nicola, Phys. Rev. Lett. 101 (2008) 190603], the transport of N$N$ globally coupled Brownian motors with diversity is investigated. By applying the mean-field approximation, the effects of the diversity disturbance and the coupling strength on the transport of the coupled system are discussed both theoretically and numerically. It is found that the diversity reduces the diffusion dramatically for small bias and increases the diffusion gradually for large bias. The collective motion shows a resonant dependence on the coupling strength in a region of small bias.