Light-driven rotary molecular motors without point chirality: a minimal design (original) (raw)
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Chiral Hydrogen Bond Environment Providing Unidirectional Rotation in Photoactive Molecular Motors
The Journal of Physical Chemistry Letters, 2013
Generation of a chiral hydrogen bond environment in efficient molecular photoswitches is proposed as a novel strategy for the design of photoactive molecular motors. Here, the following strategy is used to design a retinal-based motor presenting singular properties: (i) a single excitation wavelength is needed to complete the unidirectional rotation process (360°); (ii) the absence of any thermal step permits the process to take place at low temperatures; and (iii) the ultrafast process permits high rotational frequencies.
Journal of the American Chemical Society, 2006
The introduction of bulky substituents at the stereogenic center of light-driven second-generation molecular motors results in an acceleration of the speed of rotation. This is due to a more strained structure with elongated CdC bonds and a higher energy level of the ground state relative to the transition state for the rate-limiting thermal isomerization step. Understanding the profound influence that variation of the substituent at the stereogenic center holds over the rotational speed of the light-driven molecular motor has enabled the development of the fastest molecular motor reported thus far.
The Journal of Organic Chemistry, 2021
Synthetic molecular motors driven by E/Z photoisomerization reactions are able to produce unidirectional rotary motion because of a structural asymmetry that makes one direction of rotation more probable than the other. In most such motors, this asymmetry is realized through the incorporation of a chemically asymmetric carbon atom. Here, we present molecular dynamics simulations based on multiconfigurational quantum chemistry to investigate whether the merits of this approach can be equaled by an alternative approach that instead exploits isotopic chirality. By first considering an N-methylpyrrolidine− cyclopentadiene motor design, it is shown that isotopically chiral variants of this design undergo faster photoisomerizations than a chemically chiral counterpart, while maintaining rotary photoisomerization quantum yields of similarly high magnitude. However, by subsequently considering a pyrrolinium−cyclopentene design, it is also found that the introduction of isotopic chirality does not provide any control of the directionality of the photoinduced rotations within this framework. Taken together, the results highlight both the potential usefulness of isotopic rather than chemical chirality for the design of light-driven molecular motors, and the need for further studies to establish the exact structural circumstances under which this asymmetry is best exploited.
Journal of the American Chemical Society, 2002
Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2,3dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized. A single stereogenic center, bearing a methyl substituent, is present in each of the motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these compounds have stable isomers with pseudoaxial orientation of the methyl substituent and less-stable isomers with pseudoequatorial orientation of the methyl substituent. The photochemical and thermal isomerization processes of the motors were studied by NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations upon irradiation, each followed by a thermal helix inversion, resulting in a 360°rotation around the central double bond of the upper part with respect to the lower part. The direction of rotation is controlled by a single stereogenic center created by the methyl substituent at the upper part. The speed of rotation, governed by the two thermal steps, was adjusted to a great extent by structural modifications, with half-lives for the thermal isomerization steps ranging from t1/2 θ 233-0.67 h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria of 1:99.
Kinetic analysis of the rotation rate of light-driven unidirectional molecular motors
Physical Chemistry Chemical Physics, 2009
The combination of a photochemical and a thermal equilibrium in overcrowded alkenes, which is the basis for unidirectional rotation of light-driven molecular rotary motors, is analysed in relation to the actual average rotation rates of such structures. Experimental parameters such as temperature, concentration and irradiation intensity could be related directly to the effective rates of rotation that are achieved in solution by means of photochemical and thermal reaction rate theory. It is found that molecular properties, including absorption characteristics and photochemical quantum yields, are of less importance to the overall rate of rotation than the experimental parameters. This analysis holds considerable implications in the design of experimental conditions for functional molecular systems that will rely on high rates of rotation, and shows that average rotation rates comparable to ATPase or flagella motors are within reach assuming common experimental parameters.
In recent years, much progress has been made in the design, synthesis and operation of light-driven rotary molecular motors based on chiral overcrowded alkenes. Through consecutive cis-trans photoisomerization and thermal helix inversion steps, where the latter dictate the overall rate of rotation, these motors achieve a full 360 unidirectional rotation around the carbon-carbon double bond connecting the two (rotator and stator) alkene halves. In this work, we report quantum chemical calculations indicating that a particularly fast-rotating overcrowded alkene-based motor capable of reaching the MHz regime, can be made to rotate even faster by the substitution of a rotator methyl group with a methoxy group. Specifically, using density functional theory methods that reproduce the rate-limiting 35kJmolAˋ1thermalfree−energybarriersshownbythemethyl−bearingmotorwitherrorsof35 kJ mol À1 thermal free-energy barriers shown by the methyl-bearing motor with errors of 35kJmolAˋ1thermalfree−energybarriersshownbythemethyl−bearingmotorwitherrorsof5 kJ mol À1 only, it is predicted that this substitution reduces these barriers by a significant 15-20 kJ mol À1 . This prediction is preceded by a series of benchmark calculations for assessing how well density functional theory methods account for available experimental data (crystallographic, UV-vis absorption, thermodynamic) on the rotary cycles of overcrowded alkenes, and a detailed examination of the thermal and photochemical reaction mechanisms of the original motor of this type. Fig. 1 (a) Rotary cycle of motor 1. (b) Energy profile of the rotary cycle. Adapted from M. M. Pollard, M. Klok, D. Pijper and B. L. Feringa, Rate Acceleration of Light-Driven Rotary Molecular Motors, Adv. Funct. Mater., 2007, 17, 718-729, with permission from John Wiley and Sons. Scheme 2 Chemical structures of the trans isomers of motors 2 and 3.
A chiral molecular propeller designed for unidirectional rotations on a surface
Nature Communications, 2019
Synthetic molecular machines designed to operate on materials surfaces can convert energy into motion and they may be useful to incorporate into solid state devices. Here, we develop and characterize a multi-component molecular propeller that enables unidirectional rotations on a material surface when energized. Our propeller is composed of a rotator with three molecular blades linked via a ruthenium atom to a ratchet-shaped molecular gear. Upon adsorption on a gold crystal surface, the two dimensional nature of the surface breaks the symmetry and left or right tilting of the molecular gear-teeth induces chirality. The molecular gear dictates the rotational direction of the propellers and step-wise rotations can be induced by applying an electric field or using inelastic tunneling electrons from a scanning tunneling microscope tip. By means of scanning tunneling microscope manipulation and imaging, the rotation steps of individual molecular propellers are directly visualized, which confirms the unidirectional rotations of both left and right handed molecular propellers into clockwise and anticlockwise directions respectively.
A redesign of light-driven rotary molecular motors
Organic & Biomolecular Chemistry, 2008
Structural modification of unidirectional light-driven rotary molecular motors in which the naphthalene moieties are exchanged for substituted phenyl moieties are reported. This redesign provides an additional tool to control the speed of the motors, and should enable the design and synthesis of more complex systems. Fig. 2 Tandem Friedel-Crafts/Nazarov cyclization, followed by a Mc-Murry coupling in the synthesis of cis-3 and trans-3.
Mechanism of unidirectional motions of chiral molecular motors driven by linearly polarized pulses
The Journal of Chemical Physics, 2003
The mechanism of the unidirectional rotational motion of a chiral molecular motor driven by linearly polarized laser pulses was theoretically studied. A simple aldehyde molecule was adopted as a chiral molecular motor, in which a formyl group ͑-CHO͒ was the rotating part of the motor. Temporal evolutions of the instantaneous angular momentum averaged over an ensemble of randomly oriented motors were taken as a measure of the unidirectional motion. The contour plots of the averaged instantaneous angular momentum were obtained by using a quantum master equation approach that took into account relaxation effects and a classical trajectory approach. Two regimes are found in the contour plots. One is an intense laser field regime in which the laser-motor interaction energy exceeds the asymmetric potential barrier. In this regime, the motors are unidirectionally driven in the intuitive direction, i.e., the gentle slope of the potential. The other regime is a subthreshold laser intensity regime in which unintuitive rotational motions also occur. This unintuitive rotation is found to be a quantum effect, as indicated by contour plots calculated by taking into account temperature effects.