Light‐driven rotary molecular motors: an ultrafast optical study

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 $35 kJ mol À1 thermal free-energy barriers shown by the methyl-bearing motor with errors of $5 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.

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 Chemical Physics

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.

Chemical Physics Letters, 2000

Ab initio optical rotations have been obtained for allene, 1,3-dichloroallene, H O , and H S as a function of dihedral 2 2 2 2 Ž . angles, both at SCF and post-SCF levels. The results have been used to test a the concept that the molecular optical rotation can be written as a Ý M sin u , where u is the dihedral angle of the ith X-A-A-X type segment and M is a

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.

Chemistry - A European Journal, 2010

2009

The rotational profiles of 9-(2,3-dihydro-2-phenyl-1H-benz[e]inden-1-ylidene)-9H-fluorene, a recently developed light-driven molecular motor, have been determined in the gas phase and chloroform solution for both the singlet and the triplet electronic states. Calculations in the gas phase were performed at the (U)B3LYP/6-31G(d) and (U)MP2/6-31G(d) levels, while the effects of the solvent were examined considering both implicit and explicit solvation models through self-consistent reaction-field and quantum mechanically/molecular mechanics schemes, respectively. The free energy barriers for the thermal helix inversion obtained in the gas phase (17.0 and 13.6 kcal mol -1 at the UB3LYP and UMP2 levels, respectively) are in good agreement with the experimental estimation (21 kcal mol -1 ). On the other hand, the implicit solvation model led to a reduction of the barrier, while application of umbrella sampling with explicit chloroform molecules yielded a free energy barrier of 21.3 kcal mol -1 for the ground state. The geometrical changes produced during the rotation, which are very significant, are discussed in

2019

Molecular motors convert external energy into directional motions at the nanoscale. To date unidirectional circular rotations and linear motions have been realized but more complex directional trajectories remain unexplored on the molecular level. In this work we present a molecular motor powered by green light allowing to produce an eight-shaped geometry change during its unidirectional rotation around the central molecular axis. Motor motion proceeds in four different steps, which alternate between light powered double bond isomerizations and thermal hula-twist isomerizations. The result is a fixed sequence of populating four different isomers in a fully unidirectional trajectory possessing one crossing point. This motor system opens up new avenues for the construction and mechanisms of molecular machines and will therefore not only expand the toolbox of responsive molecular devices but enable unprecedented applications in the field of miniaturized technology in the future.

Proceedings of the National Academy of Sciences, 1997

Nature invented molecular rotatory devices such as the f lagellar motor and ATP synthase. Photoselection techniques have been frequently used to detect the rotational random walk of proteins but only rarely for the rotational drift of subunits in proteins. Pertinent theories predict an oscillatory behavior of the polarization anisotropy, r, for unidirectional rotational drift, as opposed to a monotonic relaxation of r for bidirectional random walk. The underlying assumption of an angular continuum is questionable for intersubunit rotation in proteins. We developed a theory for stepped rotatory devices. It predicts the damped oscillation of r under unidirectional drift. Damping increases with decreasing number of steps. For only three steps a quasi-monotonic relaxation of r is predicted for both random walk and drift. In photoselection experiments with active F-ATPase we observed the relaxation of r when a spectroscopic probe was attached to the central ␥-subunit. This behavior is compatible with the expectation for a three-stepped rotatory device. Rotational Random Walk and Rotational Drift in a Continuum A rotatory device at the molecular scale undergoes Brownian rotational diffusion in response to very frequent (10 13 s Ϫ1) The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ''advertisement'' in accordance with 18 U.S.C. §1734 solely to indicate this fact.