Light-driven Rotary Molecular Motors Research Articles

Theoretical Design of a Light-Driven Molecular Rotary Motor with Low Energy Helical Inversion: 9-(5-Methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene

A light-driven molecular rotary motor of 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (MPCPF) has been designed by means of ab initio complete active space self-consistent field and its second order multireference Møller-Plesset perturbation methods. In the present model molecule of MPCPF, 9H-fluorene (as a stator) and 5-methyl-2-phenyl-2-cyclopenten-1-ylidene (as a rotor) are directly linked with each other by a C═C double bond. Even by a substitution of phenyl group, MPCPF comes to have a stable P-helical MPCPF and a metastable M-helical MPCPF, and exhibits unidirectionality around the C═C double bond. In addition, interchange of the helicity can proceed with a low energy barrier through a floppy phenyl torsional motion. This is in contrast to previous light-driven molecular rotary motors where the unidirectionality is ensured by rigid and sterically overcrowded rotors. In the full rotary process of MPCPF, therefore, constancy of the rotation speed is expected to be much more improved as well as unidirectionality.

Periodic one-dimensional hopping model with transitions between nonadjacent states

A one-dimensional hopping model is useful for describing the motion of microscopic particles in a thermal noise environment. Recent experiments on the new generation of light-driven rotary molecular motors found that a motor in state i can jump forward to state i+1 or i+2 or backward to state i-1 or i-2 directly. In this paper, inspired by these experiments, such a modified periodic one-dimensional hopping model with arbitrary period N is studied theoretically. The mean velocity, effective diffusion constant, and mean dwell time in one single mechanochemical cycle are obtained. The corresponding results are illustrated and verified by being applied to the synthetic rotary molecular motors.

Reversing the direction in a light-driven rotary molecular motor

Biological rotary motors can alter their mechanical function by changing the direction of rotary motion. Achieving a similar reversal of direction of rotation in artificial molecular motors presents a fundamental stereochemical challenge: how to change from clockwise to anticlockwise motion without compromising the autonomous unidirectional rotary behaviour of the system. A new molecular motor with multilevel control of rotary motion is reported here, in which the direction of light-powered rotation can be reversed by base-catalysed epimerization. The key steps are deprotonation and reprotonation of the photochemically generated less-stable isomers during the 360° unidirectional rotary cycle, with complete inversion of the configuration at the stereogenic centre. The ability to change directionality is an essential step towards mechanical molecular systems with adaptive functional behaviour.

Rotary Molecular Motors: A Large Increase in Speed through a Small Change in Design

Reducing the steric interaction between the upper-half and the lower-half of a light-driven rotary molecular motor by decreasing the size of the aromatic moiety in the upper-half from a naphthalene to a benzothiophene results in an almost 3500 times faster rotation.

Understanding the Dynamics Behind the Photoisomerization of a Light-Driven Fluorene Molecular Rotary Motor

Light-driven molecular rotary motors derived from chiral overcrowded alkenes represent a broad class of compounds for which photochemical rearrangements lead to large scale motion of one part of the molecule with respect to another. It is this motion/change in molecular shape that is employed in many of their applications. A key group in this class are the molecular rotary motors that undergo unidirectional light-driven rotation about a double bond through a series of photochemical and thermal steps. In the present contribution we report a combined quantum chemical and molecular dynamics study of the mechanism of the rotational cycle of the fluorene-based molecular rotary motor 9-(2,4,7-trimethyl-2,3-dihydro-1H-inden-1-ylidene)-9H-fluorene (1). The potential energy surfaces of the ground and excited singlet states of 1 were calculated, and it was found that conical intersections play a central role in the mechanism of photo conversion between the stable conformer of 1 and its metastable conformer. Molecular dynamics simulations indicate that the average lifetime of the fluorene motor in the excited state is 1.40 +/- 0.10 ps when starting from the stable conformer, which increases to 1.77 +/- 0.13 ps for the reverse photoisomerization. These simulations indicate that the quantum yield of photoisomerization of the stable conformer is 0.92, whereas it is only 0.40 for the reverse photoisomerization. For the first time, a theoretical understanding of the experimentally observed photostationary state of 1 is reported that provides a detailed picture of the photoisomerization dynamics in overcrowded alkene-based molecular motor 1. The analysis of the electronic structure of the fluorene molecular motor holds considerable implications for the design of molecular motors. Importantly, the role of pyramidalization and conical intersections offer new insight into the factors that dominate the photostationary state achieved in these systems.

An Enantioselective Synthetic Route toward Second-Generation Light-Driven Rotary Molecular Motors

Controlling the unidirectional rotary process of second-generation molecular motors demands access to these motors in their enantiomerically pure form. In this paper, we describe an enantioselective route to three new second-generation light-driven molecular motors. Their synthesis starts with the preparation of an optically active alpha-methoxy-substituted upper-half ketone involving an enzymatic resolution. The subsequent conversion of this ketone to the corresponding hydrazone by treatment with hydrazine led to full racemization. However, conversion to a TBDMS-protected hydrazone by treatment with bis-TBDMS hydrazine, prepared according to a new procedure, proceeds with nearly full retention of the stereochemical integrity. Oxidation of the TBDMS-protected hydrazone and subsequent coupling to a lower-half thioketone followed by recrystallization provided the molecular motors with >99% ee. As these are the first molecular motors that have a methoxy substituent at the stereogenic center, the photochemical and thermal isomerization steps involved in the rotary cycle of one of these new molecules were studied in detail with various spectroscopic techniques.

Controlled rotary motion of light-driven molecular motors assembled on a gold film

Using circular dichroism (CD) spectroscopy, we show that light-driven rotary molecular motors based on overcrowded alkenes can function in a self-assembled monolayer on semi-transparent gold films.

Density Functional Study of the Ground and Excited State Potential Energy Surfaces of a Light-Driven Rotary Molecular Motor (3R,3′R)-(P,P)-trans-1,1′,2,2′,3,3′,4,4′-Octahydro-3,3′-dimethyl-4,4′-biphenanthrylidene

Potential energy surfaces of the ground and the first excited singlet states of the (3R,3'R)-(P,P)-trans-1,1',2,2',3,3',4,4'-octahydro-3,3'-dimethyl-4,4'-biphenanthrylidene rotary molecular motor have been investigated along the central C(4)=C(4') double bond twisting mode starting from the (P,P)-trans and from the (P,P)-cis conformations occurring in the photoisomerization cycle of this compound. The potential energy profiles obtained with the help of the state average spin restricted ensemble-referenced Kohn-Sham (SA-REKS) method feature minima on the excited state surface, the positions of which are displaced with respect to the barriers on the ground state surface toward the isomerization products, the (M,M)-cis and the (M,M)-trans conformations, respectively. The origin of these minima is analyzed and explained. The results of the present study suggest that the experimentally observed unidirectionality of photoinduced rotation in the above compound can be corroborated by the obtained profiles of the ground and excited state potential energy surfaces.

Kinetic analysis of the rotation rate of light-driven unidirectional molecular motors

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.

Light‐Driven Rotary Molecular Motors on Gold Nanoparticles

We report the synthesis of unidirectional light-driven rotary molecular motors based on chiral overcrowded alkenes and their immobilisation on the surface of gold nanoparticles through two anchors. Using a combination of (1)H and (13)C NMR, UV/Vis and CD spectroscopy, we show that these motors preserve their photochemical and thermal behaviour after they have been attached to gold nanoparticles. Furthermore, we describe the synthesis of (2)H- and (13)C-labelled derivatives that were used to verify the unidirectionality of the rotary cycle of these motors both in solution and while grafted to gold nanoparticles. Taken together, these data support the conclusion that these motors maintain their unidirectional rotary cycle when grafted to the surface of small (ca. 2 nm) gold nanoparticles. Thus, continuous irradiation of the system under appropriate conditions leads to unidirectional rotation of the upper half of the molecules relative to the entire nanoparticle.

A redesign of light-driven rotary molecular motors

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.

On the effect of donor and acceptor substituents on the behaviour of light-driven rotary molecular motors

Light-driven rotary molecular motors based on overcrowded alkenes can be substituted with electron-donating and electron-withdrawing substituents (R = OMe, Cl and CN) in direct conjugation with the central double bond (the axis of rotation) without having a significant influence on the rate-limiting, thermal isomerisation step of their rotary cycle. This indicates that in this system, it is predominantly steric factors that determine the barrier to the thermal helix inversion. In contrast, the quantum yield and photoequilibria in the photochemical step were found to be quite sensitive to the combination of substituent and solvent employed.

MOLECULAR MOTOR SPINS ON SURFACE

BY ANCHORING A CHIRAL HElical alkene onto a gold nanoparticle, chemists in the Netherlands have created the first light-driven molecular rotary motor attached to a solid surface ( Nature 2005 , 437 , 1337). This mounted molecule, the researchers say, might be a first step toward the construction of more elaborate and functional nanosized mechanical devices and perhaps systems to exploit solar energy. Fastening a rotary molecule to something solid, as Ben L. Feringa and colleagues at the University of Groningen have done, brings these spinning systems closer to becoming useful nanomachines. Putting motors on a surface is important because it should make it easier for them to do useful things, like move themselves or cargo, or change the nature of the surface in response to a stimulus, says T. Ross Kelly, a Boston College chemistry professor who made a molecular motor that runs on chemical fuel. Feringa's team uses two thiol groups to affix their molecular motor to ...