Synthetic molecular motors
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Synthetic molecular motors are nanoscale devices capable of rotation under energy input. Although the term "molecular motor" has traditionally referred to a naturally occurring protein that induces motion, some groups also use the term when referring to non-biological, non-peptide synthetic motors. Many chemists are pursuing the synthesis of such molecular motors. The prospect of synthetic molecular motors was first raised by the nanotechnology pioneer Richard Feynman in 1959 in his classic talk There's Plenty of Room at the Bottom.
The basic requirements for a synthetic motor are repetitive 360° motion, the consumption of energy and unidirectional rotation. Two efforts in this direction were published in 1999 in the same issue of Nature. For the two reports below, it is unknown whether these molecules are capable of generating torque. It is expected that reports of more efforts in this field will increase, as understanding of chemistry and physics at the nanoscale improves.
In one molecular motor (Kelly, 1999) a three-bladed triptycene rotor connected to a rigid helicene scaffold is able to rotate 120° in a 5 step reaction sequence. The bond rotation barrier for the carbon carbon covalent bond connecting the two units and acting as the axle is 25 kcal/mol (105 kJ/mol).
In the first step of the sequence a molecule of phosgene is consumed converting the triptycene aniline group in (1) into an isocyanate (2). The motor then picks up speed by thermally induced rotation which accounts for 10 kcal/mol (42 kJ/mol) (visualized in 3). This movement brings the isocyanate group in close proximity of the hydroxyl spacer mounted on the helicene part for a reaction to take place to the urethane (4).
This locks in the clockwise movement and thermal energy provides the second slow (80% conversion in 6 hours) rotation step (5). Note that the anticlockwise movement would move the two reactive groups away from each other. Finally the urethane bond is cleaved by sodium borohydride in ethanol to the original functional groups in the atropisomer (6) of the original molecule and the process can start again.
In 1999, the laboratory of Prof. Dr. Ben L. Feringa at the University of Groningen (The Netherlands) reported the creation of a monodirectional molecular rotor. Their 360° molecular motor system (Feringa, 1999) consists of a bis-helicene connected by an alkene double bond displaying axial chirality and having two stereocenters.
One cycle of unidirectional rotation takes 4 reaction steps. The first step is a low temperature endothermic photoisomerization of the trans (P, P) isomer 1 to the cis (M, M) 2 where P stands for the right handed helix and M for the left handed helix. In this process the two axial methyl groups are converted into two less sterically favorable equatorial methyl groups.
By increasing the temperature to 20 °C these methyl groups convert back exothermally to the (P, P) cis axial groups (3) in a helix inversion. Because the axial isomer is more stable than the equatorial isomer, reverse rotation is blocked. A second photoisomerization converts (P, P) cis 3 into (M, M) trans 4, again with accompanying formation of sterically unfavorable equatorial methyl groups. A thermal isomerization process at 60 °C closes the 360° cycle back to the axial positions.
A major hurdle to overcome is the large reaction half life for complete rotation in these systems which does not compare to biological systems. In the fastest system up to date with a fluorene lower half the reaction half life for thermal helix inversion is 3.2 minutes. This fluorene compound is synthesized by the Barton-Kellogg reaction and rotation around the central bond is much less sterically crowded.
The Feringa principle has been incorporated into a prototype nanocar . The car thus far synthesized has an helicene-derived engine with an oligo (phenylene ethynylene) chassis and four carborane wheels and is expected to be able to move on a solid surface with atomic force microscopy monitoring.
Interestingly the motor does not perform with fullerene wheels because they quench the photoexcited state of the motor moiety.
- ↑ Synthetic Molecular Motors Jordan R. Quinn Online Article
- ↑ Light-driven monodirectional molecular rotor Nagatoshi Koumura, Robert W. J. Zijlstra, Richard A. van Delden, Nobuyuki Harada, Prof. Dr. Ben L. Feringa Nature 401, 152-155 1999 Abstract
- ↑ Unidirectional rotary motion in a molecular system T. Ross Kelly, Harshani De Silva, Richard A. Silva Nature 401, 150-152 1999 Abstract
- ↑ Controlling the speed of rotation in molecular motors. Dramatic acceleration of the rotary motion by structural modification Javier Vicario, Auke Meetsma and Ben L. Feringa Chem. Commun., 2005, 5910–5912. Abstract
- ↑ En Route to a Motorized Nanocar Jean-François Morin, Yasuhiro Shirai, and James M. Tour Org. Lett.; 2006; 8(8) pp 1713 - 1716; (Letter) Graphical abstract
- Molecular motors