Synthetic molecular motors

Synthetic molecular motors are molecular machines 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 ^[1]. 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 nanolevel improves.

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Chemically driven rotary molecular motors

 A first example of a synthetic chemically driven rotary molecular motor was reported by Kelly and co-workers in 1999^[2]. Their system is made up from a three-bladed triptycene rotor and a [4]helicene, and is capable of performing a unidirectional 120° rotation.

This rotation takes place in five steps. First, the amine group present on the triptycene moiety is converted to an isocyanate group by condensation with a phosgene molecule (a). Thermal or spontaneous rotation around the central bond then brings the isocyanate group in proximity of the hydroxyl group located on the helicene moiety (b), thereby allowing these two groups to react with each other (c). This reaction irreversibly traps the system as a strained cyclic urethane that is higher in energy and thus energetically closer to the rotational energy barrier than the original state. Further rotation of the triptycene moiety therefore requires only a relatively small amount of thermal activation in order to overcome this barrier, thereby releasing the strain (d). Finally, cleavage of the urethane group restores the amine and alcohol functionalities of the molecule (e).

The result of this sequence of events is a unidirectional 120° rotation of the triptycene moiety with respect to the helicene moiety. Additional forward or backward rotation of the triptycene rotor is inhibited by the helicene moiety, which serves a function similar to that of the pawl of a ratchet. The unidirectionality of the system is a result from both the asymmetric skew of the helicene moiety as well as the strain of the cyclic urethane which is formed in c. This strain can be only be lowered by the clockwise rotation of the triptycene rotor in d, as both counterclockwise rotation as well as the inverse process of d are energetically unfavorable. In this respect, the preference for the rotation direction is determined by both the positions of the functional groups and the shape of the helicene, and is thus build into the design of the molecule instead of dictated by external factors.

The motor by Kelly and co-workers is an elegant example of how chemical energy can be used to induce controlled, unidirectional rotational motion, a process which resembles the consumption of ATP in organisms in order to fuel numerous processes. However, it does suffer from a serious drawback: the sequence of events that leads to 120° rotation is not repeatable. Kelly and co-workers have therefore searched for ways to extend the system so that this sequence can be carried out repeatably. Unfortunately, their attempts to accomplish this objective have not been successful and currently the project has been abandoned^[3].

Light-driven rotary molecular motors

  In 1999, the laboratory of Prof. Dr. Ben L. Feringa at the University of Groningen (The Netherlands) reported the creation of a unidirectional molecular rotor^[4]. Their 360° molecular motor system 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 long reaction time for complete rotation in these systems, which does not compare to rotation speeds displayed by motor proteins in biological systems. In the fastest system to date, with a fluorene lower half, the half-life of the thermal helix inversion is 0.005 seconds ^[5]. This compound is synthesized using the Barton-Kellogg reaction. In this molecule, the slowest step in its rotation (the thermally induced helix-inversion) is believed to proceed much more quickly because the larger tert-butyl group makes the unstable isomer even less stable than when the methyl group is used. Said differently, the unstable isomer is more destabilized than the transition state that leads to helix-inversion. The different behaviour of the two molecules is illustrated by the fact that the half-life time for the compound with a methyl group instead of a tert-butyl group is 3.2 minutes.^[6]

The Feringa principle has been incorporated into a prototype nanocar ^[7]. 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 scanning tunneling microscopy monitoring, although so far this has not been observed. Interestingly, the motor does not perform with fullerene wheels because they quench the photochemistry of the motor moiety.

References

See also

• Molecular motors
• Nanomotor