Molecular machines are ubiquitous in nature; they exhibit functions as varied as organizing the cellular cytoplasm by vesicle transport to powering the motion of cells and even driving whole body locomotion via muscle contraction (1). In stark contrast, current synthetic devices, with the exception of liquid crystals, make no use of nanoscale molecular motion. Over the last 20 years chemists have demonstrated both chemically and optically driven molecular motors (Nobel Prize 2016). Of particular interest is the electrical excitation of molecular motion, as the small dimensions of today’s electrical interconnects offer great promise for coupling external sources of energy to single molecules and realizing new function. In 2011 the Sykes group made the world’s first single molecule electric motor (2). By exploiting a unique combination of molecular chirality and mode-selective excitation of vibrations we demonstrated that directed single molecule rotational motion could be simultaneously driven and monitored using tunneling electrons from the STM tip (2-4).
We have also used alloy surface reconstructions to direct the growth of ordered arrays of molecular rotors, just like placing cogs on a pegboard, and can mechanically slow and "brake" them by altering their proximity to other molecules (5). These experiments are providing a framework for understanding coupled arrays of dipolar rotors, an important step towards tunable dielectrics and miniature signal delay lines . The work has also revealed novel methods for driving and monitoring the motion of arrays of motors, an area that may find application in the modulation of optical signals at microwave or THz frequencies. These simple molecular rotor systems have enabled us to study many important fundamental aspects of molecular rotation with unprecedented resolution and to realize the smallest molecular motor, which has captured the attention of the scientific community and the general public around the world (6).
While single molecule devices are capable of performing a number of functions from mechanical motion to simple computation. Their utility is somewhat limited, however, by difficulties associated with coupling them with either each other or with interfaces such as electrodes. Self-assembly of coupled molecular devices provides an option for the construction of larger entities that can more easily integrate with existing technologies. Most recently we have demonstrated that ordered organometallic arrays can form spontaneously by reaction of precursor molecular rotor molecules with a metal surface (7). The order and periodicity of these systems leads to complex correlated switching of the internal sub-molecular rotor units that resemble logic operations. Scanning tunneling microscopy enables simultaneous imaging (reading) and excitation (writing) of the individual rotor groups. Our results suggest that a myriad of materials with complex functions should be accessible via surface crystal engineering.
1) "Molecular Motors"M. Schliwa, G. Woehlke - Nature 2003, 422, 759-765. (link)
2) "Experimental Demonstration of a Single-Molecule Electric Motor" H. L. Tierney, C. J. Murphy, A. D. Jewell, A. E. Baber, E. V. Iski, H. Y. Khodaverdian, A. F. McGuire, Nikolai Klebanov and E. C. H. Sykes - Nature Nanotechnology 2011, 6, 625-629. (link)
3) "Regular Scanning Tunneling Microscope Tips can be Intrinsically Chiral" H. L. Tierney, C. J. Murphy and E. C. H. Sykes - Physical Review Letters 2011, 106, 010801. (link)
4) "Time-resolved studies of individual molecular rotors" A. D. Jewell, H. L. Tierney, A. E. Baber, E. V. Iski, M. M. Laha and E. C. H. Sykes - Journal of Physics-Condensed Matter 2010, 22, 264006-264016. (link)
5) "Engineering Dislocation Networks for the Directed Assembly of Two-Dimensional Rotor Arrays" D. O. Bellisario, A. E. Baber, H. L. Tierney and E. C. H. Sykes - Journal of Physical Chemistry C 2009, 113, 5895-5898. (link)
6) BBC NEWS: http://www.bbc.co.uk/news/science-environment-14763223
7) "Correlated Rotational Switching in 2D Self-Assembled Molecular Rotor Arrays" N. A. Wasio, D. P. Slough, Z. C. Smith, C. J. Ivimey, S. W. Thomas III, Y.-S. Lin, and E. C. H. Sykes - Nature Communications C 2017, 8, 16057. (link)