We are exploring the physical properties of small molecules that when adsorbed to surfaces can serve as machines at the smallest possible length scale. We hope to create a set of nanoscale molecular building blocks, consisting of single molecule pistons, levers, gears, bearings and other structures, that can be assembled into functional nanoscale mechanical devices. Further, we would like for the machines to be responsive to light or electric fields both in order to be able to control their operation remotely, and for them to be sensitive to their environment. Ultimately, we would like to achieve nanoscale remote-controllable robotics.
We have begun our work by investigating an optically powered nanoscale motor, as might propel some kind of nanoscale vehicle. An example of a possible simple nanoscale propulsion system is an inchworm style crawler. Inchworms move by coordinating the clamping and unclamping of their ends with the expansion and contraction of their midsections. Potentially, an assembly of small molecules on a surface might be able to undergo inchworm style crawling.
Azobenzene is an incredible small organic molecule that has the potential to perform as a photoswitchable nanoscale piston on a surface. Azobenzene exists in two different shapes (isomers, labeled trans and cis) and can be reversibly and reliably driven between these two shapes by shining UV and blue light on it (photoisomerization). The lengthening and contracting of azobenzene as it changes shape allows it to do mechanical work.
We want to observe single azobenzene molecules photoisomerizing on a surface and eventually assemble them into useful machines. To do this we use STM which gives us both sub-Ångstrom imaging resolution and manipulation capabilities. We have built a custom STM to do these experiments. The STM is Pan-style, with magnetic eddy-current damping, cooled to T = 30 K by a He gas-flow cryostat and housed within an ultra-high vacuum chamber. Viewports through the chamber and cryostat allow for direct LASER illumination of samples with the STM.
We attempted to observe photoswitching for azobenzene adsorbed to a gold surface. This failed. The close proximity of the azobenzene molecules to a metallic surface quenched the photoreaction.
In order to reduce the coupling between the azobenzene molecule and the surface and thereby avoid quenching, we chemically modified the azobenzene molecule (performed by our collaborators within the Jean Fréchet and Dirk Trauner groups). We added increasing numbers of tert-butyl ‘leg’ functional groups to the azobenzene molecule. The idea was that the legs would lift the azobenzene molecule away from the metal surface, and alleviate quenching.
We prepared samples of bare azobenzene (no legs), DTB-azobenzene (two legs), and TTB-azobenzene (four legs) separately adsorbed onto the Au(111) surface.
The heights of the molecules above the surface did increase with the addition of legs, as shown in the molecular profiles above measured by STM.
We illuminated all three species with UV light, and it turned out that only the tallest molecule, the 4-legged TTB-azobenzene molecule, photoswitched.
The figure above shows images of TTB-azobenzene molecules before and after illumination with UV light. Before shining light (upper panel), we see an ordered island of uniformly composed of trans molecules (molecules are added to the surface in the trans isomer, which is the ground state configuration of azobenzene). After shining UV light, many bright protrusions can be seen in the island (lower panel). Zoom-in images of the changes (inset) reveal identical changes to individual azobenzene molecules. These are cis molecules.
We can reverse the photoisomerization, as shown above for a single molecule.
Ab initio DFT calculations (performed by Steve Louie’s group) allow us to interpret our images of the photoswitched azobenzene molecules in great detail. The figure above shows the calculated structures of both the trans and cis TTB-azobenzene molecules, along with simulated STM images. It is clear that the bright central feature observed in STM images of cis molecules results from one upwards raised TTB leg.
In the future, we would like to improve our understanding of the dynamics of the photoswitching with the idea of influencing its tunability. Further, we need to learn more about the surface anchoring of the molecules and how to self-assemble or manipulate them into more complex and functional assemblies.
I. V. Pechenezhskiy, J. Cho, G. D. Nguyen, L. Berbil-Bautista, B. L. Giles, D. A. Poulsen, J. M. J. Fréchet, and M. F. Crommie, Self-Assembly and Photomechanical Switching of an Azobenzene Derivative on GaAs(110): Scanning Tunneling Microscopy Study, J. Phys. Chem. C 116, 1052–1055 (2012)
J. Cho, L. Berbil-Bautista, N. Levy, D. Poulsen, J. M. J. Fréchet, and M. F. Crommie, Functionalization, self-assembly, and photoswitching quenching for azobenzene derivatives adsorbed on Au(111), J. Chem. Phys. 133, 234707 (2010)
M. J. Comstock, D. A. Strubbe, L. Berbil-Bautista, N. Levy, J. Cho, D. Poulsen, J. M. J. Fréchet, S. G. Louie, and M. F. Crommie, Determination of Photoswitching Dynamics through Chiral Mapping of Single Molecules Using a Scanning Tunneling Microscope, Phys. Rev. Lett. 104, 178301 (2010)
N. Levy, M. J. Comstock, J. Cho, L. Berbil-Bautista, A. Kirakosian, F. Lauterwasser, D. A. Poulsen, J. M. J. Frechet, M. F. Crommie, Self-Patterned Molecular Photoswitching in Nanoscale Surface Assemblies, Nano Lett. 9, 935-939 (2009)
M. J. Comstock, N. Levy, J. Cho, L. Berbil-Bautista, M. F. Crommie, D. A. Poulsen, and J. M. J Fréchet, Measuring reversible photomechanical switching rates for a molecule at a surface, Appl. Phys. Lett. 92, 123107 (2008)
M. J. Comstock, N. Levy, A. Kirakosian, J. Cho, F. Lauterwasser, J. H. Harvey, D. A. Strubbe, J. M. J. Fréchet, D. Trauner, S. G. Louie and M. F. Crommie, Reversible photomechanical switching of individual engineered molecules at a surface, Phys. Rev. Lett. 99, 038301 (2007)
M. J. Comstock, J. Cho, A. Kirakosian and M. F. Crommie, Manipulation of azobenzene molecules on Au(111) using scanning tunneling microscopy, Phys. Rev. B 72, 153414 (2005)
A. Kirakosian, M. J. Comstock, J. Cho and M. F. Crommie, Molecular commensurability with a surface reconstruction: STM study of azobenzene on Au(111), Phys. Rev. B 71, 113409 (2005)
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