Understanding and Controlling Coupled Molecular Motion on Surfaces

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In this project funded by the Macromolecular, Supramolecular, and Nanochemistry Program of the Chemistry Division, Professor Charles Sykes and his students at Tufts University are investigating ways to couple single molecule devices, already pioneered by his research team with prior NSF funding, into molecular machines and molecular-sized devices capable of performing higher tasks. The ultimate goal of the work is the discovery of design principles for the construction of molecular machines and molecular-sized devices that can more easily integrate with existing technologies. A goal is to gain a better understanding of molecular motion. Control of molecular motion is crucial for the design of new approaches for unique new applications, including tiny molecular-sized pumps, sensors, and optoelectronics. To engage broad audiences in this project, the Sykes research team gives presentations about nanoscienceat local high schools with newly developed demonstrations. YouTube videos featuring the project results are produced. The group has developed a Science Fair between Tufts and Medford High which enables ~300 students per year to interact with graduate researchers about their science project presentations.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. This project takes a new approach using molecular self-assembly to produce 2D crystalline arrays of molecular rotors that display emergent properties like correlated rotational switching. The arrays are synthesized by the Ullmann reaction of precursor molecular rotors with a Cu (111) surface, yielding 2D networks of metal-organic complexes in which the rotary units can interact with each other. Scanning tunneling microscopy enables excitation of the individual rotor groups and the ability to study how reorientation of an individual molecular rotor affects its neighbors. By studying the effect of voltage, current, and tunneling gap distance on rotor motion, the mechanism of excitation will be explored. Altering the size and functionality of the precursor molecules enables control over the placement of the rotor units in the 2D molecular crystals and the ability to study the effect of rotor-rotor spacing and angle on correlated rotational switching. By changing the functionality of the rotor itself, both steric and dipolar coupling will be explored. Finally, chiral rotary units will be introduced as a way to induce unidirectional rotation via a flashing temperature ratchet-like mechanism.