To achieve not only detailed understanding about the mechanisms of molecular motors, but also optimize their functionality in view of upscaling, the chemical structure of these molecules will be tuned and their motor activity will be studied in dependence on their geometric and electronic structure. The latter is important as excited states play a key role in the molecular transformations. Understanding and controlling them in view of excitation energies and excited state lifetimes is a central challenge for further insight into such nanomachines. Specifically, we will change the chemical structure of the molecular motors via side groups, the first experiments are currently ongoing. These can not only modify the motor mechanism itself, e.g. by sterically changing energy barriers between different states, but also the molecular configuration when adsorbed on a surface. Furthermore, by adding side groups that do not sterically, but electronically affect the molecular structure, additional control over the motor mechanism can be achieved. Another important approach for the variation of molecular structures is the combination of several motors within the same molecule, either in small structures or in large polymers. Such assemblies are of great interest in view of cooperative effects where the nanomachine is more than just the sum of its (motor) parts. We have achieved the first results on molecule with multiple motor units incorporated in them. Voltage pulses from the STM tip can excite these molecules and lead to a change in their configuration. Beyond the molecular structures, the AMOS project will attempt to tune the environment of each individual molecule via the surface. Experiments with long-distance motion of molecules have revealed that atomic defects modulate the speed of a single molecule by orders of magnitude. Besides the option of intrinsic or extrinsic defects in and on a surface, this can be done via the surface orientation or the choice of material. We found that changes of the material have a profound effect on the motor function, due to the modified surface lattice constant. Moreover, the material choice defines the reactivity of the surface with respect to the motor molecules with great impact on molecular configurations, stabilities, adsorption energies, and electronic states lifetimes. We have obtained first results of motor molecules on ultrathin films on a metal surface, indicating substantial changes in the motor function.
