Molecular Switches and Motors Powered by Orthogonal Stimuli

Synthetic molecular machines are designed to undergo specific nanoscale movements in response to specific physical or chemical stimuli. The "engines" of molecular machines responsible for their motion are usually presented by molecular switches and motors that undergo predetermined shape transformations upon appropriate stimulation. Various types of molecular switches and motors activated by different stimuli, such as light irradiation or chemical input, have been devised and used as the core of mechanically active molecular systems. However, despite extensive research, the structural and functional complexity of state-of-the-art synthetic molecular machines does not rival that of biological or human-scale counterparts. The most apparent difference is that current molecular machines show only limited modes of translational, rotational, and oscillatory motion that are typically triggered by a single stimulus. A promising strategy for achieving more elaborate movement is to integrate multiple active parts that can be manipulated by orthogonal stimuli. However, this approach is often complicated by mutual interference between the components or between stimuli. In this review, we survey recent notable progress in the development of molecular switches and motors that exhibit distinct modes of movement triggered by orthogonal stimuli. Though this approach is challenging, it may provide insights about the innovation needed to develop next-generation molecular machines with improved maneuverability and more sophisticated functions.