As detailed in Chapter 1, molecular motors are subject to different forces and parameters when compared to motors and devices that operate at the macroscale. In particular, these molecularscale motors have no inertia and are subject to continuous Brownian motion that is random in nature. For small motors, where their size is similar to intermolecular distance, friction is “governed” by intermolecular interactions and the forces associated with such interactions. Directional motion requires three components27 and can be best described as diffusion process along a specific dimension of a potential energy surface28 where motion is achieved by successive raising and lowering of these energy barriers: 1. A randomising element (usually provided by Brownian motion). 2. An energy input, which is required to obey the second law of
thermodynamics. 3. An asymmetry in the energy potential along the direction
of movement, which can be manipulated by the operator to provide the directional motion. A chemical motor can be described as a molecule capable of
converting energy into such directed and organized motion. The first descriptions of such motors appeared simultaneously in Nature in September 1999 in two papers that presented rotary chemical
motors capable of unidirectional motion.29,30 Each motor used a different energy source to provide the motion, in one case light and in the other case chemical (a review of the broad range of chemical motors was produced by Kay et al.3). 2.1 Rotary Chemical Motors
There is little doubt that the driving force behind the design of chemical molecular motors are the beautiful and exquisitely efficient motors of biological systems and, as will be described in detail later, the archetypical rotary motor of nature is ATP synthase — this system is mentioned in many of the papers describing chemical motors. 2.1.1 Helicene-Based Motors
The first of the two letters to Nature29 describing rotary chemical motors demonstrated a motor capable of unidirectional rotation through 120° around a single chemical bond, driven by chemical energy. This motor was constructed of two sections around a chiral carbon (Fig. 2.1), one section is a 3-blade triptycene (coloured grey in Fig. 2.1), while the other is a helicene (coloured a darker shade in Fig. 2.1).