ABSTRACT
Over the years, the progress of artificial molecular machines is still in the infancy stage, and examples that do a designated work in the molecular level have yet to be demonstrated [16]. Nevertheless, previous efforts have advanced the control of molecular motions, as demonstrated with many molecular and supramolecular prototypes of machinery systems [6-8]. In general, these prototypes possess two or more distinct mechanical states (dynamic or static) that can be switched or biased by external stimuli (energy input). The machinelike movements associated with the switching processes mimic the motion of macroscopic counterparts, leading to so-called molecular rotors, [17-20] gears, [21-27] motors, [28-36] brakes, [37-42] shuttles, [43-46] and many others [47-55]. As the input of external energy is to switch reversibly among the mechanical states, these systems are better described as molecular switches or devices rather than machines [16].One important issue for the design of molecular switches is the switching efficiency among the mechanical states. The switching efficiency is defined as the percentage of molecules that is switched from one state to the other in each operation. One of the factors that affect the switching efficiency is the operation modules (mechanisms), which are related to the nature of the input energy.Typical energy sources in operating a molecular switch are chemicals (chemical energy), light (radiant energy, photons), electricity (electrical energy), and heat (thermal energy). Although scientists have attempted to manipulate heat, [56] machinery control with thermal energy is relatively energy-demanding, inefficient, and non-directional and thus not ideal. The switching efficiency of chemicals-gated molecular systems is governed by the thermodynamic and kinetic principles of molecular and supramolecular reactions. The presence of many efficient molecular and supramolecular reactions for choice is a merit of the use of chemical fuels. The main drawback of using chemical energy is the generation of chemical wastes, which should be removed to maintain an optimal condition for continued operation. In addition, the energy input by adding and distributing appropriate amount of chemicals are also time-consuming, which slows down the
switching process. In this context, photochemical and electro-chemical systems are particularly intriguing, because they are free of the problem of accumulation of chemical waste during the operation with light and electricity. Furthermore, light and electricity have several parameters for manipulation. Useful parameters include intensity, frequency, phase (polarization), and duration for light, and current intensity and electric potential for electrical energy. Table 4.1 lists the merits and drawbacks of chemicals, light, and electricity as the energy sources for an operation of molecular switches. Table 4.1 Comparison of chemicals, light, and electricity as energy sources for the operation of a molecular switch Energy source Merits DrawbacksChemicals High switching efficiency Waste generation Slow input Often equilibrated mechanical statesLight Reagent free Waste freeRemote control Rapid input
Often incomplete switching Electricity Reagent free Waste freeComplete switching (on the surface of electrode)
Non-remote controlLow stability of some redox states Among the various types of motions, rotation is a fundamental motional mode of objects, either as big as planets or as small as atoms. The energy required for the rotary motion depends on the systems. For example, Earth’s rotation results from a reservation of angular momentum of the constituent interstellar clouds, rocks, and gas upon collapses [57]. The rotation of windmill sails is driven by the energy of wind. A merry-go-round is rotated by electrical energy or manual labor. The rotation of car wheels is powered by chemicals (gasoline) or electricity (battery). In the case of molecules, internal rotations about a single bond occur as long as the thermal energy is larger than the rotation barriers.Internal Brownian rotary motion is present in all molecules containing unconstrained single bonds. The ability to control the
rotation about a specific bond in molecules or the rotation of a specific subunit in a supramolecular ensemble is a prerequisite for the development of rotation-based molecular machines. Therefore, in a so-called “molecular rotor,” rotation about a specific single bond is generally targeted. The part with larger moment of inertia at one side of the single bond is often considered as a stationary reference called “stator,” and the other part is the “rotor” component (Fig. 4.1). When the structure of a molecular rotor can be tuned by external stimuli (fuels) in a way that the rotation rate (rotation barrier) of the rotor in the accessible structures varied to a detectable extent, each of the structures corresponds to a mechanical state of the molecular rotor. In this context, the larger is the difference in the rotation rate between the mechanical states, the larger is the signal on/off ratio in the switching.
