26 Interplay of Photoisomerization and Phase Transition Events …
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such motors is of great interest. To achieve spatially unidirectional motion of a molecular motor, molecular-level chirality is key. However, it is not sufficient due to the
principle of microscopic reversibility. To realize an effective and powerful molecular motor that performs work, a nonlinear process is required in the motor system
to control the reverse process [18–20]. Recently, time-delayed chemical processes,
such as that employed in a “molecular information ratchet” [16, 21–24], have been
considered a good choice for generating molecular motors [25–28]. To date, lightpowered motors have been developed more rapidly than chemical-fueled motors.
This is partly because microscopic reversibility is not a concern for light-powered
molecular motors [25, 29–31].
However, even if excellent molecular motors are identified, the task remains to
have these motors act on another object to produce work. It is a straightforward
concept that the mechanical energy of a molecular motor in a monodispersed solution is converted immediately to thermal energy, without mentioning the physical
principle of Onsager’s reciprocal relation [25, 32]. Given the importance of friction
for actual work, it is important that the spatial and temporal scales of a working
system be considered. A relative size difference between a motor and an object is
required in a mesoscale system. The concept represents a challenge in the field of
molecular motor research. One promising approach is to place a molecular motor
on a large substrate and construct an anisotropic reaction field [33]. A pioneering
example is light-powered electron transportation through a vesicular membrane to
generate a proton-motive force [34, 35] or electromotive force [36]. In this model,
the molecular motor (generally referred to as the “sensitizer”) cyclically changes
its shape as a result of changes in structure to achieve an excited state, followed
by the generation of a radical/ion state. These cyclic changes are represented with
triangle, square, and circle symbols, respectively, in Fig. 26.2a. Another well-known
example involves the long-term rotation of small rods on cholesteric liquid crystals with light-powered molecular motor dopants [37–40]. To achieve a continuous
motion with relative size difference, synchronization of molecular motion is required
(Fig. 26.2b). If molecular-level motions are not synchronized, the system falls into a
static steady-state and no further mechanical motion is expected (Fig. 26.2c). Thus,
it is necessary to keep a system far-from-equilibrium state via a nonlinear process
[41].
26.1.2 Motion of Macroscopic Objects
In this chapter, we focus on the synchronization of molecular motors in regard to a
“photosynergetic” aspect. As mentioned above, one of the advantages of chemicalfueled molecular motors is their energy efficiency. In cooperation with thermal fluctuations, the energy efficiency of structural transitions can potentially reach up to 100%
[42]. However, it is rather difficult to accommodate the injection and elimination of
chemical substrates when constructing an artificial motorization system in a solid
phase. To date, most studies of self-organized motion using chemical processes have
451
such motors is of great interest. To achieve spatially unidirectional motion of a molecular motor, molecular-level chirality is key. However, it is not sufficient due to the
principle of microscopic reversibility. To realize an effective and powerful molecular motor that performs work, a nonlinear process is required in the motor system
to control the reverse process [18–20]. Recently, time-delayed chemical processes,
such as that employed in a “molecular information ratchet” [16, 21–24], have been
considered a good choice for generating molecular motors [25–28]. To date, lightpowered motors have been developed more rapidly than chemical-fueled motors.
This is partly because microscopic reversibility is not a concern for light-powered
molecular motors [25, 29–31].
However, even if excellent molecular motors are identified, the task remains to
have these motors act on another object to produce work. It is a straightforward
concept that the mechanical energy of a molecular motor in a monodispersed solution is converted immediately to thermal energy, without mentioning the physical
principle of Onsager’s reciprocal relation [25, 32]. Given the importance of friction
for actual work, it is important that the spatial and temporal scales of a working
system be considered. A relative size difference between a motor and an object is
required in a mesoscale system. The concept represents a challenge in the field of
molecular motor research. One promising approach is to place a molecular motor
on a large substrate and construct an anisotropic reaction field [33]. A pioneering
example is light-powered electron transportation through a vesicular membrane to
generate a proton-motive force [34, 35] or electromotive force [36]. In this model,
the molecular motor (generally referred to as the “sensitizer”) cyclically changes
its shape as a result of changes in structure to achieve an excited state, followed
by the generation of a radical/ion state. These cyclic changes are represented with
triangle, square, and circle symbols, respectively, in Fig. 26.2a. Another well-known
example involves the long-term rotation of small rods on cholesteric liquid crystals with light-powered molecular motor dopants [37–40]. To achieve a continuous
motion with relative size difference, synchronization of molecular motion is required
(Fig. 26.2b). If molecular-level motions are not synchronized, the system falls into a
static steady-state and no further mechanical motion is expected (Fig. 26.2c). Thus,
it is necessary to keep a system far-from-equilibrium state via a nonlinear process
[41].
26.1.2 Motion of Macroscopic Objects
In this chapter, we focus on the synchronization of molecular motors in regard to a
“photosynergetic” aspect. As mentioned above, one of the advantages of chemicalfueled molecular motors is their energy efficiency. In cooperation with thermal fluctuations, the energy efficiency of structural transitions can potentially reach up to 100%
[42]. However, it is rather difficult to accommodate the injection and elimination of
chemical substrates when constructing an artificial motorization system in a solid
phase. To date, most studies of self-organized motion using chemical processes have