456
Y. Kageyama
26.3 Mechanism for Self-Continuous Flipping Motion
A co-assembly of molecules 1 and 2 leads to two types of self-oscillatory flipping
motion under blue-light irradiation [53]. One type involves a single flipping event,
while the second type involves a multi-step flipping event. The mechanistic study
described in this section targets the singular flipping motion, while the multi-step
motion is described below in Sect. 26.4, and in further detail in [62]. When a dispersion of the co-assembly was analyzed in situ with X-ray diffraction (XRD), the
small-angle profile showed one-peak and two-peaks in the dark versus under bluelight irradiation, respectively (Fig. 26.6a). These results indicate that a new phase
co-exists with the original phase under light irradiation. Figure 26.6b shows the
time-course behavior of the self-oscillatory motion detected under various intensities of steady 435-nm light. Figure 26.6c shows the time-course behavior of another
assembly under 435 and 470-nm light, while Fig. 26.6d shows the time-course of
thermal behavior of the co-assembly after light irradiation. Taken together, these
results indicate that one-cycle of the flipping motion of the co-assembly consists of
four steps. Step 1 is a trans-to-cis photoisomerization event, while step 3 is a cisto-trans photoisomerization event. Meanwhile, reversal of the isomerization events
occurs due to phase transitions that occur in steps 2 and 4. These transitions are
caused by shifts in the kinetic properties of 2. Slope values for the logarithmic relationship between incident light power and the duration times of steps 1 and 3 are
1 (Fig. 26.6b), indicating that these steps are photon number-determined processes.
In other words, to shift from the photo-process events (e.g., steps 1 and 3) to the
phase transition-processes (e.g., steps 2 and 4), the isomer ratio of the assembly
must achieve a particular threshold. This means that the self-oscillation property of
this system is not chaotic, but rather represents a type of limit-cycle (Fig. 26.7).
Therefore, the assembly charges internal energy (the sum of the mechanical stress
and the enthalpy of molecular formation) up to its threshold by accepting light in
step 1. The assembly then discharges part of the energy with morphological changes
in steps 2 and 4. Overall, these events demonstrate that this assembly has the ability
to apply mechanical work to other objects periodically. For example, when the coassembly is placed in water, it applies impulses to the water which manifests as a
swimming motion [53]. Details regarding this system are reported elsewhere [63,
64].
26.4 Analogues also Exhibiting a Self-Oscillatory Flipping
Motion
The simple mechanism described above may also be observed in other assemblies
composed of photochromic compounds. However, until now, we have not investigated photochromic compounds other than azobenzene derivatives. To date, we have
successfully generated several self-oscillatory motions in co-assemblies of several
Y. Kageyama
26.3 Mechanism for Self-Continuous Flipping Motion
A co-assembly of molecules 1 and 2 leads to two types of self-oscillatory flipping
motion under blue-light irradiation [53]. One type involves a single flipping event,
while the second type involves a multi-step flipping event. The mechanistic study
described in this section targets the singular flipping motion, while the multi-step
motion is described below in Sect. 26.4, and in further detail in [62]. When a dispersion of the co-assembly was analyzed in situ with X-ray diffraction (XRD), the
small-angle profile showed one-peak and two-peaks in the dark versus under bluelight irradiation, respectively (Fig. 26.6a). These results indicate that a new phase
co-exists with the original phase under light irradiation. Figure 26.6b shows the
time-course behavior of the self-oscillatory motion detected under various intensities of steady 435-nm light. Figure 26.6c shows the time-course behavior of another
assembly under 435 and 470-nm light, while Fig. 26.6d shows the time-course of
thermal behavior of the co-assembly after light irradiation. Taken together, these
results indicate that one-cycle of the flipping motion of the co-assembly consists of
four steps. Step 1 is a trans-to-cis photoisomerization event, while step 3 is a cisto-trans photoisomerization event. Meanwhile, reversal of the isomerization events
occurs due to phase transitions that occur in steps 2 and 4. These transitions are
caused by shifts in the kinetic properties of 2. Slope values for the logarithmic relationship between incident light power and the duration times of steps 1 and 3 are
1 (Fig. 26.6b), indicating that these steps are photon number-determined processes.
In other words, to shift from the photo-process events (e.g., steps 1 and 3) to the
phase transition-processes (e.g., steps 2 and 4), the isomer ratio of the assembly
must achieve a particular threshold. This means that the self-oscillation property of
this system is not chaotic, but rather represents a type of limit-cycle (Fig. 26.7).
Therefore, the assembly charges internal energy (the sum of the mechanical stress
and the enthalpy of molecular formation) up to its threshold by accepting light in
step 1. The assembly then discharges part of the energy with morphological changes
in steps 2 and 4. Overall, these events demonstrate that this assembly has the ability
to apply mechanical work to other objects periodically. For example, when the coassembly is placed in water, it applies impulses to the water which manifests as a
swimming motion [53]. Details regarding this system are reported elsewhere [63,
64].
26.4 Analogues also Exhibiting a Self-Oscillatory Flipping
Motion
The simple mechanism described above may also be observed in other assemblies
composed of photochromic compounds. However, until now, we have not investigated photochromic compounds other than azobenzene derivatives. To date, we have
successfully generated several self-oscillatory motions in co-assemblies of several