24
H. Miyasaka et al.
2.0 s and OFF at 4.0 s. As was observed in the DAE-PMMA particles, the trapped
PQ-PAE particle showed reciprocating motion synchronized with the photochromic
reaction of PQ. The Z-position of the trapped PQ-PAE particle gradually increased
to ca. 100 nm within ca. 1 s and the particle returned to the initial Z-position in >2 s.
This positional shift along the Z-axis corresponds to the color change of the trapped
particle as shown in Fig. 1.21b.
Figure 1.21d shows dependences of the Z-displacement of the PQ-PAE microparticles on UV dose and trapping wavelength. The Z-displacement monotonically
increases with increasing UV dose for both trapping wavelength of 532 and 690 nm.
The slope of the plot for trapping wavelength at 532 nm is much larger than that
for trapping with 690-nm laser. This can be ascribed to the difference in the extinction between the two wavelengths. As shown in Fig. 1.21a, the absorption of PQ at
532 nm under UV exposure is much larger than that at 690 nm, and this difference
is attributable to the wavelength dependence of the Z-displacement.
Not only the Z-direction, but also the motion on the lateral (XY ) plane could be
triggered by the locally induced photochromic reaction. Figure 1.21e shows the lateral
motion of a trapped PQ-PAE microparticle induced by focusing the UV laser into
an edge area of the left-hand side of the particle, where a weak emission signal was
observed. This emission is presumably due to the photodegradation of PQ induced by
the intense UV irradiation. The lateral position of the PQ-PAE microparticle shifted
along X-axis by UV exposure, while the Y-displacement was negligible. This Xdisplacement is mainly attributed to the absorption force acting only on the left-hand
side of the microparticle. After turning the UV OFF, the microparticle gradually
returned to its initial position by the gradient force.
As demonstrated in this section, a new light-driven micromechanical system was
developed by combining photochromic reaction and the force of light. Reciprocating
motions along the Z-axis was achieved by switching the absorption band of microparticles. Not only along the Z-axis, but also lateral motion was induced by partial UV
irradiation. The insight obtained in the present work will introduce a new strategy
into the photomechanical control of nano/micro-objects as photosynergetic response.
1.5 Summary
We have briefly introduced our research results relating to the photosynergetic
responses in the project. The femtosecond two-pulse excitation was applied also
to the elucidation of the photoionization dynamics in solutions taking place with the
energy lower than the ionization potential in the gas phase [36] and revealed that the
solvation process of the specific electronic state regulates the ionization.
On the advanced control of the photochemical reaction by the stepwise multiphoton excitation, we extended this method to the investigation on the electron
transfer reaction in higher excited states [37, 38]. In the tetraphenylporphyrin
(ZnTPP)-substituted bridged imidazole dimer system [37], visible stepwise twophoton excitation of ZnTPP led to the formation of the charge separated state via
H. Miyasaka et al.
2.0 s and OFF at 4.0 s. As was observed in the DAE-PMMA particles, the trapped
PQ-PAE particle showed reciprocating motion synchronized with the photochromic
reaction of PQ. The Z-position of the trapped PQ-PAE particle gradually increased
to ca. 100 nm within ca. 1 s and the particle returned to the initial Z-position in >2 s.
This positional shift along the Z-axis corresponds to the color change of the trapped
particle as shown in Fig. 1.21b.
Figure 1.21d shows dependences of the Z-displacement of the PQ-PAE microparticles on UV dose and trapping wavelength. The Z-displacement monotonically
increases with increasing UV dose for both trapping wavelength of 532 and 690 nm.
The slope of the plot for trapping wavelength at 532 nm is much larger than that
for trapping with 690-nm laser. This can be ascribed to the difference in the extinction between the two wavelengths. As shown in Fig. 1.21a, the absorption of PQ at
532 nm under UV exposure is much larger than that at 690 nm, and this difference
is attributable to the wavelength dependence of the Z-displacement.
Not only the Z-direction, but also the motion on the lateral (XY ) plane could be
triggered by the locally induced photochromic reaction. Figure 1.21e shows the lateral
motion of a trapped PQ-PAE microparticle induced by focusing the UV laser into
an edge area of the left-hand side of the particle, where a weak emission signal was
observed. This emission is presumably due to the photodegradation of PQ induced by
the intense UV irradiation. The lateral position of the PQ-PAE microparticle shifted
along X-axis by UV exposure, while the Y-displacement was negligible. This Xdisplacement is mainly attributed to the absorption force acting only on the left-hand
side of the microparticle. After turning the UV OFF, the microparticle gradually
returned to its initial position by the gradient force.
As demonstrated in this section, a new light-driven micromechanical system was
developed by combining photochromic reaction and the force of light. Reciprocating
motions along the Z-axis was achieved by switching the absorption band of microparticles. Not only along the Z-axis, but also lateral motion was induced by partial UV
irradiation. The insight obtained in the present work will introduce a new strategy
into the photomechanical control of nano/micro-objects as photosynergetic response.
1.5 Summary
We have briefly introduced our research results relating to the photosynergetic
responses in the project. The femtosecond two-pulse excitation was applied also
to the elucidation of the photoionization dynamics in solutions taking place with the
energy lower than the ionization potential in the gas phase [36] and revealed that the
solvation process of the specific electronic state regulates the ionization.
On the advanced control of the photochemical reaction by the stepwise multiphoton excitation, we extended this method to the investigation on the electron
transfer reaction in higher excited states [37, 38]. In the tetraphenylporphyrin
(ZnTPP)-substituted bridged imidazole dimer system [37], visible stepwise twophoton excitation of ZnTPP led to the formation of the charge separated state via