Murakami, M. et al J. AM. CHEM. 2004, 126, 14764
Miyasaka Lab
Tomohiro Kunishi
Contents
•Photochromism
•Motivation of the present paper
•Result & Discussion
•Conclusion
Photochromism
UV light
Vis. light
Photoinduced reversible transformation in a chemical species
between two forms without changes of molecular weight.
Absorption
Refractive indices
Oxidation potential
etc
Quick change of physical properties
between two isomers
Motivation
Optical and Photonic Device
(1) Thermal stability of both isomers
(2) Low fatigue
(3) Rapid response
(4) High sensitivity
(5) Non-destructive readout capability
(5) Non-destructive read-out
capability with the high
sensitivity
needs another conditions
arising from the change of the
some outer environments that
can act as “gate” of the reaction.
Excited state reaction generally
occurs in competition with various
processes in a finite lifetime.
S1
(3) Rapid response
= (4) high sensitivity
= (2) low fatigue
hv
S0
Target Reaction
Gated-Reaction Control via Multiphoton Laser Pulse Excitation
Photochromic Reaction
Closed-form
Open-form
450 ~ 700 nm
< 360 nm
Extinction Coefficient
4
-1
-1
(10  / M cm )
4
Open-form
3
2
Reaction yield from closed-form to
open-form is only 1.3%
under steady-state light irradiation.
Closed-form
1
0
300
400
500
600
Wavelength / nm
700
Reaction yield:反応収率
Absorbance (0.2/div.)
1 ns
100 ps
80 ps
60 ps
50 ps
40 ps
30 ps
20 ps
10 ps
0 ps
-10 ps
-26 ps
400
600
800
1000
Absorbance (Normalized)
Transient absorption spectra of PT1(c) in n-hexane
excited with a 15-ps 532-nm laser pulse.
0.4
0.2
0.0
-0.2
-0.4
100 ps
10 ps
Grnd. state
-0.6
400 500 600 700 800 900 1000
Wavelength / nm
Wavelength / nm
Cycloreversion reaction completed within 100 ps.
Perfect recovery of the closed form by UV light after ps 532 nm laser pulse.
Transient absorption:過渡吸収
Time profile of trasient absorbance
< femtosecond laser >
-2
< picosecond laser >
(a)
1.5x10
Ex. at 580 nm /
Mon. at 680 nm
-2
1.0x10
-3
5.0x10
0.20
0.0
0.10
0
20
40
60
80
0.05
-3
-1.0x10
-2
-1.5x10
-2
0.00
Ex. at 580 nm /
Mon. at 620 nm
(b)
0
20
40
60
80
0.0
-2.0x10
-2
-4.0x10
-2
Ex. at 540 nm /
Mon. at 580 nm
(c)
0
20
40
60
80
Time / ps
10 ps decay / Reaction yield : 1-2 %.
No excitation wavelength effect.
0
Absorbance
Absorbance
0.0
-5.0x10
(a) 710nm
0.15
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
50
100
150
(b) 580nm
0
50
100
Time/ps
150
Time constant is almost the same.
But, remaining absorption is large.
Apparent Reaction Yield
S1
hv
ko
ΦO
kn
S0
Closed isomer Open isomer
0= k0/(kn+ko)
10ps=1/(kn+ko)
Drastic enhancement of the cycloreversion reaction yield.
1.3 % (steady-state irradiation)  40 % ( ps 532 nm laser excitation)
Conversion Efficiency
Excitation intensity dependence : conversion efficiency
at 160 ps after the excitation with a 15-ps 532-nm laser pulse.
10
Conversion efficiency is
quadratically in proportion
with the exitation intensity
0
10
-1
10
-2
Slope~2
0.01
0.1
1
2
Excitation Intensity (mJ / mm )
Conversion efficiency
-Abs590nm / Abs590nm
Two-photon process
is responsible for
the efficient bond breakage.
Conversion efficiency:変換効率
Two-photon Absorption Processes
(1)
Simultaneous two-photon absorption process
Sn
hv
I ：Peak Intensity (photon / cm2 sec)
δ : 2-photon absorption cross section
hv
S0
(2)
Ne  NgI 2
Ng : the number of the ground-state molecules
Ne : the number of the excited state molecules
Stepwise two-photon absorption process
hv
S1
hv
S0
Re-absorption of intermediate species
The competition of absorption of light
between the ground state molecule and
the intermediate species.
Effective in the case where the number
of total photon is large.
Simultaneous two-photon absorption process :同時二光子吸収
Stepwise two-photon absorption process :逐次二光子吸収
Comparison of Picosecond and Femtosecond Lasers
LASER
Wave
length
Pulse
Duration
(fwhm)
Output
/ Pulse
Peak
Energy
Peak
Energy
/ Area Size
0.5  1.0
mJ
6.7107 W
(1 mJ)
 7 109
W / cm2
 7 1010
W / cm2
PS
532 nm
FS
540 
610 nm
150 fs
5 15 mJ
6.7107 W
(10 mJ)
Ratio
Comparable
1 / 100
1 / 100
1
15 ps
10
Peak Energy
Almost the same
Total photon number
PS > FS
2
Ne


NgI
Peak Energy / Area Size ( I )
FS > PS
However 1-2 % (reaction yield : FS)
Two-photon Absorption Processes
(1)
Simultaneous two-photon absorption process
Sn
hv
I ：Peak Intensity (photon / cm2 sec)
δ : 2-photon absorption cross section
hv
S0
(2)
Ne  NgI 2
Ng : the number of the ground-state molecules
Ne : the number of the excited state molecules
Stepwise two-photon absorption process
hv
S1
hv
S0
Re-absorption of intermediate species
The competition of absorption of light
between the ground state molecule and
the intermediate species.
Effective in the case where the number
of total photon is large.
Simultaneous two-photon absorption process :同時二光子吸収
Stepwise two-photon absorption process :逐次二光子吸収
Excitation Intensity Dependence ( ps 532 nm laser )
1.0
0.4
0.5
Absorbance
Absorbance
S0 (bleaching)
S1
0.2
0.0
-0.2
-0.4
-0.6
400 500 600 700 800 900 1000
Wavelengh/nm
At 20 ps
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Sn
2
Excitation Intensity (mJ / mm )
Increase in the S1 population with an
increase in the excitation intensity. Further
increase of the exc. Intensity decreases the
S1 state population, while increasing the So
state bleaching.
S1
hn
S0
Conversion Efficiency
Conclusion
10
0
Sn
major
10
-1
Slope~2
S1
minor
10
-2
S0
closed form
0.01
0.1
1
2
Excitation Intensity (mJ / mm )
open form
Gated-Reaction Control
via Multiphoton Laser Pulse Excitation
Total photon number
↓
Stepwise abosorption process
↓
Reaction yield
PS > FS
•Optical memory
PS > FS
PS > FS
Summary
•Picosecond pulsed excitation of the closed-isomer of the
diarylethene derivatives led to the drastic enhancement of the
cycloreversion reaction.
•this enhancement is attributable to the production of the higher
excited state with a large reaction yield of the cycloreversion (50%)
Attained via a successive two-photon process.
•A new approach for one-color light control of the gated
photochromic system, which can be utilized for an erasable
memory system with nondestructive readout capability.