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

Photoinduced reversible transformation in a chemical species between two forms without changes of molecular weight.

AbsorptionRefractive indicesOxidation potential etc

Quick change of physical properties between two isomers

UV light

Vis. lightS S

F2

F2 F2

S S

F2

F2 F2

Motivation

(5) Non-destructive read-out capability with the high sensitivityneeds another conditions arising from the change of the some outer environments that can act as “gate” of the reaction.

(3) Rapid response 　 = (4) high sensitivity = (2) low fatigue

Excited state reaction generally occurs in competition with various processes in a finite lifetime.

Gated-Reaction Control via Multiphoton Laser Pulse Excitation

(1) Thermal stability of both isomers (2)  Low fatigue (3)  Rapid response (4)  High sensitivity(5) Non-destructive readout capability

Optical and Photonic Device

Photochromic Reaction

Open-form

300 400 500 600 7000

1

2

3

4

Ext

inct

ion

Co

effic

ien

t

(1

04 / M

-1cm

-1)

Wavelength / nm

Open-form

Closed-form

Reaction yield from closed-form to open-form is only 1.3% under steady-state light irradiation.

< 360 nm450 ~ 700 nm

Closed-form

Reaction yield: 反応収率

Transient absorption spectra of PT1(c) in n-hexane excited with a 15-ps 532-nm laser pulse.

400 500 600 700 800 900 1000-0.6

-0.4

-0.2

0.0

0.2

0.4

100 ps 10 ps Grnd. state

Abs

orba

nce

(Nor

mal

ized

)

Wavelength / nm

Cycloreversion reaction completed within 100 ps. Perfect recovery of the closed form by UV light after ps 532 nm laser pulse.

400 600 800 1000

-26 ps

80 ps60 ps50 ps

30 ps20 ps10 ps0 ps

100 ps

40 ps

-10 ps

1 ns

Ab

sorb

an

ce (

0.2

/div

.)

Wavelength / nm

Transient absorption: 過渡吸収

Time profile of trasient absorbance

0 50 100 150

0.00

0.05

0.10

0.15

0.20

0 50 100 150-0.6-0.5-0.4-0.3-0.2-0.10.0

(a) 710nm

(b) 580nm

Abso

rbance

Time/ps

0 20 40 60 80-1.5x10-2

-1.0x10-2

-5.0x10-3

0.0

0 20 40 60 80

-4.0x10-2

-2.0x10-2

0.0

0 20 40 60 80

0.0

5.0x10-3

1.0x10-2

1.5x10-2

(c)

(b)

Ab

sorb

an

ce

Time / ps

(a)

Ex. at 540 nm /Mon. at 580 nm

Ex. at 580 nm / Mon. at 620 nm

Ex. at 580 nm / Mon. at 680 nm

10 ps decay / Reaction yield : 1-2 %.No excitation wavelength effect.

Time constant is almost the same.But, remaining absorption is large.

< femtosecond laser > < picosecond laser >

Apparent Reaction Yield

ΦO

0= k0/(kn+ko)

10ps=1/(kn+ko)

hv

Closed isomer Open isomer

S1

kn

S0

ko

Drastic enhancement of the cycloreversion reaction yield.1.3 % (steady-state irradiation) 40 % ( ps 532 nm laser excitation)

Excitation intensity dependence : conversion efficiency at 160 ps after the excitation with a 15-ps 532-nm laser pulse.

0.01 0.1 1

10-2

10-1

100

Co

nve

rsio

n E

ffic

ien

cy

Excitation Intensity (mJ / mm2)

Conversion efficiency-Abs590nm / Abs590nm

Two-photon process is responsible forthe efficient bond breakage.

Slope~2

Conversion efficiency is quadratically in proportion with the exitation intensity

Conversion efficiency: 変換効率

Sn

S0

hv

hv

S1

S0

hv

hv

(1) Simultaneous two-photon absorption process

(2) Stepwise two-photon absorption process

Re-absorption of intermediate speciesThe competition of absorption of lightbetween the ground state molecule and the intermediate species.Effective in the case where the number of total photon is large.

I ： Peak Intensity 　 (photon / cm2 sec) δ : 2-photon absorption cross section

Ng : the number of the ground-state moleculesNe : the number of the excited state molecules

2NgINe

Two-photon Absorption Processes

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

Peak Energy Almost the sameTotal photon number PS > FSPeak Energy / Area Size ( ) FS > PS

LASER 

Wavelength

Pulse Duration(fwhm)

Output/ Pulse

Peak Energy

Peak Energy/ Area Size

PS  532 nm 15 ps0.5 1.0 mJ

6.7107 W(1 mJ)

7 109

W / cm2

FS540 610 nm

150 fs 5 15 J6.7107 W(10 J)

7 1010

W / cm2

Ratio 

Com-parable 1 / 100 1 / 100 1 10

However 1-2 % (reaction yield : FS)

2NgINe I

Two-photon Absorption Processes

Sn

S0

hv

hv

S1

S0

hv

hv

(1) Simultaneous two-photon absorption process

(2) Stepwise two-photon absorption process

Re-absorption of intermediate speciesThe competition of absorption of lightbetween the ground state molecule and the intermediate species.Effective in the case where the number of total photon is large.

I ： Peak Intensity 　 (photon / cm2 sec) δ : 2-photon absorption cross section

Ng : the number of the ground-state moleculesNe : the number of the excited state molecules

2NgINe

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 )

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.5

1.0

At 20 ps

S1

S0 (bleaching)

Excitation Intensity (mJ / mm2)

Ab

sorb

an

ce

400 500 600 700 800 900 1000-0.6

-0.4

-0.2

0.0

0.2

0.4

Ab

sorb

an

ce

Wavelengh/nm

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. S0

h

S1

Sn

Conclusion

Sn

S1

S0closed form open form

major

minor

0.01 0.1 1

10-2

10-1

100

Co

nve

rsio

n E

ffic

ien

cy

Excitation Intensity (mJ / mm2)

Total photon number PS > FS ↓Stepwise abosorption process PS > FS ↓ Reaction yield PS > FS

Gated-Reaction Control 　via 　 Multiphoton Laser Pulse Excitation

•Optical memory

Slope~2

Summary

•Picosecond pulsed excitation of the closed-isomer of thediarylethene derivatives led to the drastic enhancement of thecycloreversion 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.