Murakami, M. et al J. AM. CHEM. 2004 , 126, 14764 Miyasaka Lab Tomohiro Kunishi

1. Dynamics and Mechanisms of the Multiphoton GatedPhotochromic Reaction of Diarylethene Derivatives Murakami, M. et alJ. AM. CHEM.2004, 126, 14764 Miyasaka Lab Tomohiro Kunishi

2. Contents • Photochromism • Motivation of the present paper • Result & Discussion • Conclusion

3. 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

4. S1 hv Target Reaction S0 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. (3) Rapid response 　= (4) high sensitivity = (2) low fatigue Gated-Reaction Control via Multiphoton Laser Pulse Excitation

5. Photochromic Reaction Closed-form Open-form 450 ~ 700 nm < 360 nm Open-form Reaction yield from closed-form to open-form is only 1.3% under steady-state light irradiation. Closed-form Reaction yield:反応収率

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

7. Time profile of trasient absorbance < femtosecond laser > < picosecond laser > Ex. at 580 nm / Mon. at 680 nm Ex. at 580 nm / Mon. at 620 nm Ex. at 540 nm / Mon. at 580 nm Time constant is almost the same.But, remaining absorption is large. 10 ps decay / Reaction yield : 1-2 %. No excitation wavelength effect.

8. S1 ko kn ΦO hv S0 Closed isomer Open isomer • 0= k0/(kn+ko) 10ps=1/(kn+ko) Apparent Reaction Yield Drastic enhancement of the cycloreversion reaction yield.1.3 % (steady-state irradiation)  40 % ( ps 532 nm laser excitation)

9. Excitation intensity dependence : conversion efficiency at 160 ps after the excitation with a 15-ps 532-nm laser pulse. Conversion efficiency is quadratically in proportion with the exitation intensity Conversion efficiency -DAbs590nm / Abs590nm Slope~2 Two-photon processis responsible for the efficient bond breakage. Conversion efficiency:変換効率

10. Sn hv hv S0 hv S1 hv S0 Two-photon Absorption Processes (1) Simultaneous two-photon absorption process I ：Peak Intensity　(photon / cm2 sec) δ : 2-photon absorption cross section Ng : the number of the ground-state molecules Ne : the number of the excited state molecules (2) Stepwise two-photon absorption process 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

11. LASER Wave length Pulse Duration (fwhm) Output / Pulse Peak Energy Peak Energy / Area Size PS 532 nm 15 ps 0.5  1.0 mJ 6.7107W (1 mJ)  7 109 W / cm2 FS 540  610 nm 150 fs 6.7107W (10 mJ)  7 1010 W / cm2 5 15 mJ Ratio Com-parable 1 / 100 1 / 100 1 10 Comparison of Picosecond and Femtosecond Lasers Peak Energy Almost the same Total photon number PS > FS Peak Energy / Area Size ( ) FS > PS However 1-2 % (reaction yield : FS)

12. Sn hv hv S0 hv S1 hv S0 Two-photon Absorption Processes (1) Simultaneous two-photon absorption process I ：Peak Intensity　(photon / cm2 sec) δ : 2-photon absorption cross section Ng : the number of the ground-state molecules Ne : the number of the excited state molecules (2) Stepwise two-photon absorption process 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

13. Excitation Intensity Dependence ( ps 532 nmlaser ) Sn S1 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. hn S0

14. Conclusion Sn Slope~2 S1 major Gated-Reaction Control viaMultiphoton Laser Pulse Excitation Total photon number PS > FS ↓ Stepwise abosorption process PS > FS ↓ Reaction yield PS > FS minor S0 • Optical memory closed form open form

15. 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.