1. Multiphoton-gated cycloreversion reaction of a photochromic diarylethene derivative as revealed by femtosecond two-color and two-pulse excitation Miyasaka Lab. Tatsuhiro Nagasaka cycloreversion 開環反応 1

2. 1.Introduction What is Photochromism ? Diarylethene Derivatives (DAE) 2.Motivation 3.Experimental method Femtosecond transient absorption measurements 4.Results & Disucussion Multiphoton cycloreversion reaction 5.Summary 2

3. What is Photochromism ? UV Vis. or  Reversible transformation of a chemical species between two isomers having different absorption spectra induced in one or both directions by photoirradiation. Photochromism Isomer 異性体 : 同じ分子式だが、構造の異なる分子のこ と 3

4. UV Vis. or Δ Vis. Azobenzene Spiropyran Fulgide Diarylethene T-type (Thermally reversible) P-type (Thermally irreversible, but photochemically reversible) Examples of Photochromic Molecule M. Irie, T. Fukaminato, K. Matsuda, and S. Kobatake Chem. Rev. 114(2014) 12174-12277 4

5. Molecular Scale polarizability, ionization energy Macroscopic Scale dielectric constant, refraction factor ↓ Application optical memory, optical switch Diarylethene Derivatives (DAE) UV Vis. Open form Closed form Requirements for optical memory (1) Thermal stability (2) Fatigue resistant property (3) High sensitivity (4) Rapid response (5) Reactivity in solid (6) Non destructive read out Closed form Open form 5

6. Application to optical memory Write information Vis. Read information Open form UV Gated functions are necessary to read and erase information independently! Information Erased Closed form 6

7. Motivation h h S0S0 S1S1 SnSn Closed  ≈ 50 % Open  = 1.3% PT Read information by weak light Erase information by strong light One-photon excitation → Reactivity is low Two-photon excitation → Reactivity is high PT Gated functions can be realized by two-photon cycloreversion ! M. Murakami, H. Miyasaka, T. Okada, S. Kobatake, M. Irie J. Am. Chem. Soc. 126(2004) 14764-14772 7

8. Closed form Open form Reaction Coordinate S 0 (1A) S 2 (1B) S 1 (2A) CI Motivation General reaction scheme of DAE Internal conversion 内部転換 vibrational cooling 振動緩和 CI コニカルインターセクション Internal conversion vibrational cooling 8

9. Motivation Elucidation of the dynamics and mechanism of the two-photon cycloreversion reaction of PT. ① Femtosecond transient absorption spectroscopy measurement ↓ Investigate the initial state of S n ←S 1 transition ② Excitation wavelength dependencies of second pulse ↓ Investigate the final state of S n ←S 1 transition Closed form Open formReaction Coordinate S 0 (1A) S 2 (1B) S 1 (2A) CI SnSn Sn'Sn' 9

10. 1.Introduction What is Photochromism ? Diarylethene Derivatives (DAE) 2.Motivation 3.Experimental method Femtosecond transient absorption measurements 4.Results & Disucussion Femtosecond sequential two-photon excitation 5.Summary 10

11. Experimental method Femtosecond Transient Absorption measurements : Transient absorbance : Absorbance with pump excitation : Absorbance without pump excitation ΔA A’ A transient absorption 過渡吸収 Incident light intensity 入射光強度 transmitted light intensity 透過光強度 Absorbance : A=log(I 0 / I) I 0 : Incident light intensity of probe I : transmitted light intensity of probe 11

12. One-photon cycloreversion reaction dynamics Excitation wavelength 600 nm S 1 ←S 0 PT (n-hexane) 100 ps 10 ps 5 ps 2 ps 1 ps 0.2 ps 0.1 ps -0.3 ps 200 fs 2 ps 12 ps 840 nm 200 fs : Internal conversion CI Closed Open S 2 (1B) S 1 (2A) S 0 (1A) 12 ps : Lifetime of 2A state CI Closed Open S 2 (1B)S 1 (2A) S 0 (1A) 2 ps : Vibrational cooling CI Closed Open S 2 (1B) S 1 (2A) S 0 (1A) R.C. 12

13.  Delay : 500 ps Probe Pump2 532 nm Pump1 532 nm Picosecond two-pulse excitation 13

14. + 400 ps tt Probe 600 nm Pump1 600 nm Pump 2 530 ～ 880 nm Double pulse excitation transient absorption measurement The change of delay time ↓ Investigate the intial state of S n ←S 1 The change of Pump2 wavelength ↓ Investigate the final state of S n ←S 1 14

15. Pump1 600 nm Pump2 830 nm Probe 600 nm Two-photon cycloreversion amount / a.u. (-  Abs) 12 ps ～ Lifetime of 2A state Two-photon cycloreversion reaction dynamics 15

16.  t ≈ 0 ps  t ≈ ～ 12 ps  t >> 12ps At 830 nm Reaction Amount Large Medium Small 16

17. Two-photon cycloreversion reaction dynamics Two-photon cycloreversion amount / a.u. (-  Abs) Pump1 600 nm Pump2 530 nm Probe 600 nm 2 ps ～ 2A state increase 12 ps ～ Lifetime of 2A state 17

18.  t ≈ 0 ps  t ≈ 5 ps  t >> 12ps At 530 nm Reaction Amount Large Medium Small 18

19. Pump2 530 nm Pump2 580 nm Pump2 630 nm Pump2 680 nm Pump2 780 nm Pump2 730 nm Pump2 830 nm Pump2 880 nm Two-photon cycloreversion reaction dynamics 19

20. Summary Future plan Further measurement of other diarylethene derivatives Investigate dynamics and reactivity of higher excited state The higher excited state populated from 2A state plays an important role in the efficient cycloreversion reaction 20

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22.  Abs ∝  S1)×S1photon×{A(S1)/A(S0)+A(S1)}×  CO 22

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24. Motivation Elucidation of the dynamics and mechanism of the two-photon cycloreversion reaction of PT. ① Femtosecond transient absorption spectroscopy measurement ↓ Investigate the initial state of S n ←S 1 transition ② Excitation wavelength dependencies of second pulse ↓ Investigate the final state of S n ←S 1 transition Closed form Open form S1S1 SnSn S0S0 Sn'Sn' Reaction Coordinate 24