Ultrafast transient absorption spectroscopy of the photo-induced Z-E isomerization

1. Ultrafast transient absorption spectroscopy of the photo-induced Z-E isomerization of a photochromic furyfulgide F. Renth, M. Foca, A. Petter, F. Temps Chemical Physics Letters 428 (2006) 62-67 Miyasaka Laboratory Chikashi Ohta

2. Contents Introduction ・ Photochromism ・ Photochromism of fulgide Experiment Results and Discussion Conclusion

3. hν1 B A hν2 or Δ Photochromism Photoinduced isomerization Photoinduced reversible isomerization in a chemical species between two forms having different color. (at least, one pathway should be photoinduced.) Application Basic chemistry Quick property change Determination of a time origin at fs order Dipole moment Polarizability ・・・・・ Elucidation of the reaction mechanism Optical device Factors that dominate the reaction

4. Photochromism of fulgide UV UV VIS. UV UV E form(1E) C form(1C) Z form(1Z) cyclization / cycloreversion E-Z isomerization Properties (1) Three isomers Z form, E form, C form (2) Low fatigue (3)Thermal stability of these isomers

5. Experimental Pump detector T Probe Sample The time-resolution of the transient absorption measurement Ti:Sa femtosecond laser system Excitation : 387 nm White light: 410 nm ~ 690 nm Pulse duration : 210 fs

6. The cyclization and cycloreversionreactions a few ps UV UV UV VIS, UV <1 ps Z form(1Z) Eform(1E) C form(1C) The cyclization from the E form to the C form occurs with a time constant of a few ps Reference : J. Phys. Chem. C 2007, 111, 2730-2737 J. Phys. Chem. A 1997, 101, 502 -506 The cycloreversion from the C form to the E form occurs with a time constant of less than 1 ps purpose The dynamics of the Z-E isomerization of the furylfulgide 1Z

7. (a) -90 to 90 fs (30 fs interval time) Excited state of Z form Intermidiate state (b) 90 fs to 180 fs ΔA / 10-3 (c) 0.18 to 25 ps Z form E form Wavelength / nm Results and Discussion Transient absorbance of 1Z excited at λ=387 nm 450 nm 580 nm Excitation at 387 nm slightly negative signal the depletion of 1Z due to isomerizaton to the 1E

8. 420 nm 530 nm ΔA / 10-3 420 nm 600 nm ΔA / 10-3 Time / ps Time profiles of the transient absorbance of 1Z 220 fs dacay Z-E photoisomerization reaction takes place in this time scale 4.1 ps decay The absorption of vibrationally excited molecules Excited state τ=220 fs 300 fs rise 300 fs Intermidiate state Intermidiate state 4.1 ps E form Z form

9. An oscillatory feature 420 nm An oscillatory feature = A vibrational frequency is57 cm-1 600 nm Time / ps a corresponding torsional mode between the furan ring and succnic anhydride moity Quantum chemical calculations 60 cm-1 Z-E photoisomerization Furan ring Torsional mode Succinic anhydride Z form E form

10. Conclusions • The transient absorption in the range from λprobe=410 -690 nm was dominated by a decay on a time scale of 220 fs due to excited state absorption for all wavelengths. • A slow component present only at short wavelengths (λprobe<480 nm) was assigned to hot ground state absorption and decayed with a time constant of 4.1 ps. • The observed oscillatory behavior was interpreted as evidence for vibrational coherence involving a low-frequency torsional mode. These results indicate that the photo-induced isomerization of 1Z proceeds on a sub-ps timescale.

11. 300 fs 4.1 ps Z form E form