Ultrafast transient absorption spectroscopy of the excited-state dynamics

1. Ultrafast transient absorption spectroscopy of the excited-state dynamics of azobenzene – type metal complex Michael Busby, Pavel Matousek, Michael Towrie, Antonin Vlcek Jr. Inorganica Chimica Acta 360 (2007) 885-896 Chikashi Ohta Miyasaka Lab. M1

2. Contents Introduction Photochromism Sample Experimental Results UV – Vis absorption spectra Time resolved transient visible absorption spectra Time resolved transient infrared spectra Disucussion Summary

3. hν1 B A hν2 or Δ Photochromism photoinduced isomerization Photoinduced reversible transformation in a chemical species between two forms having different colors. (At least, one pathway should be photoinduced.) application basic chemistry The quick change of properties The determination of a time origin at fs order dipole moment polarizability ・・・・・ elucidation of the reaction mechanism optical device factors that dominate the reaction

4. Sample ~ combination use of photochromic moiety in supramolecular systm ~ fac-[Re(papy)(CO)3(bpy)]+( Re(papy)(bpy) ) (papy = trans-4-phenylazopyridine ) (bpy = 2, 2’ -bipyridine) Re complex +azobenzene (analogue) photochoromic reaction

5. Azobenzene (analogue) unit Photochromic reaction of azobenzene Cis form Trans form Azobenzene • Trans form →Cis form : UV light irradiationCis form → Trans form : UV (or visible) light irradiation or Δ • The photoisomerization undergoes on a subpicosecond time scale.

6. ReⅠ complex unit Metal to ligand charge transfer (MLCT) L Ligand (π*) L MLCT Metal (d) L ππ* Re(CO)3(bpy) Ligand (π) low – energy irradiation MLCT(bpy) + intramolecular energy transfer alternative way to trigger isomerization

7. + Azobenzene (analogue) + Re complex ON / OFF Switch Cis form emissive Trans form non emissive purpose the excited state dynamics of and Re(papy)(bpy) the development of photoresponsive functional device and optically controlled switch

8. Experimental Pump detector ΔT Probe Sample the time-resolution of the transient absorption measurement UV – Vis. transient absorption spectra Ti:Sa. femtosecond laser system ・pulse duration : 150 fs (fwhm) ・excitation wavelength : 400 nm ・probe wavelength : 420 nm ~720 nm transient IR absorption spectra Ti:Sa. femtosecond laser system ・pulse duration : 150 fs (fwhm) ・excitation wavelength : 400 nm ・probe wavelength : 1000 to 5000 cm-1

9. UV – Vis absorption spectra nπ* (free papy) ππ* (free papy) papy Free papy Re(papy)(CO)3(bpy) papy bpy Re(papy)(CO)3(bpy) Free papy the main band ππ* (papy) the shouler nπ* (papy , bpy) MLCT (papy , bpy) the main band ππ* the shoulder nπ* the strong sharp band ππ* IL (bpy)

10. papy (a) 2, 2.25, 2.75, 4 ps ΔAbsorbance Re(papy)(bpy) (b) 2, 3, 10, 100 ps 1MLCT(bpy) 3MLCT(bpy) 0.8 ps 2.5 ps 3nπ* (papy) 100 ps GS 3MLCT(bpy) UV-Vis transient absorption spectra papy (a) papy 1nπ* (papy) 1.0 ps (560 nm) (b) Re(papy)(bpy) 1nπ* (papy)

11. Transient infrared spectroscopy 3.0 ps shift 3MLCT excited state (bpy) 40 ps shift structural reorganaization of the 3nπ*(papy) 40 ps 3.0 ps 3MLCT (bpy) 3nπ*(papy) 3nπ*♯(papy) fac-[Re(papy)(CO)3(bpy)]+ N → P shift K, L, M the upward shift O, N the upward shift

12. Discussion 1ππ*, 1nπ*, 1MLCT(papy, bpy) Re(papy)(CO)3(bpy)

13. Summary • The excited dynamics of the Re complex fac-[Re(papy)(CO)3(bpy)]+ are investigated by the time-resolved measurements of visible and IR spectra. • Intersystem crossing from optically prepared Frank-Condon states (1pp*, 1MLCT and 1np*) undergoes inthe femtosecond time scale. • The 3MLCT(bpy) state of fac-[Re(Cl)(papy)(CO)3(bpy)] + undergoes a 3 psconversion to the reactive 3np* excited state. This process amounts to an energy transfer from the ReⅠ(CO)3(bpy) to the papy ligand.