Hamburg 2009 – scientific programme
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MO: Fachverband Molekülphysik
MO 20: Photochemie 1
MO 20.5: Talk
Thursday, March 5, 2009, 12:00–12:15, VMP 6 HS-G
Ultrafast Photoisomerization despite Severe Constraints in a Bridged Azobenzene with Enhanced Photochromic Properties Studied by Femtosecond Time-Resolved Spectroscopy — •Falk Renth1, Ron Siewertsen1, Rainer Herges2, Bengt Stehn-Buchheim2, and Friedrich Temps1 — 1Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel — 2Otto-Diels-Institut für Organische Chemie, Christian-Albrechts-Universität zu Kiel, Otto-Hahn-Platz 4, D-24098 Kiel
The Z-E and E-Z photoisomerizations of the severely constrained azobenzene 5,6-dihydrodibenzo[c,g][1,2]diazocine (1) following S1 excitation were studied by fs-time-resolved spectroscopy in n-hexane as solvent. Compared to azobenzene, 1 showed enhanced photochromic properties, with well-separated S1 absorption maxima at 405 nm for the thermodynamically stable 1Z and 490 nm for 1E. The broadband transient absorption measurements gave time constants of 0.1 and 0.28 ps (1Z) and <0.05 and 0.35 ps (1E). The smaller time constants could be attributed to the initial dynamics out of the Franck-Condon region. The larger time constants relate to the sub-ps radiationless deactivation and suggest barrierless reaction pathways involving a conical intersection, despite the significant steric restrictions. The data indicate high photoisomerization quantum yields. Time-dependent DFT calculations were compatible with a hula-twist isomerisation pathway. The improved photochromic properties, ultrafast photoisomerization time scales and high quantum yields make 1 a promising candidate as molecular optical switch especially at low temperatures.