Ultrafast Photoisomerization Dynamics of Intra- and Intermoleculary Bridged Azobenzenes Studied by Femtosecond Time-resolved Electronic Absorption Sepctroscopy

In this Thesis, the photophysical and -chemical characteristics of intra- and intermolecular bridged azobenzenes were investigated by UV-Vis, transient electronic absorption spectroscopy and ab initio calculations. The studies of the main chain poly(azobenzene-alt-trisiloxane) have shown that the incorporation of azobenzenes into flexible siloxane polymers lead to a moderate slowdown of the ultrafast deactivation. Furthermore, the feasibility of the light-induced back-isomerization could be proven quantitatively even in the collapsed polymer coil. For a siloxane azophane molecule, ab initio calculations clearly provided evidence for strong electronic coupling of the azobenzene moieties, which indicates impressive quantitively light-induced switching.  However, thermal back-isomerization fulfils its reactions by formation of a stable (E,Z)-isomer. The photo-induced ultrafast dynamics corroborates the moderate slowdown but features an additional sub-picosecond time component likely due to the electronic coupled chromophores. Finally, the ultrafast dynamics of the bridged oxa- and sulfurdiazocine have been fully clarified. After an excitation of the first electronic state, an ultrafast deactivation within the first picosecond was observed for both molecules. While the oxadiacozine shows a contribution of the Echair conformer as intermediate structure, the sulfur derivate hasn’t shown this feature in its ground state dynamics. For the photo-induced back-isomerization, both molecules feature an  ultrafast deactivation of the excited electronic state with a subsequent vibration cooling process of the formed photoproduct.


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