Ultrafast Photoisomerization Dynamics of Selected Molecular Switches Studied by Femtosecond Time-Resolved Electronic Absorption Spectroscopy

The task of this Thesis was the investigation of the photophysical and -chemical properties of selected multi-azobenzene (multi-AB) and heteroaromatic azobenzene systems using static UV/Vis spectroscopy, transient electronic absorption spectroscopy, and ab initio calculations. The obtained data for the carbon-linked multi-AB compounds bis(4-phenylazo)phenylmethane (C(AB)2), tris(4-phenylazo)phenylmethane (C(AB)3), and tetrakis-(4-phenylazo)phenylmethane (C(AB)4) and the phenyl-linked 1,3,5-tris-(4-phenylazo)phenylbenzene (Ph(AB)3) indicated weak exciton coupling between the chromophore subunits. The underlying ultrafast deactivation dynamics do not seem to be affected to large extents by the coupling or the number of connected AB subunits in the system.  The investigations of 2-(phenylazo)imidazole (H-PhAIm) and 1-N-methyl-2-(phenylazo)imidazole (Me-PhAIm) revealed the drastic effect on the thermal Z→E isomerization that can be caused by minimal chemical modification such as methylation. Here, a tautomerization involving two H-PhAIm molecules was proposed as fast deactivation mechanism. The methylation showed no effect on the ultrafast deactivation dynamics after S2(ππ*) excitation. Very similar time constants could be found in the investigations of the pyridine-substituted azobenzene compounds 2-(phenylazo)pyridine (2-PAPy), 3-(phenylazo)pyridine (3-PAPy), and 4-(phenylazo)pyridine (4-PAPy), as well as in 2,2'-azopyridine (2,2'-APy), 3,3'-azopyridine (3,3'-APy), and 4,4'-azopyridine (4,4'-APy). Overall, the static and transient behavior of the heteroaromatic AB compounds investigated are very similar to those of the parent AB. The investigation of aqueous NaI after 1- and 2-photon excitation to the first and second Charge-Transfer-To-Solvent states, respectively, showed an ultrafast formation of solvated electrons, followed by subsequent geminate recombination. 

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