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Heinz, Björn (2008): Alte Photoreaktionen in neuem Licht: Ultraschnelle Spektroskopie an Fulgimiden und photolabilen Schutzgruppen. Dissertation, LMU München: Faculty of Physics
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Abstract

The scope of the work presented is the investigation of photochemical reactions by means of ultrafast spectroscopy. Naturally these reactions start off in an optically bright excited state. Femtosecond time-resolved fluorescence spectroscopy is thus the method of choice to track the spectral and temporal dynamics of these emissive states. Here, an ultrafast fluorescence spectrometer based on the optical Kerr-effect serves as the appropriate tool to pursue this task. Additional information on dark states and ground states is provided by Uv-Vis transient absorption experiments. The first part of the thesis deals with a fundamental concept of mechanistic chemistry – the pericyclic reactions. The spectroscopic consequences implied within this theoretical framework are investigated by means of emission and absorption spectroscopy. The molecular probe is an indolyl-substituted fulgimide which undergoes a light-induced cyclization or cycloreversion, respectively. Both reactions feature a bi-phasic emission decay (cyclization: 0.06 ps, 0.4 ps, cycloreversion: 0.09 ps, 2.4 ps) whereas the slower component goes along with the product formation. The large difference in the slower time constants as well as the spectral properties of the corresponding emission point to the existence of different excited state pathways for both reactions. These results challenge the basic one-dimensional reaction scheme commonly used to describe pericyclic reactions. Referring to theoretical investigations, a two-dimensional reactive space is proposed to hold responsible for the different behaviour of the two isomers. The second part of the studies focuses on the dynamics of a certain type of photolabile protecting groups. These molecules are intramolecularly sensitised by a triplet energy donor, namely thioxanthone, and feature an ortho-substituted nitroaromatic as the reactive core. Investigations on the closely related energy donor xanthone reveal that photo-excitation is followed by a rapid (~ 1 ps) equilibration between the emissive singlet and a triplet state. This equilibrium holds responsible for a delayed fluorescence with a lifetime of ~ 0.1 − 1 ns and is ”switched off” by an internal conversion within the triplet manifold. These results can be directly transferred to thioxanthone and the sensitised protecting groups. The energy transfer in the latter molecules features a fast component from the initially populated triplet state (~ 100 ps) and a further slower contribution from the relaxed triplet state. Finally, the photo-reactive ortho-nitrobenzaldehyde (o-NBA) is compared with its non-reactive isomers m- and p-NBA as model systems to obtain information on the reactive core of the protecting groups. These first fluorescence experiments on monocyclic nitrated aromatics feature bi-phasic emission decays in all three cases – each with time constants of <100 fs and ~ 1 ps. For o-NBA the slower process matches the formation time of a ketene intermediate, whereas for the other isomers intersystem crossing to a triplet state is observed. The sub 100 fs process common in all three molecules is associated with a prepending 1pipi* -> 1npi* relaxation.