Alte Photoreaktionen in neuem Licht

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 1npi* relaxation.