Sensitizing mechanisms, reaction mechanisms and reactive intermediate states in photocatalytic reactions on time scales from femto- to microseconds

The development of renewable energy sources depicts a constantly
growing interdisciplinary research field. Beyond photovoltaics
chemical photocatalysis plays a small role, but is gaining more and
more importance. In photocatalysis, light serves as an energy
source for the chemical conversion of certain molecules. However,
not only the application of photocatalysis as energy source but
also the utilization of photocatalysis in chemical synthesis has
attracted a deep scien- tific interest. For the optimization of
photocatalytic systems a fundamental understanding oft the
underlying processes is more than essential. Thereby, transient
absorption spectroscopy has proved to be a very useful tool. On the
one hand, the operation of a setup for transient absorption
spectroscopy and on the other hand the systematic data evaluation
requires physical and mathe- matical skills whereas the results
cannot be interpreted without deep chemical knowledge. With- in the
framework of the present thesis the cooperation between the fields
of organic chemistry and physics has turned out as a very
productive cooperation. Sensitizing mechanisms, reaction mechanisms
and reactive intermediate states in photocatalytic reactions on
time scales from femto- to microseconds are the object of the
present work. The present thesis will prove that the analysis of
measurement data on the basis of established standard methods, such
as the fitting of a sum of exponential functions to the temporal
evolution of the measured signal, often is not sufficient for a
complete interpretation of the data. Only a data analysis precisely
adapted to the problem can lead to a fundamental understanding of
the underlying processes. In the first part of the present thesis,
the focus lies on light-induced intramolecular charge transfer
processes. Marcus Theory, which depicts the theoretical background,
will be briefly in- troduced. On the basis of a molecular
donor-bridge-acceptor system it will be shown that the damping
coefficient β is not sufficient to differ unambiguously between
coherent tunneling and incoherent hopping mechanism. Flavin-capped
DNA hairpins serve as a model for the investigation of
intramolecular charge transfer processes. After photo-excitation,
flavin induces a hole which migrates through the DNA strand. It
will be shown that an adapted base sequence allows for quantum
yields of ΦCS = 14% for long-lived charge separated states. In the
next section it will be discussed if the building blocks of the DNA
are adapted to serve as chiral backbone for enantioselective
photocatalysis. The conformation-dependent charge- transfer
dynamics in benzophenone-DNA dinucleotides will be put on solid
ground with the help of Marcus Theory. It will be shown that these
dinucleotides are generally not suited to serve as an inert
backbone for every kind of photochemical reaction. In the following
section a true bimolecular photocatalytic reaction will be
discussed. Flavin serves as photocatalyst for the conversion of an
alcohol to the corresponding aldehyde. A pre- cisely adapted data
analysis allows and exact quantification of the diffusion
controlled reaction dynamics on the ps time scale. The
understanding of the process allows optimizing the reaction
conditions. The targeted utilization of triplet chemistry within
this reaction can help to increase the quantum yield for product
formation. As photo-induced charge transfer processes have been
intensively discussed, the focus in the second part of the thesis
lies on the [2+2] photocycloaddition. As basis for the
interpretation of subsequent measurements, the [2+2]
photocycloaddition of substituted quinolones will be inves-
tigated. The formation of the cyclobutane ring in which the
quinolone triplet state plays the cen- tral role will be
characterized and quantified on the time scale from ps to ns.
Afterwards the [2+2] photocycloaddition of substituted quinolones
will be initiated by a chiral xanthone-based photocatalyst. It will
be shown that within this catalyst-substrate complex in which both
constit- uents have a distance of only few Ångströms, new
electronic properties appear. The photo- excitation of a new
electronic state not only initiates the [2+2] photocycloaddition of
the quino- lone but also depicts a new sensitizing mechanism, which
has to the author’s best knowledge not been observed in
photocatalysis of organic molecules. The quinolone triplet state
does not appear in this mechanism. The question, if this mechanism
can be transferred to other photocatalytic systems has to be
answered within the framework of further studies.