Optimale Photochemische Energiekonversion und Umgebungseffekte in Reaktiver Moleküldynamik

One of the main challenges in photochemical energy conversion is
the design of charge separating units which are able to generate a
long lived charge separated state, and to couple efficiently to an
energy storage state. In part I of this work the energy conversion
efficiency of a photochemical unit inspired by bacterial
photosynthesis is investigated. The developed model is based on
non-adiabatic multi step electron transfer to generate a
trans-membrane potential gradient. Upon optimization with multi
objective genetic algorithms, the biological strategies for high
quantum efficiency in photosynthetic reaction centers are derived,
which have to suppress loss channels such as charge recombination.
The concepts of bacterial photosynthesis are extended to the design
of artificial photochemical devices. The unified model consists of
a charge separation unit and an energy storing system whereby the
coupling between both units is assured by thermal repopulation
according to the principle of detailed balance. The complete
photosynthetic unit is characterized by the respective
current-voltage relation and an upper limit for the overall energy
efficiency is derived under AM1.5 global conditions. Such a
realistic chemical solar energy conversion system can reach
efficiencies, which are comparable to the limits of an ideal
single-junction solar cell. In Part II of this work the reactive
dynamics of two surrounding controlled photoreactions is
investigated on a microscopic scale. In general the effect of the
surrounding can be classied into intramolecular contributions, like
steric or electronic effects, and intermolecular contributions like
the solvent or the embedding in an enzyme. Both limiting cases are
examined on the basis of two generic photoreactions. The Dewar DNA
lesion follows quantitatively from the 6-4 lesion by UV-A/B
irradiation and constitutes the stable end product of continuous
solar irradiation. Here the detailed mechanism of the formally
4π-sigmatropic rearrangement is presented, which predicts that only
in the (6-4) dinucleotide the Dewar is exclusively formed from an
excited valence state, but not in the free base
5-methyl-2-pyrimidinone (5M2P) nor with a sliced backbone. The
mechanism is elucidated by the analysis of conical intersections
which show, that the photochemical deactivation of T(6-4)T is
strongly in influenced by the confinement in the dinucleotide,
leading to T(Dewar)T formation, whereas in 5M2P the photophysical
protection is ensured by a conical intersection seam. The
implementation of the ONIOM-method into the non-adiabatic mixed
quantum classical dynamics allows to follow the formation of the
T(Dewar)T lesion as well as the competing photophysical relaxation.
C=O-vibrations are identied as unambiguous spectroscopic probe of
the 4π-sigmatropic rearrangement for highly sensitive UV/VIS pump -
IR probe experiments which were successful in following the
reaction in real time. As a second photoreaction the ultrafast
phototriggered reaction of benzhydryl cations with methanol is
investigated. The mechanism of the laser induced generation of
highly reactive benzhydryl cations from the precursor molecule
diphenylmethyl chloride is derived by quantum chemical and quantum
dynamical methods. For the competing reaction channels of ion pair
and radical pair formation the interaction of different electronic
states leads to ultrafast bond cleavage. The homolytic bond
cleavage as a parallel reaction-channel is already accessible in
the FC region by the participation of lone-pairs of the Cl-leaving
group. Based on ab initio data a system Hamiltonian is derived
which is suitable to describe the multidimensional dissociation
process in a reduced reactive coordinate space. Quantum dynamical
calculations show that bond cleavage induced by a Fourier limited
femtosecond laser pulse provides the ion pair despite its higher
potential energy and the existence of conical intersections. The
subsequent bimolecular bond formation, which constitutes the second
part of the SN1 reaction, is investigated by on-the-fly molecular
dynamics simulations in a micro-solvation approach. The calculated
solvation correlation function and time resolved UV/VIS spectra are
compared to recent experimental findings. By the detailed
microscopic description the assignment of the spectral features to
different molecular events is possible. The results show that the
rising spectral signature of the generated benzhydryl cations is
not directly correlated with the bond cleavage, a fact that has to
be considered in the interpretation of the signal for a complete
understanding of the reaction mechanism.