Optimale Photochemische Energiekonversion und Umgebungseffekte in Reaktiver Moleküldynamik

Episode von Fakultät für Chemie und Pharmazie - Digitale Hochschulschriften der LMU - Teil 04/06 von Ludwig-Maximilians-Universität München · 8. Februar 2011

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.

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