Fakultät für Chemie und Pharmazie - Digitale Hochschulschriften der LMU - Teil 04/06 Archiv

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.

In this work, single-walled carbon nanotubes (SWNTs) have been studied using tip-enhanced near-field optical microscopy (TENOM). This technique provides a sub-diffraction spatial resolution of 15 nm on the basis of strong local signal enhancement, which allows for nanoscale imaging of the photoluminescence (PL) intensity and energy along single semiconducting SWNTs. Thereby, the mobility of excitons and their interaction with defects and spatial exciton energy variations can be directly visualized. Similarly, the local Raman scattering properties of metallic SWNTs have been investigated, revealing the microscopic relation of localized defects and the resulting Raman D-band intensity. The first part of the thesis presents a newly developed numerical description of exciton mobility and local quenching at defect sites, accounting also for the TENOM imaging process. This highly flexible model is used to quantitatively evaluate experimental observations such as photo-induced PL blinking and strong spatial PL intensity variations of single semiconducting SWNTs. The main finding is that exciton propagation can be described as ne-dimensional diffusion with a diffusion length of 100 nm for the studied nanotubes, determined independently from both the PL blinking characteristics and the direct visualization using high-resolution TENOM. The temporal and spatial PL variations result from efficient exciton quenching at localized defects and the nanotube ends. The second part reports on the first observation of exciton localization in SWNTs at room temperature, leading to strongly confined and bright PL emission. Localization results from narrow exciton energy minima with depths of more than 15 meV, evidenced by energy-resolved near-field PL imaging. Complementary simulations using a modified numerical model accounting for energy gradients are in good agreement, predicting a significant directed diffusion towards energy minima yielding locally enhanced exciton densities. The energy variations are attributed to inhomogeneous DNA-wrapping of the nanotubes, used for their separation during sample preparation. In the last part, the microscopic relation between the defect-induced Raman D-band and the defect density has been investigated for metallic SWNTs. The length scale of the D-band scattering process in the vicinity of defects was imaged with TENOM for the first time and found to be about 2 nm. Furthermore, localized defects have been photo-generated intentionally by the strong fields at the tip while recording the evolution of the local Raman spectrum. Based on this data, a quantitative relation could be determined, that is highly relevant for the characterization of carbon nanotubes via Raman spectroscopy.