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Schmierer, Thomas (2011): Femtosekundenspektroskopie an o-substituierten Nitrobenzolen - von Modellen zu photolabilen Schutzgruppen. Dissertation, LMU München: Fakultät für Physik



In this thesis ultrafast processes occuring in photolabile protecting groups of the ortho-nitrobenzyl type have been studied. These protecting groups enable the spatially and temporarily defined release of chemically or biologically active molecules. Due to these properties they find widespread applications, besides technical ones mainly in biochemical research. Molecules of the o-nitrobenzyl type represent the major part of photolabile protecting groups used today. Therefore, in-depth knowledge of the reaction and release mechanism is desired to improve and adapt the properties of these molecules to their applications. Despite this relevance experiments on the ultrafast behaviour of these molecules are scarce. Here, femtosecond pump probe experiments with UV/VIS, IR and Raman probing have been employed to elucidate the early processes occuring during de-protection. Model systems as well as a "real" protecting group have been investigated. Thereby, a general picture of the ultrafast processes occuring in the photoreaction of these molecules could be obtained. Excitation with uv-light to an upper singlet state leads to ultrafast internal conversion to the S1 state. Three competing processes, internal conversion, inter system crossing and the photoreaction, contribute to the decay of this state on the time scale of 1 ps. For the reaction path the formation of the first intermediate could be traced. It results from a hydrogen transfer from the substituent in o-position to the nitro group and exhibits aci-nitro like structure. In addition to the direct formation from the S1 state this intermediate is also formed via a second channel on the nanosecond time scale. This channel involves a triplet state. The "local" triplet state undergoes a hydrogen transfer reaction resulting in a triplet phased bi-radical. The recombination of this biradical then yields the aci-nitro tautomer. Once formed the aci-nitro tautomer transforms into the final nitroso product and the released functionality in a yield of 100 %. Furtheron, the similarity between the reaction mechanisms of all molecules investigated leads to the conclusion that it is universal for the group of o-nitrobenzenes. Additionally, for oNBA the influence of vibrational excitation on the stability of a ground state intermediate could be traced. The ultrafast formation of the first intermediate is about one order of magnitude faster than typical cooling times for these molecules. Therefore, the dynamics of vibrational excitation and its impact on the subsequent reaction could be studied. The vibrational excitation results in bi-phasic kinetics: a fast phase during vibrational excitation of the molecule and a slower one for the thermalized molecule.