Intra- and Intermolecular Dynamics of pi-conjugated molecules and their dimers

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In the center of this project are polycyclic aromatic hydrocarbons and heterocycles that are of relevance in material science. We will study the processes that occur in these molecules after photoexcitation. The majority of experiments will be carried out in the gas phase, but complementary studies will be conducted in solution. Beside the isolated molecules we will also study their dimers to better understand energy transfer processes in larger aggregates. Gas-phase data allow a comparison with the results of highly accurate computations and thus provide a valuable benchmark for the theory on larger aggregates. The most relevant experimental method in this project will be picosecond time-resolved spectroscopy. In addition we want to establish time-resolved photoelectron spectroscopy using imaging detection of electrons in this phase of the project. An important class of molecules that moved into the focus of our research program are organic heterocycles like 1,8-naphthalic anhydride (NDCA) and 1,8-naphthalimide (NI). These molecules exhibit a competition between internal conversion and intersystem crossing on a ps-timescale. A detailed description of the competition in these model systems will contribute to a better understanding of the photoinduced dynamics in heterocycles in general.Furthermore we will continue the work on pyrene and study the formation of excimers as a function of excitation energy and of the chemical character of the substituent. Another system of interest is the tetracene dimer, which constitutes a model system for singlet fission. In this process an excited singlet state deactivates to two triplet states, which is proposed to be an efficient route for energy transfer in optoelectronic materials. Finally we will explore the structure and dynamics of strained rings, which are models for defects in carbon structures.We note that the close cooperation with theoretical and preparative groups at our department is of particular importance for this proposal.