Photoinduced processes in nitrogen-substituted aromatic molecules and their dimers: A joint experimental and theoretical investigation .

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In this project, we will investigate the photochemistry and photophysics in aromatic molecules containing nitrogen, so-called PANH (polycyclic aromatic nitrogen-containing hydrocarbons). A characteristic of the project is the close cooperation between theory and experiment. Data on isolated molecules and dimers permit a direct comparison with high-level computations and give insight into the intrinsic molecular properties in the absence of environmental influence. Time-resolved photoionization and photoelectron imaging in a free jet will be complemented by quantum-classical dynamics simulations, an approach that permits the direct comparison with experimental observables, such as time-delay traces or photoelectron spectra. As picosecond pulses with a time-resolution of around 4 ps and a bandwidth of 15-20 cm-1 are employed, it is possible to investigate the dependence of the photodynamics on the excess energy deposited in the molecules.Although fundamental questions are in the center of our research, the program is also moti-vated by the relevance of nitrogen-containing molecules for materials science. For example, replacement of a carbon atom by a nitrogen can turn a predominantly hole-conducting polycyclic aromatic hydrocarbon (PAH) into a n-type semiconductor with improved electron conduction. Furthermore, PANH play an important role in astrochemistry and are considered as carriers of the unidentified infrared bands (UIB). In our research program we will address the following questions: a) the sequence and the character of excited electronic states in PAH is modified by the introduction of an N-atom. We will therefore investigate the electronic structure and dynamics in PANH as a function of the position of the nitrogen atom. Initially, we will study the sequence phenanthrene/ phenanthridine/ benzoquinoline and then extend our work to tetracene- and pentacene-derivatives that will be synthesized in our group. b) we will investigate the impact of the substitution by nitrogen on the structure of PANH di-mers and in particular on the formation of excimers, dimers that interact weakly in the ground state, but strongly in the electronically excited state. In particular aza-tetracenes and aza-pentacenes will be relevant, because both chromophore are considered as models for singlet fission (SF). In this process a one-electron excitation leads to the formation of two excitons, which can lead to an increased efficiency of solar cells using suitable materials. Excimer for-mation can be considered to be the first step towards SF. c) heterocycles with relevance for material science like PTCDA and PBI will be transferred into the gas phase by laser desorption. Subsequently, the photophysics of the molecules and their dimers will be investigated experimentally and theoretically.