Singlet exciton fission in polyacenes complexes: van der Waals dimers in the gas-phase and in helium nanodroplets

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The project aims at the experimental investigation of singlet exciton fission (SF) in dimers of polyacenes. SF was first suggested to explain photophysics in crystalline anthracene in 1965. Strong interest to this phenomenon appeared in the last ten years due to possibility of its application in organic photovoltaics. Estimations made in literature have shown that exploiting the SF phenomenon can increase the Shockley-Queisser limit of efficiency of single-junction solar cells by 30%. In order to optimize the SF process, it is necessary to understand its dependence on structure and monomer properties as well as to understand the nature of elementary processes involved. The most efficient SF was observed in tetracene and pentacene and their derivatives. In spite of the huge experimental material accumulated, it is still difficult to construct a non-controversial summary of experimental results and to combine experiment and theory. The main efforts of experimentalists are directed onto the studies of films or polycrystalline samples of polyacenes. The complicated morphology of these films is difficult to take into account when experimental data are interpreted, and, furthermore, efficient methods of photophysics as used in gas-phase studies cannot be applied. In this project van der Waals dimers of polyacenes are studied as the simplest electronic system containing two polyacene molecules. This is done at molecular beam conditions and as dimers isolated in helium nanodroplets. The very low temperature of 0.4K in helium nanodroplets will allow us to identify the nature of the singlet exciton due to simplification of the absorption spectra as well as to identify the so-called dark state with the use of photoelectron spectroscopy. We will apply velocity-map-imaging of photoelectrons providing information on the photophysics of SF processes at a very detailed level inaccessible in the condensed phase. Measurements of the real-time dynamics of SF process in dimers will be carried out with the use of femtosecond pump-probe techniques in both helium nanodroplets and molecular beams. The Freiburg group has a fully functional setup for producing helium nanodroplets doped with molecules and a complete set of nanosecond and femtosecond lasers, as well as extensive experience on the spectroscopy and real-time dynamics of molecular systems. The Novosibirsk group has a pulsed molecular beam setup combined with the velocity-map-imaging technique as well as extensive experience of its application for the studies of photophysics and photochemistry of van der Waals molecular complexes. Close collaboration of both groups is supposed to take place during the experimental studies carried out both in Freiburg and Novosibirsk. The Freiburg and Novosibirsk groups have already a very successful collaboration and joint publications with the results of the studies where techniques and equipment to be used in the suggested project were applied.