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

本项目旨在实验研究多聚二聚体中单线态激子裂变（SF）。在1965年首次提出用SF来解释晶体蒽的光物理现象。近十年来，由于其在有机光伏中的应用可能性，人们对这一现象产生了浓厚的兴趣。文献估计表明，利用SF现象可使单结太阳能电池效率的Shockley-Queisser极限提高30%。为了优化SF工艺，有必要了解其对结构和单体性能的依赖，以及所涉及的基本过程的性质。在四烯和并五烯及其衍生物中观察到最有效的SF。尽管积累了大量的实验资料，但构建一个对实验结果无争议的总结，并将实验与理论相结合，仍然是一件困难的事情。实验工作者的主要工作是研究聚二烯的薄膜或多晶样品。在解释实验数据时，很难考虑到这些薄膜的复杂形态，而且，在气相研究中使用的有效的光物理方法也不能应用。在这个项目中，范德华二聚体作为最简单的电子体系，包含两个多聚烯分子进行了研究。这是在分子束条件下和在氦纳米液滴中分离的二聚体中完成的。氦纳米液滴中0.4K的极低温度将使我们能够通过简化吸收光谱来识别单线态激子的性质，并利用光电子能谱来识别所谓的暗态。我们将应用光电子的速度图成像，在非常详细的水平上提供在凝聚相中无法获得的SF过程的光物理信息。利用飞秒泵浦探针技术，在氦纳米液滴和分子束中测量二聚体中SF过程的实时动力学。弗莱堡团队拥有功能齐全的设备，可以生产掺杂分子的氦纳米液滴，并有一套完整的纳秒和飞秒激光器，以及在分子系统的光谱学和实时动力学方面的丰富经验。新西伯利亚小组拥有脉冲分子束装置，结合了速度图成像技术，以及在范德华分子复合物的光物理和光化学研究中的广泛应用经验。在弗赖堡和新西伯利亚进行实验研究期间，两个小组应该进行密切合作。弗莱堡和新西伯利亚小组已经进行了非常成功的合作，并联合出版了研究结果，其中应用了拟议项目中使用的技术和设备。

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.