High resolution spectroscopy of directly excited triplet and singlet states

有机分子的光物理和光化学主要由它们的长寿命三重态决定。作为开壳系，它们各自的闭壳单重态具有较高的反应活性。在它们漫长的生命周期中，它们甚至可以通过多次碰撞克服高反应障碍。除了光化学方面，三重态（和单线态）几何形态的知识对于解释系统间交叉、三重态-三重态湮灭和理解有机发光二极管（oled）的基本过程也很重要。尽管它们非常重要，但这些状态下分子的结构大多是未知的。结构在这里被定义为键长和键角，它们决定了分子在各自状态下的三维外观。这主要是由于经典的结构测定方法（核磁共振、x射线衍射、中子散射）只适用于处于电子基态的分子，而电子基态是闭合壳层单重态。利用电子激发的光谱方法原则上可以得到结构信息，但到目前为止，仅限于激发的单重态。这种限制是由单线态-三重态跃迁的小激发截面引起的。在我们的小组中，我们获得了确定电子激发单重态分子的结构和偶极矩的经验。特别是，分子参数与旋转分辨电子光谱自动拟合的进化策略的发明促进了对大分子和弱荧光分子的研究。旋转分辨率是分子在各自电子状态下的转动惯量的关键，从转动惯量可以推断出分子的结构。在与León的Leonardo Alvarez小组的合作中，我们的目标是克服单重态的限制，记录从基态直接激发后处于三重态的分子的旋转分辨电子光谱。该技术包括观测几何形状的变化，这使得可以检测更长时间的磷光。更高的激光功率的现代单模环染料激光器允许更高的激发率。光谱的评估将使用进化算法进行更新版本的拟合程序，这将有助于确定额外的分子参数，如自旋-自旋和自旋-自旋耦合常数。

Photophysics and Photochemistry of organic molecules are mainly determined by their long-lived triplet states. As open shell systems, they have a higher reactivity as the respective closed shell singlet states. During their long life times, they can surmount even high reaction barriers through multiple collisions. Apart from photochemical aspects, the knowledge of triplet (and singlet) state geometries is important for interpretation of intersystem crossing, for triplet-triplet annihilation and for the understanding of basic processes in of organic light emitting diodes (OLEDs). Despite their great importance, the structures of the molecules in these states are mostly unknown. Structures are defined here as bond lengths and bond angles, which determine the three-dimensional appearance of a molecule in the respective state. This is mainly due to the fact that classical structure determining methods (NMR, X-ray diffraction, neutron scattering) are applicable only to molecules in their electronic ground state, which are closed shell singlet states. Spectroscopic methods with electronic excitation can yield structural information in principle, but are, up to now, restricted to excited singlet states. This restriction is caused by the small excitation cross sections of singlet-triplet transitions. In our groups, we gained experience in the determination of structures and dipole moments of molecules in electronically excited singlet states. Particularly, the invention of evolutionary strategies for the automated fit of molecular parameters to the rotationally resolved electronic spectra facilitated the investigation of large and of weakly fluorescing molecules. Rotational resolution is the key to the moments of inertia of the molecules in the respective electronic states, from which the structures can be deduced. In this cooperation with the group of Leonardo Alvarez in León we aim to surmount the restriction to singlet states and record rotationally resolved electronic spectra of molecules in their triplet states after direct excitation from the ground state. The technique comprises a change of the observation geometry, which makes in possible to detect the phosphorescence for a longer time. The higher laser power of modern single mode ring-dye lasers allows for higher excitation rates. The evaluation of the spectra will be performed with an updated version of our fitting program using evolutionary algorithms, what will facilitate the determination of additional molecular parameters like spin-spin and spin-rotation coupling constants.