High resolution spectroscopy of directly excited triplet and singlet states

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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.