Control of the entatic state by targeted optical excitation of the ligand sphere

• Applicants: Professorin Dr. Sonja Herres-Pawlis; Professor Dr. Michael Rübhausen
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413524714

The concept of the entatic state describes the ambivalent relation of a charge transfer and an accompanying structural rearrangement. It poses the fundamental question if the charge transfer is favoured by a structural pre-organisation. This question is essential for many catalytically active systems in chemistry where the catalytic properties transition metal complexes are controlled by their tailored ligand sphere. In bioinorganic chemistry, several groups have reported entatic model systems. These model systems possess structurally preorganised ligand spheres. A systematic study is impeded by the fact that systems with a tunable preorganisation are not easily chemically synthesised. We have shown earlier that the optically excited charge transfer leads to a state which corresponds to a complete transfer of charge from the transition metal to the ligand system. This project focuses thus on the targeted optical manipulation of the ligand systems of a stable model system for an entatic state in order to excite the preorganisation of the ligands. Herefore, we excite via an infrared pump beam suited ligand vibrations which translate an entatic state in another one. This process proceeds under emission of vibrational excitations. The power of the pump beam allows now the exact control of the vibrational excitation. The excitation of different infrared-active modes enables the targeted selection of different excitation patterns of preorganised systems. In contrast, the conventional excitation in the visible or ultraviolet spectral range triggers a highly complex process cascade by the excitation into highly excited electronic states which finally end up in a photoactivated entatic state. The investigation of these states will be possible in the framework of this application in a complementary way by time-resolved transient absorption measurements and time-resolved fluorescence. The determination of the short-lived structural degrees of freedom will be performed by time-resolved Raman scattering and the control of entasis will be studied by time-resolved Raman scattering after MIR pump. We will answer in this study how the dynamics of the entatic state differentiate between the electronic excitation into the charge transfer transition and the structural excitation of the ligand systems in the mid IR range. In detail, we target to clarify if the tailored excitation into eigenmodes of the ligand system facilitate the transition between entatic states. This would finally lead to the question how far entatic states can be controlled and manipulated by light.

• DFG Programme: Research Grants