Analysis of structures and reactions in isolated molecules and clusters after photo excitation by combined IR/UV spectroscopic methods

• Applicant: Privatdozent Dr. Christoph Riehn, since 5/2021
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2009 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 119584242

In this project photoinduced reactions in electronically excited states as well as in the electronic ground state will be structurally analysed in molecular beam experiments by different mass and isomer selective UV/IR spectroscopic techniques (up to quadruple resonance methods). These sophisticated methods allow an analysis of photophysical processes on a molecular level; they can be applied for isolated species as well as aggregates (e.g. generated by successive addition of water molecules). Thus the influence of an environment on structure and reactivity can be determined. A detailed structural interpretation will be drawn from a comparison with ab initio and DFT calculations.An important aspect is the experimental and theoretical analysis of proton transfer coordinates. Based on our investigations on ESPT (excited state proton transfer) reactions we will now analyse double proton transfer reactions after electronic excitation. According to theoretical predictions the first IR/UV analyses of the photo switch model 3-hydroxy-picolinic acid will be performed. Further analyses on ESPT reactions will be extended to super photo acids, for which no IR-investigations under molecular beam conditions exist. Our analysis enables us to determine the number of solvent molecules necessary for a proton transfer. Possible proton wires or IR-induced reactions can be identified. Beyond this, further methodical developments regarding the spectroscopy of overtones after electronic excitation will be addressed for a better description of the potential energy surface in electronically excited states. In another methodical development IR triggered proton transfers and rearrangement reactions in the electronic ground state will be analysed by an IR/UV probe method.A further important focus includes the analysis of the structure and reactivity of coumarin derivatives. One aspect is the self-aggregation as well as the aggregation of different coumarins (hetero dimers), for which (except for our preliminary work) no investigations under isolated molecular beam conditions have been performed so far. With regard to the reactivity of this substance class coumarin based caged compounds being able to release e.g. drug molecules after irradiation with light will be investigated. The analysis of reaction channels will be performed by our mass- and isomerselective, vibrationally resolved methods. Thus, we are able to gain insight into the mechanism of photoreactivity for caged compounds on a molecular level. With regard to synthesis, theory and spectroscopy in condensed phase cooperations with other working groups will be established.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Markus Gerhards, until 4/2021 (†)