Investigations of electronically excited states
Different electronically excited states of photoprotective and photoreactive substances are analyzed concerning their energy, structure and reactivity via combined IR/UV spectroscopy (Figure 4) in a mass selective molecular beam experiment on isolated molecules and aggregates.
Figure 1: Methods for investigation of S0 (a, b) and S1 (c ,d) state in a molecular beam experiment. After isomer selective excitation in a vibrational level of the electronically excited state the ionization takes place with the help of a second UV-photon (resonant two photon ionization, R2PI). If an IR laser beam is irradiated prior to the first UV-photon, the zero point vibrational level of the ground state is depopulated in case of a resonant vibrational excitation. A decrease of the ion signal of the R2PI process results from this (a). The irradiation of a frequency fixed IR beam prior to the IR/R2PI process leads to isomer selectivity even if the isomers possess the same excitation and ionization potential (IR/IR/R2PI, b). A vibrational spectrum of the S1 state will be obtained if the IR laser is irradiated after the first UV-photon (UV/IR/UV, c). If the first UV-photon does not lead to an isomer selective excitation, this selection can be achieved with the help of the IR/UV/IR/UV quadruple resonance method (d).
As an example of photoreactive substances, caged compounds which contain groups that release e.g. biologically active components after electronic excitation are analyzed (in cooperation with the group of PD Schaper, University Düsseldorf). Electronically excited singlet or triplet states of the caged compounds can be analyzed with respect to their fragmentions, vibrational transitions and structures. Conclusions can be drawn about the mechanism of the photoreaction from the obtained structures. We investigate primarily coumarine based caged compounds. Furthermore the aggregation behavior of coumarines under isolated respectively micro solvated conditions is analyzed.
A further photochemically interesting class of substances are flavonoids (to which the basis systems like e.g. 3-hydroxyflavone and 3-hydroxychromone belong). Flavonoids are known for their photoprotective and antioxidative effects. One of the primary steps after photoexcitation is the keto-enol-tautomerism of adjacent hydroxyl- and carbonyl groups (excited state proton transfer, ESPT). In order to obtain detailed structural information about the tautomerization coordinate, the vibrations of the electronic ground state and the electronically excited states are analyzed directly via IR/R2PI and UV/IR/UV spectroscopy. By comparing the experimentally determined frequencies with ab initio and DFT calculations, structural modifications and reaction coordinates can be identified.
Figure 5: Proton transfer in the excited state of 3-Hydroxyflavone