Berlin 2014 – wissenschaftliches Programm

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MO: Fachverband Molekülphysik

MO 6: Femtosecond Spectroscopy 1

MO 6.4: Vortrag

Dienstag, 18. März 2014, 11:30–11:45, BEBEL HS213

Ultrafast UV-induced photoisomerization of intramolecularly H-bonded β-diketones — •Pramod Kumar Verma, Patrick Nuernberger, and Tobias Brixner — Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg

The chelated enol form (H-bonded) of β-diketones usually exhibits strong absorption bands in the UV region originating from the dipole-allowed ππ^∗ transitions (S₂ ← S₀). The photochemistry following the ππ^∗ transitions can lead to multiple pathways (isomerization, tautomerization, fragmentation) and is highly sensitive to the environment of the molecule. In the gas phase, the dominant pathway is fragmentation with the loss of an OH radical. In contrast, when β-diketones in solution are subjected to UV irradiation, isomerization is the main channel while no fluorescence is detected. The absence of fluorescence suggests the presence of very efficient non-radiative processes from the lowest excited singlet state. The aim of the present study is to understand the relaxation processes of simple β-diketones after excitation to the S₂ state in solution phase. Thus, the excited-state dynamics of β-diketones have been investigated using femtosecond transient absorption with broadband UV white-light detection. Our results confirm that the formation of the non-chelated conformers on electronic excitation of the chelated form occurs on an ultrafast time scale through a dark state. We propose a model that qualitatively explains the microscopic photochemical pathways based on all the experimental and theoretical data currently available.