Düsseldorf 2007 – scientific programme

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MO: Fachverband Molekülphysik

MO 13: Photochemistry

MO 13.7: Talk

Monday, March 19, 2007, 18:00–18:15, 6B

Singlet Oxygen generation from Aromatic Endoperoxides — •Alexandra Lauer¹, Inés Corral², Leticia González², Henk Fidder¹, and Karsten Heyne¹ — ¹Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin — ²Freie Universität Berlin, Fachbereich Chemie, Takustrasse 3, 14195 Berlin

Aromatic endoperoxides are claimed to exhibit homolytic cleavage from the lowest electronic excited state, while higher electronic states lead to singlet oxygen generation. However, this lowest electronic state has never been located, and recent semi-empirical calculations even denied the existence of this state. In our contribution state-of-the-art ab initio calculations are combined with steady-state absorption and fluorescence experiments, and polarization-resolved femtosecond UV-pump/IR-probe spectroscopy, to resolve this issue. We observe the disputed S1 state of APO that leads to homolytic O-O cleavage, at 33700 cm⁻¹, in excellent agreement with our ab initio calculations. Femtosecond polarization resolved UV/IR experiments confirm that the absorption transition dipole at 36800 cm⁻¹ is indeed polarized parallel to the oxygen bridge, thereby verifying the theoretical assignment of this band to the S0→S4 transition. A mechanism for ¹O₂ generation from APO is presented, based on the ab initio calculations, that indicates how excitation to the S2 (π_CC → π_CC^*) or S4 (π_OO^* → π_CC^*) state leads to non-adiabatic population of the Sx (π_OO^* → σ_CO^*) state, which plays a decisive role in the ¹O₂ generation.