Untersuchung der Isomerisierungsdynamik von Azobenzolen und der strahlungslosen Desaktivierung von Nukleobasen mit Hilfe der Femtosekunden-Fluoreszenzspektroskopie
The isomerization dynamics of azobenzenes (AB) and the electronic deactivation dynamics of adenine (Ade) and adenosine (Ado) were investigated by femtosecond fluorescence upconversion. Trans- or \cis-AB molecules were excited to the S1 potential surface at 475 nm. Fluorescence was detected between 560 and 708 nm. Biexponential fits to the fluorescence time profiles of trans-AB yielded two time constants, which were in the range of 0.24 - 0.91 ps and 1.8 - 4.9 ps depending on the solvent and detection wavelength. For cis-AB the fluorescence time profiles could only be fitted with three exponentials with < 0.1 ps, 1 ps, and 3 - 30 ps. The viscosity dependence of the time constants was the same as in the case of the isosteric cis-stilbene molecule. For a rotationally restricted trans-AB capped with a crownether and its unrestricted analogon the time constants were 1 ps and 20 ps for both molecules, corroborating the inversion mechanism. The excited state dynamics of a push-pull substituted AB showed a strong dependence on the solvent and detection wavelength with time constants in the range of < 0.1 - 0.85 ps and 0.6 - 20.7 ps. Ade and Ado were excited to the pi\pi* potential surface at 245 - 280 nm. Fluorescence was detected between 300 - 330 nm. For 9H-Ade in H2O the time constant was observed to depend on the excitation energy. Measured time constants were about 0.3 ps for < 260 nm and about 0.66 ps for > 269 nm. In the cases of 7H-Ade and Ado no dependence on excitation energy could be observed. The averrage values were 8.4 ps (7H-Ade) and 0.31 ps (Ado). The Ade lifetimes in dioxane were constant (0.38 ps for 9H-Ade and 15.3 ps for 7H-Ade). The localization of a step in the 9H-Ade lifetimes in aqueous solution at 265 nm may hint at the opening of a pi\sigma* relaxation channel predicted in that energy range.
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