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Ultrafast excited–state dynamics of radical ions in liquid solution

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Defense Thèse de doctorat : Univ. Genève, 2011 - Sc. 4326 - 2011/05/16
Abstract The spectral and dynamic features of photoexcited radical ions have been studied by ultrafast spectroscopy. The excited state absorption spectrum, lifetimes and the influence of parameters like excess excitation energy, solvent or temperature on the photophysical behavior has been investigated for a series of chemically distinct systems chosen such as to span a range of chemical diversity, excited state energies (1 to 2 eV) and oxidation state (cationic or anionic). The radical ions have been generated either chemically or electrochemically in a home-built flow cell. The results reveal that the lifetime of the lowest electronic excited state, D1, is on the order of picoseconds only. Wurster's salts, for example, have a D1 lifetime of 0.2 ps at room temperature irrespective of solvent and nitrogen substituent but influenced by temperature and, thus, fluorescence can be detected below 120 K. By contrast, temperature does not play a role for perylene radical cation whereas the solvent does. In general, the data clearly shows that the energy gap law, often invoked to explain the lack of fluorescence of radical ions, is not obeyed. Instead conical intersections are involved whose accessibility is governed by subtle effects. The always weak the excited state absorption might explain why they have not been observed in highly exergonic photoinduced charge separation reactions associated with the Marcus inverted region, the absence of which is debated.
Keywords Radical ionsRadicalsLiquid solutionExcited state dynamicsExcited state lifetimeEnergy gap lawUltrafast spectroscopyTransient absorption spectroscopyVibrational energy relaxationVibrational relaxationInternal conversionConical intersectionTICT
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URN: urn:nbn:ch:unige-178169
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GRILJ, Jakob. Ultrafast excited–state dynamics of radical ions in liquid solution. Université de Genève. Thèse, 2011. https://archive-ouverte.unige.ch/unige:17816

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Deposited on : 2011-12-19

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