Berlin 2014 – scientific programme
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MO: Fachverband Molekülphysik
MO 23: Femtosecond Spectroscopy 3
MO 23.6: Talk
Friday, March 21, 2014, 15:15–15:30, BEBEL HS213
Individual surrounding effects – how a single counterion changes the photochemistry of phosphonium ions — •Sebastian Thallmair1,2, Christian F. Sailer2, Eberhard Riedle2, and Regina de Vivie-Riedle1 — 1Department Chemie, LMU München — 2LS Für BioMolekulare Optik, LMU München
Photochemical bond cleavage of phosphonium ions is a common tool to generate carbocations in polar and moderately polar solvents. The degree of ion pairing – hence the distance to the counterion – as well as the type of counterion influence the product formation after the bond cleavage. Ultrafast transient absorption measurements have shown, that complex counterions like SbF6− or BF4− lead to the generation of carbocations while excitation of phosphonium salts with halides (e.g. Cl−, Br−) is followed by radical formation.
First we show the influence of the different counterions on the electronic excitations at the Franck-Condon point. Whereas the anions with a high oxidation potential (e.g. SbF6−, BF4−) do not contribute to the lowest excited singulet states while Cl− or Br− reveal another behaviour. These halides are relatively easily oxidized which can be seen in their contribution to the excited states manifold via charge transfer excitations.
On the basis of these results different reaction pathways depending on the counterion are presented. Our theoretical investigations clarify that besides the design of the phosphonium precursor and the chosen solvent the counterion is an additional control knob for the photochemical bond cleavage of phosphonium salts.