Browsing by Author "Von Stein, Xavier"

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    Ultrafast photochromism in metal-organic complexes
    (Stellenbosch : Stellenbosch University, 2016-12) Von Stein, Xavier; Schwoerer, Heinrich P. H.; Bosman, Gurthwin W.; Steenkamp, Christine M.; Stellenbosch University. Faculty of Science. Dept. of Physics
    ENGLISH ABSTRACT : Dithizone (H2Dz), an analytical reagent typically used in colourimetric analysis, reacts with various transition metals to form metal dithizonate complexes. These complexes display strong absorption in the visible region of the spectrum and exhibit photochromism: a photo-induced reversible transformation of the reactant to a product form with a distinctly di erent absorption spectrum. The photo-isomerisation of a C=N bond in the dithizone's backbone is responsible for this behaviour. This mechanism was con rmed in 2011 by the rst ultra-fast study on dithizonatophenylmercury(II) (DPM), a singleliganded complex. To compliment this study, transient absorption spectroscopy was used to capture temporally and spectrally resolved spectra of the photo-induced reaction in the dithizone ligand and select two-liganded dithizontates following excitation at their absorption maxima. The ligand, as well as the two-liganded Hg(HDz)2, Pb(HDz)2 and Zn(HDz)2 complexes showed two reaction paths following photo-excitation. The rst path is associated with an evolution along the rotational isomerisation coordinate which leads to product formation and ground state recovery with a time constant of 1 ps. This is in accordance to what was found for DPM. The second reaction path leads to a re-population of the ground state with a time constant of 10 ps. A physical process could not de nitively be assigned to the second pathway, although it is speculated that it may be due to an unstable intermediate along the C=N inversion coordinate. As the 1 and 10 ps paths were found to be intrinsic to the ligand, it was concluded that the second ligand does not participate in the dynamics, at least not on times below 500 ps. The Ni(HDz)2 complex was not analysed in detail due to complexities that arise given the possibility of ligand-ligand interactions and possible metal to ligand or ligand to metal charge transfer processes.