Ultrafast photochromism in metal-organic complexes

Von Stein, P.-F. Xavier (2016-12)

Thesis (MSc)--Stellenbosch University, 2016

Thesis

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.

AFRIKAANSE OPSOMMING : Ditisoon (H2Dz), 'n reagens wat tipies gebruik word in analitiese chemie, reageer met verskeie oorgangsmetale om metaal-ditisoon komplekse te vorm. Hierdie komplekse vertoon sterk absorpsie in die sigbare gebied van die spektrum en ondergaan 'n fotochromiese reaksie: 'n foto-geïnduseerde omkeerbare transformasie van die reaktant na 'n produk vorm met 'n unieke absorpsiespektrum. Die fotoisomerisasie van 'n C=N verbinding in die ruggraat van die ditisoon is verantwoordelik vir hierdie gedrag. In 2011 is hierdie meganisme bevestig deur die eerste ultravinnige studie op ditisoonfenielkwik (II) (DPM), 'n enkel-ligand kompleks. Om hierdie studie te komplimenteer, is ultravinnige absorbsie spektroskopie gebruik om tydafhanklike spektra van die fotoreaksie in die ditisoon ligand en twee-ligand ditisoon komplekse waar te neem. Die ligand, sowel as Hg(HDz)2, Zn(HDz)2 en Pb(HDz)2 komplekse het twee reaksie paaie gevolg na foto-opwekking. Die eerste pad word geassosieer met rotasionele isomerisasie wat lei tot die vorming van die produk en herbevolking van die grondtoestand met 'n tydkonstante van 1 ps. Dit is in ooreenstemming met wat gevind is vir DPM. Die tweede reaksiepad lei na 'n herbevolking van die grondtoestand met 'n tydkonstante van 10 ps. 'n Fisiese proses kan nie met sekerheid aan die tweede reaksiepad toegeskryf word nie, hoewel daar gespekuleer word dat dit moontlik te wyte kan wees aan 'n onstabiele intermediêre toestand in die isomerisasie proses deur middel van C=N inversie. Omdat die 1 ps en 10 ps paaie intrinsieke eienskappe is van die ligand, is die gevolgtrekking gemaak dat die tweede ligand nie deelneem aan die dinamika nie. Die Ni(HDz)2 kompleks is nie in diepte geanaliseer nie, as gevolg van die kompleksiteit wat veroorsaak word deur die moontlikheid van ligand-ligand interaksies en ladingsoordragprosesse tussen die metaal en die ligand.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100303
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