Transient absorption spectroscopy of metal complexes : dithizonatophenylmercury(II) and derivatives

[摘要] ENGLISH ABSTRACT: The availability of laser systems capable of generating ultrashort optical pulses in the visiblespectral range have made it possible to study molecular species with a temporal resolutionin the order of intra-molecular dynamics which previously were not accessible. Henceforth,in this study a popular pump-probe technique called ultrafast transient absorption spectroscopy(UTAS) is employed to study the initial photochromic reaction of metal complex, dithizonatophenylmercury(II) (DPM) under various conditions. These conditions include the use ofdifferent solvents and electronically altering DPM by the addition of substituents with specificelectron affinity properties.For this study the photochromic behaviour of DPM dissolved in methanol is used as standard.Using this standard, the initial photochromic reaction after ultrafast excitation within less than100 fs caused a radiationless photoreaction with a time constant of about 2 ps, which is interpretedas C=N isomerization along the twist coordinate. It is found that during this isomerizationreaction an orthogonally twisted intermediate state was formed and observed throughits excited-state absorption, which funnels through a conical intersection onto the ground statepotential. Once here, bifurcation along pathways towards the ground states of the reactant andproduct configurations occurs. This was the first observation of photochromism for DPM in astrong polar solvent such as methanol.From the results of the test compound in methanol, comparisons to the photo-behaviour ofDPM dissolved in deuterated methanol and dichloromethane are made. What is established isthat changing the solvent resulted in slightly different decay times as well as spectral shifts inthe absorption profile which suggested that the potential energy surface of the excited state ismodified. This is similar to the results found from the electronically altered DPM species. Herethe entire photoreaction is found to either be sped up (1 ps) or slowed down (4 ps) dependingon the sample investigated.Previous studies on the photochromicity of DPM like systems concluded that apart from thephoto-activated forward reaction, a back reaction occurs in the absence of light. Therefore adetailed look at the back reaction of DPM and its derivatives are also studied. This is done bytemporally monitoring the absorption change of the photo-product as it converts back to thereactant. In doing so the lifetime of the photo-product is determined and found to differ greatlydepending on the solvent and substituent used. Lastly, in assuming that the back reactionfollows first order kinetics, an Arrhenius type measurement on the photo-product of DPM isconducted. The result of this measurement is that a potential energy barrier of 64.8 kJ/mol isovercome in the back reaction.The measurements performed and the results obtained from the photochromicity of DPM contributeto the understanding of photo-induced cis-trans isomerization reactions about a C=Ndouble bond.

[发布机构] Stellenbosch University