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

Bosman, Gurthwin Wendell (2012-12)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: The availability of laser systems capable of generating ultrashort optical pulses in the visible spectral range have made it possible to study molecular species with a temporal resolution in 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 of different solvents and electronically altering DPM by the addition of substituents with specific electron 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 than 100 fs caused a radiationless photoreaction with a time constant of about 2 ps, which is interpreted as C=N isomerization along the twist coordinate. It is found that during this isomerization reaction an orthogonally twisted intermediate state was formed and observed through its excited-state absorption, which funnels through a conical intersection onto the ground state potential. Once here, bifurcation along pathways towards the ground states of the reactant and product configurations occurs. This was the first observation of photochromism for DPM in a strong polar solvent such as methanol. From the results of the test compound in methanol, comparisons to the photo-behaviour of DPM dissolved in deuterated methanol and dichloromethane are made. What is established is that changing the solvent resulted in slightly different decay times as well as spectral shifts in the absorption profile which suggested that the potential energy surface of the excited state is modified. This is similar to the results found from the electronically altered DPM species. Here the entire photoreaction is found to either be sped up (1 ps) or slowed down (4 ps) depending on the sample investigated. Previous studies on the photochromicity of DPM like systems concluded that apart from the photo-activated forward reaction, a back reaction occurs in the absence of light. Therefore a detailed look at the back reaction of DPM and its derivatives are also studied. This is done by temporally monitoring the absorption change of the photo-product as it converts back to the reactant. In doing so the lifetime of the photo-product is determined and found to differ greatly depending on the solvent and substituent used. Lastly, in assuming that the back reaction follows first order kinetics, an Arrhenius type measurement on the photo-product of DPM is conducted. The result of this measurement is that a potential energy barrier of 64.8 kJ/mol is overcome in the back reaction. The measurements performed and the results obtained from the photochromicity of DPM contribute to the understanding of photo-induced cis-trans isomerization reactions about a C=N double bond.

AFRIKAANSE OPSOMMING: Die beskikbaarheid van laser sisteme in staat om ultrakort optiese pulse te genereer in die sigbare spektrale gebied, het die studie van molekulˆere spesies met ’n tydresolusie in die orde van intra-molekulˆere dinamika wat voorheen nie toeganklik was nie. Voortaan, in hierdie studie word ’n populˆere aktiveer-interogeer tegniek genaamd ultravinnige tydafhanklike absorpsie spektroskopie (UTAS) aangewend om die aanvanklike fotochromiese reaksie van die metaal kompleks, ditisoonfenielkwik (II) (DFK) onder verskillende kondisies te bestudeer. Hierdie kondisies sluit in die gebruik van verskillende oplosmiddels en die elektroniese verstelling van DFK deur die toevoeging van substituente met spesifieke elektron affiniteit eienskappe. Vir hierdie studie word die fotochromiese gedrag van DFK, opgelos in metanol, as standaard gebruik. Met hierdie standaard, word gevind dat die aanvanklike fotochroom reaksie na ultravinnige opwekking binne minder as 100 fs, ’n stralingslose fotoreaksie met ’n tydkonstante van 2 ps veroorsaak. Hierdie fotoreaksie word verklaar as die C=N isomerisasie met betrekking tot die draai ko¨ordinaat. Daar is ook gevind dat gedurende hierdie isomerisasie reaksie, ’n ortogonaal verdraaide oorgangstoestand gevorm word wat waargeneem is deur sy absorpsie in die opgewekte toestand wat tregter deur ’n koniese kruispunt na die potensiaal van die grondtoestand. By hierdie punt vurk die proses na die grondtoestande van die reaktant en die produk vorme. Di´e was die eerste waarneming van fotochromisme van DFK opgelos in ’n sterk polˆere oplosmiddel soos metanol. Vanuit die resultate vir die toets mengsel in metanol word vergelykings getrek tot die fotogedrag van DFK opgelos in gedeureriese metanol en dichlorometaan. Wat hieruit vasgestel is, is dat die verandering in oplosmiddel wel die fotoreaksie be¨ınvloed deurdat verskillende vervaltye en spekrale verskuiwings in die apsorpsie profiel waargeneem word. Hierdie verskille dui aan dat die potensie¨ele energie oppervlakte van die opgewekte toestand wel gemodifiseer word. Dit is ook soortgelyk aan die resultate vir die elektronies verstelde BFK spesies. Hier is gevind dat die tempo van die totale fotoreaksie `of toeneem (1 ps) `of afneem (4 ps) afhanklik van die spesifieke spesie wat bestudeer word. Vorige studies van die fotochromiese gedrag van sisteme soortgelyk aan DFK het gevind dat behalwe vir die fotogeaktiveerde voorwaartse reaksie, daar ook ’n keersy reaksie plaasvind in die afwesigheid van lig. Dus word die keersy reaksie vir DFK en sy derivate ook in detial bestudeer. In hierdie studie word die absorpsie verskille van die fotoproduk tydelik waargeneem soos dit terugkeer na die reaktant. Op hierdie manier word die leeftyd van die fotoproduk bepaal en gevind dat dit grootliks afhang van die tipe oplosmiddel en tipe substituent wat gebruik word. Ten slote, wanneer daar aanvaar word dat di´e keersy reaksie eerste orde kinetika volg, word ’n Arrhenius tipe meting op die fotoproduk uitgevoer. Die resultaat van hierdie meting is dat ’n potensie¨ele enrgie versperring van 64.8 kJ/mol oorkom word in die keersy reaksie. Die metings wat uitgevoer is en die resultate wat verkry is van die fotochromiese gedrag van DFK dra by tot die verstandhouding van lig-ge¨ınduseerde cis-trans isomerisasie reaksies rondom ’n C=N dubbelbinding.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/71913
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