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Lemke, Henrik T.,Bressler, Christian,Chen, Lin X.,Fritz, David M.,Gaffney, Kelly J.,Galler, Andreas,Gawelda, Wojciech,Haldrup, Kristoffer,Hartsock, Robert W.,Ihee, Hyotcherl,Kim, Jeongho,Kim, Kyung Hw American Chemical Society 2013 The journal of physical chemistry. A, Molecules, s Vol.117 No.4
<P>X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (∼10<SUP>12</SUP> photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2′-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is ∼160 fs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpcafh/2013/jpcafh.2013.117.issue-4/jp312559h/production/images/medium/jp-2012-12559h_0008.gif'></P>
Britz, Alexander,Gawelda, Wojciech,Assefa, Tadesse A.,Jamula, Lindsey L.,Yarranton, Jonathan T.,Galler, Andreas,Khakhulin, Dmitry,Diez, Michael,Harder, Manuel,Doumy, Gilles,March, Anne Marie,Bajno American Chemical Society 2019 Inorganic chemistry Vol.58 No.14
<P>We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)<SUB>2</SUB>]<SUP>2+</SUP> (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the <SUP>5</SUP>T<SUB>2</SUB>/<SUP>3</SUP>T<SUB>1</SUB> crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the <SUP>5</SUP>T<SUB>2</SUB> state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a <SUP>5</SUP>T<SUB>2</SUB> state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited <SUP>5</SUP>T<SUB>2</SUB> state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin <SUP>1</SUP>A<SUB>1</SUB> ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.</P><P>Time-resolved X-ray emission and absorption spectroscopies have been used to probe the excited-state electronic and geometric structure of an Fe(II) polypyridyl complex. Analysis of the data revealed that the lowest energy excited state is high-spin (<I>S</I> = 2) in character. This determination had not been possible using other experimental techniques (e.g., time-resolved optical spectroscopy), demonstrating the potential for ultrafast X-ray methods to address scientific questions that are difficult to resolve by other means.</P> [FIG OMISSION]</BR>
Kim, Kyung Hwan,Kim, Jeongho,Oang, Key Young,Lee, Jae Hyuk,Grolimund, Daniel,Milne, Christopher J.,Penfold, Thomas J.,Johnson, Steven L.,Galler, Andreas,Kim, Tae Wu,Kim, Jong Goo,Suh, Deokbeom,Moon, J The Royal Society of Chemistry 2015 Physical chemistry chemical physics Vol.17 No.36
<P>Identifying the intermediate species along a reaction pathway is a first step towards a complete understanding of the reaction mechanism, but often this task is not trivial. There has been a strong on-going debate: which of the three intermediates, the CHI<SUB>2</SUB> radical, the CHI<SUB>2</SUB>–I isomer, and the CHI<SUB>2</SUB><SUP>+</SUP> ion, is the dominant intermediate species formed in the photolysis of iodoform (CHI<SUB>3</SUB>)? Herein, by combining time-resolved X-ray liquidography (TRXL) and time-resolved X-ray absorption spectroscopy (TR-XAS), we present strong evidence that the CHI<SUB>2</SUB> radical is dominantly formed from the photolysis of CHI<SUB>3</SUB> in methanol at 267 nm within the available time resolution of the techniques (∼20 ps for TRXL and ∼100 ps for TR-XAS). The TRXL measurement, conducted using the time-slicing scheme, detected no CHI<SUB>2</SUB>–I isomer within our signal-to-noise ratio, indicating that, if formed, the CHI<SUB>2</SUB>–I isomer must be a minor intermediate. The TR-XAS transient spectra measured at the iodine L<SUB>1</SUB> and L<SUB>3</SUB> edges support the same conclusion. The present work demonstrates that the application of these two complementary time-resolved X-ray methods to the same system can provide a detailed understanding of the reaction mechanism.</P> <P>Graphic Abstract</P><P>We identify a major transient species formed in the photolysis of CHI<SUB>3</SUB> by combining time-resolved X-ray liquidography (TRXL) and time-resolved X-ray absorption spectroscopy (TR-XAS). <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5cp03686k'> </P>