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      Bromosubstituted norbornadienes and their reversible photolytic transformation to quadricyclanes

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          Two new model systems for use within the rapidly developing ultrafast time resolved x-ray scattering techniques have been prepared. Their photoisomerisation from norbornadiene to quadricyclane was found to be a suitable reaction to follow. Simulations of scattering patterns (not included in this report) showed that if heavy atoms are included in these molecular structures, then the transformation can be followed by transient X-ray scattering techniques. Two new bromosubstituted norbornadienes were synthesised and characterised. Absorption spectroscopy showed that the norbornadienes are converted quantitatively to quadricyclanes under ultraviolet irradiation. Nuclear magnetic resonance (NMR) studies showed that the process was fully reversible and that the norbornadienes could be completely recovered even without addition of catalysts. Furthermore, it was shown that the formation of quadricyclane from norbornadiene was unaffected by triplet sensitisers. The two new model systems synthesised thus are strong candidates for use in time-resolved X-ray scattering studies in both gas and condensed phases.

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          Time-resolved X-ray scattering of an electronically excited state in solution. Structure of the 3A(2u) state of tetrakis-mu-pyrophosphitodiplatinate(II).

          The structure of the (3)A(2u) excited state of tetrakis-mu-pyrophosphitodiplatinate(II) in aqueous solution is investigated by time-resolved X-ray scattering on a time scale from 100 ps to 1 micros after optical pumping. The primary structural parameter, the Pt-Pt distance, is found to be 2.74 A, which is 0.24 A shorter than the ground-state value. The contraction is in excellent agreement with earlier estimates based on spectroscopic data in solution and diffraction data in the crystalline state. As a second structural parameter, the distance between the P planes in the (3)A(2u) excited state was determined to be 2.93 A, i.e., the same as that in the ground state. This result implies that a slight lengthening of the Pt-P bond occurs following excitation.
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            Analysis of time-resolved X-ray scattering data from solution-state systems.

            As ultrafast time-resolved studies of liquid systems with the laser pump/X-ray scattering probe method have come of age over the past decade, several groups have developed methods for the analysis of such X-ray scattering data. The present article describes a method developed primarily with a focus on determining structural parameters in the excited states of medium-sized molecules (approximately 30 atoms) in solution. The general methodology is set in a maximum-likelihood framework and is introduced through the analysis of the photoactive platinum compound PtPOP, in particular the structure of its lowest triplet excited state ((3)A(2u)). Emphasis is put on structure determination in terms of model comparisons and on the information content of difference scattering signals as well as the related experimental variables. Several suggestions for improving the accuracy of these types of measurements are presented.
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              Structural Determination of a Photochemically Active Diplatinum Molecule by Time-Resolved EXAFS Spectroscopy


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                ScienceOpen Research
                05 September 2014
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                [1 ]Nano-Science Center & Department of Chemistry, University of Copenhagen, København Ø, Denmark
                [2 ]Department of Physics, Technical University of Denmark, Fysikvej 307, 2800 Kgs. Lyngby
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                [* ]Corresponding author's e-mail address: klbe@
                © 2014 P. Hammershøj et al.

                This work has been published open access under Creative Commons Attribution License CC BY 4.0 , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at .

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                Figures: 5, Tables: 0, References: 34, Pages: 7
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