As Chemistry is the Science of Molecular Change, experimental and theoretical studies of short-lived species are central to Chemistry. Time-resolved (TR) crystallography can reveal the geometry of excited states, and in studies still to be realized, the mechanism of light-induced chemical reactions. Since photoinduced biological processes are typically triggered by chemical reactions such as cis-trans isomerization, chemical and biological time-resolved diffraction are intrinsically related.

In a first series of studies we have used a stroboscopic time-resolved diffraction technique [1] to determine the geometry of the microsecond-lifetime excited states of a series of binuclear Pt, Rh and Cu metalloorganic complexes, in which large metal-metal-distance shortening occurs on excitation. The rapidly reversible bond-shortening is intermolecular in a trinuclear Cu(I) pyrazolate complex [2]. The results agree qualitatively with theory, but are not always quantitatively reproduced by calculations on the isolated molecules.

In a second phase of the work TR diffraction has been applied to species incorporated as guests in extended organic frameworks, synthesized by using the methods of crystal engineering. The corresponding solid-state dilution has several advantages and allows assessment of the effect of the matrix on ground- and excited state molecular properties. Eight different phases incorporating the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion have been prepared. They show different states of aggregation of the cations and exhibit large variations in luminescence lifetimes [3]. The ultimate goal of the work is to study the mechanism of photo-induced chemical reactions in fully-ordered three-dimensional frameworks on timescales of picoseconds and less. Specific reactions and technical developments in progress will be discussed [4].

[1] P. Coppens, I. I. Vorontsov, T. Graber, M. Gembicky, A. Yu. Kovalevsky, Acta Crystallogr. A 2005, 61, 162-172.

[2] I. I. Vorontsov, A. Yu. Kovalevsky, Y.-S. Chen, T. Graber, M. Gembicky, I. V. Novozhilova, M. A. Omary, P. Coppens, Phys. Rev. Lett. 2005, 94, 193003/1-193003/4.

[3] S.-L. Zheng, M. Gembicky, M. Messerschmidt, P. Dominiak, P. Coppens, Submitted.

[4] Research supported by the U. S. Department of Energy and the U.S. National Science Foundation.

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