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Mapping molecular motions leading to charge delocalization with ultrabright electrons

Abstract

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons1,2,3,4,5,6,7,8,9,10,11,12,13,14 or hard X-rays15,16,17,18,19 as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics20. Here we show that a recently developed, ultrabright femtosecond electron source7,8,9 makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition21,22,23,24. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources7,8,9,10,11,12,13,14 for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

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Figure 1: Insulator-to-metal first-order phase transition in (EDO-TTF)2PF6.
Figure 2: Photo-induced time-dependent structural changes monitored by ultrabright FED.
Figure 3: Reaction coordinates and their temporal evolution in the formation of TIS.

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Acknowledgements

We acknowledge the contributions of J. Stampe, M. de Jong, M. Harb and S. G. Kruglik in developing the radio-frequency cavity system and of E. Pelletier in developing the laser system. We also thank K. Iwano and M. Hoshino and K. Yonemitsu for discussions. Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation. This work was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas (grant number 20110006) and the Global Centre of Excellence (G-COE) programme for Chemistry from The Ministry of Education, Culture, Sports, Science and Technology in Japan and by Creative Scientific Research (grant number 18GS0208) from The Japan Society for the Promotion of Science.

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Authors and Affiliations

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Contributions

FED experiments were performed at University of Toronto in R.J.D.M.’s group. G. Sciaini initiated the EDO studies and conducted the work with R.J.D.M. C.L., G.M. and G. Sciaini performed the initial FED experiments at 1 kHz and 100 Hz. M.G., H.J.-R. and R.R.C. performed the final FED experiments with ultrabright electron pulses. M.G., H.J.-R., L.C.L. and A.M. did the data analysis. L.C.L. did the model structural refinement calculations and the Supplementary Video. M.G., H.J.-R. and R.R.C. performed the optical transmission measurements. K.O. and S.-y.K. performed the optical reflectivity measurements. Y.N., X.S., T.H., G. Saito and H.Y. provided the single crystals. C.L. was responsible for sample preparation for FED. M.G., L.C.L. and A.M. wrote the Supplementary Information. G. Sciaini, C.L., M.G. and R.J.D.M. wrote the manuscript with discussions among all authors.

Corresponding author

Correspondence to R. J. Dwayne Miller.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text 1-7, Supplementary Figures 1-16, Supplementary Tables 1-2 and additional references. (PDF 1868 kb)

Making the Molecular Video

Despite the enormous number of possible arrangements of atoms during a structural transition, such as occurs with changes in charge distribution or chemical processes, the interconversion from one structure to another reduces to a few key types of motions. This enormous reduction in dimensionality is what makes chemical concepts transferable from one molecule to another. This movie gives an atomic level view of this enormous reduction in complexity. The key motions involved in charge redistribution and onset of metallic properties in EDO-TTF can be mapped on to 3 highly simplified coordinates. (MP4 17723 kb)

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Gao, M., Lu, C., Jean-Ruel, H. et al. Mapping molecular motions leading to charge delocalization with ultrabright electrons. Nature 496, 343–346 (2013). https://doi.org/10.1038/nature12044

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