Every chemical reaction involves the making and breaking of atomic bonds, thus it is essential to track detailed dynamic evolutions during the processes for complete understanding of chemical reactions. The ultrafast dynamics of bond-breaking process have been intensively investigated by using time-resolved techniques, however, bond-making process have not been thoroughly studied since bimolecular nature of the bond making makes it difficult for its initiation to be synchronized with laser excitation. To scrutinize bond-making process in solution, we chose an appropriate molecular system, a gold trimer complex ([Au(CN)2-]3), in which covalent bond formation is induced by laser excitation. [Au(CN)2-]3 contains three gold atoms in a same solvent cage through a non-covalent interatomic interaction called aurophilicity. Laser excitation upon the gold trimer complex can initiate covalent bond formations between gold atoms in a same solvent cage, which makes the reaction is free from the diffusion limit. We employed femtosecond time-resolved X-ray solution scattering (TRXSS) to visualize the structural evolution during the bond formation of the gold trimer complex. With the advent of X-ray free electron laser (XFEL), time resolution of X-ray solution scattering experiment has been improved down to subpicosecond, making it possible to keep track of ultrafast bond formation of the gold complex. Moreover, we observed rotational dephasing process of an excited state of a gold trimer complex by tracking a degree of anisotropy in time-resolved X-ray scattering patterns. Finally, we performed an additional TRXSS experiment on the gold trimer complex with a better time resolution to observe subtle molecular vibrations during the bond formation process.

Revealing ultrafast structural dynamics of photo-induced bond formation process in a gold trimer complex by femtosecond X-ray solution scattering

256th National Meeting (2018)

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