Ultrafast X-ray scattering offers a structural view of excited-state charge transfer
Haiwang Yong, Xuan Xu, Jennifer M. Ruddock, Brian Stankus, Andrés Moreno Carrascosa, Nikola Zotev, Darren Bellshaw, Wenpeng Du, Nathan Goff, Yu Chang, Sébastien Boutet, Sergio Carbajo, Jason E. Koglin, Mengning Liang, Joseph S. Robinson, Adam Kirrander, Michael P. Minitti, Peter Weber
Abstract
Intramolecular charge transfer and the associated changes in molecular structure in N,N'-dimethylpiperazine are tracked using femtosecond gas-phase X-ray scattering. The molecules are optically excited to the 3p state at 200 nm. Following rapid relaxation to the 3s state, distinct charge-localized and charge-delocalized species related by charge transfer are observed. The experiment determines the molecular structure of the two species, with the redistribution of electron density accounted for by a scattering correction factor. The initially dominant charge-localized state has a weakened carbon-carbon bond and reorients one methyl group compared with the ground state. Subsequent charge transfer to the charge-delocalized state elongates the carbon-carbon bond further, creating an extended 1.634 Å bond, and also reorients the second methyl group. At the same time, the bond lengths between the nitrogen and the ring-carbon atoms contract from an average of 1.505 to 1.465 Å. The experiment determines the overall charge transfer time constant for approaching the equilibrium between charge-localized and charge-delocalized species to 3.0 ps.