Ultrafast X-ray scattering offers a structural view of excited-state charge transfer

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.

Ultrafast Intersystem Crossing Dynamics of 6-Selenoguanine in Water

“… 99 Even though this time constant almost perfectly matches the first TAS time constant, it has a different physical origin, arising from the reorganization properties of the solvent. 95 In a similar way, scattering experiments can measure intramolecular bond length distributions (see, e.g., ref ( 100 )), and hence the time constants of C=Se bond stretch (14 and 123 fs) are in principle experimentally accessible.…”

Reaction dynamics studiedviafemtosecond X-ray liquidography at X-ray free-electron lasers

“…Such ne information may be obtained through quantum mechanical treatment of X-ray scattering signals. Although such attempts have been made to analyze gas-phase time-resolved X-ray scattering data, 174,175 extracting information on changes in electronic charge distribution in solution through fs-TRXL signals has not yet been achieved and remains a future task.…”

Probing the Effect of Molecular Structure Saddling on Ultrafast Charge Migration via Time-Resolved X-ray Diffraction

“…The availability of ultrashort, intense x-ray pulses from various free-electron laser sources around the globe [21,22] has opened a new array of possibilities to extend x-ray diffraction from static to time domain. TRXD is an emerging method to probe ultrafast processes in nature with atomic-scale spatial and temporal resolutions and has triggered significant theoretical [23][24][25][26][27][28][29][30][31][32][33][34] and experimental [35][36][37][38][39] research on TRXD from different molecules in recent years. Additionally, we will analyse time-dependent electronic flux densities to understand the mechanistic details of the charge migration associated with the induced dynamics.…”