Coupled Electronic & Molecular Dynamics
Charge separation, migration, redistribution, and localization are central to efficient and robust energy storage and retrieval from chemical bonds, and more general photo-driven chemical transformations, through processes such as photosynthesis and catalysis. It has long been recognized that these processes in molecular systems are mediated by the close coupling between electronic and nuclear degrees of freedom (vibronic coupling).
However, this coupling is not well understood at the quantum level, even in simple molecules. While ultrafast optical and IR methods provide important insight on the timescales of these nonequilibrium dynamics, we have not been able to directly observe these processes to date - on fundamental timescales, and with the requisite chemical specificity.
Furthermore, conventional theoretical models based on step-wise equilibrium (e.g. IVR followed by ISC) or separation of timescales (e.g. Born-Oppenheimer approximation) are inadequate for describing electronic excited-state reactions that are inherently non-equilibrium in nature.
Advanced time-resolved X-ray scattering and spectroscopy methods enabled by XFELs (closely coupled with new theoretical approaches) will drive a qualitative advance in our understanding of excited-state chemical transformations leading to design principles for controlling these processes through directed synthesis.
One major scientific objective that underpins approaches to artificial photosynthesis is to understand how the rapid evolution of excited-state charge-density and spin-density distributions, and interaction with the solvent environment, determine non-radiative relaxation and chemical reaction pathways.
Of particular interest are charge-transfer dynamics in transition-metal complexes and assemblies for light harvesting and photocatalysis. Here the challenge is to establish design principles for achieving excited-state lifetimes and catalytic performance exploiting earth-abundant 3d metals that are presently exhibited in complexes based on precious 4d and 5d metals.
Figure 1. Hard X-ray scattering and XES studies of [Fe(bpy)3]2+ light harvesting complex: (A) potential energy surfaces indicating the initial metal-to-ligand charge-transfer (MLCT) excited state, and metal-centered (MC) states, and (B) measured electronic and atomic structural dynamics compared to a simulation. 1
Multi-modal ultrafast X-ray studies have been pioneered at LCLS, using a model iron-based molecular light harvester, [Fe(bpy)3]2+, to demonstrate the power of combining hard X-ray scattering and spectroscopy to directly quantify the initial coupling of electronic and structural configurations with atomic resolution and spin-state specificity.1
As shown in Fig. 1, time-resolved X-ray scattering follows the evolution of the Fe-N bond distance, while XES reveals the transition from the initial metal-to-ligand charge-transfer (MLCT) state, and the interconversion between metal-centered 3MC and 5MC states.
This study revealed for the first time the coherent structural motion (modulation of the Fe-N bond distance) and its direct coupling to coherent electronic transitions observed between 3MC and 5MC states (e.g. at ~300 fs) at the intersection between the respective potential energy surfaces.
Recent multi-modal LCLS studies of an iron-based photo-sensitizer further illustrates the scientific potential of this approach.2 Combined time-resolved XES and solution X-ray scattering measurements of the [Fe(bmip)2]2+ complex reveal the coherent oscillations of an Fe-ligand stretching vibration on a 3MC excited-state potential surface.
These nonequilibrium dynamics mediate the branching pathways between a longer-lived charge-transfer state (desirable for photovoltaics and photo-redox catalysis) and an undesirable relaxation pathway (through the 3MC state) in which the captured photoexcitation energy is dissipated as heat.
K. S. Kjær, T. B. Van Driel, T. C. B. Harlang, et al., "Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy", Chemical Science 10, 5749 (2019).
K. Kunnus, M. Vacher, T. C. B. Harlang, et al., "Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering", Nature Communications 11, 634 (2020).
