Probing excited state molecular reaction trajectories in photochemical processes for solar energy conversion by X-ray transient spectroscopies and scattering

The solar energy conversion processes convert energies of photons to electricity or fuels (hydrogen, methane, etc.) through molecular excited states that are highly energetic and transiently present, and undergo charge separation into holes and electron which then are used to drive chemical reactions.  The outcomes of these reactions are determined by the energetics, electronic and geometric structures of these excited state molecules. Therefore, probing transient electronic and nuclear structures of excited state molecules along the reaction coordinates will provide detailed understanding of the reaction mechanisms and guidance for molecular and materials design for optimal solar energy conversion efficiencies with minimal loss. We use ultrafast optical and X-ray spectroscopies to obtain the elemental specific electronic configurations of metal centers in various light-active materials for solar energy conversion.  The optical probe detects optical responses associated with valence excited-state reaction rates, lifetimes, reactions, and coherences between different states, while the X-ray probe detects the evolution of the electronic and geometric structures upon initiation of the reaction. This two-fold approach yields fundamental insights into reaction mechanisms that would not otherwise be available with a single technique. This combined approach has been applied to various transition metal complexes, such as metalloporphyrins, first row transition metal complex (Mn, Fe, Co, Ni, Cu and Zn) sensitizers for solar cell mimics and photocatalysts, as well as other metal containing molecules, polymers and proteins.  Our goal is to search for optimal materials for solar energy conversion through correlations of molecular structures, energetics and dynamics in light activated reactions in various systems in collaboration with organic, inorganic, polymer and biological chemists, as well as theoretical chemists and physicists.

Searching and Understanding Roles of Ultrafast Coherent Electronic and Atomic Motions in Photochemical Reactions

A newly formed research team (with Northwestern University, Argonne, North Carolina State University and University of Washington) aims at studying coherent electronic or nuclear motions in photochemical reactions and energetic, structural and dynamic factors pertinent to these coherent motions.  Here the coherence is defined as: individual constituents have well-defined phase relationships among themselves so that they are synchronized in time over extended distances and can exhibit wave-like interference effects. Such coherent phenomena for electronic and nuclear oscillations, although recognized for a long time, their definitive detection has been only made possible in the past decade because of advances in light sources, such as femtosecond lasers and X-rays, and in spectroscopic techniques, such as two-dimensional electronic spectroscopy. These advances open new opportunities of correlating dynamics, energetics and electronic/nuclear structures on time scales of coherent electronic (i.e. 10 fs) and atomic motions (i.e. vibrational periods) before excited states can significantly thermalize.  The main goal of the project is to investigate roles of these coherent motions in subsequent photochemical reactions in terms of yield, rate and directionality, which not only involve advanced experimental methods, but also new theoretical capabilities in elucidating interplays between electron and nuclear motions as well as their corresponding observables in experiments. Current systems under investigation are porphyrin and transition metal dimers with systematic tuning of electronic coupling and hybrid gold nanoparticle-dye molecule systems.  The former has shown interesting vibronic coherence and the later coherent phonon oscillations coupling to the electronic transitions of molecules on the gold nanoparticle surface.

Fundamental Electronic Processes in Organic Materials 

π-conjugated organic molecules, polymers and small molecules have played important roles in the recent advancement of organic optoelectronics and photovoltaics. We are interested in understanding the fundamental light-matter interactions in these materials (exciton formation, exciton splitting, charge carrier transport) tied to these versatile functions by spectroscopic and structural characterization techniques, and also by computational and theoretical insight. Specifically, we focus on the interplay of multiple time-scales, correlating ultrafast exciton and charge carrier dynamics with structural characterization to help explain macroscopic device properties. Common themes studied in this group include exciton generation/diffusion/dissociation, electron transfer, energy transfer, charge carrier mobility and recombination dynamics, molecular packing and domain size analysis, and electronic coherences.

Capturing Transient Protein Structures on Multiple Spatial and Temporal Scales

Metalloproteins have many different functions in cells, such as storage and transport of small molecular substrates, proteins, enzymes and signal transduction proteins. In those metalloproteins with metal center active sites, the interplay between the metal oxidation state/coordination geometry and the overall protein conformation determines the function of the protein.  Our long term objective of this project is to gain new insight into correlations between metallo activesite structures of a metalloenzyme and their functions through high resolution simultaneous structural “snapshots” of the active site during different biological relevant processes, such as electron transfer, ligand binding and protein folding.  Both transient electronic and nuclear structures of metal centered active sites as a function of the reaction time in a series of metalloproteins will be simultaneously captured using X-ray transient absorption (XTA, or transient X-ray absorption spectroscopy) spectroscopy with time resolutions from 10-13second (100 fs) to longer, while the protein conformation change along the reaction coordinates can be captured by transient small- / wide-angle X-ray solution scattering (simplified as TRXSS). Kinetics from other transient spectroscopic measurements will provide much deeper understanding of energy transduction inside the proteins during enzymatic reactions and provide guidance for modulating protein functions via structural modifications around the active sites, leading to advances in rational enzymatic function. We are also studying protein dynamics activated by pH and temperature in solution using time-resolved small angle X-ray scattering on slower time scales to provide direct structural and dynamics information for protein folding, aggregation etc.