Principal Investigators: Kelly Gaffney, Amy Cordones-Hahn
The SPC group expects to have postdoctoral and graduate student positions available starting in mid-2021. Please contact Amy Cordones-Hahn and Kelly Gaffney for more information.
Understanding photochemical reactivity in complex molecular systems with ultrafast x-ray and optical sources
The SPC group focuses on understanding, with the goal of controlling, electronic excited state dynamics of transition metal coordination complexes relevant to solar energy applications. Many of the light driven phenomena of direct relevance to efficient solar energy conversions occur on the femtosecond time scale (time-scale for bond vibration) and involve rearrangements of electronic and nuclear structure on the Ångström length scale (length-scale of the molecular bond). This makes ultrafast x-ray methods ideally suited to advancing our understanding of these ultrafast chemical processes. Specifically, we develop and exploit ultrafast x-ray methods to track charge, spin, solvation, and coordination dynamics with atomic specificity and resolution following optical excitation. These methods are combined with complementary ultrafast optical spectroscopy methods, simple inorganic synthesis, and simulation to identify the molecular properties that dictate excited state and photochemical processes.
Current SPC group members:
Research areas and recent publications:
1. Hard x-ray Kβ fluorescence spectroscopy investigations of intersystem crossing and internal conversion in 3d transition metal photosensitizers
We have developed ultrafast Kβ fluorescence to characterize the role of metal centered excited states in the relaxation dynamics of electronic excited states. Our initial effort has focused on the study photo-induced spin crossover in polypyridal Fe(II) complexes. For these molecular systems, the spin dynamics prove to be of particular importance because of the interest in using spin crossover materials in light triggered data storage and iron based dyes as earth abundant light harvesters in dye sensitized solar cells. In both cases the rate of photo-induced spin crossover proves critical, since this rate controls the switching time on the one hand and competes with charge injection on the other.
Figure 1a shows [Fe(2,2’-bipyridine)3]2+, an archetypical spin crossover complex used to develop the ultrafast Fe K Kβ fluorescence methodology at LCLS. Fe Kβ fluorescence involves the creation of an Fe 1s core hole and fluorescence corresponding to the 3p filling of the core hole (Figure 1b). This method is highly sensitive to the Fe 3d spin moment due to the strong 3p-3d exchange interaction, as shown by the difference spectra in Figure 1c for different Fe spin states. This sensitivity allows one to disentangle the charge transfer and metal centered excited states involved in the spin crossover relaxation process. Our group has applied this method to directly resolve the sequential spin crossover mechanism of [Fe(2,2’-bipyridine)3]2+ (Figure 1d). More recently, we have expanded these studies to identify ligand- and solvent-dependent relaxation mechanisms of Fe mixed ligand complexes containing a range of both cyano and polypyridyl ligands. These studies have identified mixed ligand complexes with charge transfer lifetimes enhanced by more than a factor of 500 compared to [Fe(2,2’-bipyridine)3]2+. Through collaboration, we are also applying these methods to study the photophysics of newly designed Fe photosensitizers, including those with carbene and amido-containing ligands, and extending the methodology to include ultrafast valence-to-core x-ray fluorescence, which is additionally sensitive to excited state changes in metal-ligand bonding.
1. Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers: K. Kunnus, et al. Chem. Sci. 11, 4360 (2020).
2. Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy: K. Ledbetter, et al. J. Chem. Phys. 152, 074203 (2020).
3. Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering: K. Kunnus, et al. Nat. Commun. 11, 634 (2020).
4. Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy: H. Tatsuno, et al. Angewandte Chemie 132,372 (2019).
5. Finding Intersections between Electronic Excited State Potential Energy Surfaces with Simultaneous Ultrafast X-ray Scattering and Spectroscopy: K.S. Kjaer, et al. Chem. Sci. 10, 5749 (2019).
6. Solvent Control of Charge Transfer Excited State Relaxation Pthways in [Fe(2,2’-bipyridine)(CN)4]2-: K.S. Kjaer, et al. Phys. Chem. Chem. Phys. 20, 4238 (2018).
7. Ligand Manipulation of Charge Transfer Excited State Relaxation and Spin Crossover in [Fe(2,2’-bipyridine)2(CN)2]: K.S. Kjær, et al. Struct. Dyn. 4, 044030 (2017).
8. Charge and Spin State Characterization of Cobalt Bis(o-Dioxolene) Valence Tautomers using Co Kβ X-ray Emission and L-edge X-ray Absorption Spectroscopies: H.W. Liang, et al. Inorg. Chem. 56, 737 (2017).
9. Manipulating Charge Transfer Excited State Relaxation and Spin Crossover in Iron Coordination Complexes with Ligand Substitution: W. Zhang, et al. Chem. Sci. 8, 515 (2017).
10. Tracking excited-state charge and spin dynamics in iron coordination complexes: W. Zhang, et al. Nature 509, 345 (2014).
2. Soft x-ray resonant inelastic x-ray scattering (RIXS) and soft x-ray absorption as molecular-orbital specific probes of photochemistry and photocatalysis
Femtosecond resolution RIXS has been developed as a molecular-orbital specific probe of photochemistry and photocatalysis (Figure 2). RIXS, the x-ray analogue of resonance Raman, provides atom specific information about both unoccupied and occupied frontier orbitals. We used femtosecond resolution Fe L3-edge RIXS to study the archetypical photodissociation of CO from Fe(CO)5 and the generation of the catalytically active Fe(CO)4. Our investigation demonstrated that singlet and triplet reaction channels proceed in parallel for Fe(CO)4. Prior studies have emphasized the significance of one channel versus another, but we have clearly demonstrated that both triplet and singlet forms of Fe(CO)4 occur with appreciable concentration on the sub-picosecond time scale. The ability to track the spin state of metal center excited states also provides the opportunity to investigate the role of spin in catalytic mechanisms. Spin state changes in the catalytic mechanisms of 3d transition metal catalysts have been proposed for a variety of reactions including ligand binding to Fe(CO) and Ni porphyrins, as well as bond activation by heme and non-heme iron-oxo complexes.
We also apply a combination of metal and ligand atom soft x-ray absorption to study the electronic structure and reaction mechanisms of a class of Ni-centered hydrogen evolving electro- and photo-catalysts with ‘non-innocent’ or ‘redox active’ ligands (Figure 3). X-ray absorption probes the unoccupied orbitals that participate in the reduction reactions of the catalyst, and combining the metal and ligand atomic edges provides direct sensitivity to their strong metal-ligand covalency. With the developments of high repetition rate soft x-ray sources like LCLS-II, we plan to extend these studies to the time-domain to identify changes to the electronic and nuclear structure of the catalysts upon optical excitation and during the photocatalytic reaction.
1. Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Non-Innocent Ligands by Ni and S Edge X-Ray Absorption: S. Koroidov, et al. Inorg. Chem. 57, 13167 (2018).
2. Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering: R.M. Jay, et al. J. Phys. Chem. Lett. 9, 3538 (2018).
3. Transient Metal-Centered States Mediate Isomerization of a Photochromic Ruthenium-Sulfoxide Complex: A.A. Cordones, et al. Nat. Commun. 9, 1989 (2018).
4. Fingerprints of Electronic, Spin and Structural Dynamics from Resonant Inelastic Soft X-Ray Scattering in Transient Photo-Chemical Species: J. Norell, et al. Phys. Chem. Chem. Phys. 20, 7243 (2018).
5. Anti-Stokes Resonant X-ray Raman Scattering for atom specific and excited state selective dynamics: K. Kunnus, et al. New J. Phys. 18, 103011 (2016).
6. Viewing the Valence Electronic Structure of Ferric and Ferrous Hexacyanide in Solution from the Fe and Cyanide Perspectives: K. Kunnus, et al. J. Phys. Chem. B 120, 7182 (2016).
7. Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)5 to Fe(CO)4EtOH: K. Kunnus et al. Struct. Dyn. 3, 043204 (2016).
8. Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution: P. Wernet, et al. Nature 520, 78 (2015).
3. Site-specific reaction mechanisms of model and active photocatalysts studied with time-resolved hard x-ray absorption (XAS) and solution scattering (XSS)
The reaction environment directly influences the energetics and dynamics of chemical activity. For many catalytic reactions, the activity is dominated by transition metal center that binds the reactant and facilitates the charge transfer events that enables bond formation and dissociation. Understanding the reactivity of catalysts and how the reaction environment governs the reaction dynamics requires a highly local characterization of the metal center. Hard x-ray absorption spectroscopy provides a powerful approach to atom specific characterization of electronic structure (XANES) and metal-ligand bonding (EXAFS). We are currently applying this method to study the photochemical reaction mechanisms of the Ni-centered hydrogen evolving catalysts described above, with on-going work focused on identifying their photochemical reaction intermediates. In collaboration, we have previously applied this method to study the photochemical dynamics of Cytochrome C.
Femtosecond resolution x-ray scattering provides a powerful means to probe the global structure, including the site-specific solute interactions of the metal. Equally important, the x-ray scattering signal can be unambiguously compared to molecular dynamics simulations since both methods measure the time dependent pair distribution function. We have applied this method to identify specific solute-solvent interactions in Ir dimer model photocatalysts (Figure 4). Our group was recently involved in the design and implementation of ultrafast electron solution scattering at SLAC Ultrafast Electron Diffraction facility. Further developments of this method will make it a strong complementary probe of solute structural dynamics.
1. Liquid-phase mega-electron-volt ultrafast electron diffraction: J. P. F. Nunes, et al. Struc. Dyn. 7, 024301 (2020).
2. Ultrafast X-ray Scattering Measurements of Coherent Structural Dynamics on the Ground-State Potential Energy Surface of a Diplatinum Molecule: K. Haldrup, et al. Phys. Rev. Lett. 122, 063001 (2019).
3. Anisotropy Enhanced X-Ray Scattering from Solvated Transition Metal Complexes: E. Biasin, et al. J. Synchrotron Rad. 25, 306 (2018).
4. Metalloprotein Entatic Control of Ligand-Metal Bonds Quantfied by Ultrafast X-Ray Spectroscopy: M.W. Mara et al. Science 356, 1276 (2017).
5. Atomistic Characterization of the Active-Site Solvation Dynamics of a Model Photocatalyst: T.B. van Driel, et. al. Nat. Commun. 7, 13678 (2016).
6. Femtosecond X-ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated [Co(terpy)2]2+: E. Biasin, et al. Phys. Rev. Lett. 117, 013002 (2016).
Past SPC group members:
Aniruddha Deb (Associate Research Scientist, University of Michigan)
Robert Hartsock (Engineer, CertTech)
Patrick Hillyard (Scientist, National Security Technologies)
Minbiao Ji (Professor, Fudan University)
Kasper Kjaer (Senior Consultant, BASE Life Science)
Sergey Koroidov (Researcher, Stockholm University)
Kristjan Kunnus (Associate Staff Scientist, LCLS)
Lin Li (Researcher, Stockholm University)
Winnie Liang (Co-Founder Neptune Fluid Flow Systems)
Drew Meyer (John Teagle Professional Fellow and Senior Instructor, Case Western University) Sungnam Park (Professor, Korea University)
Zheng Sun (Senior Engineer, Innovusion)
Lei Zhang (Researcher, Xi'an Jiaotong University)
Wenkai Zhang (Professor, Beijing Normal University)
Xuena Zhang (Director, Global Product Marketing at Applied Materials)