Amy Cordones-Hahn

All Publications

• Metal-Ligand Covalency in the Valence Excited States of Metal Dithiolenes Revealed by S 1s3p Resonant Inelastic X-ray Scattering. Journal of the American Chemical Society Larsen, C. B., Ledbetter, K., Nascimento, D. R., Biasin, E., Qureshi, M., Nowak, S. H., Sokaras, D., Govind, N., Cordones, A. A. 2024

  Abstract

  Metallo dithiolene complexes with biological and catalytic relevance are well-known for having strong metal-ligand covalency, which dictates their valence electronic structures. We present the resonant sulfur Kβ (1s3p) X-ray emission spectroscopy (XES) for a series of Ni and Cu bis(dithiolene) complexes to reveal the ligand sulfur contributions to both the occupied and unoccupied valence orbitals. While S K-edge X-ray absorption spectroscopy played a critical role in identifying the covalency of the unoccupied orbitals of metal dithiolenes, the present focus on XES explores the occupied density of states. For a series of [Cu(mnt)2]n- and [Ni(mnt)2]n- anions and dianions, a comparison of the nonresonant and resonant S Kβ XES spectra highlights the dramatic improvement in spectral resolution and corresponding ability to differentiate subtle changes in occupied electronic structure across the series. Furthermore, the use of resonant inelastic X-ray scattering (RIXS) probes the valence excited states and the core-valence couplings of the complexes. By employing a theoretical approach based on time-dependent density functional theory to interpret the RIXS spectra, we reveal how metal-ligand covalency influences the excited state energies and covalencies. We identify the low energy excited states as having the same symmetry as the nominal "ligand field" or "d-d" states that typically dominate the photophysics of 3d metal complexes but with significant metal-ligand charge transfer character dictated by their covalency. These results suggest that strong metal-ligand covalency can be used to influence the charge-transfer photochemistry of first row transition metal complexes.

  View details for DOI 10.1021/jacs.4c11667

  View details for PubMedID 39377493

• Oxidizing Role of Cu Cocatalysts in Unassisted Photocatalytic CO2Reduction Using p-GaN/Al2O3/Au/Cu Heterostructures. ACS nano Zoric, M. R., Basera, P., Palmer, L. D., Aitbekova, A., Powers-Riggs, N., Lim, H., Hu, W., Garcia-Esparza, A. T., Sarker, H., Abild-Pedersen, F., Atwater, H. A., Cushing, S. K., Bajdich, M., Cordones, A. A. 2024

  Abstract

  Photocatalytic CO2 reduction to CO under unassisted (unbiased) conditions was recently demonstrated using heterostructure catalysts that combine p-type GaN with plasmonic Au nanoparticles and Cu nanoparticles as cocatalysts (p-GaN/Al2O3/Au/Cu). Here, we investigate the mechanistic role of Cu in p-GaN/Al2O3/Au/Cu under unassisted photocatalytic operating conditions using Cu K-edge X-ray absorption spectroscopy and first-principles calculations. Upon exposure to gas-phase CO2 and H2O vapor reaction conditions, the composition of the Cu nanoparticles is identified as a mixture of CuI and CuII oxide, hydroxide, and carbonate compounds without metallic Cu. These composition changes, indicating oxidative conditions, are rationalized by bulk Pourbaix thermodynamics. Under photocatalytic operating conditions with visible light excitation of the plasmonic Au nanoparticles, further oxidation of CuI to CuII is observed, indicating light-driven hole transfer from Au-to-Cu. This observation is supported by the calculated band alignments of the oxidized Cu compositions with plasmonic Au particles, where light-driven hole transfer from Au-to-Cu is found to be thermodynamically favored. These findings demonstrate that under unassisted (unbiased) gas-phase reaction conditions, Cu is found in carbonate-rich oxidized compositions rather than metallic Cu. These species then act as the active cocatalyst and play an oxidative rather than a reductive role in catalysis when coupled with plasmonic Au particles for light absorption, possibly opening an additional channel for water oxidation in this system.

  View details for DOI 10.1021/acsnano.4c02088

  View details for PubMedID 39037113

• Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes. Journal of the American Chemical Society Reinhard, M. E., Sidhu, B. K., Lozada, I. B., Powers-Riggs, N., Ortiz, R. J., Lim, H., Nickel, R., Lierop, J. v., Alonso-Mori, R., Chollet, M., Gee, L. B., Kramer, P. L., Kroll, T., Raj, S. L., van Driel, T. B., Cordones, A. A., Sokaras, D., Herbert, D. E., Gaffney, K. J. 2024

  Abstract

  To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

  View details for DOI 10.1021/jacs.4c02748

  View details for PubMedID 38889309

• Observation of a Picosecond Light-Induced Spin Transition in Polymeric Nanorods. ACS nano Reinhard, M., Kunnus, K., Ledbetter, K., Biasin, E., Zederkof, D. B., Alonso-Mori, R., van Driel, T. B., Nelson, S., Kozina, M., Borkiewicz, O. J., Lorenc, M., Cammarata, M., Collet, E., Sokaras, D., Cordones, A. A., Gaffney, K. J. 2024

  Abstract

  Spin transition (ST) materials are attractive for developing photoswitchable devices, but their slow material transformations limit device applications. Size reduction could enable faster switching, but the photoinduced dynamics at the nanoscale remains poorly understood. Here, we report a femtosecond optical pump multimodal X-ray probe study of polymeric nanorods. Simultaneously tracking the ST order parameter with X-ray emission spectroscopy and structure with X-ray diffraction, we observe photodoping of the low-spin-lattice within ∼150 fs. Above a ∼16% photodoping threshold, the transition to the high-spin phase occurs following an incubation period assigned to vibrational energy redistribution within the nanorods activating the molecular spin switching. Above ∼60% photodoping, the incubation period disappears, and the transition completes within ∼50 ps, preceded by the elastic nanorod expansion in response to the photodoping. These results support the feasibility of ST material-based GHz optical switching applications.

  View details for DOI 10.1021/acsnano.3c10042

  View details for PubMedID 38833689

• Simple Preparation and Characterization of Hybrid Cobalt Phthalocyanine on Multiwalled Carbon Nanotube Electrodes ACS APPLIED ENERGY MATERIALS Chan, T., Zoric, M. R., Shandilya, A., Loeb, C. K., Barrett, J. A., Cordones, A. A., Kubiak, C. P. 2024

  View details for DOI 10.1021/acsaem.3c02953

  View details for Web of Science ID 001181209500001

• Site-specific electronic structure of covalently linked bimetallic dyads from nitrogen K-edge x-ray absorption spectroscopy. The Journal of chemical physics Ryland, E. S., Liu, X., Kumar, G., Raj, S. L., Xie, Z. L., Mengele, A. K., Fauth, S. S., Siewerth, K., Dietzek-Ivanšić, B., Rau, S., Mulfort, K. L., Li, X., Cordones, A. A. 2024; 160 (8)

  Abstract

  A nitrogen K-edge x-ray absorption near-edge structure (XANES) survey is presented for tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz)-bridged bimetallic assemblies that couple chromophore and catalyst transition metal complexes for light driven catalysis, as well as their individual molecular constituents. We demonstrate the high N site sensitivity of the N pre-edge XANES features, which are energetically well-separated for the phenazine bridge N atoms and for the individual metal-bound N atoms of the inner coordination sphere ligands. By comparison with the time-dependent density functional theory calculated spectra, we determine the origins of these distinguishable spectral features. We find that metal coordination generates large shifts toward higher energy for the metal-bound N atoms, with increasing shift for 3d < 4d < 5d metal bonding. This is attributed to increasing ligand-to-metal σ donation that increases the effective charge of the bound N atoms and stabilizes the N 1s core electrons. In contrast, the phenazine bridge N pre-edge peak is found at a lower energy due to stabilization of the low energy electron accepting orbital localized on the phenazine motif. While no sensitivity to ground state electronic coupling between the individual molecular subunits was observed, the spectra are sensitive to structural distortions of the tpphz bridge. These results demonstrate N K-edge XANES as a local probe of electronic structure in large bridging ligand motifs, able to distinctly investigate the ligand-centered orbitals involved in metal-to-ligand and ligand-to-ligand electron transfer following light absorption.

  View details for DOI 10.1063/5.0192809

  View details for PubMedID 38415835

• In situ x-ray absorption investigations of a heterogenized molecular catalyst and its interaction with a carbon nanotube support. The Journal of chemical physics Zoric, M. R., Chan, T., Musgrave, C. B., Goddard, W. A., Kubiak, C. P., Cordones, A. A. 2023; 158 (7): 074703

  Abstract

  A highly active heterogenized molecular CO2 reduction catalyst on a conductive carbon support is investigated to identify if its improved catalytic activity can be attributed to strong electronic interactions between catalyst and support. The molecular structure and electronic character of a [Re+1(tBu-bpy)(CO)3Cl] (tBu-bpy = 4,4'-tert-butyl-2,2'-bipyridine) catalyst deposited on multiwalled carbon nanotubes are characterized using ReL3-edge x-ray absorption spectroscopy under electrochemical conditions and compared to the homogeneous catalyst. The Reoxidation state is characterized from the near-edge absorption region, while structural changes of the catalyst are assessed from the extended x-ray absorption fine structure under reducing conditions. Chloride ligand dissociation and a Re-centered reduction are both observed under applied reducing potential. The results confirm weak coupling of [Re(tBu-bpy)(CO)3Cl] with the support, since the supported catalyst exhibits the same oxidation changes as the homogeneous case. However, these results do not preclude strong interactions between a reduced catalyst intermediate and the support, preliminarily investigated here using quantum mechanical calculations. Thus, our results suggest that complicated linkage schemes and strong electronic interactions with the initial catalyst species are not required to improve the activity of heterogenized molecular catalysts.

  View details for DOI 10.1063/5.0129724

  View details for PubMedID 36813711

• Dissociation of Pyridinethiolate Ligands during Hydrogen Evolution Reactions of Ni-Based Catalysts: Evidence from X-ray Absorption Spectroscopy. Inorganic chemistry Ledbetter, K., Larsen, C. B., Lim, H., Zoric, M. R., Koroidov, S., Pemmaraju, C. D., Gaffney, K. J., Cordones, A. A. 2022

  Abstract

  The protonation of several Ni-centered pyridine-2-thiolate photocatalysts for hydrogen evolution is investigated using X-ray absorption spectroscopy (XAS). While protonation of the pyridinethiolate ligand was previously thought to result in partial dechelation from the metal at the pyridyl N site, we instead observe complete dissociation of the protonated ligand and replacement by solvent molecules. A combination of Ni K-edge and S K-edge XAS of the catalyst Ni(bpy)(pyS)2 (bpy = 2,2'-bipyridine; pyS = pyridine-2-thiolate) identifies the structure of the fully protonated catalyst as a solvated [Ni(bpy)(DMF)4]2+ (DMF = dimethylformamide) complex and the dissociated ligands as the N-protonated 2-thiopyridone (pyS-H). This surprising result is further supported by UV-vis absorption spectroscopy and DFT calculations and is demonstrated for additional catalyst structures and solvent environments using a combination of XAS and UV-vis spectroscopy. Following protonation, electrochemical measurements indicate that the solvated Ni bipyridine complex acts as the primary electron-accepting species during photocatalysis, resulting in separate protonated ligand and reduced Ni species. The role of ligand dissociation is considered in the larger context of the hydrogen evolution reaction (HER) mechanism. As neither the pyS-H ligand nor the Ni bipyridine complex acts as an efficient HER catalyst alone, the critical role of ligand coordination is highlighted. This suggests that shifting the equilibrium toward bound species by addition of excess protonated ligand (2-thiopyridone) may improve the performance of pyridinethiolate-containing catalysts.

  View details for DOI 10.1021/acs.inorgchem.2c00167

  View details for PubMedID 35732599

• Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering. Inorganic chemistry Kunnus, K., Guo, M., Biasin, E., Larsen, C. B., Titus, C. J., Lee, S. J., Nordlund, D., Cordones, A. A., Uhlig, J., Gaffney, K. J. 1800

  Abstract

  Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L3 edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)2]2+ and [Fe(btbip)2]2+, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe-carbene bond length due to steric effects modify the metal-ligand bonding, including sigma/pi donation and pi back-donation. We find that sigma donation is significantly stronger in [Fe(bmip)2]2+, whereas the pi back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)2]2+ lead to approximately 1 eV destabilization of the quintet metal-centered 5T2g excited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)2]2+. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

  View details for DOI 10.1021/acs.inorgchem.1c03124

  View details for PubMedID 35029978

• Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency. The journal of physical chemistry letters Liekhus-Schmaltz, C., Fox, Z. W., Andersen, A., Kjaer, K. S., Alonso-Mori, R., Biasin, E., Carlstad, J., Chollet, M., Gaynor, J. D., Glownia, J. M., Hong, K., Kroll, T., Lee, J. H., Poulter, B. I., Reinhard, M., Sokaras, D., Zhang, Y., Doumy, G., March, A. M., Southworth, S. H., Mukamel, S., Cordones, A. A., Schoenlein, R. W., Govind, N., Khalil, M. 1800: 378-386

  Abstract

  Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron-ruthenium complex, [(CN)5FeIICNRuIII(NH3)5]-, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an 60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

  View details for DOI 10.1021/acs.jpclett.1c03613

  View details for PubMedID 34985900

• Microfluidic liquid sheets as large-area targets for high repetition XFELs. Frontiers in molecular biosciences Hoffman, D. J., Van Driel, T. B., Kroll, T., Crissman, C. J., Ryland, E. S., Nelson, K. J., Cordones, A. A., Koralek, J. D., DePonte, D. P. 2022; 9: 1048932

  Abstract

  The high intensity of X-ray free electron lasers (XFELs) can damage solution-phase samples on every scale, ranging from the molecular or electronic structure of a sample to the macroscopic structure of a liquid microjet. By using a large surface area liquid sheet microjet as a sample target instead of a standard cylindrical microjet, the incident X-ray spot size can be increased such that the incident intensity falls below the damage threshold. This capability is becoming particularly important for high repetition rate XFELs, where destroying a target with each pulse would require prohibitively large volumes of sample. We present here a study of microfluidic liquid sheet dimensions as a function of liquid flow rate. Sheet lengths, widths and thickness gradients are shown for three styles of nozzles fabricated from isotropically etched glass. In-vacuum operation and sample recirculation using these nozzles is demonstrated. The effects of intense XFEL pulses on the structure of a liquid sheet are also briefly examined.

  View details for DOI 10.3389/fmolb.2022.1048932

  View details for PubMedID 36567947

• Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering. Journal of the American Chemical Society Larsen, C. B., Braun, J. D., Lozada, I. B., Kunnus, K., Biasin, E., Kolodziej, C., Burda, C., Cordones, A. A., Gaffney, K. J., Herbert, D. E. 2021

  Abstract

  The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

  View details for DOI 10.1021/jacs.1c06429

  View details for PubMedID 34851636

• Direct observation of ultrafast hydrogen bond strengthening in liquid water. Nature Yang, J., Dettori, R., Nunes, J. P., List, N. H., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Deponte, D. P., Heinz, T. F., Kozina, M. E., Ledbetter, K., Lin, M., Lindenberg, A. M., Mo, M., Nilsson, A., Shen, X., Wolf, T. J., Donadio, D., Gaffney, K. J., Martinez, T. J., Wang, X. 2021; 596 (7873): 531-535

  Abstract

  Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2-7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04A on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch.

  View details for DOI 10.1038/s41586-021-03793-9

  View details for PubMedID 34433948

• Following Metal-to-Ligand Charge-Transfer Dynamics with Ligand and Spin Specificity Using Femtosecond Resonant Inelastic X-ray Scattering at the Nitrogen K-Edge. The journal of physical chemistry letters Jay, R. M., Eckert, S., Van Kuiken, B. E., Ochmann, M., Hantschmann, M., Cordones, A. A., Cho, H., Hong, K., Ma, R., Lee, J. H., Dakovski, G. L., Turner, J. J., Minitti, M. P., Quevedo, W., Pietzsch, A., Beye, M., Kim, T. K., Schoenlein, R. W., Wernet, P., Fohlisch, A., Huse, N. 2021: 6676-6683

  Abstract

  We demonstrate for the case of photoexcited [Ru(2,2'-bipyridine)3]2+ how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal-ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 ± 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.

  View details for DOI 10.1021/acs.jpclett.1c01401

  View details for PubMedID 34260255

• Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nature chemistry Biasin, E., Fox, Z. W., Andersen, A., Ledbetter, K., Kjar, K. S., Alonso-Mori, R., Carlstad, J. M., Chollet, M., Gaynor, J. D., Glownia, J. M., Hong, K., Kroll, T., Lee, J. H., Liekhus-Schmaltz, C., Reinhard, M., Sokaras, D., Zhang, Y., Doumy, G., March, A. M., Southworth, S. H., Mukamel, S., Gaffney, K. J., Schoenlein, R. W., Govind, N., Cordones, A. A., Khalil, M. 2021

  Abstract

  It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeiiRuiii) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of angstroms, 120-200cm-1 frequency and ~100fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

  View details for DOI 10.1038/s41557-020-00629-3

  View details for PubMedID 33589787

• Excited-State Charge Distribution of a Donor-π-Acceptor Zn Porphyrin Probed by N K-Edge Transient Absorption Spectroscopy. The journal of physical chemistry letters Cordones, A. A., Pemmaraju, C. D., Lee, J. H., Zegkinoglou, I. n., Ragoussi, M. E., Himpsel, F. J., de la Torre, G. n., Schoenlein, R. W. 2021: 1182–88

  Abstract

  Zinc porphyrin solar cell dyes with donor-π-acceptor architectures combine light absorber (π), electron-donor, and electron-acceptor moieties inside a single molecule with atomic precision. The donor-π-acceptor design promotes the separation of charge carriers following optical excitation. Here, we probe the excited-state electronic structure within such molecules by combining time-resolved X-ray absorption spectroscopy at the N K-edge with first-principles time-dependent density functional theory (TD-DFT) calculations. Customized Zn porphyrins with strong-donor triphenylamine groups or weak-donor tri-tert-butylbenzene groups were synthesized. Energetically well-separated N K-edge absorption features simultaneously probe the excited-state electronic structure from the perspectives of the macrocycle and triphenylamine N atoms. New absorption transitions between the macrocycle N atoms and the excited-state HOMO vacancy are observed, and the triphenylamine associated absorption feature blue-shifts, consistent with partial oxidation of the donor groups in the excited state.

  View details for DOI 10.1021/acs.jpclett.0c03725

  View details for PubMedID 33480697

• Structure retrieval in liquid-phase electron scattering. Physical chemistry chemical physics : PCCP Yang, J., Nunes, J. P., Ledbetter, K., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Crissman, C., Deponte, D. P., Glenzer, S. H., Lin, M., Mo, M., Rankine, C. D., Shen, X., Wolf, T. J., Wang, X. 2020

  Abstract

  Electron scattering on liquid samples has been enabled recently by the development of ultrathin liquid sheet technologies. The data treatment of liquid-phase electron scattering has been mostly reliant on methodologies developed for gas electron diffraction, in which theoretical inputs and empirical fittings are often needed to account for the atomic form factor and remove the inelastic scattering background. In this work, we present an alternative data treatment method that is able to retrieve the radial distribution of all the charged particle pairs without the need of either theoretical inputs or empirical fittings. The merits of this new method are illustrated through the retrieval of real-space molecular structure from experimental electron scattering patterns of liquid water, carbon tetrachloride, chloroform, and dichloromethane.

  View details for DOI 10.1039/d0cp06045c

  View details for PubMedID 33367391

• Photophysics of graphene quantum dot assemblies with axially coordinated cobaloxime catalysts. The Journal of chemical physics Singh, V., Gupta, N., Hargenrader, G. N., Askins, E. J., Valentine, A. J., Kumar, G., Mara, M. W., Agarwal, N., Li, X., Chen, L. X., Cordones, A. A., Glusac, K. D. 2020; 153 (12): 124903

  Abstract

  We report a study of chromophore-catalyst assemblies composed of light harvesting hexabenzocoronene (HBC) chromophores axially coordinated to two cobaloxime complexes. The chromophore-catalyst assemblies were prepared using bottom-up synthetic methodology and characterized using solid-state NMR, IR, and x-ray absorption spectroscopy. Detailed steady-state and time-resolved laser spectroscopy was utilized to identify the photophysical properties of the assemblies, coupled with time-dependent DFT calculations to characterize the relevant excited states. The HBC chromophores tend to assemble into aggregates that exhibit high exciton diffusion length (D = 18.5 molecule2/ps), indicating that over 50 chromophores can be sampled within their excited state lifetime. We find that the axial coordination of cobaloximes leads to a significant reduction in the excited state lifetime of the HBC moiety, and this finding was discussed in terms of possible electron and energy transfer pathways. By comparing the experimental quenching rate constant (1.0 * 109 s-1) with the rate constant estimates for Marcus electron transfer (5.7 * 108 s-1) and Forster/Dexter energy transfers (8.1 * 106 s-1 and 1.0 * 1010 s-1), we conclude that both Dexter energy and Marcus electron transfer process are possible deactivation pathways in CoQD-A. No charge transfer or energy transfer intermediate was detected in transient absorption spectroscopy, indicating fast, subpicosecond return to the ground state. These results provide important insights into the factors that control the photophysical properties of photocatalytic chromophore-catalyst assemblies.

  View details for DOI 10.1063/5.0018581

  View details for PubMedID 33003752

• Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers CHEMICAL SCIENCE Kunnus, K., Li, L., Titus, C., Lee, S., Reinhard, M. E., Koroidov, S., Kjaer, K. S., Hong, K., Ledbetter, K., Doriese, W. B., O'Neil, G. C., Swetz, D. S., Ullom, J., Li, D., Irwin, K., Nordlund, D., Cordones, A., Gaffney, K. J. 2020; 11 (17): 4360–73

  View details for DOI 10.1039/c9sc06272f

  View details for Web of Science ID 000532365500005

• Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers. Chemical science Kunnus, K., Li, L., Titus, C. J., Lee, S. J., Reinhard, M. E., Koroidov, S., Kjær, K. S., Hong, K., Ledbetter, K., Doriese, W. B., O'Neil, G. C., Swetz, D. S., Ullom, J. N., Li, D., Irwin, K., Nordlund, D., Cordones, A. A., Gaffney, K. J. 2020; 11 (17): 4360-4373

  Abstract

  Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)4(2,2'-bipyridine)]2- (1), [Fe(CN)4(2,3-bis(2-pyridyl)pyrazine)]2- (2) and [Fe(CN)4(2,2'-bipyrimidine)]2- (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the 1MLCT vertical energy to be varied from 1.64 eV to 2.64 eV and the 3MLCT lifetime to range from 180 fs to 67 ps. The 3MLCT lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred (3MC) excited state, as established by the Tanabe-Sugano analysis of the Fe 2p3d RIXS data. In contrast, the 3MLCT lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing 3MLCT → ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The 3MLCT → GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (hν = 1530 cm-1), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the 3MLCT and the 3MC states and suppression of the intramolecular distortion of the acceptor ligand in the 3MLCT excited state.

  View details for DOI 10.1039/c9sc06272f

  View details for PubMedID 34122894

  View details for PubMedCentralID PMC8159445

• Liquid-phase mega-electron-volt ultrafast electron diffraction STRUCTURAL DYNAMICS-US Nunes, J. F., Ledbetter, K., Lin, M., Kozina, M., DePonte, D. P., Biasin, E., Centurion, M., Crissman, C. J., Dunning, M., Guillet, S., Jobe, K., Liu, Y., Mo, M., Shen, X., Sublett, R., Weathersby, S., Yoneda, C., Wolf, T. A., Yang, J., Cordones, A. A., Wang, X. J. 2020; 7 (2): 024301

  Abstract

  The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which occur in solution. To understand the structure-function relationships mediating these processes, a technique with high spatial and temporal resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water and its network were resolved up to the 3 rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.

  View details for DOI 10.1063/1.5144518

  View details for Web of Science ID 000531214100001

  View details for PubMedID 32161776

  View details for PubMedCentralID PMC7062553

• Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy. The Journal of chemical physics Ledbetter, K. n., Reinhard, M. E., Kunnus, K. n., Gallo, A. n., Britz, A. n., Biasin, E. n., Glownia, J. M., Nelson, S. n., Van Driel, T. B., Weninger, C. n., Zederkof, D. B., Haldrup, K. n., Cordones, A. A., Gaffney, K. J., Sokaras, D. n., Alonso-Mori, R. n. 2020; 152 (7): 074203

  Abstract

  Valence-to-core x-ray emission spectroscopy (VtC XES) combines the sample flexibility and element specificity of hard x-rays with the chemical environment sensitivity of valence spectroscopy. We extend this technique to study geometric and electronic structural changes induced by photoexcitation in the femtosecond time domain via laser-pump, x-ray probe experiments using an x-ray free electron laser. The results of time-resolved VtC XES on a series of ferrous complexes [Fe(CN)2n(2, 2'-bipyridine)3-n]-2n+2, n = 1, 2, 3, are presented. Comparisons of spectra obtained from ground state density functional theory calculations reveal signatures of excited state bond length and oxidation state changes. An oxidation state change associated with a metal-to-ligand charge transfer state with a lifetime of less than 100 fs is observed, as well as bond length changes associated with metal-centered excited states with lifetimes of 13 ps and 250 ps.

  View details for DOI 10.1063/1.5139441

  View details for PubMedID 32087640

• Author Correction: Generation and characterization of ultrathin free-flowing liquid sheets. Nature communications Koralek, J. D., Kim, J. B., Bruza, P., Curry, C. B., Chen, Z., Bechtel, H. A., Cordones, A. A., Sperling, P., Toleikis, S., Kern, J. F., Moeller, S. P., Glenzer, S. H., DePonte, D. P. 2019; 10 (1): 1615

  Abstract

  The original version of this Article contained an error in Eq. (1). This has been corrected in both the PDF and HTML versions of the Article.

  View details for DOI 10.1038/s41467-019-09457-7

  View details for PubMedID 30944301

• Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy. Angewandte Chemie (International ed. in English) Tatsuno, H. n., Kjaer, K. S., Kunnus, K. n., Harlang, T. C., Timm, C. n., Guo, M. n., Chàbera, P. n., Fredin, L. A., Hartsock, R. W., Reinhard, M. E., Koroidov, S. n., Li, L. n., Cordones, A. A., Gordivska, O. n., Prakash, O. n., Liu, Y. n., Laursen, M. G., Biasin, E. n., Hansen, F. B., Vester, P. n., Christensen, M. n., Haldrup, K. n., Németh, Z. n., Sárosiné Szemes, D. n., Bajnóczi, É. n., Vankó, G. n., Van Driel, T. B., Alonso-Mori, R. n., Glownia, J. M., Nelson, S. n., Sikorski, M. n., Lemke, H. T., Sokaras, D. n., Canton, S. E., Dohn, A. O., Møller, K. B., Nielsen, M. M., Gaffney, K. J., Wärnmark, K. n., Sundström, V. n., Persson, P. n., Uhlig, J. n. 2019

  Abstract

  Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeII NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3 MLCT state, from the initially excited 1 MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3 MC state, in competition with vibrational relaxation and cooling to the relaxed 3 MLCT state. The relaxed 3 MLCT state then decays much more slowly (7.6 ps) to the 3 MC state. The 3 MC state is rapidly (2.2 ps) deactivated to the ground state. The 5 MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3 MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.

  View details for DOI 10.1002/anie.201908065

  View details for PubMedID 31602726

• Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption INORGANIC CHEMISTRY Koroidov, S., Hong, K., Kjaer, K. S., Li, L., Kunnus, K., Reinhard, M., Hartsock, R. W., Amit, D., Eisenberg, R., Das Pemmaraju, C., Gaffney, K. J., Cordones, A. A. 2018; 57 (21): 13167–75

  View details for DOI 10.1021/acs.inorgchem.8b01497

  View details for Web of Science ID 000449576900016

• Author Correction: Generation and characterization of ultrathin free-flowing liquid sheets. Nature communications Koralek, J. D., Kim, J. B., Bruza, P., Curry, C. B., Chen, Z., Bechtel, H. A., Cordones, A. A., Sperling, P., Toleikis, S., Kern, J. F., Moeller, S. P., Glenzer, S. H., DePonte, D. P. 2018; 9 (1): 2860

  Abstract

  The original version of this article omitted the following from the Acknowledgements:'P.B. was funded by the ELI Extreme Light Infrastructure Phase 2 (CZ.02.1.01/0.0/0.0/15008/0000162) from the European Regional Development Fund and the EUCALL project funded from the EU Horizon 2020 research and innovation programme under grant agreement No 654220,' which replaces the previous 'P.B. was funded by the ELI Extreme Light Infrastructure Phase 2 (CZ.02.1.01/0.0/0.0/15008/0000162) from the European Regional Development Fund.'This has been corrected in both the PDF and HTML versions of the article.

  View details for DOI 10.1038/s41467-018-05365-4

  View details for PubMedID 30018291

• UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ochmann, M., Hussain, A., von Ahnen, I., Cordones, A. A., Hong, K., Lee, J., Ma, R., Adamczyk, K., Kim, T., Schoenlein, R. W., Vendrell, O., Huse, N. 2018; 140 (21): 6554–61

  Abstract

  We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH2S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.

  View details for DOI 10.1021/jacs.7b13455

  View details for Web of Science ID 000434101100010

  View details for PubMedID 29771112

• Transient metal-centered states mediate isomerization of a photochromic rutheniumsulfoxide complex NATURE COMMUNICATIONS Cordones, A. A., Lee, J., Hong, K., Cho, H., Garg, K., Boggio-Pasqua, M., Rack, J. J., Huse, N., Schoenlein, R. W., Kim, T. 2018; 9: 1989

  Abstract

  Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.

  View details for PubMedID 29777157

• Comprehensive Experimental and Computational Spectroscopic Study of Hexacyanoferrate Complexes in Water: From Infrared to X-ray Wavelengths JOURNAL OF PHYSICAL CHEMISTRY B Ross, M., Andersen, A., Fox, Z. W., Zhang, Y., Hong, K., Lee, J., Cordones, A., March, A., Doumy, G., Southworth, S. H., Marcus, M. A., Schoenlein, R. W., Mukamel, S., Govind, N., Khalil, M. 2018; 122 (19): 5075–86

  Abstract

  We present a joint experimental and computational study of the hexacyanoferrate aqueous complexes at equilibrium in the 250 meV to 7.15 keV regime. The experiments and the computations include the vibrational spectroscopy of the cyanide ligands, the valence electronic absorption spectra, and Fe 1s core hole spectra using element-specific-resonant X-ray absorption and emission techniques. Density functional theory-based quantum mechanics/molecular mechanics molecular dynamics simulations are performed to generate explicit solute-solvent configurations, which serve as inputs for the spectroscopy calculations of the experiments spanning the IR to X-ray wavelengths. The spectroscopy simulations are performed at the same level of theory across this large energy window, which allows for a systematic comparison of the effects of explicit solute-solvent interactions in the vibrational, valence electronic, and core-level spectra of hexacyanoferrate complexes in water. Although the spectroscopy of hexacyanoferrate complexes in solution has been the subject of several studies, most of the previous works have focused on a narrow energy window and have not accounted for explicit solute-solvent interactions in their spectroscopy simulations. In this work, we focus our analysis on identifying how the local solvation environment around the hexacyanoferrate complexes influences the intensity and line shape of specific spectroscopic features in the UV/vis, X-ray absorption, and valence-to-core X-ray emission spectra. The identification of these features and their relationship to solute-solvent interactions is important because hexacyanoferrate complexes serve as model systems for understanding the photochemistry and photophysics of a large class of Fe(II) and Fe(III) complexes in solution.

  View details for DOI 10.1021/acs.jpcb.7b12532

  View details for Web of Science ID 000432754100013

  View details for PubMedID 29613798

• Generation and characterization of ultrathin free-flowing liquid sheets NATURE COMMUNICATIONS Koralek, J. D., Kim, J. B., Bruza, P., Curry, C. B., Chen, Z., Bechtel, H. A., Cordones, A. A., Sperling, P., Toleikis, S., Kern, J. F., Moeller, S. P., Glenzer, S. H., DePonte, D. P. 2018; 9: 1353

  Abstract

  The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas-dynamic forces to generate free-flowing, sub-micron, liquid sheets which are two orders of magnitude thinner than anything previously reported. Optical, infrared, and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 μm  down to less than 20 nm, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources.

  View details for DOI 10.1038/s41467-018-03696-w

  View details for Web of Science ID 000429521200005

  View details for PubMedID 29636445

  View details for PubMedCentralID PMC5893585

• Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2 '-bipyridine)(CN)(4)](2-) PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kjaer, K. S., Kunnus, K., Harlang, T. B., Van Driel, T. B., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Biasin, E., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Uhlig, J., Sundstom, V., Nemeth, Z., Szemes, D., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Warnmark, K., Persson, P., Cordones, A. A., Gaffney, K. J. 2018; 20 (6): 4238–49

  Abstract

  The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.

  View details for PubMedID 29364300

• Picosecond sulfur K-edge X-ray absorption spectroscopy with applications to excited state proton transfer STRUCTURAL DYNAMICS Van Kuiken, B. E., Ross, M. R., Strader, M. L., Cordones, A. A., Cho, H., Lee, J., Schoenlein, R. W., Khalil, M. 2017; 4 (4): 044021

  Abstract

  Picosecond X-ray absorption (XA) spectroscopy at the S K-edge (∼2.4 keV) is demonstrated and used to monitor excited state dynamics in a small organosulfur molecule (2-Thiopyridone, 2TP) following optical excitation. Multiple studies have reported that the thione (2TP) is converted into the thiol (2-Mercaptopyridine, 2MP) following photoexcitation. However, the timescale and photochemical pathway of this reaction remain uncertain. In this work, time-resolved XA spectroscopy at the S K-edge is used to monitor the formation and decay of two transient species following 400 nm excitation of 2TP dissolved in acetonitrile. The first transient species forms within the instrument response time (70 ps) and decays within 6 ns. The second transient species forms on a timescale of ∼400 ps and decays on a 15 ns timescale. Time-dependent density functional theory is used to identify the first and second transient species as the lowest-lying triplet states of 2TP and 2MP, respectively. This study demonstrates transient S K-edge XA spectroscopy as a sensitive and viable probe of time-evolving charge dynamics near sulfur sites in small molecules with future applications towards studying complex biological and material systems.

  View details for DOI 10.1063/1.4983157

  View details for Web of Science ID 000402004800022

  View details for PubMedID 28529962

  View details for PubMedCentralID PMC5422206

• Light-Induced Radical Formation and Isomerization of an Aromatic Thiol in Solution Followed by Time-Resolved X-ray Absorption Spectroscopy at the Sulfur K-Edge JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ochmann, M., von Ahnen, I., Cordones, A. A., Hussain, A., Lee, J., Hong, K., Adamczyk, K., Vendrell, O., Kim, T., Schoenlein, R. W., Huse, N. 2017; 139 (13): 4797–4804

  Abstract

  We applied time-resolved sulfur-1s absorption spectroscopy to a model aromatic thiol system as a promising method for tracking chemical reactions in solution. Sulfur-1s absorption spectroscopy allows tracking multiple sulfur species with a time resolution of ∼70 ps at synchrotron radiation facilities. Experimental transient spectra combined with high-level electronic structure theory allow identification of a radical and two thione isomers, which are generated upon illumination with 267 nm radiation. Moreover, the regioselectivity of the thione isomerization is explained by the resulting radical frontier orbitals. This work demonstrates the usefulness and potential of time-resolved sulfur-1s absorption spectroscopy for tracking multiple chemical reaction pathways and transient products of sulfur-containing molecules in solution.

  View details for DOI 10.1021/jacs.6b12992

  View details for Web of Science ID 000398764000032

  View details for PubMedID 28219243

• Soft X-ray spectroscopy studies of adsorption and reaction of CO in the presence of H-2 over 6 nm MnO nanoparticles supported on mesoporous Co3O4 SURFACE SCIENCE Ralston, W. T., Musselwhite, N., Kennedy, G., An, K., Horowitz, Y., Cordones, A. A., Rude, B., Ahmed, M., Melaet, G., Alayoglu, S. 2016; 648: 14–22

  View details for DOI 10.1016/j.susc.2015.12.006

  View details for Web of Science ID 000374198600004

• Electronic and nuclear contributions to time-resolved optical and X-ray absorption spectra of hematite and insights into photoelectrochemical performance Energy & Environmental Science Hayes, D., Hadt, R. G., Emery, J. D., Cordones, A. A., Martinson, A. F., Shelby, M. L., Fransted, K. A., Dahlberg, P. D., Hong, J., Zhang, X., Kong, Q., Schoenlein, R. W., Chen, L. X. 2016; 9 (12): 3754–69

  View details for DOI 10.1039/c6ee02266a

  View details for Web of Science ID 000392915500016

• X-rays only when you want them: optimized pump-probe experiments using pseudo-single-bunch operation JOURNAL OF SYNCHROTRON RADIATION Hertlein, M. P., Scholl, A., Cordones, A. A., Lee, J. H., Engelhorn, K., Glover, T. E., Barbrel, B., Sun, C., Steier, C., Portmann, G., Robin, D. S. 2015; 22: 729–35

  Abstract

  Laser pump-X-ray probe experiments require control over the X-ray pulse pattern and timing. Here, the first use of pseudo-single-bunch mode at the Advanced Light Source in picosecond time-resolved X-ray absorption experiments on solutions and solids is reported. In this mode the X-ray repetition rate is fully adjustable from single shot to 500 kHz, allowing it to be matched to typical laser excitation pulse rates. Suppressing undesired X-ray pulses considerably reduces detector noise and improves signal to noise in time-resolved experiments. In addition, dose-induced sample damage is considerably reduced, easing experimental setup and allowing the investigation of less robust samples. Single-shot X-ray exposures of a streak camera detector using a conventional non-gated charge-coupled device (CCD) camera are also demonstrated.

  View details for DOI 10.1107/S1600577515001770

  View details for Web of Science ID 000353920300037

  View details for PubMedID 25931090

  View details for PubMedCentralID PMC4416684

• Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy FARADAY DISCUSSIONS Karslioglu, O., Nemsak, S., Zegkinoglou, I., Shavorskiy, A., Hartl, M., Salmassi, F., Gullikson, E. M., Ng, M. L., Rameshan, C., Rude, B., Bianculli, D., Cordones, A. A., Axnanda, S., Crumlin, E. J., Ross, P. N., Schneider, C. M., Hussain, Z., Liu, Z., Fadley, C. S., Bluhm, H. 2015; 180: 35–53

  Abstract

  We describe a new in operando approach for the investigation of heterogeneous processes at solid/liquid interfaces with elemental and chemical specificity which combines the preparation of thin liquid films using the meniscus method with standing wave ambient pressure X-ray photoelectron spectroscopy [Nemšák et al., Nat. Commun., 5, 5441 (2014)]. This technique provides information about the chemical composition across liquid/solid interfaces with sub-nanometer depth resolution and under realistic conditions of solution composition and concentration, pH, as well as electrical bias. In this article, we discuss the basics of the technique and present the first results of measurements on KOH/Ni interfaces.

  View details for DOI 10.1039/c5fd00003c

  View details for Web of Science ID 000358249600002

  View details for PubMedID 25912462

• Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface JOURNAL OF PHYSICAL CHEMISTRY LETTERS Siefermann, K. R., Pemmaraju, C. D., Neppl, S., Shavorskiy, A., Cordones, A. A., Vura-Weis, J., Slaughter, D. S., Sturm, F. P., Weise, F., Bluhm, H., Strader, M. L., Cho, H., Lin, M., Bacellar, C., Khurmi, C., Guo, J., Coslovich, G., Robinson, J. S., Kaindl, R. A., Schoenlein, R. W., Belkacem, A., Neumark, D. M., Leone, S. R., Nordlund, D., Ogasawara, H., Krupin, O., Turner, J. J., Schlotter, W. F., Holmes, M. R., Messerschmidt, M., Minitti, M. P., Gul, S., Zhang, J. Z., Huse, N., Prendergast, D., Gessner, O. 2014; 5 (15): 2753-2759

  Abstract

  Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

  View details for DOI 10.1021/jz501264x

  View details for Web of Science ID 000340222200044

  View details for PubMedID 26277975

• Linking On-State Memory and Distributed Kinetics in Single Nanocrystal Blinking JOURNAL OF PHYSICAL CHEMISTRY B Cordones, A. A., Knappenberger, K. L., Leone, S. R. 2013; 117 (16): 4241–48

  Abstract

  Memory effects in single nanocrystal fluorescence blinking are investigated as a function of the on-state kinetics for CdSe/ZnS quantum dots and CdSe nanorods. The on-state duration probability distributions for single nanocrystal blinking traces are characterized by an inverse power law, which crosses over to exponential decay for long on-state durations. The correlations of subsequent on-state durations (Rlog,on) are found to decrease for nanocrystals that display earlier crossover times and smaller power law coefficients. Specifically, Rlog,on increases from 0.14 ± 0.02 to a saturation value of 0.44 ± 0.01 for nanocrystals with average crossover times of ~100 ms to more than 5.0 s, respectively. The results represent the first link between memory effects and blinking kinetics and are interpreted in the framework of two competing charge trapping mechanisms. A slow fluctuation-based trapping mechanism leads to power-law-distributed on durations and significant memory effects; however, the additional contribution of an ionization-induced trapping pathway is found to induce crossover to exponential decay and decreased memory. Monte Carlo simulations of nanocrystal blinking based on the two trapping mechanisms reproduce the experimental results, suggesting that the power law component and the memory effects correlate with a fluctuation-based mechanism. This effect is found to be universal, occurring for two nanocrystal morphologies and in blinking data measured using a wide range of continuous and pulsed excitation conditions.

  View details for DOI 10.1021/jp3041549

  View details for Web of Science ID 000318211600010

  View details for PubMedID 22967127

• Effect of Thermal Annealing in Ammonia on the Properties of InGaN Nanowires with Different Indium Concentrations JOURNAL OF PHYSICAL CHEMISTRY C Hahn, C., Cordones, A. A., Andrews, S. C., Gao, H., Fu, A., Leone, S. R., Yang, P. 2013; 117 (7): 3627-3634

  View details for DOI 10.1021/jp311685x

  View details for Web of Science ID 000315432100050

• Time-Resolved X-Ray Photoelectron Spectroscopy Techniques For Real-Time Studies Of Interfacial Charge Transfer Dynamics 22nd International Conference on the Application of Accelerators in Research and Industry (CAARI) Shavorskiy, A., Cordones, A., Vura-Weis, J., Siefermann, K., Slaughter, D., Sturm, F., Weise, F., Bluhm, H., Strader, M., Cho, H., Lin, M., Bacellar, C., Khurmi, C., Hertlein, M., Guo, J., Tyliszczak, T., Prendergast, D., Coslovich, G., Robinson, J., Kaindl, R. A., Schoenlein, R. W., Belkacem, A., Weber, T., Neumark, D. M., Leone, S. R., Nordlund, D., Ogasawara, H., Nilsson, A. R., Krupin, O., Turner, J. J., Schlotter, W. F., Holmes, M. R., Heimann, P. A., Messerschmidt, M., Minitti, M. P., Beye, M., Gul, S., Zhang, J. Z., Huse, N., Gessner, O. AMER INST PHYSICS. 2013: 475–479

  View details for DOI 10.1063/1.4802374

  View details for Web of Science ID 000319997900095

• Mechanisms for charge trapping in single semiconductor nanocrystals probed by fluorescence blinking CHEMICAL SOCIETY REVIEWS Cordones, A. A., Leone, S. R. 2013; 42 (8): 3209–21

  Abstract

  Semiconductor nanocrystal optical and charge transport properties are largely influenced by the trapping of charge carriers on the nanocrystal surface. Charge trapping increases the non-radiative exciton decay pathways, thus decreasing the fluorescence quantum yield, and it also impedes efficient charge transfer at the nanocrystal interface. On a single nanocrystal basis, charge trapping causes interruptions in the otherwise continuous fluorescence known as fluorescence intermittency or blinking. In this tutorial review we examine the relationship between charge trapping and fluorescence blinking. The fluorescence microscopy techniques and statistical analysis methods used to measure single nanocrystal blinking are introduced. The development of numerous blinking mechanisms is reviewed, as is the physical nature of charge trapping sites. An overview of blinking experiments used to probe specific mechanisms for charge carrier trapping is presented. Finally, a summary and outlook are offered. Although the detailed mechanism is not fully understood, blinking experiments are found to provide direct evidence for several charge trapping mechanisms and report on changes to the nature and distribution of charge trapping sites.

  View details for DOI 10.1039/c2cs35452g

  View details for Web of Science ID 000316869700003

  View details for PubMedID 23306775

• Probing the Interaction of Single Nanocrystals with Inorganic Capping Ligands: Time-Resolved Fluorescence from CdSe-CdS Quantum Dots Capped with Chalcogenidometalates JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Cordones, A. A., Scheele, M., Alivisatos, A., Leone, S. R. 2012; 134 (44): 18366–73

  Abstract

  Fluorescence intermittency and excited-state decay measurements are carried out on single CdSe-CdS core-shell quantum dots (QD) stabilized with chalcogenidometalates (ChaMs, In(2)Se(4)(2-), or Sn(2)S(6)(4-))(-). The results are used to probe the nature and distribution of charge trapping sites in the QD local environment. A comparison is made between capping by a neutral organic ligand (oleylamine) and a small inorganic ligand with high charge density (ChaMs). Overall, shorter on-state durations and longer off-state durations are observed for the ChaMs. These results indicate an increased density of charge trapping sites and increased stabilization of surface-trapped charges. By varying the thickness of the CdS shell, we identified hole trapping by the ligand as the dominant charging mechanism in ChaM-capped QDs. Faster excited-state decay rates are measured for the ChaM-capped QDs, highlighting the role of strongly stabilized trapped charges in this system. Using cyclic voltammetry measurements of the ChaMs, an energy level diagram is constructed relating the ChaMs and CdSe-CdS-QDs that explains their superior performance as active layers in photodetectors.

  View details for DOI 10.1021/ja3071732

  View details for Web of Science ID 000310720900039

  View details for PubMedID 23072613

• CdSe/ZnS quantum dot intermittency in N,N '-diphenyl-N,N '-bis(3-methylphenyl)-(1,1 '-biphenyl)-4,4 '-diamine (TPD) CHEMICAL PHYSICS LETTERS Bixby, T. J., Cordones, A. A., Leone, S. R. 2012; 521: 7–11

  View details for DOI 10.1016/j.cplett.2011.11.051

  View details for Web of Science ID 000298967100002

• Direct Measurement of Off-State Trapping Rate Fluctuations in Single Quantum Dot Fluorescence NANO LETTERS Cordones, A. A., Bixby, T. J., Leone, S. R. 2011; 11 (8): 3366–69

  Abstract

  Fluorescence decay times measured during the off-state of single CdSe/ZnS quantum dot blinking are found to decrease with increasing off-state duration, contradicting the charging model widely considered to explain the blinking phenomenon. The change in the nonradiative process of a short off-state duration compared to a long one is investigated here through simultaneous measurement of fluorescence decay and blinking behavior. The results are investigated in the framework of two models based on fluctuating trapping rates.

  View details for DOI 10.1021/nl2017674

  View details for Web of Science ID 000293665600053

  View details for PubMedID 21732607

• Evidence for Multiple Trapping Mechanisms in Single CdSe/ZnS Quantum Dots from Fluorescence Intermittency Measurements over a Wide Range of Excitation Intensities JOURNAL OF PHYSICAL CHEMISTRY C Cordones, A. A., Bixby, T. J., Leone, S. R. 2011; 115 (14): 6341–49

  View details for DOI 10.1021/jp2001223

  View details for Web of Science ID 000289215400023