Stanford Advisors

All Publications

  • Integrated Quantum-Classical Protocol for the Realistic Description of Solvated Multinuclear Mixed-Valence Transition-Metal Complexes and Their Solvatochromic Properties. Journal of chemical theory and computation Prampolini, G., Andersen, A., Poulter, B. I., Khalil, M., Govind, N., Biasin, E., Pastore, M. 2023


    Linear cyanide-bridged polymetallic complexes, which undergo photoinduced metal-to-metal charge transfer, represent prototypical systems for studying long-range electron-transfer reactions and understanding the role played by specific solute-solvent interactions in modulating the excited-state dynamics. To tackle this problem, while achieving a statistically meaningful description of the solvent and of its relaxation, one needs a computational approach capable of handling large polynuclear transition-metal complexes, both in their ground and excited states, as well as the ability to follow their dynamics in several environments up to nanosecond time scales. Here, we present a mixed quantum classical approach, which combines large-scale molecular dynamics (MD) simulations based on an accurate quantum mechanically derived force field (QMD-FF) and self-consistent QMD polarized point charges, with IR and UV-vis spectral calculations to model the solvation dynamics and optical properties of a cyano-bridged trinuclear mixed-valence compound (trans-[(NC)5FeIII(μ-CN)RuII(pyridine)4(μ-NC)FeIII(CN)5]4-). We demonstrate the reliability of the QMD-FF/MD approach in sampling the solute conformational space and capturing the local solute-solvent interactions by comparing the results with higher-level quantum mechanics/molecular mechanics (QM/MM) MD reference data. The IR spectra calculated along the classical MD trajectories in different solvents correctly predict the red shift of the CN stretching band in the aprotic medium (acetonitrile) and the subtle differences measured in water and methanol, respectively. By explicitly including the solvent molecules around the cyanide ligands and calculating the thermal averaged absorption spectra using time-dependent density functional theory calculations within the Tamm-Dancoff approximation, the experimental solvatochromic shift is quantitatively reproduced going from water to methanol, while it is overestimated for acetonitrile. This discrepancy can likely be traced back to the lack of important dispersion interactions between the solvent cyano groups and the pyridine substituents in our micro solvation model. The proposed protocol is applied to the ground state in water, methanol, and acetonitrile and can be flexibly generalized to study excited-state nonequilibrium solvation dynamics.

    View details for DOI 10.1021/acs.jctc.3c01084

    View details for PubMedID 38086742

  • Uncovering the 3d and 4d Electronic Interactions in Solvated Ru Complexes with 2p3d Resonant Inelastic X-ray Scattering. Inorganic chemistry Poulter, B. I., Biasin, E., Nowak, S. H., Kroll, T., Alonso-Mori, R., Schoenlein, R. W., Govind, N., Sokaras, D., Khalil, M. 2023


    The electronic structure and dynamics of ruthenium complexes are widely studied given their use in catalytic and light-harvesting materials. Here we investigate three model Ru complexes, [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4-, with L3-edge 2p3d resonant inelastic X-ray scattering (RIXS) to probe unoccupied 4d valence orbitals and occupied 3d orbitals and to gain insight into the interactions between these levels. The 2p3d RIXS maps contain a higher level of spectral information than the L3 X-ray absorption near edge structure (XANES). This study provides a direct measure of the 3d spin-orbit splittings of 4.3, 4.0, and 4.1 eV between the 3d5/2 and 3d3/2 orbitals of the [RuIII(NH3)6]3+, [RuII(bpy)3]2+, and [RuII(CN)6]4- complexes, respectively.

    View details for DOI 10.1021/acs.inorgchem.3c00919

    View details for PubMedID 37314410

  • Revealing core-valence interactions in solution with femtosecond X-ray pump X-ray probe spectroscopy. Nature communications Weakly, R. B., Liekhus-Schmaltz, C. E., Poulter, B. I., Biasin, E., Alonso-Mori, R., Aquila, A., Boutet, S., Fuller, F. D., Ho, P. J., Kroll, T., Loe, C. M., Lutman, A., Zhu, D., Bergmann, U., Schoenlein, R. W., Govind, N., Khalil, M. 2023; 14 (1): 3384


    Femtosecond pump-probe spectroscopy using ultrafast optical and infrared pulses has become an essential tool to discover and understand complex electronic and structural dynamics in solvated molecular, biological, and material systems. Here we report the experimental realization of anultrafast two-color X-ray pump X-ray probe transient absorption experiment performed in solution. A 10fs X-ray pump pulse creates a localized excitation by removing a 1s electron from an Fe atom in solvated ferro- and ferricyanide complexes. Following the ensuing Auger-Meitner cascade, the second X-ray pulse probes the Fe 1s3p transitions in resultant novel core-excited electronic states. Careful comparison of the experimental spectra with theory, extracts +2eV shifts in transition energies per valence hole, providing insight into correlated interactions of valence 3d with 3p and deeper-lying electrons. Such information is essential for accurate modeling and predictive synthesis of transition metal complexes relevant for applications ranging from catalysis to information storage technology. This study demonstrates the experimental realization of the scientific opportunities possible with the continued development of multicolor multi-pulse X-ray spectroscopy to study electronic correlations in complex condensed phase systems.

    View details for DOI 10.1038/s41467-023-39165-2

    View details for PubMedID 37291130

  • 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


    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

  • Resonant Inelastic X-ray Scattering Calculations of Transition Metal Complexes Within a Simplified Time-Dependent Density Functional Theory Framework. Journal of chemical theory and computation Nascimento, D. R., Biasin, E., Poulter, B. I., Khalil, M., Sokaras, D., Govind, N. 2021


    We present a time-dependent density functional theory (TDDFT) approach to compute the light-matter couplings between two different manifolds of excited states relative to a common ground state in the context of 4d transition metal systems. These quantities are the necessary ingredients to solve the Kramers-Heisenberg (KH) equation for resonant inelastic X-ray scattering (RIXS) and several other types of two-photon spectroscopies. The procedure is based on the pseudo-wavefunction approach, where the solutions of a TDDFT calculation can be used to construct excited-state wavefunctions, and on the restricted energy window approach, where a manifold of excited states can be rigorously defined based on the energies of the occupied molecular orbitals involved in the excitation process. Thus, the present approach bypasses the need to solve the costly TDDFT quadratic-response equations. We illustrate the applicability of the method to 4d transition metal molecular complexes by calculating the 2p4d RIXS maps of three representative ruthenium complexes and comparing them to experimental results. The method can capture all the experimental features in all three complexes to allow the assignment of the experimental peaks, with relative energies correct to within 0.6 eV at the cost of two independent TDDFT calculations.

    View details for DOI 10.1021/acs.jctc.1c00144

    View details for PubMedID 33909424

  • Revealing the bonding of solvated Ru complexes with valence-to-core resonant inelastic X-ray scattering. Chemical science Biasin, E., Nascimento, D. R., Poulter, B. I., Abraham, B., Kunnus, K., Garcia-Esparza, A. T., Nowak, S. H., Kroll, T., Schoenlein, R. W., Alonso-Mori, R., Khalil, M., Govind, N., Sokaras, D. 2021; 12 (10): 3713-3725


    Ru-complexes are widely studied because of their use in biological applications and photoconversion technologies. We reveal novel insights into the chemical bonding of a series of Ru(ii)- and Ru(iii)-complexes by leveraging recent advances in high-energy-resolution tender X-ray spectroscopy and theoretical calculations. We perform Ru 2p4d resonant inelastic X-ray scattering (RIXS) to probe the valence excitations in dilute solvated Ru-complexes. Combining these experiments with a newly developed theoretical approach based on time-dependent density functional theory, we assign the spectral features and quantify the metal-ligand bonding interactions. The valence-to-core RIXS features uniquely identify the metal-centered and charge transfer states and allow extracting the ligand-field splitting for all the complexes. The combined experimental and theoretical approach described here is shown to reliably characterize the ground and excited valence states of Ru complexes, and serve as a basis for future investigations of ruthenium, or other 4d metals active sites, in biological and chemical applications.

    View details for DOI 10.1039/d0sc06227h

    View details for PubMedID 34163645

    View details for PubMedCentralID PMC8179428

  • A Highly Sensitive Nonenzymatic Glucose Biosensor Based on the Regulatory Effect of Glucose on Electrochemical Behaviors of Colloidal Silver Nanoparticles on MoS₂†. Sensors (Basel, Switzerland) Anderson, K., Poulter, B., Dudgeon, J., Li, S. E., Ma, X. 2017; 17 (8)


    A novel and highly sensitive nonenzymatic glucose biosensor was developed by nucleating colloidal silver nanoparticles (AgNPs) on MoS₂. The facile fabrication method, high reproducibility (97.5%) and stability indicates a promising capability for large-scale manufacturing. Additionally, the excellent sensitivity (9044.6 μA mM-1 cm-2), low detection limit (0.03 μM), appropriate linear range of 0.1-1000 μM, and high selectivity suggests that this biosensor has a great potential to be applied for noninvasive glucose detection in human body fluids, such as sweat and saliva.

    View details for DOI 10.3390/s17081807

    View details for PubMedID 28783068

    View details for PubMedCentralID PMC5579834