Letter pubs.acs.org/JPCL
Time-Resolved X‑ray Spectroscopy in the Water Window: Elucidating Transient Valence Charge Distributions in an Aqueous Fe(II) Complex Benjamin E. Van Kuiken,†,⊥,⧫ Hana Cho,‡,§,#,⧫ Kiryong Hong,§ Munira Khalil,† Robert W. Schoenlein,*,‡,⊗ Tae Kyu Kim,*,§ and Nils Huse*,∥ †
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States Ultrafast X-ray Science Laboratory, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, Busan 46241, Republic of Korea ∥ Department of Physics, University of Hamburg, Max Planck Institute for the Structure and Dynamics of Matter, and Center for Free-Electron Laser Science, 22761 Hamburg, Germany ‡
S Supporting Information *
ABSTRACT: Time-resolved nitrogen-1s spectroscopy in the X-ray water window is presented as a novel probe of metal−ligand interactions and transient states in nitrogencontaining organic compounds. New information on iron(II) polypyridyl complexes via nitrogen core-level transitions yields insight into the charge density of the photoinduced high-spin state by comparing experimental results with time-dependent density functional theory. In the transient high-spin state, the 3d electrons of the metal center are more delocalized over the nearest-neighbor nitrogen atoms despite increased bond lengths. Our findings point to a strong coupling of electronic states with charge-transfer character, facilitating the ultrafast intersystem crossing cascade in these systems. The study also highlights the importance of local charge density measures to complement chemical interaction concepts of charge donation and back-bonding with molecular orbital descriptions of states.
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containing systems would therefore provide new information on reaction pathways in organic and inorganic chemistry. We have exploited nitrogen 1s → 2p core-level transitions as a probe of transient valence charge distributions in an aqueous solute to demonstrate the feasibility and usefulness of timeresolved nitrogen 1s spectroscopy in solution. We choose aqueous Fe(II) tris(2,2′-bipyridine), hereafter termed [Fe(bpy)3]2+, as a prototypical Fe(II) polypyridyl complex exhibiting a rapid spin transition to a transient highspin (HS) state via an optically active metal-to-ligand chargetransfer (MLCT) excitation. The spin transition is very puzzling because it is complete with a few hundred femtoseconds while involving the excitation of at least two electrons in rapid succession. The formation of the metastable quintet state holds important clues about the spin-crossover (SCO) mechanism and the response of the solvent environment. The molecular structure and spin evolution have been characterized by time-resolved X-ray spectroscopy of the metal center, but a comprehensive understanding of metal−ligand interactions in the ground and excited states of [Fe(bpy)3]2+ requires probing
he ability to follow chemical reactions in solution with atomic resolution and element specificity promises new insight into mechanisms of chemical transformation. Timeresolved X-ray probes by means of scattering and spectroscopy constitute an important step in this endeavor by providing more direct information than optical spectroscopy on local structure, valence charge distributions, and spin states in metalcontaining systems.1−7 The ability to probe the valence charge density of chemical elements and even distinguish individual atoms in different chemical environments within an ensemble of the same atomic species is unique to core-level spectroscopy using X-ray photons. Time-resolved X-ray spectroscopy allows for the interrogation of materials in transient states, thereby facilitating the understanding of reaction pathways.8−10 Of particular interest and challenge is the time-resolved X-ray spectroscopy in the solution phase when the solute contains or consists entirely of light elements such as nitrogen and oxygen. While these elements provide essential function to transitionmetal complexes and (bio)organic compounds, the rich information content that transient core-level spectroscopy provides over the course of a chemical reaction has not been exploited. Among the light elements, nitrogen atoms often constitute important heteroatoms in organic compounds and nearest neighbors in transition-metal complexes. Characterizing the evolution of site-specific valence charge density in nitrogen© 2016 American Chemical Society
Received: November 9, 2015 Accepted: January 4, 2016 Published: January 4, 2016 465
DOI: 10.1021/acs.jpclett.5b02509 J. Phys. Chem. Lett. 2016, 7, 465−470
Letter
The Journal of Physical Chemistry Letters
of excited states has not been explicitly discussed nor probed in detail except for one theoretical study.29 In the following, we present the first transient N-1s spectroscopy of solvated molecules. We exploit the sensitivity of the nitrogen 1s → 2p transition to local valence charge density changes to characterize the quintet state of aqueous [Fe(bpy)3]2+. Solution-phase X-ray absorption spectra of the ground state and transient differential spectra were measured at the ultrafast soft X-ray beamline of the Advanced Light Source. The general layout of the time-resolved experiment and details of the sample cell and the experimental chamber have been described in previous publications.18,21 Details specific to this experiment can be found in the Supporting Information (SI). The experimental ground-state N-1s spectrum in Figure 2A (black dots and line) shows a distinct core-level resonance
changes in ligand electronic structure, which has motivated our particular choice of the molecular system. The structure of aqueous [Fe(bpy)3]2+ is shown in Figure 1A. The central Fe ion is surrounded by six nitrogen atoms in
Figure 1. (A) Structure of the Fe(II) complex. (B) Simplified orbital diagram for an Fe(II) compound of Oh symmetry in LS and HS configuration. (C) Experimental probe scheme using the N-1s → 2p dipole-allowed core-level transition.
quasi-octahedral (Oh) symmetry. Upon optical charge-transfer excitation from the low-spin (LS) singlet ground state, the system undergoes a fast spin cascade to a quintet HS state depicted in Figure 1B. We have illustrated the probing scheme in Figure 1C in which N-1s electrons are excited into the unoccupied π-orbitals of the pyridine ligand. The reaction pathway of polypyridyl Fe(II) complexes has been the object of intense debate for over 2 decades, but even very recently, contradicting reports of the intermediate steps of the reaction pathway have been published.11,12 The ultrafast nature of the LS → HS transition was first demonstrated by subpicosecond optical studies of Hendrickson’s group.13 Subsequent ultrafast work by Monat and McCusker directly linked the transient spectrum to the HS state of analogous SCO compounds.14−16 Several studies have reported time-resolved measurements of spin state conversions in Fe(II) polypyridyl complexes using triplet-state fluorescence, X-ray absorption, and X-ray emission spectroscopy.17−21 Structurally, timeresolved extended X-ray absorption fine structure (EXAFS) spectroscopy revealed symmetric ligand cage dilation identical to that of SCO analogues, yielding Fe−N bond length changes of 0.2 Å within 300 fs.22−25 This ultrafast structural alteration accompanying the electronic and spin state changes acts as an impulsive perturbation, creating vibrational coherence in the excited HS state, with subsequent vibrational energy relaxation occurring on subpicosecond to picosecond time scales.26−28 The picture of the reaction pathway that seems to emerge is that of a spin cascade; the initially excited 1MLCT state relaxes to the 3MLCT state with a time constant of ∼30 fs.17 Subsequent electronic relaxation to the manifold of metalcentered states occurs with a time constant < 150 fs.15,20 Recent results from femtosecond X-ray emission spectroscopy suggest that the population of an intermediate metal-centered triplet state before the metastable HS is populated, while ultraviolet spectroscopy suggests a direct population of the metastable quintet state.11,12 However, the nature of the valence charge distribution that might reveal the origin of the efficient coupling
Figure 2. (A) Experimental ground-state absorption spectrum of aqueous [Fe(bpy)3)]2+ at the nitrogen K-edge (black line) and calculated N-1s spectra of the LS and HS states (blue and red lines). Plotted below are the transient differential absorption spectra at 150 ps pump−probe delay (dots) and the predicted difference spectrum (green line). (B) Transients at indicated probe energies (solid and open circles) and a fit of a single-exponential decay with a time constant of 665 ± 30 ps. (C,D) Charge density differences between initial and core-excited states for the dominant N-1s transitions of the LS and HS states at 399.1 and 398.8 eV, respectively.
at 399.1 eV, well below the continuum edge (not visible), and weaker transitions at 400.2 and 401.5 eV. The differential absorption spectrum at 150 ps delay following 400 nm excitation is plotted in Figure 2A below the ground-state spectrum. The dispersive line shape with increased absorption below and reduced absorption at the main ground-state absorption feature indicates a shift of the N-1s transitions to lower energy. The characteristic recovery of the LS ground state of aqueous [Fe(bpy)3]2+ is revealed by the experimental transients in Figure 2B (solid and open circles), which were recorded at the indicated energies. A single-exponential model with a time constant of 665 ± 30 ps convolved with a Gaussian X-ray pulse of 70 ps fwhm accurately describes both transients. This time constant is identical to published lifetimes23 of the HS state of aqueous [Fe(bpy)3]2+ at room temperature. We performed quantum chemical calculations to interpret the experimental data. Density functional theory (DFT) was 466
DOI: 10.1021/acs.jpclett.5b02509 J. Phys. Chem. Lett. 2016, 7, 465−470
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The Journal of Physical Chemistry Letters
of metal−ligand bonding in Fe(II) systems have been well described with empirical σ-donation and π-back-bonding concepts. The details of these interactions have been investigated by X-ray spectroscopy and atomic multiplet theory.18,30 A benefit of N-1s spectroscopy is a fairly accurate description by means of TD-DFT using a molecular orbital picture.31 When analyzing the computational results, we find molecular orbitals below the HOMOs, clearly reflecting the chemical bond concepts of σ-donation (Figure S3 bottom). These orbitals of σ-symmetry allow the nitrogen atoms to delocalize charge density toward the metal center. Similarly, π-backbonding manifests in occupied molecular orbitals of πsymmetry character delocalized from the metal center toward the ligand nitrogen atoms (Figure S3 top). Upon photoexcitation of the MLCT band, the system undergoes intersystem crossing and assumes the HS configuration (Figure 1B) in which four of the five Fe-3d orbitals are only half filled, increasing the Fe−N bond length and redistributing the electron density. Accordingly, a new N-1s transition to a final state appears in which the core-excited electron is promoted into a ligand π*-orbital involving a small contribution from the half-filled t2g orbitals (Figure 2D). The degeneracy is formally lifted in the HS state because of Jahn− Teller effects (resulting from the incommensurability of an octahedral orbital arrangement and the ground-state D3 symmetry). However, the main N-1s transitions at around 398.8 eV remain close in energy, resulting in a similar line shape. While TD-DFT provides a many-electron approach for calculation of N-1s excitations, comparison of our results with one-electron studies of N K-edge XAS provides an explanation of the 0.3 eV shift in the transition energy. The relationship between the charge on the nitrogen atom and N-1s → π* excitation energies has been well-established by ΔSCF calculations in the case of metal porphyrins.32 An increase in electron density on the nitrogen atom (more negative charge) is correlated with a decrease in excitation energy. It was shown that decreases in excitation energy were due to increases (less negative) in the 1s orbital energy. Qualitatively, this maybe be understood by the fact that the 1s electrons are more screened from the nuclear charge when the electron density at the nitrogen is greater. In our case, the results listed in Table 1
used to predict the singlet and quintet geometric structure as well as orbital populations, and time-dependent DFT (TDDFT) was employed to calculate the XAS spectra of each state. At first glance, calculating the N-1s spectra of the HS quintet state involves the daunting task of dealing with a state involving two electronic excitations (one valence and one core). However, the experimentally observed quintet state is the lowest-lying state within its spin manifold, making its structure and electron density accessible by ground-state theories. The SI contains specific details about the electronic structure calculations, including information about basis sets, functionals, and the energy calibration of calculated XAS spectra. The resulting predicted spectra of the LS and HS states of [Fe(bpy)3]2+ are superimposed on the experimental data in Figure 2A and B. The main absorption feature of the N-1s spectrum is well reproduced by the calculations. The similarity between the experiment and theory is good, and only the amplitude of the ground-state transition at 401.5 eV is overestimated by theory, leading to a slight discrepancy between the experimental and predicted differential spectra. The dispersive line shape of the experimental transient differential spectrum (0.5 eV peak-to-peak measure) is due to a shift of the main N-1s resonance by 0.3 eV between the two calculated spectra. The difference charge densities in Figure 2C and D depict the predicted charge distribution of the coreexcited N-1s electron. These plots indicate that the most intense feature in the spectrum corresponds to N-1s → pyridine-π* transitions, which arises from a single excitation from each nitrogen atom. In the case of the HS state, it can be seen from Figure 2D that there is greater participation of the Fe-3d orbitals in the difference density. This is due to the fact that the spin transition leaves Fe-3d orbitals with π symmetry partially unoccupied. The ground-state line shape at 399.1 eV is due to a six-fold degenerate N-1s → pyridine-π* transition (see the SI for stick spectra). In the corresponding LS (t2g)6 configuration (Figure 1B), all six Fe-3d electrons are paired, occupying orbitals of nonbonding character as is anticipated for quasi-octahedral transition-metal complexes and illustrated by the corresponding isosurfaces of these orbitals in Figure 3A. Moreover, the details
Table 1. Natural Populations and Fe−N Bond Lengths singlet 1A1 element
nat. pop.
Fe Nave
0.999 −0.491
quintet 5T2g
Fe−N /Å
nat. pop.
Fe−N /Å
2.020
1.33 −0.546
2.215
indicate that natural populations of the nitrogen atoms become more negative (i.e., increased electron density) while the natural populations of the metal ion are more positive (decreased electron density). The increased electron density on the N atoms is accompanied by an increase (less negative) in the N-1s orbital energy by ∼0.4 eV. While changes in the valence orbitals and core−hole effects may also influence the observed transition energy, our results are consistent with previous one-electron analyses of N 1s → π* transitions in aromatic ligand systems of metal−organic systems.32 Having established that the increase in electron density on the nitrogen results in the observed shift in the 1s → π* transition, we now consider how the SCO relates to this
Figure 3. Highest occupied molecular orbitals (HOMOs) of the singlet ground (top) and quintet excited states (bottom) based on DFT calculations. HOMOs of Fe-t2g (A) and Fe-eg (B) character suggest that the former are nonbonding and highly confined on the metal center while the latter orbitals in panel B are the formally antibonding σ*-orbitals. 467
DOI: 10.1021/acs.jpclett.5b02509 J. Phys. Chem. Lett. 2016, 7, 465−470
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The Journal of Physical Chemistry Letters
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