ABSTRACT
This report will describe our recent studies of transition metal complex structural dynamics on the fs and ps time scales using an X-ray free electron laser source, Linac Coherent Light Source (LCLS). Ultrafast XANES spectra at the Ni K-edge of nickel(ii) tetramesitylporphyrin (NiTMP) were measured for optically excited states at a timescale from 100 fs to 50 ps, providing insight into its sub-ps electronic and structural relaxation processes. Importantly, a transient reduced state Ni(i) (π, 3dx2-y2) electronic state is captured through the interpretation of a short-lived excited state absorption on the low-energy shoulder of the edge, which is aided by the computation of X-ray transitions for postulated excited electronic states. The observed and computed inner shell to valence orbital transition energies demonstrate and quantify the influence of the electronic configuration on specific metal orbital energies. A strong influence of the valence orbital occupation on the inner shell orbital energies indicates that one should not use the transition energy from 1s to other orbitals to draw conclusions about the d-orbital energies. For photocatalysis, a transient electronic configuration could influence d-orbital energies up to a few eV and any attempt to steer the reaction pathway should account for this to ensure that external energies can be used optimally in driving desirable processes. NiTMP structural evolution and the influence of the porphyrin macrocycle conformation on relaxation kinetics can be likewise inferred from this study.
Subject(s)
Coordination Complexes , Electrons , Lasers , Molecular Conformation , Porphyrins/chemistry , Quantum Theory , X-RaysABSTRACT
Pyrazolate-bridged dinuclear Pt(II) complexes represent a series of molecules with tunable absorption and emission properties that can be directly modulated by structural factors, such as the Pt-Pt distance. However, direct experimental information regarding the structure of the emissive triplet excited state has remained scarce. Using time-resolved wide-angle X-ray scattering (WAXS), the excited triplet state molecular structure of [Pt(ppy)(µ-t-Bu2pz)]2 (ppy = 2-phenylpyridine; t-Bu2pz = 3,5-di-tert-butylpyrazolate), complex 1, was obtained in a dilute (0.5 mM) toluene solution utilizing the monochromatic X-ray pulses at Beamline 11IDD of the Advanced Photon Source. The excited-state structural analysis of 1 was performed based on the results from both transient WAXS measurements and density functional theory calculations to shed light on the primary structural changes in its triplet metal-metal-to-ligand charge-transfer (MMLCT) state, in particular, the Pt-Pt distance and ligand rotation. We found a pronounced Pt-Pt distance contraction accompanied by rotational motions of ppy ligands toward one another in the MMLCT state of 1. Our results suggest that the contraction is larger than what has previously been reported, but they are in good agreement with recent theoretical efforts and suggest the ppy moieties as targets for rational synthesis aimed at tuning the excited-state structure and properties.
ABSTRACT
Photoexcited Nickel(II) tetramesitylporphyrin (NiTMP), like many open-shell metalloporphyrins, relaxes rapidly through multiple electronic states following an initial porphyrin-based excitation, some involving metal centered electronic configuration changes that could be harnessed catalytically before excited state relaxation. While a NiTMP excited state present at 100 ps was previously identified by X-ray transient absorption (XTA) spectroscopy at a synchrotron source as a relaxed (d,d) state, the lowest energy excited state (J. Am. Chem. Soc., 2007, 129, 9616 and Chem. Sci., 2010, 1, 642), structural dynamics before thermalization were not resolved due to the â¼100 ps duration of the available X-ray probe pulse. Using the femtosecond (fs) X-ray pulses of the Linac Coherent Light Source (LCLS), the Ni center electronic configuration from the initial excited state to the relaxed (d,d) state has been obtained via ultrafast Ni K-edge XANES (X-ray absorption near edge structure) on a time scale from hundreds of femtoseconds to 100 ps. This enabled the identification of a short-lived Ni(I) species aided by time-dependent density functional theory (TDDFT) methods. Computed electronic and nuclear structure for critical excited electronic states in the relaxation pathway characterize the dependence of the complex's geometry on the electron occupation of the 3d orbitals. Calculated XANES transitions for these excited states assign a short-lived transient signal to the spectroscopic signature of the Ni(I) species, resulting from intramolecular charge transfer on a time scale that has eluded previous synchrotron studies. These combined results enable us to examine the excited state structural dynamics of NiTMP prior to thermal relaxation and to capture intermediates of potential photocatalytic significance.
Subject(s)
Metalloporphyrins/chemistry , Electrons , Kinetics , Models, Molecular , Molecular Conformation , Quantum Theory , X-Ray Absorption SpectroscopyABSTRACT
Copper(I) diimine complexes have emerged as low cost replacements for ruthenium complexes as light sensitizers and electron donors, but their shorter metal-to-ligand-charge-transfer (MLCT) states lifetimes and lability of transient Cu(II) species impede their intended functions. Two carboxylated Cu(I) bis-2,9-diphenylphenanthroline (dpp) complexes [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(COOH)2)](+) and [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(Φ-COOH)2)](+) (Φ = tolyl) with different linker lengths were synthesized in which the MLCT-state solvent quenching pathways are effectively blocked, the lifetime of the singlet MLCT state is prolonged, and the transient Cu(II) ligands are stabilized. Aiming at understanding the mechanisms of structural influence to the interfacial charge transfer in the dye-sensitized solar cell mimics, electronic and geometric structures as well as dynamics for the MLCT state of these complexes and their hybrid with TiO2 nanoparticles were investigated using optical transient spectroscopy, X-ray transient absorption spectroscopy, time-dependent density functional theory, and quantum dynamics simulations. The combined results show that these complexes exhibit strong absorption throughout the visible spectrum due to the severely flattened ground state, and a long-lived charge-separated Cu(II) has been achieved via ultrafast electron injection (<300 fs) from the (1)MLCT state into TiO2 nanoparticles. The results also indicate that the TiO2-phen distance in these systems does not have significant effect on the efficiency of the interfacial electron-transfer process. The mechanisms for electron transfer in these systems are discussed and used to develop new strategies in optimizing copper(I) diimine complexes in solar energy conversion devices.
ABSTRACT
Determining the electronic and geometric structures of photoexcited transient species with high accuracy is crucial for understanding their fundamental photochemistry and controlling their photoreactivity. We have applied X-ray transient absorption spectroscopy to measure the XANES and EXAFS spectra of a dilute (submillimolar) solution of the osmium(II) polypyridyl complex [Os(bpy)2dcbpy](PF6)2 (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) (OsL2L') in methanol at the Os LIII edge. We have obtained spectra of superb quality for both the ground state and the photoinduced (3)MLCT excited state that have allowed us not only to extract detailed information about the Os 5d orbitals but also to resolve very small differences of 0.010 ± 0.008 Å in the average Os-N bond lengths of the ground and excited states. Theoretical calculations using a recently developed DFT-based approach support the measured electronic structures and further identify the nature of the molecular orbitals that contribute to the main absorption bands in the XANES spectra.
ABSTRACT
We report time-resolved X-ray absorption near edge structure (TR-XANES) measurements at the Eu L3 edge upon photoexcitation of several Eu(III)-based luminescent lanthanide complexes. We find an unambiguous signature of the 4f intrashell excitation that occurs upon energy transfer from the photoactive organic antennas to the lanthanide species. Phenomenologically, this observation provides the basis for direct investigation of a crucial step in the energy transfer pathways that lead to sensitized luminescence in lanthanide-based dyes. Interestingly, the details of the TR-XANES feature suggest that the degree of 4f-5d hybridization may itself vary depending on the excited state of the Eu(III) ion.
ABSTRACT
The photodissociation and recombination of CO and 1-methylimidazole (Im) from iron protoporphyrin IX (FePP-ImCO) dissolved in a 30% v/v aqueous solution of Im was studied using ultrafast optical transient absorption (TA) and X-ray transient absorption (XTA) spectroscopies. FePP-ImCO was shown to lose the CO ligand upon excitation at the Q bands, with 3.8 ps vibrational cooling and 21.6 ps intersystem crossing time constants derived from optical TA experiments, followed by ligation of a second Im on the nanosecond time scale. The penta-coordinate FePP-Im intermediate which forms following CO dissociation adopts a square pyramidal geometry with a "domed" iron center that is reminiscent of that formed upon loss of CO from carbonmonoxymyoglobin (MbCO). Unlike MbCO, which typically retains its newly generated penta-coordinated geometry until CO recombination, FePP can adopt a hexa-coordinate geometry by binding an additional Im ligand (FePP-(Im)2), allowing the porphyrin to exist in the low-spin electronic state even without the CO attached. The second Im ligand remains bound until CO recombination occurs with a time constant of 283 µs. The photodissociated states of FePP-ImCO and MbCO 100 ps after photoexcitation have similar iron site geometries, implying that the protein matrix in MbCO maintains minimum potential energy in the heme center despite the large-scale reorganization in the protein secondary and tertiary structure that arises from the dynamic active site/matrix interaction.
Subject(s)
Ligands , Protoporphyrins/chemistry , X-Ray Absorption Spectroscopy , Carbon Monoxide/chemistry , Electrons , Imidazoles/chemistry , Kinetics , Myoglobin/chemistry , Myoglobin/metabolism , Photolysis , Solutions/chemistry , Spectrophotometry, Ultraviolet , Water/chemistryABSTRACT
The molecular and electronic structures of the transient intermediates generated from the photolysis of trirutheniumdodecacarbonyl, Ru3(CO)12, by ultrafast UV (351 nm) laser excitation were investigated using X-ray transient absorption (XTA) spectroscopy. The electronic configuration change and nuclear rearrangement after the dissociation of carbonyls were observed at ruthenium K-edge X-ray absorption near edge structure and X-ray absorption fine structure spectra. Analysis of XTA data, acquired after 100, 200, and 400 ps and 300 ns time delay following the photoexcitation, identified the presence of three intermediate species with Ru3(CO)10 being the most dominating one. The results set an example of applying XTA in capturing both transient electronic and nuclear configurations in metal clusters simulating catalysts in chemical reactions.
ABSTRACT
Upon electrochemical oxidation of the precursor complexes [Cp*Ir(H(2)O)(3)]SO(4) (1) or [(Cp*Ir)(2)(OH)(3)]OH (2) (Cp* = pentamethylcyclopentadienyl), a blue layer of amorphous iridium oxide containing a carbon admixture (BL) is deposited onto the anode. The solid-state, amorphous iridium oxide material that is formed from the molecular precursors is significantly more active for water-oxidation catalysis than crystalline IrO(2) and functions as a remarkably robust catalyst, capable of catalyzing water oxidation without deactivation or significant corrosion for at least 70 h. Elemental analysis reveals that BL contains carbon that is derived from the Cp* ligand (â¼ 3% by mass after prolonged electrolysis). Because the electrodeposition of precursors 1 or 2 gives a highly active catalyst material, and electrochemical oxidation of other iridium complexes seems not to result in immediate conversion to iridium oxide materials, we investigate here the nature of the deposited material. The steps leading to the formation of BL and its structure have been investigated by a combination of spectroscopic and theoretical methods. IR spectroscopy shows that the carbon content of BL, while containing some C-H bonds intact at short times, is composed primarily of components with CâO fragments at longer times. X-ray absorption and X-ray absorption fine structure show that, on average, the six ligands to iridium in BL are likely oxygen atoms, consistent with formation of iridium oxide under the oxidizing conditions. High-energy X-ray scattering (HEXS) and pair distribution function (PDF) analysis (obtained ex situ on powder samples) show that BL is largely free of the molecular precursors and is composed of small, <7 Å, iridium oxide domains. Density functional theory (DFT) modeling of the X-ray data suggests a limited set of final components in BL; ketomalonate has been chosen as a model fragment because it gives a good fit to the HEXS-PDF data and is a potential decomposition product of Cp*.
ABSTRACT
X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (â¼10(12) photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2'-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is â¼160 fs.
ABSTRACT
The effects of structural constraints on the metal-to-ligand charge transfer (MLCT) excited state structural dynamics of cuprous bis-2,9-diphenyl-phenanthroline ([Cu(I)(dpp)(2)](+)) in both coordinating acetonitrile and noncoordinating toluene were studied using X-ray transient absorption (XTA) spectroscopy and density functional theory (DFT) calculations. The phenyl groups attached to the phenanthroline ligands not only effectively shield the Cu(I) center from solvent molecules, but also force a flattened tetrahedral coordination geometry of the Cu(I) center. Consequently, the MLCT state lifetime in [Cu(I)(dpp)(2)](+) is solvent-independent, unlike the previously studied 2,9-methyl substituted bis-phenanthroline Cu(I) complex. The MLCT state of [Cu(I)(dpp)(2)](+) still undergoes a "pseudo Jahn-Teller distortion," with the angle between the two phenanthroline ligand planes decreased further by 7°. The XTA results indicate that, in the MLCT excited state of [Cu(I)(dpp)(2)](+), the phenyls at the 2, 9 positions of the phenanthroline rotate, breaking the π-π interaction with the phenanthroline ligands without ever rotating in-plane with the phenanthroline ligands. Hence, the transferred electron density from the Cu(I) center is localized on the phenanthroline moiety with no charge density present on the phenyl rings. The insight about the effect of the structural constraints on the MLCT state properties will guide the design of Cu(I) diimine complexes with suitable excited-state properties to function as earth-abundant dye sensitizers for solar electricity generation.
ABSTRACT
Although understanding the structural dynamics associated with ligand photodissociation is necessary in order to correlate structure and function in biological systems, few techniques are capable of measuring the ultrafast dynamics of these systems in solution-phase at room temperature. We present here a detailed X-ray transient absorption (XTA) study of the photodissociation of CO-bound myoglobin (Fe(II)CO-Mb) in room-temperature aqueous buffer solution with a time resolution of 80 ps, along with a general procedure for handling biological samples under the harsh experimental conditions that transient X-ray experiments entail. The XTA spectra of (Fe(II)CO-Mb) exhibit significant XANES and XAFS alterations following 527 nm excitation, which remain unchanged for >47 µs. These spectral changes indicate loss of the CO ligand, resulting in a five-coordinate, domed heme, and significant energetic reorganization of the 3d orbitals of the Fe center. With the current experimental setup, each X-ray pulse in the pulse train, separated by ~153 ns, can be separately discriminated, yielding snapshots of the myoglobin evolution over time. These methods can be easily applied to other biological systems, allowing for simultaneous structural and electronic measurements of any biological system with both ultrafast and slow time resolutions, effectively mapping out all of the samples' relevant physiological processes.
Subject(s)
Carbon Monoxide/chemistry , Heme/chemistry , Myoglobin/chemistry , Carbon Monoxide/metabolism , Electrons , Ferrous Compounds/chemistry , Myoglobin/metabolism , Photolysis , Protein Structure, Tertiary , Solutions/chemistry , Temperature , Thermodynamics , X-Ray Absorption SpectroscopyABSTRACT
Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of synâ configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the antiâ biradical.
Subject(s)
Coordination Complexes/chemistry , Photosensitizing Agents/chemistry , Models, Molecular , Molecular Conformation , Oxidation-Reduction , Photochemical Processes , Spectrophotometry, Infrared , Time Factors , X-Ray Absorption SpectroscopyABSTRACT
We describe our implementation of a high repetition rate (54 kHz-6.5 MHz), high power (>10 W), laser system at the 7ID beamline at the Advanced Photon Source for laser pump/x-ray probe studies of optically driven molecular processes. Laser pulses at 1.06 µm wavelength and variable duration (10 or 130 ps) are synchronized to the storage ring rf signal to a precision of ~250 fs rms. Frequency doubling and tripling of the laser radiation using nonlinear optical techniques have been applied to generate 532 and 355 nm light. We demonstrate that by combining a microfocused x-ray probe with focused optical laser radiation the requisite fluence (with <10 µJ/pulse) for efficient optical excitation can be readily achieved with a compact and commercial laser system at megahertz repetition rates. We present results showing the time-evolution of near-edge x-ray spectra of a well-studied, laser-excited metalloporphyrin, Ni(II)-tetramesitylporphyrin. The use of high repetition rate, short pulse lasers as pump sources will dramatically enhance the duty cycle and efficiency in data acquisition and hence capabilities for laser-pump/x-ray probe studies of ultrafast structural dynamics at synchrotron sources.
ABSTRACT
The excited-state structure of a dinuclear platinum(II) complex with tert-butyl substituted pyrazolate bridging units, [Pt(ppy)(µ-(t)Bu(2)pz)](2) (ppy = 2-phenylpyridine; (t)Bu(2)pz = 3,5-di-tert-butylpyrazolate) is studied by X-ray transient absorption (XTA) spectroscopy to reveal the transient electronic and nuclear geometry. DFT calculations predict that the lowest energy triplet excited state, assigned to a metal-metal-to-ligand charge transfer (MMLCT) transition, has a contraction in the Pt-Pt distance. The Pt-Pt bond length and other structural parameters extracted from fitting the experimental XTA difference spectra from full multiple scattering (FMS) and multidimensional interpolation calculations indicates a metal-metal distance decrease by approximately 0.2 Å in the triplet excited state. The advantages and challenges of this approach in resolving dynamic transient structures of nonbonding or weak-bonding dinuclear metal complexes in solution are discussed.
Subject(s)
Organoplatinum Compounds/chemistry , Platinum/chemistry , Pyrazoles/chemistry , Quantum Theory , Dimerization , Models, Molecular , Molecular Structure , Organoplatinum Compounds/chemical synthesis , X-Ray Absorption SpectroscopyABSTRACT
Transient molecular structures along chemical reaction pathways are important for predicting molecular reactivity, understanding reaction mechanisms, as well as controlling reaction pathways. During the past decade, X-ray transient absorption spectroscopy (XTA, or LITR-XAS, laser-initiated X-ray absorption spectroscopy), analogous to the commonly used optical transient absorption spectroscopy, has been developed. XTA uses a laser pulse to trigger a fundamental chemical process, and an X-ray pulse(s) to probe transient structures as a function of the time delay between the pump and probe pulses. Using X-ray pulses with high photon flux from synchrotron sources, transient electronic and molecular structures of metal complexes have been studied in disordered media from homogeneous solutions to heterogeneous solution-solid interfaces. Several examples from the studies at the Advanced Photon Source in Argonne National Laboratory are summarized, including excited-state metalloporphyrins, metal-to-ligand charge transfer (MLCT) states of transition metal complexes, and charge transfer states of metal complexes at the interface with semiconductor nanoparticles. Recent developments of the method are briefly described followed by a future prospective of XTA. It is envisioned that concurrent developments in X-ray free-electron lasers and synchrotron X-ray facilities as well as other table-top laser-driven femtosecond X-ray sources will make many breakthroughs and realise dreams of visualizing molecular movies and snapshots, which ultimately enable chemical reaction pathways to be controlled.
ABSTRACT
Time-resolved transient absorption spectroscopy was used to investigate the primary geminate recombination and cage escape of alkyl radicals in solution over a temperature range from 0 to 80 degrees C. Radical pairs were produced by photoexcitation of methyl, ethyl, propyl, hexylnitrile, and adenosylcobalamin in water, ethylene glycol, mixtures of water and ethylene glycol, and sucrose solutions. In contrast to previous studies of cage escape and geminate recombination, these experiments demonstrate that cage escape for these radical pairs occurs on time scales ranging from a hundred picoseconds to over a nanosecond as a function of solvent fluidity and radical size. Ultrafast cage escape (<100 ps) is only observed for the methyl radical where the radical pair is produced through excitation to a directly dissociative electronic state. The data are interpreted using a unimolecular model with competition between geminate recombination and cage escape. The escape rate constant, k(e), is not a simple function of the solvent fluidity (T/eta) but depends on the nature of the solvent as well. The slope of k(e) as a function of T/eta for the adenosyl radical in water is in near quantitative agreement with the slope calculated using a hydrodynamic model and the Stokes-Einstein equation for the diffusion coefficients. The solvent dependence is reproduced when diffusion constants are calculated taking into account the relative volume and mass of both solvent and solute using the expression proposed by Akgerman (Akgerman, A.; Gainer, J. L. Ind. Eng. Chem. Fundam. 1972, 11, 373-379). Rate constants for cage escape of the other radicals investigated are consistently smaller than the calculated values suggesting a systematic correction for radical size or coupled radical pair motion.
Subject(s)
Ethylene Glycol/chemistry , Sucrose/chemistry , Thermodynamics , Vitamin B 12/chemistry , Water/chemistry , Free Radicals/chemistry , Models, Molecular , Molecular Conformation , Photochemistry , Quantum Theory , Solvents/chemistry , TemperatureABSTRACT
Transient absorption spectroscopy has been used to elucidate the nature of the S1 intermediate state populated following excitation of cob(III)alamin (Cbl(III)) compounds. This state is sensitive both to axial ligation and to solvent polarity. The excited-state lifetime as a function of temperature and solvent environment is used to separate the dynamic and electrostatic influence of the solvent. Two distinct types of excited states are identified, both assigned to pi3d configurations. The spectra of both types of excited states are characterized by a red absorption band (ca. 600 nm) assigned to Co 3d --> 3d or Co 3d --> corrin pi* transitions and by visible absorption bands similar to the corrin pi-->pi* transitions observed for ground state Cbl(III) compounds. The excited state observed following excitation of nonalkyl Cbl(III) compounds has an excited-state spectrum characteristic of Cbl(III) molecules with a weakened bond to the axial ligand (Type I). A similar excited-state spectrum is observed for adenosylcobalamin (AdoCbl) in water and ethylene glycol. The excited-state spectrum of methyl, ethyl, and n-propylcobalamin is characteristic of a Cbl(III) species with a sigma-donating alkyl anion ligand (Type II). This Type II excited-state spectrum is also observed for AdoCbl bound to glutamate mutase. The results are discussed in the context of theoretical calculations of Cbl(III) species reported in the literature and highlight the need for additional calculations exploring the influence of the alkyl ligand on the electronic structure of cobalamins.
ABSTRACT
Femtosecond to nanosecond transient absorption spectroscopy is used to investigate the photolysis of 5'-deoxyadenosylcobalamin (coenzyme B12, AdoCbl) bound to glutamate mutase. The photochemistry of AdoCbl is found to be inherently dependent upon the environment of the cofactor. Excitation of AdoCbl bound to glutamate mutase results in formation of a metal-to-ligand charge transfer intermediate state which decays to form cob(II)alamin with a time constant of 105 ps. This observation is in contrast to earlier measurements in water where the photohomolysis proceeds through an intermediate state in which the axial dimethylbenzimidazole ligand appears to have dissociated, and measurements in ethylene glycol where prompt bond homolysis is observed (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188). The quantum yield for formation of stable radical pairs in the enzyme is found to be phi = 0.05 +/- 0.03, and the resulting intrinsic rate constants for geminate recombination and "cage escape" are 1.0 +/- 0.1 and 0.05 +/- 0.03 ns(-1), respectively. The rate constant for geminate recombination is 30% less than that observed for AdoCbl in water or ethylene glycol. This reduction is insufficient to account for the 10(12)-fold increase in the homolysis rate observed when substrate is bound to the protein. Finally, the protein provides a cage to prevent diffusive loss of the adenosyl radical; however, the ultimate yield for long-lived radicals is determined by the evolution from a singlet to a triplet radical pair as proposed for AdoCbl in ethylene glycol.