X-ray Transient Absorption Research Articles

A reversible photochromic covalent organic framework

Covalent organic frameworks are a type of crystalline porous materials that linked through covalent bond, and they have numerous potential applications in adsorption, separation, catalysis, and more. However, there are rarely relevant reported on photochromism. Fortunately, a hydrazone-linked DBTB-DETH-COF is rapidly generated through ultrasound method. The DBTB-DETH-COF is found to exhibit reversible photochromism (at least 50 cycles) from yellow to olive in the presence of light and air, and subsequently back to the original color upon heating. In addition, the structure of DBTB-DETH-COF remains unchanged after 15 days of light illumination. Furthermore, the reason of photochromic process is discussed by electron paramagnetic resonance, X-ray photoelectron spectroscopy, electrochemistry characterizations and transient absorption measurements. The reversible photochromic DBTB-DETH-COF can be used as anti-counterfeiting ink and optical switch in the presence of air. This work expands a stable organic photochromic material and broadens the applications of COFs.

Few-femtosecond soft X-ray transient absorption spectroscopy with tuneable DUV-Vis pump pulses

Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics and for creating superpositions of valence states in quantum systems. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Here, we present, to our knowledge, an unprecedented temporal resolution and pump tuneability for UV-Vis-pumped soft X-ray transient absorption spectroscopy. We have combined few-fs deep-UV to visible tuneable pump pulses from resonant dispersive wave emission in hollow capillary fiber with attosecond soft X-ray probe pulses from high harmonic generation. We achieve sub-5-fs time resolution, sub-fs interferometric stability, and continuous tuneability of the pump pulses from 230 to 700 nm. We demonstrate that the pump can initiate an ultrafast photochemical reaction and that the dynamics at different atomic sites can be resolved simultaneously. These capabilities will allow studies of the fastest electronic dynamics in a large range of photochemical, photobiological and photovoltaic reactions.

Measurement of coherent vibrational dynamics with X-ray Transient Absorption Spectroscopy simultaneously at the Carbon K- and Chlorine L2,3- edges

X-ray Transient Absorption Spectroscopy (XTAS) is a powerful probe for ultrafast molecular dynamics. The evolution of XTAS signal is controlled by the shapes of potential energy surfaces of the associated core-excited states, which are difficult to directly measure. Here, we study the vibrational dynamics of Raman activated CCl4 with XTAS targeting the C 1s and Cl 2p electrons. The totally symmetric stretching mode leads to concerted elongation or contraction in bond lengths, which in turn induce an experimentally measurable red or blue shift in the X-ray absorption energies associated with inner-shell electron excitations to the valence antibonding levels. The ratios between slopes of different core-excited potential energy surfaces (CEPESs) thereby extracted agree very well with Restricted Open-Shell Kohn-Sham calculations. The other, asymmetric, modes do not measurably contribute to the XTAS signal. The results highlight the ability of XTAS to reveal coherent nuclear dynamics involving < 0.01 Å atomic displacements and also provide direct measurement of forces on CEPESs.

Structural Evolution of Photoexcited Methylcobalamin toward a CarH-like Metastable State: Evidence from Time-Resolved X-ray Absorption and X-ray Emission.

CarH is a protein photoreceptor that uses a form of B12, adenosylcobalamin (AdoCbl), to sense light via formation of a metastable excited state. Aside from AdoCbl bound to CarH, methylcobalamin (MeCbl) is the only other example─to date─of photoexcited cobalamins forming metastable excited states with lifetimes of nanoseconds or longer. The UV-visible spectra of the excited states of MeCbl and AdoCbl bound to CarH are similar. We have used transient Co K-edge X-ray absorption and X-ray emission spectroscopies in conjunction with transient absorption spectroscopy in the UV-visible region to characterize the excited states of MeCbl. These data show that the metastable excited state of MeCbl has a slightly expanded corrin ring and increased electron density on the cobalt, but only small changes in the axial bond lengths.

High-Spin State of a Ferrocene Electron Donor Revealed by Optical and X-ray Transient Absorption Spectroscopy.

Ferrocene is one of the most common electron donors, and mapping its ligand-field excited states is critical to designing donor-acceptor (D-A) molecules with long-lived charge transfer states. Although 3(d-d) states are commonly invoked in the photophysics of ferrocene complexes, mention of the high-spin 5(d-d) state is scarce. Here, we provide clear evidence of 5(d-d) formation in a bimetallic D-A molecule, ferrocenyl cobaltocenium hexafluorophosphate ([FcCc]PF6). Femtosecond optical transient absorption (OTA) spectroscopy reveals two distinct electronic excited states with 30 and 500 ps lifetimes. Using a combination of ultraviolet, visible, near-infrared, and short-wave infrared probe pulses, we capture the spectral features of these states over an ultrabroadband range spanning 320 to 2200 nm. Time-dependent density functional theory (DFT) calculations of the lowest triplet and quintet states, both primarily Fe(II) (d-d) in character, qualitatively agree with the experimental OTA spectra, allowing us to assign the 30 ps state as the 3(d-d) state and the 500 ps state as the high-spin 5(d-d) state. To confirm the ferrocene-centered high-spin character of the 500 ps state, we performed X-ray transient absorption (XTA) spectroscopy at the Fe and Co K edges. The Fe K-edge XTA spectrum at 150 ps shows a red shift of the absorption edge that is consistent with an Fe(II) high-spin state, as supported by ab initio calculations. The transient signal detected at the Co K-edge is 50× weaker, confirming the ferrocene-centered character of the excited state. Fitting of the transient extended X-ray absorption fine structure region yields an Fe-C bond length increase of 0.25 ± 0.1 Å in the excited state, as expected for the high-spin state based on DFT. Altogether, these results demonstrate that the high-spin state of ferrocene should be considered when designing donor-acceptor assemblies for photocatalysis and photovoltaics.

Efficient deep-blue electroluminescence from Ce-based metal halide

Rare earth ions with d-f transitions (Ce3+, Eu2+) have emerged as promising candidates for electroluminescence applications due to their abundant emission spectra, high light conversion efficiency, and excellent stability. However, directly injecting charge into 4f orbitals remains a significant challenge, resulting in unsatisfied external quantum efficiency and high operating voltage in rare earth light-emitting diodes. Herein, we propose a scheme to solve the difficulty by utilizing the energy transfer process. X-ray photoelectron spectroscopy and transient absorption spectra suggest that the Cs3CeI6 luminescence process is primarily driven by the energy transfer from the I2-based self-trapped exciton to the Ce-based Frenkel exciton. Furthermore, energy transfer efficiency is largely improved by enhancing the spectra overlap between the self-trapped exciton emission and the Ce-based Frenkel exciton excitation. When implemented as an active layer in light-emitting diodes, they show the maximum brightness and external quantum efficiency of 1073 cd m−2 and 7.9%, respectively.

Structure-enabled long-lived charge separation in single crystals of an asymmetric donor-acceptor perylenediimide cyclophane.

We report the synthesis and characterization of a covalently linked asymmetric cyclophane comprising a 1,7-di(pyrrolidin-1'-yl)perylene-3,4,9,10-bis(dicarboximide) (pyrPDI) and 1,6,7,12-tetra(4'-t-butylphenoxy)perylene-3,4,9,10-bis(dicarboximide) (tpPDI), which absorbs light from 400-750 nm. Single crystals of pyrPDI-tpPDI were analyzed by using X-ray diffraction and transient absorption microscopy. The crystal structure contains several types of intermolecular donor-acceptor interactions (pyrPDI-pyrPDI, tpPDI-tpPDI, and pyrPDI-tpPDI) in addition to the covalently installed intramolecular interaction. Following photoexcitation of the pyrPDI-tpPDI single crystal, the transient absorption data show that charge separation occurs in τ = 21 ps, which is about nine times faster than in toluene solution, while charge recombination occurs in τ > 2 μs, which is more than 400 times longer than in solution. The faster charge separation in the single crystals results from the intermolecular donor-acceptor pyrPDI-tpPDI interactions, while the greatly enhanced charge-separated state lifetime is a consequence of charge transport through the intermolecular π-stacks. These results demonstrate the utility of pre-organizing donor-acceptor structural motifs to elicit specific crystal morphologies that can lead to enhanced photogenerated charge carrier lifetimes for solar energy conversion.

Time-Resolved X-ray Absorption Spectroscopy: An MCTDH Quantum Dynamics Protocol.

Expressions for linear and nonlinear spectroscopy simulation in the X-ray window in which the time evolution of a photoexcited molecular system is treated via quantum dynamics are derived. By leveraging on the peculiar properties of core-excited/ionized states, first- and third-order response functions are recast in the limit of time-scale separation between the extremely short core-state lifetime and the (comparably longer) electronic-state transfer and nuclear vibrational motion. This work is a natural extension of Segatta et al. (J. Chem. Theory Comput. 2023, 19, 2075-2091), in which some of the present authors coupled MCTDH quantum dynamics to spectroscopy simulation at different levels of sophistication. Full quantum dynamics and approximate expressions are compared by simulating X-ray transient absorption spectroscopy at the carbon K-edge in the pyrene molecule.

Pushing the heterometal doping limit while preserving long-lived charge separation in a Ti-based MOF photocatalyst

This study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe3+-doped titanium-based metal organic framework (MOF), xFeMIL125-NH2, as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts.

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two-Dimensional Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) with mobile charges have attracted significant attention due to their potential applications in photoelectric devices, chemical resistance sensors, and catalysis. However, fundamental understanding of the charge transport pathway within the framework and the key properties that determine the performance of conductive MOFs in photoelectric devices remain underexplored. Herein, we report the mechanisms of photoinduced charge transport and electron dynamics in the conductive 2D M-HHTP (M=Cu, Zn or Cu/Zn mixed; HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) MOFs and their correlation with photoconductivity using the combination of time-resolved terahertz spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and density functional theory (DFT) calculations. We identify the through-space hole transport mechanism through the interlayer sheet π-π interaction, where photoinduced hole state resides in HHTP ligand and electronic state is localized at the metal center. Moreover, the photoconductivity of the Cu-HHTP MOF is found to be 65.5 S m-1 , which represents the record high photoconductivity for porous MOF materials based on catecholate ligands.

Mapping the Ultrafast Mechanistic Pathways of Co Photocatalysts in Pure Water through Time-resolved X-ray Spectroscopy.

Nanosecond time-resolved X-ray (tr-XAS) and optical transient absorption spectroscopy (OTA) are applied to study 3 multimolecular photocatalytic systems with [Ru(bpy)3]2+photoabsorber, ascorbic acid electron donor and Co catalysts with methylene (1), hydroxomethylene (2) and methyl (3) amine substituents in pure water. OTA and tr-XAS of 1 and 2 show that the favored catalytic pathway involves reductive quenching of the excited photosensitizer and electron transfer to the catalyst to form a CoII square pyramidal intermediate with a bonded aqua molecule followed by a CoI square planar derivative that decays within ~8 µs. By contrast, a CoI square pyramidal intermediate with a longer decay lifetime of ~35µs is formed from an analogous CoII geometry for 3 in H2O. These results highlight the protonation of CoI to form the elusive hydride species to be the rate limiting step and show that the catalytic rate can be enhanced through hydrogen containing pendant amines that act as H-H bond formation proton relays.

Precious-Metal-Free CO2 Photoreduction Boosted by Dynamic Coordinative Interaction between Pyridine-Tethered Cu(I) Sensitizers and a Co(II) Catalyst.

Improving the photocatalytic efficiency of a fully noble-metal-free system for CO2 reduction remains a fundamental challenge, which can be accomplished by facilitating electron delivery as a consequence of exploiting intermolecular interactions. Herein, we have designed two Cu(I) photosensitizers with different pyridyl pendants at the phenanthroline moiety to enable dynamic coordinative interactions between the sensitizers and a cobalt macrocyclic catalyst. Compared to the parent Cu(I) photosensitizer, one of the pyridine-tethered derivatives boosts the apparent quantum yield up to 76 ± 6% at 425 nm for selective (near 99%) CO2-to-CO conversion. This value is nearly twice that of the parent system with no pyridyl pendants (40 ± 5%) and substantially surpasses the record (57%) of the noble-metal-free systems reported so far. This system also realizes a maximum turnover number of 11 800 ± 1400. In contrast, another Cu(I) photosensitizer, in which the pyridine substituents are directly linked to the phenanthroline moiety, is inactive. The above behavior and photocatalytic mechanism are systematically elucidated by transient fluorescence, transient absorption, transient X-ray absorption spectroscopies, and quantum chemical calculations. This work highlights the advantage of constructing coordinative interactions to fine-tune the electron transfer processes within noble-metal-free systems for CO2 photoreduction.

Synchrotron based transient x-ray absorption spectroscopy for emerging solid-state energy materials

The rational design of cutting-edge materials for an efficient solar energy conversion process is a challenging task, which demands a fundamental understanding of the mechanisms operative during the photoinduced physical and chemical reactions. In response to these issues, progress in the field has steered attention toward the use of time-resolved spectroscopic techniques to resolve the multiple intermediate species involved in these photoinduced reactions. Thanks to the advent of pump–probe technique, which leads to the development of various time-resolved spectroscopic methods, significant progress has been made in understanding the photophysical and photochemical properties (e.g., excited state dynamics, charge transfer mechanism, charge separation dynamics, etc.) of energy materials. Synchrotron-based x-ray transient absorption (XTA) spectroscopy is one of the most important time-resolved techniques to unravel the direct correlation of the material structure with their photophysical properties owing to its unique capability in directly observing electronic and structural evolution simultaneously. The aim of this work is to provide a systematic overview of the recent progress in using XTA for capturing the structural dynamics associated with excited state and charge separation dynamics in emerging solid-state energy materials.

Photon-shot-noise-limited transient absorption soft X-ray spectroscopy at the European XFEL.

Femtosecond transient soft X-ray absorption spectroscopy (XAS) is a very promising technique that can be employed at X-ray free-electron lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here, a dedicated setup for soft X-rays available at the Spectroscopy and Coherent Scattering (SCS) instrument at the European X-ray Free-Electron Laser (European XFEL) is presented. It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot by shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, an imaging detector capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst is employed, and allows a photon-shot-noise-limited sensitivity to be approached. The setup and its capabilities are reviewed as well as the online and offline analysis tools provided to users.

Ultrafast X-ray Spectroscopy of Intersystem Crossing in Hexafluoroacetylacetone: Chromophore Photophysics and Spectral Changes in the Face of Electron-Withdrawing Groups.

Intersystem crossings between singlet and triplet states represent a crucial relaxation pathway in photochemical processes. Herein, we probe the intersystem crossing in hexafluoro-acetylacetone with ultrafast X-ray transient absorption spectroscopy at the carbon K-edge. We observe the excited state dynamics following excitation with 266 nm UV light to the 1ππ* (S2) state with element and site-specificity using a broadband soft X-ray pulse produced by high harmonic generation. These results are compared to X-ray spectra computed from orbital optimized density functional theory methods. It is found that the electron-withdrawing fluorine atoms decongest the X-ray absorption spectrum by enhancing separation between features originating from different carbon atoms. This facilitates the elucidation of structural and electronic dynamics at the chromophore. The evolution of the core-to-valence resonances at the carbon K-edge reveals an ultrafast population transfer between the 1nπ* (S1) and 3ππ* (T1) states on a 1.6 ± 0.4 ps time scale, which is similar to the 1.5 ps time scale earlier observed for acetylacetone [ J. Am. Chem. Soc. 2017, 139, 16576-16583, DOI: 10.1021/jacs.7b07532]. It therefore appears that terminal fluorination has little influence on the intersystem crossing rate of the acetylacetone chromophore. In addition, the significant role of hydrogen-bond opened and twisted rotational isomers is elucidated in the excited state dynamics by comparison of the experimental transient X-ray spectra with theory.

Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core–hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe–Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides─TiO2, α-Cr2O3, β-MnO2, α-Fe2O3, Co3O4, NiO, CuO, and ZnO─are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core–valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Asynchronous x-ray multiprobe data acquisition for x-ray transient absorption spectroscopy.

Laser pump X-ray Transient Absorption (XTA) spectroscopy offers unique insights into photochemical and photophysical phenomena. X-ray Multiprobe data acquisition (XMP DAQ) is a technique that acquires XTA spectra at thousands of pump-probe time delays in a single measurement, producing highly self-consistent XTA spectral dynamics. In this work, we report two new XTA data acquisition techniques that leverage the high performance of XMP DAQ in combination with High Repetition Rate (HRR) laser excitation: HRR-XMP and Asynchronous X-ray Multiprobe (AXMP). HRR-XMP uses a laser repetition rate up to 200 times higher than previous implementations of XMP DAQ and proportionally increases the data collection efficiency at each time delay. This allows HRR-XMP to acquire more high-quality XTA data in less time. AXMP uses a frequency mismatch between the laser and x-ray pulses to acquire XTA data at a flexibly defined set of pump-probe time delays with a spacing down to a few picoseconds. AXMP introduces a novel pump-probe synchronization concept that acquires data in clusters of time delays. The temporally inhomogeneous distribution of acquired data improves the attainable signal statistics at early times, making the AXMP synchronization concept useful for measuring sub-nanosecond dynamics with photon-starved techniques like XTA. In this paper, we demonstrate HRR-XMP and AXMP by measuring the laser-induced spectral dynamics of dilute aqueous solutions of Fe(CN)6 4- and [FeII(bpy)3]2+ (bpy: 2,2'-bipyridine), respectively.

Real-time observation of the Woodward-Hoffmann rule for 1,3-cyclohexadiene by femtosecond soft X-ray transient absorption.

The stereochemistry of pericyclic reactions is explained by orbital symmetry conservation, referred to as the Woodward-Hoffmann (WH) rule. Although this rule has been verified using the structures of reactants and products, the temporal evolution of the orbital symmetry during the reaction has not been clarified. Herein, we used femtosecond soft X-ray transient absorption spectroscopy to elucidate the thermal pericyclic reaction of 1,3-cyclohexadiene (CHD) molecules, i.e., their isomerization to 1,3,5-hexatriene. In the present experimental scheme, the ring-opening reaction is driven by the thermal vibrational energy induced by photoexcitation to the Rydberg states at 6.2 eV and subsequent femtosecond relaxation to the ground state of CHD molecules. The direction of the ring opening, which can be conrotatory or disrotatory, was the primary focus, and the WH rule predicts the disrotatory pathway in the thermal process. We observed the shifts in K-edge absorption of the carbon atom from the 1s orbital to vacant molecular orbitals around 285 eV at a delay between 340 and 600 fs. Furthermore, a theoretical investigation predicts that the shifts depend on the molecular structures along the reaction pathways and the observed shifts in induced absorption are attributed to the structural change in the disrotatory pathway. This confirms that the orbital symmetry is dynamically conserved in the ring-opening reaction of CHD molecules as predicted using the WH rule.

Direct Z-scheme heterojunction of PCN-222/CsPbBr3 for boosting photocatalytic CO2 reduction to HCOOH

Exploring photocatalysts with efficient charge separation to boost CO2 reduction to valuable chemicals is of great significance. Herein, we develop a PCN-222/CsPbBr3 hybrid with direct Z-scheme heterojunction for high-efficiency photoreduction of CO2 to HCOOH. By delicately controlling the interface of the PCN-222/CsPbBr3 hybrid, effective electron transfer from CsPbBr3 to PCN-222 is achieved, leading to the construction of an internal electric field (IEF). The photo-induced electrons transfer from PCN-222 to CsPbBr3 under visible light irradiation derived from IEF is confirmed by in-situ X-ray photoelectron spectroscopy (XPS), transient absorption (TA), and Kelvin probe force microscopy (KPFM), resulting in the impactful spatial separation of electron-hole pairs. The optimized PCN-222/CsPbBr3 catalyst with direct Z-scheme heterojunction finally offers a 7.0- and 2.1-times enhancement in CO2 photoreduction to HCOOH compared to pristine PCN-222 and CsPbBr3, respectively. This work provides a facile strategy for developing superior perovskite-based CO2RR photocatalysts with both high catalytic activity and selectivity of value-added products.

Photochemical and Photophysical Dynamics of the Aqueous Ferrate(VI) Ion.

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as "green" alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications.