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The intrinsic photophysical origin of lead halide perovskites (LHPs) that are used successfully in optolectronic applications remains hotly debated. Here, by using ultrafast X-ray transient absorption spectroscopy, we successfully tracked the fate of photogenerated charge carriers at room temperature within the thin films of two classic LHPs, namely, MAPbBr3 (MA = CH3NH3) and FAPbBr3 [FA = CH(NH2)2]. We clearly observed in both thin films that the hole polaron is formed by localizing the photogenerated hole at the Br 4p orbital and concurrently distorting the local structure surrounding the Br atom after the photoexcitation. Furthermore, the larger FA cation in the cavity of the [PbBr6]4- octahedral framework induces a stronger hole polaron effect due to the hybridization of its p orbital into valence and conduction bands, correlating with the slower charge carrier recombination dynamics. Our direct experimental observation of the localized hole polaron in perovskites should advance the fundamental comprehension of charge carrier behavior within LHPs and their related devices.

The use of 'water-in-salt' electrolytes has considerably expanded the electrochemical window of aqueous lithium-ion batteries to 3 to 4 volts, making it possible to couple high-voltage cathodes with low-potential graphite anodes1-4. However, the limited lithium intercalation capacities (less than 200 milliampere-hours per gram) of typical transition-metal-oxide cathodes5,6 preclude higher energy densities. Partial7,8 or exclusive9 anionic redox reactions may achieve higher capacity, but at the expense of reversibility. Here we report a halogen conversion-intercalation chemistry in graphite that produces composite electrodes with a capacity of 243 milliampere-hours per gram (for the total weight of the electrode) at an average potential of 4.2 volts versus Li/Li+. Experimental characterization and modelling attribute this high specific capacity to a densely packed stage-I graphite intercalation compound, C3.5[Br0.5Cl0.5], which can form reversibly in water-in-bisalt electrolyte. By coupling this cathode with a passivated graphite anode, we create a 4-volt-class aqueous Li-ion full cell with an energy density of 460 watt-hours per kilogram of total composite electrode and about 100 per cent Coulombic efficiency. This anion conversion-intercalation mechanism combines the high energy densities of the conversion reactions, the excellent reversibility of the intercalation mechanism and the improved safety of aqueous batteries.

Excited with copper: A multi‐method approach was used to determine the ground and excited state structures of a series of CuI photosensitizers containing additional donor moieties. For the first time a pentacoordinated copper center, corresponding to a 4+1‐fold coordination mode, was found in the excited state of the homoleptic complexes. A systematic series of four novel homo‐ and heteroleptic CuI photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers., publishedVersion

Light‐emitting diodes (LEDs) made with quasi‐2D/3D and layered perovskites have undergone an unprecedented surge as their external quantum efficiency (EQE) is rapidly approaching other lighting technologies. Manipulating the charge recombination pathway in semiconductors is highly desirable for improving the device performance. This study reports high‐performance layered perovskites LEDs with benzyl ring as spacer where radiative recombination lifetime is longer, compared with much shorter alkyl chain spacer yields. Based on detailed optical and X‐ray absorption spectroscopy measurements, direct signature of charges localization is observed near the band edge in exchange with the shallow traps in benzyl organics containing layered perovskites. As a result, it boosts the photoluminescence intensity by 7.4 times compared to that made with the alkyl organics. As a demonstration, a bright LED made with the benzyl organics with current efficiency of 23.46 ± 1.52 cd A−1 is shown when the device emits at a high brightness of 6.6 ± 0.93 × 104 cd m−2. The average EQE is 9.2% ± 1.43%, two orders of magnitude higher than the device made with alkyl organics. The study suggests that the choices of organic spacers provide a path toward the manipulation of charge recombination, essential for efficient optoelectronic device fabrications., The charge recombination pathways in layered perovskites are closely related to the choice of the organic spacers. Using phenylethylamine as organic spacer greatly extends the charge recombination lifetime by localizing the hole near the band edge. Efficient Ruddlesden–Popper layered perovskites light emitting‐diodes are thus demonstrated.

Chemistry - a European journal 26(47), 10801 - 10810 (2020). doi:10.1002/chem.202001393, Time���resolved X���ray (Tr���XAS) and optical transient absorption (OTA) spectroscopy on the pico���microsecond timescale coupled with density functional theory calculations are applied to study the light���induced spin crossover processes of a Fe���based macrocyclic complex in solution. Tr���XAS analysis after light illumination shows the formation of a seven���coordinated high���spin quintet metastable state, which relaxes to a six���coordinated high���spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long���lived decay lifetime of approximately 42.6 ��s. Comparative studies with a non���macrocyclic counterpart illustrate a significantly shortened approximately 568���fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven���coordinated high���spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design., Published by Wiley-VCH, Weinheim

International audience; Inspired by the sulphur-rich environment found in active hydrogenase enzymes, a novel Ni-based proton reduction catalyst with pentadentate N2S3 ligand was synthesized. When coupled with a [Ru(bpy)3] 2+ photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared to a homologous counterpart with a tetradentate N2S2 ligand. The mechanistic pathway of photo-induced hydrogen evolution was comprehensively analysed through optical transient absorption (OTA) and time-resolved X-ray absorption spectroscopy (tr-XAS) revealing important electronic and structural changes in the catalytic system during photo-irradiation. It was found that the Ni (II) catalyst undergoes a photo-induced metal-centred reduction to form a Ni (I) intermediate bearing a distorted square bipyramidal geometry. Further kinetic analyses pointed out differences in charge separation dynamics between the pentadentate and tetradentate homologues.

Covalent organic framework (COF) represents an emerging class of porous materials that have exhibited great potential in various applications, particularly in catalysis. In this work, we report a newly designed 2D COF with incorporated Re complex, which exhibits intrinsic light absorption and charge separation (CS) properties. We show that this hybrid catalyst can efficiently reduce CO2 to form CO under visible light illumination with high electivity (98%) and better activity than its homogeneous Re counterpart. More importantly, using advanced transient optical and X-ray absorption spectroscopy and in situ diffuse reflectance spectroscopy, we unraveled three key intermediates that are responsible for CS, the induction period, and rate limiting step in catalysis. This work not only demonstrates the potential of COFs as next generation photocatalysts for solar fuel conversion but also provide unprecedented insight into the mechanistic origins for light-driven CO2 reduction.

Zeolitic imidazolate frameworks (ZIFs) with open-shell transition metal nodes represent a promising class of highly ordered light harvesting antennas for photoenergy applications. However, their charge transport properties within the framework, the key criterion to achieve efficient photoenergy conversion, are not yet explored. Herein, we report the first direct evidence of a charge transport pathway through node-to-node communication in both ground state and excited state ZIFs using the combination of paramagnetic susceptibility measurements and time-resolved optical and X-ray absorption spectroscopy. These findings provide unprecedented new insights into the photoactivity and charge transport nature of ZIF frameworks, paving the way for their novel application as light harvesting arrays in diverse photoenergy conversion devices.

Cobalt–carbon bond cleavage is crucial to most natural and synthetic applications of the cobalamin class of compounds, and here we present the first direct electronic and geometric structural characteristics of intermediates formed following photoexcitation of methylcobalamin (MeCbl) using time-resolved X-ray absorption spectroscopy (XAS). We catch transients corresponding to two intermediates, in the hundreds of picoseconds and a few microseconds. Highlights of the picosecond intermediate, which is reduced in comparison to the ground state, are elongation of the upper axial Co–C bond and relaxation of the corrin ring. This is not so with the recombining photocleaved products captured at a few microseconds, where the Co–C bond almost (yet not entirely) reverts to its ground state configuration and a substantially elongated lower axial Co–NIm bond is observed. The reduced cobalt site here confirms formation of methyl radical as the photoproduct.

Metal–organic frameworks (MOFs) have emerged as novel scaffolds for artificial photosynthesis due to their unique capability in incorporating homogeneous photosensitizer and catalyst to their robust heterogeneous matrix. In this work, we report the charge separation dynamics between molecular Ru-photosensitizer and Pt-catalyst, both of which were successfully incorporated into a Zr-MOF that demonstrates excellent activity and stability for light-driven H2 generation from water. Using optical transient absorption (OTA) spectroscopy, we show that charge separation in this hybrid MOF occurs via electron transfer (ET) from Ru-photosensitizer to Pt-catalyst. Using Pt L3-edge X-ray transient absorption (XTA) spectroscopy, we observed the intermediate reduced Pt site, directly confirming the formation of charge separated state due to ET from Ru-photosensitizer and unraveling their key roles in photocatalysis.

Spinel nickel ferrite (NiFe2O4) emerges as a promising low-cost catalyst for water splitting but it usually shows low catalytic activity because of its limited number of active sites and poor conductivity. For the first time, herein we have successfully overcome its weaknesses using defect engineering approach by creating oxygen vacancies in NiFe2O4. The existence of oxygen vacancy not only shifts up the d-band center, strengthens the adsorption of H2O, and thus provides more active catalytic sites, but also tunes the electron configuration and creates massive number of defective donor states in the band gap to facilitate charge transfer processes. The optimal defective catalyst showed significantly enhanced catalytic OER performance with an OER overpotential as low as 0.35 V at 10 mA cm−2 and a Tafel slope of only ∼40 mV dec−1. Moreover, the impressive specific mass and area current density of 17.5 A g−1 and 0.106 A m−2 at 1.58 V vs. RHE have been achieved, which are ∼23 and ∼36 times higher than that of defect-free counterpart, respectively.

In this work, we report the direct correlation of photoinduced carrier dynamics and electronic structure of CuInS2 (CIS) nanoparticles (NPs) using the combination of multiple spectroscopic techniques including steady-state X-ray absorption spectroscopy (XAS), optical transient absorption (OTA), and X-ray transient (XTA) absorption spectroscopy. XAS results show that CIS NPs contain a large amount of surface Cu atoms with ≪four-coordination, which is more severe in CIS NPs with shorter nucleation times, indicating the presence of more Cu defect states in CIS NPs with smaller size particles. Using the combination of OTA and XTA spectroscopy, we show that electrons are trapped at states with mainly In or S nature while holes are trapped in sites characteristic of Cu. While there is no direct correlation of ultrafast trapping dynamics with NP nucleation time, charge recombination is significantly inhibited in CIS NPs with larger particles. These results suggest the key roles that Cu defect sites play in carri...

The presence of long-lived charge-separated excited states in metal–organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these charge-separated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100(Fe), composed of Fe3-μ3-oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ3-oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to furthe...

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of V-k center (Br-2(-) dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of, similar to 59 mu s, which is similar to 3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.

International audience; Inspired by the sulphur-rich environment found in active hydrogenase enzymes, a novel Ni-based proton reduction catalyst with pentadentate N 2 S 3 ligand was synthesized. When coupled with a [Ru(bpy) 3 ] 2+ photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared to a homologous counterpart with a tetradentate N 2 S 2 ligand. The mechanistic pathway of photo-induced hydrogen evolution was comprehensively analysed through optical transient absorption (OTA) and time-resolved X-ray absorption spectroscopy (tr-XAS) revealing important electronic and structural changes in the catalytic system during photo-irradiation. It was found that the Ni (II) catalyst undergoes a photo-induced metal-centred reduction to form a Ni (I) intermediate bearing a distorted square bipyramidal geometry. Further kinetic analyses pointed out differences in charge separation dynamics between the pentadentate and tetradentate homologues.

We report the light-induced electronic and geometric changes taking place within a heteroleptic CuI photosensitizer, namely [(xant)Cu(Me2 phenPh2 )]PF6 (xant=xantphos, Me2 phenPh2 =bathocuproine), by time-resolved X-ray absorption spectroscopy in the ps-μs time regime. Time-resolved X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis enabled the elucidation of the electronic and structural configuration of the copper center in the excited state as well as its decay dynamics in different solvent conditions with and without triethylamine acting as a sacrificial electron donor. A three-fold decrease in the decay lifetime of the excited state is observed in the presence of triethylamine, showing the feasibility of the reductive quenching pathway in the latter case. A prominent pre-edge feature is observed in the XANES spectrum of the excited state upon metal to charge ligand transfer transition, showing an increased hybridization of the 3d states with the ligand p orbitals in the tetrahedron around the Cu center. EXAFS and density functional theory illustrate a significant shortening of the Cu-N and an elongation of the Cu-P bonds together with a decrease in the torsional angle between the xantphos and bathocuproine ligand. This study provides mechanistic time-resolved understanding for the development of improved heteroleptic CuI photosensitizers, which can be used for the light-driven production of hydrogen from water.

Investigating the photoinduced electronic and structural response of bistable molecular building blocks incorporating transition metals in solution phase constitutes a necessary stepping stone for steering their properties toward applications and performance optimizations. This work presents a detailed X-ray transient absorption (XTA) spectroscopy study of a prototypical spin crossover (SCO) complex [FeII(mbpy)3]2+ (where mbpy = 4,4′-dimethyl-2,2′-bipyridine) with an [FeIIN6] first coordination shell in water (H2O) and acetonitrile (CH3CN). The unprecedented data quality of the XTA spectra together with the direct fitting of the difference spectra in k space using a large number of scattering paths enables resolving the subtle difference in the photoexcited structures of an FeII complex in two solvents for the first time. Compared to the low spin (LS) 1A1 state, the average Fe–N bond elongations for the photoinduced high spin (HS) 5T2 state are found to be 0.181 ± 0.003 A in H2O and 0.199 ± 0.003 A in CH3...

Artificial conversion of sunlight to chemical fuels has attracted attention for several decades as a potential source of clean, renewable energy. We recently found that CdSe quantum dots (QDs) and simple aqueous Ni2+ salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen. Ultrafast transient absorption spectroscopy studies of electron transfer (ET) processes from the QDs to the Ni catalysts reveal extremely fast ET, and provide a fundamental explanation for the exceptional photocatalytic H2 activity. Additionally, by studying H2 production of the Ni catalyst with CdSe/CdS nanoparticles of various structures, it was determined that surface charge density plays an important role in charge transfer and ultimately H2 production activity.

In Fig. 3e of this Letter, the labels "Br-Cl1" and "Br-Cl2" should read "Br-Br1" and "Br-Br2", respectively. In the Methods section 'Preparation of electrodes', the phrase "anhydrous LiBr/LiCl was replaced by LiBr·H2O (99.95%; Sigma-Aldrich) and LiCl (99.95%; Sigma-Aldrich)" should read "anhydrous LiBr/LiCl was replaced by LiBr·H2O (99.95%; Sigma-Aldrich) and LiCl·H2O (99.95%; Sigma-Aldrich)". These errors have been corrected online.