Roaming-mediated isomerization is a universal reaction mechanism in photochemistry, yet solvent-dependent pathways of roaming intermediates remain poorly understood, particularly for environmentally relevant halogen compounds involved in ozone depletion. Here, using femtosecond time-resolved X-ray solution scattering, we resolve the solvent-dependent roaming dynamics of CHBr3 in methanol and methylcyclohexane. By combining multi-method experimental analysis, machine learning-assisted ab initio molecular dynamics simulations, and density functional theory calculations, we uncover distinct solvent-steered reaction pathways. In methanol, roaming enhances solute-solvent interactions, leading to solvolysis before a stable isomer forms. In methylcyclohexane, roaming facilitates isomerization to a long-lived iso-CHBr2-Br product. Direct dissociation into CHBr2 + Br competes with both pathways in either solvent. By tracking bond-length oscillations and angular dynamics in real time, we visualize how the condensed-phase environment governs the branching ratio between competing pathways. Our findings establish solute-solvent interactions as key factors controlling roaming-mediated reactions in CHBr3, with broad implications for photochemical outcomes in solution.

Multi-electron donation remains a challenge for CO2 photocatalytic conversion to multi-electron products due to the efficient Auger recombination or annihilation at multiple excitation conditions for conventional molecules or semiconductor photocatalysts. In this paper, we demonstrated possible multi-electron donation within a quantum dot (QD)/metal complex hybrid photocatalyst system when multiple metal complexes are attached to one QD. Structural characterization first confirmed the number of [Re(4,4 '-R-bpy)(CO)3Br] catalysts (bpy = 2,2 ' bipyridine) attached per QD. The time-dependent density functional theory (TD-DFT) calculation identified that photoexcited electrons directly reside on the ligand of the metal complexes. Combining the studies from transient visible and infrared spectroscopies, we reveal that the efficient multi-electron transfer from one excited QD can be achieved when two metal complexes are anchored to one QDs with an electron injection time shorter than one ps. The transferred electrons are localized at the Re-complex while the holes are delocalized in the QD with a long lifetime. This can guarantee efficient multi-electron donation during photocatalytic reactions. Consequently, such multiple catalysts attachment facilitates the CO2 photocatalytic reduction, where unconventional methane production involving the donation of eight electrons has been significantly enhanced with an enhanced CH4 evolution rate of 130 mu mol/g/h and apparently quantum yield of 1.7 % in acetonitrile medium with triethanolamine as sacrificial electron donor. This work establishes a strategy to control CO2 reduction products via tuning the multi-electron donation pathways through molecular engineering.

In charge-transfer (CT) chromophores built around transition metal ions, energy losses caused by relaxation from the absorbing high-energy excited-state (HE-ES) to the metastable lowest-energy excited-state (LE-ES) inherently limits their functionalities. With emerging hot CT chromophores, intramolecular charge delocalization is utilized before complete relaxation to the LE-ES. However, rationalizing their design is largely contingent upon understanding their ultrafast dynamics in multidimensional energy landscapes. Here, transient optical absorption and X-ray emission spectroscopies are combined to probe the photoinduced dynamics in a prototypical Ni(II) porphyrin with electronic and spin sensitivities on the femtosecond timescale. Measurements across the HE-ES → LE-ES relaxation resolve an intermediate CT step involving Ni(II) and the porphyrin ring. Full-dimensional trajectory surface hopping dynamics simulations unveil clear correlations between CT and bifurcations in the excited-state landscape. These results establish Ni(II) porphyrins and other open-shell metalloporphyrins as a rich platform for developing hot CT chromophores to target challenging endergonic photoreactions.

Metalloporphyrins with open d-shell ions can drive biochemical energy cycles. However, their utilization in photoconversion is hampered by rapid deactivation. Mapping the relaxation pathways is essential for elaborating strategies that can favorably alter the charge dynamics through chemical design and photoexcitation conditions. Here, we combine transient optical absorption spectroscopy and transient X-ray emission spectroscopy with femtosecond resolution to probe directly the coupled electronic and spin dynamics within a photoexcited nickel porphyrin in solution. Measurements and calculations reveal that a state with charge-transfer character mediates the formation of the thermalized excited state, thereby advancing the description of the photocycle for this important representative molecule. More generally, establishing that intramolecular charge-transfer steps play a role in the photoinduced dynamics of metalloporphyrins with open d-shell sets a conceptual ground for their development as building blocks capable of boosting nonadiabatic photoconversion in functional architectures through "hot" charge transfer down to the attosecond time scale.

Effective photoinduced charge transfer makes molecular bimetallic assemblies attractive for applications as active light induced proton reduction systems. For a more sustainable future, development of competitive base metal dyads is mandatory. However, the electron transfer mechanisms from the photosensitizer to the proton reduction catalyst in base metal dyads remain so far unexplored. We study a Fe-Co dyad that exhibits photocatalytic H2 production activity using femtosecond X-ray emission spectroscopy, complemented by ultrafast optical spectroscopy and theoretical time-dependent DFT calculations, to understand the electronic and structural dynamics after photoexcitation and during the subsequent charge transfer process from the FeII photosensitizer to the cobaloxime catalyst. Using this novel approach, the simultaneous measurement of the transient Kalpha X-ray emission at the iron and cobalt K-edges in a two-colour experiment is enabled making it possible to correlate the excited state dynamics to the electron transfer processes. The methodology, therefore, provides a clear and direct spectroscopic evidence of the Fe->Co electron transfer responsible for the proton reduction activity.

The effective conversion of carbon dioxide (CO2) into valuable chemical fuels relies significantly on the donation of multiple electrons. Its efficiency is closely linked to both the density and lifetime of excited charge carriers. In this study, a hybrid catalyst system comprising covalently bonded InP/ZnS quantum dots (QDs) and Re-complexes is showcased. The electronic band alignment between the QDs and the Re-complexes is revealed to dominate the multi-electron transfer process for photocatalytic conversion to methane (CH4). Notably, the size of the QDs is found to be a determining parameter. Among the three QD sizes investigated, transient absorption spectroscopy studies unveil that rapid multi-electron transfer from the QDs to the Re-catalyst occurs in smaller QDs (2.3 nm) due to the substantial driving force. Consequently, the photocatalytic conversion of CO2 to CH4 is significantly enhanced with a turnover number of 6, corresponding to the overall apparent quantum yield of approximate to 1%. This research underscores the possibilities of engineering multi-electron transfer by manipulating the electronic band alignment within a catalytic system. This can serve as a guide for optimizing photocatalytic CO2 reduction.

Recently, cesium bismuth halide perovskites have emerged as potential substitutes to their counterparts, cesium lead halide perovskites, owing to their low toxicity. However, the photophysics of cesium-bismuth halides nanocrystals (NCs) have not yet been fully rationalized because their structures remain highly debated. The ultraviolet-visible (UV-vis) absorption along with other photophysical properties such as the nature and lifetime of the excited states vary considerably across the previous reports. Here, we successfully synthesize pure Cs3BiBr6 and Cs3Bi2Br9 NCs via a modified hot-injection method, where the structure can be easily controlled by tuning the reaction temperature. The UV-vis absorption spectrum of the pure Cs3Bi2Br9 NCs features two characteristic peaks originating from the absorption of the first exciton and second exciton, respectively, which ultimately clarifies the debate in the previous reports. Using femtosecond transient absorption spectroscopy, we systematically investigate the excited state dynamics of the Cs3Bi2Br9 NCs and reveal that the photoexcited carriers undergo a self-trapping process within 3 ps after excitation. More intriguingly, the Cs3Bi2Br9 NCs prepared by this method show much better photostability than those prepared by the ligand-assisted reprecipitation process. Photodetectors based on these Cs3Bi2Br9 NCs show a sensitive light response, demonstrating the definite potential for breakthrough optoelectronic applications.

The concept of hot carrier solar cells (HCSCs) has been proposed as a promising yet elusive path toward high-performance photovoltaics (PV), capable of surpassing the Shockley-Queisser limit by recycling energy that would otherwise be lost during thermalization. Lead halide perovskites (LHPs) have emerged as highly promising materials for PV applications. The reports of slow hot carrier (HC) cooling in these materials have ignited discussions of their potential in realizing HCSCs. Here we summarize the key findings regarding HC dynamics in LHPs, as revealed by numerous studies using advanced time-resolved spectroscopies. We also emphasize the interconnected mechanisms involved in HC cooling. In addition, we discuss the influence of nanostructuring on HC cooling dynamics in LHPs and suggest that two-dimensional LHPs, with their inherent quantum well structure, might exhibit modified phononic features and altered carrier-phonon coupling strength. We conclude by critically discussing the prospects for HCSCs, taking into account our current understanding of HC cooling and excitation conditions under sunlight.

Recently, research on the edge states of 2D lead halide perovskites (LHPs) has been attracting much attention. The lower‐energy edge state (LES) is believed to provide an efficient pathway for the dissociation of photoexcited excitons. However, the mechanism of the LES formation remains controversial, and studies that establish precisely the local electronic properties are lacking. Herein, the first study of spatially resolved electronic structures in 2D LHP single‐crystal flakes by X‐ray photoemission electron microscopy is presented, specifically identifying the contribution from the edge area. The results show that blueshifts occur in the Pb 5d core‐level peaks at the edge area compared to the interior area with much less difference in I 4d core‐level peaks. The shift becomes more pronounced as n varies from 1 to 3 (≈0.2–1.0 eV). This phenomenon is attributed to the surface restructuring of the edge area induced by the release of mechanical strain through lattice expansion. This work provides an important reference on the origin of the LES of 2D LHPs and is beneficial for future optoelectronic device applications.

Single‐ion magnets (SIMs) constitute the ultimate size limit in the quest for miniaturizing magnetic materials. Several bottlenecks currently hindering breakthroughs in quantum information and communication technologies could be alleviated by new generations of SIMs displaying multifunctionality. Here, ultrafast optical absorption spectroscopy and X‐ray emission spectroscopy are employed to track the photoinduced spin‐state switching of the prototypical complex [Co(terpy) 2 ] 2+ (terpy = 2,2′:6′,2″‐terpyridine) in solution phase. The combined measurements and their analysis supported by density functional theory (DFT), time‐dependent‐DFT (TD‐DFT) and multireference quantum chemistry calculations reveal that the complex undergoes a spin‐state transition from a tetragonally elongated doublet state to a tetragonally compressed quartet state on the femtosecond timescale, i.e., it sustains ultrafast Jahn‐Teller (JT) photoswitching between two different spin multiplicities. Adding new Co‐based complexes as possible contenders in the search for JT photoswitching SIMs will greatly widen the possibilities for implementing magnetic multifunctionality and eventually controlling ultrafast magnetization with optical photons.

CsPbI 3 nanocrystals (CPI NCs) have become a trending research topic due to their impressive potential in functional optoelectronic devices and optical gain applications. Their optical responses are governed by carrier dynamics, which is greatly influenced by temperature and corresponding phase structure due to the effects of inherently electron‐phonon coupling. Notably, CPI NCs have been identified to adopt an unexpectedly stable cubic phase from room temperature to liquid helium temperature. Here, using in situ cryogenic electron diffraction measurements, it is unambiguously demonstrated that CPI NCs undergo consecutive cubic‐tetragonal‐orthorhombic phase transitions from 298 to 100 K. The corresponding temperature‐dependent multiexcitonic dynamics are investigated in each phase by combining time‐resolved photoluminescence and transient absorption spectroscopy. In addition to the temperature dependency, the lifetime of both excitons and biexcitons evidently depends on the phase structures of the CPI NCs, highlighting the crucial effect of crystal structure on the carrier dynamics. Moreover, the biexciton binding energy increases with higher crystal symmetry due to the decrease of the dielectric constant. The findings shed light on the structural phase transition and its relationship to the carrier dynamics in all‐inorganic perovskite NCs, which provides critical insight into the structure‐performance relationship in CPI NCs for promising applications in optoelectronic devices.

X-ray free-electron lasers (XFEL) provide unique opportunities for hard X-ray emission spectroscopy (XES) in solution phase by enabling femtosecond temporal resolution. The high brilliance and MHz repetition rate of such sources drive the development of specialized sample delivery systems to overcome the challenges of sample degradation, as well as major advances of multiple-gain detectors to fully utilize the high dynamic range required for X-ray detection. This paper addresses the experimental limitations that arise in a typical XES experiment performed on fast liquid jets at the MHz repetition rate of the European XFEL. Using a low-noise gain-switching JUNGFRAU detector allows taking advantage of the high dynamic range and high sensitivity, but restricts a single-shot detection due to the limited frame rate. The gain transition regions of the detector response may exhibit nonlinear behavior that can be identified and efficiently corrected using the internal reference-based normalization and linearization approaches described here.

Hot carrier (HC) cooling accounts for the significant energy loss in lead halide perovskite (LHP) solar cells. Here, we study HC relaxation dynamics in Mn-doped LHP CsPbI3 nanocrystals (NCs), combining transient absorption spectroscopy and density functional theory (DFT) calculations. We demonstrate that Mn2+ doping (1) enlarges the longitudinal optical (LO)-acoustic phonon bandgap, (2) enhances the electron-LO phonon coupling strength, and (3) adds HC relaxation pathways via Mn orbitals within the bands. The spectroscopic study shows that the HC cooling process is decelerated after doping under band-edge excitation due to the dominant phonon bandgap enlargement. When the excitation photon energy is larger than the optical bandgap and the Mn2+ transition gap, the doping accelerates the cooling rate owing to the dominant effect of enhanced carrier-phonon coupling and relaxation pathways. We demonstrate that such a phenomenon is optimal for the application of hot carrier solar cells. The enhanced electron-LO phonon coupling and accelerated cooling of high-temperature hot carriers efficiently establish a high-temperature thermal quasi-equilibrium where the excessive energy of the hot carriers is transferred to heat the cold carriers. On the other hand, the enlarged phononic band-gap prevents further cooling of such a quasi-equilibrium, which facilitates the energy conversion process. Our results manifest a straightforward methodology to optimize the HC dynamics for hot carrier solar cells by element doping.

Rhenium(I)-carbonyl-diimine complexes have emerged as promising photocatalysts for carbon dioxide reduction with covalent organic frameworks recognized as perfect sensitizers and scaffold support. Such Re complexes/covalent organic frameworks hybrid catalysts have demonstrated high carbon dioxide reduction activities but with strong excitation energy-dependence. In this paper, we rationalize this behavior by the excitation energy-dependent pathways of internal photo-induced charge transfer studied via transient optical spectroscopies and time-dependent density-functional theory calculation. Under band-edge excitation, the excited electrons are quickly injected from covalent organic frameworks moiety into catalytic Rhenium I center within picosecond but followed by fast backward geminate recombination. While under excitation with high-energy photon, the injected electrons are located at high-energy levels in Rhenium I centers with longer lifetime. Besides those injected electrons to Rhenium I center, there still remain some long-lived electrons in covalent organic frameworks moiety which is transferred back from Rhenium I . This facilitates the two-electron reaction of carbon dioxide conversion to carbon monoxide.

We investigate HC cooling in two-dimensional perovskite single-crystals by applying two complementary ultrafast techniques and observe spatial sensitivity of cooling.

Metal halide perovskites have emerged as promising materials for optoelectronic applications in the last decade. A large amount of effort has been made to investigate the interplay between the crystalline lattice and photoexcited charge carriers as it is vital to their optoelectronic performance. Among them, ultrafast laser spectroscopy has been intensively utilized to explore the charge carrier dynamics of perovskites, from which the local structural information can only be extracted indirectly. Here, we have applied a time-resolved X-ray diffraction technique to investigate the structural dynamics of prototypical two-dimensional lead-free halide perovskite Cs3Bi2Br9 nanoparticles across temporal scales from 80 ps to microseconds. We observed a quick recoverable (a few ns) photoinduced microstrain up to 0.15% and a long existing lattice expansion (∼a few hundred nanoseconds) at mild laser fluence. Once the laser flux exceeds 1.4 mJ/cm2, the microstrain saturates and the crystalline phase partially transfers into a disordered phase. This photoinduced transient structural change can recover within the nanosecond time scale. These results indicate that photoexcitation of charge carriers couples with lattice distortion, which fundamentally affects the dielectric environment and charge carrier transport.

Visualizing on the atomic scale the full extent of the electronic and structural changes that are triggered by charge separation and subsequent charge transport is crucial for developing the rational design of novel sensitizers and catalysts. The rapid progress of ultrafast X-ray techniques, both at synchrotrons (100 ps) and at X-ray free electron laser facilities (sub-ps) have equipped the scientific community with novel analytical tools that are capable of delivering unique feedback with spin and elemental sensitivity about the highly-correlated nonadiabatic dynamics that follow photoabsorption. The present talk will review the technical state-of-the art and the ongoing developments that are currently taking place. The talk will also highlight several of the recent results that have been obtained for intramolecular and interfacial processes of relevance for the function and optimization of advanced materials.

Rhenium(I)-carbonyl-diimine complexes are promising photocatalysts for CO 2 reduction. Covalent organic frameworks (COFs) can be perfect sensitizers to enhance the reduction activities. Here we investigated the excited state dynamics of COF (TpBpy) with 2,2'-bipyridine incorporating Re(CO) 5 Cl (Re-TpBpy) to rationalize the underlying mechanism. The time-dependent DFT calculation first clarified excited state structure of the hybrid catalyst. The studies from transient visible and infrared spectroscopies revealed the excitation energy-dependent photo-induced charge transfer pathways in Re-TpBpy. Under low energy excitation, the electrons at the LUMO level are quickly injected from Bpy into Re I center (1–2 ps) followed by backward recombination (13 ps). Under high energy excitation, the hot-electrons are first injected into the higher unoccupied level of Re I center (1–2 ps) and then slowly relax back to the HOMO in COF (24 ps). There also remains long-lived free electrons in the COF moiety. This explained the excitation energy-dependent CO 2 reduction performance in our system.

The physical origin of sub-band gap photoluminescence in Ruddlesden-Poppers two-dimensional (2D) lead halide perovskites (LHPs) is still under debate. In this paper, we studied the photoluminescence features from two different facets of 2D LHP single crystals: the in-plane facet (IF) containing the 2D inorganic layers and the facet perpendicular to the 2D layers (PF). At the IF, the free carriers (FCs) dominate due to the weak electron-phonon coupling in a symmetric lattice. At the PF, the strain accumulation along the 2D layers enhances the electron-phonon coupling and facilitates self-trapped exciton (STE) formation. The time-resolved PL studies indicate that free carriers (FCs) at the IF can move freely and display the trapping by the intrinsic defects. The STEs at the PF are not likely trapped by the defects due to the reduced mobility. However, with increasing STE density, the STE transport is promoted, enabling the trapping of STE by the intrinsic defects.

We present a small-angle X-ray scattering (SAXS) study of the anisotropic photoinduced growth of silver (Ag) nanoprisms in aqueous dispersions. The growth of nearly spherical (<10 nm) Ag particles into large (>40 nm) and thin (<10 nm) triangular nanoprisms induced by 550 nm laser is followed in terms of particle size using indirect and direct methods for irradiation times up to 150 min. During the process, the surface-to-volume ratio of the particles decreased. The SAXS data of the initial solution fit well to the model of polydisperse spheres with pronounced average diameters around 7.4 nm and 10 nm. The data after 45 min irradiation fit well to the model containing approximately the same amount of the initial particles and the end product, the nanoprisms.