Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined

Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy

Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2′-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals

Carotenoid-protein interaction alters the S-1 energy of hydroxyechinenone in the Orange Carotenoid Protein

The Orange Carotenoid Protein (OCP) is a photoactive water soluble protein that is crucial for photoprotection in cyanobacteria. When activated by blue-green light, it triggers quenching of phycobilisome fluorescence and regulates energy flow from the phycobilisome to the reaction center. The OCP contains a single pigment, the carotenoid 3'-hydroxyechinenone (hECN). Binding to the OCP causes a conformational change in hECN leading to an extension of its effective conjugation length. We have determined the S-1 energy of hECN in organic solvent and compared it with the S-1 energy of hECN bound to the OCP. In methanol and n-hexane, hECN has an S-1 energy of 14,300 cm(-1), slightly higher than carotenoids with shorter conjugation lengths such as zeaxanthin or beta-carotene; this is consistent with the proposal that the presence of the conjugated carbonyl group in hECN increases its Si energy. The S-1 energy of hECN in organic solvent is independent of solvent polarity. Upon binding to the OCP, the S-1 energy of hECN is further increased to 14,700 cm(-1), underscoring the importance of protein binding which twists the conjugated carbonyl group into s-trans conformation and enhances the effect of the carbonyl group. Activated OCP, however, has an S-1 energy of 14,000 cm(-1), indicating that significant changes in the vicinity of the conjugated carbonyl group occur upon activation. (C) 2012 Elsevier B.V. All rights reserved

Quantum dot photodegradation due to CdSe-ZnO charge transfer: Transient absorption study

We study changes in ultrafast transient absorption due to photodegradation of quantum dots attached to ZnO nanowire. The time-resolved measurements reveal impact of photodegradation on three distinct kinetic components present in transient absorption tau similar to 7 ps, 80 ps, and 7.5 ns). In addition, we observe superlinear dependence of photodegradation rate on concentration of excited electrons. The data are used to evaluate the mean electron back-recombination time of similar to 1 mu s. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729382

KiMoPack : A python Package for Kinetic Modeling of the Chemical Mechanism

Herein, we present KiMoPack, an analysis tool for the kinetic modeling of transient spectroscopic data. KiMoPack enables a state-of-the-art analysis routine including data preprocessing and standard fitting (global analysis), as well as fitting of complex (target) kinetic models, interactive viewing of (fit) results, and multiexperiment analysis via user accessible functions and a graphical user interface (GUI) enhanced interface. To facilitate its use, this paper guides the user through typical operations covering a wide range of analysis tasks, establishes a typical workflow and is bridging the gap between ease of use for less experienced users and introducing the advanced interfaces for experienced users. KiMoPack is open source and provides a comprehensive front-end for preprocessing, fitting and plotting of 2-dimensional data that simplifies the access to a powerful python-based data-processing system and forms the foundation for a well documented, reliable, and reproducible data analysis

Balancing Electron Transfer and Surface Passivation in Gradient CdSe/ZnS Core-Shell Quantum Dots Attached to ZnO

Core-shell (CS) quantum dots (QDs) are promising light absorbers for solar cell applications mainly because of their enhanced photostability compared with bare QDs. Moreover, the superb photostability can be combined with a low number of defects by using CSQDs with a gradient composition change from the core to the shell. Here, we study electron injection from the gradient CSQDs to ZnO nanoparticles. We observe the typical exponential injection rate dependence on the shell thickness (beta = 0.51 angstrom(-1)) and discuss it in light of previously published results on step-like CSQDs. Despite the rapid drop in injection rates with shell thickness, we find that there exists an optimum thickness of the shell layer at similar to 1 nm, which combines high injection efficiency (>90%) with a superior passivation of QDs