Excitation-energy transfer is a key step in processes such as photosynthesis that convert light into other forms of energy. Time-dependent density functional theory (DFT) in real time is ideal for ...the first-principles simulation of such processes due to its computational efficiency. We here demonstrate how real-time DFT can be used for analyzing excitation-energy transfer from first-principles. We discuss several measures of energy transfer that are based solely on the time-dependent density, are well founded in the DFT framework, allow for intuitive understanding and visualization, and reproduce important limiting cases of an analytical model. We demonstrate their usefulness in calculations for model systems, both with static nuclei and in the context of DFT-based Ehrenfest dynamics.
Angle-resolved photoemission spectroscopy has been developed to a very high accuracy. However, effects that depend sensitively on the state of the emitted photoelectron were so far hard to compute ...for real molecules. We here show that the real-time propagation approach to time-dependent density functional theory allows us to obtain final-state effects consistently from first principles and with an accuracy that allows for the interpretation of experimental data. In a combined theoretical and experimental study we demonstrate that the approach captures three hallmark effects that are beyond the final-state plane-wave approximation: emission perpendicular to the light polarization, circular dichroism in the photoelectron angular distribution, and a pronounced energy dependence of the photoemission intensity.
The rare earths (REs) are a family of 17 elements that exhibit pronounced chemical similarities as a group, while individually expressing distinctive and varied electronic properties. These atomistic ...electronic properties are extraordinarily useful and motivate the application of REs in many technologies and devices. From their discovery to the present day, a major challenge faced by chemists has been the separation of RE elements, which has evolved from tedious crystallization to highly engineered solvent extraction schemes. The increasing incorporation and dependence of REs in technology have raised concerns about their sustainability and motivated recent studies for improved separations to achieve a circular RE economy.
Lanthanide Photocatalysis Qiao, Yusen; Schelter, Eric J
Accounts of chemical research,
11/2018, Letnik:
51, Številka:
11
Journal Article
Recenzirano
Conspectus The use of earth-abundant, cheap, potent, and readily available lanthanide photocatalysts provides an opportunity to complement or even replace rare and precious metal photosensitizers. ...Moreover, lanthanide photosensitizers have been demonstrated for the generation of a variety of reactive species, including aryl radicals, alkyl radicals, and others, by single-electron-transfer (SET) and hydrogen atom transfer (HAT) pathways under mild reaction conditions. Some lanthanide photocatalysts have unprecedented reducing power from their photoexcited states, achieving the activation of challenging organic substrates that have not otherwise been activated by reported organic or transition-metal photosensitizers. In this Account, we describe our recent advances in the rational design and strategic application of lanthanide photo(redox)catalysis. Our research goals include understanding the photophysics of lanthanide luminophores and incorporating them into new photocatalysis. Among the lanthanides, we have focused on cerium because of the doublet to doublet 4f → 5d excitation and emission, which affords good conservation of energy without losses through spin-state changes, as well as a large natural abundance of that element. We have performed structural, spectroscopic, computational, and reactivity studies to demonstrate that luminescent Ce(III) guanidinate–amide complexes can mediate photocatalytic C(sp3)–C(sp3) bond forming reactions. Taking advantage of the strong reducing power of the cerium excited states and the cerium–halogen bond forming enthalpies, we determined that the reactive, excited-state cerium metalloradical abstracts chloride anion from benzyl chloride to generate the benzyl radical. To control and predict the photocatalytic reactivities, we have also performed photophysical and photochemical studies on a series of mixed-ligand Ce(III) guanidinate–amide and guanidinate–aryloxide complexes to establish structure–property relationship for Ce(III) photocatalysts. We discovered that the emission color is directly related to ligand type and rigidity of the coordination sphere and that the photoluminescent quantum yield is correlated to variation in steric encumbrance around the cerium centers. The low excited-state reduction potentials (E 1/2 * ≈ −2.1 to −2.9 V versus Cp2Fe0/+) and relatively fast quenching rates (k q ≈ 107 M–1 s–1) toward aryl halides enabled the Ce(III) guanidinate–amide complexes to participate in photocatalytic C(sp2)–C(sp2) bond forming reactions through either inner-sphere or outer-sphere SET processes. We have also reported a simple, potent, and air-stable ultraviolet A photoreductant, the hexachlorocerate(III) anion (CeIIICl63–). This complex is a potent photoreductant (E 1/2 * ≈ −3.45 V versus Cp2Fe0/+) and exhibits a fast quenching kinetics (k q ≈ 109–1010 M–1 s–1) toward organohalogens. The CeIVCl62− redox partner can also act as a potent photo-oxidant though a (presumably) long-lived chloride-to-cerium(IV) charge transfer excited state (ε = ∼6000 M–1 cm–1), that was used to turnover photocatalytic dechlorination and Miyuara borylation reactions. With CeIIICl63–, we achieved efficient photoinduced dehalogenation and borylation of unactivated aryl chlorides with broad substrate scope, through formally two-photon cycles where both Ce(III) and Ce(IV) act as photocatalysts. Lanthanide photoredox catalysis is now being applied in several contexts for reactions including photocatalytic dehydrogenation of amines, alkoxy-radical-mediated C–C bond cleavage and amination of alkanols, and C–H activation of alkanes. Overall, simple and potent lanthanide photocatalysis is expected to find practical applications in organic synthesis, pharmaceutical development, and small molecule activation.
The first photoinduced carbon(sp2)–heteroatom bond forming reaction by a rare‐earth‐metal photoreductant, a Miyaura borylation, has been achieved. This simple, scalable, and novel borylation method ...that makes use of the hexachlorocerate(III) anion (CeIIICl63−, derived from CeCl3) has a broad substrate scope and functional‐group tolerance and can be conducted at room temperature. Combined with Suzuki–Miyaura cross‐coupling, the method is applicable to the synthesis of various biaryl products, including through the use of aryl chloride substrates.
Salt of the earth: The first photoinduced Miyaura borylation mediated by a rare‐earth‐metal photoreductant, namely the hexachlorocerate(III) anion (CeIIICl63−), is reported. This operationally simple, scalable, and practical borylation method has a broad substrate scope and functional‐group tolerance.
The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising Ce
Cl
(OR)
Ce
, ...cerium(IV); OR, -OCH
or -OCCl
CH
was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by Ce
-OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C-H) activation of alkanes is instead mediated by the photocatalyst NEt
CeCl
(NEt
, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C-H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of Cl•alcohol adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.
Inorganic materials are critical components of clean energy technology. For example, rare earths are key for the function of electric car batteries and in permanent magnets used in wind turbines, and ...palladium helps to reduce harmful exhaust in automotive three-way catalysts. Many of the critical elements for these materials are of low abundance in the earth’s crust, found in few places globally, and/or require energy- and resource-intensive purification. By comparison, many of these elements are concentrated in waste electrical and electronic equipment, which represents an attractive secondary resource. Inorganic chemists are ideally positioned to develop new chemistry and greener processes that are more efficient and use less hazardous reagents to separate high-value metals from waste electronics. The purpose of this Viewpoint is to highlight recent discoveries in fundamental inorganic chemistry that can contribute to new recycling technologies for gold, lithium, palladium, germanium, and rare earths, especially using simple approaches in solid–liquid extraction. Such fundamental studies are expected to help close metal supply chain loops and create circular economies.
Two complete mixed-ligand series of luminescent CeIII complexes with the general formulas (Me3Si)2NC(N i Pr)2 x CeIIIN(SiMe3)23–x (x = 0, 1-N; x = 1, 2-N, x = 2, 3-N; x = 3, 4) and (Me3Si)2NC(N i ...Pr)2 x CeIII(OAr)3–x (x = 0, 1-OAr; x = 1, 2-OAr, x = 2, 3-OAr; x = 3, 4) were developed, featuring photoluminescence quantum yields up to 0.81(2) and lifetimes to 117(1) ns. Although the 4f → 5d absorptive transitions for these complexes were all found at ca. 420 nm, their emission bands exhibited large Stokes shifts with maxima occurring at 553 nm for 1-N, 518 nm for 2-N, 508 nm for 3-N, and 459 nm for 4, featuring yellow, lime-green, green, and blue light, respectively. Combined time-dependent density functional theory (TD-DFT) calculations and spectroscopic studies suggested that the long-lived 2D excited states of these complexes corresponded to singly occupied 5d z 2 orbitals. The observed difference in the Stokes shifts was attributed to the relaxation of excited states through vibrational processes facilitated by the ligands. The photochemistry of the sterically congested complex 4 was demonstrated by C–C bond forming reaction between 4-fluoroiodobenzene and benzene through an outer sphere electron transfer pathway, which expands the capabilities of cerium photosensitizers beyond our previous results that demonstrated inner sphere halogen atom abstraction reactivity by 1-N.
Partial directed coherence is a powerful tool used to analyze interdependencies in multivariate systems based on vector autoregressive modeling. This frequency domain measure for Granger-causality is ...designed such that it is normalized to 0,1. This normalization induces several pitfalls for the interpretability of the ordinary partial directed coherence, which will be discussed in some detail in this paper. In order to avoid these pitfalls, we introduce renormalized partial directed coherence and calculate confidence intervals and significance levels. The performance of this novel concept is illustrated by application to model systems and to electroencephalography and electromyography data from a patient suffering from Parkinsonian tremor.
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