Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic ...spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.
For 200 years, the ‘closed box’ analogy of intracranial pressure (ICP) has underpinned neurosurgery and neuro-critical care. Cushing conceptualised the Monro-Kellie doctrine stating that a change in ...blood, brain or CSF volume resulted in reciprocal changes in one or both of the other two. When not possible, attempts to increase a volume further increase ICP. On this doctrine’s “truth or relative untruth” depends many of the critical procedures in the surgery of the central nervous system. However, each volume component may not deserve the equal weighting this static concept implies. The slow production of CSF (0.35 ml/min) is dwarfed by the dynamic blood in and outflow (∼700 ml/min). Neuro-critical care practice focusing on arterial and ICP regulation has been questioned. Failure of venous efferent flow to precisely match arterial afferent flow will yield immediate and dramatic changes in intracranial blood volume and pressure. Interpreting ICP without interrogating its core drivers may be misleading. Multiple clinical conditions and the cerebral effects of altitude and microgravity relate to imbalances in this dynamic rather than ICP per se. This article reviews the Monro-Kellie doctrine, categorises venous outflow limitation conditions, relates physiological mechanisms to clinical conditions and suggests specific management options.
During World War II, the United States helped vanquish the Axis powers by converting its enormous economic capacities into military might. Producing nearly two-thirds of all the munitions used by ...Allied forces, American industry became what President Franklin D. Roosevelt called "the arsenal of democracy." Crucial in this effort were business leaders. Some of these captains of industry went to Washington to coordinate the mobilization, while others led their companies to churn out weapons. In this way, the private sector won the war—or so the story goes.Based on new research in business and military archives, Destructive Creation shows that the enormous mobilization effort relied not only on the capacities of private companies but also on massive public investment and robust government regulation. This public- private partnership involved plenty of government-business cooperation, but it also generated antagonism in the American business community that had lasting repercussions for American politics. Many business leaders, still engaged in political battles against the New Deal, regarded the wartime government as an overreaching regulator and a threatening rival. In response, they mounted an aggressive campaign that touted the achievements of for-profit firms while dismissing the value of public-sector contributions. This probusiness story about mobilization was a political success, not just during the war, but afterward, as it shaped reconversion policy and the transformation of the American military-industrial complex.Offering a groundbreaking account of the inner workings of the "arsenal of democracy, " Destructive Creation also suggests how the struggle to define its heroes and villains has continued to shape economic and political development to the present day.
Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet ...excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley–Queisser limit.
Methylammonium lead halide (MAPbX3) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic ...instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase‐pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor‐processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead‐free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.
Call me MA‐Bi‐I! A lead‐free alternative to hybrid lead perovskites is explored for solar cells: methylammonium bismuth iodide (see figure). This material exhibits higher air stability than hybrid lead perovskites, while demonstrating optoelectronic properties promising for solar absorbers.
Optical upconversion via sensitized triplet-triplet exciton annihilation converts incoherent low-energy photons to shorter wavelengths under modest excitation intensities. Here, we report a ...solid-state thin film for infrared-to-visible upconversion that employs lead sulphide colloidal nanocrystals as a sensitizer. Upconversion is achieved from pump wavelengths beyond λ = 1 μm to emission at λ = 612 nm. When excited at λ = 808 nm, two excitons in the sensitizer are converted to one higher-energy state in the emitter at a yield of 1.2 ± 0.2%. Peak efficiency is attained at an absorbed intensity equivalent to less than one sun. We demonstrate that colloidal nanocrystals are an attractive alternative to existing molecular sensitizers, given their small exchange splitting, wide wavelength tunability, broadband infrared absorption, and our transient observations of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.
Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each ...generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.
DJ-1 is a member of the large and functionally diverse DJ-1/PfpI superfamily and has homologs in nearly all organisms. Because of its connection to parkinsonism and cancer, human DJ-1 has been ...intensely studied for over a decade. The current view is that DJ-1 is a multifunctional oxidative stress response protein that defends cells against reactive oxygen species and mitochondrial damage, although the details of its biochemical function remain unclear. A conserved cysteine residue in DJ-1 (Cys106) is both functionally essential and subject to oxidation to the cysteine-sulfinate and cysteine-sulfonate. Consequently, the oxidative modification of Cys106 has been proposed to allow DJ-1 to act as a sensor of cellular redox homeostasis and to participate in cytoprotective signaling pathways in the cell. This review explores the current evidence for the role of cysteine oxidation in DJ-1 function, with emphasis on emerging models for how oxidative modification may regulate DJ-1's protective function and also contribute to dysfunction and disease.
This article is a written version of the Presidential Address I gave at the annual meeting of the National Council on Measurement in Education (NCME) in April 2017. It is a call to NCME members (and ...others who read this, of course) to rebalance their focus so that classroom assessments are seen as being at least as important as large-scale assessments for education (in fact, in my view, they are more important). The article reviews research literature about the effects of classroom assessment to establish its importance for education. Then, the roles of large-scale assessment are reviewed, and, in particular, it is noted how these can have negative results when the large-scale assessments are not well aligned with sound curriculum and instructional and assessment practices grounded in theories of learning. In the next two sections (a) the idea of a learning progression is described as a way to facilitate the coherence between classroom and large-scale assessment and (b) the idea of a 'roadmap' is described, being the assessment components of the learning progression. This is followed by a description of an example of such a roadmap, developed for the Assessing Data Modeling and Statistical Reasoning project using the BEAR Assessment System (BAS). Finally, a concluding discussion reviews the ways that the coherence between large-scale and classroom assessments can be achieved using the BAS, and hence make measurement more important for education. Author abstract
Purpose - The purpose of this paper is to advance supply chain network theory by applying theoretical and empirical developments in complex network literature to the context of supply chains as ...complex adaptive systems. The authors synthesize these advancements to gain an understanding of the network properties underlying efficient supply chains. To develop a suitable theory of supply chain networks, the authors look to mirror the properties of complex network models with real-world supply chains.Design methodology approach - The authors review complex network literature drawn from multiple disciplines in top scientific journals. From this interdisciplinary review a series of propositions are developed around supply chain complexity and adaptive phenomena.Findings - This paper proposes that the structure of efficient supply chains follows a "scale-free" network. This proposal emerges from arguments that the key properties of efficient supply chains are a short characteristic path length, a high clustering coefficient and a power law connectivity distribution.Research limitations implications - The authors' discussion centres on applying advances found in recent complex network literature. Hence, the need is noted to empirically validate the series of propositions developed in this paper in a supply chain context.Practical implications - If efficient supply chains resemble a scale-free network, then managers can derive a number of implications. For example, supply chain resilience is derived by the presence of hub firms. To reduce the vulnerability of supply chains to cascading failures, it is recognized that managers could build in redundancy, undertake a multi-sourcing strategy or intermediation between hub firms.Originality value - This paper advances supply chain network theory. It offers a novel understanding of supply chains as complex adaptive systems and, in particular, that efficient and resilient supply chain systems resemble a scale-free network. In addition, it provides a series of propositions that allow modelling and empirical research to proceed.
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