Solar hydrogen conversion represents a clean and economic approach to addressing global energy and environmental issues, for which efficient photocatalysts are heavily pursued. Lead halide ...perovskites are promising candidates for efficient phtocatalysts in solar hydrogen generation due to their attractive properties in light absorption, photogenerated charge transportation, and utilization. However, photocatalytic applications of lead halide perovskites are limited owing to their poor stability in the presence of water or other polar solvent environment. This work presents the rational control of surface ligands in achieving a good balance between stability and photocatalytic activity of CsPbBr3 quantum dots (QDs). Detailed studies reveal that the deliberate surface ligands engineering is crucial for maximizing the photocatalytic activity of CsPbBr3 QDs while maintaining good QD stability. A certain amount of surface ligands protect the CsPbBr3 QDs from decomposition in moisture during the photocatalytic reaction while still enabling efficient charge transfer for photocatalytic reactions on the surface of QDs. The well‐controlled CsPbBr3 photocatalyst shows efficient visible light‐driven H2 generation with outstanding stability (≥160 h).
Rational control of surface ligands of lead halide perovskite quantum dots leads to stable and efficient photocatalytic solar hydrogen generation in a gas/solid reaction setup. This concept may provide opportunities for a broad range of metal halide perovskite quantum dots for photocatalytic H2 generation and extend potential applications to the removal of volatile pollutants and air purification.
Influenza infects an estimated 9–35 million individuals each year in the United States and is a contributing cause for between 12,000 and 56,000 deaths annually. Seasonal outbreaks of influenza are ...common in temperate regions of the world, with highest incidence typically occurring in colder and drier months of the year. Real-time forecasts of influenza transmission can inform public health response to outbreaks. We present the results of a multiinstitution collaborative effort to standardize the collection and evaluation of forecasting models for influenza in the United States for the 2010/2011 through 2016/2017 influenza seasons. For these seven seasons, we assembled weekly real-time forecasts of seven targets of public health interest from 22 different models. We compared forecast accuracy of each model relative to a historical baseline seasonal average. Across all regions of the United States, over half of the models showed consistently better performance than the historical baseline when forecasting incidence of influenza-like illness 1 wk, 2 wk, and 3 wk ahead of available data and when forecasting the timing and magnitude of the seasonal peak. In some regions, delays in data reporting were strongly and negatively associated with forecast accuracy. More timely reporting and an improved overall accessibility to novel and traditional data sources are needed to improve forecasting accuracy and its integration with real-time public health decision making.
In sunlit waters, the fate of fluoroquinolone antibiotics is significantly impacted by photodegradation. The mechanism of how natural organic matter (NOM) participates in the reaction has been ...frequently studied but still remains unclear. In this work, the interactions between the excited triplet state of the fluoroquinolone antibiotic norfloxacin (3NOR*) and a variety of NOM extracts were investigated using time-resolved laser spectroscopy. The observed transient absorption spectrum of 3NOR* showed a maximum at ca. 600 nm, and global fitting gave a lifetime of 1.0 μs for 3NOR* in phosphate buffer at pH = 7.5. Quenching of 3NOR* by Suwannee River hydrophobic acids (HPO), Beaufort River HPO, and Gartempe River HPO yielded rate constants of 1.8, 2.6, and 4.5 (×107 molC –1 s–1) respectively, whereas HPO from South Platte River unexpectedly increased the lifetime of 3NOR* with an as yet unknown mechanism. Concurrent photodegradation experiments of NOR (5 μM) in the presence of these NOM were also performed using a sunlight simulator. In general, the effects of NOM on the photodegradation rate of NOR were in agreement with observations from transient absorption studies. We suggest that adsorption of NOR to NOM is one of the major factors contributing to the observed quenching. These results yield a new insight into the likely role of NOM in sunlight-induced degradation of micropollutants.
The development of photocatalytic reactions has provided many novel opportunities to expand the scope of synthetic organic chemistry. In parallel with progress towards uncovering new reactivity, ...there is consensus that efforts focused on providing detailed mechanistic insight in order to uncover underlying excited‐state reactions are essential to maximise formation of desired products. With this in mind, we have investigated the recently reported sensitization‐initiated electron transfer (SenI‐ET) reaction for the C−H arylation of activated aryl halides. Using a variety of techniques, and in particular nanosecond transient absorption spectroscopy, we are able to distinguish several characteristic signals from the excited‐state species involved in the reaction, and subsequent kinetic analysis under various conditions has facilitated a detailed insight into the likely reaction mechanism.
Sensitization‐initiated electron transfer (SenI‐ET) for photocatalysis has been investigated under various reaction conditions by nanosecond transient absorption (ns‐TA) spectroscopy. By characterising the kinetic behaviour of the different excited‐state species involved, detailed insight into the reaction mechanism has been obtained.
Seasonal influenza results in substantial annual morbidity and mortality in the United States and worldwide. Accurate forecasts of key features of influenza epidemics, such as the timing and severity ...of the peak incidence in a given season, can inform public health response to outbreaks. As part of ongoing efforts to incorporate data and advanced analytical methods into public health decision-making, the United States Centers for Disease Control and Prevention (CDC) has organized seasonal influenza forecasting challenges since the 2013/2014 season. In the 2017/2018 season, 22 teams participated. A subset of four teams created a research consortium called the FluSight Network in early 2017. During the 2017/2018 season they worked together to produce a collaborative multi-model ensemble that combined 21 separate component models into a single model using a machine learning technique called stacking. This approach creates a weighted average of predictive densities where the weight for each component is determined by maximizing overall ensemble accuracy over past seasons. In the 2017/2018 influenza season, one of the largest seasonal outbreaks in the last 15 years, this multi-model ensemble performed better on average than all individual component models and placed second overall in the CDC challenge. It also outperformed the baseline multi-model ensemble created by the CDC that took a simple average of all models submitted to the forecasting challenge. This project shows that collaborative efforts between research teams to develop ensemble forecasting approaches can bring measurable improvements in forecast accuracy and important reductions in the variability of performance from year to year. Efforts such as this, that emphasize real-time testing and evaluation of forecasting models and facilitate the close collaboration between public health officials and modeling researchers, are essential to improving our understanding of how best to use forecasts to improve public health response to seasonal and emerging epidemic threats.
Ligand-sensitized, luminescent lanthanide(III) complexes are of considerable importance because their unique photophysical properties (microsecond to millisecond lifetimes, characteristic and narrow ...emission bands, and large Stokes shifts) make them well suited as labels in fluorescence-based bioassays. The long-lived emission of lanthanide(III) cations can be temporally resolved from scattered light and background fluorescence to vastly enhance measurement sensitivity. One challenge in this field is the design of sensitizing ligands that provide highly emissive complexes with sufficient stability and aqueous solubility for practical applications. In this Account, we give an overview of some of the general properties of the trivalent lanthanides and follow with a summary of advances made in our laboratory in the development of highly luminescent Tb(III) and Eu(III) complexes for applications in biotechnology. A focus of our research has been the optimization of these compounds as potential commercial agents for use in homogeneous time-resolved fluorescence (HTRF) technology. Our approach involves developing high-stability octadentate Tb(III) and Eu(III) complexes that rely on all-oxygen donor atoms and using multichromophore chelates to increase molar absorptivity; earlier examples utilized a single pendant chromophore (that is, a single “antenna”). Ligands based on 2-hydroxyisophthalamide (IAM) provide exceptionally emissive Tb(III) complexes with quantum yield values up to ∼60% that are stable at the nanomolar concentrations required for commercial assays. Through synthetic modification of the IAM chromophore and time-dependent density functional theory (TD-DFT) calculations, we have developed a method to predict absorption and emission properties of these chromophores as a tool to guide ligand design. Additionally, we have investigated chiral IAM ligands that yield Tb(III) complexes possessing both high quantum yield values and strong circularly polarized luminescence (CPL) activity. To efficiently sensitize Eu(III) emission, we have used the 1-hydroxypyridin-2-one (1,2-HOPO) chelate to create remarkable ligands that combine excellent photophysical properties and exceptional aqueous stabilities. A more complete understanding of this chromophore has been achieved by combining low-temperature phosphorescence measurements with the same TD-DFT approach used with the IAM system. Eu(III) complexes with strong CPL activity have also been obtained with chiral 1,2-HOPO ligands. We have also undertaken kinetic analysis of radiative and nonradiative decay pathways for a series of Eu(III) complexes; the importance of the metal ion symmetry on the ensuing photophysical properties is clear. Lastly, we describe a Tb(III)-IAM compoundnow carried through to commercial availabilitythat offers improved performance in the common HTRF platform and has the potential to vastly improve sensitivity.
A series of highly luminescent europium(III) complexes which exhibit photoluminescence from the Eu(III) center following energy transfer from the UV absorbing organic sensitizer have been ...investigated using a combination of ultrafast optical transient absorption and Eu L3 X-ray transient absorption techniques. We have previously demonstrated that the latter can be used as a signature of 4f–4f excitation responsible for the photoluminescence in these Eu(III) coordination complexes, but the long time scale of the earlier measurements did not allow direct observation of the ligand-to-metal energy transfer step, preventing a determination of the sensitization mechanism. Here, we provide the first direct experimental verification that Dexter electron exchange from the ligand triplet state is the dominant energy transfer mechanism in these photoluminescent systems. Moreover, the optical transient absorption results obtained herein imply that energy transfer for all three compounds has near unity yield, regardless of differences in the sensitization efficiencies, suggesting that the variations in the sensitization efficiencies are determined almost entirely by differences in the ligand-centered intersystem crossing rates. The implications for the rational design of more effective photoluminescent lanthanide complexes are discussed.
Light up the POMs: A luminescent lanthanoid complex with polyoxometalate (POM) and organic ligands has been structurally characterized (see picture). Comparison of this octanuclear TbIII complex of ...2‐picolinate and tungstoarsenate ligands with a dinuclear relative reveals the role of the organic ligands as chromophores, identifies the luminescent Tb centers, and determines the relationship between POM coordination mode and luminescence quenching.
To achieve activatable triplet–triplet annihilation (TTA) upconversion, we linked a diiodoBodipy triplet photosensitizing unit and perylene triplet energy acceptor/annihilation/emitter using a ...disulfide bond (dyad BP-1), which can be selectively cleaved by thiols. For comparison, a reference dyad featuring a shorter and more chemically robust 1,2,3-triazole linker between the two components was also prepared (dyad BP-2). The photophysical properties of these compounds have been studied using steady-state and time-resolved transient spectroscopies; forward singlet energy transfer and backward triplet energy transfer (ping-pong energy transfer) were observed. For BP-1, the rate for forward intramolecular Förster resonance energy transfer from perylene to diiodoBodipy is k FRET = 1.9 × 108 s–1, while the backward triplet–triplet energy-transfer (TTET) process from diiodoBodipy to perylene was slightly slower, with k TTET = 3.7 × 107 s–1. For BP-2, faster energy-transfer kinetics were determined (k FRET = 3.1 × 108 s–1 and k TTET = 8.4 × 107 s–1, respectively). Interestingly, we found the FRET rate constant is more critically dependent on the length of the linker than the corresponding TTET process, which may have important implications for the design of supramolecular TTA architectures. Lastly, upon cleavage of the disulfide bond in BP-1, intramolecular FRET was effectively shut down, and instead intermolecular TTET was observed, allowing for thiol-activatable TTA upconversion with an improvement in upconversion quantum yield from 0.03% to 0.5% in the presence of thiols.
Over the years, achieving efficient electroluminescence (EL) while simultaneously having low light amplification thresholds under optical excitation has been the key to progression toward the ...long‐thought objective of electrically pumped organic lasers. While significant progress in this regard has been made for organic semiconductors emitting in the blue–green region of the visible spectrum, organic laser dyes with low‐energy emission (>600 nm) still suffer from high amplified spontaneous emission (ASE) thresholds and low external quantum efficiencies (EQEs) in devices. Herein, low ASE thresholds and efficient EL are reported from a solution‐processable organic laser dye dithiophenyl diketopyrrolopyrrole (DT‐DPP). The ASE threshold of 4 µJ cm−2 at the wavelength of 620 nm is obtained while making constructive use of triplet excitons by doping DT‐DPP in a green‐emitting host matrix, which exhibits thermally activated delayed fluorescence (TADF). The organic light‐emitting diode fabricated from this system gives a high EQE of 7.9% due to the efficient utilization of triplet excitons. Transient EL studies further show that a high reverse intersystem crossing rate is crucial in achieving lasing under electrical pumping from such TADF‐assisted fluorescent systems.
Thermally activated delayed fluorescence (TADF)‐assisted fluorescence is successfully utilized to boost light‐amplification properties and device efficiencies for a diketopyrrolopyrrole derivative. An extremely low amplified spontaneous emission (ASE) threshold (4 µJ cm−2) is achieved along with high device external quantum efficiencies (8%). The triplet exciton confinement on the TADF is demonstrated to reduce singlet‐triplet annihilation losses while allowing the generated triplet excitons to be harvested.
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