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"Schmidt, Timothy"
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Future and challenges for hybrid upconversion nanosystems
To improve the efficiency of photon upconversion, a hybrid approach of combining organic dyes and inorganic nanoparticles is proving successful, especially in the form of dye-sensitized lanthanide-doped upconversion nanoparticles, nanoparticle-sensitized molecular triplet–triplet annihilation systems and metal–organic-framework nanoparticles. In this Review, we survey the latest advances and examine the key factors affecting upconversion performance, such as spectral overlap, core–shell design and the management of triplet excitons and quenchers at the interface between materials. Although issues such as stability, triplet-state quenching, concentration quenching and reabsorption must still be overcome, smart designs of hybrid nanosystems offer exciting opportunities for applications such as solar photovoltaic devices, deep-tissue biomedical imaging, optogenetics and nanomedicine among others.
Journal Article
The electronic structure of benzene from a tiling of the correlated 126-dimensional wavefunction
The electronic structure of benzene is a battleground for competing viewpoints of electronic structure, with valence bond theory localising electrons within superimposed resonance structures, and molecular orbital theory describing delocalised electrons. But, the interpretation of electronic structure in terms of orbitals ignores that the wavefunction is anti-symmetric upon interchange of like-spins. Furthermore, molecular orbitals do not provide an intuitive description of electron correlation. Here we show that the 126-dimensional electronic wavefunction of benzene can be partitioned into tiles related by permutation of like-spins. Employing correlated wavefunctions, these tiles are projected onto the three dimensions of each electron to reveal the superposition of Kekulé structures. But, opposing spins favour the occupancy of alternate Kekulé structures. This result succinctly describes the principal effect of electron correlation in benzene and underlines that electrons will not be spatially paired when it is energetically advantageous to avoid one another.
The electronic structure of benzene has been a test bed for competing theories along the years. Here the authors show via quantum chemistry calculations that the wavefunction of benzene can be partitioned into tiles which show that the two electron spins exhibit staggered Kekulé structures.
Journal Article
Chromosome 1q21 abnormalities in multiple myeloma
Gain of chromosome 1q (+1q) is one of the most common recurrent cytogenetic abnormalities in multiple myeloma (MM), occurring in approximately 40% of newly diagnosed cases. Although it is often considered a poor prognostic marker in MM, +1q has not been uniformly adopted as a high-risk cytogenetic abnormality in guidelines. Controversy exists regarding the importance of copy number, as well as whether +1q is itself a driver of poor outcomes or merely a common passenger genetic abnormality in biologically unstable disease. Although the identification of a clear pathogenic mechanism from +1q remains elusive, many genes at the 1q21 locus have been proposed to cause early progression and resistance to anti-myeloma therapy. The plethora of potential drivers suggests that +1q is not only a causative factor or poor outcomes in MM but may be targetable and/or predictive of response to novel therapies. This review will summarize our current understanding of the pathogenesis of +1q in plasma cell neoplasms, the impact of 1q copy number, identify potential genetic drivers of poor outcomes within this subset, and attempt to clarify its clinical significance and implications for the management of patients with multiple myeloma.
Journal Article
Photochemical upconversion of near-infrared light from below the silicon bandgap
Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. Although systems have been developed for upconverting light from photon energies in the near-infrared, upconversion from below the silicon bandgap has been out of reach. Here, we demonstrate an upconversion composition using PbS semiconductor nanocrystal sensitizers that absorb photons below the bandgap of silicon and populate violanthrone triplet states below the singlet oxygen energy. The triplet-state violanthrone chromophores luminesce in the visible spectrum following energy delivery from two singlet oxygen molecules. By incorporating organic chromophores as ligands onto the PbS nanocrystals to improve energy transfer, we demonstrate that violanthrone upconverts in the absence of oxygen by the triplet–triplet annihilation mechanism. The change in mechanism is shown by exploiting the magnetic field effect on triplet–triplet interactions.Photochemical upconversion of light with photon energy below the silicon bandgap has remained elusive, but the feat has now been demonstrated using PbS semiconductor nanocrystals and violanthrone.
Journal Article
Endothermic singlet fission is hindered by excimer formation
Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.
Journal Article
Calculating curly arrows from ab initio wavefunctions
Despite being at the heart of chemical thought, the curly arrow notation of reaction mechanisms has been treated with suspicion—the connection with rigorous molecular quantum mechanics being unclear. The connection requires a view of the wavefunction that goes beyond molecular orbitals and rests on the most fundamental property of electrons. The antisymmetry of electronic wavefunctions requires that an
N
-electron wavefunction repeat itself in 3
N
dimensions, thus exhibiting tiles. Inspection of wavefunction tiles permits insight into structure and mechanism. Here, we demonstrate that analysis of the wavefunction tile along a reaction coordinate reveals the electron movements depicted by the curly arrow notation for several reactions. The Diels–Alder reaction is revealed to involve the separation and counter propagation of electron spins. This unprecedented method of extracting the movements of electrons during a chemical reaction is a breakthrough in connecting traditional depictions of chemical mechanism with state-of-the-art quantum chemical calculations.
Despite being essential to organic chemistry, the curly arrow notation of reaction mechanisms has been treated with suspicion due to its unclear connection with quantum mechanics. Here, the authors show that analysis of wavefunction 'tiles' along a reaction coordinate reveals the electron motion depicted by curly arrows.
Journal Article
The sphingosine 1-phosphate receptor S1P2 maintains the homeostasis of germinal center B cells and promotes niche confinement
The environmental cues involved in regulating germinal center size are not fully understood. Cyster and colleagues show that the sphingosine 1-phosphate receptor S1P
2
controls the survival and localization of B cells in germinal centers by antagonizing signaling by the kinase Akt and follicular chemoattractants.
Mice deficient in sphingosine 1-phosphate receptor type 2 (S1P
2
) develop diffuse large B cell lymphoma. However, the role of S1P
2
in normal germinal center (GC) physiology is unknown. Here we show that S1P
2
-deficient GC B cells outgrew their wild-type counterparts in chronically established GCs. We found that antagonism of the kinase Akt mediated by S1P
2
and its downstream mediators Gα
12
, Gα
13
and p115RhoGEF regulated cell viability and was required for growth control in chronically proliferating GCs. Moreover, S1P
2
inhibited GC B cell responses to follicular chemoattractants and helped confine cells to the GC. In addition, S1P
2
overexpression promoted the centering of activated B cells in the follicle. We suggest that by inhibiting Akt activation and migration, S1P
2
helps restrict GC B cell survival and localization to an S1P-low niche at the follicle center.
Journal Article