Highly efficient spin-conversion effect leading to energy up-converted electroluminescence in singlet fission photovoltaics

Free charge generation in donor-acceptor (D-A) based organic photovoltaic diodes (OPV) progresses through formation of charge-transfer (CT) and charge-separated (CS) states and excitation decay to the triplet level is considered as a terminal loss. On the other hand a direct excitation decay to the triplet state is beneficial for multiexciton harvesting in singlet fission photovoltaics (SF-PV) and the formation of CT-state is considered as a limiting factor for multiple triplet harvesting. These two extremes when present in a D-A system are expected to provide important insights into the mechanism of free charge generation and spin-character of bimolecular recombination in OPVs. Herein, we present the complete cycle of events linked to spin conversion in the model OPV system of rubrene/C60. By tracking the spectral evolution of photocurrent generation at short-circuit and close to open-circuit conditions we are able to capture spectral changes to photocurrent that reveal the triplet character of CT-state. Furthermore, we unveil an energy up-conversion effect that sets in as a consequence of triplet population build-up where triplet-triplet annihilation (TTA) process effectively regenerates the singlet excitation. This detailed balance is shown to enable a rare event of photon emission just above the open-circuit voltage (VOC) in OPVs.

Generation of Cationic Two-Coordinate Group-13 Ligand Systems by Spontaneous Halide Ejection: Remarkably Nucleophile-Resistant (Dimethylamino)borylene Complexes

Spontaneous ejection of chloride from a three-coordinate boron Lewis acid can be effected by employing very electron rich metal substituents and leads to the formation of a sterically unprotected terminal (dimethylamino)borylene complex that has a short metal-boron bond and remarkable resistance to attack by nucleophilic and protic reagents.

Group fission in Nilgiri langurs (Presbytis johnii)

During a field study on habituated groups of wild Nilgiri langurs (Presbytis johnii), four cases of group fission were observed which differed markedly from group changes reported in other species of the same genus. All fissions occurred in groups with more than one mature male and roughly coincided with the onset of loud call vocalization by the second mature male group member. As a result of the fission the founder group factions acquired the structure of a onemale group. The filial factions consisted of both mature males and females and occupied, at least temporarily, a part of, or an area adjacent to, the founder group. During fission, the amount of aggressive interactions between the two mature males increased. However, the majority of agonistic interactions involved ritualized threat and intimidation behavior without physical aggression. In at least three cases the two mature males involved in the fission had not joined recently but had lived in these groups for three years or more. Although the presence of all male bands and single males in the two study populations indicate that male replacement by invasive takeovers may occur, it is suggested that in Nilgiri langurs, noninvasive fissions are probably another common mechanism for the foundation and maintenance of bisexual one-male groups.

(Dimethylamino)borylene and Related Complexes of Electron-Rich Metal Fragments: Generation of Nucleophile-Resistant Cations by Spontaneous Halide Ejection

Spontaneous halide ejection from a three-coordinate Lewis acid has been shown to offer a remarkable new route to cationic metal complexes featuring a linear, multiply bonded boron-donor Ligand. The exploitation of electron-rich [CpM(PR3)(2)] fragments within boryl systems of the type LnMB(hal)NR2 leads to the spontaneous formation in polar solvents of chemically robust borylene complexes, [LnM(BNR2)](+), with exceptionally low electrophilicity and short M-B bonds. This is reflected by M-B distances (ca. 1.80 angstrom for FeB systems) which are more akin to alkyl-/aryl-substituted borylene complexes and, perhaps most strikingly, by the very low exothermicity associated with the binding of pyridine to the two-coordinate boron center (Delta H = -7.4 kcal mol(-1), cf. -40.7 kcal mol(-1) for BCl3). Despite the strong pi electron release from the metal fragment implied by this suppressed reactivity and by such short M-B bonds, the barrier to rotation about the Fe=B bond in the unsymmetrical variant [CpFe(dmpe)(BN{C6H4OMe-4}Me)](+) is found to be very small (ca. 2.9 kcal mol(-1)). This apparent contradiction is rationalized by the orthogonal orientations of the HOMO and HOMO-2 orbitals of the [CpML2](+) fragment, which mean that the M-B pi interaction does not fall to zero even in the highest energy conformation.

Spontaneous self-assembly of designed cyclic dipeptide (Phg-Phg) into two-dimensional nano- and mesosheets

In this study, we present the spontaneous self-assembly of designed simplest aromatic cyclic dipeptides of (L-Phg-L-Phg) and (D-Phg-L-Phg) to form highly stable two-dimensional (2D) nano- and mesosheets with large lateral surface area. Various microscopy data revealed that the morphology of 2D mesosheets resembles the hierarchical natural materials with layered structure. Solution and solid-state NMR studies on cyclo(L-Phg-L-Phg) revealed the presence of strong (N-H-O) hydrogen-bonded molecular chains supported by aromatic pi-pi interactions to form 2D mesosheets. Interestingly, cyclo(D-Phg-L-Phg) self-assembles to form single-crystalline as well as non-crystalline 2D rhomboid sheets with large lateral dimension. X-ray diffraction analysis revealed the stacking of (N-H-O) hydrogen-bonded molecular layers along c-axis supported by aromatic pi-pi interactions. The thermogravimetric analysis shows two transitions with overall high thermal stability attributed to layered hierarchy found in 2D mesosheets.

Spontaneous formation of branched nanochains from room temperature molten amides: visible and near-IR active, SERS substrates for non-fluorescent and fluorescent analytes

Highly stable, branched gold nanoworms are formed spontaneously in an acetamide-based room temperature molten solvent without any additional external stabilizing or aggregating agent. The nanoworms can be anchored onto solid substrates such as indium tin oxide (ITO) without any change in morphology. The anchored nanoworms are explored as substrates for surface enhanced Raman scattering (SERS) studies using non-fluorescent 4-mercaptobenzoic acid (4-MBA) and fluorescent rhodamine 6G (R6G) as probe molecules. The anchored nanostructured particles respond to near IR (1064 nm) as well as visible (785, 632.8 and 514 nm) excitation lasers and yield good surface enhancement in Raman signals. Enhancement factors of the order 10(6)-10(7) are determined for the analytes using a 1064 nm excitation source. Minimum detection limits based on adsorption from ethanolic solutions of 1028 M 4-MBA and aqueous solutions of 1027 M R6G are achieved. Experimental Raman frequencies and frequencies estimated by DFT calculations are in fairly good agreement. SERS imaging of the nanostructures suggests that the substrates comprising of three dimensional, highly interlinked particles are more suited than particles fused in one dimension. The high SERS activity of the branched nanoworms may be attributed to both electromagnetic and charge transfer effects.

Splicing Functions and Global Dependency on Fission Yeast Slu7 Reveal Diversity in Spliceosome Assembly

The multiple short introns in Schizosaccharomyces pombe genes with degenerate cis sequences and atypically positioned polypyrimidine tracts make an interesting model to investigate canonical and alternative roles for conserved splicing factors. Here we report functions and interactions of the S. pombe slu7(+) (spslu7(+)) gene product, known from Saccharomyces cerevisiae and human in vitro reactions to assemble into spliceosomes after the first catalytic reaction and to dictate 3' splice site choice during the second reaction. By using a missense mutant of this essential S. pombe factor, we detected a range of global splicing derangements that were validated in assays for the splicing status of diverse candidate introns. We ascribe widespread, intron-specific SpSlu7 functions and have deduced several features, including the branch nucleotide-to-3' splice site distance, intron length, and the impact of its A/U content at the 5' end on the intron's dependence on SpSlu7. The data imply dynamic substrate-splicing factor relationships in multiintron transcripts. Interestingly, the unexpected early splicing arrest in spslu7-2 revealed a role before catalysis. We detected a salt-stable association with U5 snRNP and observed genetic interactions with spprp1(+), a homolog of human U5-102k factor. These observations together point to an altered recruitment and dependence on SpSlu7, suggesting its role in facilitating transitions that promote catalysis, and highlight the diversity in spliceosome assembly.

Spontaneous Lorentz violation in gauge theories

Frohlich, Morchio and Strocchi long ago proved that the Lorentz invariance is spontaneously broken in QED because of infrared effects. We develop a simple model where the consequences of this breakdown can be explicitly and easily calculated. For this purpose, the superselected U(1) charge group of QED is extended to a superselected ``Sky'' group containing direction-dependent gauge transformations at infinity. It is the analog of the Spi group of gravity. As Lorentz transformations do not commute with Sky, they are spontaneously broken. These Abelian considerations and model are extended to non-Abelian gauge symmetries. Basic issues regarding the observability of twisted non-Abelian gauge symmetries and of the asymptotic ADM symmetries of quantum gravity are raised.

Spontaneous assembly of iridium nanochain-like structures: surface enhanced Raman scattering activity using visible light

A facile, environmentally friendly approach to synthesize branched Ir nanochain-like structures under mild conditions, using polyfunctional capping molecules in an aqueous medium is reported; the nanostructures exhibit a surface plasmon resonance peak (SPR) in the visible region and serve as an active substrate for surface enhanced Raman scattering studies.

Spontaneous Breaking of Time-Reversal Symmetry in Strongly Interacting Two-Dimensional Electron Layers in Silicon and Germanium

We report experimental evidence of a remarkable spontaneous time-reversal symmetry breaking in two-dimensional electron systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge). Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease rapidly with decreasing doping in the Si: P and Ge: P delta layers, suggesting an effect driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of intrinsic local spin fluctuations at low doping, providing a microscopic mechanism for spontaneous lifting of the time-reversal symmetry. Our experiments suggest the emergence of a new many-body quantum state when two-dimensional electrons are confined to narrow half-filled impurity bands.

Frequency-domain stimulated and spontaneous light emission signals at molecular junctions

Using a diagrammatic superoperator formalism we calculate optical signals at molecular junctions where a single molecule is coupled to two metal leads which are held at different chemical potentials. The molecule starts in a nonequilibrium steady state whereby it continuously exchanges electrons with the leads with a constant electron flux. Expressions for frequency domain optical signals measured in response to continuous laser fields are derived by expanding the molecular correlation functions in terms of its many-body states. The nonunitary evolution of molecular states is described by the quantum master equation. (C) 2014 AIP Publishing LLC.

Response to ``Comment on `Frequency-domain stimulated and spontaneous light emission signals at molecular junctions''' J. Chem. Phys. 142, 137101 (2015)]

In a recent work [U. Harbola, B. K. Agrawalla, and S. Mukamel, J. Chem. Phys. 141, 074107 (2014)], we have presented a superoperator (Liouville space) diagrammatic formulation of spontaneous and stimulated optical signals from current-carrying molecular junctions. We computed the diagrams that contribute to the spontaneous light emission SLE (fluorescence and Raman) signal using a diagrammatic method which clearly distinguishes between the Raman and the fluorescence contributions. We pointed out some discrepancies with the work of Galperin, Ratner and Nitzan (GRN) [M. Galperin, M. A. Ratner and, A. Nitzan, J. Chem. Phys. 130, 144109 (2009)]. In their response [M. Galperin, M. A. Ratner and A. Nitzan, “Comment on‘ Frequency-domain stimulated and spontaneous light emission signals at molecular junctions’” [J. Chem. Phys. 141, 074107 (2014)], J. Chem. Phys. 142, 137101 (2015)] to our work, GRN have argued that there are no differences in the choice of Raman diagrams in both works. Here we reply to their points and show where the differences exist.

Spontaneous Lorentz violation: the case of infrared QED

It is by now clear that the infrared sector of quantum electrodynamics (QED) has an intriguingly complex structure. Based on earlier pioneering work on this subject, two of us recently proposed a simple modification of QED by constructing a generalization of the U(1) charge group of QED to the ``Sky'' group incorporating the well-known spontaneous Lorentz violation due to infrared photons, but still compatible in particular with locality (Balachandran and Vaidya, Eur Phys J Plus 128:118, 2013). It was shown that the ``Sky'' group is generated by the algebra of angle-dependent charges and a study of its superselection sectors has revealed a manifest description of spontaneous breaking of the Lorentz symmetry. We further elaborate this approach here and investigate in some detail the properties of charged particles dressed by the infrared photons. We find that Lorentz violation due to soft photons may be manifestly codified in an angle-dependent fermion mass, modifying therefore the fermion dispersion relations. The fact that the masses of the charged particles are not Lorentz invariant affects their spin content, and time dilation formulas for decays should also get corrections.