Synthesis and characterization of monodisperse oligo(fluorene-co-bithiophene)s

A series of monodisperse oligo(9,9-di-n-octylfluorene-co-bithiophene)s (OFbTs) with molecular lengths of up to 19.5 nm and molecular weights up to 7025 g mol(-1) has been synthesized by a divergent/convergent approach involving Stille coupling reactions. Stille coupling is quite efficient in preparing this class of oligomers, and even the molecule with nine fluorene units and eight bithiophene units (F9Th16) can be synthesized in a yield as high as 70 %. Because of easy functionalization of the thiophene ring at its alpha position, no additional protecting group allowing activation for further reaction is necessary. However, the synthetic routes must be optimized to eliminate contamination of the targeting compounds with the homocoupling product of the organotin reagents. Synthesis of the longest oligomer F13Th24 in a relative large quantity is limited by its low yield due to the pronounced ligand-exchange side reactions of the starting materials and reaction intermediates. All oligomers longer than F4Th6 are nematic mesomorphs and exhibit enhanced glass transition temperature and clearing point with increasing molecular length, as revealed by differential scanning calorimetry and polarizing optical microscopy. Absorption and photoluminescence (PL) measurements reveal that OFbTs are well-conjugated systems with an effective conjugation length longer than the length of F13Th24.     

Syntheses, structures, magnetism, and optical properties of lutetium-based interlanthanide selenides

Ln3LuSe6 (Ln = La, Ce), beta-LnLuSe3 (Ln = Pr, Nd), and LnxLu4-xSe6 (Ln = Sm, Gd; x = 1.82, 1.87) have been synthesized using a Sb2Se3 flux at 1000 degrees C. Ln3LuSe6 (Ln = La, Ce) adopts the U3ScS6-type three-dimensional structure, which is constructed from two-dimensional 2(infinity)[Ln3Se6](3-) slabs with the gaps between these slabs being filled by octahedrally coordinated Lu(3+) ions. The series of beta-LnLuSe3 (Ln = Pr, Nd) are isotypic with UFeS3. Their structures include layers formed from LuSe6 octahedra that are separated by eight-coordinate Ln(3+) (Ln = Pr, Nd) ions in bicapped trigonal prismatic environments. Sm1.82Lu2.18Se6 and Gd1.87Lu2.13Se6 crystallize in the disordered F-Ln2S3 type structure with the eight-coordinate bicapped trigonal prismatic Ln(1) ions residing in the one-dimensional channels formed by three different double chains via edge- and corner-sharing. These double chains are constructed from Ln(2)Se7 monocapped trigonal prisms, Ln(3)Se6 octahedra, and Ln(4)S6 octahedra, respectively. The magnetic susceptibilities of beta-PrLuSe3 and beta-NdLuSe3 follow the Curie-Weiss law. Sm1.82Lu2.18Se6 shows van Vleck paramagnetism. Magnetic susceptibility measurements show that Gd1.87Lu2.13Se6 undergoes an antiferromagnetic transition around 4 K. Ce3LuSe6 exhibits soft ferromagnetism below 5 K. The optical band gaps for La3LuSe6, Ce3LuSe6, beta-PrLuSe3, beta-NdLuSe3, Sm1.82Lu2.18Se6, and Gd1.87Lu2.13Se6 are 1.26, 1.10, 1.56, 1.61, 1.51, and 1.56 eV, respectively.     

Density functional theory study of the platinum-catalyzed cyclopropanation reaction with olefin

A computational study of the platinum-catalyzed cyclopropanation reaction with olefin is presented. The model system is formed by an ethylene molecule and the active catalytic species, which forms from a CH2 fragment and the Cl2Pt(PH3)2 complex. The results show that the active catalytic species is not a metal-carbene of the type (PH3)2Cl2Pt=CH2 but two carbenoid complexes which can exist in almost two degenerate forms, namely (PH3)2Pt(CH2Cl)Cl (carbenoid A) and (PH3)Pt(CH2PH3)Cl2 (carbenoid B). The reaction proceeds through three pathways: methylene transfer, carbometalation for carbenoid A, and the reaction of a monophosphinic species for carbenoids (A and B). The most favored reaction channel is methylene transfer pathway for (PH3)Pt(CH2PH3)Cl2 (carbenoid B) species with a barrier of 31.32 kcal/mol in gas phase. The effects of dichloromethane, THF, and benzene solvent are investigated with PCM method. For carbenoid A, both methylene transfer and carbometalation pathway barriers to reaction become remarkably lower with the increasing polarity of solvent (from 43.25 and 52.50 kcal/mol for no solvent to 25.36 and 38.53 kcal/mol in the presence of the dichloromethane). In contrast, the reaction barriers for carbenoid B via the methylene transfer path hoist 6.30 kcal/mol, whereas the barriers do not change significantly for the reaction path of a monophosphinic species for carbenoids (A and B).     

Unlocking the Remarkable Influence of Intramolecular Group Rotation for Third-order Nonlinear Optical Properties

This work exposes for the first time the remarkable influence of intramolecular group rotation on third-order nonlinear optical (NLO) performance. In order to prove the role of group rotation, we designed and synthesized two photo-response compounds tetramethyl 5,5′-(((diazene-1,2-diylbis(4,1-phenylene))bis(oxy))bis(methylene))diisophthalate (1) and 5,5′-(((diazene-1,2-diylbis(4,1-phenylene))bis(oxy))bis(methylene))diisophthalic acid (2) and investigated their NLO performance under different substituent (benzyloxy group) rotation states. 1 and 2 have dynamic benzyloxy group rotation in dilute solution and shows reverse saturated absorption (RSA). When the benzyloxy group rotation of 1 and 2 was restricted by PMMA, their NLO performance not only converted into saturated absorption (SA) and NLO refraction behaviours, but also hardly changed after isomerization. Interestingly, we also restricted the benzyloxy group rotation in solution to a certain extent through photo-induced trans→cis isomerization, and found that the NLO performances of cis isomers of 1 and 2 exhibit SA and positive refraction and are similar to those of 1 -PMMA and 2 -PMMA. This work provides a new exploratory method for studying the influencing factors of third-order NLO performance.     

Electronic properties of monolayer copper selenide with one-dimensional moiré patterns

Strain engineering is a vital way to manipulate the electronic properties of two-dimensional (2D) materials. As a typical representative of transition metal mono-chalcogenides (TMMs), a honeycomb CuSe monolayer features with one-dimensional (1D) moiré patterns owing to the uniaxial strain along one of three equivalent orientations of Cu(111) substrates. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/S) experiments and density functional theory (DFT) calculations, we systematically investigate the electronic properties of the strained CuSe monolayer on the Cu(111) substrate. Our results show the semiconducting feature of CuSe monolayer with a band gap of 1.28 eV and the 1D periodical modulation of electronic properties by the 1D moiré patterns. Except for the uniaxially strained CuSe monolayer, we observed domain boundary and line defects in the CuSe monolayer, where the biaxial-strain and strain-free conditions can be investigated respectively. STS measurements for the three different strain regions show that the first peak in conduction band will move downward with the increasing strain. DFT calculations based on the three CuSe atomic models with different strain inside reproduced the peak movement. The present findings not only enrich the fundamental comprehension toward the influence of strain on electronic properties at 2D limit, but also offer the benchmark for the development of 2D semiconductor materials.     

Unified mechanism behind the even-parity ground state and neutron halo of 11Be

Using the axially deformed relativistic Hartree-Fock-Bogoliubov (D-RHFB) model, we explore the mechanism behind the parity inversion and halo occurrence in ¹¹Be, which are well reproduced by the RHF Lagrangian PKA1. It is illustrated that evidently enhanced deformation effects by the π-pseudo-vector and ρ-tensor couplings in PKA1 are crucial for correctly describing both the even-parity ground state (GS) and the neutron halo of ¹¹Be. Coupling with the deformation, the intrude \begin{document}$ 1d_{5/2} $\end{document} component largely enhances the couplings between the even-parity orbit \begin{document}$ 1/2_2^+ $\end{document} and the nuclear core to ensure an even-parity GS, whereas the \begin{document}$ 2s_{1/2} $\end{document} component therein dominates the halo formation in ¹¹Be. Moreover, the deformed halo in ¹¹Be is found to be stabilized by the attractive inherent correlations between the \begin{document}$ 1d_{5/2} $\end{document} and \begin{document}$ 2s_{1/2} $\end{document} components of the halo orbit \begin{document}$ 1/2_2^+ $\end{document}, instead of pairing correlations, which paves a new way for understanding the halo pictures in deformed unstable nuclei.     

Epitaxial fabrication of AgTe monolayer on Ag(111) and the sequential growth of Te film

Transition-metal chalcogenides (TMCs) materials have attracted increasing interest both for fundamental research and industrial applications. Among all these materials, two-dimensional (2D) compounds with honeycomb-like structure possess exotic electronic structures. Here, we report a systematic study of TMC monolayer AgTe fabricated by direct depositing Te on the surface of Ag(111) and annealing. Few intrinsic defects are observed and studied by scanning tunneling microscopy, indicating that there are two kinds of AgTe domains and they can form gliding twin-boundary. Then, the monolayer AgTe can serve as the template for the following growth of Te film. Meanwhile, some Te atoms are observed in the form of chains on the top of the bottom Te film. Our findings in this work might provide insightful guide for the epitaxial growth of 2D materials for study of novel physical properties and for future quantum devices.     

Monte Carlo study of the magnetic properties and magnetocaloric effect of an AFM/FM BiFeO_3/Co bilayer

The magnetic properties and magnetocaloric effect of an antiferromagnetic/ferromagnetic(AFM/FM)BiFeO_3/Co bilayer with mixed-spin(5/2,3/2) have been studied based on Monte Carlo simulation.The magnetization, susceptibility, and critical temperature are investigated under various exchange couplings and an external magnetic field. In particular, the influence of exchange couplings and an external magnetic field on the magnetic entropy change, adiabatic temperature change, and the relative cooling power(RCP) are studied. The simulation results indicated that the decrease of the exchange coupling and the increase of external magnetic fields can cause an increase of magnetic entropy change, adiabatic temperature change, and RCP. In addition, the hysteresis loops of the system are presented for different exchange couplings and temperatures.     

A comparison between repeating bursts of FRB 121102 and giant pulses from Crab pulsar and its applications

There are some similarities between bursts of repeating fast radio bursts (FRBs) and giant pulses (GPs) of pulsars. To explore possible relations between them, we study the cumulative energy distributions of these two phenomena using the observations of repeating FRB 121102 and the GPs of Crab pulsar. We find that the power-law slope of GPs (with fluence≥130 Jy·ms) is 2.85±0.10. The energy distribution of FRB 121102 can be well fitted by a smooth broken power-law function. For the bursts of FRB 121102 above the break energy (1.22 ×10³⁷ erg), the best-fitting slope is {2.90}_{-0.44}^{+0.55}, similar to the index of GPs at the same observing frequency (∼1.4 GHz). We further discuss the physical origin of the repeating FRB 121102 in the framework of the super GPs model. And we find that the super GPs model involving a millisecond pulsar is workable and favored for explaining FRB 121102 despite that the magnetar burst model is more popular.     

Self-organized criticality in multi-pulse gamma-ray bursts

The variability in multi-pulse gamma-ray bursts(GRBs)may help to reveal the mechanism of underlying processes from the central engine.To investigate whether the self-organized criticality(SOC)phenomena exist in the prompt phase of GRBs,we statistically study the proper ties of GRBs with more than 3 pulses in each burst by fitting the distributions of several observed physical variables with a Markov Chain Monte Carlo approach,including the isotropic energy E_iso,the duration time T,and the peak count rate P of each pulse.Our sample consists of 454 pulses in 93 GRBs observed by the CGRO/BATSE satellite.The best-fitting values and uncertainties for these power-law indices of the differential frequency distributions are:α_E^d=1.54±0.09,α_T^d=1.82_-0.15^+0.14andα_P^d=2.09_-0.19^0.18,while the power-law indices in the cumulative frequency distributions are:α_E^c=1.44_-0.10^+0.08,α_T^c=1.75_-0.13^0.11andα_P^c=1.99_-0.19^+0.16.We find that these distributions are roughly consistent with the physical framework of a Fractal-Diffusive,Self^Organized Criticality(FD-SOC)system with the spatial dimension S=3 and the classical diffusionβ=1.Our results support that the jet responsible for the GRBs should be magnetically dominated and magnetic instabilities(e.g.,kink model,or tearing-model instability)lead the GRB emission region into the SOC state.