Theoretical Insight into the Influence of Molecular Ratio on the Stability, Mechanical Property, Solvent Effect and Cooperativity Effect of HMX/DMI Cocrystal Explosive

Molecular dynamics method was employed to study the binding energies of the selected crystal planes of the 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane（HMX）/1,3-dimethyl-2-imidazolidinone（DMI） cocrystal in different molecular molar ratios. The mechanical properties were estimated in different molar ratios. Solvent effects were evaluated and the cooperativity effects were discussed in the HMX···HF···DMI ternary by using the M06-2x/6-311＋G（2df,2p） and MP2（full）/6-311＋G（2df,2p） methods. The results indicate that the substituted patterns（020） and（100） own the highest binding energies. The stabilities of cocrystals in the 1：1 and 2：1 ratios are the greatest, and thus the HMX/DMI cocrystals prefer cocrystallizing in the 1：1 and 2：1 molar ratios, which have good mechanical properties. The sensitivity change of cocrystal originates from not only the formation of intermolecular interaction but also the increment of bond dissociation energy of the N–NO2 bond. The cooperativity effect appears in the linear complex while the anti-cooperativity effect is found in the cyclic system. DMI binding to HMX is not energetically and structurally favored in the presence of HF. This is perhaps the reason that the solvent with large dielectric constant weakens the stability of the HMX/DMI cocrystals. Therefore, the solvents with low dielectric constants should be chosen in the preparation of HMX/DMI cocrystals.

Multi-ion cooperative processes in Yb^(3+) clusters

Cooperative luminescence(CL)occurs in spectral regions in which single ions do not have energy levels.It was first observed more than 40 years ago,and all results reported so far are from a pair of ions.In this work,upconverted CL of three Yb^(3+) ions was observed in the ultraviolet(UV)region under near-infrared(NIR)excitation.The UV CL intensity showed a cubic dependence on the NIR pump power,whereas the luminescence lifetime was nearly one-third the luminescence lifetime of single Yb^(3+) ions.The triplet CL(TCL)has a clear spectral structure,in which most emission peaks are consistent with the self-convoluted spectra from single Yb^(3+) ions.Blue shifts were observed for certain peaks,indicating complex interactions among the excited Yb^(3+) ions.The probability of the TCL process versus the average distances among three Yb^(3+) ions was derived via the first-and second-order corrections to the wave functions of lanthanide ions,indicating that the formation of Yb^(3+) clusters containing closely spaced ions favors the occurrence of the multi-ion interaction processes.Furthermore,the cooperative sensitization of one Gd^(3+) ion by four excited Yb^(3+) ions(Yb^(3+)-tetramer)was demonstrated experimentally,which exhibited a novel upconversion mechanism—cluster sensitization.Our results are intriguing for further exploring quantum transitions that simultaneously involve multiple ions.

Synthesis, structural and spectroscopic studies on GdBO_3:Yb^(3+)/Tb^(3+)@SiO_2 core-shell nanostructures

The presented study concerned up-converting core/shell type nanomaterials based on lanthanide（Ⅲ） ions, Ln（Ⅲ）, doped orthoborates. The system studied composed of the GdBO3 doped with Yb^3＋/Tb^3＋ luminescent core ensured an effective cooperative sensitization up-conversion, resulting in a bright green luminescence. The silica coating process was performed by a modified St？ber method, which resulted in the formation of core-shell nanostructures, making them suitable for bioapplications. The nanophosphors and nanocomposites were obtained by various methods, such as co-precipitation in the presence of Triton X-100 and micelle synthesis with ethylenediaminetetraacetic acid（EDTA） as organic modifiers/surfactants. The synthesized nanomaterials were characterized with the use of powder X-ray diffraction（XRD）, infrared light absorption with Fourier transform FT-IR spectra, transmission electron microscopy（TEM）, up-conversion emission spectra under IR light, as well as excitation spectra, emission spectra and fluorescence lifetimes under UV light, and their photophysical properties were compared.