EPR study of Mn2+ doped nanocrystalline PbF2

Nanocrystalline samples of PbF2 doped with 0.05, 0.1, 0.4 and 1 mol% Mn2+, used as paramagnetic probe, were prepared by inert gas condensation technique. All the samples were vacuum annealed at different temperatures to get different grain sizes. The X-ray diffraction studies showed the dominant content of -PbF2 phase with a fractional quantity of -PbF2. Thermal stability and sublattice melting were studied by TGA and DSC respectively. EPR measurements were made on all these samples at 77 and 300 K. The EPR spectra of all samples were found to contain well resolved sextet arising from the Mn2+ ions that occupied the cubic sites of Pb2+ ion of PbF2 lattice. The lower concentration of the Mn2+ ions (0.05 and 0.1 mol%) clearly monitored the Pb2+ environment in the PbF2 lattice. The 0.4 mol% showed the presence of only the cubic sites with a minor concentration of the orthorhombic sites. The spectra corresponding to 1 mol% Mn2+ clearly showed two different components. The isotropic nature of the 1 mol% as-prepared sample implied that there was no cluster formation and hence this EPR spectrum was taken as the single ion spectrum. The annealed samples contain two spectral components; one is from the isolated single ions and the other one from the Mn2+ clusters. The spectral component of Mn2+ clusters was obtained by subtracting the spectrum for the as-prepared sample for the spectra of annealed samples. The extracted cluster phase spectra and the pure spectrum from the as-prepared sample were then combined to simulate the entire set of experimental spectra. The simulated spectra were found to be in good agreement with the experimental data. The g values obtained were in the range very close to the free electron g factor as the electrons are in the S state (L=0).     

An efficient deprotection method for 5′-[O,O-bis(pivaloyloxymethyl)]-(E)-vinylphosphonate containing oligonucleotides

5′-(E)-Vinylphosphonate (VP) is an effective bioisostere of the natural 5′-monophosphate in small interfering RNAs (siRNAs). Solid-phase synthesis of VP-siRNAs requires the use of appropriately protected VP-phosphoramidites in combination with optimal oligonucleotide deprotection conditions. Addition of 3% (v) neat diethylamine to the standard aqueous ammonia deprotection conditions allows clean and rapid one-step deprotection of 5′-[O,O-bis(pivaloyloxymethyl)] (POM)-protected VP oligonucleotides, minimizing side reactions and impurities, which broadly enhances the scope of VP oligonucleotide synthesis.     

Crystal and molecular structure ofN-methylphenazinium bis(benzene-1,2-dithiolato)nickelate(III). Structure of (C13H11N2)[Ni(S2C6H4)2]

The crystal and molecular structure of the title compound, (C₁₃H₁₁N₂)[Ni(S₂C₆H₄)₂], is reported. Crystals are triclinic, space groupP¯1(No.2) withZ=2 in a unit cell of dimensionsa=9.363(1),b=10.148(1),c=13.623(1) Å,=85.64(1),=107.33(1), and =113.10(1)°. The structure was solved by the heavy-atom method and refined by the method of full-matrix least squares toR=0.036 for 2874 unique reflections. The metal atom of the anion has an approximate square-planar configuration. The metal atoms occupy the origins and A-face centers of the lattice. The anions with the metal atoms in the origins stack along thec-axis, the interplanar separation being thec-axis lattice repeat of 13.623(1) Å. The anions with the metal atoms in theA-face centers and the cations form a mixed stack along the short diagonal of theab-plane. The short interplanar distances (C–A=A–C=3.43 Å and C-C=3.91 Å) in the mixed stack (CAC-CAC) show the possibility of unusual electrical behavior. There is no evidence of direct or indirect metal-metal interaction.This work was supported by the Department of Science and Technology, Government of India, New Delhi, for which we are grateful. One of the authors (PK) also expresses his thanks to the U.G.C., New Delhi and Pachaiyappas College, Madras, for the award of FIP Fellowship.     

Triplet acetylenes as synthetic equivalents of 1,2-bicarbenes: phantom n,pi state controls reactivity in triplet photocycloaddition

Diaryl acetylenes, in which one of the aryl groups is either a pyridine or a pyrazine, undergo efficient triplet state photocycloaddition to 1,4-cyclohexadiene with formation of 1,5-diaryl substituted tetracyclo[3.3.0.0(2,8).0(4,6)]octanes (homoquadricyclanes). In the case of pyrazinyl acetylenes, the primary homoquadricyclane products undergo a secondary photochemical rearangement leading to diaryl substituted tricyclo[3.2.1.0(4,6)]oct-2-enes. Mechanistic and photophysical studies suggest that photocycloaddition proceeds through an electrophilic triplet excited state whereas the subsequent rearrangement to the tricyclooctenes proceeds through a singlet excited state. Chemical and quantum yields for the cycloaddition, in general, correlate with the electron acceptor character of aryl substituents but are attenuated by photophysical factors, such as the competition between the conversion of acetylene singlet excited state into the reactive triplet excited states (intersystem crossing: ISC) and/or to the radical-anion (photoelectron transfer from the diene to the excited acetylene: PET). Dramatically enhanced ISC between pi-pi S(1) state and "phantom" n,pi triplet excited state is likely to be important in directing reactivity to the triplet pathway. The role of PET can be minimized by the judicious choice of reaction conditions (solvent, concentration, etc.). From a practical perspective, such reactions are interesting because "capping" of the triple bond with the polycyclic framework orients the terminal aryl (4-pyridyl, 4-tetrafluoropyridyl, phenyl, etc.) groups in an almost perfect 60 degrees angle and renders such molecules promising supramolecular building blocks, especially in the design of metal coordination polymers.     

Photoexcitation and relaxation properties of a spin-crossover solid in the case of a stable high-spin state

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L --> H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample.     

Ortho effect in the Bergman cyclization: interception of p-benzyne intermediate by intramolecular hydrogen abstraction

Intramolecular hydrogen atom (H-atom) abstraction from the o-OCH3 group effectively intercepts the p-benzyne intermediate in the Bergman cycloaromatization of 2,3-diethynyl-1-methoxybenzene (1) before this intermediate undergoes either retro-Bergman ring opening or external H-atom abstraction. This process leads to the formation of a new diradical and renders the cyclization step essentially irreversible. Chemical and kinetic consequences of this phenomenon were investigated through the combination of computational and experimental studies.     

Crystal and molecular structure of bromo-bis(bis-diphenylphosphino-ethylene)-nickel(II)-tetraphenylborate

The Ni(II) complex of the ligandcis-1,2[bis(diphenylphosphino)ethylene] (VPP), [Ni(VPP)₂Br]BPh₄, crystallizes in the orthorhombic space groupP2₁2₁,2, witha=25.971(6),b=24.508 (6),c=19.642(5) Å,Z=8. The structure was determined by the procedure DIRDIF from Cu K automatic diffractometer X-ray data and refined by block-diagonal least-squares toR=11% for 5572 reflections. A question on the space group assignment betweenP2₁2₁2 andP2₁2₁2₁ was resolved only after the complete structure had been determined. The detailed crystal and molecular structure is presented.     

Temperature dependent study of the single crystal electronic spectrum of octahedral Ni2+ ion subjected to distortions in weak fields

Electronic spectra of two high spind ⁸ nickel complexes, Ni(en)₃Cl₂.2H₂O and Ni(OAc)₂.4H₂O, are reported. Polarized spectra were measured at 298 K. Temperature dependent spectra were measured using unpolarized light down to 20 K. The spectra at 20 K are well resolved revealing the features of spin-forbidden transitions and vibrational fine structure on some of the bands. Some of the spectra are deconvoluted to separate out the overlapping bands. The assignments are made usingD ₃ symmetry for Ni(en)₃Cl₂.2H₂O complex whileD ₄ symmetry is used for Ni(OAc)₂.4H₂O. The energies of the spectroscopic states are calculated and compared with the observed transition energies. The distortion parameters are determined.     

Adaptable Optical Microwaveguides From Mechanically Flexible Crystalline Materials

Flexible organic crystals (elastic and plastic) are important materials for optical waveguides, tunable optoelectronic devices, and photonic integrated circuits. Here, we present highly elastic organic crystals of a Schiff base, 1-((E)-(2,5-dichlorophenylimino)methyl)naphthalen-2-ol ( 1 ), and an azine molecule, 2,4-dibromo-6-((E)-((E)-(2,6-dichlorobenzylidene)hydrazono)methyl)phenol ( 2 ). These microcrystals are highly flexible under external mechanical force, both in the macroscopic and the microscopic regimes. The mechanical flexibility of these crystals arises as a result of weak and dispersive C−H⋅⋅⋅Cl, Cl⋅⋅⋅Cl, Br⋅⋅⋅Br, and π⋅⋅⋅π stacking interactions. Singly and doubly-bent geometries were achieved from their straight shape by a micromechanical approach using the AFM cantilever tip. Crystals of molecules 1 and 2 display a bright-green and red fluorescence (FL), respectively, and selective reabsorption of a part of their FL band. Crystals 1 and 2 exhibit optical-path-dependent low loss emissions at the termini of crystal in their straight and even in extremely bent geometries. Interestingly, the excitation position-dependent optical modes appear in both linear and bent waveguides of crystals 1 and 2 , confirming their light-trapping ability.