Effect of Axial Ligands on Easy-Axis Anisotropy and Field-Induced Slow Magnetic Relaxation in Heptacoordinated FeII Complexes

A rational approach to modulating easy-axis magnetic anisotropy by varying the axial donor ligand in heptacoordinated Fe^II complexes has been explored. In this series of complexes with formulae of [Fe(H₄L)(NCS)₂] ⋅ 3 DMF ⋅ 0.5 H₂O ( 1 ), [Fe(H₄L)(NCSe)₂] ⋅ 3 DMF ⋅ 0.5 H₂O ( 2 ), and [Fe(H₄L)(NCNCN)₂] ⋅ DMF ⋅ H₂O ( 3 ) [H₄L=2,2′-{pyridine-2,6-diylbis(ethan-1-yl-1-ylidene)}bis(N-phenylhydrazinecarboxamide)], the axial positions are successively occupied by different nitrogen-based π-donor ligands. Detailed dc and ac magnetic susceptibility measurements reveal the existence of easy-axis magnetic anisotropy for all of the complexes, with 1 [U_eff=21 K, τ₀=1.72×10⁻⁶ s] and 2 [U_eff=25 K, τ₀=2.25×10⁻⁶ s] showing field-induced slow magnetic relaxation behavior. However, both experimental studies and theoretical calculations indicate the magnitude of the D value of complex 3 to be larger than those of complexes 1 and 2 due to the axial bond angle being smaller than that for an ideal geometry. Detailed analysis of the field and temperature dependences of relaxation time for 1 and 2 has revealed that multiple relaxation processes (quantum tunneling of magnetization, direct, and Raman) are involved in slow magnetic relaxation for both of these complexes. Magnetic dilution experiments support the role of intermolecular short contacts.     

Alignment of Axial Anisotropy in a 1D Coordination Polymer shows Improved Field Induced Single Molecule Magnet Behavior over a Mononuclear Seven Coordinated FeII Complex

Herein, we report a CN‐bridged alternating Fe^II?Ni^II 1D chain to ensure the alignment of axial anisotropy and improve the single molecule magnet (SMM) behavior in seven coordinated Fe^II compound. The chain was constructed from hepta coordinated Fe(II) complex as an anisotropic building unit and diamagnetic nickel tetra cyanate as a bridging ligand. The magnetic measurements show the easy‐axis anisotropy of the seven coordinated Fe(II) complex and field induced SMM behavior with spin reversal energy barrier U_eff=61(2) K (42 cm^?1) and pre‐exponential relaxation time τ₀=1.9×10^?8 s. The detailed analysis of the relaxation dynamics discloses that the Orbach process plays an important role in slow relaxation of magnetization for this compound. Notably, this example represents a remarkable energy barrier observed in hepta coordinated Fe(II) SMMs. The ab initio calculations estimate the magnitude of axial anisotropy and show the parallel orientation of the anisotropic axis throughout the 1D polymeric chain. In addition, it is also reported that the presence of weak π accepter ligands in the distorted axial position enhance the easy‐axis anisotropy.     

Self tapering and uptapering of a self guided laser beam in an absorbing/gain medium with nonlinearity

The absorption/gain induced self tapering and uptapering of a self guided Gaussian laser beam when the beam propagates in a saturable nonlinear medium has been discussed. Using the WKBJ and abberationless paraxial ray approximation a beam width equation has been derived. Conditions for tapering/uptapering for both cases i.e., for absorption and gain have been discussed. The beam width of tapered/uptapered beam depends on the rate of absorption/gain, and the length of absorption/gain region along the beam propagation direction. The importance of the present study is pointed out.     

77Se n.m.r. studies of organoselenium compounds: III—substituent effects in 4,4′-disubstituted diphenyl selenides and 4,4′-disubstituted diphenylmethanes studied by 1H, 13C and 77Se spectroscopy

⁷⁷Se N.m.r. spectra of 4,4′-disubstituted diphenyl selenides have been obtained. The chemical shifts could be correlated with substituent constants of the Hammett type or with Swain and Lupton two-parameter equations. No correlations were observed between these ⁷⁷Se shifts and the ¹³C shifts of the CH₂ group of 4,4′-disubstituted diphenylmethanes, the latter shifts showing no correlation with substituent constants. On the other hand linear correlation was observed between the ⁷⁷Se shifts of the 4,4′-disubstituted diphenyl selenides and the ¹H shifts of the CH₂ group of the diphenylmethanes, the ¹³C shifts of their 1-carbons and the ¹⁹F shifts of 4-substituted fluorobenzenes.     

Light and thermally induced metastabilities in electrochemically etched nanocrystalline porous silicon

We observe that light soaking for short durations and thermal quenching in nanocrystalline porous silicon (PS) produce metastable states. These metastable states show higher dark and photo currents, large photoluminescence and a weaker electron spin resonance (ESR) signal. However, long exposures to light produce the opposite effect. The metastable states are stable against sub-band gap light exposures. These metastable states can be removed by annealing at 150°C for 1 h. ESR shows the presence of a-Si phase (g ～ 2.0058, 6.4 G) in PS sample, but it is not sufficient to explain all the experimental results. Rather, our experiments suggest that light soaking causes more than one type of defects in porous silicon. The structural changes involving the movement of hydrogen present at the surface of PS or at the PS/a-Si interface may be responsible for these effects.     

Generation of ultrasonic waves in water by an elliptical Gaussian laser beam

The paper presents an analysis of ultrasonic wave generation in water by intensity modulated elliptical Gaussian laser beams. It is found that generated acoustic radiation is highly directional both in polar and azimuthal directions. An increase in the assymmetry of the transverse intensity distribution from the Gaussian dependence enhances the source directivity considerably. An obvious conclusion is that elliptical Gaussian beams are better choice in applications where it is desired to communicate in an approximately prescribed small solid angle.     

Adsorption of Cr(VI) and Se(IV) from their aqueous solutions onto Zr4+-substituted ZnAl/MgAl-layered double hydroxides: effect of Zr4+ substitution in the layer

Hydrotalcite-like compounds (layered double hydroxides, LDHs) containing varying amounts of Al(3+), Zr(4+), and Zn(2+) or Mg(2+) in the metal hydroxide layer have been synthesized and characterized by various physicochemical methods. The adsorption behavior of uncalcined (as-synthesized) and calcined LDHs have been investigated for Cr(2)O(7)(2-) and SeO(3)(2-). The mixed oxides, obtained on calcination at 450 degrees C, exhibit high adsorption capacities for Cr(2)O(7)(2-) (1.6-2.7 meq/g) and SeO(3)(2-) (1.1-1.5 meq/g), where adsorption occurs through rehydration. Substitution of Zr(4+) in the LDHs, for either M(2+) or Al(3+) ions, increases the adsorption capacity up to 20%, thus providing an alternative way to enhance the adsorption capacity of this type of material. The high adsorption capacity of these materials could be successfully used for removal of undesirable anions from water and also for synthesis of intercalated materials with tailored acidobasicity.     

Far-field spectral behaviour and spectral switching of super-Gaussian pulses

We have presented the far-field spectral behaviour of a super-Gaussian pulse diffracted through a rectangular aperture. It has been found that the on-axis modified power spectrum is always blue shifted, whose value increases with the increase in the value of the super-Gaussian parameter (m). For off-axis case, at some critical angle of diffraction, the diffraction induced far-field power spectrum is split into two lines of equal height. The far-field power spectrum is blue shifted when the diffraction angle is greater than the critical angle, whereas it is red shifted when the diffraction angle is less than the critical angle, i.e., spectral switching occurs. The critical angle for switching significantly increases with the increase in the super-Gaussian parameter (m). An important finding is that the switching frequency, the blue and red shift frequencies can be tuned by varying m.