We design and investigate 11 different bis-oxoverdazyl diradicals connected by various aromatic couplers for their magnetic properties. The intramolecular magnetic exchange coupling constants (J) have been calculated using a broken symmetry approach in DFT framework. The J values are explained using spin polarization maps and magnetic orbitals. Isotropic hyperfine coupling constants (hfcc's) have been calculated for all the species in vacuum. The computed hfcc values also support intramolecular magnetic interactions. It has been found that some of the diradicals have ferromagnetic character while the others are antiferromagnetic in nature.
Here, in this work we have designed a molecular bridge structure which can be used as a spin filter where the prototypical highly ferromagnetic m-phenylene connected bis(aminoxyl) diradical is used as a bridging fragment between two semi-infinitely widened gold (Au) electrodes along the [100] direction. A state-of-the-art non-equilibrium Green function's (NEGF) method coupled with the density functional theory (DFT) was carried out on this two-probe molecular bridge system to understand its electrical spin transport characteristics. The spin current at various bias voltages from 0.00 V to 4.00 V at intervals of 0.20 V for this Au-diradical-Au molecular junction is evaluated. We also quantify the bias-dependent spin injection coefficients (BDSIC) at different bias voltages and also the spin-filter efficiency at equilibrium, i.e., at zero bias voltage. Also plots of BDSIC vs. voltage, the up- and down-spin current vs. voltage (I-V) curves, and density of states (DOS) at zero bias voltage are evaluated.
The spin blocker capacity of borazine is investigated. Specifically, meta-B-B, meta-N-N and para-B-N connected borazines are used as spin-blocker couplers comprised of a pair of radicals: two iminonitroxides (IN); IN and tetrathiafulvalene radical cations (TTF); or two TTFs. Density functional theory (DFT) is used to elucidate the spin blocker capacity of the linkage-specific (meta or para) borazine-coupler and elaborate the role of the lowest unoccupied molecular orbital (LUMO) in magnetic-exchange. Furthermore, a qualitative relation between different magnetic aromaticity indices is made using both nuclear-independent chemical shift (NICS) and the harmonic oscillator model of aromaticity (HOMA). The NICS values are calculated at the centre of the borazine spacer fragment of these diradical species and then also at 0.5 Å increments of the virtual probe from this centre position up to an orthogonal distance of 2.0 Å from the centre. The HOMA values are calculated for the borazine ring fragment in these diradicals. Based on the HOMA and NICS values, it is evident that the borazine exhibits less aromatic character than benzene itself - due to the polar nature of B-N π-bonding. The LUMO mediated spin-exchange between the two consecutive singly occupied molecular orbitals (SOMOs) is explicitly discussed and confirmed to play a pivotal role. The parity of the coupler pathways, i.e. even or odd number of bonds along a selected pathway, between radical moieties is an important factor in predicting the nature and extent of magnetic exchange for these diradicals. Surprisingly, borazine does not always act as a spin-coupling blocker - rather in some cases the coupling is enhanced as compared to a homoatomic (carbon-based) benzene coupler.
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