Suranjan Shil

A DFT Study on the Magnetostructural Property of Ferromagnetic Heteroverdazyl Diradicals with Phenylene Coupler

We have theoretically designed five different m-phenylene coupled high-spin bis-heteroverdazyl diradicals and their analogous p-phenylene coupled low-spin positional isomers. The geometry-based aromaticity index, harmonic oscillator model of aromaticity (HOMA) values for both the couplers (local HOMA), and the whole diradicals (global HOMA) have been calculated for all the diradicals. We also qualitatively relate these HOMA values with the intramolecular magnetic exchange coupling constants (J), calculated using a broken symmetry approach within unrestricted density functional theory framework. Structural aromaticity index HOMA of linkage specific benzene rings in our designed diradical systems shows that the aromatic character depends on the planarity of the molecule and it controls the sign and magnitude of J. The predicted J values are explained on the basis of spin polarization maps, average dihedral angles, and magnetic orbitals. The effect of the spin leakage phenomenon on magnetic exchange coupling constant and that on HOMA values of certain phosphaverdazyl systems has been explicitly discussed. In addition, a similar comparison is made between the calculated exchange coupling constants and corresponding HOMA values. The main novelty of this work stands on the consideration of the aromatic behavior by means of the geometrical index HOMA. We also estimate another aromaticity index, nucleus independent chemical shift (NICS) values for the phenylene coupler in each diradical to measure aromaticity and compare its value with that of HOMA. The ground state stabilities of these diradicals have also been compared.

Photoinduced antiferromagnetic to ferromagnetic crossover in organic systems.

Magnetization reversal is important for different technological applications. Photoinduced magnetization reversal is easier to implement than conventional reversal methods. Here, we theoretically design and investigate the photomagnetic property of azobenzene based diradical systems, where trans isomers convert into corresponding cis forms upon irradiation with light of appropriate wavelength. The coupling constant values have been estimated using the broken symmetry approach in the density functional theory framework. In each case, the trans isomer is found to be antiferromagnetic, while the cis form is ferromagnetic in nature. Therefore, photoinduced magnetic crossover from antiferromagnetic to ferromagnetic regime would be observed. This is a new observation in case of the systems of organic origin. Importance of such systems for photomagnetic switches, sensors, high density data storage, spin valves, and semiconductor spintronic materials have also been discussed with support from density of state analysis, singly occupied molecular orbital-singly occupied molecular orbital energy gaps and spin density plots.

Performance of the Widely Used Minnesota Density Functionals for the Prediction of Heat of Formations, Ionization Potentials of Some Benchmarked First Row Transition Metal Complexes

We have computed and investigated the performance of Minnesota density functionals especially the M05, M06, and M08 suite of complementary density functionals for the prediction of the heat of formations (HOFs) and the ionization potentials (IPs) of various benchmark complexes containing nine different first row transition metals. The eight functionals of M0X family, namely, the M05, M05-2X, M06-L, M06, M06-2X, M06-HF, M08-SO, and M08-HX are taken for the computation of the above-mentioned physical properties of such metal complexes along with popular Los Alamos National Laboratory 2 double-ζ (LANL2DZ) basis set. Total 54 benchmark systems are taken for HOF calculation, whereas the 47 systems among these benchmark complexes are chosen for the calculation of IPs because of lack of experimental results on rest of the seven systems. The computed values of HOFs and IPs are compared with the experimental results obtained from the literature. The deviation of these computed values from the actual experimental results is calculated for each eight different M0X functionals to judge their performances in evaluating these properties. Finally, a clear relationship between the exchange correlation energy of eight M0X functionals and their efficiency are made to predict the different physical properties.

Role of the coupler to design organic magnetic molecules: LUMO plays an important role in magnetic exchange

We have designed seven organic diradicals with polyacene couplers to show the effect of the configuration, aromaticity [estimated with the help of Nucleus Independent Chemical Shift (NICS(0) and NICS(I)), and Harmonic Oscillator Model of Aromaticity (HOMA)] and HOMO–LUMO gap of the couplers on the exchange coupling constant of the diradicals. It has been observed that the linear polyacenes are less aromatic compared to the corresponding angular ones. We have correlated aromaticity indexes NICS and HOMA to explain the change of aromaticity for structures having the same number of carbon and hydrogen atoms and rings. The diradicals with linear couplers manifest stronger exchange coupling constants compared to those with angular couplers. It has been found that the NICS value cannot adequately address the aromaticity of the polyacenes, whereas the HOMA value can reliably account for the observations. Here, we have found the HOMO–LUMO gap is the determining factor for the extent of the magnetic exchange coupling constant in the diradicals. It has been found that not only the energy value of the LUMO, but also its occupation number and spatial position, play an important role in magnetic exchange in diradicals. Thus, the role of the LUMO in magnetic exchange has been firmly established through this work. The magneto-structural correlation has also been studied to establish the mechanism of magnetic interaction.

A Perspective on Designing Chiral Organic Magnetic Molecules with Unusual Behavior in Magnetic Exchange Coupling.

A total of nine diradical-based organic chiral magnetic molecules with allene and cumulene couplers have been theoretically designed, and subsequently, their magnetic property has been studied by density functional theory. It is found that with an increase in length of the coupler, a remarkable increase in spin density within the coupler takes place. An increase in the length of the coupler reduces the energy of LUMO, and a smaller HOMO-LUMO gap facilitates stronger magnetic coupling and thereby a higher magnetic exchange coupling constant (J). This observation is supported by the occupation number of natural orbitals.

Electric field induced tuning of molecular conformation to acquire spintronics property in biphenyl systems

We have theoretically studied the effect of an external electric field on biphenyl, methylene substituted biphenyl monoradical and diradical systems. It has been found that the molecular conformation changes after the application of an external electric field. Thus, it is possible to tune molecular conformation reversibly only by external electrical stimuli. The HOMO–LUMO gap of the diradical reduces under an external electric field. The molecules with a low HOMO–LUMO gap can be used as unimolecular rectifiers if the HOMO and LUMO have a different spatial location on the molecule, or as a molecular conductor if the HOMO and LUMO are in the same spatial position. It has been observed that the diradical can be used as both a unimolecular rectifier and a molecular conductor depending upon the field strength. Now, the insulator molecule can be switched to a molecular rectifier or a molecular conductor and vice versa by an external electric field. The spin topology in a radical depends on the strength and direction of the applied electric field. Consequently, the extent of magnetic coupling in a diradical system changes with the external electric field. We have calculated the transmission spectra and I–V curve for the diradical at different dihedral angles. At a lower dihedral angle the diradical shows efficient spin filter behavior; within the range of bias voltage of 1 V and 1.5 V, the spin filtering efficiency of the diradical is achieved up to 99%. It is now possible to use a single molecule for multifarious applications like a unimolecular rectifier, spin filter and a molecular conductor depending upon the strength of the applied electric field.

3-Epihydroxy lup-20(29)-en-19(28)-olide: partial synthesis, antitopoisomerase activity, and 3D molecular docking

A novel method for the partial synthesis of the rare triterpenoid, 3-epihydroxy lup-20(29)-en-19(28)-olide, and 1 from betulinic acid is reported. The binding efficiency, mode of binding for different compounds to the central catalytic domain of topoisomerase IIα, was calculated from a complete 3D molecular docking study on the crystal structure of the enzyme (1bgw, pdb). The compounds 1, 2b, and 2d showed a dose-dependent inhibition of catalytic activity of topoisomerase IIα.