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Title: Theoretical Investigation of Magnetic and Conducting Properties of Substituted Silicon Chains. I. Hydrogen and Oxo-Verdazyl Ligands
Authors: HANSDA, S
Issue Date: 2012
Citation: JOURNAL OF PHYSICAL CHEMISTRY C, 116(23)12725-12737
Abstract: In this work, we investigate the magnetic and conducting properties of polysilene chains attached to the stable free radical oxo-Verdazyl (o-VER) via C-linkage, by using quantum chemical and solid state methods. Calculations are first carried out on 39 possible monomers in their triplet and broken symmetry states following the density functional methodology UB3LYP and using Gaussian 03 and 09 codes. The magnetic exchange coupling constant (J) is calculated for each species. Here, the magnetic exchange coupling constant J equals the negative of the ratio of the energy difference and <S-2> difference between the low-spin and the high-spin ground states, in accordance with the Heisenberg spin exchange Hamiltonian. Geometry is optimized using the 6-31G(d) basis set. Single-point calculations on the triplet and broken symmetry states are performed with the optimized geometries and 6-311G(d,p) basis set. Calculations on three series of molecules of interest are finally done by employing the 6-311++G(d,p) basis set. Planar silicon chains having o-VER groups at alternative positions have intramolecular ferromagnetic (FM) interaction (with positive J). The nonalternative arrangement shows antiferromagnetic (AFM) coupling (with negative J). These results directly agree with the spin alternation rule. However, alternatively placed o-VER groups that are arranged as a stack perpendicular to the axis of the silicon chain show from weak AFM to weak FM intramolecular interaction. Negative J values are calculated for distorted as well as exactly parallel stacks of o-VER without methyl groups. We find positive J values for distorted stacks and negative J for parallel stacks, both with methyl groups retained in o-VER These observations for the stacked radicals are rationalized in terms of a competition between through-chain FM and through-space AFM spin interactions. In the second step, solid state calculations are performed on the planar FM polymer (without methyl groups in o-VER) and two AFM chains with o-VER groups stacked in parallel (without and with methyl groups) by using the CRYSTAL09 code. These are based on the UB3LYP methodology and 6-21G(d) basis set. From these calculations, we predict the first polymer, with estimated J of the order of 30 cm(-1), to be a semiconductor with band gap 1.27 eV. The stacked two, both antiferromagnetic with J = -27 and -35 cm(-1), are predicted to be, respectively, a conductor and a semiconductor, the latter with a band gap of only 0.57 eV. The distorted stack version of the third polymer is likely to be ferromagnetic and a semiconductor, like the first polymer, though they considerably differ in structural geometry.
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