Spectral Faraday rotation in tripyridinium bis[tetrachloroferrate(III)] chloride molecular magnet at room temperature due to electronic transition

•PIM in a molecular magnet is attributed to electronic transitions between HOMOs and LUMOs.•The molecule is simulated first-principally by DFT methods and MOs are calculated.•PIM is calculated considering paramagnetic transitions. The physical mechanism responsible for Faraday rotation (FR) in magnetic molecules under the irradiation of visible light is under intense investigations. In this paper, a quantum mechanical description of the Faraday rotation is provided based on the electronic transitions between the molecular states. The method is applied on bis[tetrachloroferrate(III)]chloride magnetic molecule showing significant FR under irradiation of the visible light (λ=[450,775]nm) at room temperature. Use of dielectric tensor for the molecule in terms of oscillator strengths between HOMOs (highest occupied molecular orbital) and LUMOs (lowest unoccupied molecular orbital) leads to magnetization curve as a function of frequency. The results are obtained in view of quantitative ab initio simulation of electronic structure in the framework of density functional theory (DFT), and the wave functions are calculated making use of Gaussian 03 software within the scheme of B3LYP hybrid functional approximation. The simulation method works for discrete molecules in acetonitrile solvent and considers effect of the solvent as well. The refractive index and the oscillator strengths between molecular states are calculated using the simulated wave functions and the angle of rotation versus wavelength is presented. Comparison of the theoretical results with those of the experiment justifies reasonably the role of electronic transitions between molecular states which have not been emphasized as a mechanism of Faraday rotation in iron-based complexes.