Experimental and Theoretical Insights into the Optical and Vibrational Behavior of a Zero-Dimensional Transition Metal-Based Tetrachloridocobaltate Hybrid

Abstract

This work presents a combined experimental and theoretical study of the molecular structure, vibrational, and optical properties of tris(1,8-octanediammonium) bis(tetrachloridocobaltate) dichloride (C₈H₂₂N₂)₃(CoCl₄)₂Cl₂. The crystal structure consists of discrete [CoCl₄]²⁻ anions surrounded by organic (C₈H₂₂N₂)²⁺ cations and uncoordinated Cl⁻ ions, resulting in a zero-dimensional structure. Optical properties were investigated using UV–visible absorption and photoluminescence (PL) spectroscopy at room temperature. The UV–visible absorption spectrum exhibits an absorption edge near 280 nm, corresponding to an optical band gap of 4.42 eV. The PL spectrum shows a strong blue-green emission centered at 470 nm (2.63 eV). Additional characterization was performed using Raman and infrared spectroscopy at room temperature. For reliable assignment of vibrational modes, the IR and Raman spectra were calculated using density functional theory (DFT) at the B3LYP/LanL2DZ level. To gain deeper insight into the electronic structure, DFT and time-dependent DFT (TD-DFT) calculations were carried out, enabling evaluation of the density of states (DOS) and estimation of the band gap. The theoretical results show good agreement with the experimental observations, as evidenced by the close correspondence between calculated and measured absorption spectra. These results demonstrate that the optical properties arise from a synergistic interplay between the inorganic [CoCl₄]²⁻ units and the organic sublattice. In addition to offering a less toxic alternative to lead-based systems, this material shows promising potential for optoelectronic and photonic applications.