Tailoring electronic structure and light-matter interaction in a 0D hybrid chlorozincate (C10H9N2)2ZnCl4 with a direct-forbidden optical transition and low structural disorder.

The organic-inorganic hybrid compound (C10H9N2)2ZnCl4 was synthesized by a hydrothermal route and comprehensively characterized through structural, thermal, spectroscopic, optical, and theoretical approaches. Single-crystal X-ray diffraction revealed a centrosymmetric monoclinic structure (space group C2/c) consisting of discrete tetrahedral [ZnCl4]2- anions separated by protonated 2,2'-bipyridin-1-ium cations, giving rise to a zero-dimensional supramolecular architecture. The crystal packing is stabilized by a cooperative network of N-H⋯Cl and C-H⋯Cl hydrogen bonds together with weak π-related interactions, as confirmed by Hirshfeld surface analysis. Thermal analysis revealed a reversible first-order phase transition near 489 K, while melting occurred above 533 K. The FTIR and Raman spectra were interpreted with the support of DFT calculations, which satisfactorily reproduce the principal vibrational features of the compound. Optical investigations revealed absorption bands associated with π → π*, n → π*, ligand-to-metal charge-transfer (LMCT), and charge-transfer (CT) transitions. Diffuse reflectance analysis demonstrated that the optical absorption edge is best described by a direct-forbidden electronic transition, yielding an optical band gap of 1.92 eV, whereas the low Urbach energy (0.168 eV) indicates a low degree of structural disorder. Frontier molecular orbital, density of states (DOS), and partial density of states (PDOS) analyses provide qualitative insight into the electronic structure and the respective contributions of the organic cations and inorganic anions to the frontier electronic states. The combination of structural robustness, thermal stability, visible-light absorption, and narrow-band-gap semiconducting behavior highlights the potential of this lead-free hybrid chlorozincate for optoelectronic and photonic applications.