Molecular self-assembly of 1D linear and 2D T-shape polyiodide arrangements in two organic ammonium triiodide salts: supramolecular structures and density functional theory-based optical properties

Abstract

The preparation of two tri-iodide salts, C₇H₁₁N₂⁺·I₃⁻·0.5H₂O (1) and (C₂H₅)₄N⁺·I₃⁻ (2), synthesized by the slow diffusion method in solution, is reported. Their structures have been characterized by means of single-crystal X-ray diffraction. They consist of two different arrangements of tri-iodide ions, organized in a one-dimensional linear chain for 1 and in two-dimensional T-shaped sheets for 2, stabilized by 4-dimethylaminopyridinium and tetraethylammonium ligands. An asymmetry of the tri-iodide anions is observed in both structures, and is associated to the I⋯I halogen and hydrogen bonding interactions. The generation of the resulting supramolecular networks for these compounds was found to be driven by hydrogen-bond assisted self-assembly. The crystal structures of the two compounds were found to be supported essentially by H⋯I interactions between organic molecules and tri-iodide anions. For compound 2, an additionally stabilization by O⋯H/H⋯O interactions due to the presence of water molecules occurs. The dominant intermolecular interactions and their directionality have been examined by Hirshfeld surfaces and fingerprint analysis. Both compounds were characterized further by FT-IR and Raman spectroscopy, TGA/DTG and UV-visible absorption by means of thin film methods. The target compounds were also studied computationally using density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches to explore their potential performances and to rationalize the experimental results. An electronic transition from 3σ_g to has been proven to dominate intramolecular charge transfer occurring for both 1 and 2. This electronic communication leads to an HOMO–LUMO energy gap of 4.208 eV for 1 and 4.255 eV for 2.