Modeling the Photochromism of S-Doped Sodalites Using DFT, TD-DFT, and SAC-CI Methods

S-doped sodalite minerals of the Na₈Al₆Si₆O₂₄(Cl,S)₂ formula, also known as hackmanites, are computationally investigated for the first time, in order to understand their photochromic properties. With combined periodic boundary conditions and embedded cluster-type approaches, this paper brings a theoretical overview of the photochromism mechanism, also called tenebrescence in geology. Time-dependent density functional theory (TD-DFT) calculations of sodalite systems containing electrons trapped in Cl vacancies showed an absorption spectrum and a simulated color in agreement with experiment. This modeling highlights the huge effect of the F center's environment such as the direct contribution of the β cage on the trapped electron and a strong vibronic coupling of the absorption spectrum. TD-DFT and post-Hartree-Fock (SAC-CI) calculations were also operated on S₂^2--containing systems in order to determine the exact mechanism of coloration and discoloration, supporting that the key step is a direct through-space charge transfer between the S₂^2- ion and a Cl vacancy. The geometry modification induced by this charge transfer leads to a large electronic reorganization stabilizing the F center, thus explaining the high stability of the colored state of the mineral.