Optical and Magnetic Properties of Yellow Chromophore S2•- in Ultramarine Pigments: CASPT2 and DFT Studies

Disulfur radical anion S2•- attracts attention as a yellow chromophore, orange emitter, and participant in many industrially relevant chemical and electrochemical processes, e.g., in metal-sulfur batteries. The reactive S2•- radical can be stabilized by embedding in protective matrices, such as ultramarine pigments, where it accompanies other sulfur-radical species, mostly S3•-. Despite the apparent simplicity of S2•-, the assignment of its electron paramagnetic resonance (EPR) signal in ultramarines remains controversial. To elucidate this topic, we computationally investigate S2•- embedded in cluster models of aluminosilicate cages in ultramarines. We find that the optical properties of S2•- can be reliably treated using time-dependent density functional theory (DFT). However, to correctly predict the g tensor of either isolated or embedded S2•-, we find it necessary to employ the multiconfigurational complete active space method (CASSCF/CASPT2), combined with a variational treatment of strong spin-orbit coupling (SOC), stemming from orbital near-degeneracy. The g tensor of S2•- in ultramarine is predicted to be strongly anisotropic, nearly axial, and notably environment-dependent, which may contribute to the difficulties in its unambiguous experimental characterization. Our results help clarify the assignment of the S2•- EPR signal, elucidating some partially correct experimental data and identifying other interpretations as erroneous. This study should also be relevant for modeling other dichalcogenide radicals embedded in solid-state matrices.