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

Abstract

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.