Computational refinement of the puzzling red tetrasulfur chromophore in ultramarine pigments

Abstract

We investigated the cryptic red chromophore, accompanying the blue S₃˙⁻ radical in ultramarine pigments, which usually was tentatively assigned to an unspecified isomer of either neutral S₄ or ionic S₄˙⁻ species. To reveal its identity, we performed the first systematic density functional studies on periodic and large cluster models of red ultramarines, considering several S₄/S₄˙⁻ isomers embedded in aluminosilicate cages. For both neutral and charged tetrasulfides the most stable confined isomer is the planar C_2v one. The only plausible candidate for the red chromophore among the tetrasulfur species is the planar C_2v isomer of the neutral S₄ molecule, which, apart from being thermodynamically preferable, strongly absorbs green light and its vibrational modes match very well with the available Raman data. The C_2v-S₄˙⁻ radical, if present at all in red ultramarines, could be identified by strong absorption in the near infrared region and possibly by the slightly larger isotropic value of the g tensor than that of the S₃˙⁻ radical.