Computational refinement of the puzzling red tetrasulfur chromophore in ultramarine pigments

We investigated the cryptic red chromophore, accompanying blue S3- radical in ultramarine pigments, which usually was tentatively assigned to the 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 C2v one. Generally, the properties of embedded anionic S4- radicals (energetic stability order, electronic excitations) deviate more strongly from the gas phase values than it is found in the case of embedded and free neutral S4 molecules. However, some notable differences in the position of vibrational modes are induced by the environment also for the neutral species. The only plausible candidate for red chromophore among the tetrasulfur species is the planar C2v isomer of neutral S4 molecule, which, apart from being thermodynamically preferable, strongly absorbs green lights and its vibrational modes match very well the available Raman data. The C2v–S4- radical, if present at all in red ultramarines, could be identified by strong absorption in near infrared region and possibly by the slightly larger isotropic value of g tensor than this of S3- radical.