Structural, Optical, and Magnetic Properties of Ultramarine Pigments: A DFT Insight

The ultramarine pigments are among the most widely used coloring materials since the antiquity till present times. Despite many experimental studies, the characterization of ultramarines is still incomplete. In this work, we reported for the first time the density functional theory results obtained for realistic periodic and large cluster models of ultramarines with blue S3– and yellow S2– chromophores. Periodic calculations provided insight into Sn– siting inside aluminosilicate cages, normally not resolved well in experimental structural data. All electron calculations performed on large cluster models showed that the optical properties of S3– ions depend little on their orientation within cavities, unless strong distortion from free S3– ion C2v symmetry is enforced by the lattice. No magnetic coupling between S3– species occupying adjacent cages was found. Upon the present results, observed differences in the averaging of electron resonance signals should be rather ascribed to different S3– dynamical effects. Though the quantitative computational treatment of S2– systems is more challenging because of near orbital degeneracy, the qualitative results show that the electronic structure and spectroscopic properties of embedded S2– radicals are more sensitive to the environment than in the case of S3– species.