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.