The absorption wavelengths of sulfur chromophors of ultramarines calculated by time-dependent density functional theory

Abstract

The neutral tetrasulfur and the trisulfur and tetrasulfur anion radicals were studied by density functional theory (DFT) employing the hybrid functionals B3LYP, PBE0 and PB95 and large basis sets. The spectral data were obtained by time-dependent density functional theory (TD-DFT). The calculated absorption wavelengths for the calculated vertical electronic transitions in the visible region were compared with the wavelengths of the absorption maxima of ultramarines and of synthetic sulfur compounds measured in matrices or in solution. The chromophore of ultramarine blue was considered as a test case and the chromophore of ultramarine red studied in several model calculations. The calculated transition energies and intensities favour the cis tetrasulfur chromophore of red ultramarine. The excellent agreement between the theoretical and experimental absorption wavelengths of the color bands supports earlier assignments. The electronic structure of cis and trans tetrasulfur is discussed in terms of ylidic and biradical resonance structures by natural resonance theory. The singlet ground states display RKS/UKS instability. The broken-symmetry UKS structure is of lower energy than the RKS structure, which does, however, not influence the molecular and electronic structure significantly.