Cyclotriphosphazenes with sulfur-containing side groups: refractive index and optical dispersion

Two types of cyclotriphosphazenes with sulfur-containing side groups were synthesized for an examination of their optical properties including refractive index, optical dispersion, and absorption in the UV-vis region. In the first type, the residue from ethanethiol was introduced through P-S bonding. In the second type, the residues from alcohols that contain alkyl sulfide units were introduced through P-O bonding. A high content of sulfur and phosphorus atoms and thus a high refractive index of 1.6161 were achieved in the first case: however, the optical dispersion increased due to the absorption by the P-S bonds. This increase in optical dispersion was avoided in the second case, where the phosphazene skeleton showed no absorption down to 230 nm. The molar refraction and the molar volume of the -P=N- repeating unit were evaluated, and these demonstrated the advantage of the cyclotriphosphazene skeleton as a building block for optical materials with high refractive indices. In addition, the transparency of the phosphazene skeleton is beneficial for low optical dispersion.