Dinaphtho[2,1-b:1′,2′-d]thiophenes as High Refractive Index Materials Exploiting the Potential Characteristics of “Dynamic Thiahelicenes”

Dinaphtho[2,1-b:1′,2′-d]thiophene (DNT) is one of the “dynamic thiahelicenes” that is obtained as a racemate due to its low energy barrier for enantiomerization. Careful evaluation of both the metastable DNT film and the stable films of DNT/polymethyl metacrylate (PMMA) blends revealed that DNT has an extra-high refractive index of 1.808 at 633 nm, which is rarely achieved in organic small molecules. The fundamental factors responsible for the unusual observation of DNT were explored by comparing the properties of DNT with those of a flat-shaped dibenzothiophene (DBT) both in experiments and DFT calculations, and by analysing them with the Lorenz-Lorentz equation. It became clear that the unique properties of DNT were based on its flexible dynamic thiahelicene structure, leading to the metastable film-forming ability, the high compatibility with PMMA, and the exhibition of an extra-high refractive index caused by the dense packing in the amorphous film. Chemical modification of DNT provided new halogen-free vinyl monomers, which could be easily polymerized to form vinyl polymers with high refractive indices of 1.72-1.74, as well as film-forming ability, high thermal stability, and high transparency in the visible region. Thus, we could demonstrate that the DNT structure is a new promising candidate for organic polymeric high refractive index materials.