Electronic and optical properties of C16S8 and C16S4Se4 molecules and crystals

Abstract

Hetero[8]circulenes have been proposed as promising fluorescent emitters for organic light-emitting diodes and as emerging materials in the construction of organic solar cells. Among them, octathio[8]circulene (C₁₆S₈) and its derivative tetrathiotetraseleno[8]circulene (C₁₆S₄Se₄) crystals have been highlighted as efficient charge transport materials in field-effect devices. Using density functional theory, we investigate in this paper the electronic and optical properties of the C₁₆S₈ and C₁₆S₄Se₄ molecules and crystals in order to revise previously reported controversial experimental data and report highly reproducible new results. We find that formation of the crystals results in a significant band gap decrease (∼0.6–0.7 eV) caused by relatively strong intermolecular interactions. A partial replacement of S atoms with Se atoms also leads to a small band gap reduction. The C₁₆S₈ crystal demonstrates a band gap of 3.32 eV, while a band gap of 3.20 eV is found for C₁₆S₄Se₄. Both C₁₆S₈ and C₁₆S₄Se₄ compounds are optically transparent in the visible region, confirming the absence of red coloration reported previously in one of experimental papers. The C₁₆S₈ and C₁₆S₄Se₄ crystals demonstrate anisotropic electronic, mechanical and optical properties. These findings might initiate future experimental studies of the red color origin for C₁₆S₈ and provide insight for the design and engineering of C₁₆S₈ and C₁₆S₄Se₄ based charge transport and optical devices.