What makes brickwork crystal structures favourable? A case study on methylthiolated arenes and heteroarenes for high-mobility molecular semiconductors

Abstract

Among various types of crystal structures of molecular semiconductors, a brickwork structure, a two-dimensional π-stacking structure, has been regarded as a promising platform for high-mobility material. The brickwork structure is a kind of “unnatural” crystal structure for parent polycyclic aromatic hydrocarbons (PAHs), and thus, to realize the brickwork structures, appropriate molecular modification is mandatory. The conventional molecular design for the brickwork structures was to introduce (trialkylsilyl)ethynyl groups on the acene and heteroacene cores. On the other hand, we have recently found that regio-selective methylthiolation on PAHs is also a promising strategy. Thus developed 1,3,6,8-tetrakis(methylthio)pyrene and 1,3,8,10-tetrakis(methylthio)peropyrene gave single-crystal field-effect transistors (SC-FETs) showing ultrahigh mobility of 30 cm² V⁻¹ s⁻¹, which are among the highest mobilities reported for molecular semiconductors. However, not all the methylthiolated PAHs with the brickwork structures showed such promising performances as molecular semiconductors, which prompted us to quantify the brickwork structures for a better understanding of the system. In the present Highlights, we first describe the quantitative description of brickwork crystal structures. This enabled us not only to classify the range of molecules with brickwork crystal structures in terms of dimensionality but also to clarify the relationship between the typical brickwork structure with the P space group and inclined brickwork structures and, even more, pitched π-stacking crystal structures with different molecular arrangments. Then, the roles of methylchalcogeno groups were discussed in realizing the brickwork crystal structures, in comparison to the halogen counterparts. Finally, the molecular design principle for the brickwork crystal structure is discussed. With these analyses and discussions, we would like to share our viewpoints with readers to develop better molecular designs for high-performance molecular semiconductors.