Backbone-shape engineering of fused-ring electron-deficient molecular semiconductors for unipolar n-type organic transistors: synthesis, conformation changes, and structure–property correlations

Abstract

Backbone-shape engineering is one important strategy towards high-mobility molecular semiconductors. However, it has been seldom employed in modifying fused-ring electron-deficient small molecules, which can provide useful molecular structure–property correlations for developing high-performance n-type organic semiconductors. In this work, we design and synthesize a series of n-type fused-ring small molecules based on ladder-type dithienothiophen-pyrrolobenzothiadiazole (TPBT) derivatives, namely BTPOCl-c, BTPOCl-s, and BTPOCl-w. Due to the different backbone symmetry and length of TPBT derivatives, the above molecules demonstrate three shapes, i.e. C-, S-, and W-backbone shape. Intriguingly, all molecules show a slightly helical conformation between the central cores and end-groups. However, the dihedral angles in the helical conformation are different, which gradually become smaller in the trend of BTPOCl-c < BTPOCl-s < BTPOCl-w. Therefore, changing the backbone shape from C to S and then to W is conducive to enhancing the backbone planarity. Consequently, BTPOCl-w-based unipolar n-type organic transistors display the highest electron mobility among the three molecules. The above result is reasonably explained by BTPOCl-w's deepest energy levels, highest crystallinity with the shortest π–π stacking distance, and larger grain size with fewer grain boundaries. We believe that backbone-shape engineering is a promising approach for developing advanced n-type molecular semiconductors.