α‑Modified Naphthodithiophene DiimidesMolecular Design Strategy for Air-Stable n‑Channel Organic Semiconductors

Molecular modifications of naphtho­[2,3-<i>b</i>:6,7-<i>b</i>′]­dithiophene diimide (NDTI) by introducing electron-deficient substituents, such as <i>p</i>-(trifluoromethyl)­phenyl-, 5-pyrimidyl-, and chlorine groups, on the thiophene α-positions were examined to develop superior n-channel organic semiconductors for organic thin-film transistors (OTFTs). Among newly developed NDTI derivatives, <i>N</i>,<i>N</i>′-dioctyl-2,7-dichloro-NDTI (<b>5</b>) was found to be a superior semiconductor over <i>N</i>,<i>N</i>′-dioctyl-NDTI (<b>1a</b>). The OTFTs based on <b>5</b> showed electron mobility as high as 0.73 cm² V^–1 s^–1, whereas the mobility of α-unsubstituted <b>1a</b> based OTFTs was 0.05 cm² V^–1 s^–1. The improved mobility by the introduction of chlorine groups can be explained by the electronic structure in the solid state. In contrast to an one-dimensional (1D) electronic structure of <b>1a</b> elucidated by single crystal X-ray analysis and theoretical calculations, <b>5</b> can be characterized as a two-dimensional (2D) bricklayer structure, in which the chlorine groups at the thiophene α-positons play a critical role. In the packing structure of <b>5</b>, there exist intermolecular tape-like arrays connected by intermolecular Cl···OC contacts in the side-by-side direction of the π-stacking columns with the face-to-face intermolecular interaction. In fact, intermolecular lowest unoccupied molecular orbital (LUMO) overlaps estimated by the theoretical calculations suggest the 2D-like electronic structure, which can well explain the better performances in the OTFT devices than those of <b>1a</b>-based ones. From these results, it can be concluded that the chlorination on the thiophene α-positions of the NDTI core is an effective approach to improve performances of NDTI-based n-channel materials by controlling the electronic structures of materials both at the molecular (i.e., highest occupied molecular orbital (HOMO) and LUMO energy level) and the solid-state levels (intermolecular orbital overlaps)