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

Molecular modifications of naphtho­[2,3-b:6,7-b′]­dithiophene diimide (NDTI) by introducing electron-deficient substituents, such as p-(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, N,N′-dioctyl-2,7-dichloro-NDTI (5) was found to be a superior semiconductor over N,N′-dioctyl-NDTI (1a). The OTFTs based on 5 showed electron mobility as high as 0.73 cm2 V–1 s–1, whereas the mobility of α-unsubstituted 1a 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 1a elucidated by single crystal X-ray analysis and theoretical calculations, 5 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 5, 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 1a-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).