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).