Modulating the Alkylation Position on Terminal Thiophene Ring of Naphtho[2,3-b:6,7-b′] Bithieno[2,3-d] Thiophene (NBTT) for High-Performance Organic Optoelectronic Devices

Several thiophene terminated thienoacenes with high mobilities in organic thin-film transistors (OTFTs) have been reported; however, the structure–property relationship of thiophene terminated thienoacenes was unclear, especially the impact of α or β position substitution of terminal thiophene ring on molecular packing and physicochemical properties. Here, we report the synthesis and characterization of a six-ring-fused naphtho[2,3- b:6,7- b ′] bithieno[2,3- d ] thiophene (NBTT) and its derivatives 2,8-dioctyl-naphtho[2,3- b:6,7- b ′] bithieno [2,3- d ] thiophene (2,8-C8NBTT) and 3,9-dioctyl-naphtho[2,3- b:6,7- b ′] bithieno [2,3- d ] thiophene (3,9-C8NBTT). It is found that the alkylation on terminal thiophene ring can effectively tune the molecular stacking from a cofacial herringbone stacking mode (NBTT) to layer-by-layer packing (2,8-C8NBTT and 3,9-C8NBTT). Impressively, a hopping to “band-like” charge transport mechanism evolution of vacuum deposited films is realized by modulating the alkylation position on the terminal thiophene rings. As a result, the OTFTs based on 2,8-C8NBTT characterized by a “band-like” transport presents the highest mobility of 3.58 cm 2 V –1 s –1 together with a remarkably high current on/off ratio around 10 9 . Furthermore, organic phototransistors (OPTs) based on 2,8-C8NBTT thin film also exhibits higher photosensitivity ( P ) of 2.0 × 10 8, photoresponsivity ( R ) of 3.3 × 10 3 A W –1, and detectivity ( D *) of 1.3 × 10 16 Jones than those based on NBTT and 3,9-C8NBTT.