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 alpha or beta 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[2,3b: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 cm2 V-1 s-1 together with a remarkably high current on/off ratio around 109. Furthermore, organic phototransistors (OPTs) based on 2,8-C8NBTT thin film also exhibits higher photosensitivity (P) of 2.0 x 108, photoresponsivity (R) of 3.3 x 103 A W-1, and detectivity (D*) of 1.3 x 1016 Jones than those based on NBTT and 3,9-C8NBTT.