Synthesis of Asymmetric Benzotrithiophene/Benzotriselenophene Building Blocks and Their Donor–Acceptor Copolymers: Chalcogen Effect on Face-on/Edge-on Orientations and Charge Transport

Two asymmetric 5-octadecanyl benzotrithiophene (BTT) and benzotriselenophene (BTS) were designed and synthesized through a key step of palladium-catalyzed intramolecular C–H arylation. The two-dimensional stannylated BTT and BTS monomers were then polymerized with a DPP comonomer by Stille coupling to afford two new donor–acceptor polymers, PBTTDPP and PBTSDPP. With the chalcogen effect of selenium, PBTSDPP exhibited a higher-lying HOMO level and a more red-shifted absorption than the corresponding PBTTDPP. The crystalline domains of both PBTTDPP and PBTSDPP were analyzed via GIWAXS measurements, revealing perpendicular π–π stacking of the main-chain backbone and lamellar stacking of side-chain interdigitation. The dimensions of the π–π stacking and lamellar stacking of the crystalline domains were estimated by calculating their corresponding correlation lengths (Lc). We discovered that the ratio between the correlation lengths of lamellar stacking (Ll) and π–π stacking (Lπ), denoted as (RL), plays a crucial role in determining the edge-on/face-on polymer orientations of substrates. PBTTDPP, with a larger RL of 2.56, predominantly exhibits a face-on orientation, while PBTSDPP, which has a stronger π–π interaction due to Se inclusion, leads to a smaller RL of 1.36, preferring an edge-on orientation. After thermal annealing at 210 °C for 10 min, PBTSDPP forms higher-order crystalline domains, increasing its mobility from 0.04 to 0.17 cm2 V–1 s–1. Despite its face-on orientation, the pristine PBTTDPP, with a higher molecular weight, exhibits the best mobility of 0.61 cm2 V–1 s–1 due to efficient intrachain charge transport and good connectivity between different nanodomains. However, thermal annealing of PBTTDPP leads to a decrease in π–π stacking distance from 3.67 to 3.41 Å, facilitating charge transport via face-on π-stacking hopping perpendicular to the source-to-drain direction. Consequently, the mobility of annealed PBTTDPP is reduced to 0.18 cm2 V–1 s–1. This work demonstrates that the use of benzotrichalcogenophenes in the polymer backbone can fine-tune intermolecular interactions, controlling the orientations in crystalline nanodomains that ultimately determine the charge transport properties.