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 (L_c). We discovered that the ratio between the correlation lengths of lamellar stacking (L_l) and π-π stacking (L_π), denoted as (R_L), plays a crucial role in determining the edge-on/face-on polymer orientations of substrates. PBTTDPP, with a larger R_L 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 R_L 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.