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-catalyzedintramolecular C-H arylation. The two-dimensional stannylatedBTT and BTS monomers were then polymerized with a DPP comonomer byStille coupling to afford two new donor-acceptor polymers,PBTTDPP and PBTSDPP. With the chalcogen effect of selenium, PBTSDPPexhibited a higher-lying HOMO level and a more red-shifted absorptionthan the corresponding PBTTDPP. The crystalline domains of both PBTTDPPand PBTSDPP were analyzed via GIWAXS measurements, revealing perpendicular & pi;-& pi; stacking of the main-chain backbone and lamellarstacking of side-chain interdigitation. The dimensions of the & pi;-& pi;stacking and lamellar stacking of the crystalline domains were estimatedby calculating their corresponding correlation lengths (L (c)). We discovered that the ratio between the correlationlengths of lamellar stacking (L (l)) and & pi;-& pi; stacking (L (& pi;)), denoted as (R (L)), plays a crucial rolein determining the edge-on/face-on polymer orientations of substrates.PBTTDPP, with a larger R (L) of 2.56, predominantlyexhibits a face-on orientation, while PBTSDPP, which has a stronger & pi;-& pi; interaction due to Se inclusion, leads to asmaller R (L) of 1.36, preferring an edge-onorientation. After thermal annealing at 210 & DEG;C for 10 min, PBTSDPPforms higher-order crystalline domains, increasing its mobility from0.04 to 0.17 cm(2) V-1 s(-1). Despite its face-on orientation, the pristine PBTTDPP, with a highermolecular weight, exhibits the best mobility of 0.61 cm(2) V-1 s(-1) due to efficient intrachaincharge transport and good connectivity between different nanodomains.However, thermal annealing of PBTTDPP leads to a decrease in & pi;-& pi;stacking distance from 3.67 to 3.41 & ANGS;, facilitating charge transportvia face-on & pi;-stacking hopping perpendicular to the source-to-draindirection. Consequently, the mobility of annealed PBTTDPP is reducedto 0.18 cm(2) V-1 s(-1). This work demonstrates that the use of benzotrichalcogenophenesin the polymer backbone can fine-tune intermolecular interactions,controlling the orientations in crystalline nanodomains that ultimatelydetermine the charge transport properties.