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

Abstract

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.

Enhancement of Conjugated Polymer Microstructure and Mixed-Conducting Properties via Chalcogenophene Heteroatom Substitution

“…p­(g3T2-S) presents an intermediate case showing distinct π-stack and lamellar scattering both in-plane and out-of-plane (Figure b), indicating a relatively equal mix of face- and edge-on 2D crystallites, both with some distribution of orientation angles about perfectly edge- or face-on. Such chalcogenophene heteroatom directed crystallite orientation was not wholly unexpected as it has been observed before in other systems. , …”

Synthesis and charge-transport properties of novel π-conjugated polymers incorporating core-extended naphtho[2,1-b:3,4-b′]dithiophene diimides

“…Given the presence of low crystallinity, unsatisfactory hole mobility was thus achieved by the three NTI-based polymers when compared with the state-of-the-art polymer systems, for example, DPP- and NDI-based polymers. 37,67…”

Designer Conjugation-Break Spacer That Boosts Charge Transport in Semiconducting Terpolymers

“…Conjugated polymers (CPs) offer appealing advantages such as good solution processability, excellent mechanical flexibility, diverse molecular design, and tunable optoelectronic properties over inorganic silicon, which make them promising for applications in organic field-effect transistors (OFETs), − organic solar cells, − organic light-emitting diodes, and other electronic devices. − Great advances have been made in improving the charge carrier transport properties of CPs, primarily through the development of innovative electroactive building blocks. − It is well understood that rigid structures and coplanar geometries are favorable for strengthening intermolecular interactions and promoting tight interchain stacking, both being essential for effective intermolecular charge carrier transport. − In addition, in order to maximize the conjugation length to facilitate intrachain charge transport, those building blocks are designed to possess a fully π-conjugated skeleton, typically composed of sp 2 -hybridized carbons which constitute alternating single and double carbon–carbon bonds. − Constrained by these conventional perceptions, nonconjugated building blocks are typically excluded from being used for high-mobility CPs as they are electronically not favorable for charge delocalization and transport. In fact, for the rare instances where conjugation-break spacers (CBSs) have been incorporated into the backbone of CPs, the attention has been solely directed toward the use of flexible, nonconjugated moieties such as aliphatic or amide-containing segments, for regulating the solution processability and mechanical property of polymers. ,, While inclusion of such flexible linkers was successful in adjusting the solubility and film stretchability, , such modifications have inevitably compromised charge carrier transport after disrupting π-electron delocalization along the polymer backbone and weakening interchain interactions, potentially deterring further strategic use of CBS in CPs (Scheme a). , …”