Synthesis of Crown Ether End-Capped PM6 Copolymers with Tunable Molecular Weight for Organic Solar Cells

Abstract

Poly-[(2,6-(4,8-bis­(5-(2-ethylhexyl-3-fluoro)­thiophen-2-yl)-benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-c:4′,5′-c′]­dithiophene-4,8-dione))] (PM6) is one of the star molecules in organic solar cells (OSCs). However, PM6 has been suffering from molecular weight control and unreacted terminal groups. Thus, synthesizing high-quality polymers without batch-to-batch variations has been challenging for many years. Herein, a one-step protocol of both end-capping and polymerization at the same time was developed to prepare end-capped PM6 with molecular weight controlled by changing the monomer ratio. Specifically, non-end-capped PM67 and a series of PM6En with different molecular weights were synthesized and characterized. Our research findings indicated that (1) these copolymers’ number-average molecular weights (M ns) and weight-average molecular weights (M ws) were measured to be almost linearly increased. Although textbooks have mentioned the nonequivalent ratio monomer strategy to regulate the molecular weight of polymers, the end-capping yield has not been reported in the research of OSCs due to a lack of efficient experiments and instruments to measure the low percentage of end groups in the polymer. (2) Since the selected end-caping reagent 4′-bromobenzo-18-crown-6 (EBr) can be easily distinguished from the PM6 main backbone, we first measured the end-capping yield by both nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOF-MS). We applied this approach for the first time to synthesize polymers with reasonable end-capping yields (40–97%) for OSCs. (3) Surprisingly, 18-crown-6 can reduce end-group defects, lower the polydispersity index (PDI) of the polymer, and achieve suitable morphology and mobility. As a result, the end-capped PM6E7-based device exhibited the best stability and power conversion efficiency (PCE) compared to those of the other end-capped or non-end-capped polymers. Both the end groups and molecular weight significantly affect the performance of the OSCs. PM6 and other copolymers could be finely tuned by subtly changing the molecular weights or end-cap groups with functional building blocks. Our research will stimulate the development of OSCs and other applications of copolymers.

Single-Molecule Study Reveals Ion-Dependent Conformational Change and Nanomechanical Property of Crown Ether-Based Polymer

“…Supramolecular polymeric networks (SPNs) are polymeric networks constructed by both covalent and noncovalent interactions. , SPN-based materials combine the advantages of both covalent and noncovalent polymers through efficient functionalization, and they have a wide range of applications. − The dynamic noncovalent supramolecular interactions in the SPNs endow the corresponding materials with high processability and excellent stimulus-responsiveness. − Among the numerous supramolecular interactions, the outstanding selectivity and tunability make the host–guest interactions one of the widely studied topics in supramolecular chemistry. − There are many macrocycle-based host–guest systems, such as crown ethers, − cyclodextrins, , cucurbit­[n]­urils, , pillararenes, , calixarenes, heteracalixaromatics, corona­[n]­arenes, and beltarenes . Among them, crown ethers, the first generation of macrocyclic hosts, play a key role in the development of host–guest and supramolecular chemistry.…”