Parameters influencing the molecular weight of 3,6‐carbazole‐based D‐π‐A‐type copolymers

Abstract

Condensation copolymerization reactions of carbazole 3,6-diboronate with 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (DTBT) only produce low-molecular-weight donor (D)-p-acceptor (A) copolymers. High-molecular-weight copolymers for use in optoelectronic devices are necessary for achieving extended p-conjugation and for controlling the copolymer processibility. To elucidate the cause of the persistently low molecular weight, we synthesized three 3,6-carbazole-based D-A copolymers using copolymerizations of N-9 0heptadecanyl-3,6-carbazole with DTBT, N-9 0 {2-[2-(2-methoxyethoxy)-ethoxy]-ethyl}-3,-6-carbazole with DTBT, and N-9 0 -heptadecanyl-3,6-carbazole with alkyl-substituted DTBT. We investigated several parameters for their influence on molecular copolymer weight, including the conformation of the chain during growth, the solubility of the monomers, and the dihedral angles between the donor and acceptor units. Size exclusion chromatography, UV-vis absorption spectroscopy, and computational studies revealed that the low molecular weights

Suppressing charge recombination by incorporating 3,6‐carbazole into poly[9‐(heptadecan‐9‐yl)‐9H‐carbazole‐2,7‐diyl‐alt‐(5,6‐bis‐(octyloxy)‐4,7‐di(thiophen‐2‐yl)benzo[1,2,5]‐thiadiazole)‐5,5‐diyl]

“…The reduced performances resulted first from the low molecular weight. As we reported previously, the π–π interactions were strong, even in solution, and limited the formation of copolymers with a high molecular weight during the copolymerization reaction. Although the strong π–π stacking interactions improved intermolecular charge transport in the copolymers, an excess of 3,6‐carbozole units, which formed partial negative charges at the conjugation breaks and did not participate in the intermolecular interactions, contributed to the temporary trapping of holes generated in the blend.…”

“…The incorporation of 3,6‐carbazole units into the copolymers was expected to induce the strong intramolecular charge transfer (ICT), and, therefore, a conjugation break in the center of the backbone. The conjugation break was expected to interfere with chain growth as demonstrated previously . The molecular weights of the copolymers did not decrease as the 3,6‐carbazole 3 content was increased to 10 mol %, yielding molecular weights in the range of 22.0–23.2 kDa ( M w / M n = 1.8–2.6).…”

“…Previously, we also reported that 3,6‐carbazole copolymerized with bis (bromothienyl)benzo‐thiadiazole (DTBT) induced conjugation breaks in the middle of the carbazole units of the resulting copolymers. The polymers revealed a highly coplanar structure that formed strong intramolecular charge complexes with strong π–π stacking between the conjugation breaks on 3,6‐carbazole . This concept was applied to poly[9,9‐dioctylfluorene‐alt‐(bisthienyl)‐benzo‐thiadiazole]s (PFDTBT) and poly[ N −9′‐heptadecanyl‐2,7‐carbazole‐alt‐(bisthienyl)‐benzothiadiazole]s (PCDTBT) .…”

Simultaneously Grasping and Self‐Organizing Photoactive Polymers for Highly Reproducible Organic Solar Cells with Improved Efficiency

“…Therefore, incorporation of 3,6‐carbazole‐ DTBT units (3,6‐carbazole moiety) having a significantly high T g value into conjugated polymers based on 2,7‐carbazole could lead simultaneous grasping and self‐organizing between polymer chains under a thermal annealing condition. This could not only provide a highly stable morphology but also an improved hole mobility26 in a blend of donor‐acceptor (D‐ π ‐A)‐type conjugated polymer and a fullerene derivative.…”

“…The presence of 3,6‐carbazole in P(C 49 C 1 ‐DTBT) clearly affects its electronic delocalization in the film state. Indeed, the presence of 3,6‐carbazole also increases the electron‐donating properties of the 2,7‐carbazole repeat units and the intramolecular charge transfer (ICT) along the polymer backbone, which results in the slight increase in the absorption maximum 25–27. The optical energy gap of P(C 49 C 1 ‐DTBT) was measured from the absorption onset in the film spectrum and found to be the same as that of P(2,7C‐DTBT) (1.88 eV).…”