1,6-Naphthodipyrrolidone-based donor–acceptor polymers with low bandgap

Abstract

Donor–acceptor (D–A) copolymers P1–P4 containing 1,6-naphthodipyrrolidone as acceptor unit are synthesized and characterized. Copolymers P1 and P2 are prepared upon Suzuki coupling of 3,8-di(4-bromophenyl)-1.6-bis(dodecyl)-2,7-dioxo-1,2,6,7-tetrahydronaphtho[1,2-b:5,6-b′]dipyrrole (M1) or 3,8-di-(4-bromophenyl)-1.6-bis(2-octyldodecyl)-2,7-dioxo-1,2,6,7-tetrahydronaphtho[1,2-b:5,6-b′]dipyrrole (M2) with the donor comonomer 2,2-(9,9-dioctyl-9H-fluoren-2,7-diyl)-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M3). Polymers P3 and P4 are prepared upon Stille coupling of M1 or M2 with the donor comonomer 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl-bistrimethylstannane M4. The two monomers M1 and M2 exhibit a deep purple color in dichloromethane with high extinction coefficients up to 4.9*104 L mol−1 cm−1. Optical, electrochemical and thermal properties of the polymers are characterized. The polymers exhibit a broad UV/vis absorption range from 400 to 850 nm, low optical bandgaps (1.27–1.47 eV), low LUMO levels (especially for P1 and P2), a high thermal stability with glass temperature Tg up to 224 °C, and 5% weight loss up to 402 °C. Density functional theory calculations indicate that the backbone of the 1,6-NDP-polymers is rather planar. Photostability studies indicate a high stability against UV-irradiation in the solid state, but only a moderate stability upon UV irradiation in solution.