The Use of Cyclopenta[2,1‐b;3,4‐b′]dithiophene Analogues for the Development of Low‐Bandgap Materials

Abstract This paper presents the synthesis and characterization of low‐bandgap materials based on cyclopenta[1,2‐ b ;3,4‐ b ′]dithiophene (CPDT). An array of electron‐poor monomers is synthesized and used to prepare homo‐ and alternating donor–acceptor copolymers. This yields polymers with low bandgaps ( E g = 1.12–1.23 eV) and broad light absorption (400–1100 nm). The influence of the electron‐withdrawing substituent and the donor material on the polymer properties is studied. It is demonstrated that the low bandgap is not a result of intramolecular charge transfer, but of a transition localized in the electron‐poor CPDT monomer. The bandgap can be correlated with the substituent on the CPDT monomer and is relatively independent of other parameters, resulting in materials with an easily tunable bandgap.