Carbazole-based copolymers: Effects of conjugation breaks and steric hindrance

A series of carbazole-based D-π-A copolymers were synthesized to investigate the influences of conjugation length and structural distortion on intramolecular charge transfer (CT) complexation between the donor (D) and acceptor (A) components. Carbazole presents two possible linkage sites, the 2,7-and 3,6-positions, which led to significant differences in the thermal, photophysical, electrochemical, and electrical properties of the copolymers due to the positioning of the electron-rich nitrogen atom with respect to the copolymer backbone. The copolymers were comprehensively characterized using TGA, DSC, UV-vis, and photoluminescence spectroscopy, cyclic voltammetry, and DFT calculations. P(3,6C-DTBT), which was linked by a thienyl-2′,1′, 3′-benzothiadiazole (DTBT) group at the 3,6-positions of the carbazoles so as to directly involve the electron-rich nitrogen atoms in conjugation, exhibited conjugation breaks in the middle of the carbazole units. The breaks resulted in a robust coplanar structure with an extraordinarily low oxidation potential and the ability to stably generate excitons, in contrast with P(2,7C-DTBT), which was linked by DTBT at the 2,7-positions of the carbazole. Two additional hexyl substituents at the 4-position of the thiophene in the DTBT groups of P(2,7C-HDTBT) and P(3,6C-HDTBT), which were identical to P(2,7C-DTBT) and P(3,6C-DTBT), respectively, except for the presence of the substituents, introduced steric hindrance between the D and A units, thereby breaking the coplanarity. Finally, the hole mobilities of the 3,6-carbazole-based copolymers were 1 order of magnitude higher than those of 2,7-carbazole-based copolymers, measured in hole-only devices. This result indicated the presence of stable radical cations and dications at the nitrogen atoms of the copolymers. This work deepens our understanding of carbazole-based D-π-A copolymers and provides insight into the design of novel materials for optoelectronic devices.