Synthesis of benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers with enhanced photoluminance stability

The benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers with perfluoroalkyl side chain were synthesized by Suzuki coupling reaction. The copolymers were characterized and investigated via Fourier infrared spectroscopy (FTIR), hydrogen nuclear magnetic resonance spectroscopy ( 1 HNMR), cyclic voltammetry (CV), ultraviolet absorption spectroscopy (UV-vis) and photoluminance spectroscopy (PL). The results showed that benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers with different amounts of perfluoroalkyl structural units were successfully synthesized. Because of the long fluoroalkyl side chain in the copolymers, the maximum ultraviolet absorption wavelength of the benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers was blue shifted compared with that of non-fluorine-containing copolymer of poly(benzothiadiazole-9,9-dioctylfluorene) (PF0BT10). The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the copolymers were lowered and the energy bandgap ( E gap ) was narrowed. Because of the self-assembly of the long perfluoroalkyl side chain, the benzothiadiazole-9,9-bis(perfluorohexylethyl propionate) fluorene copolymers exhibit excimer emission at 550 nm. Due to the protective effect of the long perfluoroalkyl side chain on the main chain, the photoluminance stability of the copolymers was significantly improved. After being placed in an atmosphere of about 70−75% relative humidity for 210 min, the photoluminance intensity of benzothiadiazole-9,9- bis(perfluorohexylethyl propionate) fluorene copolymers at 530 nm had 54 % of the initial photoluminance intensity, while only 36 % of the initial photoluminance intensity at 530 nm for PF0BT10.