Effect of fluorination pattern and extent on the properties of PCDTBT derivatives

Herein, we report the synthesis of a series of fluorinated dithienyl carbazole-alt-benzothiadiazoles (PCDTBT analogues) and the characterisation of their optical, electrochemical, thermal and molecular organisation in the solid state. The polymers were decorated with fluorine on either the benzothiadiazote unit, carbazole unit or both to yield PCDTffBT, PCffDTBT and PCffDTffBT, respectively. The copolymers displayed decomposition temperatures in excess of 350 degrees C. PCDTffBT, PCffDTBT and PCffDTffBT displayed optical band gaps of 1.86, 1.82 and 1.88 eV, respectively. It was speculated this was a consequence of the higher molecular weight of PCffDTBT relative to the other polymers. PCffDTBT and PCffDTffBT displayed shallower HOMO levels relative to PCDTffBT; a consequence of fluorinating the carbazote-donor moiety. XRD studies confirmed that fluorinating the benzothiadiazole-acceptor moiety improves molecular ordering by promoting pi-pi stacking of polymer backbones in solid state. Interestingly, fluorinating the carbazole-donor unit does not improve pi-pi stacking of polymer backbones.