Impact of ethynylene linkers on the optical and electrochemical properties of benzothiadiazole based alternate conjugated polymers

The effect of ethynylene spacers on the bandgap of alternating polymers, comprising 4,7-linked benzothiadiazole units and 2,7-linked fluorene, 2,7-linked carbazole or 2,6-linked anthracene repeat units has been investigated. The three novel polymers PFDEBT, PCDEBT and PPADEBT were prepared via the Sonogashira coupling reaction. The optical, electrochemical and thermal properties of the resulting polymers were compared and analysed. All polymers displayed low solubility in common organic solvents and have moderate molecular weights. Optical studies revealed that all the new ethynylene based-polymers displayed large bandgaps in excess of 2.1 eV. Results highlighted that incorporation of acetylene units between the benzothiadiazole electron accepting units and the other electron donor units over polymer chains leads to wide bandgaps as a result of the electron accepting properties of the acetylene units. The HOMO levels of the resulting polymers are unaffected by the different donor moieties used. However, varying the electron donor units can perturb the electron accepting ability of the main chain of polymers in this series and their LUMO levels. Anthracene-based polymer (PPADEBT) displayed the lowest LUMO level, while the fluorene-based polymer (PFDEBT) displayed the highest LUMO level. All polymers showed good stability to thermal degradation. The amorphous nature of these polymers was confirmed with powder X-ray diffraction studies.