Isolation of π-conjugated system through polyfluorene from electronic coupling with side-chain substituents by cardo structures

The electronic coupling via the cardo structure in polyfluorene (PFs) was investigated. The series of fluorene units alternatively having alkoxyphenyl as an electron-donating group (EDG) and/or alkyl benzoate as an electron-withdrawing group (EWG) at the cardo carbon were synthesized. From the investigation of optical properties of the polymers containing these fluorene units, it was found that the electronic states of the substituents at the cardo carbons and the PF main chains should be less influenced by the introduction of EDG and/or EWG at the cardo structure. Furthermore, these preservation effects in the cardo-PFs were observed in the film states even after the thermal treatment. We conclude that the electronic structures of the PF main chain are highly preserved from the correlations with the substituents at the cardo carbons. This is the first example, to the best of our knowledge, to survey the systematic information on the electronic structures of the cardo-PFs and offer the preservation effect of the optical properties from the introduction of EDGs and EWGs at the cardo carbon.