Syntheses and optical properties of conjugated polymers containing cross-conjugated oxadiazole units

In this paper, we report the detailed synthesis and optical properties of three new conjugated polymers with cross-conjugated oxadiazole segments containing ester substituents. It was found that, due to the electron-withdrawing nature of the four ester substituents, the π* state of the oxadiazole segments was lowered to such an extent that it has a lower energy than the π* state of the PPV backbone. Consequently, a charge-transfer (CT) excited state becomes the lowest-lying excited state in these polymers. As a result, these polymers exhibit dramatically different fluorescence behavior and much lower photoluminescence (PL) quantum efficiencies than the previously reported polymer containing t-butyl substituents. Although these polymers with their low PL quantum efficiencies are not suitable for LED applications, they may be highly photoconductive materials.