Three novel indenofluorene based cationic conjugated oligomers, PIFPC, PIFTC, and PIF2TC, have been designed and synthesized. The optical and electrochemical behaviors of these oligomers have been studied. These high charge density oligomers exhibit good water solubility as high as 12.6mg/mL. Furthermore, the absorption and emission properties of this type of oligomers can be easily tuned by incorporating appropriate comonomers. The application of these water-soluble indenofluorene based oligomers as new chemosensors for Hg 2+ detection is reported, and the results indicate that these oligomer sensors exhibit a high selectivity and sensitivity for Hg 2+ detection without interference from other metal ions.