A highly soluble coumarin monomer, 3,7-dibromo-4-octyloxycoumarin, was developed as the novel building block for π−conjugated polymers. Sonogashira coupling reaction, dehalogenation reaction, and Suzuki-Miyaura coupling reaction were carried out by using this new monomer as the main chain unit of π−conjugated polymer for polymerization. Introduction of a long alkoxy chain into coumarin monomer unit significantly improved the solubility of thus obtained polymers. The absorption and emission spectra of polymers showed red-shift compared to those of monomer, indicating the expansion of the π−conjugated system in the polymer main chain. Especially the polymers synthesized via Sonogashira coupling formed excimer in film state. The polymers showed acid-base responsibility with the addition of KOHaq and HClaq.