Soluble poly[(1,3-phenyleneethynylene)-alt-tris(2,5-dialkoxy-1,4-phenyleneethynylene)] derivatives (5) have been synthesized by using the Heck-type coupling reaction. Even with a significant increase in the p-phenyleneethynylene content, the copolymers exhibit a random-coil conformation in THF solution, with a Mark-Houwink exponent determined to be a approximate to 0.78. As a result of the extended conjugation length of the chromophore, the absorption and emission lambda(max) values of 5 are notably red-shifted (by about 30-40 nm) from that of poly[(1,3-phenyleneethynylene)-alt-(2,5-dialkoxy-1,4-phenyleneethynylene)] derivatives (2). The fluorescence quantum efficiency of 5 is estimated to be phi(fl) approximate to 0.50, slightly higher than that of 2 (phi(fl) approximate to 0.44). The fluorescence of 5 in the solid state is strong, indicating its potential for various device applications. LEDs based on 5 emitted green-yellow EL with an external quantum efficiency of 0.013%.