Low-band-gap, π-conjugated polymers composed of π-excessive thiophene and π-deficient benzothiadiazole and quinoxaline units were prepared in high yields by a polycondensation method using the Sonogashira cross-coupling reaction. The copolymers were characterized by NMR, IR, UV, gel permeation chromatography, and elemental analysis. The absorption and emission in solution and in the solid state of the polymers revealed that the polymers generated a π-stacked structure in the solid state, and the polymer molecules in the film were ordered. Thin films of poly[3-dodecylthiophen-2,5-diylethynylene-(benzo[1,2,5]thiadiazole-4,7-diyl)ethynylene] (P-1), poly[3,4-didodecylthiophen-2,5-diylethynylene-(benzo[1,2,5]thiadiazole-4,7-diyl)ethynylene] (P-2), poly[3-dodecylthiophene-2,5-diylethynylene-(2,3-dipyridine-2-ylquinoxaline-5,8-diyl)ethynylene] (P-3), and poly[3,4-didodecylthiophene-2,5-diylethynylene-(2,3-dipyridine-2-ylquinoxaline-5,8-diyl)ethynylene] (P-4) exhibited an optical band gap of ∼1.85–2.08 eV. The absorption spectra of P-2 and P-4 at different temperatures also revealed significant effects of the structure on the molecular interactions.