In this study, five small band gap thiophene (TH)-thienopyrazine (TP) conjugated copolymers were synthesized by Stille-coupling reaction. The polymer structures consisted of one to four thiophene rings with the TP of different side groups provided a systematical investigation on the structure-electronic property relationship. The absorption maxima of the polymer films decreased from 850 to 590 nm as the thiophene moieties increased from thiophene to quaterthiophene. The optical and electrochemical band gaps of the studied poly[2,3-didodecyl-5-(thiophen-2-yl)thieno[3,4-b]pyrazine] (PTHTP-C12) were 0.97 and 0.78 eV, respectively, indicating a significant intramolecular charge transfer. The theoretical geometry and electronic properties of the TH-TP copolymers by the density functional theory at the B3LYP level and 6-31G(d) basis set suggested that the bond length alternation enlarged with enhancing the thiophene content and resulted in the variation on the polymer band gap. The relatively small theoretical effective mass of poly(TH-alt-TP) also indicated its potential applications for field transistor applications. Our study demonstrates the tunable electronic properties of small band gap copolymers by the thiophene content and the resulted geometry variation. Such polymers could be potentially used for near-infrared electronic and optoelectronic devices. (c) 2007 Wiley Periodicals, Inc.