The effect of the substituent groups (alkyl or aryl) on the structure, electronic, optical properties, ionization potentials (IPs), electron affinities (EAs), and reorganization energy of the donor-acceptor monomers 5,8-di-2-thienyl-quinoxaline (T[Q]T), 4,9-di-2-thienylpyrazine[2,3-g]quinoxaline (T[PQ]T) were studied theoretically. The lowest-lying absorption in assigned to pi ->pi* transition, and the fluorescence can be described as originating from the (1)[pi pi*]excited state. The lowest-lying absorption and emission spectrum of T[Q]T and T[PQ]T with alkyl groups exhibit blue-shifted, while T[Q]T and T[PQ]T with aryl groups exhibit the opposite result. The extra absorption bond at 400 nm of T[Q]T-Bph is contributed by the pi-pi* transitions between the biphenyl and acceptor fragment. Orbital compositions transfer coefficient (chi) of the donor in LUMOs is reduced with the aryl groups on the acceptor, which illuminates that the aryl contributes to intramolecular charge transfer, and the result is in accord with the analysis of reorganization energy. IPs is brought down by both of the alkyl and aryl groups, but EAs is raised only by aryl, therefore, aryl is conductive to forming excitons for D-A-D molecule. Consequently, T[Q]T and T[PQ]T with aryl groups are more reasonable monomers of donor-acceptor copolymers as a solar cell materials comparing with the alkyl-introduced ones. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2099-2107, 2010