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Four acceptor-donor organic conjugated molecules based on thieno [3,4-b]pyrazine-terthienyls were analyzed in order to explore the effect of the donor substituent on their molecular structures, electronic and optical properties. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD/DFT) calculations were carried out employing the B3LYP hybrid functional in combination with the 6-31G(d,p) basis set. The results suggests that the addition of electron-donating substituents to the conjugated molecules can diminish their energy gap value, which is beneficial to the photon harvesting. The lowest-lying absorption spectra of compounds substituted with electron donor groups exhibited a red-shift and a high oscillation factor compared with the unsubstituted molecule. Additionally, the ionization potential (IP), electron affinity (EA), reorganization energy (λ) and open-circuit voltage (V oc ) of the molecules were evaluated. According to these values, the molecules show good photovoltaic properties, and efficient charge transfer for hole and electron and balanced charges.

A variety of organic donor–acceptor–donor materials based on thienylbenzothiadiazole (BTD[Formula: see text]–5) combined with different [Formula: see text]-conjugated systems are studied by density functional theory (DFT) and time-dependent DFT (TD-DFT) for the ground- and excited-state properties, respectively, using B3LYP and the 6-31G(d, p) basis set. The effect of different electron-donor groups on the structural, electronic and optoelectronic properties is studied. To provide for the bandgap and to guide the synthesis of novel low bandgap materials, we applied quantum chemistry techniques to calculate the difference in the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital LUMO energies. However, we have studied the effect of the reduction and oxidation properties on the electronic excitation transitions for all compounds. The emission energies have been obtained from TD-DFT calculations performed on the excited-state optimized S1 geometries. The theoretical results suggest that both the introduction of electron-donor groups and the doping process contribute significantly to the electronic and optoelectronic properties of the alternating donor–acceptor–donor conjugated systems studied.