In this work, we theoretically design a new π-conjugated thieno[3,4-b]pyrazine derivative intended for organic electronics. The geometric, electronic, optical, photovoltaic, and nonlinear optical (NLO) properties of the studied molecule were investigated using calculations based on density functional theory (DFT) and its time-dependent variant (TD-DFT) at the B3LYP/6-31G(d,p) level. The structural modification of the reference molecule results in a small energy gap (Eg = 1.68 eV) and a significant enhancement in optical absorption in the visible region, with maxima observed at λmax = 877 nm (TD-B3LYP) and λmax = 668 nm (CAM-B3LYP).

Designing high-efficiency sensitizers remains a central challenge for dye-sensitized solar cells (DSSCs). In this work, nineteen donor motifs commonly employed in the literature were screened to identify optimal donors and to assess the impact of donor substitution on the reference dye D0 (2-cyano-3-[2′-(2″-(4″-(dimethylamino)phenyl)ethynyl)thieno[3,2-b]thiophen-5′-yl]acrylic acid) with the goal of improving device performance. Using DFT/TD-DFT with the CAM-B3LYP functional, we characterized nineteen literature-derived donor motifs via their electronic absorption spectra and key photovoltaic descriptors. Donor substitution, most notably with D1, D3, D5, D6, and D15, substantially improves the electronic and optical properties, yielding lower HOMO energies, a reduced band gap (Egap), enhanced absorption intensity, and a bathochromic shift of the main band. Performance indicators, notably the electron-injection free energy (ΔGinj) and open-circuit voltage (Voc), indicate spontaneous, thermodynamically favorable electron injection from the excited dye into the TiO₂ conduction band.