Molecular Design and DFT/TD-DFT Investigation of N3 Derived Ruthenium(II) Azopyridine Complexes for Dye-sensitized Solar Cells

Renewable energy technologies are increasingly important for meeting rising global energy needs while reducing reliance on polluting fossil fuels. Among them, Dye-Sensitised Solar Cells (DSSCs) are attractive third-generation photovoltaic devices because they are inexpensive to produce, relatively simple to fabricate, and can perform well under low-light conditions. Their efficiency, however, depends largely on the sensitising dye used to capture sunlight and transfer electrons. This study explores ways to improve the well-known ruthenium-based N3 dye. Drawing on the structures of N3 and the α- and β-isomers of RuCl₂(azpy)₂, eight new ruthenium(II) complexes, labelled D1–D8, were designed by modifying bipyridine, azopyridine, NCS, and chloride ligands. Their structural, electronic, optical, and photovoltaic properties were evaluated using DFT and TD-DFT calculations at the B3LYP/LANL2DZ level. The analysis examined molecular geometry, frontier orbitals, energy gaps, reactivity indicators, absorption spectra, and parameters related to electron injection into TiO₂. Among the eight proposed dyes, D1 and D7 showed the most promising results. Both absorb strongly in the visible region, provide favourable conditions for electron injection into TiO₂, and display electronic characteristics that could support efficient charge separation. Importantly, these improvements were achieved without substantially altering the coordination environment around the ruthenium centre. Overall, the findings suggest that ruthenium complexes containing azopyridine ligands could be effective sensitisers for DSSCs. They also offer useful theoretical guidance for designing improved ruthenium-based dyes. Experimental synthesis and photovoltaic testing will be needed to confirm these predictions.