Quinoxaline-Fused Porphyrins for Dye-Sensitized Solar Cells

5,10,15,20-Tetrakis(2,4,6-trimethylphenyl)-6‘-carboxyquinoxalino[2,3-β]porphyrinatozinc (II) (ZnQMA) and 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-6‘,7‘-dicarboxyquinoxalino[2,3-β]porphyrinatozinc (II) (ZnQDA) have been synthesized to evaluate the effects of β,β‘-carboxyquinoxalino moieties on the structure and optical, electrochemical, and photovoltaic properties of the porphyrins. Both ZnQMA and ZnQDA exhibited broadened and red-shifted light absorption in UV−visible absorption spectra compared with 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrinatozinc (II) (ZnP). ZnQMA and ZnQDA also showed decrease in the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap due to the extension of the porphyrin π-system. From the results of ¹H NMR spectroscopy and DFT calculations, ZnQMA and ZnQDA were found to adopt saddle and planar structures, respectively. ZnQMA-sensitized TiO₂ solar cell with TiO₂ nanoparticles (P25) revealed the power conversion efficiency (η) of 5.2%, whereas ZnQDA-sensitized cell showed η = 4.0%. The superior performance of the ZnQMA-sensitized solar cell to the ZnQDA-sensitized one is originated from both the more favorable electron injection and charge collection efficiency.