Benzoporphyrins bearing pyridine or pyridine-N-oxide anchoring groups as sensitizers for dye-sensitized solar cell

Novel benzoporphyrins bearing pyridine or pyridine-N-oxide groups were prepared through a concise method based on a Pd-0 catalyzed cascade reaction. These benzoporphyrins were examined as sensitizers for dye-sensitized solar cells. Vicinal pyridine and vicinal pyridine-N-oxide groups were introduced as new types of anchoring/acceptor groups for dye-sensitized solar cells for the first time. While all the porphyrins showed solar to electricity conversion, benzoporphyrins bearing pyridine-N-oxide anchoring groups displayed higher conversion efficiency than benzoporphyrins bearing pyridine-anchoring groups. Opp-dibenzoporphyrins display broadened and red-shifted UV-vis absorption and emission bands as compared with those of the monobenzoporphyrins, which arises from the fusion of one more benzene ring and the attachment of two more electron-withdrawing groups to the porphyrin beta-positions. Cyclic voltammetry (CV) data and DFT calculation data obtained for these porphyrins agree well with their UV-vis absorption and fluorescence spectroscopic data. The HOMO energy level derived from the first oxidation potentials indicate that regeneration of the resulting porphyrin radical cation by the redox mediator (I-/I-3(-)) is thermodynamically feasible for all these benzoporphyrin sensitizers (3, 5, 8 and 10). On the other hand, excited state energy levels of these benzoporphyrins calculated from the CV data, the UV-vis and fluorescence spectroscopic data are all slightly lower than the energy level of the conduction band of TiO2, suggesting insufficient driving force for efficient electron injection from the porphyrin excited singlet state to the conduction band of TiO2.