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Keywords:

  • Porphyrinoids;
  • Dyes;
  • Sensiti­zers;
  • Solar cells;
  • Nanotubes;
  • Transition metals

Abstract

The 2-(1-acetyl-2-oxopropyl)-5,10,15,20-tetraphenylporphyrin free base (H2TPP-AOP) and its transition-metal complexes (MTPP-AOP, M = Cu2+, Zn2+) have been synthesized. Single-crystal X-ray diffraction analysis revealed that the 1-acetyl-2-oxopropyl (AOP) group is attached to a β-pyrrolic position through the methylene group. The mean plane of the AOP is almost perpendicular to the porphyrin ring. The zinc(II) complex crystallizes as a five-coordinate species with one methanol/water solvate in the axial position, whereas the copper(II) complex is four-coordinate. These compounds exhibit strong absorption in the UV and visible regions. The fluorescence lifetimes are 1.85 and 8.0 ns for the free base and zinc(II) complex, respectively. Density functional calculations indicate that these porphyrins are energetically suitable for electron injection from their lowest-unoccupied molecular orbitals to the conduction band of the titanium dioxide semiconductor. However, the energy conversion efficiencies of the three porphyrin-sensitized solar cells are low, that is, 0.14, 0.015 and 0.021 % for ZnTPP-AOP, CuTPP-AOP and H2TPP-AOP, respectively. The poor photovoltaic performance is ascribed to the low dye-loading of these porphyrins on the titanium dioxide surface. The neighbouring phenyl groups exert sufficient steric hindrance on the AOP to limit its binding to the TiO2 semiconductor.