Supramolecular nanostructured assemblies of different types of porphyrins with fullerene using TiO₂ nanoparticles for light energy conversion

TiO₂ nanoparticles were modified with porphyrin derivatives, 5-[4-benzoic acid]-10,15,20-tris[3,5-di-tert-butylphenyl]-21H,23H-porphyrin (Ar-H₂P-COOH), 5-[4-benzoic acid]-10,20-tris[3,5-di-tert-butylphenyl] -21H,23H-porphyrin (H-H₂P-COOH), and 5,10,15,20-tetra[4-benzoic acid]-21H,23H-porphyrin (H₂P-4COOH). The porphyrin-modified TiO ₂ nanoparticles were deposited on nanostructured OTE/SnO₂ electrode together with nanoclusters of fullerene (C₆₀) in acetonitrile-toluene (3/1, v/v) using an electrophoretic deposition technique to afford the porphyrin-modified TiO₂ composite electrode denoted as OTE/SnO₂/(porphyrin-modified TiO₂ nanoparticle+C ₆₀)_n. The porphyrin-modified TiO₂ composite electrodes have efficient light absorbing properties in the visible region, exhibiting the photoactive response under visible light excitation using I ₃^-/I^- redox couple. The incident photon-to-photocurrent efficiency (IPCE) values of supramolecular nanostructured electrodes of porphyrin-modified TiO₂ nanoparticles with fullerene [OTE/SnO₂/(Ar-H₂P-COO-TiO₂+C₆₀) _n, OTE/SnO₂/(H-H₂P-COO-TiO₂+C ₆₀)_n, and OTE/SnO₂/(H₂P-4COO- TiO₂+C₆₀)_n] are much larger than those of the reference systems of porphyrin-modified TiO₂ nanoparticles without C₆₀ [OTE/SnO₂/(Ar-H₂P-COO-TiO₂) _n, OTE/SnO₂/(H-H₂P-COO-TiO₂) _n, and OTE/SnO₂/(H₂P-4COO-TiO₂) _n]. In particular, the maximum IPCE value (41%) is obtained for OTE/SnO₂/(H-H₂P-COO-TiO₂+C₆₀) _n under the bias potential of 0.2 V versus SCE. This indicates that the formation of supramolecular complexes between porphyrins and fullerene on TiO₂ nanoparticles plays an important role in improvement of the light energy conversion properties.