Organization of supramolecular assemblies of fullerene, porphyrin and fluorescein dye derivatives on TiO₂ nanoparticles for light energy conversion

TiO₂ nanoparticles were modified with a porphyrin derivative, 5-[4-benzoic acid]-10,15,20-tris[3,5-di-tert-butylphenyl]-21H,23H-porphyrin (H₂P-COOH), and fluorescein derivatives, 9-[2-(3-carboxy-9,10- diphenyl)anthryl]-2,7-difluoro-6-hydroxy-3H-xanthen-3-one (DPAX-COOH) and 2′,7′-difluorofluorescein (FL-COOH). The dye-modified TiO ₂ nanoparticles were deposited on nanostructured OTE/SnO₂ (OTE: optically transparent electrodes) together with nanoclusters of fullerene (C₆₀) from acetonitrile/toluene (3:1, v/v) using an electrophoretic deposition technique. The dye-modified TiO₂ composite electrodes [OTE/SnO₂/(dye + C₆₀)_n] have broad as well as high absorbance properties in the visible region, exhibiting the photo response under visible light excitation using I3-/I- redox couple. The incident photon to photocurrent efficiency (IPCE) for these electrodes increases in order: OTE/SnO₂/(H₂P)_n < OTE/SnO ₂/(H₂P-COO-TiO₂)_n < OTE/SnO ₂/(H₂P-COO-TiO₂ + C₆₀). The IPCE value can be further improved by replacing H₂P-COOH with a fluorescein derivative containing an electron donor moiety: DPAX-COOH (DPA: diphenylanthracene). The maximum IPCE value (42%) is obtained for OTE/SnO ₂/(DPAX-COO-TiO₂ + C₆₀)_n under the bias of 0.2 V vs. SCE.