Artificial Photosynthesis System Using Mg Chlorophyll-a Conjugated Nanocrystalline TiO₂ Film Electrode via the Axial Imidazole-4-Acetic Acid Ligand

Mg chlorophyll-a (MgChl-a) with imidazole-4-acetic acid (Im) as the axial ligand, MgChl-a(Im) conjugated to a nanocrystalline TiO₂ film onto optical transparent substrate electrode (OTE) is prepared and the photovoltaic properties of the nanocrystalline TiO₂ film through an axial Im ligand of MgChl-a are studied. MgChl-a conjugated to nanocrystalline TiO₂ film electrode is prepared by using two different methods. One is the MgChl-a conjugated to nanocrystalline TiO₂ film electrode using carboxyl group of Im of MgChl-a(Im) (MgChl-a(Im)/TiO₂). The other is the MgChl-a conjugated to nanocrystalline TiO₂ film electrode using the coordination of Im immobilized onto TiO₂ to the axial site of MgChl-a (MgChl-a/Im-TiO₂). In MgChl-a (Im)/TiO₂ and MgChl-a/Im-TiO₂, the fluorescence of MgChl-a is effectively quenched by the electron injection from the excited singlet state of MgChl-a into the conduction band of TiO₂ particles via the axial ligand of Im. The photocurrent action spectra of MgChl-a (Im)/TiO₂, MgChl-a/Im-TiO₂ and MgChl-a/TiO₂ electrodes are similar to that of the UV-vis absorption spectra in the methanol solution and incident photon to current efficiency (IPCE) values reach a maximum around the wavelength of absorption maximum (660–680 nm). IPCE value of MgChl-a/TiO₂ electrode (0.65% at 680 nm) is lowest among those of MgChl-a(Im)/TiO₂ (1.65% at 680 nm) and MgChl-a/Im-TiO₂ (1.74% at 660 nm) electrodes. These results indicate that MgChl-a anchored on nanocrystalline TiO₂ film through the axial Im ligand is the efficient visible sensitization for photovoltaic injection cell based on the nanocrystalline TiO₂ film electrode.