Visible-Light-Induced Hydrophilic Effect in an Ultrathin Hybrid Film of Titania Nanosheet and an Optical Active Ruthenium(II) Complex Cation

Hybrid multilayers of titania nanosheet, amphiphilic alkylammonium cation, and (rac- or Λ-)Ru(II) complex cation have been prepared by a modified Langmuir−Blodgett technique. When a solution of octadecylammonium chloride (CH 3 (CH 2 ) 17 NH 3 + Cl −: ODAH + Cl − ) was spread at an interface between the air and an aqueous dispersion of the negatively charged nanosheets, the nanosheets were adsorbed onto the floating monomolecular film of ODAH + to form a hybrid film at the interface. The surface of the hybrid film deposited on a substrate was immersed in an aqueous [Ru(bpy) 3 ]Cl 2 solution. By repeating these procedures, the hybrid multilayer was fabricated. Electronic and infrared spectra of the multilayers indicated layer-by-layer deposition of the hybrid monolayers. X-ray diffraction patterns of the films showed that the layer structure for the film of Λ-[Ru(bpy) 3 ] 2+ was perturbed rather than that of rac-[Ru(bpy) 3 ] 2+ . The second-harmonic generation (SHG) from the hybrid film of Λ-[Ru(bpy) 3 ] 2+ was much more intense than that from the film of rac-[Ru(bpy) 3 ] 2+, because the former system included noncentrosymmetric component of the optical active (enantiomeric) Ru(II) complex. The IR spectral data for the film of Λ-[Ru(bpy) 3 ] 2+ indicated that the water content in the film increased by the illumination of visible light at 465 nm. However, only a small increase in the water content was observed for the film of rac-[Ru(bpy) 3 ] 2+ when it was illuminated by the visible light. These results were explained by the following mechanism; the visible light irradiated to the hybrid film of Λ-[Ru(bpy) 3 ] 2+ was changed to the ultraviolet light by the SHG effect of the film, which induced superhydrophilicity on the surface of the titania nanosheet. The water content in the film increased, so that the layer structure was perturbed. On the contrary, the SHG-inactive film of rac-[Ru(bpy) 3 ] 2+ kept the layer structure.