Electron Injection and Energy-Transfer Properties of Spiropyran–Cyclodextrin Complexes Coated onto Metal Oxide Nanoparticles: Toward Photochromic Light Harvesting

The photochromic and spectroscopic properties of spiropyran−γ-cyclodextrin inclusion complexes coated onto the surface of zirconia and titania nanoparticles (NPs) are examined. After ultraviolet (UV) irradiation, the colorless spiropyran (SP) isomer contained within the cyclodextrin (CD) is converted to the colored merocyanine (MC) isomer, which absorbs light in the 400–600 nm region. A significant increase in the excited-state lifetime for MC-CD on zirconia NPs (τ_MC = 1.32 ns) is observed compared to MC in solution (τ_MC = 0.21 ns), indicating that MC → SP photoisomerisation is hindered by the surrounding CD. Excitation of MC-CD on titania NPs results in electron injection into the titania conduction band with a quantum yield of 0.70. Excitation of MC-CD on zirconia NPs when a squaraine-based dye sensitizer is also attached to the NP surface results in energy transfer to the dye sensitizer with a quantum yield of 0.65. The prolonged excited-state lifetime of the encapsulated MC-CD is responsible for the high electron injection and energy-transfer quantum yields. The SP-CD/MC-CD system has potential applications for photochromic, light-harvesting electrodes in dye-sensitized solar cells