Photosensitization of Colloidal Titania Particles by Electron Injection from an Excited Organic Dye−Antennae Function

Electron transfer from an excited organic dye−antennae device, the diester formed between fluorescein and anthracene-9-carboxylic acid (FL−AN), to colloidal TiO 2 particles has been examined by absorption, photoluminescence, and laser flash spectroscopies and by electron spin resonance techniques under steady-state laser excitation. This diester FL−AN is chemisorbed to the TiO 2 particles through the phenolic function of the fluorescein moiety; the anthracene component of FL−AN acts as the energy donor component, which is not in contact with the semiconductor particle surface. The apparent association constant ( K app ) is ca. 200 mol -1 L. Exciting the fluorescein side with visible light (around 470 nm) induces electron injection from its singlet excited state onto the conduction band of TiO 2 ( k et = 2.30 × 10 8 s -1 ). Excitation of the anthracene moiety with ultraviolet light (λ ≈ 355 nm) first causes energy transfer from its singlet excited state to the lowest excited singlet state of fluorescein (Φ enT = 0.98, and k enT = 2.23 × 10 10 s -1 ), following which electron injection occurs from the fluorescein singlet to the conduction band of TiO 2 at a rate otherwise identical to direct excitation of fluorescein. This greatly improves sensitization efficiency in applications to solar energy conversion. Electron spin resonance and laser flash spectroscopies demonstrated formation of the radical cation FL •+ −AN on electron injection.