Photosensitized Electron-Transfer Reactions in β-Cyclodextrin Aqueous Media: Effects on Dissociation of Ground-State Complexes, Charge Separation, and H₂ Evolution

Zinc(II) meso-tetrakis[1-(3-sulfonatopropyl)-4-pyridino]porphyrin, ZnTPSPyP (2), forms a ground state complex with anthraquinone-2-sulfonate, AQS^- (1). The formation of the complex results in internal static quenching of the excited S state of the sensitizer and recombination of the photoproducts in the cage structure. In the presence of β-cyclodextrin (β-CD), the complex is separated due to selective association of AQS^- to the receptor cavity. This process leads to the effective decay of excited ZnTPSPyP to the T state that undergoes diffusional quenching by β-CD-bound AQS^-. The electron-transfer photoproducts are stabilized against back-electron-transfer reaction and AQHS^-· can be accumulated under continuous illumination in the presence of cysteine as the electron donor. Photoreduction of N,N’-dioctyl-4,4’-bipyridinium, C₈V²⁺, with Ru(bpy)₃ ²⁺ as the sensitizer and Na₂EDTA as the donor leads to the formation of a dimer aggregate (C₈V^+·)₂. The aggregate is inactive in H₂evolution. In the presence of β-CD, the aggregate formation is prevented due to the selective association of C₈V^+· monomer to the cyclodextrin cavity. High quantum yields for H₂ evolution in the presence of a Pt-colloid are observed with β-CD, ϕ = 4 × 10^–2. Flash photolysis studies reveal that the association of C₈V^+· to the β-CD stabilizes the intermediate photoproducts against back-electron-transfer reactions.