Photoinduced Hydrogen Evolution by a Zwitterionic Diquat Electron Acceptor. The Functions of SiO₂ Colloid in Controlling the Electron-Transfer Process

Photosensitized hydrogen evolution from a basic aqueous SiO₂ colloid (pH 9-10) is accomplished with N,N′-bis-(3-sulfonatopropyl)-2,2,-bipyridinium (DQS⁰, 1) and colloidal platinum as mediating catalysts. In this system Ru(bpy)₃²⁺ acts as a photosensitizer and triethanolamine (TEOA) as ultimate electron donor. No hydrogen formation is observed in a homogeneous aqueous solution under similar conditions. The SiO₂ colloid affects the formation and stabilization of the intermediate photoproducts, Ru(bpy)₃³⁺ and DQS^-·, by means of electrostatic interactions. The electric potential of the particles assists the separation of the products from the initial “encounter cage complex” and results in the repulsion of the reduced product, DQS^-·, from the colloidal interface. Consequently, the recombination rate of DQS^-. with the oxidized product Ru(bpy)₃³⁺ is retarded. The electrostatic functions of the colloid are confirmed by alteration of the ionic strength and pH of the colloid solution. The structure of DQS⁰ was determined by X-ray crystallography. The compound crystallizes in space group P2₁/n with unit cell dimensions of a = 10.392 (1) Å, b = 22.390 (3) Å, c = 8.235 (1) Å, β = 95.07 (2)°, V = 1909 (1) ų, and Z = 4.