ENERGY AND ELECTRON-TRANSFER REACTIONS OF THE MLCT STATE OF RUTHENIUM TRIS(BIPYRIDYL) WITH MOLECULAR-OXYGEN - A LASER FLASH-PHOTOLYSIS STUDY

The quenching by molecular oxygen of the excited state of RU(bpy)(3)(2+) in a series of aqueous solutions with different proton concentrations has been investigated using the techniques of laser flash photolysis and time-resolved near-infrared emission spectrophotometry. A quantum yield of singlet oxygen (Phi(Delta)) = 0.53 +/- 0.02 was obtained which was independent of the proton concentration. In addition, chemical bleaching upon excitation for all the acidic solutions (> 1 M) was noted. This was shown to be due to the formation of Ru(bpy)(3)(3+), released from the solvent cage by protonation of the concomitantly formed superoxide radical anion. The quantum yield of the oxidized complex (Phi(f)) was shown to be dependent on the proton concentration in the solution. The limiting quantum yield at infinite acid concentration of 0.49 +/- 0.05 was obtained. The data are consistent with a scheme that requires the collision complex formed between the excited state and oxygen to undergo energy and electron transfer with equal efficiency but requires the presence of protons in high concentration to release the electron transfer products from the solvent cage.