530 nm‐Laser Photolysis Studies of the Photo Reduction of Tris (2,2‐bipyridine)‐ruthenium(II) by Organic Donors

The reduction of the excited charge‐transfer state of tris(2,2'‐bipyridine)‐ruthenium(II), Ru(bipy) 3 2+ by organic donors (D) in methanol was studied by laser photolysis technique. The reduction follows the general schema: The rate of reduction is diffusion controlled for all donors except diphenylamine for which a rate constant of 2.5 · 10 7 M −1 s −1 was obtained. The reduction potential of *Ru(bipy) 3 2+ was determined experimentally to lie close to the thermodynamic limit of 0.85 V. ‐ Reaction (1) and subsequent dark reactions were examined in detail for D = phenothiazine (PTH). For this donor the efficiency of electron transfer was found to be 0.6 ± 0.2. The spectrum of Ru(bipy) 3 + obtained from these studies shows a maximum at λ = 510 nm with an extinction coefficient of 1.8 · 10 4 M −1 cm −1 . Ru(bipy) 3 + was found to decay exponentially with a time constant of 1.1 · 10 5 s −1 . At higher PTH concentrations, this reaction competes with electron transfer from Ru(bipy) 3 + to PTH for which a rate constant of 3.4 · 10 7 M −1 s −1 was obtained.