METAL-COMPLEX SENSITIZED PHOTOOXIDATION OF PHENOL IN AQUEOUS-SOLUTION

The luminescent excited states of Cr(bpy)33+ (bpy = 2,2'-bipyridine) a nd Ru(bpz)32+ (bpz = 2,2'-bipyrazine) are good oxidants with E0 value s in water of 1.44 and 1.68 V, respectively; Ru(bpy)32+ is a weaker o xidant with E0 = 0.93 V. *Cr(bpy)33+ and *Ru(bpz)32+ are quenched by phenol; Ru(bpy)32+ is not. The three excited states are quenched by O 2 with the formation of O-1(2). The continuous photolysis of air-satur ated acidic solutions of Cr(bpy)33+ and phenol yields Cr(bpy)2 (OH2)23 + and 1,4-benzoquinone as the only photoproducts as determined by HPLC . In the absence of air, only the aquation of the photosenzitizer is m anifest. No Cr(bpy)32+ is detected in pulsed-laser flash photolysis, i ndicating that the cage escape yield (eta(ce)) for the electron transf er reaction is < 0.01. For Ru(bpz)32+, eta(ce) for the formation of R u(bpz)3+ and the phenoxyl radical is 0.48, and the products of the rea ction in the absence of air are hydroxyphenyl dimers. With Ru(bpy)32 as the photosensitizer in aerated solution, the product is benzoquino ne. We conclude that benzoquinone is generated via a O-1(2) pathway, w hile the hydroxyphenyl dimers arise via redox quenching. The results d emonstrate how the products of the photosensitization reaction can be fine-tuned through the utilization of alternative mechanisms that take advantage of the excited state properties of the coordination complex .