Polymerization of acrylamide photoinitiated by tris(2,2 '-bipyridine) ruthenium(II)-amine in aqueous solution: Effect of the amine structure

The photopolymerization of acrylamide (AA) initiated by the metallic comple x tris(2,2'-bipyridine)ruthenium(II) [Ru(bPY)(3)(+2)] in the presence of al iphatic and aromatic amines as co-initiators was investigated in aqueous so lution. Aromatic amines, which are good quenchers of the emission of the me tal-to-ligand-charge-transfer excited state of the complex, are more effect ive co-initiators than those that do not quench the luminescence of Ru(bpy) (3)(+2), such as aliphatic amines and aniline. Laser-flash photolysis exper iments show the presence of the reduced form of the complex, Ru(bpy)(3)(+1) , for all the amines investigated. For aliphatic amines, the yield of Ru(bp y)(3)(+1) increases with temperature, and on the basis of these experiments , a metal-centered excited state is proposed as the reactive intermediate i n the reaction with these amines. The decay of the transient Ru(bpy)(3)(+1) is faster in the presence of AA. This may be understood by an electron-tra nsfer process from Ru(bpy)(3)(+1) to AA, regenerating Ru(bpy)(3)(+2) and pr oducing the radical anion of AA. It is proposed that this radical anion pro tonates in a fast process to give the neutral AA radical, initiating in thi s way the polymerization chain. (C) 2001 John Wiley & Sons, Inc.