Evaluation of electronic interaction in matrix elements for photoinduced for photoinduced electron transfer processes within mixed-valence complexes

The photochemical properties of ''alkane-bridged'' binuclear complexes, [(bec)(2)Ru-II(L-C-n-L)Ru-II/III(bpy)(2)](4+/5+) (where bec = 4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine, bpy = 2,2'-bipyridine, and n = 3, 4, 5, 10) and [(dmb)(2)Ru-II(L-C-n-L)Os-II/III(L-alpha)(2)](4+/5+) (where dmb = 4,4'-dimethyl-2,2'-bipyridine, L-alpha = bpy or dmb and n = 3, 4, 5), in which the bridging ligand (L-C-n-L) was made of two [2-(2-pyridyl)-1-benzimidazolyl] groups (L) linked by an alkane chain, have been investigated using time-resolved nano-and picosecond laser spectroscopy, In the Ru(II)-Os(II) binuclear complexes, a metal-to-ligand charge transfer (MLCT) state of the Ru(II) moiety, ((CT)-C-3)Ru, was quenched by ((CT)-C-3)Ru-->Os(II) energy transfer. The intramolecular energy transfer rates for n = 3, 4, and 5 are well understood by assuming the Forster-type mechanism provided that the methylene chains are in a nearly extended form. In the mixed-valence binuclear complexes, Ru(II)-M(III) (M = Ru or Os), the decay of the ((CT)-C-3)Ru excited state was accelerated by electron transfer (ET) to the adjacent M(III) site giving the valence-isomer state, Ru(III)-M(II), which decayed to The ground state of the original form via back ET. Forward ET rates (k(et)) have been determined from the decay rate of the ((CT)-C-3)Ru excited state (k(es)) and the production yields of Ru(III)-M(II), phi(et), and back ET rates (k(b)) were obtained from the decay of Ru(III)-M(II), The value of phi(et) (less than or equal to 0.5 +/- 0.1) decreases along with the decreasing number of methylene groups or temperature, indicating the existence of another process quenching the ((CT)-C-3)Ru excited state. From the temperature dependence of k(et) and k(b), the electronic interaction matrix elements (H-rp) have been estimated using a nonadiabatic ET theory. H-rp for the forward ET process was found to be enhanced by a factor of 2 in the compounds in which the excited electron locates on the bridging ligand in [(dmb)(2)Ru-II(L-C-n-L)Os-III(L-alpha)(2)](5+) compared to that for [(bec)(2)Ru-II(L-C-n-L)Ru-III(bpy)(2)](5+) (n = 4, 5) in which the electron locates on the remote ligands. On the other hand, the matrix elements for back ET were found to be the same in either case, independent of metal ions and remote ligands.