The effect of heavy atoms on photoinduced electron injection from nonthermalized and thermalized donor states of M(II)-polypyridyl (M=Ru/Os) complexes to nanoparticulate TiO(2) surfaces: an ultrafast time-resolved absorption study.

We have synthesized ruthenium(II)- and osmium(II)-polypyridyl complexes ([M(bpy)(2)L](2+), in which M=Os(II) or Ru(II), bpy=2,2'-bipyridyl, and L=4-(2,2'-bipyridinyl-4-yl)benzene-1,2-diol) and studied the interfacial electron-transfer process on a TiO(2) nanoparticle surface using femtosecond transient-absorption spectroscopy. Ruthenium(II)- and osmium(II)-based dyes have a similar molecular structure; nevertheless, we have observed quite different interfacial electron-transfer dynamics (both forward and backward). In the case of the Ru(II)/TiO(2) system, single-exponential electron injection takes place from photoexcited nonthermalized metal-to-ligand charge transfer (MLCT) states. However, in the case of the Os(II)/TiO(2) system, electron injection takes place biexponentially from both nonthermalized and thermalized MLCT states (mainly (3)MLCT states). Larger spin-orbit coupling for the heavier transition-metal osmium, relative to that of ruthenium, accounts for the more efficient population of the (3)MLCT states in the Os(II)-based dye during the electron-injection process that yields biexponential dynamics. Our results tend to suggest that appropriately designed Os(II)-polypyridyl dye can be a better sensitizer molecule relative to its Ru(II) analogue not only due to much broader absorption in the visible region of the solar-emission spectrum, but also on account of slower charge recombination.