The effect of heavy atoms on photoinduced electron injection from nonthermalized and thermalized donor states of MII–polypyridyl (M=Ru/Os) complexes to nanoparticulate TiO₂ surfaces: an ultrafast time-resolved absorption study

Abstract

We have synthesized ruthenium(II)–and osmium(II)–polypyridyl complexes ([M(bpy)₂L]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 TiO₂ 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/TiO₂ 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/TiO₂ system, electron injection takes place biexponentially from both nonthermalized and thermalized MLCT states (mainly ³MLCT states). Larger spin–orbit coupling for the heavier transition-metal osmium, relative to that of ruthenium, accounts for the more efficient population of the ³MLCT 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.