Excited-State Relaxation of Ruthenium Polypyridyl Compounds Relevant to Dye-Sensitized Solar Cells

Remarkably little is known about metal-to-ligand charge transfer (MLCT) excited-state relaxation pathways for the ruthenium polypyridyl compounds commonly utilized in dye-sensitized solar cells. Herein, we report variable-temperature photoluminescence studies of compounds of the general type cis-Ru(LL)(2)(X)(2), where LL is a bipyridyl ligand and X is CN- or NCS-, and contrast them with the well-known Ru(bpy)(3)(2+) and Os(bpy)(3)(2+), where bpy is 2,2'-bipyridine, to identify relaxation pathways. In fluid acetonitrile and propylene carbonate solutions, excited-state relaxation was found to obey a first-order kinetic model. An Arrhenius analysis revealed internal conversion to two different states, assigned to an upper MLCT excited state and a ligand field excited state. Relaxation through the upper MLCT excited state typically displayed pre-exponential factors of 10(7)-10(8) s(-1) with activation energies of 400-900 cm(-1), while relaxation rates through ligand field states occurred with 10(14)-10(15) s(-1) and activation energies of 4000-5000 cm(-1). Nonradiative decay through LF states was sensitive to the ligand identity, but in a manner that was not fully consistent with the spectrochemical series. Excited-state relaxation of cis-Ru(dcbH(2))(2)(NCS)(2), where dcbH2 is 4,4'-(CO2H)(2)-2,2'-bipyridine, sometimes termed N3, anchored to mesoporous TiO2 or ZrO2 thin films immersed in CH3CN occurred through the upper MLCT excited state with activational parameters in surprisingly good agreement with those abstracted from data measured in fluid solution. An important finding from these studies is that the population of dissociative ligand field excited states is unlikely to lead to unwanted photochemistry of dye-sensitized solar cells based on cis-Ru(LL)(2)(NCS)(2)-type compounds at room temperature