Elucidating Band-Selective Sensitization in Iron(II) Polypyridine-TiO2 Assemblies

Iron(II) polypyridines represent a cheaper and nontoxic alternative to analogous Ru(II) polypyridine dyes successfully used as photosensitizers in dye sensitized solar cells (DSSCs). We employ density functional theory (DFT) and time-dependent DFT (TD-DFT) to study ground and excited state properties of [Fe(bpy)(CN)(4)](2-), [Fe(bpy-dca)-(CN)(4)](2-), and [Fe(bpy-dca)(2)(CN)(2)] complexes, where bpy = 2,2'-bipyridine and dca = 4,4'-dicarboxylic acid. Quantum dynamics simulations are further used to investigate the interfacial electron transfer (JET) between the excited Fe(II) dyes and a TiO2 nanoparticle. All three dyes investigated display two bands in the visible region of the absorption spectrum, with the major transitions corresponding to the metal-to-ligand charge transfer states. The calculated IET rates from the particle states created by the excitation of the lower energy absorption band are comparable to or slower than the rate of the excited state decay into the nonemissive, metal-centered states of the Fe(II) dyes (similar to 100 fs), indicating that the JET upon the excitation of this band is unlikely. Several particle states in the higher energy absorption band display JET rates at or below 100 fs, suggesting the possibility of the JET between the Fe(II)-sensitizer and TiO2 nanoparticle upon excitation with visible light. Our results are consistent with the previous experimental work on Fe(II) sensitizers (Ferrere, S. Chem Mater. 2000, 12, 1083) and elucidate the band selective nature of the IET in these compounds.