Charge-Transfer Studies of Iron Cyano Compounds Bound to Nanocrystalline TiO2 Surfaces

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K4[Fe(CN)6] and Na2[Fe(LL)(CN)4], where LL = bpy (2,2‘-bipyridine), dmb (4,4‘-dimethyl-2,2‘-bipyridine), or dpb (4,4‘-diphenyl-2,2‘-bipyridine). Coordination of Fe(CN)64- to the TiO2 surface results in the appearance of a broad absorption band (fwhm ∼ 8200 cm-1) centered at 23800 ± 400 cm-1 assigned to an Fe(II) → TiO2 metal-to-particle charge-transfer (MPCT) band. The absorption spectra of Fe(LL)(CN)42- compounds anchored to TiO2 are well modeled by a sum of metal-to-ligand charge-transfer (MLCT) bands and a MPCT band. Pulsed light excitation (417 or 532 nm, ∼8 ns fwhm, ∼2−15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO2(e-), FeIII], kinj > 108 s-1. Charge recombination is well described by a second-order equal concentration kinetic model and requires milliseconds for completion. A model is proposed wherein sensitization of Fe...