Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO2 surfaces

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K-4[Fe(CN)(6)] and Na-2[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)(6)(4-) to the TiO2 surface results in the appearance of a broad absorption band (fwhm similar to 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)(4)(2-) 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, similar to8 ns fwhm, similar to2-15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO2(e(-)), Fe-III], k(inj) > 10(8) 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(LL)(CN)(4)(2-)/TiO2 occurs by MPCT and MLCT pathways, the quantum yield for the latter being dependent on environment. The solvatochromism of the materials allows the reorganization energies associated with charge transfer to be quantified. The photocurrent efficiencies of the sensitized materials are also reported.