Electron Transfer from Organic Aminophenyl Acid Sensitizers to Titanium Dioxide Nanoparticle Films

Electron transfer from three conjugated amino-phenyl acid dyes to titanium and aluminum oxide nanocrystalline films was studied by using transient absorption spectroscopy with sub 20 fs time-resolution over the visible spectral region. All the dyes attached to TiO(2) showed long-lived ground state bleach signals indicative of formation of new species. Global analysis of the transient kinetics of the dyes on TiO(2) revealed stimulated emission decays of about 40 fs and less than 300 fs assigned to electron injection. The same dyes on Al(2)O(3) substrates displayed long stimulated emission decays (ns) suggesting that electron transfer is blocked in this high band gap semiconductor. For two of the dyes (NK1 and NK2) with amino methyl terminal groups cation formation was seen at 670 nm probe wavelength, in the onset region of cation absorption predicted by time-dependent density functional theory calculations, with dynamics matching that of stimulated emission decay. Early excited-state dynamics observed in the NK7 dye bound to both TiO(2) and Al(2)O(3), with pulse limited rise and 30-40 fs decay times, was assigned to rearrangement of charge in the amino phenyl moiety and/or possibly isomerization, which competes with electron injection in NK7 sensitized TiO(2) films. This additional relaxation channel could be the reason for the low efficiency reported for the NK7 sensitized solar cell.