Dynamics of ultrafast photoinduced heterogeneous electron transfer, implications for recent solar energy conversion scenarios

The general case of a heterogeneous electron transfer reaction is realized by ultrafast electron transfer from a photo-excited molecule to a wide continuum of electronic acceptor states. Two different theoretical model calculations addressing the injection dynamics have recently been presented. The first scenario predicts a wide energy distribution for the injected electron via excitations of high-energy vibrational modes in the ionized molecule, whereas the second scenario ascribes the width to thermal fluctuations. We present experimental data at different temperatures and identify the valid injection scenario for perylene/TiO2 systems. The results are discussed in view of recent solar energy conversion scenarios

Measurement of Temperature-Independent Femtosecond Interfacial Electron Transfer from an Anchored Molecular Electron Donor to a Semiconductor as Acceptor

Modified perylene chromophores were adsorbed with virtually constant reaction distance on the surface of a spongelike TiO2 electrode. Interfacial electron transfer was probed with femtosecond resolution in ultrahigh vacuum via transient absorption and fluorescence up-conversion measurements. Identical time constants were measured for the decay of the reactant and rise of the product states. The dominant fast time constant was 190 fs. It remained constant between 300 and 22 K

Measurement of Ultrafast Photoinduced Electron Transfer from Chemically Anchored Ru-Dye Molecules into Empty Electronic States in a Colloidal Anatase TiO₂ Film

Electron transfer from the excited electronic singlet state of chemisorbed ruthenium(II) cis-di(isothiocyanato)- bis(2,2´-bipyridyl-4,4´-dicarboxylate) into empty electronic states in a colloidal anatase TiO2 film was measured as a transient absorption signal of the injected hot electrons with a rise time <25 fs. Optical absorption of the anchored dye molecules led to the excited singlet state of the dye with a small admixture of charge transfer states. The electron transfer reaction reported here did not involve redistribution of vibrational excitation energy and was thus completely different from the well-known Marcus-Levich-Jortner-Gerischer type of electron transfer in the case of weak electronic interaction. It was also not a direct optical charge transfer transition from the donor to the acceptor level but rather an electron transfer reaction with an ultrashort but finite reaction time

Experimental results and basic considerations concerning injection and transport of electrons in the dye-sensitized colloidal sponge-type anatase TiO₂ electrode

Electron transfer over the reaction distance corresponding to three saturated bonds occurs within 70 fs from the excited singlet state of anchored perylene chromophores to empty electronic levels in a sponge-type anatase TiO2 electrode. Injected electrons can be consumed in three different types of recombination processes at the interface with the electrolytic contact. Electron transport through the electrode follows a potential gradient that is set-up in the dark, corresponding to the difference between the Fermi levels of the two contact materials. It is dominated by multiple trapping and trap filling processes