The general case of a heterogeneous electron-transfer reaction is realized by ultrafast electron transfer from a light-absorbing molecule to a wide continuum of electronic acceptor states, realizing the so-called wide band limit. Experimental data obtained for perylene dye/TiO2 systems confirm the predictions of fully quantum mechanical model calculations of the dynamics. The energy distribution of the injected electron shows an energy loss due to excitations of high-energy (quantum) vibrational modes in the ionized perylene moiety. The electron-transfer mechanism is non-adiabatic and the reaction is ultrafast, for example, with a time constant of 9 fs for the COOH anchor-bridge group. The underlying strong coupling of the electronic states to high-energy vibrational modes is a characteristic feature of sensitizer molecules.

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.

The energy distribution of electrons injected into acceptor states on the surface of TiO 2 was measured with femtosecond two-photon photoemission. Shape and relative energetic position of these distribution curves with respect to the corresponding donor states, i.e. of perylene chromophores in the first excited singlet state attached via different bridge-anchor groups to the TiO 2 surface, were compared with the predictions of different theoretical models for light-induced ultrafast heterogeneous electron transfer (HET). Gerischer’s early scenario for light-induced HET was considered and two recent explicit calculations, i.e. a fully quantum mechanical analytical model and a time-dependent density functional theory model based on molecular dynamics simulations for the vibrational modes were also considered. Based on the known vibrational structure in the photoionization spectrum of perylene in the gas phase and that measured in the linear absorption spectra of the perylene chromophores anchored on the TiO 2 surface the energy distribution curves for the injected electrons were fitted assuming the excitation of the dominant 0·17 eV vibrational mode in the ionized perylene chromophore leading to a corresponding Franck-Condon dictated progression in the energy distribution curves. Each individual peak was fitted with a Voigt profile where the Lorentzian contribution was taken from the time-resolved HET data and the Gaussian contribution attributed to inhomogeneous broadening. The measured room temperature energy distribution curves for the injected electrons are explained with the fully quantum mechanical model for light-induced HET with the high energy, 0·17 eV, skeletal stretching mode excited in the ionized perylene chromophore. The corresponding energy distribution of the injected electrons is fully accommodated in acceptor states on the TiO 2 surface fulfilling the wide band limit.

Electron transfer from a molecular level to empty continuum levels of a substrate is described theoretically. Using a quasicontinuum approach to model the substrate, analytical expressions pertaining to the time-dependent probability among the various levels of the substrate is presented along with its extension to coherently excited molecular vibrational modes. Hidden time scales and dynamics are revealed in the analysis and possible experiments to observe the new results are suggested. We note the applicability of the model to the description of a variety of other phenomena that are formally similar to the electron injection problem, although pertaining to different physics.

Two-photon photoemission (2PPE) spectra related to sub-100-fs heterogeneous electron transfer from perylene to TiO2 are calculated. The approach accounts for the dominant intramolecular vibration of perylene as well as for the band structure of TiO2 described in a tight-binding model. The focus is on the influence of the pump and probe laser pulse duration, with the pump laser originating charge injection and the probe laser causing the photoemission process. The latter may proceed directly from the photoexcited molecule or, after charge injection, from the TiO2 conduction band. The time-dependent Schrodinger equation which describes charge injection and accounts for the pump pulse is solved exactly within a time interval of about 250 fs. The action of the probe pulse is considered in linear response theory. While the vibrational structure in the 2PPE spectra broadens with decreasing pump pulse length, it is found that this structure is largely preserved when varying the probe pulse duration. In order to estimate dephasing caused by intramolecular vibrational energy redistribution in perylene and electron phonon coupling in TiO2, a density matrix scheme is also introduced describing heterogeneous electron transfer and the photoelectron emission processes. A finite escape depth for electrons at the TiO2 surface is finally taken into account to evaluate its influence on the spectra.

Perylene chromophores were adsorbed on the Ag(110) surface via surface bond formation of the –CH2–SH group that was covalently attached to the perylene moieties. The lowest excited singlet state of perylene was populated with a laser pulse of 440nm wavelength. The decay of this donor state due to ultrafast electron transfer to the Ag electrode was probed with a second, delayed laser pulse centered at 280nm that gave rise to a photoemission signal detected in ultra-high-vacuum by a spectrometer. The fit to the measured data revealed a time constant of 15fs for the electron injection reaction.

A complete picture of different interfacial electron transfer dynamics has been obtained from transient absorption and two-photon photoemission data when inserting different anchor/bridge groups between the excited organic donor and the electrode surface.

Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the organic chromophore perylene into the inorganic semiconductor rutile TiO2 was investigated with femtosecond time-resolved two-photon photoemission (2PPE). With 2PPE one can address adsorbates at coverages far below a monolayer on single crystal surfaces. With the same chromophore perylene fixed with different anchor and bridge groups at the surface of rutile TiO2(110) the corresponding 2PPE transients revealed the relevant parameters that characterize the contributing processes. Instantaneous optical injection on one hand and slow injection over a long distance on the other hand were realized. Direct optical charge transfer was realized with the chromophore catechol that is known to form a charge transfer. complex with Ti atoms on the surface of TiO2. The slow injection cases were realized by inserting rigid-molecular bridges. Comparison of the different 2PPE signals with corresponding transient absorption (TA) signals for the identical systems revealed the physical processes and time scales that control the 2PPE transients. On the surface of the single crystals only one long time constant was measured via 2PPE also in the case of a-long rigid bridge/anchor group in contrast to a broad distribution of time constants observed for the same molecules anchored in the nm-size cavities of an anatase TiO2 film measured via TA. The broad distribution of time constants in the latter measurements can be attributed to different microscopic environments giving rise to different distances between the chromophore and the nearest TiO2 wall.

Absorption spectra related to heterogeneous electron transfer are analyzed with the focus on direct charge transfer transition from the surface attached molecule into the semiconductor band states. The computations are based on a model of reduced dimensionality with a single intramolecular vibrational coordinate but a complete account for the continuum of conduction band states. The applicability of this model to perylene on TiO2 has been demonstrated in a series of earlier papers. Here, based on a time-dependent formulation, the absorbance is calculated with the inclusion of charge transfer excitations. A broad parameter set inspired by the perylene TiO2 systems is considered. In particular, the description generalizes the Fano effect to heterogeneous electron transfer reactions. Preliminary simulations of measured spectra are presented for perylene-catechol attached to TiO2.

A broad distribution of time constants was found for photoinduced heterogeneous electron transfer (PHET) from the excited-state of a perylene chromophore when the latter was attached via long rigid bridge/anchor groups to the inner walls of nanometer-size cavities formed in a colloidal anatase TiO2 layer. In contrast, in the same environment PHET was dominated by only one short time constant when the perylene chromophore was attached via a short anchor/bridge group. The same results were obtained irrespective of the specific chemical composition of the short or long rigid anchor/bridge groups. To verify that the set of different time constants was caused by different microscopic environments in the nanometer-cavities, PHET was also measured for the same perylene compounds on the (110) surface of TiO2 rutile single crystals, employing here the more sensitive femtosecond two-photon photoemission technique in place of transient absorption. On the surface of the single crystals only one long time constant was measured for PHET also in the case of the long rigid bridge/anchor groups. Thus, the broad distribution of time constants observed in the nanometersize cavities for the long rigid bridge/anchor groups can be attributed to different microscopic environments giving rise to different distances between the chromophore and the nearest TiO2 wall. Consequences of this pathway dependent PHET are discussed for the design of dye molecules and electrodes in dye-sensitized solar cells.

Two dye sensitized solar cells (DSC) can be joined to form a tandem cell with two separate absorption ranges for the two different absorber materials. This can enhance the solar conversion efficiency and in particular the photovoltage of the DSC. Water splitting appears as a realistic long term target. The DSC tandem can be realized as n-n junction employing known dye molecules with optimal absorption spectra. Dye molecules with elongated shapes can be realized by covalently attaching a conducting bridge group terminated by an anchor group to a desired chromophore. Due to the long conducting bridge group separating the hole state of the dye from the surface of the semiconductor recombination is slowed down. The ordered molecular structure can be self-assembled on the recently introduced rod or cylinder shaped oxide electrodes but will not slow down recombination in the nm-cavities of the conventional TiO2 Graetzel electrode.

Ultrafast interfacial electron transfer from the donor orbital of organic chromophores into empty electronic acceptor states of a semiconductor and of a metal was investigated by two-photon photoemission spectroscopy (2PPE). Experimental tools and procedures have been developed for carrying out wet-chemistry preparation of the molecule/solid interface. The organic chromophore perylene was investigated with several different bridge/anchor groups on TiO2(110). One perylene compound was investigated for comparison on Ag(110). Angle and polarization dependent 2PPE measurements revealed the orientation of the perylene chromophore on the surface as controlled by the adsorption geometry of the respective anchor group on TiO2. UPS measurements gave the position of the HOMO level of the chromophore with respect to the Fermi level of the solid. The donor level of each molecule was found high enough to fulfill the "wide band limit" of heterogeneous electron transfer dynamics. Time constants for heterogeneous electron transfer were extracted from 2PPE transients. A difference by a factor of four was found, 13 fs against 47 fs, when a conjugated bond was exchanged for a saturated bond in the otherwise identical bridge group. The two different contributions to the 2PPE transients arising firstly from the excited state of the chromophore and secondly from the injected electrons were separated by measuring the latter contribution separately in the case of instantaneous interfacial electron transfer realized with catechol as adsorbate. The time scales measured for the electron transfer step and for the subsequent electron escape process from the surface into the bulk of TiO2 showed both good agreement with recent theoretical predictions of other groups for these systems.

Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the large organic chromophore perylene into the inorganic semiconductor rutile TiO2 was investigated with femtosecond time-resolved two-photon photoemission (TR-2PPE). The strength of the electronic interaction between the chromophore and the semiconductor was varied by inserting different anchor/bridge groups that functioned either as electronic wire or electronic tunnelling barrier. Both anchor groups, i.e. carboxylic and phosphonic acid, formed strong chemical bonds at the TiO2 surface. The perylene chromophore with the different anchor/bridge groups was adsorbed from solution in a dedicated ultra-high-vacuum (UHV) chamber. The adsorption geometry of the chromophore perylene was determined from angle and polarization dependent two-photon photoemission (2PPE) signals and was found to be very different for the two different anchor/bridge groups. The measured adsorption geometries are compatible with recent DFT (density functional theory) calculations by P. Persson and co-workers [M. Nilsing, S. Lunell, P. Persson, L. Ojamäe, Phosphonic acid adsorption at the TiO2 anatase (1 0 1) surface investigated by periodic hybrid HF-DFT computations, Surf. Sci. 582 (2005) 49–60]. Two different processes contributed to the TR-2PPE transients, firstly electron transfer from the chromophore to the electronic acceptor states on the surface and secondly escape of the electrons from the surface into the bulk of the semiconductor. The latter escape process was measured separately by making the interfacial electron injection process instantaneous when the chromophore catechol was employed in place of the perylene compounds. The thus measured electron escape behavior was governed by the same time constants that have recently been predicted by Prezhdo and coworkers from time dependent DFT calculations [W.R. Duncan, W.M. Stier, O.V. Prezhdo, Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the Alizarin-TiO2 interface, J. Am. Chem. Soc. 127 (2005) 7941–7951]. The HET times derived from the 2PPE transients showed very good agreement with HET times measured via transient absorption (TA) on anatase TiO2 layers. The measured energy distribution of the 2PPE signals for the injected electrons suggests that a high density of electronic acceptor states is operative in both systems and is spread over an at least 1 eV wide energy range. The acceptor states are tentatively identified with surface states created through the formation of chemical bonds between the anchor groups of the organic molecules and surface atoms of the semiconductor.