Photoinduced Heterogeneous Electron Transfer Research Articles

Vibrational Spectroscopy on Photoexcited Dye-Sensitized Films via Pump-Degenerate Four-Wave Mixing

Molecular sensitization of semiconductor films is an important technology for energy and environmental applications including solar energy conversion, photocatalytic hydrogen production, and water purification. Dye-sensitized films are also scientifically complex and interesting systems with a long history of research. In most applications, photoinduced heterogeneous electron transfer (HET) at the molecule/semiconductor interface is of critical importance, and while great progress has been made in understanding HET, many open questions remain. Of particular interest is the role of combined electronic and vibrational effects and coherence of the dye during HET. The ultrafast nature of the process, the rapid intramolecular vibrational energy redistribution, and vibrational cooling present complications in the study of vibronic coupling in HET. We present the application of a time domain vibrational spectroscopy-pump-degenerate four-wave mixing (pump-DFWM)-to dye-sensitized solid-state semiconductor films. Pump-DFWM can measure Raman-active vibrational modes that are triggered by excitation of the sample with an actinic pump pulse. Modifications to the instrument for solid-state samples and its application to an anatase TiO2 film sensitized by a Zn-porphyrin dye are discussed. We show an effective combination of experimental techniques to overcome typical challenges in measuring solid-state samples with laser spectroscopy and observe molecular vibrations following HET in a picosecond time window. The cation spectrum of the dye shows modes that can be assigned to the linker group and a mode that is localized on the Zn-phorphyrin chromophore and that is connected to photoexcitation.

(Invited) Contactless Photo-Electrochemistry of Dye-Sensitized TiO2 Nanoparticles Floating at Electrified Water-Oil Interfaces

Certain soft interfaces formed between aqueous and organic electrolyte solutions of low miscibility (e.g., trifluorotoluene) are electrochemically active in the sense that it is possible to precisely control the Galvani potential difference between the two adjacent liquids (i.e., to “polarise” or electrify the interface), and thus drive charge transfer reactions. In comparison to solid electrode-electrolyte interfaces, where typically only electron transfer may be probed, charge transfer reactions at soft interfaces may include simple ion transfer, assisted ion transfer, heterogeneous electron transfer, or photoinduced heterogeneous electron transfer (PET), see Figure 1 [1]. PET represents a novel method of potentially harnessing solar energy at electrified soft interfaces. By exploiting (i) energy trapped from visible light by a photoactive interfacial film (e.g., a porphyrin dye or a dye-sensitized TiO2 nanoparticle) and (ii) the electrochemical driving force provided by electrification of the soft interface, a solar fuel such as H2 or H2O2 may be evolved by PET across the soft interface using a weak electron donor in the oil phase, e.g., ferrocene. Crucially, all photo-electrochemical techniques applied at traditional solid electrode-electrolyte interfaces (voltammetry, impedance, photo-current transients, etc.) are applicable at polarised soft interfaces [2-3]. The photochemical properties of TiO2 nanoparticles have been extensively studied in homogeneous phases for a variety of processes including water splitting and the photocatalytic destruction of pollutants. Mesoporous films based on TiO2 nanoparticles have also been studied as photoactive materials. In this regard, dye sensitization of wide bandgap semiconductors has been extensively studied in the field of photovoltaics. Despite the overwhelming literature on the photochemistry of TiO2 colloids, little is yet known about the properties of these particles floating as “photo-electrodes” at molecular water-oil interfaces, with only a small number of published articles on the topic [4-7]. In this presentation the photo-electrochemistry of TiO2 nanoparticles floating at electrified water-oil interfaces under blue LED illumination will be detailed. The experimental techniques employed (cyclic voltammetry, photocurrent transient data) will be able to provide valuable fundamental information on the rates of charge transfer between redox species in solution and the semiconductors floating at the interface. The studies will explore new possibilities for contactless photo-electrochemistry of nanoparticles in the presence of a static electric field, with potential applications in solar energy conversion to solar fuels and storage (ECS) and photocatalytic pollutant degradation.

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.

Pathway-Dependent Electron Transfer for Rod-Shaped Perylene-Derived Molecules Adsorbed in Nanometer-Size TiO2 Cavities

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 nanometer-size 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.

Theory of ultrafast photoinduced heterogeneous electron transfer

Ultrafast heterogeneous electron transfer (HET) between a molecule attached to a semiconductor surface and the conduction band of the semiconductor is discussed theoretically with emphasis on the perylene–TiO2 system. The used description accounts for the specialty of the molecule, i.e. its particular electronic level scheme together with its vibrational degrees of freedom. The band continuum of the semiconductor is included and the approach is ready to describe different optical excitation and detection processes. Using a diabatic-state like separation of the whole system into molecular and semiconductor states, femtosecond photoinduced dynamics are studied. Since the HET is ultrafast standard rate theories cannot be applied. Instead, the respective time-dependent Schrödinger equation governing the electron–vibrational wave function is solved. Based on this approach and using a time-dependent formulation, the steady state linear absorption is calculated. Parameters of perylene attached to nano-structured TiO2 via different bridge–anchor groups are adjusted by a comparison with measured spectra. A direct charge transfer excitation into the conduction band continuum is included into the description. This time-dependent formulation of the absorbance is confronted with a direct formulation in the frequency domain using the molecular Green's function. Then, it is explained how to observe the energetic distribution of the injected electron which carries signatures of the molecular vibrations in a two-photon photon emission spectrum. Some speculations on a laser pulse control of ultrafast HET are finally given.

Theory of photoinduced heterogeneous electron transfer

We consider electron injection into the conduction band of a semiconductor, from an electronically excited state of a dye molecule, adsorbed on its surface. For arbitrary width of the conduction band, the survival probability of the excited state can be calculated using a Green's-function approach. We show that the existence of a split-off state can play an important role in the total injection probability. In the wide band limit, the survival probability decays exponentially, but for finite band widths it does not. We further investigate the effect of vibrations on the process. A Green's operator technique may be used to solve this too exactly. We show that the problem may be reduced to a non-Hermitian eigenvalue problem for the vibrational states alone. Exact results can be obtained for arbitrary bandwidth and for a few vibrational degrees of freedom. In the wide band limit, the dynamics is particularly simple and we find that (1) the survival probability of the excited state is unchanged by the inclusion of vibrational motion, but (2) each vibrational state now has a finite lifetime. Numerical results are presented for the effects of reorganization energy, energy of the injecting level, and the variation of the matrix element for the electron injection, on the survival probability of the electron in the excited state. As an illustration of the approach, we also present results of numerical calculation of the absorption spectrum of perylene adsorbed on TiO(2) and compare it with experimental results.

Ultrafast heterogeneous electron transfer reactions: Comparative theoretical studies on time- and frequency-domain data

Recent theoretical studies on linear absorption spectra of dye-semiconductor systems [perylene attached to nanostructured TiO2, L. Wang et al., J. Phys. Chem. B 109, 9589 (2005)] are extended here in different respects. Since the systems show ultrafast photoinduced heterogeneous electron transfer the time-dependent formulation used to compute the absorbance is also applied to calculate the temporal evolution of the sub-100 fs charge injection dynamics after a 10 fs laser-pulse excitation. These studies complement our recent absorption spectra fit for two perylene bridge-anchor group TiO2 systems. Moreover, the time-dependent formulation of the absorbance is confronted with a frequency-domain description. The latter underlines the central importance of the self-energy caused by the coupling of the dye levels to the semiconductor band continuum. The used model is further applied to study the effect of different parameters such as (1) the dependence on the reorganization energies of the involved intramolecular transitions, (2) the effect of changing the transfer integral which couples the excited dye state with the band continuum, and (3) the effect of the concrete form of the semiconductor band density of states. Emphasis is also put on the case where the charge injection level of the dye is near or somewhat below the band edge. This nicely demonstrates the change from a structureless absorption to a well-resolved vibrational progression including characteristic shifts of the absorption lines which are a direct measure for the dye-semiconductor coupling.

Theoretical Study of Ultrafast Heterogeneous Electron Transfer Reactions at Dye−Semiconductor Interfaces: Coumarin 343 at Titanium Oxide

A theoretical study of photoinduced heterogeneous electron transfer in the dye-semiconductor system coumarin 343-TiO(2) is presented. The study is based on a generic model for heterogeneous electron transfer reactions, which takes into account the coupling of the electronic states to the nuclear degrees of freedom of coumarin 343 as well as to the surrounding solvent. The quantum dynamics of the electron injection process is simulated employing the recently proposed multilayer formulation of the multiconfiguration time-dependent Hartree method. The results reveal an ultrafast injection dynamics of the electron from the photoexcited donor state into the conduction band of the semiconductor. Furthermore, the mutual influence of electronic injection dynamics and nuclear motion is analyzed in some detail. The analysis shows that--depending on the time scale of nuclear motion--electronic vibrational coupling can result in electron transfer driven by coherent vibrational motion or vibrational motion induced by ultrafast electron transfer.

Absorption Spectra Related to Heterogeneous Electron Transfer Reactions: The Perylene TiO2System

Linear absorption spectra of dye-semiconductor systems (perylene attached to nanostructured TiO2) are studied theoretically and experimentally. The systems show ultrafast photoinduced heterogeneous electron transfer (HET). By applying a time-dependent formulation of the absorbance, the theoretical analysis of the measured data is carried out. The respective electron-vibrational wave packet propagation fully accounts for the electronic coupling to the conduction band continuum of TiO2 and is based on a single-reaction-coordinate model (corresponding to a perylene in-plane C-C stretching vibration with a quantum energy of 1370 cm(-1)). By the insertion of different bridge-anchor groups, the electronic coupling responsible for HET is varied. The dye absorbance in a solvent and the trends in the line broadening of the vibrational progression due to the coupling to the conduction band continuum are reproduced for all investigated types of bridge-anchor groups. HET rates deduced from the calculations on the absorbance displaying line broadenings follow the qualitative trend obtained from transient absorption spectra.

Photoinduced Electron Transfer of 5,10,15,20- Tetraphenylporphyrinato Zinc(II) at the Polarized Water/1,2-Dichloroethane Interface

The photocurrent at the polarized water/1,2-dichloroethane (DCE) interface was successfully observed in the presence of a lipophilic sensitizer, 5,10,15,20-tetraphenylporphyrinato zinc (ZnTPP), in the organic phase. The photocurrent transient responses were apparently affected by the employed organic supporting electrolyte: tetrapenthylammonium tetraphenylborate (TPnATPB) or tris(tetraoctylammonium)tungstophosphate ((TOcA)3PW12O40). The photocurrent measured in the TPnATPB system exhibited rather slow responses associated with the ion transfer of photoproducts. On the other hand, the photoinduced heterogeneous electron transfer could be observed in the use of (TOcA)3PW12O40. The photocurrent intensity in the (TOcA)3PW12O40 system exhibited an apparent pH dependence and the photoreduction of hydrogen ions probably took place at the water/DCE interface. By analyzing the real and imaginary components of the photocurrent depending on the photoexcitation frequency, we roughly estimated the phenomenological rate constants of the product separation (k(ps)) and recombination (k(rec)) processes as log(k(ps)/s(-1)) = 1.5 +/- 0.2 and log(k(rec)/s(-1)) = 1.8 +/- 0.1, respectively.

Theory of ultrafast photoinduced heterogeneous electron transfer: Decay of vibrational coherence into a finite electronic–vibrational quasicontinuum

Photo-induced electron transfer from a surface attached dye molecule to the band levels of a semiconductor is modeled via an electronic–vibronic quasicontinuum. The description enables one to obtain a fairly accurate expression for the decay of the excited molecular state, including initial vibronic coherences. The model accounts for (a) the effect of a finite band width, (b) variations in reorganization energy and electronic coupling, (c) various energetic positions for the injecting level, (d) different initial vibrational wave packets in the excited state, and (e) two vibrational modes participating in the electron transfer process. Most cases are studied numerically and can be reasonably well understood from the obtained decay expression.

Two-Photon Photoemission Study of Heterogeneous Electron Transfer: C6F6on Cu(111)

Photoinduced heterogeneous electron transfer (ET) constitutes the key step in a number of chemical processes, such as solar energy conversion, photocatalysis, and surface photochemistry. We present a general strategy to investigate photoinduced heterogeneous ET using two-photon photoemission (2PPE) spectroscopy. In this report, we focus on the energetics in photoinduced heterogeneous ET in monolayer and bilayers of hexafluorobenzene on Cu(111). We found that for monolayer C6F6, which likely adsorbs flat on the surface, the ET resonance is not visible in 2PPE spectrum. However, for second-layer C6F6, a strong molecular resonance is observed. Photon energy dependence shows that the shift in electron kinetic energy (ΔEkin) from the molecular resonance scales with Δhν, unlike that from the occupied surface state, which scales with 2Δhν. This provides evidence that the observed molecular resonance results from electron transfer to an unoccupied molecular orbital located at 2.9 eV above the Fermi level (or 1.8 ...

Solar energy conversion using dye-sensitised liquid|liquid interfaces

A novel approach to solar energy conversion is presented on the basis of photoinduced heterogeneous electron transfer at polarised interfaces between two immiscible electrolyte solutions. Preliminary studies at the water|1,2-dichloroethane interface sensitised by water-soluble zinc porphyrin heterodimers exhibited conversion efficiencies around 0.1%, corresponding to a photocurrent to absorbed photon flux ratio close to unity. The conversion efficiency is increased in the presence of redox couples acting as supersensitiser in the aqueous phase. The characteristic power relationship for these interfaces is also presented, and the possibility of solar cells where no solid electrodes are directly involved in the photogeneration and transport of charge carriers is envisaged.