The dynamics of photoinduced heterogeneous electron transfer between a series of ferrocene derivatives and the heterodimer zinc meso-tetrakis(p-sulfonatophenyl)-porphyrin (ZnTPPS4-) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP4+) were studied at the polarized water/1,2-dichloroethane interface. The photocurrent responses originating from the heterogeneous quenching of the heterodimer showed a well-defined dependence on the formal Gibbs energy of electron transfer (Δ ). The use of various ferrocene derivatives with different redox potentials and potentiostatic control over the Galvani potential difference across the interface allowed modifying over a range of 1 eV. The photocurrent as a function of can be unambiguously described in terms of a Marcus-type behavior of the phenomenological bimolecular electron-transfer rate constant ( ). The solvent reorganization energy was estimated to be 1.05 eV, from which an average distance of 0.8 nm between the redox species can be evaluated within the framework of the Marcus model for sharp liquid/liquid boundary. These studies also provided an estimate of the activation-less limit of of 3 × 10-19 cm4 s-1, which reflects a rather nonadiabatic behaviour of the charge-transfer process. The origin of this nonadiabaticity is connected to the average distance separating the redox species across the interface. Finally, the implications of the observed potential dependence of on current debates about structure and potential distribution across the interface are briefly highlighted.

The initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the water|1,2-dichloroethane (DCE) interface have been studied by ultrafast time-resolved spectroscopy and dynamic photoelectrochemical measurements. Photoexcitation of the water-soluble ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS(4)(-)) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP(4+)) leads to a charge-separated state of the form ZnTPPS(3)(-)-ZnTMPyP(3+) within 40 ps. This charge-separated state is involved in the heterogeneous electron injection to acceptors in the organic phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estimations of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodynamic driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This analysis also suggests that the effective redox potential of BQ at the liquid|liquid boundary is shifted by 0.6 V toward positive potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is associated with the formation of hydrogen bonds at the liquid|liquid boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed.

The adsorption of ions at the interface between two immiscible electrolyte solutions (ITIES) is primarily controlled by the potential distribution across the interface, which in turn is influenced by the adsorption process. In the present paper, we simulate the effect of the adsorption of charged species on the charge distribution at the ITIES based on the classical description of the interface employing the Gouy-Chapman model. The inner layer is considered as a charged plane, where the ionic adsorption takes place. The potential at this plane is determined by the electro-neutrality condition. Various adsorption isotherms are considered, including potential dependent isotherms based on the Langmuir and Frumkin adsorption models. The potential distribution and the charge density profile are derived by solving the Poisson-Boltzman equation numerically. We show that the charge distribution in the interfacial region is significantly affected by the adsorption of ionic species. Under certain conditions, the adsorption results in a nonmonotonic potential distribution with a potential trap at the interface.

The voltage induced assembly and photoreactivity of cadmium selenide (CdSe) nanoparticles protected by mercaptosuccinic acid are studied at the polarisable interface between water and 1,2-dichloroethane electrolyte solutions. Cyclic voltammograms and admittance measurements show an increase of the interface excess charge associated with the adsorption of CdSe nanoparticles as the Galvani potential difference is tuned to negative values with respect to the potential in the organic phase. Within the potential range where the nanoparticles are adsorbed, band-gap illumination leads to heterogeneous electron transfer from CdSe nanoparticles to electron acceptors located in the organic phase. The interfacial Galvani potential difference plays an important role in these phenomena, as it affects the interfacial density of the nanoparticles, as well as the driving force for the electron transfer. The photocurrent efficiency also strongly depends on the formal redox potential of the electron acceptor, indicating that the heterogeneous photoreaction is kinetically controlled. The interfacial electron transfer occurs via depopulation of the deep trap states in the band gap. Analysis of the photocurrent transient responses reveals that the magnitude of the instantaneous photocurrent upon illumination is determined by the kinetics of heterogeneous electron transfer, while photogenerated holes are swiftly captured by species present in the aqueous phase. The photocurrent decay upon constant illumination is associated with the diffusion of the acceptor to the interfacial region. From the phenomenological point of view, the photoelectrochemical behaviour of CdSe nanoparticles can be compared to a self-assembled ultrathin p-type semiconductor photoelectrode.

The heterogeneous photoinduced electron transfer involving dyes adsorbed at the interface between two immiscible electrolyte solutions and redox molecules located in the adjacent phase manifests itself as photocurrent responses under potentiostatic conditions. Photocurrent transients as functions of the light intensity and bias potential allow the extraction of insightful information on the kinetics of the various processes associated with the photoinduced reaction. Previous analyses of this type of responses were based on phenomenological models that did not consider mass transport. In the present paper, we develop a generalised model for photocurrent transients taking into account the diffusion of reacting species to the interface. Comparison with the experimental data confirms that the responses can be described adequately by applying stationary conditions to the surface concentration of the photoactive species. Mechanistic aspects associated with the nature of the photocurrent relaxation on the microsecond time scale are examined. In particular, the dependence of the transient response on the light intensity indicates that charge recombination proceeds mainly as a first order reaction from an interfacial geminate ion pair. Coupled ion transfer reactions involving the photoproducts can also contribute to the photocurrent, depending on the formal ion transfer potential of the corresponding species.