Dynamic photoelectrochemistry at porphyrin-sensitised liquid/liquid interfaces

Abstract

The interface between two immiscible electrolyte solutions provides a well-defined framework for fundamental studies of heterogeneous charge transfer processes. Recent developments in experimental methodology have allowed addressing the kinetics of electron transfer across these molecular junctions. Among these advances, photoinduced heterogeneous electron transfer has provided interesting insights into the elementary processes at dye-sensitised liquid|liquid interfaces. In the present work, we show that the self-organisation of a variety of water-soluble dyes at the water|1,2-dichloroethane interface can be followed by admittance and photocurrent measurements in the presence of a redox couple in the adjacent phase. The specific adsorption manifests itself by changes in the surface excess charge as well as in the dielectric properties of the interface. Photocurrent data allow extracting Gibbs energies of adsorption in the range of -30 to -45 kJ mol-1. The average molecular orientation of the adsorbed photoactive species is estimated from the photocurrent dependence on the angle of light polarisation in total internal reflection. The results show a clear correlation between the orientation of the transition dipole and the distribution of the peripheral charged groups responsible for the hydrophilic nature of the dyes. The anionic zinc meso-tetrakis(p-sulphonatophenyl)porphyrin and the cationic zinc meso-tetrakis(N-methylpyridyl)porphyrin undergo spontaneous association in solution, leading to electrically neutral surface active heterodimer species of particular interest in the investigation of the kinetics of heterogeneous electron transfer processes. Indeed, the surface coverage of heterodimer appears independent of the Galvani potential difference between the two phases. Hence the photocurrent dependence on the applied potential can be directly related to changes in the kinetics of electron transfer. While the depopulation of the S1 state of the monomers via fluorescence and intersystem crossing occurs in the nanosecond range, the excited state of the heterodimer features a fast decay within 100 ps. This decay leads to the formation of an intermolecular charge transfer state with lifetime extending into the microsecond time scale. The dynamics of photoinduced heterogeneous electron transfer between ferrocene or quinone derivatives and the heterodimer are studied at the polarised water|1,2-dichloroethane interface. The photocurrent responses originating from the heterogeneous quenching of the porphyrin complex show a well-defined dependence on the formal Gibbs energy of electron transfer (ΔGo'et). The photocurrent as a function of ΔGo'et can be unambiguously described in terms of a Marcus-type behaviour of the phenomenological bimolecular electron transfer rate constant (kIIet). The solvent reorganisation energy is 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 provide an estimate of the activation-less limit of kIIet of 3·10-19 cm4 s-1, which reflects a rather non-adiabatic behaviour of the charge transfer process. In the presence of ferrocene derivatives, back electron transfer from the reduced porphyrin complex introduces a relaxation of the photocurrent in the millisecond time scale. In the case of electron acceptors such as 1,4-benzoquinone, the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidised porphyrin complex as well as interfacial protonation of the semiquinone radical anion. The dynamics of charge separation, back electron transfer, coupled ion transfer and diffusion of redox species to the liquid|liquid interface can be resolved in the time and frequency domains by photocurrent transient measurements as well as intensity modulated photocurrent spectroscopy.