Photoinduced electron transfer at liquid/liquid interfaces - Part II. A study of the electron transfer and recombination dynamics by intensity modulated photocurrent spectroscopy (IMPS)

The dynamics of photoresponses associated with heterogeneous quenching of zinc tetrakis(carboxphenyl)porphyrin (ZnTPPC) and ferrocene derivatives at the water/1,2-dichloroethane interface were studied by intensity modulated photocurrent spectroscopy (IMPS). The contribution arising from the electron injection, recombination-product separation competition and the attenuation associated with the uncompensated resistance and interfacial capacitance (RC) time constant of the cell were deconvoluted in the frequency domain. The flux of electron injection was described in terms of a competition between the relaxation of the porphyrin excited state and the electron transfer step. Experimental results in the presence of ferrocene and diferrocenylethane confirmed that as the Galvani potential difference is increased, the phenomenological electron transfer rate constant increases and the ZnTPPC coverage at the liquid/liquid junction decreases. Furthermore, the recombination rate constant decreases with increasing potentials, while the product separation rate constant did not show a clear potential dependence. Photocurrent studies were extended to the electron donors dimethylferrocene and trianisylamine, as well as to the electron acceptor tetracyanoquinodimethane. The results obtained clearly indicate that the Gibbs energy of activation for the charge transfer process is affected by the Galvani potential difference. It is suggested that the electron transfer dynamics are dependent on the local electric field generation by the specifically adsorbed ZnTPPC. The general expressions for the frequency dependent photocurrents at liquid/liquid interfaces are also introduced.