Photo-induced charge separation in TiO2/porphyrin bilayers studied by time-resolved microwave conductivity

The electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study photo-induced interfacial charge separation in bilayers consisting of a smooth, 80 mu layer of anatase TiO2 onto which is spin-coated a 60 nm layer of a tetraphenyl porphyrin derivative with either unsubstituted or para-carboxy-substituted phenyl moieties; "TPP" or "TPPC". No significant difference in sensitization efficiency was observed between TPP and TPPC; both gave a maximum charge separation efficiency per incident photon of ca 1% on irradiation in their Soret band (430 nm). The low efficiency indicates that interfacial charge transfer occurs mainly for excited states produced in the first adsorbed monolayer of the sensitizer. Contrary to previous suggestions, specific interactions between the carboxy groups of TPPC and the TiO2 surface do not appear to favor more efficient charge separation.