The interaction of Zn(II) porphyrin (ZnPP) with colloidal TiO2 was studied by absorption and fluorescence spectroscopy. The fluorescence emission of ZnPP was quenched by colloidal TiO2 upon excitation of its absorption band. The quenching rate constant ( ) is ？M？1？s？1. These data indicate that there is an interaction between ZnPP and colloidal TiO2 nanoparticle surface. The quenching mechanism is discussed on the basis of the quenching rate constant as well as the reduction potential of the colloidal TiO2. And the mechanism of electron transfer has been confirmed by the calculation of free energy change by applying Rehm-Weller equation as well as energy level diagram. 1. Introduction Wide-band gap semiconductor particles such as TiO2 have been widely used for different applications in photocatalysis and the environment [1, 2].Over the past decades, considerable interest has been shown in the modification of TiO2 semiconductors by organic dyes to extend the photoresponse to visible light owing to their potential application in solar energy conversion [3–5]. Dye sensitization is considered to be an efficient method to modify the photo response properties of TiO2 particles. The dyes used are erythrosine B , rose Bengal , metal porphyrin [8–10], and so forth. Porphyrins, (including metal-free porphyrins, metalloporphyrins and supramolecular porphyrins)  are recognized to be the most promising sensitizers . The chemistry of porphyrin derivatives has played an important role especially during the past decade in particular branches of new materials science, and many researchers have undertaken projects on the synthesis of variously substituted compounds to obtain new functional materials [13–15]. Metalloporphyrin may be an appropriate candidate because of its high absorption coefficient within the solar spectrum and its good chemical stability in comparison to that of other dyes. They are highly effective photocatalysts due to their very strong absorption in the 400？nm–450？nm region (Soret band) and in the 500？nm–700？nm region (Q-bands) and, in fact, the presence of p-electrons affords the condition for electron transfer during the photoreaction. In the present work we have investigated the electron transfer from excited ZnPP (see Scheme 1) to the conduction band of TiO2 colloid by using absorption and fluorescence spectroscopy. Scheme 1: Structure of ZnPP. 2. Materials and Methods 2.1. Materials Zn(II) porphyrin and tetrabutyl titanate were purchased from Aldrich. The doubly distilled water was used for preparing the solutions. All measurements
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