Adsorption behavior and orientation of tetrakis(methylpyridiniumyl)porphyrin on silica

The adsorption of 5,10,15,20-tetrakis(4-N-methylpyridiniumyl)porphyrin (H(2)TMPyP) from aqueous solutions on silica has been studied by reflectometry, while order parameters of the orientation distribution of the adsorbed porphyrins have been obtained by total internal reflection fluorescence (TIRF). The initial adsorption rate of the positively charged porphyrins on the negative surface is transport-controlled. Subsequently, the adsorption continues at a lower rate, leading to significantly higher adsorbed amounts in the presence of electrolyte than from pure water. From a combination of reflectometer and TIRF results it is found that ordering in the adsorption layer is primarily determined by steric and electrostatic interactions between the adsorbed molecules and not by charge interactions between the surface and the porphyrin molecules. In the presence of buffer (screening lateral electrostatic interactions) the H(2)TMPyP molecules are more or less randomly oriented at low surface coverage, while, at higher surface coverage or in the absence of background electrolyte, a broad orientation distribution around an angle of ca. 45 degrees between the porphyrin macrocycle and the surface is found. From TIRF measurements at various solution viscosities it is found that the orientation of the adsorbed porphyrins does not change on the time scale of fluorescence.