UV-VIS, FLUORESCENCE, AND EPR STUDIES OF PORPHYRINS IN BILAYERS OF DIOCTADECYLDIMETHYLAMMONIUM SURFACTANTS

The incorporation characteristics of three simple tetraarylporphyrins into bilayers of dioctadecyldimethylammonium surfactants was studied by UV-vis, fluorescence, and EPR spectroscopy. The porphyrins used are tetraphenylporphyrin (TPP), tetrakis(4-(hexadecyloxy)phenyl)porphyrin (THPP), and tris(4-(hexadecyloxy)phenyl)(4-methylpyridinium) porphyrin tosylate (TrHPyP). At low porphyrin to surfactant molar ratios (<5 x 10(-4)) the porphyrins show a strong fluorescence. Increasing this ratio to 5 x 10(-3) causes a change in the UV-vis spectra and a decrease of the fluorescence intensity. Time-resolved fluorescence measurements indicated that the latter is due to the formation of non-fluorescent porphyrin aggregates. The spectral characteristics of the porphyrin aggregates are discussed in terms of the formation of different types of aggregates. The location of the porphyrins within the bilayers was investigated by fluorescence quenching experiments using iodide, 9(10)-bromooctadecanoic acid, and 16-bromohexadecanoic acid. These experiments suggest that THPP is located in the middle of the bilayer and TrHPyP near the aqueous interface. For the parent compound TPP no well-defined position was found. EPR spectroscopy on cast films of dioctadecyldimethylammonium surfactants with the copper derivatives of the porphyrins incorporated (porphyrin-to-surfactant ratio = 5 x 10(-3)) revealed a clear anisptropic distribution of the latter molecules in the cast bilayers. The angles between the porphyrin normals and the bilayer normal were determined by comparison of experiment with simulated spectra. In the case of THPP and TrHPyP these angles agree very well with an arrangement in which the long porphyrin axis lies parallel to the bilayer surface. The arrangement of TPP aggregates could not be established.