Determination of porphyrin dimer in a mixed monolayer of porphyrin/phospholipid transferred on ITO electrodes

The Langmuir-Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air \ water interfac e onto optically transparent indium-tin oxide (ITO) electrodes. The surface concentration of porphyrin, Gamma, transferred on the ITO surface, has bee n obtained by integration of the reduction current from the cyclic voltammo grams. The experimental Gamma values ranged from Gamma(m) = 5.19 X 10(-11) mol cm(-2), and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Gamma(d) = 8.65 X 10 (-11) mol cm(-2), and equivalent to the total amount of the porphyrin molec ules at the air I water interface under a compression of 35 mN m(-1). Prior to the electrochemical experiments, the transmission spectrum was recorded . The surface concentration obtained of the porphyrin is not directly propo rtional to the transmission of the film, Delta T. This phenomenon is assign ed to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on a n intact ITO electrode is altered with respect to that found at the air \ w ater interface. A simple model has been developed to evaluate the contribut ion of monomer and dimer phases of the porphyrin in the mixed monolayer. Fu rthermore spectroscopic measurements with linearly polarized light under ob lique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces. (C) 2000 Elsevier Science S.A. All rights reserved.