Orientation of Porphyrin Moieties in Langmuir−Blodgett Films of Tetraphenylporphyrin Vinyl Monomers and Their Polymers

Abstract

The monolayer behavior and formation of Langmuir−Blodgett (LB) films were compared between the vinyl monomers of silver(II) tetraphenylporphyrin (AgTPP) and their polymers. In both cases, eicosanoic acid was mixed at [eicosanoic acid]/[AgTPP unit] = 1.5 ratio to reduce the aggregation of the porphyrins. From the isotherms of the monolayers at an air−water interface, the porphyrin moieties in the monomers occupied a larger area than those in the polymers. It indicated that the porphyrin groups were more compact in the polymers because they were attached to a polymer chain. Applying the electron spin resonance and polarized UV−vis absorption spectroscopies for the LB films deposited on a hydrophobic glass plate, the orientation of the AgTPP moieties was estimated. As a result, three orientations of porphyrin planes existed in the monomer films: the angle between the film normal and the transition dipole (or the porphyrin plane), φ, was 90° for one orientation and 39−59° for the other two orientations. Two orientations existed in the polymer films: φ was 39−59° for these two. The porphyrin whose plane oriented at φ = 90° was considered to be the one that was squeezed out of the fatty acid layers. Such porphyrin moieties did not exist in the polymer films. These results have shown that the polymer main chain plays an important role in stabilizing the orientation of porphyrin moieties.