MONOLAYER STUDIES OF 5-(4-CARBOXYPHENYL)-10,15,20-TRITOLYL-PORPHYRIN–I. OPTICAL STUDIES OF FILMS AT THE AIR-WATER INTERFACE and OF FILMS TRANSFERRED ONTO SOLID SUBSTRATES?

Abstract— The title compound forms well-behaved monomolecular films at the air-water interface. The surface pressure-molecular area isotherms change with the pH of the subphase in a manner which suggests that the carboxylic acid group acts as the hydrophylic portion of the molecule with a pK_a of –7.3. In compressed monolayers the porphyrin ring appears to be oriented so that the plane of the ring is perpendicular to the surface. Spectroscopic studies of single monolayer films transferred to quartz slides using the Langmuir-Blodgett technique indicate that three distinct species are present in the films, with the amount of each phase depending on the pH of the subphase. One species, present at low pH, is assigned as a monomer on the basis of its optical and fluorescence spectra and its fluorescence lifetime. At low pH this monomer species co-exists with another non-fluorescent aggregated species. For films formed on subphases with pH > 7.3, these two species are converted to a single, weakly fluorescent species which exhibits an unusual absorption spectrum. We postulate that this third species is a constrained aggregate but rule out the possibility of a face-to-face dimer on spectroscopic grounds. For films at the air-water interface specular reflection, indicative of a smooth, highly absorbant film, is observed from monolayers of the title compound. Visual examination of this phenomenon proved to be very useful in assessing the completeness of spreading and the collapse point. Under certain conditions a distinct macroscopic structure is observed in the monolayer film. This structure is interpreted as evidence for the presence of two or more two-dimensional crystalline phases. Support for this view comes from previous measurements of specular reflection on single crystals of tetraphenylporphyrin. There is no indication that variations in the macroscopic structure of the films have any significant effect on the microscopic properties such as the surface pressure-area isotherms.