Two-Dimensional Pigment Monolayer Assemblies for Light-Harvesting Applications: Structural Characterization at the Air/Water Interface with X-ray Specular Reflectivity and on Solid Substrates by Optical Absorption Spectroscopy

Abstract

X-ray specular reflectivity at the liquid/gas interface of dihexadecyl phosphate (DHDP) on pure H2O and on a series of pigment-containing aqueous solutions are reported along with visible absorption spectra of corresponding monomolecular Langmuir−Blodgett films on quartz substrates. Molecular level interpretation of the reflectivity from DHDP on pure water reveals that at large surface pressure (>10 mN/m), the film is closely packed with practically untilted hydrocarbon chains and hydrated phosphate headgroups. On solutions containing either water-soluble cationic tetraazaphthalocyanines or tetrapyridylporphyrins, significant changes in the organization of the lipid with respect to that on pure water are found. Total film thicknesses are larger and consistent with the adsorption of a single pigment layer contiguous to the headgroup, whereas the hydrocarbon tail region is shorter, suggestive of tilted alkyl chains. In addition, film thicknesses for phthalocyanine-containing films suggest formation of an iodide counterion layer underneath the plane containing the pigments. This sharply contrasts the interfacial profile obtained for porphyrin-containing films, in which the iodide counterions appear to exist within the pigment plane. Visible absorption spectra of all transferred films indicate a closely packed single pigment layer, consistent with the reflectivity results. The optical spectra of the pigment are preserved (in relation to the aqueous solution monomer spectra) in the transferred film, indicating a suppression of pigment aggregation. Reflectivity measurements at large molecular areas on pure water indicate that DHDP forms an inhomogeneous film, suggestive of phase segregation; on the pigment-containing solutions, DHDP induces (through attractive Coulomb interactions) the adsorption of a homogeneous monopigment layer. The existence of a complete pigment monolayer over the measured surface pressure−molecular area (π−A) isotherms has been evidenced by both X-ray reflectivity and visible optical studies. Preservation of pigment functionality has been demonstrated through the process of Coulomb association of the chromophores with charged lipid monolayer headgroups at the air/water interface. The potential for applications as model photosynthetic antennae will be discussed.