2-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/

X-ray specular reflectivity at the liquid/gas interface of dihexadecyl phosphate (DHDP) on pure H2O and on a series of pigment-containing aq ueous solutions are reported along with visible absorption spectra of corresponding monomolecular Langmuir-Blodgett films on quartz substrat es. Molecular level interpretation of the reflectivity from DHDP on pu re water reveals that at large surface pressure (>10 mN/m), the film i s closely packed with practically untilted hydrocarbon chains and hydr ated phosphate headgroups. On solutions containing either water-solubl e cationic tetraazaphthalocyanines or tetrapyridylporphyrins, signific ant changes in the organization of the lipid with respect to that on p ure water are found. Total film thicknesses are larger and consistent with the adsorption of a single pigment layer contiguous to the headgr oup, whereas the hydrocarbon tail region is shorter, suggestive of til ted alkyl chains. In addition, film thicknesses for phthalocyanine-con taining films suggest formation of an iodide counterion layer undernea th the plane containing the pigments. This sharply contrasts the inter facial profile obtained for porphyrin-containing films, in which the i odide counterions appear to exist within the pigment plane. Visible ab sorption spectra of all transferred films indicate a closely packed si ngle pigment layer, consistent with the reflectivity results. The opti cal spectra of the pigment are preserved (in relation to the aqueous s olution monomer spectra) in the transferred film, indicating a suppres sion of pigment aggregation. Reflectivity measurements at large molecu lar 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 adsorptio n of a homogeneous monopigment layer. The existence of a complete pigm ent monolayer over the measured surface pressure-molecular area (pi-A) isotherms has been evidenced by both X-ray reflectivity and visible o ptical studies. Preservation of pigment functionality has been demonst rated through the process of Coulomb association of the chromophores w ith charged lipid monolayer headgroups at the air/water interface. The potential for applications as model photosynthetic antennae will be d iscussed.