Palladium porphyrin containing zirconium phosphonate Langmuir-Blodgett films

The substituted tetraphenylporphyrins palladium 5,10,15,20-tetrakis(2',3',5',6'-tetrafluorophenyl-4'-octadecyloxyphosphonic acid)porphyrin (16) and palladium 5,10,15-tris(2',6'-dichlorophenyl)-20-(2',3',5',6'-tetrafluorophenyl-4'-octadecyloxyphosphonic acid)porphyrin (17) have been studied as Langmuir monolayers and as zirconium phosphonate Langmuir-Blodgett (LB) films. Using a three-step deposition technique, symmetric and alternating zirconium phosphonate bilayers and multilayers were prepared. In these films, the porphyrin constituent resides in the hydrophobic region of the monolayer and the phosphonate substituents bind zirconium ions in the hydrophilic part. Films of the pure porphyrins and of mixtures with octadecylphosphonic acid (OPA) were prepared. Langmuir monolayers were characterized with pressure vs area isotherms and reflectance UV-vis spectroscopy. LB films were studied with transmittance UV-vis and X-ray diffraction. Control over chromophore interaction was achieved by chemical modification of the amphiphiles and by selection of appropriate transfer conditions. For example, reduced aggregation was seen in LB films of the tetraphosphonic acid substituted porphyrin 16 transferred at mean molecular areas (MMA) larger than the area per molecule of the substituted porphyrin. In these films, the porphyrin macrocycles are nonaggregated and oriented parallel to the surface. In contrast, the monophosphonic acid substituted 17 aggregates under all of the deposition conditions studied. The stability of the porphyrin LB films was examined by exposing the films to refluxing chloroform. UV-vis absorbance after immersion in chloroform confirmed conclusions that in films of 17, many of the chromophores are not tethered to the inorganic network and are easily removed, whereas in films of 16, all molecules bind to the zirconium phosphonate extended network, making these films very resilient.