Arrangement of a hydrophobically shielded porphyrin, 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin, in octadecylamine Langmuir-Blodgett multilayers

A hydrophobically shielded porphyrin, 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin (TBP), and 5,10,15,20-tetra(p-tolyl)porphyrin (TTP) were comparatively studied in octadecylamine (ODA)-based monolayers and Langmuir-Blodgett mono- and multilayer films. Regardless of its concentration, TTP was found to undergo severe aggregation resulting in approximately a 100- fold decrease of the fluorescence lifetime relative to homogeneous solutions. In contrast, TBP exhibited far less tendency to aggregate. In the case of diluted films, where the TBP/ODA ratio was 1:99 or lower, two-component emission decay curves were observed with the lifetimes of 9 and 4 ns, which can be attributed to the coexistence of monomeric and dimeric forms of the porphyrin. An increase in TBP concentration led to a gradual decrease of the lifetimes; however, they remained longer than 1.5 ns even at a TBP/ODA ratio as high as 1:3. With TBP/ODA ratios less than 1:4, the isotherms exhibited a phase transition at approximately 20 mN/m. As may be suggested from the molecular areas, TBP molecules can generally be adopted in two forms when embedded in an ODA matrix. One form presumes monomeric molecules oriented perpendicular to the film plane, and the other form comprises face-to-face dimers with porphyrin macrocycles aligned parallel to the film plane.