Enhanced Aggregation Behavior of Antimony(V) Porphyrins in Polyfluorinated Surfactant/Clay Hybrid Microenvironment

Abstract

Polyfluorinated surfactant (C_nF_2n+1CONH(CH₂)₂N⁺(CH₃)₂C₁₆H₃₃ Br^-; designated as CnF-S, where n = 1−3)/clay hybrid compounds have been shown to provide polyfluorinated microcavities in the clay interlayers with molecular dimensions. The aggregation behavior of a water-soluble porphyrin (tetra(4-sulfonatophenyl)porphyrinatoantimony(V); Sb(V)TSPP) that cointercalates with the surfactant in the polyfluorinated microcavity was investigated. One of the key findings in this study was that the absorption spectra of Sb(V)TSPP molecules that intercalate in the C3F-S polyfluorinated surfactant/clay hybrid microstructures are drastically changed upon dispersion in benzene. The monomer Soret absorption band of Sb(V)TSPP (422 nm) was observed to split into both a longer (438 nm) and a shorter (388 nm) wavelength component. These spectral changes are dependent on the adsorbed amount of Sb(V)TSPP and can be accounted for on the basis of dimer formation. The absorption and emission measurements suggest that two types of dimers (J and H dimers) are formed in the polyfluorinated surfactant/clay hybrid interlayers. It was also found that when the adsorbed amount of surfactant molecules decreased, i.e., when the volume of the polyfluorinated microcavity in the interlayer increased, then the dimerization of Sb(V)TSPP was enhanced. In the case of C2F-S and C1F-S polyfluorinated/clay hybrid compounds, similar spectral behavior arising from dimerization was observed. In contrast, in the case of the hydrocarbon analogues (C3H-S) and cetyltrimethylammonium bromide (CTAB)/clay hybrid compounds, the absorption and emission arising from Sb(V)TSPP indicates that it essentially retains its monomer character. Small-angle X-ray scattering experiments revealed that the clearance space (distance between the layers) for the hybrid compounds in benzene increases compared to that of the solid hybrid compounds. This result indicates a penetration of the benzene molecules into the hybrid layers. It is proposed that the aggregation mechanism of Sb(V)TSPP in polyfluorinated surfactant/clay hybrid compounds obeys the following sequence: (1) formation of a polyfluorinated environment of interlayers that have very weak intermolecular interactions among the surfactant molecules; (2) upon penetration of the benzene solvent molecules, swelling of the hybrid compounds and solvation of Sb(V)TSPP in the interlayers; (3) expulsion of Sb(V)TSPP molecules from the polyfluorinated assemblies into the interlayer space; and (4) migration of Sb(V)TSPP molecules to the microcavities, with concomitant dimer (H and J) formation.