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    Organization of Cationic Porphyrins in Mixed Langmuir-Blodgett Films. An Absorption and Steady-State Fluorescence Study

    Year: 2002

    Journal: Langmuir 2002, 18, 5772-5781, 20111221

    Authors: Maria Isabel Viseu, Amelia M. Gonzalves da Silva, Patri­cia Antunes, and Silvia M. B. Costa

    Organizations: Centro de Quica Estrutural, Complexo I, Instituto Superior Tenico, Avenida Rovisco Pais, 049-001 Lisboa, Portugal

    Two cationic porphyrins, the amphiphilic meso-tetra(4-N-stearylpyridyl)porphine (PO1) and the hydrophilic meso-tetra(4-N-methylpyridyl)porphine (TMPyP or PO2), were incorporated in ionic diluent matrixes containing sodium hexadecyl sulfate (SHS) and dioctadecyldimethylammonium bromide(DODAB) at the air-water interface and in Langmuir-Blodgett films. All systems containingPO1show a preferential interaction of SHS with DODAB, instead of SHS with PO1. The cationic system PO1/SHS/DODAB at a 1:4:4 molar ratio behaves as the separatePO1plusSHS+DODABmonolayers and forms H-type porphyrin aggregates at high π. The neutral system PO1/SHS/DODAB at a 1:8:4 molar ratio separates into PO1 + 4SHS plus SHS + DODAB monolayers, forming a PO1 + 4SHS bilayer at high π in which the porphyrin macrocycles can adopt "planar" and "nonplanar" conformations. The quenching efficiency of the PO1 fluorescence was found to be generally higher in the cationic, rather than neutral, monolayers and also to increase with the porphyrin surface concentration and number of deposited layers. Systems containing PO2 are generally not very stable at the air-water interface. The system PO2/4SHS forms H-type PO2 aggregates (or H-dimers) at low π and dissolves in water at high π. The system PO2/4SHS/4DODAB separates into the soluble PO2 plus the neutral SHS + DODAB monolayer. Only the neutral system PO2/8SHS/4DODABis quite stable at the air-water interface because of favorable (cooperative) electrostatic and hydrophobic interactions.