Behaviour of the water-soluble meso-tetra(4-methylpyridyl)porphine in mixed monolayers and in Langmuir–Blodgett films

Abstract

The water-soluble tetracationic meso-tetra(4-methylpyridyl)porphine (PO2) was incorporated in mixed Langmuir monolayers (LM) with the anionic surfactant sodium hexadecylsulfate (SHS) and stearic acid (SA). The stability of the monolayer and the molecular packing of PO2 vary with the surface pressure and amount of stearic acid in the monolayer. The aggregation or tilting of PO2, occurring at the long transition during the compression run at the air–water interface, is reversible for all the ternary systems studied (PO2/SHS/SA). The organization of PO2 in LM, deduced from the π–A isotherms, is confronted with the spectroscopic data of LB monolayers and multilayers on silica substrates. At surface pressures below the long transition only one layer transfers onto the solid substrate whatever the number of vertical transfer strokes performed, while at high surface pressures LB films with several layers were formed. All absorption spectra of LB films exhibit a Soret maximum at λ₁ ≈ 433 nm, which is ascribed to a nearly flat conformation of the PO2 ring. LB monolayers and multilayers of systems PO2/4SHS/4SA and PO2/4SHS/8SA, formed at high π(50 mN m⁻¹), show an additional absorption band centred at λ₂ ≈ 405 nm, which may account for a nonplanar conformation of PO2 with the pyridinium rings out of the porphine plane. The results suggest that the planar form of PO2 is imposed in the first layer by direct contact in a parallel orientation to the solid substrate, while the nonplanar form may coexist or even prevail in upper layers of LB films formed at high π.