Structural Study of Intermolecular Interactions in Pure and Diluted Films of a Redox-Active Phthalocyanine
Abstract
This paper reports on a structural study of pure and diluted homolayers of an amphiphilic vanadyl porphyrazine (VOS18), which were built up by the Langmuir-Blodgett technique. Electron spin resonance measurements on pure homolayers show that the macrocycles lie flat on the substrate. Even for low dilution ratios, Langmuir films of VOS18 diluted by stearic acid exhibit a very good cohesion (up to 50 mN m−1), which is greater than the collapse pressure of both pure films. Pressure vs. area curves recorded for progressively diluted films indicate that the stearic acid climbs onto the macrocycle, up to four or five equivalents. This phenomenon explains the good cohesion of the mixed film. It is related to the formation of a carboxylate salt, as monitored by IR spectroscopy, where the stearate ion replaces the bromide ion in the vicinity of the vanadyl porphyrazine. This chemical reaction in the monolayer phase goes with an increased organization of the fatty chains, as shown by IR linear dichroism.