Molecular engineering: highly ordered Langmuir-Blodgett films based on a cobalt phthalocyanine and its axial complexation
Résumé
An amphiphilic cobaltous phthalocyanine, cobalt tetraoctadecyltetrapyridino[ 3,4-b:3'"'-g3'',4''-Z3'",4"'-q] porphyrazinium bromide (CoS$_{I8}$Br$_4$) has been synthesized. This amphiphilic macrocycle can be organized in the solid state by the Langmuir-Blodgett (LB) method, giving rise to highly ordered LB multilayers. The conjugated macrocycles lie flat on the substrate with a distribution width less than 10°. This peculiar geometry, rather unusual for phthalocyanines, is confirmed by different spectroscopic studies, in particular by anisotropic ESR measurements. This powerful technique indicates also that the cobaltous ion is axially complexed by a water molecule. Diluting the initial spreading solution of the amphiphilic macrocycle by a fatty acid gives access to an homogeneous mixed monolayer, where four fatty carboxylates have climbed onto each macrocycle. The formation of this supermolecule arises from an ion exchange within the mixed monolayer. In the same way, diluting the cobaltous phthalocyanine with an amphiphilic Lewis base such as octadecylimidazole or tricosanylamine allows in situ formation of the pentavalent complexes by axial interaction between the nitrogen doublet of the Lewis bases and the d$_z$² orbital of the cobaltous ion. So, the two chemically active sites of the initial amphiphilic macrocycle can be pertinently used to build up new molecular assemblies