Pseudo-crown functionalized copper salen complexes forming electroactive polymers: Rationalization of Ba2+ interaction using XAS and DFT

A series of electroactive films based on the Cu(salen) motif was produced on Pt electrodes by potentiodynamic polymerization of [Cu(3-MeOsalen)] (1), [Cu(3-MeOsaltMe)] (2), [Cu(3-MeOsalpd)] (3) and [Cu(3-MeOsalophen)] (4). Coulometric assay showed coverages to be in the range Γ ∼ 90–530 nmol cm−2. During polymerization and subsequent redox cycling, the films had distinct i-E signatures, despite the fact that the imine bridge is notionally outside the site of electroactivity (the conjugated polymer spine). A combination of DFT and XAS was used to explore underlying structural reasons for the differing electrochemical signatures and the facility of the films to complex solution phase Ba2+ (a model for metal ion sensing). DFT was able to provide finer details, notably between the N- and O-donors in the Cu local environment, but its application was restricted to monomeric entities. The most significant structural effect of the nature of the imine bridge involvement was the extent of distortion from planarity of the salen moiety. This in turn alters the size of the pseudo-crown pocket formed by the two O-donors shared by the Cu and the two O-donors of the methoxy groups, such that the Ba2+ ion can only lie within the molecular plane in the cases of 1 and 2. XANES shows the Cu to be invariably present in an essentially square planar environment, irrespective of imine bridge, monomer or polymer environment, or Ba2+ complexation. The relevance of these outcomes to design criteria for materials of relevance to metal ion sensing is discussed.