Further insights into hydrophobic interactions between ferrocenyl-tamoxifen drugs and non-polar molecular architectures at electrode surfaces

The hydrophobic interactions between ferrocenyl compounds, based on the tamoxifen skeleton (ferrocifens), and non-polar molecular architectures, such as cyclodextrin (CD) cavities and unsubstituted n-alkane chains, were investigated in water and alcohol/water mixtures by cyclic voltammetry through three different approaches. In the first one, the organometallic complex was covalently attached to a glassy carbon electrode surface whereas the CD was dissolved in homogeneous solution. The second one consisted in preparing a self-assembled monolayer of a mixture of synthesized per-6-thio-beta-CD and of pentanethiol chains at a gold electrode surface, whereas the ferrocifen complex was initially present in solution. In the third approach, the gold electrode surface was modified only with alkane thiol monolayers having various lengths. As expected, the complexation between the ferrocene moiety of the tamoxifen derivative and the CD cavity became stronger as the solvent polarity increased. Interestingly, two different electrochemical behaviors could be evidenced for the neutral starting complex and for the electrogenerated cationic ferrocenium derivative owing to their different polarities. Importantly, the reduction process of the cationic complex followed a distinct behavior on adsorbed CDs and on alkane chains. Notably, the presence of two aromatic rings in the ferrocenyl structure proved to favor its facile penetration through short alkyl chains (CnH2n+1, n <= 5). For much longer chains (16 <= n < 5) a faradaic oxidation current was still observed for ferrocifen though progressively diminished. (C) 2009 Elsevier B.V. All rights reserved.