Synthesis and characterization of bis(S-acetylthio)-derivatized europium triple-decker monomers and oligomers.

Abstract

We report the synthesis of monomers, dimers, trimers, and oligomers of triple-decker (TD) complexes bearing S-acetylthio groups at the termini: AcS-(TD)(n)()-SAc. Each TD was of type (Pc)Eu(Pc)Eu(Por), where H(2)Pc = tetra-tert-butylphthalocyanine and H(2)Por is a meso-tetraarylporphyrin bearing functional groups at the 4-aryl position such as ethynyl, TMS-ethynyl, TIPS-ethynyl, or iodo. The TD arrays were prepared by Sonogashira- and Glaser-type coupling reactions, affording 1,4-diphenylethyne or 1,4-diphenylbutadiyne linkers joining the TDs. Each TD array exhibited high solubility in organic solvents such as CHCl(3) or CH(2)Cl(2). Self-assembled monolayers (SAMs) of all the TDs were prepared on Au substrates and investigated via a variety of electrochemical techniques aimed at determining redox potentials, rates of electron transfer under applied potential, and rates of charge retention in the absence of applied potential. The electrochemical measurements were accompanied by ellipsometric studies aimed at determining SAM thickness and, hence, the orientation of the complexes with respect to the surface plane. All of the TD SAMs exhibit robust, reversible voltammetry yielding four well-resolved waves in the potential range of 0 to +1.6 V (corresponding to the mono-, di-, tri-, and tetracations). The electron-transfer rates for the various oxidation states of all of the TD SAMS are similar and in the 10(4)-10(5) s(-)(1) range. The charge-dissipation rates (measured in terms of a charge-retention half-life) are also similar and are in the 10-60 s range. These rates are influenced by both the packing density of the molecules and the orientation of the molecules on the surface. The full body of data supports the view that all of the dithio-derivatized TD complexes assume a similar geometry on the surface. In particular, the complexes are oriented with their linkers/macrocycle planes generally parallel with the surface, unlike monothio-derivatized analogues, which are in a more perpendicular geometry. The parallel geometry of the dithio-derivatized TDs is qualitatively consistent with covalent attachment to Au via both thiols.