| Abstract
| - We report the synthesis of monomers, dimers, trimers, and oligomers of triple-decker (TD) complexes bearingS-acetylthio groups at the termini: AcS−(TD)n−SAc. Each TD was of type (Pc)Eu(Pc)Eu(Por), where H2Pc =tetra-tert-butylphthalocyanine and H2Por is a meso-tetraarylporphyrin bearing functional groups at the 4-aryl positionsuch 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 TDarray exhibited high solubility in organic solvents such as CHCl3 or CH2Cl2. Self-assembled monolayers (SAMs) ofall the TDs were prepared on Au substrates and investigated via a variety of electrochemical techniques aimed atdetermining redox potentials, rates of electron transfer under applied potential, and rates of charge retention in theabsence of applied potential. The electrochemical measurements were accompanied by ellipsometric studies aimedat determining SAM thickness and, hence, the orientation of the complexes with respect to the surface plane. Allof the TD SAMs exhibit robust, reversible voltammetry yielding four well-resolved waves in the potential range of0 to +1.6 V (corresponding to the mono-, di-, tri-, and tetracations). The electron-transfer rates for the variousoxidation states of all of the TD SAMS are similar and in the 104−105 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 areinfluenced by both the packing density of the molecules and the orientation of the molecules on the surface. Thefull body of data supports the view that all of the dithio-derivatized TD complexes assume a similar geometry onthe surface. In particular, the complexes are oriented with their linkers/macrocycle planes generally parallel withthe surface, unlike monothio-derivatized analogues, which are in a more perpendicular geometry. The parallelgeometry of the dithio-derivatized TDs is qualitatively consistent with covalent attachment to Au via both thiols.
- Understanding the electron-transfer properties of redox-active molecules on surfaces impacts numerous disciplines. Triple-decker (TD) sandwich coordination complexes exhibit four cationic oxidation states over an accessible potential range (0 to +1.6 V). Dithio-derivatized TD arrays (monomer, dimer, trimer, oligomer) have been immobilized on Au surfaces. The electron-transfer rates for the various oxidation states of all of the TDs are similar (104−105 s-1) while all charge-retention half-lives are in the 10−60 s range.
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