New Crown Annelated Tetrathiafulvalenes:  Synthesis, Electrochemistry, Self-Assembly of Thiol Derivatives, and Metal Cation Recognition

The synthesis of new S2O4-crown annelated tetrathiafulvalene (TTF) derivatives substituted with one terminal thiol group is described. Self-assembled monolayers (SAMs) of these compounds have been assembled on gold and platinum surfaces, the latter substrate giving improved quality films. SAMs of TTF derivative 16b are the most stable of those we have studied. Electrochemical data for SAMs of 5a, 5b, 8, 16a, and 16b in acetonitrile reveal two reversible one-electron waves, typical of the TTF system; the current increased linearly with scan rate, indicating a surface wave response. SAMs of 8, 16a, and 16b exhibited an electrochemical response in aqueous electrolytes, which was observed between 50 and 100 cycles. Moreover, if the potential scanned was limited to the first TTF oxidation, the cyclic voltammetry response was observed for at least 1000 cycles. Metal complexation by the crown ionophore of the SAMs in acetonitrile has been monitored by a positive shift in the first oxidation potential of the TTF unit (maximum ΔE 1 1/2 = 80 mV for Ag+). We also report the X-ray crystal structure of TTF−crown derivative 21 bearing two hydroxymethyl substituents, synthesized during the course of this work. The structure is characterized by infinite chains of molecules linked by strong intrachain hydrogen bonds between the terminal hydroxy groups.