Redox, water oxidation, acid - base chemistry and electrochemical syntheses based on tetrathiafulvalene
thesisposted on 08.02.2017 by Ahmed, Shaimaa Mohamed Adil
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The redox chemistry of tetrathiafulvalene (TTF) and its cation radical and dication have been studied systematically in acetonitrile (0.1M Bu4NPF6) in the presence of water and light. In acetonitrile (100%) containing supporting electrolyte, neutral TTF undergoes two electrochemically and chemically reversible one-electron oxidation steps. The reversible formal potentials associated with The TTF0/+•/2+ are -74 and 311 mV vs. Fc0/+ (Fc = ferrocene). TTF reacts with water to form HTTF+ and TTF+• reacts with water in the presence of light to give TTF, HTTF+ and O2, implying that TTF+• is photoreduced to TTF and water is oxidized to oxygen and protons. Experiments with a Clark electrode confirm the evolution of oxygen. TTF2+ is not stable under ambient conditions and spontaneously reacts with water to generate TTF+• and HTTF+. Voltammetric measurements reveal the details of the redox chemistry for TTF, TTF+•, TTF2+ and HTTF+ in the presence of water and light. The acid-base chemistry of TTF, TTF+• and TTF2+ in acetonitrile in the presence of trifluoroacetic acid (TFA) and ethereal HBF4 also have been examined. On addition of acid, TTF reacts to form HTTF+, while TTF+• and TTF2+ do not react with acid. The underlying mechanism for the redox and the acid-base chemistry of TTF and its cations has been proposed and supported using digital simulations. A large body of thermodynamic and kinetic information has been revealed from the comparison of the experimental and simulated data. Electrochemical synthesis of two new materials based on TTF with MO42- or WO42- in aqueous and mixed acetonitrile/H2O (90:10 v/v) solutions have been undertaken. In aqueous media, (TTF)2MoO4 and (TTF)2WO4 have been electrochemically synthesized via the oxidation of TTF solid modified electrode surface and placed in contact with aqueous solutions of MoO42- or WO42-, respectively. TTF can also be electrochemically oxidized in acetonitrile/H2O (90:10 v/v) in the presence of 1mM Na2MoO4 or Na2WO4 to form (TTF)2MoO4 and (TTF)2WO4., respectively The electrocrystallization of these compounds has been monitored using cyclic voltammetry, surface plasmon resonance and electrochemical quartz crystal microbalance. Furthermore, a wide range of microscopic and spectroscopic techniques have been employed to characterize these newly electro-generated compounds. Two new dimers, TTF2(NO3) and TTF2(NO3)2 have been electrochemically synthesized in aqueous media via oxidation of TTF-modified electrode surfaces, placed in contact with a aqueous nitrate solution. The solid-solid state transformation to these materials has been explored by cyclic voltammetry and chronoamperometry. The solid-solid state transformation involves oxidation of TTF to TTF+•accompanied by the ingress of aqueous nitrate anions from the bulk solution to the TTF+•. This reaction is governed by a nucleation and growth mechanism and is independent of the electrode material and the identity of the counter cation. However it is strongly dependent on nitrate concentration and the scan rate of voltammetry. Microscopic (SEM) and spectroscopic techniques, such as, UV-vis, FT-IR and Raman have been used to characterize the electrocrystallized TTF-NO3 based materials. Large amplitude Fourier transformed ac (FTAC) voltammetry technique was used to study the very fast electron transfer for TTF, its radical cation TTF+• and that of dication TTF2+ using a relatively high frequency (233 Hz). Difficulties associated with the use of higher frequencies at large area electrode surface is also discussed in detail. Comparison of the experimental data with simulations was used to extract the heterogeneous charge transfer rate constant for each redox process (i.e. TTF0/+•, TTF+•/2+) and in both the anodic and cathodic scan directions. Finally, the synthesis and characterization of a new compound containing TTF with phosphorylated sugar has been undertaken. The electrochemical conversion of solid TTF into (TTF)2G6P using a TTF-modified electrode in contact with aqueous Na2G6P electrolyte is reported. In particular, crystalline TTF was immobilized onto a glassy carbon surface and then electrochemically oxidized to the TTF+• in the presence of aqueous Na2G6P as the electrolyte to form the (TTF)2G6P product. Cyclic voltammetry and chronoamperometry were used to probe the mechanism of the solid–solid state phase interconversion from TTF into (TTF)2G6P. Microscopic (SEM) and spectroscopic techniques, such as, UV-vis, FT-IR and Raman have been utilized to characterize the electrocrystallized (TTF)2G6P bio-materials.