Preparation and characterization of some molecular and glassy semiconductors.

Abstract

This work concerns mainly the characterization of oxonol dyes, cyanine dyes and their complexes, especially with tetrathiafulvalene (TTF). Some work has also been done on titanium (IV) doped iron (III) oxide. Cyclic voltammetry shows that TTF and its derivative dibenzo TTF can be reversibly oxidized to its radical cation and dication in non-aqueous solution. The equilibrium potentials for two one-electron oxidation steps in acetonitrile/tetrabutylammonium perchlorate (TBAP) were measured and found to be E(0,+.) = 0.302V and E(+.,++) = 0.770V for TTF and E(0,+.) = 0.540V and E(+.,++) = 0.900V for dibenzo TTF. Unlike cyanine dyes, oxonols are irreversibly oxidized and equilibrium potentials were measured by extrapolation of potential-sweep rate curves. Also using cyclic voltammetry, the rates of electron-transfer were calculated for TTF and dibenzo TTF as well as an electrochemical reduction series for some charge-transfer compounds relative to hydrogen reduction potential.The complex TTF/oxonol dye was prepared electrochemically at the optimum conditions. Its solubility and sensitivity towards pH changes were measured. Cyclic voltammetry shows that the complex is irreversibly oxidized and at lower voltages compared to its constituents TTF and oxonol dye. Electrical measurements show that the complex is a semiconductor and the energy gap is 0.516 eV. First ionization potentials measured by photoelectron spectroscopy for TTF, dibenzo TTF and TTF-oxonol dye complex give approximately the same value which suggests that, the solution chemistry of these conpounds is different from their gaseous state behaviour. MNDOcalculations were used for geometry optimization, charge-density distribution, heats of formation and ionization potentials. Coupling constants and g-factors were calculated for TTF, its dibenzo derivative and its dye complexes using esr spectroscopy. Charge-transfer couplexes, particularly TTF/TCNQ show some degree of response towards polluting gases such as sulphur dioxide and carbon monoxide as determined by their conductivity changes when the gases were passed over the surface. They may therefore be used as gas sensors if certain modifications in structure or electrode design could be made.Titanium (IV) Iron (III) oxide mixture shows a reasonable conductivity of 1.4 x 10 ft cm but unfortunately a compressed pellet electrode is not mechanically stable. This problem can be solved by sintering the oxides with glass powder which resulted in a high resistance electrode in which some pH/voltage measurements have been done.