P. Bäuerle

Bioelectrocatalysed reduction of nitrate utilizing polythiophene bipyridinium enzyme electrodes

Abstract Enzyme electrodes in several configurations including the enzyme nitrate reductase are applied in the bioelectrocatalysed reduction of nitrate to nitrite. One enzyme electrode configuration includes the physical immobilization of nitrate reductase in a polythiophene-bipyridinium matrix formed by electropolymerization of 1-methyl-1′-(3-thiophene-3-yl)-pentyl-4,4′-bipyridinium-bis-hexafluorophosphate. The bipyridinium units of the polymer act as electron mediators between the electrode and the enzyme. The concentration of bioelectrogenerated nitrite depends on the concentration of nitrate in the electrolyte solution. A calibration curve representing the rate of nitrite formation as a function of nitrate concentration in the electrolysis cell allows quantitative analysis of nitrate in aqueous solutions. Two other enzyme-electrode configurations include copolymerization of thiophene-bipyridinium and thiophene-3-acetic acid, onto a gold electrode and layered deposition of two polymers composed of thiophene-bipyridinium and thiophene-3-acetic acid onto the electrode. Covalent linkage of the enzyme nitrate reductase to the carboxylic acid residues of the different polymer matrices results in active enzyme electrodes for bioelectrocatalysed reduction of nitrate to nitrite.

Synthesis and structural characterization of alkyl oligothiophenes—the first isomerically pure dialkylsexithiophene

Regioselective bromination of alkylated bi-and ter-thiophenes affords key building blocks for the synthesis of higher, isomerically pure alkyl oligothiophenes. For all new compounds, unequivocal structural assignment is based on a detailed analysis of the fully coupled 13C, 1H NMR spectra.

Orientation and mobility in ultrathin oligothiophene films: UV-Vis, IR and fluorescence studies

Abstract Thin films of oligothiophenes αn Ts are prepared by vapour deposition on fused silica and crystalline sapphire substrates, and characterized with respect to molecular orientation by polarized fluorescence, UV-Vis and IR absorption spectroscopy. Isolated αn Ts are adsorbed in the submonolayer region with their long molecular axes parallel to the surface plane. In the monolayer region, the αn Ts organize with their long axes perpendicular to the surface plane forming aggregates of the pin cushion type with a mean tilt angle of θ = 15 ± 3° relative to the normal of the macroscopic surface. Terminal alkylation of αn T or multilayer coverage results in a loss of orientation with respect to the surface. Heating the substrate during film deposition either reduces or enhances the molecular order depending on temperature and molecular chain length.

UV/Vis and IR spectroscopic studies on molecular orientation in ultrathin films of polythiophene model compounds

Abstract The characteristic differences shown by optical spectra of oligothiophenes (OTs) in films on silica surfaces and in solution are explained by the Molecular Exciton Model. Thus unsubstituted OTs in monolayers are arranged in a side-by-side manner, whereas substitution with terminal alkyl groups destroys this alignment, thereby reducing the π-overlap of adjacent molecules. With the orientation of the transition moments within the molecular framework known from stretched sheet experiments polarized UV/Vis and IR spectra reveal that the long molecular axis of the unsubstituted OTs in monolayers on silica is directed perpendicularly to the surface, while alkyl substituted oligomers do not show any anisotropy. This influence of the alkyl groups and the reduced order in multilayers of unsubstituted OTs both suggest a decisive role of the interaction between surface and π-electron-system for the molecular arrangement in thin films.

Design, synthesis and assembly of new thiophene-based molecular functional units with controlled properties

Abstract Individual aspects in the field of ‘molecular electronics’ are addressed using new materials which are based on conjugated thiophenes as key building blocks. Among then, functionalized polythiophenes represent prototype compounds in which the three-dimensional network of ‘molecular wires’ can interact with the specific functional groups. Corresponding structurally defined oligothiophenes serve as model compounds and allow the correlation of various properties with structure. In addition, novel properties and superior processibility of these oligomers lead to interesting applications. Light emitting diodes and metal/organic/metal nanostructures are constructed by evaporation of thin oligomer films. A clear dependence of electroluminescence and electrical transport properties on chain length is found. Finally, the supramolecular assembly of functionalized (oligo)thiophenes is achieved by oriented adsorption on single crystal surfaces and self-organization in highly ordered LB films.

Study of charge carrier injection and luminescence processes in oligothiophene-based light-emitting diodes

Abstract Light-emitting diodes (LEDs) with end-capped oligothiophenes (EC n T, n =5–7) as the active material were fabricated by vacuum sublimation. The metal/EC n T/ITO LEDs were studied for EC n T layers of different thicknesses (30–200 nm) and for different metals (Mg, Al, In) as top electrodes. Electroluminescence (EL) and photoluminescence (PL) spectra, current-voltage ( IV ) curves and quantum efficiencies were measured for a wide range of temperatures (20–300 K). The IV curves cannot be described by a standard tunnelling or thermionic injection mechanism for charge carriers at Schottky barriers alone. Instead, we have evidence that field- and temperature-dependent mobilities in the organic bulk material are important here. From EL spectra of LEDs with a double-layer film we derive that light emission arises from a region close to the cathode (less than 30 nm).

Third generation of conducting polymers: Spectroelectrochemical investigations on viologen functionalized poly (3‐alkylthiophenes)

Results of in situ electron spin resonance, optical absorption, and Fourier transform infrared spectroscopic studies during electrochemical doping and undoping processes of viologen functionalized poly (3‐alkylthiophenes) are reported. Effects of the functional group on the electronic properties of the polythiophene backbone in its conducting state are observed resulting in localization of the charge carriers. Radicals on the viologen side group show hyperfine structured electron spin resonance signals indicating a rapid motional averaging of anisotropic electron‐nuclear couplings. Experimental results are discussed by comparison with other substituted polythiophenes in terms of conformational effects of the acceptor group on the conducting polymer backbone. A structural model with counterion complexation between polythiophene backbone and the viologen functional unit is presented.

New functionalized polythiophenes

Abstract The development of a series of new functionalized polythiophenes prepared by electropolymerization is described. The corresponding monomers were synthesized starting with ω-haloalkyl substituted mono-, bi- and terthiophenes which were subsequently further reacted to yield viologens and ferrocenes. The characterization of both monomers and polymers revealed a dependence of the properties on the type of heterocyclic unit involved in the monomer, on the steric and electronic influence of the functional group and on the ratio of functional groups per monomeric units.

Pulsed-EPR on the photoexcited triplet state of C60 in fluid solution: electron transfer from end-capped quaterthiophene and C60-radical anion formation

Abstract The photoexcited triplet state of C 60 in fluid toluene solution was investigated by pulsed-EPR spectroscopy in a temperature range T = 200–300 K. The time evolution of the triplet EPR signal is found to be determined by the spin-lattice relaxation time T 1 and the triplet decay. In the presence of the electron donor end-capped quaterthiophene the C 60 -triplet decay is quenched by electron transfer reaction. The temperature dependence of the triplet quenching rate k q suggests the formation of an exciplex intermediate. Moreover we observe the formation of the C 60 -radical anion following electron transfer. The kinetics and polarization of the C 60 - EPR signal are discussed.

Temperature dependent spectroelectrochemical measurements on end-capped oligothiophenes

Abstract ‘End-capped’ oligothiophenes have been synthesized as new model compounds for polythiophene. Due to the blocking of the reactive α- and β-positions of both terminal thiopene units with a cycloalkane ‘cap’ more precise characterization of the oligomers in various oxidation states becomes possible. Spectroelectrochemical investigations were carried out on a homologues series of ‘end-capped’ oligothiophenes in a thin-layer optical device providing the possibility of temperature variation during measurements. The optical absorption spectra of the electrochemically prepared oxidation states show a clear dependence on chain length and an unusual temperature dependence. The results will be discussed partially in terms of molecular polarons and bipolarons in polythiophene and in terms of π-dimerization.