Andreas Petr

Chain-growth polymerization of unusual anion-radical monomers based on naphthalene diimide: a new route to well-defined n-type conjugated copolymers.

Strongly electron-deficient (n-type) main-chain π-conjugated polymers are commonly prepared via well-established step-growth polycondensation protocols which enable limited control over polymerization. Here we demonstrate that activated Zn and electron-deficient brominated thiophene-naphthalene diimide oligomers form anion-radical complexes instead of conventional Zn-organic derivatives. These highly unusual zinc complexes undergo Ni-catalyzed chain-growth polymerization leading to n-type conjugated polymers with controlled molecular weight, relatively narrow polydispersities, and specific end-functions.

In situ uv-vis esr spectroelectrochemistry

Simultaneous in situ measurements by both ESR and UV-vis spectroscopy have been carried out at the same electrode for the first time during a single electrochemical experiment in order to spectroscopically characterize the electrochemical reaction products. The experimental technique, including a special TE102 optical ESR cavity and an electrochemical cell for both ESR and UV-vis spectroscopy in transmission, is described. It is shown that the UV-vis absorbance measured in situ, as well as the ESR intensity characteristics of the electrochemical system under study, can be understood in terms of the faradaic current. The reliability of this system is proved by measuring the organic redox couple of methyl-substituted p-phenylenediamine and by comparing calculated and experimental curves.

The redox mechanism of polyaniline studied by simultaneous ESR-UV-vis spectroelectrochemistry

Abstract The electrochemical redox process to charge and discharge polyaniline layers is studied by simultaneous Electron Spin Resonance (ESR) and UV–vis spectroscopic measurements. For this purpose, optically transparent electrodes were applied under potential-controlled conditions. The use of a calibrated manganese ESR standard ensures the calculation of the absolute numbers of free spins in the polymer layer during electrochemical doping. A general procedure is given to analyse the simultaneously recorded electrochemical and spectroscopic data. The procedure proposed following the time dependence of charge, ESR absorption and the absorbance demonstrates that the electrochemical charging/discharging mechanism of polyaniline can be described on the base of three oxidation states. In this way, the separated UV–vis spectra and the time/potential dependence of each redox state can be shown. Only little changes are observed in the shape of the separated spectra using the polaron/bipolaron model in comparison to the polaron/ σ -dimer model. A square ladder scheme of the redox mechanism is discussed, considering neutral, polaronic and bipolaronic states and the protonation equilibria. Furthermore, experimental evidence for the Faradaic nature of the “capacitive” current plateau is presented.

Electrochemical synthesis of electroactive polytriphenylamine

In this paper the electrochemical synthesis of polytriphenylamine is shown for the first time. The electrochemical oxidation of triphenylamine (TPA) in nonpolar solvents leads to the formation of oligomers of TPA in solution and the formation of a polymer film with TPA as repeating unit. Cyclic voltammetry of these electrochemically produced TPA films is compared to that of chemically produced polytriphenylamine what demonstrates a similar electrochemical behaviour. From FTIR spectra of the polymer film it is concluded that the electrochemically formed polymer is more crosslinked than the chemically produced one. Therefore, it is a very stable redox polymer and a promising candidate for application in batteries, electrochromic displays, sensors or organic electronic devices.

Charge transfer and doping at organic/organic interfaces

We studied the electronic properties of technically relevant organic/organic interfaces using photoemission spectroscopy. Representatives of organic semiconductors from the family of the phthalocyanines were evaporated onto PEDOT:PSS [mixture of poly-3,4-ethylenedioxy-thiophene and polystyrenesulfonate] thin films, which are often applied as electrode material in organic semiconductor devices. Besides the formation of interface dipoles we observe energetic shifts of the electronic levels of the semiconductors, which are explained by a doping of the organic semiconductor near the interface due to a mixing of the two organic materials within the interface region.

Radical Dimerization of 5,5′‐Diphenyl‐3,3′,4,4′‐tetramethoxy‐2,2′‐bipyrrole: π Dimer in the Crystal, σ Dimer in Solution.

The crystalline, spinless π dimer (1.+ ⋅PF6 )2 was obtained by the oxidation of bipyrrole 1 with ferrocenium hexafluorophosphate. The diamagnetic species generated electrochemically in solution was identified as the σ dimer by NMR spectroscopy.

In situ EPR/UV–VIS spectroelectrochemistry of polypyrrole redox cycling

The redox behaviour of polypyrrole layers prepared by the electrochemical oxidation of pyrrole in acetonitrile solutions was studied using insitu EPR/UV–VIS spectroelectrochemistry. Various supporting electrolytes: p-toluene sulfonic acid, tetrabutylammonium perchlorate (TBAP) and tetraethylammonium p-toluene sulfonate (TEATos), were used. Optically transparent indium tin oxide (ITO)-coated glass and gold mesh electrodes were used as electrode materials. Using the calibrated manganese EPR standard, the quantitative time dependences of the three oxidation states in the polymer layer were obtained during redox cycling. The separation of the superimposed UV–VIS spectra was carried out by a least-squares method. The different mechanisms of the polymer oxidation, the polaron/bipolaron route and polaron/dimer route, are compared.

Highly ordered phthalocyanine thin films on a technically relevant polymer substrate

We have studied the molecular orientation of well-known representatives of organic semiconductors from the family of the phthalocyanines [copper phthalocyanine (CuPc) and its perfluorinated relative (CuPcF16)] on a conducting polymer thin film using polarization-dependent x-ray absorption spectroscopy. As a polymer substrate PEDOT:PSS [a mixture of poly-3,4-ethylenedioxy-thiophene (PEDOT) and polystyrenesulfonate (PSS), which is often applied as an electrode material in (all-)organic semiconductor devices] was spin coated onto indium-tin-oxide substrates. Even if the interfaces themselves are relatively ill defined (we found recently a mixing of the two organic materials and charge-transfer processes), a very high degree of molecular ordering is observed in the 20–50nm thick phthalocyanine films.

The Nature of the Charge Carriers in Polyazulene as Studied by in Situ Electron Spin Resonance -UV -Visible -Near-Infrared Spectroscopy

In situ spectroelectrochemistry is of high importance for the characterization of doping reactions in pi-conjugated polymers. In this paper we present the results of simultaneous ESR and UV-vis-NIR measurements performed in situ during electrochemical p- and n-doping of polyazulene (PAz). In previous studies on p-doping of PAz the assignment of the optical absorption bands to specific charge carriers have been somewhat controversial, therefore the aim of this study is to clarify the nature of the doping-induced charge carriers and their corresponding optical absorption bands by in situ ESR-UV-vis-NIR spectroelectrochemistry. PAz was polymerized in two different potential ranges in order to obtain films with different structures and morphologies. On the basis of our spectroelectrochemical results we propose that polarons and polaron pairs are formed during p-doping in the two different types of PAz films electrodeposited on ITO. For studying n-doping of PAz, a Pt electrode was used. The ESR signal first decreased in intensity at low doping levels and then increased in intensity at higher doping levels pointing to the formation of new paramagnetic species. At high negative potentials there occurred an additional line broadening of the ESR signal indicating the existence of rather localized negative charge carriers.

Electrochemically doped polymeric anode for improving the performance of molecular organic light-emitting diodes

The doping level of spin-coated poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) films was adjusted quantitatively by an electrochemical doping/dedoping process in toluenesulfonic acid containing solution. The increase in doping level leads to an improvement of hole injection from the PEDT:PSS/ITO (indium–tin oxide) anode to the hole transporting layer, which is attributed to the enhanced work function of PEDT. The performance of vapor-deposited tri(8-hydroxyquinoline) aluminum (Alq3) based organic light-emitting devices with these anodes is shown to depend strongly on the doping level of PEDT:PSS. The device, built on a highly oxidized anode by electrochemical modification (electrochemical equilibrium potential of 0.59 V vs Ag/AgCl), shows a reduction of the operating voltage by 2 V, as well as remarkable enhancement of the luminance compared to the device on a pristine polymer anode.