Malgorzata Zagorska

UV–VIS–NIR and Raman spectroelectrochemistry of regioregular poly(3-octylthiophene): comparison with its non-regioregular analogue

Spectroelectrochemical behaviour of regioregular poly(3-octylthiophene) has been investigated using UV–VIS–NIR and Raman spectroscopies. Static and dynamic UV–VIS–NIR spectroelectrochemical experiments combined with cyclic voltammetry show that oxidative doping of the regioregular polymer is a two-step process in which polarons (radical cations) are first created which then recombine to bipolarons (dications). This two-step oxidative doping mechanism is corroborated by FT Raman spectroelectrochemical studies which show significant changes in the positions and intensities of the Raman bands coinciding with the first and second oxidation peaks in cyclic voltammetry. These changes can be interpreted in terms of the doping induced formation of quinoid sequence of bands in the oxidized polymer. Vibrational calculations carried out for undoped and doped poly(3-octylthiophene) gave a very good agreement between the calculated Raman band frequencies and those recorded experimentally for the regioregular polymer.

Alternate copolymers of head to head coupled dialkylbithiophenes and oligoaniline substituted thiophenes: preparation, electrochemical and spectroelectrochemical properties

New processable, electroactive, alternate copolymers consisting of dialkylbithiophene units and oligoanilinethiophene units have been prepared by post-polymerization functionalization of a specially prepared precursor polymer, namely poly[(4,4″-dioctyl-2,2′∶5′,2″-terthiophene-3′-yl)ethyl acetate], carried out via its hydrolysis and consecutive branching aniline dimer or tetramer through the amidation reaction. The precursor polymer is interesting by itself because it gives a very clear spectroelectrochemical response over a very narrow potential range. The proposed method enables the preparation of regiochemically better defined alkylthiophene-oligoanilinethiophene copolymers with higher content of oligoaniline side groups as compared to previously used methods. Cyclic voltammetry investigations combined with UV–vis–NIR, EPR and Raman spectroelectrochemistry show that both the oligoaniline side groups and poly(thienylene) main chain are electrochemically active. Significant differences for the side group electrochemistry are observed in acidified and nonacidified electrolytes making the prepared new copolymer a good candidate for electrochromic applications in diversified electrolytes.

Effect of molecular weight on electronic, electrochemical and spectroelectrochemical properties of poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene)

Poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene), obtained from its corresponding monomer by oxidative polymerization with FeCl3, has been fractionated into five fractions of reduced polydispersity, covering the Mn range from 1.50 kDa to 10.50 kDa (measured vs. polystyrene standards). The effect of Mn on spectroscopic, electrochemical, spectroelectrochemical and electrical transport properties has been investigated. Fractions of growing Mn show an increasing bathochromic shift of the band originating from the π–π* transition in the neutral polymer with the appearance of a clear vibrational structure for the two highest molecular fractions. The onset of oxidative doping determined from the cyclic voltammogram shifts towards lower potentials with increasing molecular weight. A similar trend is observed for doping induced near infrared bands, which shift towards lower energies (higher wavelengths) with increasing molecular weight and appear at lower potentials in spectroelectrochemical experiments. Finally, a comparison of the FET mobility in two transistors fabricated under identical conditions from polymer fractions differing in their molecular weight shows that a ca. fourfold increase of Mn (from 2.40 kDa to 10.50 kDa) results in a two orders of magnitude increase in the carriers’ mobility (from μsat = 4 × 10−5 cm2 V−1 s−1 to μsat = 2 × 10−3 cm2 V−1 s−1). The obtained results underline the importance of the control of the macromolecular parameters in the preparation of electronic and electrochemical devices from poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene).

Synthesis and spectroscopic characterization of polypyrrole containing ferrous cyanide anions

Abstract Conducting polypyrrole-ferrous cyanide films or powders can be obtained by electrochemical or chemical oxidation of pyrrole in solutions containing ferrous and/or ferric cyanide anions. Mossbauer effect studies show that Fe(CN) 4− 6 is the only iron species detectable in electrochemically prepared polypyrrole films. Ferrous cyanide anion is also the dominant dopant in the samples synthesized chemically. Elemental analysis and IR studies confirm the presence of iron cyanide anions in the samples. The IR studies indicate the existence of defects in polypyrrole chains possibly consisting of C=N groupings in dehydrogenated pyrrole rings. Solid state 13 C NMR spectra of chemically prepared polypyrrole-iron cyanide anions system exhibits a double peak structure: a broad peak centered at 126 ppm and a small peak at 104 ppm. The conductivity of polypyrrole-ferrous cyanides is lower than observed in other conducting polypyrrole-based systems and decreases approximately twice upon 30 day period of exposure to air.

Efficient electroluminescence in regioregular polyalkylthiophene light-emitting diodes

Abstract The regioregular poly(3-alkylthiophenes) (PATs) were used as active materials in polymer-based light-emitting diodes (LEDs). Bright green electroluminescence (EL), peaked at 540 nm, was observed with quantum efficiency of about 0.5% photons/electron. Polymers with different lengths of alkyl side groups were used. This fact however did not change EL yield and peak position. Additionally, optical absorption and photoluminescence spectra are presented. Finally, the presented data are referred to the recently published theoretical results on the photoexcited species in oligothiophenes.

Esters of 5-sulfo-i-phthalic acid as new dopants improving the solution processibility of polyaniline: spectroscopic, structural and transport properties of the doped polymer

Abstract Three esters of 5-sulfo- i -phthalic acid, namely di( n -amyl) (ASIP), di(2-butoxyethyl) (BESIP) and di(2-butoxy-2-ethoxyethyl) (BEESIP) 5-sulfo- i -phthalates, were synthesized and tested as polyaniline doping agents with the goal of improving its solution processibility. Polyaniline protonated with these acidic esters is soluble in chloroform, diethyl ketone, hexafluoro-2-propanol, m -cresol and dichloroacetic acid. Solutions of PANI(BESIP) 0.5 in dichloroacetic acid (DCAA) are especially interesting because their spectra are strongly concentration-dependent, showing the change in polymer conformation upon the dilution. The spectra of diluted solutions exhibit features characteristic of the conformation that favors localized charge carriers, whereas with increasing polymer concentration, features ascribed to delocalized charge carriers begin to dominate the spectrum. Diffractograms of PANI(BESIP) 0.5 and PANI(BEESIP) 0.5 films cast from DCAA, in addition to a broad amorphous halo, show a sharp Bragg reflection at low angles (2 θ =2–4°), which is indicative of self-organized supramolecular structures previously observed for other polyelectrolyte like systems with amphiphilic counterions. The highest conductivity (85 S/cm at RT) was obtained for PANI(BESIP) 0.5 .

Grafting of oligoaniline on CdSe nanocrystals: spectroscopic, electrochemical and spectroelectrochemical properties of the resulting organic/inorganic hybrid

We have synthesised a new organic/inorganic hybrid, in which electroactive aniline tetramer is grafted on the surface of quasi-monodispersed CdSe nanocrystals. The grafting reaction consists of two steps. First initial nanocrystal surface ligands (TOPO) are exchanged for 4-formyldithiobenzoate linker ligands, which contain an anchor function (the dithioate group) and an aniline grafting function (the aldehyde group). The condensation reaction between the primary amine group of aniline tetramer and the aldehyde group of the linker ligand results in the grafting of the former with simultaneous formation of an azomethine linkage between both reagents. The grafting reaction is nearly quantitative as demonstrated by 1H NMR, FTIR and XPS studies. Voltammetric, UV-visible and EPR spectroelectrochemical studies of the hybrid compound show that after the grafting process the polyconjugated molecule retains its electrochemical activity i.e. electrochemical switching between the completely reduced state (leucoemeraldine), the semi-oxidised state (emeraldine) and the completely oxidised state (pernigraniline) is possible.

Electrochemical studies of polypyrrole doped with ferrocyanide anions

Abstract Thin films of polypyrrole-ferrocyanide have been characterized using a rotating ring-disk electrode. It has been shown that the total amount of ferrocyanide is removed from the system in the first reduction-oxidation cycle if an appropriately low potential is applied during polypyrrole polymerization. If the polymerization potential reaches higher values than 0.6 V versus SCE in aqueous solutions, a significant irreversibility of the redox reaction results, leading in turn to the existence of non-removable anions in the polymer film. The linear relationship between the amount of released dopant registered at the ring and the film thickness indicates a homogeneous distribution of the dopant within the polypyrrole matrix.

Mixed alkylthiophene-based heterocyclic polymers containing oxadiazole units via electrochemical polymerisation: spectroscopic, electrochemical and spectroelectrochemical properties

Symmetric alkylthiophene-based mixed heterocyclic trimer and pentamer, containing central oxadiazole units, have been prepared. Because of the electron-withdrawing properties of oxadiazole, the trimer cannot be electropolymerised and undergoes an oxidative-type destruction at high potentials. In contrast, the pentamer readily polymerises, giving a short chain polymer. Both trimer and pentamer exhibit strong photoluminescence with a maximum at 399 nm (13% quantum yield) and 467 nm (46% quantum yield), respectively. The polymer resulting from the electropolymerisation of the pentamer is also luminescent with the maximum of the excitation band at 528 nm (33% quantum yield). The polymer can be oxidatively doped as demonstrated by cyclic voltammetry, showing a clear anodic peak at 0.62 V versus Ag/Ag+ and its cathodic counterpart at 0.56 V, associated with the undoping process. The significantly higher potential of the oxidative doping of the prepared mixed heterocyclic polymer, as compared to the poly(alkylthiophene) homopolymer of similar molecular weight, is caused by the presence of the oxadiazole unit, which lowers the electron density in the π-electron system of the oligothiophene subunit and makes its oxidation more difficult. The spectroelectrochemical investigation of the polymer is consistent with its voltammetric behaviour, exhibiting doping-induced bleaching of the band originating from the π-π* transition and simultaneous growth of the bipolaron bands. The observed clear and reversible spectroelectrochemical behaviour makes the developed polymer a promising candidate for applications in electrochromic devices or electrochemical sensors.

The role of chain and dopant engineering in the preparation of processible conducting polymers with desired properties

We discuss spectroscopic, structural and processing peculiarities of polyaniline (PANI) doped with two types of protonating agents, namely diesters of 4-phthalosulfonic acid (abbreviated as DEPSA) and diesters of sulfosuccinic acid (abbreviated as DESSA). In out approach PANI doped with DEPSA or DESSA is considered as a comb-shaped macromolecular system in which dopant anions are regarded as side groups attached to the main chain via ionic bonds. This type of supramolecular architecture facilitates the self-organization of doped macromolecules into layered or in some cases lamellar structures. Plasticizing ester groups, present in the dopant anions, lower the glass transition temperature, T g , which in all cases is below room temperature. This low T g induces several features which previously were not reported for doped PANI. First, the polymer can be stretched at room temperature up to three times its original length. Second, it exhibits low temperature termochromism which is manifested by an abrupt transformation of the UV-vis-NiR spectrum characteristic of delocalized charge carriers into a spectrum typical of localized charge carriers. An interesting correlation can he found between the temperature of the onset of this low temperature thermochromism, the temperature of the metal to insulator transition determined from the σ = f(T) relationship and the temperature of the freezing of the dopant molecular motion derived from the analysis of the temperature dependence of elastic neutron scattering intensity. The coincidence of these three temperatures underlines a key role of the dopant motion in the transport properties of low T g , plasticized PANI.