Wojciech Domagala

In situ EPR spectroelectrochemical studies of paramagnetic centres in poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-butylenedioxythiophene) (PBuDOT) films

In situ EPR spectroelectrochemical studies of two members of poly(3,4-alkylenedioxythiophene) family of polymers, namely poly(3,4-ethylenedioxythiophene) – PEDOT and poly(3,4-butylenedioxythiophene) – PBuDOT have been performed with an aim to investigate the effect of doping and subsequent dedoping on charge carriers in these polymers. The objectives of investigations were to characterise the type of paramagnetic centres appearing in the polymer and to observe the influence of doping on their concentration and other spectroscopic properties. It was found that the evolution of spins in both polymers upon doping follows a trend similar to that observed for other thiophene-based polymers. The characteristic points of these curves are different however for each polymer. The maximum spin concentration in PEDOT takes place at 0.2 V and for PBuDOT it is at 0.35 V. While generally similar in behaviour during doping, the behaviour of these two polymers upon dedoping is markedly different. For PBuDOT the concentration of spins follows a more or less reverse path of the doping curve whereas in PEDOT a hysteresis of concentration of spins appears. Similar trends are observed for the signals’ ΔBpp widths. Both polymers display distinct narrow EPR lines at the end of the reduction half-cycle suggesting that a noteworthy concentration of spins exist in them even in the fully reduced (dedoped) state. Analysis of the EPR spectra of the polymers in their doped states revealed their complex character. It was found that these complex spectra could be decomposed into two separate components – one Gaussian and one Lorentzian. Based on EPR microwave saturation studies the Gaussian component was attributed to highly mobile polarons which are confined in isolated spin packets and the Lorentzian one to delocalised less mobile polarons, homogeneously distributed within the material, which thus could be associated with the electrons of conductivity.

ESR spectroelectrochemistry of poly(3,4-ethylenedioxythiophene) (PEDOT)

Abstract In situ ESR spectroelectrochemical studies of poly(3,4-ethylenedioxythiophene) (PEDOT) have been performed, in an attempt to take a closer look at species responsible for the conductivity of the polymer in the doped state. A series of ESR spectra at progressively changed potentials applied to the polymer film in the oxidation and subsequently, reduction half-cycles were recorded. The results reveal distinct ESR lines with a noteworthy concentration of spins in the reduced state of the polymer and marked changes in both the intensities and Δ B pp widths of the ESR signal across the studied potential range, indicating non-trivial changes in the character of charge carriers with changing potential. Also, interesting phenomena like the potential hysteresis of the spin concentration and of Δ B pp linewidths between the oxidation and reduction cycles of the polymer are observed. The presence of residual spins in the polymer in the reduced state may indicate that at least to some partial extent, PEDOT chains exist in the quinoid rather than benzoid configuration in the dedoped state. Hysteresis of spectroscopic parameters may imply that certain hindrance factors like slow anion expulsion speed accompany the dedoping process of the polymer.

Synthesis of new, highly luminescent bis(2,2’-bithiophen-5-yl) substituted 1,3,4-oxadiazole, 1,3,4-thiadiazole and 1,2,4-triazole

Summary A new synthetic approach towards the preparation of functionalised, soluble, donor–acceptor (DA) alkylbithiophene derivatives of oxadiazole, thiadiazole and triazole is reported. Taking advantage of the Fiesselmann reaction, reactive bithiophene synthons having alkyl or alkoxy substituents at designated positions are prepared. Following a synthetic strategy, featuring the bottom-up approach, sequential structural elements are built, starting from a simple thiophene compound, until the target molecule is obtained, all in good yield. Supplementing the well established methods of oxadiazole and thiadiazole synthesis, efficient ring closure reaction affording a 4H-1,2,4-triazole unit is presented. All target ambipolar compounds display strong photoluminescence with measured quantum yields up to 0.59. Modification of the demonstrated synthetic routes may be exploited for the preparation of longer, specifically functionalised oligothiophenes, coupled to other heteroaromatic cores.

Electrochemical studies of selected regioregular oligooctylthiophenes in solution and in thin film solid state

Abstract Electrochemical studies of a series of regioregular 3-octylthiophene oligomers with 5–8 thiophene rings, chlorine end-capped at one α terminal position, have been conducted in solution and in thin film state. Results indicate gradual coalescing of the oxidation potentials of neutral and radical cation oligomer molecule, indicative of coming close to sufficient length of the molecule to allow for two separate radical cations to exist along its chain. With increasing number of thiophene rings in the molecule, further quasi-reversible oxidation reactions upon which trication and tetracation were formed have been observed at ever decreasing potentials. It was also observed that the oxidation of radical cations comes about more easily than that of electron-paired cations, the latter process being strongly dependent on the length of oligothiophene chain. Thin film electrochemistry revealed that the processes taking place during redox reactions differ from those in solution. Sharp oxidation peaks were observed which were attributed to conformational changes taking place in the oligomer molecules upon transition to the conductive form, alongside the films oxidative doping process. To shed some light on the issue of charge carriers appearing in the film upon its oxidation, supplemental ESR spectroelectrochemical measurements were carried out and confirmed that, radical cations are generated in the film.

The role of structural and electronic factors in shaping the ambipolar properties of donor–acceptor polymers of thiophene and benzothiadiazole

The influence of different thiophene donor units on electrochemical and spectroscopic properties of benzothiadiazole based donor–acceptor π-conjugated organic materials is studied. Two different structure modification vectors of the donor units are being considered – one addressing the intermolecular interactions through off-conjugation side chain architecture, and the other focusing on intramolecular interactions tuned by in-conjugation substituents. Electrochemical and simultaneous in situ EPR-UV-Vis-NIR spectroelectrochemical studies of the oxidative (p-) and reductive (n-) doping processes, which are responsible for the optoelectronic properties of these materials, revealed their disparate course and dissimilar effects of redox reactions of the conjugated π-bond. While p-doping prevalent species were found to comprise intensively interacting spin bearing and spinless charge carriers, the n-doping state was found to involve only one type of negatively charged carrier, with spin carrying species being selectively generated at due cathodic potentials. No spin pairing of these negative polarons was observed with their increasing population behaving like a collection of localised charge carriers. Qualitative and quantitative comparisons between the p- and n-doping carrier populations provided independent support for the spin pairing phenomena of positive charge carriers. Steric effects of varying alkyl side chain substitution have demonstrated predominant impact on the electrochemical properties of investigated polymers, and, thereto related, stability of n-doped state, while mesomeric effects of different 3,4-ethylenechalcogenide thiophene functionalities have been found to shape the energy level related spectral properties of these polymers, with particular reference to p-doping induced charged states. These findings provide new insights into the factors requiring attention during structure tailoring of donor–acceptor assemblies for organic optoelectronic applications.

Fused H-shaped tetrathiafulvalene–oligothiophenes as charge transport materials for OFETs and OPVs

A series of hybrid tetrathiafulvalene–oligothiophene compounds has been synthesised, where the tetrathiafulvalene unit is fused at each side to an end-capped oligothiophene chain of varying length (terthiophene, quinquithiophene and septithiophene). Each hybrid structure (1–3) has been studied by cyclic voltammetry and triple EPR-UV-Vis-NIR spectroelectrochemistry in the case of the quinquithiophene compound (2). Comparison is made with the corresponding half-units, which lack the fulvalene core and contain just one oligothiophene chain. The highest hole mobility of quinquithiophene–TTF 2 was obtained from field effect transistors (8.61 × 10−3 cm2 V−1 s−1); its surface morphology was characterised by tapping mode atomic force microscopy and a power conversion of 2.5% was achieved from a bulk heterojunction organic solar cell device using PC71BM as the acceptor.

Systematic elongation of thienyl linkers and their effect on optical and electrochemical properties in carbazole–BODIPY donor–acceptor systems

Synthesis, spectral and electrochemical properties of a series of new panchromatic BODIPY donor–acceptor–donor derivatives, comprising carbazole conjugated with systematically elongated framework by thiophene – based linkers were investigated. It has been found that elongation of the π-system with one thiophene unit shifts the spectra toward near infrared, while incorporation of next thiophene derivatives has only limited influence on the lowest energy transition, causing no further shift of absorption maxima. Elongation of π-conjugation of the donor arm tunes the electron properties, influencing the ionization potential (IP) and electron affinity (EA) values of the molecules. The experimental results are supported by quantum chemical computations revealing that electron density of SOMO orbital is low at the 3-positions of terminal carbazole units in BODIPY push–pull oligomers while elongating their backbone, thus preventing them to undergo electrochemical polymerization giving reversibly p- and n-dopable products. Presented study tests the influence and limitations of incorporation of electron – rich substituents for designing panchromatic BODIPY systems.

Spectroelectrochemistry of poly(3-hexylthiophenes) in solution

The first comprehensive spectroelectrochemical account of the behaviour of regioregular (RR-P3HT) and statistical (ST-P3HT) poly(3-hexylthiophenes) in solution is presented, in contrast to the many reports dealing with P3HT films merely deposited from solution. The conducted experiments revealed that the two types of P3HTs behave in sharply different ways upon the application of electrochemical stimuli: ST-P3HT readily precipitates at mildly oxidative potentials, while the precipitation of the RR-P3HT takes place to a much lesser extent, even at higher potentials. The two polymers, studied via UV–Vis–NIR–EPR spectroelectrochemistry, exhibited properties mostly in line with earlier reports. Further study revealed that RR-P3HT largely remains in solution, even in its doped form, whereas only traces of the doped ST-P3HT are observed in solution in identical conditions. The high concentration of the doped RR-P3HT in solution can be explained by its ability to form soluble polymer agglomerates, in which the positive charge of the p-doped chains is stabilised by and delocalised over neighbouring, interacting undoped chains. These conclusions are consistent with SEM micrographs, which show that after cycling the potential of the electrode in a solution of ST-P3HT, a uniform layer is formed, covering most of the surface of the electrode, whereas in the case of RR-P3HT surface coverage is marginal and formed layer has the appearance of veined blotches.Graphical abstract

The influence of oxygen conditioning effect on the permeation properties of polyaniline membranes

ABSTRACT The effect of oxygen conditioning on the performance of polyaniline (PANI) membranes is presented and discussed. The presence of the conditioning effect in such kind of polymer matrix was confirmed by measuring a variation in spin concentration using electron paramagnetic resonance (EPR) spectroscopy. The observed decrease in spin concentration indicates the presence of the dissolved oxygen incompletely removed from PANI matrix. After the prolonged contact between oxygen and PANI membrane, a pronounced decrease in membrane’s selectivity was observed, which may be explained by the firm and permanent interaction between oxygen molecules and nitrogen paramagnetic centers of PANI and could be considered as a kind of PANI irreversible doping process.

Electrochemistry and In Situ EPR Spectroelectrochemistry of Poly(3,4-ethylenedithiothiophene)

Polymer films of poly(3,4-ethylenedithiothiophene) obtained by electrochemical oxidative polymerisation of the corresponding monomer have been investigated. p-Doping process of the polymer was characterized by in situ EPR spectroelectrochemisty. Compared to its close relative – PEDOT, the polymer features two distinct voltammetric peaks. The results of our studies indicate that both represent faradaic processes and that each manifests generation of charge carriers in the polymer.