Publications

Temperature and Molecular Weight Dependent Hierarchical Equilibrium Structures in Semiconducting Poly(3-hexylthiophene)

We report on structural investigations of a series of regioregular poly(3-hexylthiophene) with well-defined molecular weight (5−19 kg/mol) using DSC, small angle and wide-angle X-ray scattering, and AFM. With increasing temperature, we identify three ordered phases, namely 3D crystalline, 2D crystalline with disordered side chains, and a layered phase of smectic symmetry, followed by complete melting. Although all samples crystallize in extended chain conformation, the lower molecular weight material exhibits a lower crystallinity, most likely caused by noncrystallizable end groups. The crystallinity increases strongly with increasing molecular weight, which could be a possible explanation for the known dependence of charge transport properties on molecular weight.

Synthesis and Characterization of Monocarboxylated Poly(3-hexylthiophene)s via Quantitative End-Group Functionalization

We report the quantitative conversion of bromine end groups in regioregular poly(3-hexylthiophene)s (P3HTs) and the characterization of the resulting monocarboxylated P3HTs (P3HT-COOHs) carrying one carboxylic acid group at their chain ends. The monocarboxylation for three different chain lengths is carried out, and the resulting P3HT-COOHs are characterized with size exclusion chromatography, matrix-assisted laser desorption ionization spectroscopy with time-of-flight detection mass spectroscopy, and UV−vis spectroscopy. The thermal properties and crystallinity in bulk and thin films were studied in a comparison between P3HT and P3HT-COOHs. Differential scanning calorimetry and wide-angle X-ray scattering support the increasing crystallinity for the higher molecular weight samples. Preliminary OFET measurements show a good charge carrier mobility in the range of 10−3 cm2/(V s) for P3HT-COOHs with molecular weights of 5000 and 10 800 g/mol.

Undoped and doped poly(tetraphenylbenzidine) as sensitive material for an impedimetric nitrogen dioxide gas dosimeter

This article presents a nitrogen dioxide (NO2) detecting gas dosimeter based on poly(tetraphenylbenzidine) poly(TPD) as nitrogen oxide (NOx) sensitive layer. Gas dosimeters are suitable devices to determine reliably low levels of analytes over a long period of time. During NOx exposure, the analyte molecules are accumulated irreversibly in the sensing layer of the dosimeter enhancing the conductivity of the hole conducting poly(TPD), which can be measured by impedance spectroscopy. Due to their possibility for low cost production by simple printing techniques and very good physical, photochemical, and electrochemical properties, poly(TPD)s are suitable for application in gas dosimeters operated at room temperature. We studied the effect of doping with a Co(III)-complex in combination with a conducting salt on the dosimeter behavior. Compared to the undoped material, a strong influence of the doping can be observed: the conductivity of the sensing material increases significantly, the noise of the signal decreases and an unwanted recovery of the sensor signal can be prevented, leading to a NOx detection limit <10 ppm.

Excited state dynamics and conformations of a Cu(<scp>ii</scp>)-phthalocyanine-perylenebisimide dyad

We investigate the excited state dynamics and the conformations of a new molecular donor-bridge-acceptor system, a Cu(ii)-phthalocyanine (CuPc) covalently linked via a flexible aliphatic spacer to a perylenebisimide (PBI). We performed time-resolved polarization anisotropy and pump-probe measurements in combination with molecular dynamics simulations. Our data suggest the existence of three conformations of the dyad: two more extended, metastable conformations with centre-of-mass distances >1 nm between the PBI and CuPc units of the dyad, and a highly stable folded structure, in which the PBI and CuPc units are stacked on top of each other with a centre-of-mass distance of 0.4 nm. In the extended conformations the dyad shows emission predominantly from the PBI unit with a very weak contribution from the CuPc unit. In contrast, for the folded conformation the PBI emission of the dyad is strongly quenched due to fast energy transfer from the PBI to the CuPc unit (3 ps) and subsequent intersystem-crossing (300 fs) from the first excited singlet state of CuPc unit into its triplet state. Finally, the CuPc triplet state is deactivated non-radiatively with a time constant of 25 ns.

Semi‐Interpenetrating Network Electrolytes Utilizing Ester‐Functionalized Low <i>T</i>_g Polysiloxanes in Lithium‐Metal Batteries

Abstract Solid polymer electrolytes (SPE) obtained from polyesters are viable alternatives to polyethylene oxide‐based materials, especially for room‐temperature applications. In SPEs, the ion conduction is dependent on the polymer segmental mobility and is thus facilitated by low glass transition temperature ( T g ). Here, the study synthesizes an ester‐funtionalized polysiloxane‐based polymer electrolyte with an exceptionally low T g of −76 °C, resulting in a high ionic conductivity of 2.6 × 10 −5 S cm −1 at room temperature and a lithium transference number of 0.72. However, the low T g and consequently low mechanical stability require reinforcement to promote the formation of stable lithium‐electrolyte interfaces in lithium plating stripping experiments and stable battery cycling in lithium‐metal batteries (LMBs). For this, the SPE is incorporated into a network structure to yield a semi‐interpenetrating network electrolyte (SPE20‐SIPN) which results in significantly improved storage modulus by three orders of magnitude and ionic conductivity is maintained upon crosslinking. The SPE20‐SIPN exhibits stable cycling for up to 50 cycles with fluctuation (voltage noise) in some of the cells. A combination of crosslinking and nanoparticle addition (SPE20‐N10‐SIPN) overcomes the voltage noise and results in high coulombic efficiencies and high capacity retention above 80% for 200 cycles in solvent‐free, all‐solid‐state LMBs at 30 °C.

Efficient hybrid polymer/titania solar cells sensitized with carboxylated polymer dye

Swallow-Tail Substituted Liquid Crystalline Perylene Bisimides: Synthesis and Thermotropic Properties

Tailor-made synthesis and structure-property relationship of several swallow-tail N-substituted perylene bisimide (PBI) dyes are presented. PBI derivatives were synthesized by two distinct synthetic approaches, the details being evaluated herein. All the PBIs carry either alkyl swallow-tail or oligoethylenglycolether (OEG) swallow-tail moieties as N-substituents, and many of them are unsymmetrically substituted. We avoided substitution at bay positions of the perylene core to maintain the planarity and strong pi-pi interactions, which favor intermolecular order and charge carrier transport. The thermotropic behavior, which is strongly influenced by the nature of the substituents was investigated using differential scanning calorimetry (DSC), polarization optical microscopy (POM), and X-ray diffraction measurements (XRD). The introduction of OEG swallow-tail units facilitates thermotropic liquid crystalline behavior in most cases and the unsymmetrical substitution allowed the tuning of the mesophase-width. The mesophases exhibit characteristic columnar hexagonal (Col(h)) packing arising from pi-pi interactions between cofacially orientated perylene molecules. Thus, the inherent tendency of PBI molecules for crystallization could be effectively suppressed by incorporating flexible OEG swallow-tail units only at imide positions. This molecular design was crucial to obtain liquid crystallinity and intracolumnar long-range order. The substituents did not influence the electronic energy levels such as HOMO and LUMO.

Unravelling the conformations of di-(perylene bisimide acrylate) by combining time-resolved fluorescence-anisotropy experiments and molecular modelling

We compare the results from time-resolved fluorescence anisotropy experiments and molecular modelling on perylene bisimide acrylate dimers which allows us to connect the observed spectral signatures unambiguously with the non-stacked and two (parallel and anti-parallel) stacked conformations. For the parallel stacked conformation the experimental data can be reproduced quantitatively using a model that assumes structural relaxation in the electronically excited state of the stacked aggregate. For the non-stacked conformation we find quantitative agreement between experiment and modelling only if a fast hopping of the electronic excitation between the perylene bisimide subunits is taken into account.

Influence of Composition of Amphiphilic Double-Crystalline P3HT-<i>b</i>-PEG Block Copolymers on Structure Formation in Aqueous Solution

A series of poly(3-hexylthiophene)-block-poly(ethylene glycol) (P3HT-b-PEG) with constant P3HT block length block and variable PEG block lengths are presented. Alkyne-functionalized P3HT with high absolute molecular weight of 11.4 kg/mol is combined with azide-functionalized PEGs via copper-catalyzed alkyne–azide cycloaddition (CuAAC). The resulting P3HT-b-PEG block copolymers have PEG weight fractions between 15 and 64 wt %. In bulk materials the crystallinity in the conjugated block is similar to pure P3HT, while the crystallinity of PEG is influenced with decreasing PEG block length. On addition of MeOH as a nonsolvent for the P3HT block, these block copolymers are able to form stable micellar aggregates if the PEG fraction is >31 wt %. In AFM, DLS, and cryo-TEM, P3HT-b-PEG micelles are found to have a spherical or cylindrical shape with diameters of around 25 nm and lengths between 40 nm and some hundred nanometers. It is found that short PEG blocks lead to bigger block copolymer micelles. Thus, a correlation of composition on solution structure and its consequences on the crystallization of both blocks is given.

Comparative Study of the Mechanical Reinforcement by Blending, Filling, and Block Copolymerization in Bottlebrush Polymer Electrolytes

Poly(ethylene glycol) (PEG)-based bottlebrush polymer electrolytes exhibit improved room-temperature ionic conductivity and reduced crystallinity compared to those of semicrystalline poly(ethylene oxide) (PEO). However, these graft copolymers suffer from low mechanical stability. Therefore, we synthesized a PEG-based bottlebrush polymer having a polynorbornene backbone using ring-opening metathesis polymerization, and it was mechanically reinforced using three strategies: (a) by blending with a polynorbornene (PNb) homopolymer, (b) filling with TiO2 nanoparticles, or (c) via block copolymerization with a PNb segment. All three systems were converted to solid polymer electrolytes by adding LiTFSI, and their thermal, mechanical, and detailed electrochemical properties in symmetrical Li/SPE/Li cells over a large number of cycles are given. All solid-state lithium metal battery (Li/SPE/LFP) cells were fabricated, and charge/discharge cycles as well as the cycling behavior were comparatively studied. It was found that block copolymerization resulted in the highest storage modulus above 0.1 Hz and overall ionic conductivity (in the whole range of 25 to 80 °C) compared to those of the other two strategies. Furthermore, the highest accessible discharge capacities (159 mA h g–1) and highest capacity retention of 88% after 50 cycles were also achieved with the block copolymer concept.

Synthesis and Characterization of Bifunctional Polymers Carrying Tris(bipyridyl)ruthenium(II) and Triphenylamine Units

The synthesis, characterization, and properties of a highly soluble bifunctional polymer are described in which a tris(bipyridyl)Ru(II) unit acts as dye and triphenylamine units act as charge transport moieties. First a macroligand, a bipyridine carrying two poly(4-bromostyrene) chains, was synthesized by atom transfer radical polymerization (ATRP) of 4-bromostyrene in bulk using CuCl/PMDETA as the catalytic system and bis(chloromethyl) bipyridine as the initiator. The target polymer was then obtained via a polymer amination reaction in which the bromophenyl group was converted into a triphenylamine followed by metallation of the bipyridine unit of the macroligand with Ru(II) bis(bipyridine). The reaction conditions of ATRP and polymer amination reaction were optimized, and the degree of conversion for both steps was determined by gas chromatography (GC) analysis of rest monomer content and elemental analysis of unreacted bromine, respectively. The control in molecular weight was achieved maintaining a narrow distribution in the desired low molecular weight range of bulk polymerization of 4-bromostyrene. The polymer amination reaction using the Pd(OAc)2 and P(t-Bu)3 system was found to be very efficient, and the reaction was complete within 2 h. The metallation reaction could be followed by UV/vis spectroscopy. MALDI-TOF MS of the three polymers was carried out to obtain absolute molecular weights and their distribution. A comparison of these molecular weights gave additional information about the degree of polymer amination and metallation reaction. The thermal properties of the different polymers suggest that the thermal stability as well as the glass transition temperature increases from the starting macroligand which carries poly(4-bromostyrene) chains to the intermediate polymer having poly(vinyltriphenylamine) chains and finally to the bifunctional Ru(II) polymer complex.

Solid-state dye-sensitized solar cells fabricated with nanoporous TiO2 and TPD dyes: Analysis of penetration behavior and I–V characteristics

Spectral tuning of light emitting diodes with phenyl-thiophenes

Determination of the Crystallinity of Semicrystalline Poly(3-hexylthiophene) by Means of Wide-Angle X-ray Scattering

Temperature-dependent small-angle and wide-angle X-ray scattering (SAXS/WAXS) measurements on a series of chemically well-defined and highly regioregular poly(3-hexylthiophenes) were analyzed to determine absolute values of the crystallinities. The analysis is based on the evaluation of the scattered intensity from the amorphous regions providing an easy and fast method for the determination of the crystallinity in the class of side chain substituted polymers. The resulting values are in the range of 68–80% at room temperature depending on the molecular weight. Based on these values, an extrapolated reference melting enthalpy of a 100% crystalline material was determined (ΔHm∞ = 33 ± 3 J/g) for use in DSC measurements. For higher molecular weights a decrease of the crystallinity was observed which can be explained by the onset of chain folding as deduced from the analysis of the SAXS patterns. An in-depth analysis based on Ruland’s method showed that the crystalline regions of P3HT exhibit a large amount of internal disorder.

Electroluminescent Behavior of a Homologous Series of Phenylenevinylene Oligomers

Oligomeric LED systems have received little attention compared to that devoted to polymeric LED systems. This paper focuses on the electroluminescent behavior of a homologous series of six phenylenevinylene oligomers—their molecular structure is illustrated in the Figure—in LEDs in combination with a comparative study of related cyclic voltammetry data.

Synthesis, mesomorphism and electrochemical properties of tetrasubstituted zinc and copper phthalocyanines

We describe the synthesis of highly soluble, tetrasubstituted zinc and copper phthalocyanines exhibiting liquid crystalline properties. These compounds were characterized using elemental analysis, MS, 1H-NMR, FTIR and UV-Vis absorption spectroscopy. The liquid crystalline properties of phthalocyanines (Pcs) were studied by polarizing optical microscopy and differential scanning calorimetry. All of the four compounds exhibit thermotropic columnar mesophases and two of them show clear hexagonal ordering. Additionally, these Pcs display lyotropic behaviour. The clearing temperatures lie above 300 °C and on cooling down the mesophase can be “frozen” into a glassy state. The cyclic voltammetry studies of these phthalocyanines show electrochemical stability and reversible redox behaviour. The Pcs without any azo groups in the ligands exhibit both reversible oxidation and reduction, whereas those with azo groups show only reversible reduction steps. The low HOMO energy values of about 5 eV as calculated from oxidation potential make these compounds interesting as p-type organic semiconductors for electro-optical applications.

Synthesis and properties of novel hole transport materials for electroluminescent devices

Abstract We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5‐tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4‐ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between −4.97 and −5.16 eV. By using a polymeric TPD as hole transport layer and tris(8‐quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cd/m 2 were obtained under ambient conditions.

High-Performance Organic Electrochemical Transistors Based on Conjugated Polyelectrolyte Copolymers

A new generation of polythiophene-based polyelectrolytes is reported to address fundamental issues in organic electrochemical transistors (OECTs). In such devices, the semiconductor must be able to transport and store ions and possess simultaneously a very high electronic mobility. For this, the ion-conducting 6-(thiophen-3-yl) hexane-1-sulfonate tetramethylammonium monomer (THS–TMA+) is copolymerized with the hole-conducting 3-hexylthiophene (3HT) to obtain copolymers, PTHS–TMA+-co-P3HT 1–3 with a gradient architecture. The copolymers having up to 50 mol % 3HT content are easily oxidizable and are crystalline. Consequently, for the copolymers, a higher stability in water is achieved, thus reducing the amount of cross-linker needed to stabilize the film. Furthermore, OECTs using copolymers with 75 and 50 mol % of PTHS–TMA+ content exhibit 2–3 orders of magnitude higher ON/OFF ratio and an extremely lower threshold voltage (−0.15 V) compared to PTHS–TMA+. Additionally, high volumetric capacitance (C* > 100 F/cm3) is achieved, indicating that the ion transport is not hampered by the hydrophobic 3HT up to 50 mol %, for which a very high OECT hole mobility of 0.017 cm2/(V s) is also achieved. Thus, the concept of copolymerization to combine both ionic and electronic charge transport in an organic mixed conductor offers an elegant approach to obtain high-performance OECT materials.