Publications

Room temperature liquid crystalline perylene diester benzimidazoles with extended absorption

The synthesis, characterization and thermotropic properties of novel asymmetrically substituted discotic molecules, perylene diester benzimidazoles (PDBIs), are presented. PDBIs were designed with an imidazole unit at 3,4 positions and a bisester moiety at 9,10 positions of the perylene tetracarboxylic acid core. By attaching linear or branched aliphatic substituents at the ester moiety and two alkyl or alkoxy substituents at the benzimidazole unit, sufficient solubility and the flexibility to obtain mesophases was guaranteed. Thermotropic behaviour, which is strongly influenced by the nature of the respective substituents at the diester and benzimidazole moiety, was investigated using differential scanning calorimetry (DSC), polarization optical microscopy (POM) and X-ray diffraction measurements (XRD). All PDBIs under investigation self-organize into liquid crystalline columnar hexagonal phases (Colh), among them PDBI-3 even at room temperature. Also the formation of a room temperature columnar plastic phase (Colhp) and the formation of a lamellar phase was observed. Due to extension of the π-conjugation system, the absorption of these well soluble discogens is significantly extended to longer wavelengths in the visible regime up to 680 nm.

Fluorescent Dye‐Labeled Polymers Carrying Triphenylamine, Styrene, or Acrylate Pendant Groups

Abstract Summary: An alkoxyamine initiator carrying a perylene bisimide unit suitable for the nitroxide mediated controlled radical polymerization of different monomers was synthesized. The synthesis, characterization, and properties of a series of polymers obtained from monomers such as 4‐vinyltriphenylamine, styrene, and different acrylates using this initiator are described. The controlled nature of the polymerization was demonstrated by time‐dependent measurements of the conversion and the molecular weight. The incorporation of the single fluorescent unit to the polymer chain end was verified by MALDI‐TOF MS. The perylene bisimide acts as an electron acceptor with a strong fluorescence. Since 4‐vinyltriphenylamine is a donor monomer, the resulting polymers exhibit photoluminescence quenching due to electron transfer between the donor polymer chain and the acceptor moiety. The perylene bisimide moiety shows aggregation via π‐π stacking which was studied using UV‐vis and fluorescence spectroscopy. By controlling the polymer chain length, the stacking of the perylene bisimide can be controlled. The LUMO and HOMO levels of the perylene bisimide initiator and the dye‐labeled polymer were determined by cyclic voltammetry as −3.7 and −6.0 eV, respectively. With this approach tailor‐made fluorescent dye‐labeled polymers with desired architecture, low polydispersity, and controlled molecular weight can be obtained as model systems for electron and energy transfer studies. Schematic representation of electron transfer in dye‐labeled poly(vinyltriphenylamine). magnified image Schematic representation of electron transfer in dye‐labeled poly(vinyltriphenylamine).

Modular Synthesis and Structure Analysis of P3HT-<i>b</i>-PPBI Donor–Acceptor Diblock Copolymers

While donor–acceptor block copolymers (BCPs) are widely discussed to be optimal model materials for studying the morphology-dependent processes in organic photovoltaics, the observation of the desired microphase-separated structures is rare. We present a novel modular approach for the synthesis of donor–acceptor block copolymers offering a fast and flexible way for chemical modifications leading to microphase separation accompanied by confined crystallization of the individual blocks. Two donor–acceptor BCPs of poly(3-hexylthiophene)-b-polyperylene bisimide (P3HT-b-PPBI 1 and 2) were synthesized incorporating P3HT as a donor and a polystyrene with two different pendant perylene bisimides (PBI-N3 1 and 2) as acceptor blocks to show the flexibility of the synthetic approach. The synthesis combines Kumada catalyst transfer polymerization (KCTP), controlled radical polymerization, and click chemistry to obtain highly comparable polymers via a modular approach. We synthesized poly(3-hexylthiophene) (P3HT) with a high molecular weight (Mn,SEC = 18300 g mol–1), in a controlled manner, introduced a chain transfer end group by click chemistry to form a macroinitiator, and subsequently polymerized propargyloxystyrene by sequential polymerization. In a postpolymerization click reaction, the polystyrene block was quantitatively grafted with two different PBI acceptor units. We obtained diblock copolymers with 70 wt % of the PBI block and 30 wt % P3HT. The BCPs were characterized in bulk by temperature-dependent small- and wide-angle X-ray scattering (SAXS/WAXS) in combination with differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The same materials were also characterized in thin films by grazing incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). The attachment of the PBI units to the PS backbone was found to slow down the molecular dynamics, evidenced by an increase of the glass transition temperature (Tg). Despite the high Tg, the BCP morphology is dominated by microphase separation in the molten state. While the BCPs adopt a cylindrical morphology, the range of order is limited by the slow molecular dynamics. Subsequent crystallization of the individual blocks during cooling results in confined crystallization within the cylindrical, microphase-separated morphology. The results emphasize the importance of molecular dynamics for the formation of a well-ordered microphase-separated BCP morphology.

Photosensitizers in Solar Energy Conversion

Long-term switching of single photochromic triads based on dithienylcyclopentene and fluorophores at cryogenic temperatures

Photochromic molecules can be reversibly converted between two bistable forms by light. These systems have been intensively studied for applications as molecular memories, sensing devices, or super-resolution optical microscopy. Here, we study the long-term switching behavior of single photochromic triads under oxygen-free conditions at 10 K. The triads consist of a photochromic unit that is covalently linked to two strong fluorophores that were employed for monitoring the light-induced conversions of the switch via changes in the fluorescence intensity from the fluorophores. As dyes we use either perylene bisimide or boron-dipyrromethen, and as photochromic switch we use dithienylcyclopentene (DCP). Both types of triads showed high fatigue resistance allowing for up to 6000 switching cycles of a single triad corresponding to time durations in the order of 80 min without deterioration. Long-term analysis of the switching cycles reveals that the probability that an intensity change in the emission from the dyes can be assigned to an externally stimulated conversion of the DCP (rather than to stochastic blinking of the dye molecules) amounts to 0.7 ± 0.1 for both types of triads. This number is far too low for optical data storage using single triads and implications concerning the miniaturization of optical memories based on such systems will be discussed. Yet, together with the high fatigue resistance, this number is encouraging for applications in super-resolution optical microscopy on frozen biological samples.

Interface modifications in solid state dye-sensitized TiO 2 solar cells

The concept of solid-state dye-sensitized TiO₂ solar cell in which the hole transport medium is an organic semiconductor is critically studied by examining the anode-TiO₂ interface and dye-hole conductor interface. The importance and the role of a compact hole-blocking TiO₂ layer in between the anode and the mesoporous layer is extensively studied by preparing this layer by spray pyrolysis using an automated procedure which guarantees reproducibility in obtaining constant thickness and quality of this crucial layer as seen in the current-voltage characteristics of the solar cells. To characterize the rectifying behavior of the blocking layer, cells with the structure, fluorinated tin oxide (FTO)/blocking TiO₂ layer/hole conductor/Au, were prepared and their current (I)-voltage (U) properties were investigated. Solid state solar cells were also prepared with different blocking layer thicknesses and their photovoltaic properties were investigated in order to study the influence of the blocking layer thickness on solar cell performance. In order to improve the dye-hole conductor interface, novel multifunctional molecules carrying dye units and triphenylamine moieties were synthesized and their influence as interface modifiers were studied. This interface modification results in doubling the external quantum efficiency of current conversion via improved charge transfer at the dye-hole conductor interface. Moreover, the recombination processes at this interface is drastically suppressed which leads to higher open circuit voltage and consequently higher power conversion efficiencies.

Environmental exposure enhances the internalization of microplastic particles into cells

Microplastic particles ubiquitously found in the environment are ingested by a huge variety of organisms. Subsequently, microplastic particles can translocate from the gastrointestinal tract into the tissues likely by cellular internalization. The reason for cellular internalization is unknown, since this has only been shown for specifically surface-functionalized particles. We show that environmentally exposed microplastic particles were internalized significantly more often than pristine microplastic particles into macrophages. We identified biomolecules forming an eco-corona on the surface of microplastic particles, suggesting that environmental exposure promotes the cellular internalization of microplastics. Our findings further indicate that cellular internalization is a key route by which microplastic particles translocate into tissues, where they may cause toxicological effects that have implications for the environment and human health.

A Combinatorial Study of the Dependence of Organic LED Characteristics on Layer Thickness

Optimization of the device structure of organic light-emitting diodes (OLEDs) is required for improved reliability and performance. An apparatus is described that enables a matrix of OLEDs to be fabricated in a single experiment, i.e., under comparable conditions, on a single substrate by combinatorial techniques. It is demonstrated that the influence of layer thickness variation of one or more layers on the device performance can be easily studied—as illustrated on the cover of this issue. The technique is also suitable for the fast screening and optimization of materials.

Elucidating the Effect of Interfacial Interactions on Crystal Orientations in Thin Films of Polythiophenes

Molecular orientation is crucial for improving the efficiency of organic electronic devices. In many ordered materials, orientation is achieved by directional crystallization on a substrate but usually not in semicrystalline conjugated polymers. Specifically, a full face-on molecular alignment of poly(3-hexylthiophene) (P3HT) on a substrate has not been realized so far. Here, it is found that P3HT crystallized on graphene exhibits a double-layered edge-on and face-on crystal orientation with edge-on crystals formed on the top surface. This finding is interpreted as a result of two competing interfacial interactions of P3HT chains with graphene and vacuum. By modifying the side-chain chemical composition in poly[3-(6-bromohexyl)]thiophene, the influence of the interface to vacuum can be reduced, resulting in full face-on orientation in films with a thickness of up to 26 nm. These results demonstrate that directed crystallization can be used to control the orientation of semicrystalline functional polymers in thin films given that the interaction with both interfaces is taken into account.

Emitting Species of Poly(3-hexylthiophene): From Single, Isolated Chains to Bulk

The photophysical properties of films of alkyl-substituted polythiophenes are governed by a subtle interplay between intra- and interchain electronic couplings. The intramolecular properties, however, are still not entirely clear because polythiophenes possess a strong tendency to form π-stacked aggregate structures with appreciable interchain couplings. Here we employ low-temperature single-molecule photoluminescence spectroscopy on isolated regioregular poly(3-hexylthiophene), P3HT, chains with different, but well-defined molecular weights to reveal the intrachain properties of their emitting sites. We find that the inhomogeneous distribution function of the zero-phonon lines (ZPL) is very narrow (<480 cm–1, 60 meV), which indicates a low degree of torsional disorder of the P3HT backbone on length scales of the emitting sites (despite a large mean dihedral angle). Moreover, the single-chain ZPLs are exclusively located in the high energy tail of the corresponding spectrum of a disordered ensemble. Using concentration-dependent measurements in combination with time-dependent density functional theory, we show that this spectral shift stems from aggregation-induced partial planarization and concomitant electronic coupling between segments of neighboring P3HT chains.

Dynamic Distortions of Quasi-2D Ruddlesden–Popper Perovskites at Elevated Temperatures: Influence on Thermal and Electronic Properties

Ruddlesden-Popper hybrid halide perovskites are quasi-two-dimensional materials with a layered structure and structural dynamics that are determined by the interplay between the organic and inorganic layers. While their optical properties are governed by confinement effects, the atomistic origin of thermal and electronic properties of these materials is yet to be fully established. Here we combine computational and experimental techniques to study A₂PbI₄ (A = butylammonium (BA), phenethylammonium (PEA)) Ruddlesden-Popper perovskites and compare them with the quintessential perovskite CH₃NH₃PbI₃. We use first-principles density functional theory, molecular dynamics simulations based on machine-learned interatomic potentials, thermal measurements, temperature-dependent Raman spectroscopy, and ultraviolet photoelectron spectroscopy to probe the thermal and electronic properties of these materials at elevated temperatures. Our molecular dynamics simulations demonstrate that dynamic fluctuations in the organic sublattice determine the bulk-average distortions of these materials at room temperature, explaining significant differences in their electronic density of states close to the Fermi level. Furthermore, by analyzing the organic layer dynamics in BA₂PbI₄ we provide a mechanistic explanation for the phase transition of this material at 274 K and observations from Raman measurements. Our results highlight the role of the organic interlayer for the electronic and thermal transport properties of Ruddlesden-Popper perovskites, paving the way for the design of new hybrid materials for tailored applications.

Bodipy dyes as a potential sentisizer for dye sensitized solar cells

Reference EPFL-CONF-191254View record in Web of Science Record created on 2013-12-13, modified on 2017-05-12

Highly Efficient Doping of Conjugated Polymers Using Multielectron Acceptor Salts

Chemical doping is a vital tool for tuning electronic properties of conjugated polymers. Most single electron acceptors used for p-doping necessitate high dopant concentrations to achieve good electrical conductivity. However, high-molar doping ratios hamper doping efficiency. Here a new concept of using multielectron acceptor (MEA) salts as dopants for conjugated polymers is presented. Two novel MEA salts are synthesized and their doping efficiency towards two polymers differing in their dielectric properties are compared with two single electron acceptors such as NOPF₆ and magic blue. Cutting-edge methods such as ultraviolet photoelectron spectroscopy/X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, and density of states analysis in addition to UV-vis-NIR absorption, spectroelectrochemistry, and Raman spectroscopy methods are used to characterize the doped systems. The tetracation salt improves the conductivity by two orders of magnitude and quadruples the charge carrier concentration compared to single electron acceptors for the same molar ratio. The differences in charge carrier density and activation energy on doping are delineated. Further, a strong dependency of the carrier release on the polymer polarity is observed. High carrier densities at reduced dopant loadings and improved doping efficacies using MEA dopants offer a highly efficient doping strategy for conjugated polymers.

Nanostructures of n-Type Organic Semiconductor in a p-Type Matrix via Self-Assembly of Block Copolymers

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTNanostructures of n-Type Organic Semiconductor in a p-Type Matrix via Self-Assembly of Block CopolymersStefan M. Lindner and Mukundan ThelakkatView Author Information Makromolekulare Chemie I, Universität Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany Cite this: Macromolecules 2004, 37, 24, 8832–8835Publication Date (Web):October 30, 2004Publication History Received7 September 2004Revised18 October 2004Published online30 October 2004Published inissue 1 November 2004https://pubs.acs.org/doi/10.1021/ma0481656https://doi.org/10.1021/ma0481656rapid-communicationACS PublicationsCopyright © 2004 American Chemical SocietyRequest reuse permissionsArticle Views2697Altmetric-Citations145LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Aromatic compounds,Copolymers,Dyes and pigments,Hydrocarbons,Polymers Get e-Alerts

Liquid‐Crystalline Perylene Diester Polymers with Tunable Charge‐Carrier Mobility

Abstract New classes of liquid‐crystalline semiconductor polymers based on perylene diester benzimidazole and perylene diester imide mesogens are reported. Two highly soluble side‐chain polymers, poly(perylene diester benzimidazole acrylate) (PPDB) and poly(perylene diester imide acrylate) (PPDI) are synthesized by nitroxide‐mediated radical polymerization (NMRP). PPDB shows n‐type semiconductor performance with electron mobilities of 3.2 × 10 −4 cm 2 V −1 s −1 obtained in a diode configuration by fitting the space‐charge‐limited currents (SCLC) according to the Mott–Gurney equation. Interestingly, PPDI performs preferentially as a p‐type material with a hole mobility of 1.5 × 10 −4 cm 2 V −1 s −1 , which is attributed to the less electron‐deficient perylene core of PPDI compared to PPDB. Optical properties are investigated by UV‐vis and fluorescence spectroscopy. The extended π‐conjugation system due to the benzimidazole unit of PPDB leads to a considerably broader absorption in the visible region compared to PPDI. HOMO and LUMO levels of the polymers are also determined by cyclic voltammetry; the resulting energy band‐gaps are 1.86 eV for PPDB and 2.16 eV for PPDI. Thermal behavior and liquid crystallinity are studied by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction measurements. The results indicate liquid‐crystalline order of the polymers over a broad temperature range. These thermal, electrical, and optical properties make the perylene side‐chain polymers attractive materials for organic photovoltaics.

Influence of ω-Bromo Substitution on Structure and Optoelectronic Properties of Homopolymers and Gradient Copolymers of 3-Hexylthiophene

The nature of side chains and substituents influence the resulting electronic and structural properties of conjugated polymers. Here, we report fundamental studies on the influence of a ω-bromohexyl side chain in polythiophene (resulting in P3BrHT) as well as in a series of gradient copolymers using 3-hexylthiophene (3HT) comonomer. The rate of polymerization of 3BrHT is slower compared to that for 3HT. Temperature dependent absorption and emission show that, with increasing content of 3BrHT, aggregation, critical transition temperature, and the fraction of aggregates decrease. Wide angle X-ray scattering shows that P3BrHT has a triclinic crystal lattice; the crystallinity is almost half of that of P3HT and an increase in the crystallinity and a gradual monotonic decrease of the a and b crystal lattice parameters with decreasing 3BrHT content in copolymers is observed. Finally, the consequences of change in aggregation and crystallinity on charge transport are elucidated using OFET, giving a general explanation for the influence of a ω-bromo substituent in polythiophene copolymers.

&lt;title&gt;Aromatic ethers with 1,3,5-triazine units as hole-blocking/electron-transport materials in LEDs&lt;/title&gt;

Various fluoro-functionalized aromatic 1,3,5-triazine monomers were prepared. A series low molar mass and poly- (1,3,5-triazine)-ethers were synthesized by a condensation reaction. The polymers as well as the low molar mass compounds have excellent thermal stability and are amorphous. In order to examine the potential to apply these compounds in organic electroluminescent devices, the redox properties were studied by cyclic voltammetry. It was found that the monomers have high electron affinity and reach LUMO values in the range of -2.7 to -3.1 eV. Additionally high oxidation stability with HOMO values lower than -6.4 eV follows hole blocking capabilities. This opens the possibility to utilize 1,3,5-triazine containing materials as electron injection/hole blocking layer in LEDs. First LED results are in agreement to these high electron affinities.

<scp>EDOT</scp>‐diketopyrrolopyrrole copolymers for high bulk hole mobility and near infrared absorption

ABSTRACT To obtain novel low‐bandgap materials with tailored hole‐transport properties and extended absorption, electron rich 3,4‐ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4‐ c ]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X‐ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space‐charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene‐flanked copolymer PDPP[T] 2 ‐EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10 −4 cm 2 V −1 s −1 as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well‐known hole transporting polymer poly(3‐hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54 , 639–648