A New Solution to an Old Problem: Synthesis of Unsubstituted Poly(<i>para</i>-phenylene)

Unsubstituted and structurally well-defined poly­(<i>para</i>-phenylene) (PPP) has been long-desired as an organic semiconductor prototype of conjugated polymers. To date, several attempts to synthesize unsubstituted, pristine, high-molecular-weight PPP have failed. Here we solved this synthetic problem by a versatile precursor route. Suzuki polymerization of kinked disubstituted 1,4-dimethoxycyclohexadienylene monomers yielded a well-soluble, nonaromatic precursor polymer. Its solubility allowed processing by spin-coating into nanometer-thick films. Subsequent additive-free thermal treatment induced aromatization and led to exclusively para-connected, highly fluorescent PPP with a length of about 75 phenylene units

Tuning Packing and Solubility of Donor (D)–Acceptor (A) Polymers by <i>cis</i>–<i>trans</i> Isomerization within Alkenyl Side Chains

The impact of alkenyl substituents on the behavior of cyclopentadithiophene–benzothiadiazole (CDT–BTZ) donor (D)–acceptor (A) polymers in organic field-effect transistors (OFETs) and on the supramolecular organization was investigated. Linear <i>cis</i>- and <i>trans-</i>alkenes were attached to the donor unit of CDT–BTZ polymers to demonstrate the dependence of supramolecular ordering and solubility in organic solvents on chemical conformation. The layer interdigitation of the substituents differed due to shape disparities between <i>cis-</i> and <i>trans-</i>alkenes. While <i>trans-</i>alkenes exhibit zigzag structures that are beneficial for close packing, <i>cis</i>-alkenes are curved and thus possess a less regular shape that is disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies, which revealed shorter intermolecular distances for the polymer with <i>trans-</i>alkene substituents even in comparison to analogous polymers with saturated alkyl substituents. Furthermore, the isomerization of the <i>cis</i>-substituents toward their <i>trans-</i>conformers allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies

Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics

Solution-processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets of high quality. The electrochemically exfoliated graphene (EG) contains a high yield (>80%) of one- to three-layer graphene flakes with high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□ for a single EG sheet). Due to the solution processability of EG, a vacuum filtration method in association with dry transfer is introduced to produce large-area and highly conductive graphene films on various substrates. Moreover, we demonstrate that the patterned EG can serve as high-performance source/drain electrodes for organic field-effect transistors

Solution-Processed Bio-OLEDs with a Vitamin-Derived Riboflavin Tetrabutyrate Emission Layer

Solution processed biomaterials are required for the active component to develop printed biodegradable and biocompatible optoelectronic devices. Ideal film formation is crucial for the fabrication of multilayer thin film sandwich devices. We report on the characterization of thin films of the riboflavin-derived biomaterial riboflavin tetrabutyrate and its utilization in an organic light-emitting diode. We show that the nonsolution processable precursor can form homogeneous and smooth films with the addition of tailored side groups that change its solubility. We demonstrate by grazing incidence wide-angle X-ray scattering that this chemical derivative reduces the crystallinity and enhances emission, likely by suppressing π–π stacking interactions. Organic light-emitting diodes with a poly­(9-vinylcarbazole)–emissive riboflavin tetrabutyrate bilayer structure yield a maximum luminance of 10 cd/m² and external quantum efficiency of 0.02% with a 640 nm peak orange exciplex emission. External quantum efficiency measurements of a photodiode affirm the exciplex formation

Solubility Modulation of Polyfluorene Emitters by Thermally Induced (Retro)-Diels–Alder Cross-Linking of Cyclopentadienyl Substituents

For cost-efficient organic electronic devices, the consecutive deposition of active layers by solution-based processes is a key benefit. We report a synthetic approach enabling solubility reduction of bis­(cyclopentadienyl)-substituted polyfluorenes as emissive layers in organic light-emitting diodes (OLEDs). Thermally induced retro-Diels–Alder reaction liberates free cyclopentadiene as “protecting group” and pending cyclopentadienyl units, which cross-link the polymer strands upon cooling via [4+2] cycloadditions. The activation temperature is tuned in the range of 180–250 °C through alkyl, alkoxy, or ester linkages. Ultimately, macrocyclic self-protected bis­(cyclopentadienylene) moieties avoid extrusion of volatile cyclopentadiene during activation. The solvent resistance of the emissive layers after cross-linking is examined by absorption spectroscopy and white light scanning interferometry. The influence of the desolubilization procedure on the performance of solution-processed OLEDs is investigated