Research

The FEM group works on intelligent polymeric and organic materials which combine electronic properties (e.g., high mobility) with a second functionality (e.g., an ultra-low modulus for biocompatible electronics). We conduct research in the following four areas.

Materials for Information and Energy Storage

Wolf type-III polymers are organometallic hybrid materials with a high potential for (opto)electronic applications due to their cooperative redox properties, yet so far, only redox-inactive Pt containing polymers were investigated in depth. The FEM group incorporates redox-active metal complexes in conjugated polymers to obtain charge conducting Wolf-III polymers. Group 8 transition metals can be switched between two stable oxidation states without inducing significant structural changes in the ligand sphere, enabling, e.g., direct intra-chain doping, or to switch the polymers between a low and high conductance state.

Recently we published the first report of a soluble Wolf-III polymer semiconductor (PSC) with Ru(II) centres. The PSC is redox-active due to the reversible oxidation of the metal centre, while the solubilizing alky substituents on the organic monomer allow the processing from solution. The polymer can be coated into smooth thin films and has semiconducting properties in an organic field transistor (OFET). Cyclic voltammographic studies in solution indicate the formation of a mixed valence species.

Soft Polymer Electronics

Stretchable electronics is an interdisciplinary research area with very high application potential, e.g., regarding wearable electronics, soft robotics, the Internet of Things (IoT), Industry 4.0, or on-skin sensors for e-Health.

Realizing stretchable electronics requires all electronic components to be electronically active and simultaneously mechanically soft and stretchable. Polymer semiconductors (PSCs) are very promising as they have a low modulus compared to their inorganic counterparts (0.1 - 1 GPa for typical PSCs), yet they are still orders of magnitudes away from the softness of human skin (0.1 - 10 MPa). Furthermore, PSCs are brittle and structurally fragile, prohibiting their application in mechanically strenous environments, or leading to the break down of electric functionality.

We work on PSCs with a very low modulus matching the one of human skin. Currently, our focus are block copolymers combining PSCs with biocompatible elastomers. We synthesized triblock co-polymers (TBCs) by covalently endcapping poly-diketo-pyrrolopyrrole-thienothiophene (PDPP-TT), a high-performance D-A PSC, with two elastomeric polydimethylsiloxanes (PDMS) blocks to combine favorable electrical and mechanical properties in one system. This leads to the nanophase segregation of both components, while preserving the features of both moieties in their respective domains. The TBCs are ultra-soft and durable: the TBC with the highest PDMS content has a modulus in the range of mammalian skin and shows no crack formation up to 85% strain. Also, the polymer maintains electronic functionality over more than 1500 cycles at 50% strain.

Polymer-enabled MALDI MS and MS Imaging

MALDI mass spectrometry imaging (MSI) is an emerging clinicopathological imaging tool enabling to visualize the spatial distribution of molecules in a 2D sample, for example in a cancer biopsy. Unlike other imaging methods, MALDI MSI works without target-specific radioactive or immunohistochemical labeling and thus allows e.g., the creation of molecular maps to directly investigate metabolic pathways and illnesses.
Developing matrix systems able to support measurements in the metabolic low molecular weight (LMW) area remains a challenge: Currently, MALDI MSI uses small molecule matrices developed for classic MALDI MS, which generally do not support LMW measurements. Also, as MALDI MSI measurements take considerably longer to record, matrices have to be fully stable under ultra-high vacuum, and as the matrix is coated on top of the sample instead of being co-crystallized with the analyte, it is necessary to fabricate uniform matrix layers with good analyte extraction abilities.

We recently showed that conjugated polymers are excellent MALDI silent (i.e., no or only few matrix related peaks) for LMW compounds, and furthermore allow measurements in both positive and negative mode. By designing amorphous and semi-crystalline derivatives, we could show that analyte incorporation takes place in the amorphous part of the polymer film, instead of the generally assumed co-crystallisation.

Currently we work on the interface between polymer matrix and analyte, e.g., to understand analyte incorporation, extraction and ionization mechanism, and develop new polymer matrices for high-resolution imaging. Long-term, our aim is to realize reactive polymer matrices.

Interfaces and Molecular Machines

The precise manipulation of a solid state device's electrical output is a fundamental challenge in organic electronics and nanotechnology, and modulating interfaces in the device layout, e.g., metal/organic unit, is a core point to achieve this. Furthermore, by electronically coupling stimuli responsive units, multifunctional devices are accessible, e.g., photo-responsive memories or logic circuits.

In the area of unimolecular electronics, we are interested in the design of functional nanomachines. Together with the CFAED group Unimolecular Machines, we recently could call in question the general assumption that nanocars require a high dipole moment for efficient lateral mobility. We continue to design new structures to elucidate the influencing factors, and will participate as the German team GAZE (German Azulene Explorer) in the second international Nanocar-Race in 2022. In collaboration with the FET open project Mechanics with Molecules (MEMO), we synthesize molecular rotors, which can be chemically anchored on an Au(111) surface. Our target is the development of structures for unidirectional rotation, and we recently reported a unidirectional rotor at the temperature of 5 K.

Also, we seek to create hybrid materials by binding conjugated molecules to coinage metals using mechanically stable and highly conductive anchoring groups. Recently, we showed that N-heterocyclic carbenes can be used as starting points to initiate Kumada-type polymerization. NHC-Au complexes were synthesized and regioregular poly(3-hexylthiophenes) with narrow weight distributions grown from the active aryl bromide, before gold nanoparticles were obtained via direct reduction. In comparison with Au NPs stabilized by PEG-SH, the synthesized P3HT-NHC@Au NP systems show excellent thermal and electrochemical redox stabilities as well as enhanced electron delocalization across the organic/inorganic interface. We currently work on the modification of electrodes to modulate contact resistance, and on the incorporation of switchable interfaces.