Research field A: Synthesis and self-assembly

On-surface synthesis of parallel aligned W-shaped graphene nanoribbons.

Coordinator: Bart Jan Ravoo, Organic Chemistry Institute

Inspired by nature, complex materials are generated in a hierarchical bottom-up fabrication process from a diversity of molecular building blocks. The precise and at the same time dynamic arrangement of the molecular components in terms of space and time defines the structure and the function of the resulting material. Within this research field, both established approaches of organic, supramolecular and macromolecular chemistry as well as new yet to be developed procedures are applied. Relevant new synthetic procedures for soft nanomaterials developed within the SoN include a) the preparation of novel tailor-made synthetic and biological molecular building blocks, b) the synthesis of 2D materials on surfaces, and c) the integration of bottom-up and top-down fabrication methods to prepare structured surfaces. 

Reference: Electronic Structure of Spatially Aligned Graphene Nanoribbons on Au(788); S. Linden, D. Zhong, A. Timmer, N. Aghdassi, J. H. Franke, H. Zhang, X. Feng, K. Müllen, H. Fuchs, L. Chi, H. Zacharias; Phys. Rev. Lett. 108, 216801 (2012)

Functionalization of a gold dot-structured surface with different organic molecules.

Lifeng Chi, Physics Institute:
Nanostructured biochemical surfaces

Patterned surfaces with micro/nano sized structures have been demonstrated to be a powerful platform for providing molecular engineering of cellular environments. Increased efforts have been made to study the influence of structured surfaces on cell behaviors such as adhesion, differentiation, proliferation and so on. These studies aim at creating biomimetic environments with defined chemical and physical properties in order to gain deeper understandings on how cells interact with their environment.

We are well experienced in the fabrication of structured surfaces with feature sizes ranging from micrometers down to nanometers, based on bottom-up and top-down strategies, as well as their combinations, and further chemical modifications. The application of these structured surfaces in cell culture has been conducted in our group in the recent years to demonstrate their influence on cell behaviors. For instance, osteoblasts, phytopathegenic fungi Magnaporthe Grisea and Pucinnia Graminis cells show clear alignment along topography patterns made by Langmuir-Blodgett Lithography (LBL); Human embryonic stem cells (hESC), which have the ability to differentiate into three germ layers, differentiate preferably towards neuronal lineage on structured surfaces with linear shapes; the growth of some cancer cells can be effectively depressed on nanostructured surfaces.

In future works, we are going to focus on the fate of embryonic stem cells and cancer cells on structured surfaces - in collaboration with the Max-Planch-Institute of Molecular Biomedicine (MPI Münster) and medical/biological groups of WWU - together with chemical modifications of the surfaces. Furthermore the application of external forces such as mechanical forces and electrical fields is in planning. Based on the understanding of the mechanisms behind the effects, the long term goal is to develop applicable surfaces/interfaces for clinical regenerative medicine.

Relevant preliminary work:

  1. “Tunable organic hetero-patterns via molecule diffusion control.” H. Wang, W. Wang, L. Li, M. Hirtz, C. G. Wang, Y. Wang, Z. Xie, H. Fuchs, L. Chi, Small 2014, 10, 3045-3049.
  2. “Addressable organic structure by anisotropic wetting.” W. Wang, C. Du, L. Li, H. Wang, C. Wang, Y. Wang, H. Fuchs, L. Chi, Adv. Mater. 2013, 25, 2018-2023.
  3. “Patterned nucleation control in vacuum deposition of organic molecules.” W. C. Wang, D. Y. Zhong, J. Zhu, F. Kalischewski, R. F. Dou, K. Wedeking, Y. Wang, A. Heuer, H. Fuchs, G. Erker, L. F. Chi, Phys. Rev. Lett. 2007, 98, 225504.

Reference: “Tunable multicolor ordered patterns with two dye molecules.” W. Wang, C. Du, H. Bi, Y. Sun, Y. Wang, C. Mauser, E. Da Como, H. Fuchs, L. Chi, Adv. Mater. 2010, 22, 2764-2769.

Schematic model of a minimal self-replicating cycle.
© A. Dieckmann (2009)

Nikos Doltsinis, Institute of Solid State Theory:
Multiscale simulation of self-replicating systems

Rational control of the cell growth: The basic requirement of molecular self-replication - an essential feature of living organisms - is studied be means of synthetic organic self-replicating systems. Therefore, the building blocks of the self-replicators are chosen to be as small as possible to enable an investigation by quantum mechanical calculations and molecular-dynamical simulations. Insights into the rational design of effective replicators are obtained from systematic chemical variation studies.

Relevant preliminary work:

  1. “Elucidating the origin of diastereoselectivity in a self-replicating system: Selfishness vs. altruism.” A. Dieckmann, S. Beniken, C. D. Lorenz, N. L. Doltsinis, G. von Kiedrowski, Chem. Eur. J. 2011, 17, 468-480.
  2. “Unravelling a fulvene based replicator: Experiment and theory in interplay.” A. Dieckmann, S. Beni-ken, C. D. Lorenz, N. L. Doltsinis, G. von Kiedrowski, J. Syst. Chem. 2010, 1, 10.

Reference: A. Dieckmann 2009

Harald Fuchs, Physics Institute:
Surface assisted chemical reactions: Chemistry beyond the test-tube

A variety of approaches was investigated to control the structure and yield of chemical reactions. Stereochemistry, based, e.g. on enantioselective reaction and chirality, and molecular recognition processes which represent a key in all biological self-assembly processes leading to complex multilevel systems, allow to design highly specific chemical products. They are based on reactions which require special geometrical conditions such as multiple bond formation according to the key-lock principle, which are usually not applicable in systems exhibiting mirror symmetry and single bond reactions. While these concepts are already widely used in conventional solvent chemistry two other, however, much less investigated effects are cooperativity/synergy requiring helper systems (the best ones known are catalysts), and the influence of dimensionality/spatial confinement. Even far less investigated is the combination of these two conditions.

Ideally, the two concepts, which will not work in a conventional 3-test-tube environment, may be perfectly studied under well controlled vacuum conditions on clean surfaces. Depending of the local structure of surfaces, such as metallic single crystals with a simple crystallographic structure or exhibiting a nanostructure after having undergone, e.g. a surface reconstruction, may significantly change the reactivity of molecular reactants on the surface. In addition the surface itself might undergo a re-ordering process due to the presence of the molecular system prior to the reaction, resulting in optimal spatial conditions for the molecular educts for driving them from a state of usually unfavored molecular reactions to preferred ones. In this way, for example, chemical reactions within 1-D channels or 2-D patches may occur fully self-consistently in the sense that the whole system (molecule + surface) finds its global minimum in the energetic hyper-plane without local external influence. Kinetic and thermodynamic effects can be used to tune these effects. First experiments demonstrated that the reactivity even in fairly simple, usually non-reactive alkane systems can be dramatically increased at very mild conditions making use of this approach (e.g. CH-activation).

The concept of applying cooperative/synergetic surface chemistry and its study at the single molecular level is just at the beginning and requires intense cooperation between surface physics, allowing to monitor chemical states during and after reactions by spectroscopic and microscopy techniques, and preparative as well as theoretical chemistry.

Reference: H.Fuchs

POPC/POPS lipid doble layer generated by vesicle spreading on Mica.

Volker Gerke, Institute of Medical Biochemistry, ZMBE:
Membrane/cytoskeleton dynamics during cell migration of controlled surfaces

The migration of cells on 2D substrates is basically controlled by the interaction of cellular integrin receptors with components of the extracellular matrix. Beside this 2D migration cell movements within a 3D environment exist, which is of presumably high relevance within tissues or in the course of the embryonic development. In both cases, 2D and 3D cell migration, the cellular actomyosin complex plays a superior role regarding the direction of movement. On the one hand, the formation of a lamellipodia at the front end and the effective contraction at the back end of the migrating cell is enabled. On the other hand, a selective regulation of the connection between the cortical actomyosin network and the plasma membrane causes the formation of membrane ruffles (blebs), which seem to be of particular importance for migrating through tissue. Numerous proteins and protein complexes control the at this point relevant connection of the actin cytoskeleton with the plasma membrane, i.a. members of the ezrin-radixin-moesin (ERM) family, which can directly bridge actin filaments and lipids of the plasma membrane. We envision to mimic the transition from a 2D to a 3D environment applying appropriate model systems (solid-supported matrices) and to study whether and how the behavior of cell migration changes at that transition. Particularly, the distribution and activation of ERM proteins based on fluorescence microscopic techniques is analyzed.

Relevant preliminary work:

  1. “Modes of ezrin-membrane interaction as a function of PIP2 binding and pseudophosphorylation.” V. Shabardina, C. Kramer, B. Gerdes, J. Braunger, A. Cordes, J. Schäfer, I. Mey, D. Grill, V. Gerke, C. Steinem, Biophys J. 2016, 110, 2710-2719.
  2. “Ezrin interacts with the scaffold protein IQGAP1 and affects its cortical localization.” R. C. Nammalwar, A. Heil, V. Gerke, Biochim. Biophys. Acta 2015, 1853, 2086-2094.
  3. “Characterization of the Ca2+-regulated ezrin-S100P interaction and its role in tumor cell migration.” J. Austermann, A. R. Nazmi, C. Müller-Tidow, V. Gerke, J. Biol. Chem. 2008, 283, 29331-29340.

Reference: “Phosphatidylserine Membrane Domain Clustering Induced by Annexin A2/S100A10 Heterotetramer.” M. Menke, V. Gerke, C. Steinem, Biochemistry 2005, 44, 15296-15303.

Numerous characteristics render modified nanoparticles attractive for catalysis
© Quelle: F. Glorius

Frank Glorius, Organic Chemistry Institute:
Functionalized nanoparticles

The focus of the Glorius group within the structure of the SoN is on the selective synthesis and characterization of metal and metal oxide nanoparticles and their application in important, particularly asymmetric catalytic reactions.

Despite the advanced development of the field of asymmetric catalysis (Nobel Prize for Chemistry 2001), only very few of these reactions are applicable to an industrial environment/scale. Problems arise from high costs, an insufficient space-time yield, low robustness and range of applications as well as difficulties regarding purification and recovery/recycling. However, in the area of heterogeneous catalysis, which is in industrial terms very popular, asymmetric variations are missing. Responsible for that lack is the in many times little understanding of heterogeneous catalysis (black box system) and poor structural manipulability.

Nanoparticles (NPs) could represent an interesting approach to bridge homogeneous and heterogeneous catalysis. Due to the small particle sizes high surface area-to-volume ratios are available, which is the reason for the high chemical/catalytic activity. Moreover, NPs can be influenced in their structure either in the course of their synthesis or afterwards. Applying chiral organic molecules (chiral modifier) to change the surface and to yield efficient catalysts for asymmetric catalysis is, up to now, a rather young and less investigated field. The modifier should be strongly bound to the NP, enhance the catalytic activity (active principle) and based on an active interaction with the substrates result in high activities and enantioselectivities. Furthermore, ideally the NPs should be stable and not lose any metal which is the basis for a long-term recycling of the catalyst. In our work we aim for a systematic investigation of NP synthesis, different modifiers and reactions as well as the underlying mechanisms and thus promote this economically and ecologically highly important, rather young research field.

Relevant preliminary work:

  1. “Asymmetric Nanocatalysis: N-Heterocyclic Carbenes as Chiral Modifiers of Fe3O4/Pd nanoparticles.” K. V. S. Ranganath, J. Kloesges, A. H. Schäfer, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 7786-7789; Angew. Chem. 2010, 122, 7952-7956.
  2. “Modular Bidentate Hybrid NHC-Thioether Ligands for the Stabilization of Palladium Nanoparticles in Various Solvents.” A. Rühling, K. Schaepe, L. Rakers, B. Vonhören, P. Tegeder, B. J. Ravoo,* F. Glorius*, Angew. Chem. Int. Ed. 2016, 55, 5856-5860; Angew. Chem. 2016, 128, 5950-5955.
  3. “Ballbot-type motion of N-heterocyclic carbenes on gold surfaces.” G. Wang, A. Rühling, S. Amirjalayer, M. Knor, J. B. Ernst, C. Richter, H.-J. Gao, A. Timmer, H.-Y. Gao, N. L. Doltsinis, F. Glorius*, H. Fuchs*, Nat. Chem. 2017, 9, 152-156.

Reference: F. Glorius

© [3]

Svetlana Gurevich, Institute for Theoretical Physics:
Evolution of surface geometries by self-assembly

Open systems far from equilibrium are capable to spontaneously form temporal, spatial, spatiotemporal and functional structures. Examples include hydrodynamic instabilities, chemical reactions at diffusion as well as laser systems. Dissipative self-assembly processes play a central role in growth processes of both the non-living and the living nature. In this context, dynamic processes within the cytoskeleton of a single cell involving changes of the cell surface, but also growth processes and morphological instabilities of cell associations, like the tumor growth, are important to mention.
The working group ‘self-assembly and complexity’ at the Institute for Theoretical Physics of the WWU Münster deals with the theory of the emergence of spatiotemporal ordered structures, the origin of temporal complexity and chaos, as well as the analysis of turbulent fields. The main focus lies on the investigation of the universal properties of structure formation and its theoretical analysis by methods of non-linear dynamics (bifurcation theory, chaos theory and theory of partial differential equations) and statistical physics (stochastic processes, non-equilibrium statistics).

In many cases the dynamics of complex liquids and soft matter is interface-dominated, i.e., controlled by capillarity and/or wettability. Examples include (driven) droplets on homo- and heterogeneous substrates, (active) liquid crystals and colloidal suspensions, self-propelled droplets, and multicomponent multilayer. An important objective is to understand the structure-forming interaction of the various interdependent advective and diffusive transport processes and phase transitions.

Experimental techniques like Langmuir--Blodgett transfer where a surfactant layer is transferred from a bath onto a plate, dip-coating or vapor deposition are widely used to create coatings of a precise thickness and/or structure. Using the self-assembly of microscopic building blocks into macroscopic structures facilitates the production of coatings with highly regular patterns over large areas. One aim is to understand how the basic properties of each system lead to the formation of particular functional patterns, as this allows one to develop ways to control the patterning process, e.g., by means of prestructured substrates or external fields.

Relevant preliminary work:

  1. “Structure formation by dynamic self-assembly.” L. Li, M. H. Köpf, S. V. Gurevich, R. Friedrich, L. Chi, Small 2012, 8, 488-503.
  2. “Instabilities of layers of deposited molecules on chemically stripe patterned substrates: Ridges vs. drops.” C. Honisch, T.-S. Lin, A. Heuer, U. Thiele, S. Gurevich, Langmuir 2015, 31, 10618-10631.
  3.  "Effect of pre-patterned surfaces on self-organized LB patterns.” J. Zhu, M. Wilczek, M. Hirtz, J. Gao, W. Wang, H. Fuchs, S.V. Gurevich, L. Chi, Adv. Mater. Interf. 2016, 3, 1600478.

Reference: 3.

Figure: Top:  AFM phase image of a branched vertical LB pattern on a homogeneous Si surface (a) and   AFM phase images of LB patterns on prestructured surfaces with different wavelength of the prestructure, showing (b) 1:1, (c) 2:1 locking to the prestructure. Bottom: Snapshots of 2d simulations of the generalized Cahn-Hilliard model in the case of vertically aligned prestructures calculated for different periodicities. Branches that occur without a prestructure disappear for a vertically aligned prestructure. Different periodicities lead to different locking ratios like 1:1 or 2:1.

Two-dimensional free energy landscape of a cytosine-rich short DNA strand. It is expressed via the first two eigenvectors, resulting from a principal component analysis. Configuration (a) corresponds to the i-motif, (b) and (c) to the hairpin structures and (e) to the fully extended chain.
© WWU / Heuer

Andreas Heuer, Institute of Physical Chemistry:
Molecular dynamical and Monte Carlo simulations of soft nanomaterials: From the understanding of free energy landscapes to hysteresis effects

In the subject area of controllable nanomaterials molecular dynamical simulation of different experimentally relevant systems determining the free energy landscape in dependence to external parameters, such as pH and electrochemical potential, shall be performed. For this purpose, our working group has currently developed algorithms, e.g. in the area of metadynamics, which also allow the investigation of complex molecular systems. Furthermore, applying appropriate model systems it shall be examined in how far potential hysteresis effects, which can appear e.g. during the variation of external parameters, are present in free energy landscapes. Therefore, it is crucial to identify an appropriate reaction coordinate as a basis for the free energy landscape on the one hand, and to additionally reveal hidden complexity in the respective orthogonal coordinates on the other hand. If necessary Monte Carlo simulations of lattice models on even larger scales shall be performed, for example to study the influence and reversibility of external disturbances on gels.

Relevant preliminary work:

  1. “Calculation of free energy landscapes: A Histogram Reweighted Metadynamics approach.” J. Smiatek, A. Heuer, J. Comput. Chem. 2011, 32, 2084-2096.
  2. “Systematic detection of hidden complexities in the unfolding mechanism of a cytosine-rich DNA strand.” J. Smiatek, D. Janssen-Mueller, R. Friedrich, A. Heuer, Physica A 2014, 394, 136-144.
  3. “Anomalous approach to thermodynamic equilibrium: structure formation of molecules after vapor deposition.” P. K. Jana, C. Wang, R. L. Jack, L. Chi, A. Heuer, Phys. Rev. E 2015, 92, 052402.

Reference: “Stable Conformations of a Single Stranded Deprotonated DNA i-Motif.” J. Smiatek, C. Chen, D. Liu, A. Heuer, J. Phys. Chem. B 2011, 115, 13788-13795.

Schematic representation of a DNA double helix with metal-mediated base pairs
© J. Müller

Jens Müller, Institute of Inorganic and Analytical Chemistry:
Synthetic nucleic acids

The research focus of the Müller group is the bioinorganic chemistry of nucleic acids with a particular emphasis on the generation, characterization as well as application of nucleic acids including metal-mediated base pairs. Within a metal-mediated base pair, the hydrogen bonds between two complementary nucleobases are formally replaced by coordinate bonds to a central metal ion. The synthetic nucleic acid serves as template for the specific arrangement of metal ions along the helical axis. As this approach enables a site-specific arrangement of metal ions with high precision, interesting chemical and physical properties arise for the resulting constructs.

One of the main research areas is the development of new synthetic nucleosides for metal-mediated base pairs, thus combining metal-based functionality with the evolutionary optimized, reversible self-assembly of nucleic acids. For example, we succeeded in the generation of a metal-mediated base pair including two metal ions and, for that reason, exhibiting luminescent properties. An additional focus is the development of applications based on metal-modified nucleic acids. Possible applications are sensors for various analytes, supramolecular networks with metal-based functionality, catalysts in asymmetric catalysis and many more. We developed a nucleic acid that is able to recognize via metal-mediated base pair formation one oligonucleotide from a mixture of natural oligonucleotides that differ by a single nucleoside only. Moreover, our synthetic nucleic acids including artificial ligand-based nucleosides can be applied for a controlled aggregation of DNA to form supramolecular structures. A further application is the generation of DNA-stabilized metal nanoclusters.

Relevant preliminary work:

  1. “Solution structure of a DNA double helix with consecutive metal-mediated base pairs.” S. Johannsen, N. Megger, D. Böhme, R. K. O. Sigel, J. Müller, Nat. Chem. 2010, 2, 229-234.
  2. “A metal-mediated base pair that discriminates between the canonical pyrimidine nucleobases.” B. Jash, P. Scharf, N. Sandmann, C. Fonseca Guerra, D. A. Megger, J. Müller, Chem. Sci. 2017, doi: 10.1039/c6sc03482a.

Reference: J.Müller

Stepwise preparation of polymer-shelled vesicles by using cyclodextrin vesicles as supramolecular templates and an adamantine-functinalized poly(acrylic acid) additive anchored via host-guest interaction.
© B.J. Ravoo

Bart Jan Ravoo, Organic Chemistry Institute:
Synthetic vesicles

The general objective of our research is applying molecules as building blocks for the construction of soft materials and nanoscale structures by means of self-assembly. The formation of complex and dynamic superstructures based on various small molecules results in chemical systems featuring new properties that go beyond the actual sum of the single components. Our group works on two main topics: Biomimetic supramolecular chemistry and surface modifications via molecular self-assembly.
In the field of biomimetic supramolecular chemistry we investigate the self-assembly of molecules and colloids in aqueous solution. We employ non-covalent interactions to generate larger structures from molecular building blocks. Multiple weak interactions result in a strong and selective multivalent interaction. One main research topic involves receptor molecules incorporated vesicles, such as cyclodextrins. The multivalent recognition of guest molecules on the vesicle surface, and the according interaction among the vesicles, serve as biomimetic model system for the biological cell-cell recognition and innovative container for drugs. A further research topic is based in the development of synthetic carbohydrate receptors. Applying a dynamic-combinatorial approach oligomers and macrocyclic carbohydrate receptors are formed from peptidic building blocks. These receptors are capable to effectively and selectively bind carbohydrates both in water and on membrane surfaces. In the area of surface modifications by self-assembly, we investigate the preparation and the resulting properties of molecular monolayers on solid substrates. We combine bottom-up self-assembly with top-down lithography, for example to structure surfaces by microcontact printing with reactive molecular inks. Microcontact printing is applied to prepare chemical and biological surface templates including protein, nucleotide and carbohydrate based chips. In the long-term we aim at the synthesis of adaptive surfaces.

Relevant preliminary work:

  1. "Fabrication of hydrophilic polymer nanocontainers by use of supramolecular templates." A. Samanta, M. Tesch, U. Keller, J. Klingauf, A. Studer, B. J. Ravoo, J. Am. Chem. Soc. 2015, 137, 1967– 1971.
  2. "Rewritable Polymer Brush Micropatterns Grafted by Triazolinedione Click Chemistry." O. Roling, K. De Bruycker, B. Vonhören, L. Stricker, M. Körsgen, H. F. Arlinghaus, B. J. Ravoo, F. E. Du Prez, Angew. Chem. 2015, 127, 13319–13323; Angew. Chem. Int. Ed. 2015, 54, 13126–13129.
  3. Nadjas UCNP folgt in Kürze

Reference: 1.

Synthesis, photophysical and scanning tunnel spectroscopic investigation of photofunctional Pt(II) complexes
© C.A. Strassert

Cristian A. Strassert, Physics Institute:
Photofunctional nanomaterials

Transition metal complexes and organic dyes with long-lived triplet states can be employed for the light-driven generation of reactive oxygen species (ROS). The phototriggered production of ROS can be used for therapy, e.g. for the inactivation of neoplastic diseases and bacterial infections.  The therapeutic effect should be combined with imaging capabilities by labelling the affected areas. Ideally, a single molecular species (or nanoparticle) should be able to   produce ROS, to show a defined chemotherapeutic effect and to aid the diagnosis by selectively recognizing their biological target.

C. A. Strassert and his group develop new photofunctional coordination compounds and nanomaterials for optoelectronics and theranostics. In particular, nanostructured multifunctional systems are developed, which can be used in organic light-emitting diodes1 but also for the optical labelling and light-driven inactivation of antibiotic resistant bacteria2. The new materials are investigated regarding their photophysical properties and optoelectronic performance. Their photobiological properties as well as their use in functional microscopy are explored as well3.

Relevant preliminary work:

  1. “Scanning-Tunneling-Spectroscopy-Directed Design of Tailored Deep-Blue Emitters.” J. Sanning, P. Ewen, L. Stegemann, J. Schmidt, C. G. Daniliuc, T. Koch, N. L. Doltsinis, D. Wegner, C. A. Strassert, An-gew. Chem. Int. Ed. 2015, 54, 786-791.
  2. “Silicon(IV) Phthalocyanine-Decorated Cyclodextrin Vesicles as a Self-Assembled Phototherapeutic Agent against MRSA.” A. Galstyan, U. Kauscher, D. Block, B. J. Ravoo, C. A. Strassert, ACS Appl. Mater. Interfaces 2016, 8, 12631-12637.
  3. “Spatiotemporally Resolved Tracking of Bacterial Responses to ROS-Mediated Damage at the Sin-gle-Cell Level with Quantitative Functional Microscopy.” A. Barroso Peña, M. C. Grüner, T. Forbes, C. Denz, C. A. Strassert, ACS Appl. Mater. Interfaces 2016, 8, 15046-15057.

Reference: 1.

Schematic presentation and scanning tunneling microscopy images of a) the light-induced Glaser coupling on Ag(111),[2] b) the decarboxylative coupling on Cu(111) and c) the bisacylperoxide formation on Au(100)
© A. Studer

Armido Studer, Organic Chemistry Institute:
On-surface reactions in ultrahigh vacuum

The possibility to build up complex nanostructures by self-assembly and subsequent chemical connection of single molecular building blocks in a specific manner (bottom-up principle) has gained great importance in the field of modern nanotechnology. The carrier surfaces used for this purpose enable the preparation of monomolecular thin polymeric structures on the given surfaces. Notably, the surface can be regarded as a new reaction medium in modern synthesis and the so-called on-surface chemistry provides the potential for the formation of two-dimensional macro-molecules. Moreover, new reactions have been discovered that are not accessible with classical methods using solvent-based or gas phase chemistry.

For several years, the Studer group in close collaboration with colleagues from the physics department (mainly Prof. H. Fuchs) has been successfully working in the field of on-surface chemistry.

The know-how of the Münster chemists enables the design and synthesis of the needed molecular building blocks according to the desired requirements. Specific introduction of functional groups into the chosen monomer entities allows to control the target on-surface reactions and to influence the physical properties of the generated polymeric structures.

Following this approach, the Studer/Fuchs working groups in joint efforts were able to use well established solution-phase chemical reactions such as the Glaser coupling of alkynes or the decarboxylative coupling of carboxylic acids for on-surface polymerizations. The successfully generated polymeric structures were investigated by scanning tunneling microscopy with molecular resolution. Along these lines, unexpected and up to now unknown reactions like the dehydrogenative coupling of carboxylic acids via bisacylperoxide formation were discovered and used for the on-surface preparation of linear polymeric chains with a length of up to 100 nm.

Relevant preliminary work:

  1. “Glaser coupling at metal surfaces.” H.-Y. Gao, H. Wagner, D. Zhong, J.-H. Franke, A. Studer, H. Fuchs, Angew. Chem. Int. Ed. 2013, 52, 4024-4028; Angew. Chem. 2013, 125, 4116–4120.
  2. „Photochemical glaser coupling at metal surfaces.”H.-Y. Gao, D. Zhong, H. Mönig, H. Wagner, P. A. Held, A. Timmer, A. Studer, H. Fuchs, J. Phys. Chem. C 2014, 118, 6272–6277.
  3. “Decarboxylative polymerization of 2,6-naphthalenedicarboxylic acid at surfaces.” H.-Y. Gao, P. A. Held, M. Knor, C. Mück-Lichtenfeld, J. Neugebauer, A. Studer, H. Fuchs, J. Am. Chem. Soc. 2014, 136, 9658–9663.
  4. “On-Surface Domino Reactions: Glaser Coupling and Dehydrogenative Coupling of a Biscarboxylic Acid To Form Polymeric Bisacylperoxides.” P. A. Held, H.-Y. Gao, L. Liu, C. Mück-Lichtenfeld, A. Timmer, H. Mönig, D. Barton, J. Neugebauer, H. Fuchs, A. Studer, Angew. Chem. Int. Ed. 2016, 55, 9777-9782; Angew. Chem. 2016, 128, 9929–9934.

Reference: 2.-4.

Elektron Spinfilterung durch organisierte oligo dsDNA.
© H. Zacharias

Helmut Zacharias, Physics Institute
Electron transfer in functional organic films

We are investigating energy, charge and spin transfer processes in organized functional organic films on, e.g., SiC, a bio-inert substrate material of interest also for electronic applications. To understand the interplay between the adsorbates and the substrate as well as interactions within functional constituents of adsorbates the lifetimes of occupied and unoccupied states are investigated. SiC substrates functionalized via organic linkers with several organic dyes by wet chemistry as well as evaporation in vacuum have been characterized by inverse photoemission, X-ray photoelectron spectroscopy, and picosecond time-resolved fluorescence microscopy. The latter already yields spatially resolved positions of fluorophores.

Ultrafast electron transfer processes are studied by time-resolved two-photon photoemission (2PPE), employing laser radiation with pulse durations of a few tens of femtoseconds. Standard 2PPE spectroscopy does not yield any spatial information, because the recorded signal integrates over the whole light spot on the sample. In order to investigate structural dependencies of electron transfer processes a photoemission electron microscope (PEEM) is employed. This may yield a spatial resolution of 20 nm or better, depending on the type of instrument. In the experiments we use an optical parametric amplifier (OPA) pumped by a Ti:sapphire laser with pulse energies up to 1.1 mJ, a pulse duration of 30 fs and repetition rates between 1 and 10 kHz. The OPA is continuously tunable from 230 nm to 2700 nm with pulse durations between 30 fs and 120 fs and pulse energies between 2 µJ and 250 µJ, depending on the desired output wavelength. A motorized delay stage with a Michelson-type interferometer splits the resulting pulses into two separate ones and thus introduces an adjustable delay with femtosecond resolution.

Lifetime measurements allow to sensitively assess different binding environments. It is therefore expected that changes in the binding of non-covalently bound buildings blocks of functional materials can be revealed.

Relevant preliminary work:

  1. “Spin selectivity in electron transmission through self-assembled monolayers of double-stranded DNA.” B. Göhler, V. Hamelbeck, T. Z. Markus, M. Kettner, G. F. Hanne, Z. Vagner, R. Naaman, H. Zacharias, Science 2011, 331, 894-897.
  2. “Femtosecond Electron Dynamics in Dye-Functionalized 6H-SiC(0001).” N. F. Kleimeier, D. K. Bhow-mick, H. Zacharias, J. Phys. Chem. C 2015, 119, 27489-27495.
  3. “Fluorescence Properties of Perylene and Pyrene Dyes Covalently Linked to 6H-SiC(0001) and Silicate Surfaces.” D. K. Bhowmick, L. Stegemann, M. Bartsch, C. Strassert, H. Zacharias, J. Phys. Chem. C  2016, 120, 3275-3288.

Reference: H. Zacharias.