Our primary goal is the utilization of charged molecules and their fragments – generated within a mass spectrometer – for the synthesis of condensed phase matter. We are able to use a large variety of experimental and theoretical methods for our research due to the excellent support of many experts and experienced research groups around the world.

Research Profile Junior Research Group Warneke. Graphics: Sascha Steinbrink
Research Profile Junior Research Group Warneke. Graphics: Sascha Steinbrink

Chemistry with molecular fragment ions, preparative mass spectrometry

Mass spectrometry is usually known to chemists as a very sensitive analytical tool. In the mass spectrometer, energy can be transferred to charged molecules in the gas phase to excite them. During this process, molecular fragments are often generated which cannot be synthesized by classical preparative methods or which cannot be stabilized in a condensed phase environment. Occasionally, these charged fragments are highly reactive or show some catalytic activity. However, since these ions with exceptional chemical properties could be generated only in very small amounts until recently, their potential applications have been rarely considered. We use different spectroscopic, mass-spectrometric and theoretical methods for characterization of molecular fragments and their reaction products and accumulate them via “ion soft-landing” on a macroscopic scale. To achieve this goal, we build a newly developed instrument which allows us to deposit gaseous ions gently on surfaces. We explore new methods to stabilize reactive ions on surfaces and to use them for the activation of inert bonds. Additional goals are the preparation of new catalytically active surfaces and metastable, self-organizing material layers, which show hitherto unknown physical and chemical properties.

In addition to our laboratory at WOI we operate a laboratory for mass-selective molecular ion deposition together with the IOM.

Overview on our current projects

Gaseous molecular ions, as they are commonly known from mass spectrometry investigations, are standing in the focus of our research. While such ions are usually used solely for analytical applications, we aim to use them in a preparative manner. We study gaseous ions with spectroscopic, mass spectrometric and theoretical methods to understand their properties in detail. We are building ion soft-landing instruments which allow us to deposit mass-selected gaseous ions with high ion currents on surfaces. This process generates material layers on surfaces which cannot be generated using other preparative methods.

Molecular ion deposition for the generation of peptide and polymer surface layers

Our project within the SFB TRR 102 focuses on two major scientific questions: i) can ion soft-landing be employed to initialize polymerization on surfaces by the co-deposition of monomers and polymerization starters (e.g. radicals, anions or cations) and ii) how do large polypeptides behave on surfaces after their mass-selected deposition with respect to agglomeration and how do reactive fragment ions react with such peptides? For example, we probed the radical dianion [B12Br11O]2-• as a polymerization starter by soft-landing it onto layers of the co-deposited monomer of 4-vinylbenzenesulfonate [C8H7SO3]- (see Figure 1). We explore how reaction conditions including layer thickness, spatial overlap of the ion beams on the target and total ion current influences the chain length of observed polymers. In addition, we are extending our instrument for this project to enable ion soft-landing into ionic liquids.

Figure 1: Collision induced dissociation and Structure of 4-vinylbenzenesulfonate. Graphics: S. Kawa
Figure 1: Collision induced dissociation and Structure of 4-vinylbenzenesulfonate. Graphics: S. Kawa

Figure 1. a) Collision induced dissociation (CID) of [B12Br11NO2]2- resulting in the reactive radical dianion [B12Br11O]2-•. b) Structure of 4-vinylbenzenesulfonate.

We prepare layers of soft-landed peptides with different coverages for STM investigations (performed with cooperation partners at Leibniz-institute for surface modification (IOM)). We study, how fragment ions of different nature bind to the different functional groups of peptides.

Ion Soft-landing of Undercoordinated Metal Complexes and their Application in Novel Materials for Dihydrogen Isotope Separation

We explore the preparation of alternative materials for hydrogen isotope separation containing under-coordinated transition metal cations. Free coordination sites in metal containing ionic complexes are generated by fragmentation reactions in the gas phase. We search for methods which allow the stabilization of these fragment ions after their soft-landing on surfaces. One ion that is intensively studied by us is the molecular fragment ion [Ru(bpy)2]2+ deposited from the gas phase. [Ru(bpy)2]2+ is generated by collision-induced dissociation (CID). The vacant binding sites are reactive and bind to various reagents on the surface. We explore the codeposition with weakly coordinating anions in order to stabilize the reactive fragment. The stabilized undercoordinated metal center may exhibit some hydrogen isotope separation capability, as it is known from undercoordinated metal centers in metal organic frameworks.

(Robert Schiewe)

 

Figure: Structure of the fragment ion Ru(bpy)2]2+. (Robert Schiewe)
Figure: Structure of the fragment ion Ru(bpy)2]2+. (Robert Schiewe)

Functional particle-induced nanofabrication with layers of mass-selected ions

Nanotechnology is a key field of materials science with the mission to continuously advance established nanostructuring processes and to explore novel and high-performance materials for use at the nanoscale. Conventional nanotechnology is usually based on photolithography, in which structures are transferred into photosensitive material using short-wavelength light and masks. However, the achievable minimum structure size is closely related to the wavelength of the light used and poses significant challenges to production engineering when using extreme ultraviolet or even shorter wavelength radiation. In complementary approaches, mask-free processes based on direct-write deposition of material by particle-induced chemistry (high-energy electrons or ions) have been investigated. Beams of such particles can be very tightly focused and therefore allow structure sizes in the range below 1 nm. So far, however, these processes have been limited to the use of volatile precursors.

We aim to use layers, generated by deposition of mass-selected ions, as substrates for particle-induced nanofabrication. The ion soft-landing process enables to use the substance class of complex molecular ions for layer preparation from the gas phase followed by nanostructuring. This allows for the generation of nanomaterials that are not available via conventional nanofabrication or from volatile precursor compounds. In particular, we are investigating particle-induced solubility contrasts in layers of organometallic ions that enable nanostructures that could find application, for example, as nanoscale humidity sensors with high sensitivity.

(Dr. M. Rohdenburg)

Process of particle-induced nanofabrication based on layers of mass-selected ions on the example of tris(bipyridine)ruthenium(II) dications. Graphics: M. Rohdenburg
Process of particle-induced nanofabrication based on layers of mass-selected ions on the example of tris(bipyridine)ruthenium(II) dications. Graphics: M. Rohdenburg

Fragment ions as new "building blocks" for chemical synthesis of molecules

We explore the formation of fragment ions, characterize their chemical properties and accumulate reaction products which can hardly be accessed by known classical preparative methods. Mass spectrometers offer a variety of possibilities to cleave bonds in the gas phase. Therefore, a large variety of “building blocks” is available. Our goal is to establish these ions for chemical synthesis. We have studied the unique properties of molecular fragment ions, for example, on fragments of type [B12X11]- (X=halogen,CN), so-called "superelectrophilic anions".

Representative publications

  • "Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to B12Cl11]-" Angew. Chem. Int. Ed. 2017, 56, 7980 – 7985.
    https://doi.org/10.1002/anie.201702237
Optical microscopy images of ion soft-landing deposits.

Central image: Deposited ion [B12Cl12]2-, surface: fluorinated alkane thiol on gold, background pressure: 10-5 mbar. An exchange of these parameters affects a significant change in the layers optical appearance,
a) Exchange fluorine with hydrogen in the alkane thiol,
b) background pressure 10-8 mbar
c) Exchange of the halogen substituents of the deposited ion ([B12I12]2-).

The deposition of mass-selected ions offers unique possibilities to generate material by deposition of one type of ion – without simultaneous deposition of a counterion. Usually, ions are discharged during deposition. A surface layer which contains only equally charged ions cannot exist due to strong repulsive forces. However, if highly electronically stable anions are deposited on special surfaces, interesting phenomena can be observed. The layer on the surface built from charged mass-selected ions accumulates molecules from the background of the ion soft-landing instrument. Upon contact with air, these metastable material layers change their morphology within timeframes of seconds to hours and show complex self-organization processes. The microstructures of the self-organized layers can be very different. They are dependent on the type of ion, the surface and the vacuum conditions during the deposition (see microscopy images). The access of charge which accumulates in the layer during the deposition appears to drive these phenomena. Our goal is to understand the physical background of the observed self-organizing processes, to discover (electro-)chemical reactions which may occur within the layers and to control these processes.

We explore the preparation of new functional surface layers using gaseous ions at the Leibnitz-Institute of Surface Engineering (IOM Leipzig). New polymers and catalytically active surfaces with potential applications are standing in the focus of our research.

Representative publications

Physical & chemical properties and molecular interactions with closo-borate anions

The substance class of closo-borate anions plays an important role in our research for several reasons:

1. Fragments of closo-borate anions show a fascinating gas phase ion chemistry
2. Ion deposition under preservation of charge is possible, and
3. Closo-borate anions are weakly coordinating super-stable anions which can stabilize highly reactive cations. These cations may be deposited "on top" using ion soft-landing.

Next to these applications in our group, closo-borate anions are of interest to some medical research fields. For example, boron neutron capture therapy (BNCT) has high potential for the treatment of inoperable tumors. We use gas phase spectroscopic, mass spectrometric and theoretical methods to characterize newly synthesized closo-borate anions, which become available within our cooperation network. First, we study the isolated ions and then their interactions with other molecules, for example with counterions, one or several solvent molecules or in host-guest complexes. These results help closing the knowledge gap between the properties of the isolated ions in the gas phase and the salts in solution. This knowledge also serves as a baseline for designing new closo-borate compounds with target-oriented properties.

Representative publications

  • "Properties of perhalogenated {closo-B10} and {closo-B11} multiply charged anions and a critical comparison with {closo-B12} in the gas and condensed phases" Phys. Chem. Chem. Phys. 2019, 21, 5903-5915. DOI: 10.1039/C8CP05313H
  • "Evidence for an intrinsic binding force between dodecaborate dianions and receptors with hydrophobic binding pockets" Chem. Commun., 2016, 52, 6300 – 6303.
    DOI: 10.1039/C6CC01233G
     
  • "Protic anions [H(B12X12)]- (X = F-I) that act as Brønsted acids in the gas phase" Chem. Eur. J. 2015, 21, 5887-5891. DOI: 10.1002/chem.201500034

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