Research

Electrospray is not only a method to transfer ions from solution to the gas phase, but also to initiate highly accelerated reactions at the surfaces of the charged droplets that form during the electrospray process. We have recently successfully combined the research fields of “reaction acceleration in charged microdroplets” and “ion soft-landing” to obtain intermediates in high ion yields from the electrospray process and to chemically convert them on surfaces. [1] Our research in this area will focus on droplet reactions of pre-charged ions, which have found little attention in this research field so far.

Mass spectrum, isolation of all [B₁₂(CN)₁₁]^– ions with a nominal m/z value of 416 in a cryogenic ion trap in which neon was introduced

Fragment ions are valuable “building blocks” for us. We use various methods of computational chemistry as well as different mass spectrometric and gas phase spectroscopic methods (in cooperation) to gain a fundamental understanding of fragment ions. For example, this led to the discovery of so-called “super-electrophilic anions”. [2] With these fragments, we were able to expand noble gas chemistry: We generated the first stable argon-containing molecular anion at room temperature which was also able to bind neon up to 50 K (Fig. 2). [3,4] Furthermore, the direct functionalization of saturated alkanes could be performed with these ions. [5,6] Various related radical ions are currently being investigated for use in molecular syntheses and layer preparation.

Fig. 3: Analytical results demonstrating the generation of new substances by fragment ion deposition, a) mass spectrometric detection of a trianion generated by binding of an anionic fragment to a dianion, b) IR spectroscopic detection and c) NMR spectroscopic detection of the [B₁₂Br₁₁N₂]^– ion.

By depositing highly reactive anionic fragments on layers containing dianions, we were able to generate covalently bound trianions ([B₂₄I₂₃]^3-) (Fig. 3a). [7] The synthesis of new substances by direct binding of unreactive “raw materials” such as N₂ is possible with fragment ions: The ion [B₁₂Br₁₁N₂]^- was generated by an ion-molecule reaction of the fragment [B₁₂Br₁₁]^- with N₂, deposited and subsequently characterized by IR and NMR spectroscopy (Fig. 3b, c). By sequentially landing these anions with cations, we generated a charge-balanced molecular salt on the surface, which could be used in subsequent reactions. Thus, for the first time, an ion-molecule reaction was realized in a mass spectrometer as part of a multistep synthesis. [8] Our future research in this field will focus on controlling the selectivity of reactive fragments in molecular layers, such as targeted binding to specific functional groups. First published results are available [9] and further publications are in preparation.

Many models of ion deposition assume that the ions are discharged when they hit a conductive surface. However, stable molecular pre-charged ions can remain charged during deposition. It is intuitively understandable that a condensed layer cannot consist of ions of only one polarity. Nevertheless, visible layers often form on surfaces when only one type of ion is mass-selected and deposited. [10] We investigate the electron transfer processes that occur in such cases between the deposited layer and the surface, which lead to charge balancing within the layer. These include complex reactions, e. g. polymerization, and the formation of counterions from background molecules (DFG joint project).

a) Schematic representation: STM tip and landed complex on the surface, b) imaging and c) scanning tunneling spectroscopy (differential plot of current vs. voltage), for details see text.

We investigate the deposition of undercoordinated metal complexes for binding to surface-bound molecules, such as self-assembled monolayers (SAMs) and porous coordination polymers. Furthermore, the stabilization of undercoordinated metal complexes with weakly coordinating counterions on surfaces is investigated. These methods may open new possibilities for the generation of (photo)catalytically active coating materials as well as for gas separation and sensor technology.  The functionalization of biomolecules by binding fragment ions will be investigated in the future as part of an “Exploration Grant” from the Boehringer Ingelheim Foundation. In cooperation with the Monakhov-Group at the Leibniz Institute for Surface Modification (IOM), we are researching “soft-landing” of polyoxometalate anions with voltage-responsive, multi-step resistive switching behavior for potential applications in molecular electronics. Fig. 4a schematically shows a landed host-guest complex during a scanning tunneling microscopy (STM) investigation. Fig. 4b shows STM imaging of individual landed particles and Fig. 4c shows the gradual reduction in resistance measured by the tunneling current between the tip and the surface. [11] The gradual change in resistance is attributed to reduction of the highly oxidized vanadium centers. The deposition of corresponding switching units on nanoelectrodes is planned as a future direction of this research.