New ways of controlling and analyzing organic reactions on silicon surfaces – Publications by A8 (Koert/Dürr) & B5 (Höfer/Mette) and A6 (Tonner)

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In a joint effort, the groups of SFB-projects A8 and B5 used scanning tunneling microscopy for controlling the final products of a textbook-type reaction of organic molecules on silicon surfaces. A detailed understanding of the driving forces of these reactions are obtained by means of energy decomposition analysis as developed in SFB-project A6.

The isomers naphthalene (left) and azulene (right) bind very differently to a copper surface: While naphthalene forms a weak bond (physisorption), azulene engages in a strong chemical bond with substantial charge transfer (chemisorption).

Ether cleavage on silicon is the surface analogue of an SN2 reaction; SN2 reactions represent the textbook example for how to control solution-based chemical reactions by means of steric hinderance or the choice of solvent. In an STM study published in Angewandte Chemie, the team around Gerson Mette and Michael Dürr has now shown that tip-induced ether cleavage on Si(001) leads to additional final products which are not obtained by thermal activation. Moreover, different final products can be selectively addressed by different excitation channels, either direct excitation by electron transfer or multiple excitation of vibrational modes. As the two channels can be selectively addressed by the tunneling bias, a new way of reaction control was achieved.

In parallel, the advances in the theoretical description of these systems are illustrated in a review article by Lisa Pecher and Ralf Tonner. Within the framework of density functional theory, the chemists of A8 successfully applied energy decomposition analysis to extended systems in order to derive bonding concepts for molecules on surfaces. This allows to interpret experimental results and predict new reaction schemes.

Publications

G. Mette, A. Adamkiewicz, M. Reutzel, U. Koert, M. Dürr, and U. Höfer,
Controlling an SN2 reaction by electronic and vibrational excitation ‐ tip‐induced ether cleavage on Si(001)
Angew. Chemie Int. Ed. 58/11 (2019) 3417-3420 DOI: 10.1002/anie.201806777
L. Pecher and R. Tonner,
Deriving bonding concepts for molecules, surfaces, and solids with energy decomposition analysis for extended systems
WIREs Comput. Mol. Sci. (2018) (21pp) DOI:10.1002/wcms.1401

See also joint press release by the universities of Gießen and Marburg under the auspices of the Forschungscampus Mittelhessen ( in German).

Contact

Prof. Dr. Michael Dürr
Justus-Liebig-Universität Gießen
SFB 1083 project A8
Tel.: 0641 993490
EMAIL

Molecular topology critically controls metal-organic interfaces in electronic devices – Publication by A4 (Gottfried), A6 (Tonner), & A12 (Tautz/Bocquet/Kumpf)

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In organic electronic devices, such as modern displays with organic light-emitting diodes (OLEDs), organic materials connect to metal electrodes. The resulting metal–organic interfaces, which are in the focus of SFB 1083, determine important performance parameters such as rates of charge-carrier injection. Precise control over the interface properties, especially the wave-function overlap and the energy-level alignment, is therefore critical for rational improvement of organic electronic devices. Here, the SFB 1083 projects A4, A6 (both Univ. Marburg) and A12 (at FZ Jülich), together with groups in Utrecht (NL), Warwick (UK) and Erlangen (DE), show that the properties of metal-organic interfaces depend strongly on the linking pattern of the atoms in the organic material.

In organic semiconductors, the carbon atoms are typically laid out in a honeycomb-like sheet of abutting six-sided rings. If the sheet contains no odd-numbered rings, it is described as an “alternant topology.” Researchers rarely consider nonalternant topologies, which occur when the structure contains, for example, five- or seven-sided rings. To elucidate the influence of the topology on the interaction with a metal surface, the authors compare the aromatic hydrocarbon naphthalene to its nonalternant isomer, azulene, and study their interactions with a copper surface.

Azulene-like nonalternant 5-7 structural element embedded in a graphene lattice (right), compared to the ideal graphene lattice left. The figure shows sections through the charge density for both systems, according to DFT calculations. The 5-7 element accumulates negative charge (red) at the 5-membered ring and positive charge (blue/white) at the 7-membered ring. Copyright by CC-BY 4.0.

Benedikt Klein and his co-workers find that azulene forms a much stronger and shorter bond to copper than naphthalene. Spectroscopic analysis of the electronic structure reveals that azulene forms a true chemical bond and receives negative charge from the surface, whereas naphthalene bonds only weakly and does not exchange charge. Theoretical analysis reveals that the influence of the topology on the electronic structure, especially the lowest unoccupied molecular orbital, is responsible for the different behavior. This comprehensive analysis of a surface chemical bond was only possible through a multi-technique approach, which involved a collaboration between six research groups from experiment and theory, including three from SFB 1083. Important contributions were made by the groups of Ingmar Swart (Utrecht, NL), Reinhardt Maurer (Warwick, UK), and Wolfgang Hieringer (Erlangen, DE).

Based on their findings, the authors propose that the incorporation of nonalternant structural elements can be used to control and optimize performance-related properties of functional metal–organic interfaces.

Publication

B. P. Klein, N. J. van der Heijden, S. R. Kachel, M. Franke, C. K. Krug, K. K. Greulich, L. Ruppenthal, P. Müller, P. Rosenow, S. Parhizkar, F. C. Bocquet, M. Schmid, W. Hieringer, R. J. Maurer, R. Tonner, C. Kumpf, I. Swart, and J. M. Gottfried, Molecular topology and surface chemical bond: alternant versus nonalternant aromatic systems as functional structural elements, Physical Review X 9/1(2019) 011030 (17pp) DOI:10.1103/PhysRevX.9.011030

Contact

Prof. Dr. Michael Gottfried
Philipps-Universität Marburg
SFB 1083 project A4
Tel.: 06421 28-22541
EMAIL

“Frauenförderpreis 2018” for Prof. Dr. Stefanie Dehnen (A9)

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Professor Dehnen with Prof. Dr. Katharina Krause (president of Philipps-Universität Marburg), Prof. Dr. Carmen Bickle, and Dr. Nina Schumacher (women and equal opportunity officer). (Photo: Henrik Isenberg)

As part of a special event organized for the last day of November, Prof. Dr. Stefanie Dehnen, PI of SFB-project A9 and Professor of Inorganic Chemistry, and Prof. Dr. Carmen Birkle, Professor for American Studies, jointly received the “Frauenförderpreis” of the Philipps-Universität Marburg.

The “Frauenförderpreis” of Philipps-Universität Marburg is awarded every two years since 1998 and worth 2500 EUR. It recognizes Prof. Dr. Stefanie Dehnen and Prof. Dr. Carmen Birkle for their strong mentorship and in the case of Professor Dehnen in particular for her participation as a mentor for early-career female academic staff in the Hessen-wide project “SciMento” and her enduring engagement for a family-friendly research environment.

In balancing a professorship and family-life with four young children, Stefanie Dehnen is living proof that having both, a research career and a family is possible.