On-surface polymerization of organic monomers into covalent networks
M Lackinger et al 2011 J. Phys. D. Appl. Phys. 44 464011
One- and two-dimensional surface-supported molecular structures have been extensively studied by different complementary surface science techniques over the last two decades. Those systems are typically stabilized by various types of different intermolecular interactions such as van-der-Waals, hydrogen bonds, and metal coordination. For many systems, these comparatively weak and thus reversible bonds promote the emergence of long-range ordered networks. Yet, the weakness of intermolecular bonds can also be seen as a disadvantage, since many applications require mechanically and thermally robust systems or demand strong electronic coupling between the molecular building blocks. All those requirements can be fulfilled by covalently interlinking single molecules into large and extended super-molecules. Carrying out these reactions directly on a crystalline substrate in a surface chemical approach offers many advantages, such as for instance using surface material and crystallographic orientation as additional parameters to control and template the polymerization reaction. On the other hand, the surface chemical approach poses many new research questions and the reaction mechanisms are often not very well understood.
In summary, on-surface polymerization of monomers into covalently interlinked networks is an emerging, highly dynamic research branch with extremely exciting possibilities and new challenges.
The aim of this review is to provide an overview over recent research efforts described in the literature. Different synthetic approaches employ different types of reactions and are also carried out in different experimental environments. For instance radical addition and condensation reactions are both utilized for the synthesis and offer both different advantages and disadvantages to the surface chemist. Because the irreversibility and strength of newly formed bonds renders defect correction, either during growth or in a post-processing procedure, very intricate, a main challenge in the field is the synthesis of long-range ordered systems with low defect density and large domain size. Since many preparation protocols yield covalently interlinked but poorly ordered structures, the STM as a high-resolution real space technique is particularly useful.
Images: (above left) Covalent networks from condensation [1] and (right) covalent networks from radical addition[2].
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A STM perspective on covalent intermolecular coupling reactions on surfaces
M Lackinger et al 2011 J. Phys. D. Appl. Phys. 44 464011
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About the author
Dr Markus Lackinger and his experimental research group study surface-supported molecular nanostructures in a joint venture between the Technical University Munich and the Deutsches Museum, where the labs are located. Besides being the largest science and technology museum in Germany, the Deutsches Museum is also a research institution with original nanoscience as a new research branch. The research group's main analytical tool and field of expertise is scanning tunneling microscopy both at the liquid-solid interface and also under ultrahigh-vacuum conditions. Over recent years, covalently interlinked organic structures have become an important research focus. In addition, the group is interested in supramolecular self-assembly with focus on porous systems and host-guest chemistry, and develops scientific instruments, such as scanning probe microscopes, preparation tools, and deposition sources.
Images: Dr Markus Lackinger (above left) and Prof. Dr Wolfgang M Heckl (above right).
Group website: http://www.2d-materials.com/
[1] Reprinted with permission from (N. A. A. Zwaneveld et al 2008 J. Am. Chem. Soc., 130, 6678-6679.). Copyright (2008) American Chemical Society.
[2]M. Bieri et al 2009 Chem. Commun.,45 6919-21. Reproduced by permission of The Royal Society of Chemistry.
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