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Nanostructures

The subgroup „nanostructures“ has a great interest in the synthesis of novel organic building blocks for the formation of supramolecular networks via self assembly. Over the past few years, the importance of supramolecular assemblies increased significantly, which is illustrated by the impressive and still rapidly increasing number of publications in this field of research. However, when studying previous work on this field, one will notice that most examples are limited to achiral substrates. Within our group we want to overcome this limitation and prepare chiral networks.
The best known application of highly porous structures is located in gas storage. Here, extraordinary pure organic networks which consist exclusively of light elements have a great application potential, in particular in the automobile industry. Porous materials are also widely used in opto-electronics as well as in asymmetric catalysis. New applications like the separation of racemic gases are currently examined.
Based on trityl moieties, novel organic building blocks have been prepared and structurally investigated. Substituted hexaphenyl-p-xylene (1,4-ditritylbenzene) as well as extended analogues thereof were prepared. Furthermore, a new family based on a 1,3,5-tritritylbenzene motif, connecting three trityl groups through a formal mesitylene unit, was developed. Both families were further converted through six- and nine-fold substitution reactions, respectively, to yield potent molecular building blocks for supramolecular assemblies (Figure 1).

SFB 1176 – Molecular Structuring of Soft Matter

The Cooperative Research Centre (SFB) 1176 „Molecular Structuring of Soft Matter“ investigates the design of highly defined polymeric materials along the chain, on or in a plane as well as in three-dimensional space. The application of polymeric materials is primarily defined by their molecular structure. Therefore, the aim of the SFB 1176 is the synthesis of such materials with a so far unmatched degree of structural control in 1, 2 and 3 dimensions, to achieve a precise placement of functionalities, uniformity of the chain length or a high sequence control.

The Bräse group deals with the synthesis of suitable monomers or tectones for the buildup of linear, conjugated polymers (project A4), the functionalization of silicon- and graphite-surfaces for applications in Lithium-Ion-Batteries (project B2), planar triarylamine layers for OLED-applications (project C2), as well as the synthesis of COFs (project C5) and functionalized SURGELs (project C6). As core molecules we utilize derivatives of tetraphenylmethane and adamantane, substituted triphenylbenzenes, triarylamines, biphenyl as well as terphenyl and analogue compounds. For further details regarding the projects or the SFB 1176 in general please visit: http://www.sfb1176.de/

Supramolekulare Überstrukturen

Different tetraphenylmethane- and 1,3,5,7-tetraphenyladamantane cores
Abbildung 1
Tetraphenylmethane- (left), 1,3,5,7-tetrakisphenyladamantane- (middle) and bistritylbenzene skeleton (right) with different sticky-sites R.
Schema 1
Scheme 1: Fourfold click reactions for the efficient synthesis of methane and adamantane building blocks.[2]
Schema 2
Schema 2: Fourfold cross-coupling for the synthesis of rigid methane and adamantane building blocks.[3]
Schema 3
Scheme 3: Fourfold asymmetric 1,2-additions to methane building blocks.[4]

 

Our research focuses on the synthesis of tetrahedral and pseudo octahedral organic building blocks for supramolecular superstructures. Therefore we developed a general method to synthesize simple tetraphenylmethane-, 1,3,5,7-tetraphenyladamantane- and bistritylbenzene frameworks that are functionalized with different sticky-sites afterwards. These sticky-sites are functional groups that can interact with other molecules based on the principle of molecular self assembly. Among others we managed to synthesize the tetraazide derivatives as well as exact measurements for the calculation of their energetic potentials.[1]

In addition we developed a general approach to methane- and adamantane building blocks based on the Huisgen 1,3-dipolar cycloaddition, a click reaction (scheme 1).[2]

The same strategy has been applied in different organometallic cross-couplings and also enabled interesting tetrahedral building blocks for supramolecular frameworks efficiently (scheme 2).[3]

Our group is one of the first to perform fourfold asymmetric 1,2-additions at appropriate precursors to obtain peripheral-chiral building blocks (scheme 3).[4] These building blocks assemble to enantiomeric pure networks.

 


Verschiedene Bistritylbenzol-Cores

Schema 4
Scheme 4: Synthesis of functionalized bistritylbenzene derivatives.
Schema 5
Scheme 5: Synthesis of functionalized tristritylbenzene derivatives.

Following the work on the tetrahedral cores we could transfer the synthetic strategy of tetraphenylmethane onto the so far barely mentioned bistritylbenzene (scheme 4) and therefore provide a new basic building block for the synthesis of covalent organic frameworks. In this regard we could synthesize various bistritylbenzene derivatives that are used as direct precursors for porous organic frameworks.[5][(Plietzsch, Schade et al. 2013)]

Further on, some tristritylbenzene derivatives (scheme 5) that are very suitable for the synthesis of three-dimensional frameworks were obtained.


Supramolekulare Netzwerke

Abbildung 2
Figure 2: Three different 3D DNA building block hybrids.[6,7]
Abbildung 3
Figure 3: Calculated diamond-PPN network with a porosity of 88–94%.[8]
Schema 6
Scheme 6: Hyper-cross-linked-polymers through the application of click chemistry.[9]
Schema 7
Scheme 7: Synthetic strategy for HPCs with bistritylbenzene cores.

In collaboration with Prof. Clemens Richert (University of Stuttgart) we prepared methane-, adamantane- and hexakis(p-hydroxyphenyl)xylene building blocks that contain DNA-chains connected through phosphate- or triazole groups (figure 2). Through Watson-Crick base pairing a new material was obtained with the highest melting point for DNA-nanostructures reached so far.[6,7]

In collaboration with Prof. Hong-Cai Zhou (College Station, Texas) we prepared a series of porous polymer networks (PPNs) via Yamamoto and Eglinton coupling. The structures based on methane- and adamantane monomers show good selectivity for the storage of methane carbon dioxide mixtures (figure 3).[8]

In collaboration with Dr. Angiolina Comotti and Prof. Piero Sozzani (Università degli Studi di Milano Bicocca) we prepared hyper-cross-linked-polymers (HCPs) via click chemistry (scheme 6).[9] The network based on adamantane shows good carbon storage capacities at low pressure.

Although click chemistry found big applications in biochemistry and polymer chemistry this is one of the first porous purely organic networks that were generated by click chemistry.[10,11]

Most recently it was possible to obtain three-dimensional porous networks with some bistritylbenzene derivatives after appropriate optimization of the reaction conditions by using the mentioned synthetic strategy for the tetrahedral building blocks (scheme 7) and transferring this strategy onto the pseudo octahedral equivalent. With the Huisgen 1,3-dipolar cycloaddition we obtained three HCPs with a BET-surface of up to 725 m2/g.[12]

Future studies should expand our works in the field of porous materials. These three-dimensional networks are possible to store gases like CO2 selectively, to separate gas mixtures or to serve as catalysts.


SURMOFs, PSM and SURGELs

Control of growth and properties of structures on a length scale down to molecular dimensions is one of the major challenges in nanotechnology. Metal-organic frameworks (MOFs) which are coordination polymers consisting of organic ligands linked together by metal ions, are very promising systems due to the virtually unlimited flexibility in their design.
In addition to properties of the framework itself (electrochromic, magnetic, and storage materials), MOFs can be loaded with other molecular compounds by employing a guest-host chemistry which increases the technological potential in a variety of different fields, in particular with respect to catalysis and hydrogen storage.

SURMOFs
Abbildung 4
Figure 4: Schematic representation of LPE process for the growth of SURMOFs.
Further applications emerge when MOFs are attached to surfaces. Surface-mounted metal-organic frameworks (SURMOFs) are obtained via an innovative and efficient approach by using surfaces to initiate and control the growth of MOFs (Figure 4). The method is based on epitaxially grown metal−organic frameworks deposited on modified substrates using liquid-phase epitaxy (LPE).

PSM

The properties of MOFs relevant for their various applications, such as storage, separation, shape selective catalysis, and molecular recognition, can be tuned and optimized in a rather straightforward fashion. For these cases, the so-called post-synthetic modification (PSM) offers an alternative, because the addition of the target function is carried out after the MOF lattice is formed. In particular, numerous studies have shown that MOFs built from organic linkers exhibiting functional groups, such as −NH2, −OH, −NO2, and −N3, are well-suited for the PSM process.

SURGELs
Abbildung 5
Figure 5: Cross-linking process illustration during SURGELs synthesis.

The surface-grafted gels (SURGELs) are an interesting class of 3D, highly porous, covalently bound polymer films, obtained by cross-linking the organic struts within a SURMOF (Figure 5). Combining the advantages of the MOF class of materials with that of a covalently connected gel, they offer enormous variability for introducing functional groups into the framework while featuring a pronounced stability against water and the absence of metal ions. These SURGELs can be loaded with bioactive compounds and applied as bioactive coatings and provide a drug-release platform in in vitro cell culture studies.


Literatur

[1] C. I. Schilling, S. Bräse, Org. Biomol. Chem. 2007, 5, 3586–8. Stable organic azides based on rigid tetrahedral methane and adamantane structures as high energetic materials.

[2] O. Plietzsch, C. I. Schilling, M. Tolev, M. Nieger, C. Richert, T. Muller, S. Bräse, Org. Biomol. Chem. 2009, 7, 4734–43. Four-fold click reactions: Generation of tetrahedral methane- and adamantane-based building blocks for higher-order molecular assemblies.

[3] C. I. Schilling, O. Plietzsch, M. Nieger, T. Muller, S. Bräse, European J. Org. Chem. 2011, 2011, 1743–1754. Fourfold Suzuki-Miyaura and Sonogashira Cross-Coupling Reactions on Tetrahedral Methane and Adamantane Derivatives

[4] O. Plietzsch, C. I. Schilling, M. Nieger, T. Muller, S. Bräse, Tetrahedron: Asymmetry 2010, 21, 1474–1479. Asymmetric synthesis of chiral tectons with tetrapodal symmetry: fourfold asymmetric reactions

[5] O. Plietzsch, A. Schade, A. Hafner, J. Huuskonen, K. Rissanen, M. Nieger, T. Muller, S. Bräse, European J. Org. Chem. 2013, 2013, 283–299. Synthesis and Topological Determination of Hexakis-Substituted 1,4-Ditritylbenzene and Nonakis-Substituted 1,3,5-Tritritylbenzene Derivatives: Building Blocks for Higher Supramolecular Assemblies

[6] M. Meng, C. Ahlborn, M. Bauer, O. Plietzsch, S. a Soomro, A. Singh, T. Muller, W. Wenzel, S. Bräse, C. Richert, Chembiochem 2009, 10, 1335–9. Two base pair duplexes suffice to build a novel material.

[7] A. Singh, M. Tolev, M. Meng, K. Klenin, O. Plietzsch, C. I. Schilling, T. Muller, M. Nieger, S. Bräse, W. Wenzel, et al., Angew. Chem. Int. Ed. Engl. 2011, 50, 3227–31. Branched DNA that forms a solid at 95 °C.

[8] W. Lu, D. Yuan, D. Zhao, C. I. Schilling, O. Plietzsch, T. Muller, S. Bra¨se, J. Guenther, J. Blu¨mel, R. Krishna, et al., Chem. Mater. 2010, 22, 5964–5972. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation

[9] O. Plietzsch, C. I. Schilling, T. Grab, S. L. Grage, A. S. Ulrich, A. Comotti, P. Sozzani, T. Muller, S. Bräse, New J. Chem. 2011, 35, 1577. Click chemistry produces hyper-cross-linked polymers with tetrahedral cores

[10] T. Muller, S. Bräse, Angew. Chemie 2011, 123, 12046–12047. Klick-Chemie findet ihren Weg in kovalente poröse organische Materialien

[11] T. Muller, S. Bräse, Angew. Chem. Int. Ed. Engl. 2011, 50, 11844–5. Click chemistry finds its way into covalent porous organic materials.

[12] A. Schade, L. Monnereau, T. Muller, S. Bräse, ChemPlusChem 2014, in press. Hexaphenyl-p-xylene, a rigid pseudo-octahedral core for 3D porous frameworks


Weitere Literatur:

T. Muller, S. Bräse, RSC Advances 2014, 4, 6886-6907. Tetraedral organic molecules as monomers for porous supramolecular networks.

L. Monnereau, M. Nieger, T. Muller, S. Bräse, Adv. Funct. Mat. 2014, 24, 1054-1058. Tetrakis(4-thiylphenyl)methane: Origin of a Reversible 3D-Polymer

M. Tsotsalas, J. Liu, B. Tettmann, S. Bräse, S. Grosjean, A. Shahnas, Z. Wang, C. Azucena, M. Addicoat, T. Heine, J. Lahann, J. Overhage, H. Gliemann, C. Wöll, J. Am. Chem. Soc. 2014, 136, 8-11. Fabrication of highly uniform gel-coatings by conversion of surface-anchored metal-organic framework

Z. Wang, H. K. Arslan, J. Liu, S. Grosjean, T. Hagendorn, H. Gliemann, S. Bräse, C. Wöll, Langmuir 2013, 15958-15964. Post-synthetic modification of molecular frameworks using click chemistry: the importance of strained C-C triple bonds

Jinxuan Liu, Binit Lukose, Osama Shekhah, Hasan Kemal Arslan, Peter Weidler, Hartmut Gliemann, Stefan Bräse, Sylvain Grosjean, Adelheid Godt, Xinliang Feng, Klaus Müllen, Ioan-Bogdan Magdau, Thomas Heine & Christof Wöll, Sci. Rep. 2012, 2, 921. A novel series of isoreticular metal-organic frameworks: realizing metastable structures by liquid phase epitaxy.

Lars Heinke, Murat Cakici, Marcel Dommaschk, Sylvain Grosjean, Rainer Herges, Stefan Bräse, and Christof Wöll, ACS Nano 2014, 8, 1463-1467. Photoswitching in Two-Component Surface Mounted Metal-Organic Frameworks: Optically Triggered Release from a Molecular Container.

Mitglieder

Dr. Christin Bednarek
Dr. Sylvain Grosjean
Dr. Ksenia Kutonova
Mathias Lang
Yannick Matt
Isabelle Wessely

Frühere Mitglieder

Philipp Beyler
Christoph Hussal
Dr. Laure Monnereau
Dr. Thierry Muller
Dr. Philippe Pierrat
Dr. Oliver Plietzsch
Dr. Céline Réthoré
Dr. Alexandra Schade
Dr. Christine I. Schilling
Dr. Stefan Seifermann