Molecular weaving via surface-templated epitaxy of crystalline coordination networks

One of the dream reactions in polymer chemistry is the bottom-up, self-assembled synthesis of polymer fabrics, with interwoven, one-dimensional fibres of monomolecular thickness forming planar pieces of textiles. We have made a major step towards realizing this goal by assembling sophisticated, quadritopic linkers into surface-mounted metal-organic frameworks. By sandwiching these quadritopic linkers between sacrificial metal-organic framework thin films, we obtained multi-heteroepitaxial, crystalline systems. In a next step, Glaser-Hay coupling of triple bonds in the quadritopic linkers yields linear, interwoven polymer chains. X-ray diffraction studies revealed that this topochemical reaction leaves the MOF backbone completely intact. After removing the metal ions, the textile sheets can be transferred onto different supports and imaged using scanning electron microscopy and atomic-force microscopy. The individual polymer strands forming the two-dimensional textiles have lengths on the order of 200 nm, as evidenced by atomic-force microscopy images recorded from the disassembled textiles

Synthesis of Functionalized Azobiphenyl- and Azoterphenyl- Ditopic Linkers : Modular Building Blocks for Photoresponsive Smart Materials

Modular synthesis of structurally diverse functionalized azobiphenyls and azoterphenyls for the realization of optically switchable materials has been described. The corresponding synthesis of azobiphenyls and azoterphenyls by stepwise Mills/Suzuki-Miyaura cross-coupling reaction, proceeds with high yields and provides facile access to a library of functionalized building blocks. The synthetic methods described herein allow combining several distinct functional groups within a single unit, each intended for a specific task, such as 1) the -N=N- azobenzene core as a photoswitchable moiety, 2) aryls and heteroaryls, functionalized with carboxylic acids or pyridine at its peripheries, as coordinating moieties and 3) varying substitution, size and length of the backbone for adaptability to specific applications. These specifically designed azobiphenyls and azoterphenyls provide modular bricks, potentially useful for the assembly of a variety of polymers, molecular containers and coordination networks, offering a high degree of molecular functionality. Once integrated into materials, the azobenzene system, as a side group on the organic linker backbone, can be exploited for remotely controlling the structural, mechanical or physical properties, thus being applicable for a broad variety of 'smart' applications.Peer reviewe

Crystalline assembly of perylene in metal–organic framework thin film: J-aggregate or excimer? Insight into the electronic structure

The spatial orientation of chromophores defines the photophysical and optoelectronic properties of a material and serves as the main tunable parameter for tailoring functionality. Controlled assembly for achieving a predefined spatial orientation of chromophores is rather challenging. Metal–organic frameworks (MOFs) are an attractive platform for exploring the virtually unlimited chemical space of organic components and their self-assembly for device optimization. Here, we demonstrate the impact of interchromophore interactions on the photophysical properties of a surface-anchored MOF (SURMOF) based on 3,9-perylenedicarboxylicacid linkers. We predict the structural assembly of the perylene molecules in the MOF via robust periodic density functional theory calculations and discuss the impact of unit topology and π-π interaction patterns on spectroscopic and semiconducting properties of the MOF films. We explain the dual nature of excited states in the perylene MOF, where strong temperature-modulated excimer emission, enhanced by the formation of perylene J-aggregates, and low stable monomer emission are observed. We use band-like and hopping transport mechanisms to predict semiconducting properties of perylene SURMOF-2 films as a function of inter-linker interactions, demonstrating both p-type and n-type conduction mechanisms. Hole carrier mobility up to 7.34 cm2/Vs is predicted for the perylene SURMOF-2. The results show a promising pathway towards controlling excimer photophysics in a MOF while controlling charge carrier mobility on the basis of a predictive model

Influence of molecular structure on phase transitions: A study of self-assembled monolayers of 2-(aryl)-ethane thiols

Self-assembled monolayers (SAMs) prepared on Au(111) substrates from solutions of omega-(4'-methylbiphenyl-4-yl)ethane thiol (CH3(C6H4)(2)(CH2)(n)SH, n = 2, BP2), at room temperature and subsequently annealed at temperatures of up to 423 K were studied using scanning tunneling microscopy, low-energy electron diffraction, high-resolution X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy. Upon annealing a phase transition occurs from the low-temperature (5 root 3 x 3) structure common to all SAMs prepared from the series of BPn homologues with n = even studied so far, to a new structure which is markedly different from the high-temperature phases of the higher BPn homologues. Although its basic structure can be approximated by a (2 root 3 x 2) unit cell, the regular occurrence of line defects running exclusively along the 11 (2) over bar direction is the most characteristic feature of this new phase. Irrespective of these defects the phase transition dramatically improves the stability of the BP2 monolayer as demonstrated by exchange experiments. In contrast to BP2, SAMs made from the closely related 2-phenylethane thiol (C6H5(CH2)(2)SH, P2) do not show any phase transition. The differences between BP2, its higher homologues, and P2 highlight the subtleties of the interplay of different factors determining the structure of a SAM.</p

Rational Design of Two-Dimensional Nanoscale Networks by Electrostatic Interactions at Surfaces

The self-assembly of aromatic carboxylic acids and cesium adatoms on a Cu(100) surface at room temperature has been investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy. The highly ordered molecular nanostructures are comprised of a central ionic coupling motif between the anionic carboxylate moieties and Cs cations that generate distinctive chiral arrangements of the network structures. The primary electrostatic interaction results in highly flexible bond lengths and geometries. The adsorbate-substrate coupling is found to be important for the determination of the structures. With the use of rod-like carboxylic linker molecules, the dimension of the porous networks can be tuned through the variation of the aromatic backbone length