ABSTRACT
A two-dimensional (2D) covalent monolayer based on [4 + 4] cycloaddition reactions between adjacent anthracene units was synthesized at an air/water interface. For structural analysis, tip-enhanced Raman spectroscopy (TERS) provides direct evidence for the covalent bonds formed between monomer molecules. For the first time, progress of the photopolymerization reaction was monitored by irradiation (λ = 385 nm) of the monomer monolayer for different times, based on averaged TER spectra extracted from maps. In addition, a 2D polymerization on a Au (111) substrate was realized, which opens up new possibilities for such chemical transformations. This work uses TERS as a minimally invasive tool to investigate how the reaction conditions affect polymerization conversion. We show that the high sensitivity and the high spatial resolution of TERS can be used to estimate the crystallinity of 2D covalent monolayers, which is a key question in polymer synthesis.
ABSTRACT
In this work we prepare Langmuir-Blodgett monolayers with a trifunctional amphiphilic anthraphane monomer. Upon spreading at the air/water interface, the monomers self-assemble into 1 nm-thin monolayer islands, which are highly fluorescent and can be visualized by the naked eye upon excitation. In situ fluorescence spectroscopy indicates that in the monolayers, all the anthracene units of the monomers are stacked face-to-face forming excimer pairs, whereas at the edges of the monolayers, free anthracenes are present acting as edge groups. Irradiation of the monolayer triggers [4 + 4]-cycloadditions among the excimer pairs, effectively resulting in a two-dimensional (2D) polymerization. The polymerization reaction also completely quenches the fluorescence, allowing to draw patterns on the monomer monolayers. More interestingly, after transferring the monomer monolayer on a solid substrate, by employing masks or the laser of a confocal scanning microscope, it is possible to arbitrarily select the parts of the monolayer that one wants to polymerize. The unpolymerized regions can then be washed away from the substrate, leaving 2D macromolecular monolayer objects of the desired shape. This photolithographic process employs 2D polymerizations and affords 1 nm-thin coatings.
ABSTRACT
In this work we present one of the rare cases of single-crystal-to-single-crystal (SCSC) linear polymerizations, resulting in a novel ladder-type polymer. The polymerization is based on the photoinduced [4+4]-cycloaddition reactions between trifunctional anthracene-based monomers. The careful design of the monomer anthraphane-tri(OMe), results in perfectly stacked anthracene pairs in the crystal structure, with Schmidt's distances d=3.505-3.666â Å and shift s=1.109â Å, allowing a selective linear polymerization in quantitative yields and in a matter of minutes, without compromising the integrity of the single crystals. The obtained polyanthraphane-tri(OMe), reveals moreover a very interesting and unprecedented case of stereoisomerism, which is characteristic for polyanthraphanes.
ABSTRACT
Polymer chemistry has recently welcomed a new addition to its field: the planar macromolecules known as two-dimensional polymers (2DPs). These topologically planar and crystalline monolayer covalent sheets are reminiscent of molecular fishermen's nets and apart from being conceptually very interesting for the field of macro-molecular chemistry, they also show some potential applications as novel 2D materials. This article reviews how the field has developed five years after the first 2DP was synthesised in 2012. After a brief historical introduction, the main synthetic approaches will be discussed providing concrete examples of 2DPs and highlighting the challenges associated with the synthesis and especially structural characterisation of these fascinating macro-molecules. Finally an overview on their potential applications such as membranes for gas separation, rewritable molecular paper and miniaturisation of optical devices will be presented.
ABSTRACT
The novel hydrocarbon propeller-shaped D3h-symmetric cyclophane (3), "anthraphane", was prepared through a revisited and optimized gram-scale synthesis of the key building block anthracene-1,8-ditriflate 7. Anthraphane has a high tendency to crystallize and single crystals in size ranges of 100-200 µm are easily obtained from different solvents. The crystallization behavior of 3 was extensively studied to unravel packing motifs and determine whether the packing can be steered into a desired direction, so to allow topochemical photopolymerization. SC-XRD shows that anthraphane packs in layers irrespective of the solvent used for crystallization. However, within the layers, intermolecular arrangements and π-π interactions of the anthracene units vary strongly. Four interaction motifs for the anthracene moieties are observed and discussed in detail: two types of exclusively edge-to-face (etf), a mixture of edge-to-face and face-to-face (ftf), and no anthracene-anthracene interaction at all. To elucidate why an exclusive ftf stacking was not observed, electrostatic potential surface (EPS) calculations with the semiempirical PM3 method were performed. They show qualitatively that the anthracene faces bear a strong negative surface potential, which may be the cause for this cyclophane to avoid ftf interactions. This combined crystallographic and computational study provides valuable insights on how to create all-ftf packings.
ABSTRACT
Synthetic covalent monolayer sheets and their subclass, two-dimensional polymers are of particular interest in materials science because of their special dimensionality which renders them very different from any bulk matter. However, structural analysis of such entities is rather challenging, and there is a clear need for additional analytical methods. The present study shows how tip-enhanced Raman spectroscopy (TERS) can be performed on monomer monolayers and the covalent sheets prepared from them by [4 + 4]-cycloaddition to explore rather complex structural and mechanistic issues. TERS is a surface analytical method that combines the high lateral resolution of scanning probe microscopy (SPM) with a greatly enhanced Raman scattering intensity. The high spatial resolution (<60 nm) and the significantly improved sensitivity (contrast factor of >4000) compared to confocal Raman microscopy provides new insights into the formation of this new and exciting material, namely significant consumption of the reactive units (anthracenes) and exclusion of the alternative [4 + 2]-cycloaddition. Moreover, due to the high lateral resolution, it was possible to find a first spectroscopic hint for step growth as the dominant mechanism in the formation of these novel monolayer sheets. In addition, TERS was used to get first insights into the phase behavior of a comonomer mixture.
ABSTRACT
By using structurally similar amphiphilic monomers, it is shown that compressed monolayers of varying amounts of such monomers at the air/water interface can be converted by photo-irradiation into the corresponding covalently connected monolayer sheets. Since one of the monomers carries three anthracene units and the other three 1,8-diaza-anthracene units, the growth reaction is proposed to take place through photochemically achieved [4+4]-cycloaddition between pairs of these units that are co-facially (face-to-face) arranged, to furnish the corresponding covalent dimers. While evidence for both homodimers is amply available, the existence of the heterodimer needs to be established with the help of a model reaction to support the conceptual aspect of this work, copolymerization in two dimensions. The sheet copolymers exhibit substantial robustness in that they can be spanned over 20 × 20 µm(2)-sized holes without rupturing under their own weight. X-ray photoelectron spectroscopy (XPS) studies reveal that the monomers are incorporated into the sheet copolymers according to feed. These results establish existence of the first covalent sheet copolymer, which is considered a step ahead towards novel 2D materials.
Subject(s)
Anthracenes/chemistry , Aza Compounds/chemistry , Photochemical Processes , Polymers/chemistry , Cyclization/radiation effects , Dimerization , Microscopy, Electron, Scanning , Molecular Structure , Photoelectron Spectroscopy , Surface Properties/radiation effects , ThermodynamicsABSTRACT
Vapor-phase post-synthetic modification (VP-PSM) is herein described. VP-PSM is a tool that overcomes limitations of standard PSM methods by giving a higher yield in short reaction times and will give more flexibility in designing metal-organic frameworks with functionalization for chemical and physical applications.
