Marco Servalli

Approaching two-dimensional copolymers: photoirradiation of anthracene- and diaza-anthracene-bearing monomers in Langmuir monolayers.

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.

Minimally invasive characterization of covalent monolayer sheets using tip-enhanced Raman spectroscopy.

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.

Synthesizing molecular fishing nets

The power of organic chemistry is opening new possibilities for building customized two-dimensional materials.

Challenges and Perspectives in the Macromolecular Flatland

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 fishermens 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.

Single-Crystal-to-Single-Crystal (SCSC) Linear Polymerization of a Desymmetrized Anthraphane

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 Schmidts distances d=3.505-3.666 Å and shift s=1.109 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.

Photochemical Creation of Covalent Organic 2D Monolayer Objects in Defined Shapes via a Lithographic 2D Polymerization

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.

Anthraphanes: a New Class of Potential Monomers for the Synthesis of Two-Dimensional Polymers

XIV Abstract Since the isolation of graphene in 2004, a single atom-thick molecular sheet of carbon, the research field on this revolutionary natural 2D material has literally exploded, revealing its peculiar and outstanding properties such as its enormous tensile strength and electrical conductivity. These interesting properties are the result of the molecular structure of graphene and particularly its confinement in two dimensions. Graphene is thus expected to gain a huge societal impact. Considerable interest has also arisen for other 2D materials such as hexagonal boron nitride (h-BN), with its structure analogous to graphene, and inorganic metal chalcogenides such as MoS2, WS2, MoSe2 and NbSe2. However, the aforementioned 2D materials are mostly of inorganic nature and therefore lack the versatility that organic chemistry can offer, in terms of functionalities and chemical modification. The synthesis of organic 2D polymers by mild recipes was first achieved in 2012 and since then, considerable interest in the field has developed. They are defined as free-standing, macromolecular sheets with one-monomer-unit thickness and a periodical internal structure. However, their synthesis can be very challenging, in fact, four years later, the reported cases of successfully synthesised 2D polymers can still be counted on the fingers of one hand. The biggest synthetic challenge relies in having a controlled growth reaction confined into two dimensions and a periodical polymeric structure. The two successful methods for synthesising 2D polymers rely into the preorganisation and polymerisation of the monomers in layered single crystals and pre-organisation and polymerisation at the air/water interface. As the field of 2DPs is still in its infancy, there is a need for new monomer and polymer systems, to widen the field and to better understand the potential properties and applications of these new materials. In particular, it is desirable to have a versatile monomer structure, which can be employed for both the single crystal and the air/water interface approach, so that a direct comparison of the two methods can be done in terms of feasibility, structural perfection of the polymer obtained and its characterisation. For this purpose, a novel anthracene-based monomer’s family was designed and synthesised: the “anthraphanes” 1, 2 and 3. These monomers all share the same basic skeletal structure, but according to their functionalities they can be in principle used for the single crystal approach and/or the air/water interface approach. Chapter 1 provides a general introduction to 1D and 2D polymers and reviews the currently available synthetic 2D polymer systems, as well as the methods to prepare them. General criteria for designing monomers for the synthesis of 2D polymers will also be discussed.