Fabrication of tailor-made 2D crystalline and freestanding supramolecular- and metal-organic networks : use of interfacial assembly of amphiphilic calix[4]arenes

This thesis reports new strategies for the fabrication of two-dimensional (2D) free-standing and crystalline tailor-made organic networks via bottom-up assembly of calix[4]arene organic building blocks. Three major areas of work were conducted in this project: 1- Formation of novel 2D metal-organic coordination networks (MOCNs) of calix[4]arene amphiphiles, i.e. a 2D MOCN of C3-p-carboxy-calixarene. 2- Fabrication of the first reported free-standing and crystalline linker-free 2D supramolecular organic network (SON) through dipole-dipole interactions between C3-p-me-cyano-calixarene building units. 3- In-situ synthesis of a bilayered MOCN of C3-p-me-carboxy-calixarene synthons with control over the degree of bilayer formation. The physical and chemical properties of 2D atomically ordered organic and inorganic architectures, including strong in-plane and weak out-of-plane bonding, are unique and distinct to those of the corresponding 3D structures. Thus, 2D materials have both fundamental and applied importance to various applications ranging from superconductors to catalysis.1 Rational design and synthesis of 2D hybrid frameworks that enables control over the resulting structures and functionalities can be achieved by applying the concepts of supramolecular and reticular chemistry.2 Numerous 2D supramolecular organic networks have been produced by taking advantage of non-covalent interactions between organic building units. In addition to supramolecular interactions, coordination bonds have also been employed to link organic building blocks and single metal/metal complex nodes to synthesize 2D architectures, namely 2D MOCNs.3 A wide range of possible organic and inorganic building blocks that possess coordination centers and electron donator/acceptor properties and containing various selective functional groups and catalytic centers have been used to fabricate 2D architectures with desirable structures, properties and functions.3 Several bottom-up techniques for the fabrication of 2D materials have been reported, including wet chemistry synthesis such as direct crystal growth from solvothermal solution, chemical vapor deposition, surfactant-directed synthesis and interfacial self-assembly reactions.2 In bottom-up fabrication methods, the interfaces - where the reaction happens - plays a pivotal role in the nucleation and growth of 2D materials. Interfacial self-assembly is d iii riven by reduction of the interfacial energy between the media.4 Assembly of nanomaterials at interfaces with various geometries (e.g. nanosheets) has led the development of advanced applications in many sectors such as polymer-based photovoltaic devices, magnetic data storage media, polymer composites, oil and mineral refining, coatings, detergents and pharmaceuticals.5 In spite of the great progress in the field of 2D organic networks, there is a lack of practical and versatile strategies to design, fabricate and characterize free-standing and crystalline 2D nano-architectures. The research work described in this thesis led to development of a new approach to produce free-standing and crystalline 2D organic networks of calix[4]arene amphiphilic macrocycles. This work demonstrates 2D organic networks with specific desirable properties and functions can be fabricated by tailoring the structure of the amphiphilic calix[4]arene backbone. Indeed, this technique provides precise control over the organization of the molecular building blocks down to the atomic level, i.e. enables molecular design. The first chapter of this thesis reports the first example of a 2D MOCN of calix[4]arene organic building blocks with tunable crystallinity. We show that carefully functionalized calix[4]arene macrocycles containing carboxy functional groups at the upper rim and propyl chains at the lower rim form a free-standing and crystalline MOCN upon coordination with Cu2+ ions at the air-liquid interface. The second chapter provides the first report of linker-free 2D SON that is stabilized via synergistic interactions of the dipole-dipole forces between calix[4]arene derivative building blocks. A functionalized calix[4]arene synthon bearing methyl cyano functional groups at the upper rim and propyl chains at the lower rim was designed. This amphiphilic calix[4]arene self assembles and forms a crystalline 2D SON at the air-water interface. The crystalline layer can be transferred from the interface onto a solid substrate, while maintaining the homogeneity and crystallinity of the layer. Furthermore, free-standing layers of the 2D SON exhibit high stability and can span perforated substrates with pores as large as 3 × 3 µm. Electron diffraction analysis confirmed the crystallinity of the free-standing 2D SON. The in-situ formation of a bilayered MOCN, at the air-liquid interface, from a functionalized calix[4]arene carrying methyl carboxy groups at the upper rim and propyl chains at the lower rim is reported in the third chapter. Calix[4]arene derivatives are shown to form monolayers or bilayers upon changing the transition metal ion in the subphase. In the presence of Cu2+ ions, a free-standing and crystalline MOCN bilayer forms, in which the degree of bilayer iv formation can be kinetically controlled. Substituting the Cu2+ ions with Ni2+ ions results in formation of a crystalline and free-standing monolayer of MOCN. These observations confirmed that the conformational freedom and flexibility of the modified calix[4]arene building components enable adoption of different coordination geometries in the MOCN, depending on the transition metal ion employed. These highly stable 2D calix[4]arene-based organic networks can potentially be applied as a new generation of sample supports for transmission electron microscopy imaging and X-ray analysis of single biomolecules/assemblies and single-magnetic nanoparticles. Indeed, some examples of biomolecule, assembly and nanoparticle deposition experiments on free-standing coordination networks of calix[4]arene amphiphiles are presented in this thesis. Further work is currently underway to extend this research and define advanced methods to enhance the structural analysis of single biomolecules and the magnetic properties of nanoparticles deposited on free-standing monolayers of calixarenes and related compounds. Keywords: supramolecular chemistry, reticular chemistry, 2D metal-organic coordination network, 2D supramolecular-organic network, self-assembly, Langmuir-Blodgett technique, calix[4]arene amphiphile, amphiphilic bilayers.