Dissipative self-assembly of redox-responsive porous supramolecular networks

Abstract

Porous materials, such as Metal-Organic Frameworks (MOFs), Covalent-Organic frameworks (COFs), and Supramolecular Organic Frameworks (SOFs), are of great scientific interest for a variety of applications such as molecular recognition, sensing, transport, separation, and catalysis. MOFs and COFs are crystalline materials, showing porosity in the solid state, and they are typically insoluble and/or instable in solution, which hampers their applications in homogeneous media. In contrast, SOFs are soluble porous network structures formed by solution self-assembly of opportunely designed building blocks. Since SOFs exhibit their porosity in solution, they are more suitable for applications in drug delivery and energy related fields. The formation of supramolecular structures and assembled materials is due to molecular self-assembly based on non-covalent interactions. In dissipative self-assembly (DSA), structures and materials are formed far from thermodynamic equilibrium through the constant exchange of energy and matter with the external environment. In fact, in DSA, the formation of the assembled state is not spontaneous, but requires the activation (and deactivation) of the building blocks by a supply of energy, often provided by the addition of chemical fuels. The ultimate goal of our research project is to develop novel porous supramolecular networks formed from ferrocene-functionalised zinc porphyrins and β-CD dimers (acting as ditopic linker molecules), which can interact thanks to β-CD/ferrocene host-guest interactions. The aim is to exploit the redox-responsive nature of this host-guest interaction to implement DSA in these systems, where the disassembly/assembly of the network can be controlled through the addition of redox chemical fuels. To this aim, we first prepared a novel Fc-functionalised Zn-porphyrin through convergent synthesis and using ‘click’ chemistry. Then we studied the assembly behaviour in aqueous solution of this porphyrin by UV-Vis and DLS, both alone and in mixture with β-CD or β-CD dimer. In addition, as a comparison, we studied the assembly behaviour of another porphyrin (previously synthesized), bearing longer spacer groups. The results showed that the porphyrin we synthesized self-assembles into larger aggregate structures than the porphyrin with longer spacers, and, in the presence of the β-CD dimer, forms a population of aggregates with a higher hydrodynamic diameter. However, further studies need to be carried out to determine if this observation can be ascribed to the formation of extended supramolecular networks