Conjugated Metal Complex Nanosheets: Rational Design, Bottom-up Synthesis, Characterization and Application Studies

Nanosheet materials have attracted significant attention because of their unique electronic and physical properties, which derive from their two-dimensional (2D) nature. Graphene and other 2D inorganic sheet assemblies of metal chalcogenides (e.g. TiO2, TiNbO5, MoS2, WS2, etc.) are prominent nanosheet materials which are widely exploited in electronics, photonics, and spintronics technologies, because 2D materials possess close affinity with those used in the silicon-based electronics. Most of these nanosheets have been synthesized via exfoliation of bulk layered materials, which is a top-down method. In contrast, “bottom-up” nanosheets are generally synthesized directly from organic compounds, ligands, and/or metal ion connectors. Such examples include surface metal- organic frameworks and covalent organic frameworks. Their composition, structure, and properties can be tuned at will by tailoring the constituent components used. This is a strong point of using bottom-up nanosheet against the top-down congener. However, application studies of the bottom-up 2D metal complex nanosheets are still very rare.

In this project, we initiate a new programme of designing and synthesizing a new family of bis(dipyrrinato) metal complex 2D nanosheets by the facile bottom-up approach, which can be monometallic or bimetallic in nature. Both single-layer and multilayer 2D nanosheets will be developed depending on the fabrication method (i.e. gas-liquid or liquid-liquid interfacial synthesis). The ligand structure can be easily altered in terms of spacer lengths between dipyrrinato unit and central core as well as the number of arms. Bis(dipyrrinato) metal complexes can be modified to show significant π-conjugation and intense absorptions in the visible and near-infrared region, which will be useful for the construction of photoresponsive and semiconductive nanosheets. We can easily adjust the hole and electron mobilities of the compounds by using electron-rich and electron- poor chromophores in the ligands, respectively. Therefore, bottom-up nanosheets with different topological structures, pore sizes, surface areas and advanced functionalities (e.g. tunable optical and magnetic properties) will be prepared by different monomer combinations. With our strengths in synthetic chemistry, polymer and metal complex chemistry, the properties and device performance of these metal-organic nanosheets can be fine-tuned through chemical modulations of the ligand structures and metal ions. The present work fills in a relatively under-investigated area of bottom-up 2D metal complex nanosheets. It is anticipated that an attractive class of nanosheets with novel topological structures and advanced functional properties can be developed which can find high-tech applications in electronics and spintronics.