Porphyrins are ideally suited to the growth of functional molecular networks. Structural versatility coupled with desirable optical and chemical properties have seen porphyrins find applications in areas including dye-sensitised solar cells, photodynamic therapy and the design of electro-catalytic surfaces. This talk will present two aspects of our research into the growth of two-dimensional porphyrin networks: (1) the formation of imine stabilised 2D covalent-organic frameworks and their post-growth chemical modification; and (2) the use of nucleobase functional groups to control the complex self-assembly of multi-component porphyrin networks. The growth of 2D covalent-organic frameworks (2D-COFs) via imine formation is a widely applied technique. In the first half of my talk I will present results showing the formation of highly ordered monolayer and bi-layer 2D-COFs grown using either freebase or metal containing tetra(4-aminophenyl)porphyrins. The resulting 2D-COF layers were characterised using scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). A potential limiting factor to the application of 2D-COF thin-films is the low stability of imine bonds with respect to hydrolysis. We demonstrate a post-growth chemical modification approach that reduces imines to secondary amines. This process dramatically increases the stability of the framework layers while maintaining the highly ordered structure of the 2D-COF. In the final part of my talk I will present the synthesis and 2D self-assembly of nucleobase functionalised porphyrins. Tetra(phenylthymine) zinc porphyrin (Zn-tetra-TP) and tetra(phenyladenine) porphyrin (tetra-AP) have been synthesised and their self-assembly at a liquid-solid interface investigated using STM. Self-assembly of mono-component mixtures of these porphyrins produces an open porous network for Zn-tetra-TP, and a more close-packed network for tetra-AP. However, mixtures of the two components self-assemble into a structure with p4 symmetry consisting of alternating Zn-tetra-TP and tetra-AP molecules in a chessboard type pattern. Molecular mechanics (MM) simulations based on drift corrected STM images suggest hydrogen bonding in the mixed networks is in the form adenine-thymine (ATAT) quartets, see figure 1. This result opens up the possibility of using selective interactions between nucleobase functional groups to drive the 2D self-assembly of complex 2D porphyrin networks.

Two-dimensional porphyrin networks: From nucleobase driven self-assembly to covalent-organic frameworks

2019 Spring National Meeting

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