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Final Report Summary - TOPCHEM (Topologically Confined Chemical Reactions Performed Within Substrate-Supported Porous Molecular Architectures)

TOPCHEM brings together aspects from condensed matter physics, surface chemistry, supramolecular chemistry, and catalysis in order to study the physical and chemical processes of on-surface reactions. Improving the selectivity and efficiency of chemical reactions is of obvious importance to the fine chemical and pharmaceutical industries, as well as providing economic and environmental benefits. Within TOPCHEM the interactions between molecules confined within well-defined self-assembled molecular templates, supported on catalytically active substrates under ultra-high vacuum (UHV) conditions, will be investigated. This approach builds upon the concepts of topological chemistry (topochemistry) where solid-state reaction products are determined by the geometric alignment of reactant molecules. A ‘next-generation’ topochemical methodology will be applied to drive on-surface reactions towards specific products by utilising the 2D size confinement provided by porous molecular architectures, thus providing a new methodology for the control of reaction products.

Within this work a catalytically active surface will be templated with a molecular overlayer, providing nanoscale reactive regions with specific geometries to be used as the size- and shape-specific reaction vessels. An important aspect of TOPCHEM is to identify a suitable molecular template for these reactions. Within TOPCHEM we shall consider three routes: (1) Hydrogen-bonded supramolecular structures – providing a flexible route towards templates with a range of pore dimensions. (2) Porphyrin Nanorings – covalently bonded circular structures with a range of pore sizes depending on the number of porphyrin units within the ring. (3) The atomic structure of the catalytic surface itself – investigating the effect of local surface topography.

In order to acquire a comprehensive understanding of the processes involved in controlling on-surface reactions complementary approaches will employed; the use of molecular self-assembly to form templates, on-surface synthesis to produce product structures, scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) studies to provide characterisation of the molecular arrangements.

Within TOPCHEM three distinct areas are outlined:
(1) A new methodology for controlling the selectivity of on-surface reactions via ‘topochemistry’: Molecular templates will be investigated as a way of controlling the number of molecules available within each pore and the geometry in which they align. Using this control of initial conditions on-surface reactions may, in principle, be steered along a specific reaction pathway towards a specific product. For example, an on-surface covalent coupling reaction performed on a flat surface with a reactant molecule with a trigonal symmetry would produce an extended 2D hexagonal structure. The use of steric confinement within a porous molecular template should drive the reaction towards smaller non-extended structures.

(2) Use of topochemistry to produce multi-component products in a one-step reaction: Porous template molecules, such as porphyrin-based cyclic polymers, can potentially be used to give rise to pores with ‘large’ diameters (>10 nm – large on a single-molecule scale!). Such molecules will need to be synthesised ex-situ and then transferred from solution to the catalytically active substrate. TOPCHEM will seek to form templated surface using these molecules and will look to investigate if ‘one-step’ synthetic pathway for multi-component molecular species may be conducted within these pores.

(3) Topochemical reactions at the liquid-surface interface: The reactions described in (1) and (2) will be transferred to a liquid or ambient environment with the aim of investigating the possibility of using alternative substrates which are not limited to use under a vacuum environment. This objective will move from a model system to a reaction environment more relevant to industrial processes.

During the initial stages of TOPCHEM work was undertaken to characterise two different types of on-surface reactions: Glaser coupling – based on molecules functionalised with acetylene groups, and Ullmann coupling – based on molecules functionalised with halogens (e.g. bromine or iodine). These reactions were observed to proceed on metallic substrates and result in 1D or 2D covalently bonded polymer structures. In both of these reactions the initial stage is the breaking of chemical bonds to produce an activated species and a leaving group (hydrogen and a halogen in the respective cases of Glaser and Ullmann coupling). In the case of Ullmann coupling an intermediate structure consisting of the activated species and metal atoms (metal organic framework - MOF) was observed. Glaser coupling has the benefit of only having hydrogen as a leaving group (halogen atoms from Ullmann coupling can remain on the surface as contaminants). In these initial experiments a novel ‘rapid-heating’ deposition technique (developed as part of TOPCHEM) was used to deposit functionalised porphyrin monomers which then underwent thermally activated Glaser coupling to produce 1D-porphyrin polymers (known to possess interesting electrical and optical properties). The surface structures were characterised via a combination of STM and nc-AFNM allowing determination of the covalent chemical bonding between molecules as well as the conformation (‘3D-shape’) of the individual molecules. This work is the first example of on-surface synthesis of porphyrin polymers via Glaser coupling.

Following the characterisation of these two on-surface reactions self-assembled hydrogen-bonded porous molecular networks were formed upon the catalytically reactive surface, in order to give rise to a ‘template’ within which the reactions could occur. Combining two organic molecules on metallic surfaces (PTCDI and melamine on Au and Ag substrates) allowed two types of porous structure to be produced. These structures allowed templates with hexagonal and parallelogram pores to be produced and were observed to effect the reaction pathway as compared to the non-templates surface. Molecules trapped within the pores underwent the first stage of the Ullmann coupling reaction (breaking of the carbon-halogen bond), with subsequent formation of extended networks being hindered due to the confinement within the network pores. The porous template appears to stabilise an intermediate chemical species (not observed on the non-templated surface) and these experiments provide the first results demonstrating how an on-surface Ullmann reaction may be controlled via a template structure.

The study of on-surface reactions with multiple components will require templated surfaces with large pore dimensions. As part of TOPCHEM the deposition of cyclic porphyrin rings onto Ag(111) was successfully achieved via use of an electrospray deposition technique (enabling these large molecules to be transferred directly from solution to substrates held in a vacuum environment). This is the first step in allowing reactions with larger molecules such as porphyrins (which can be selected to exhibit optical/electronic/magnetic properties) to be conducted within well-defined templates and as such is an important step towards controlling on-surface synthesis.

The final section of TOPCHEM is to explore the feasibility of moving away from studying the above systems under a vacuum environment. Experiments were conducted to form templated metallic surfaces under ambient conditions via solution deposition of the cyclic porphyrin structures. In addition, studies were conducted on the use of hexagonal boron nitride (hBN) as a substrate for on-surface reactions. Results showed that molecular structures formed under vacuum conditions were stable when removed to ambient conditions. This offers a promising route for the creation of templated structures which may be stable in conditions suitable for applications.

The results produced over the course of the TOPCHEM project have provided proof-of-principle for the use of templated catalytic surfaces as a way of controlling on-surface reactions. To date the TOPCHEM project has produced three key results (1) Demonstration of a novel Glaser based surface reaction between functionalised porphyrins on Ag(111). (2) The control of the reactivity of a reactive molecule on Ag(111) by the use of hydrogen-bonded supramolecular networks. (3) Use of Ag(110) and Ag(111) surfaces to affect an on-surface reaction via the topography of the catalytically active substrate. All of these findings are of interest to the scientific community and manuscripts on these topics are published or in preparation. The initial research outputs, collaborations formed, and understanding of the processes underpinning surface confined topochemical reactions will be used as a platform for the continued study of this important new methodology which offers the potential to control chemical reactions.

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