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Polymer chains formed on Ag(111) by on-surface synthesis. (a) Chemical structure of porphyrin monomer. (b) Scheme for on-surface reaction. STM images of (c) close-packed 1D polymer islands after deposition and annealing at 120 1C (blue arrow shows row direction and position of lineprofile in (e), white arrow shows 'hair-pin' bend), (d) close-packed island; linear to curved transition indicated. (e) Line-profile acquired from (c) with dimensions and polymer model. Image parameters: tunnel current, I = 30 pA, sample voltage, V = À1.80 V.
Source publication
One-dimensional polymer chains consisting of π-conjugated porphyrin units are formed via Glaser coupling on a Ag(111) surface. Scanning probe microscopy reveals the covalent structure of the products and their ordering. The conformational flexibility within the chains is investigated via a comparision of room temperature and cryogenic measurements.
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Citations
... Ag (111) 44,45 and Cu(111). 43 Similarly, other competing dehydrogenation processes of the alkyne group, as well as hydrogen transfer within the alkyne group assisted by a single Au adatom, shown in Figure S10, have a much larger activation barrier compared to the hydrogen transfer from the amine to central carbon atoms of the alkyne segment. ...
The synthesis of N-fused heterocycles is a crucial element in organic chemistry. However, it involves multiple steps that add complexity and reduce overall yields. Here, we introduce the high-yield on-surface synthesis of N-heterocyclic compounds employing the intramolecular hydroamination of alkyne-functionalized molecular precursors under mild thermal conditions in an ultra-high vacuum environment on an Au(111) surface. This method offers an efficient preparation of two discrete N-heterocyclic derivatives using a single gold(0)-atom catalyst. Notably, the synthesis introduces two pyrrole groups into quinoidal-based precursor, enabling the formation of two fused pyrrolo-benzoquinonediimine compounds with tailored electronic band-gap not achievable in solution chemistry. To analyze the resulting reaction products, we utilized scanning tunneling microscopy and non-contact atomic force microscopy with single bond resolution, comparing these products to those obtained through traditional solution methods. We further performed computational studies to elucidate detailed mechanistic insights into the on-surface reaction course.
... However, the connection between aromaticity and on-surface reactivity of organic molecules remains largely unexplored. While various reactions of (polycyclic) aromatic molecules [20][21][22] and, in particular, coupling of porphyrins [23][24][25][26][27][28][29][30][31] , have been studied on surfaces, none of these studies identifies the role of aromaticity in the reactions. Recent investigations report differences in the reactivity of functional groups depending on whether these groups are linked to an aromatic ring system 32,33 . ...
Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, its role remains largely unexplored in on-surface chemistry, where the interaction with the substrate can alter the electronic and geometric structure of the adsorbates. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity in the reactions by scanning tunnelling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, which show a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights are helpful to understand, and in turn design, reactions with aromatic species in on-surface chemistry and heterogeneous catalysis.
... Although the advancement in nanoscale imaging techniques such as scanning tunnelling microscopy (STM) has enabled the determination of the pathways of polymerization reactions running on surfaces [11][12][13]25,26,[30][31][32][33][34], selection of the optimal monomeric building block has still been problematic in certain cases. This refers especially to the number and intramolecular distribution of halogen substituents in PAHs monomers, aiming at the synthesis of the polymer with predefined architecture. ...
On-surface synthesis of C-C covalent low-dimensional nanomaterials is a promising method of obtaining structures with tailored and novel physicochemical and electric properties. In this contribution, the Monte Carlo simulation approach was proposed to predict the topology of metal-organic (MO) intermediates formed in the Ullmann homocoupling of halogenated isomers of tetracene. The coarse-grained model of polyaromatic hydrocarbons (PAH) haloderivatives and divalent copper adatoms on a metallic crystal surface (111) was used, where locations of substituents in the molecules were encoded as active centres with directional C-Cu interactions. The computations were performed for various structural isomers of tetracene, from disubstituted to tetrasubstituted units. As a result, diverse superstructures were obtained, such as dimers, trimers, and other oligomers, chains and ladders, and metal-organic networks, both chiral and achiral. Additionally, for the prochiral linkers, simulations of the racemic mixtures were performed. Our study provided useful insight into the influence of substituents’ position and the carbon backbone’s size on the topology of the modelled precursor architectures.
... 19 However, the connection between aromaticity and on-surface reactivity of organic molecules remains largely unexplored. While various reactions of (polycyclic) aromatic molecules, [20][21][22] and in particular coupling of porphyrins, [23][24][25][26][27][28][29][30][31] have been studied on surfaces, none of these studies identi es the role of aromaticity in the reactions. Recent investigations report differences in the reactivity of functional groups depending on whether these groups are linked to an aromatic ring system. ...
Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, in on-surface chemistry – where the interaction with the substrate can significantly alter the electronic and geometric structure of the adsorbates – the role of aromaticity is not understood. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity of the reactions by scanning tunneling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, revealing a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights will help to design and understand reactions in on-surface chemistry and heterogeneous catalysis.
... A range of alternative deposition techniques have been developed 10 , and we have demonstrated that porphyrin oligomers, polymers and macrocyclic systems can be deposited via an electrospray deposition technique [11][12][13] . In addition, porphyrin polymers may be formed by using a range of on-surface synthesis techniques [14][15][16][17] , including Glaser coupling of porphyrin monomers 18 . ...
... The two 'brighter contrast' features are assigned to the terminal porphyrin units (functionalised with thioester bridges), with the two central features assigned to di-tert-butyl phenyl (di-tBuPh) functionalised porphyrin units. The separation between the porphyrin units is found to be 1.34 ± 0.08 nm, in good agreement with our previous measurements for similar systems containing butadiyne linked porphyrins 11,18,29 . ...
... We first consider the more densely-packed phase (labelled as I) shown in Fig. 2b: 0.119 ± 0.008 molecules nm -2 . Based on a comparison between the spatially-resolved features in the STM images and a space-filling model based on the van der Waals radii of S4-l-P4 it can be seen that the extended structures are stabilised by the interaction of the di-tBuPh groups in a similar manner to that observed for porphyrin polymer chains where the pendant groups were exclusively di-tBuPh units; see Fig. 2c (highlighted in orange) 18 . The unit cell for this interdigitated (I) structure is measured to be 6.0 ± 0.1 nm by Fig. 2a). ...
Polymeric structures based on porphyrin units exhibit a range of complex properties, such as nanoscale charge transport and quantum interference effects, and have the potential to act as biomimetic materials for light-harvesting and catalysis. These functionalities are based upon the characteristics of the porphyrin monomers, but are also emergent properties of the extended polymer system. Incorporation of these properties within solid-state devices requires transfer of the polymers to a supporting substrate, and may require a high-degree of lateral order. Here we show that highly ordered self-assembled structures can be formed via a simple solution deposition protocol; for a strapped linear porphyrin oligomer adsorbed on a highly oriented pyrolytic graphite (HOPG) substrate. Two distinct molecule–molecule interactions are observed to drive the formation of two molecular phases (‘Interdigitated’ and ‘Bridge-stabilised’) characterised by scanning tunnelling microscopy, providing information on the unit cell dimensions and self-assembled structure. The concentration dependence of these phases is investigated, and we conclude that the bridge-stabilised phase is a thermodynamically stable structure at room temperature.
... This new environment has the potential to facilitate alternative reaction pathways to those available in solution. An example of this, in the case of Glaser-type coupling, is the use the steric hindrance induced from 2D confinement to reduce branching reactions in the synthesis of linear structures 13,14 . To fully utilise this alternative environment a detailed understanding of the underlying mechanistic processes underpinning these reactions is required. ...
The on-surface synthesis of covalently bonded materials differs from solution-phase synthesis in several respects. The transition from a three-dimensional reaction volume to quasi-two-dimensional confinement, as is the case for on-surface synthesis, has the potential to facilitate alternative reaction pathways to those available in solution. Ullmann-type reactions, where the surface plays a role in the coupling of aryl-halide functionalised species, has been shown to facilitate extended one- and two-dimensional structures. Here we employ a combination of scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and X-ray standing wave (XSW) analysis to perform a chemical and structural characterisation of the Ullmann-type coupling of two iodine functionalised species on a Ag(111) surface held under ultra-high vacuum (UHV) conditions. Our results allow characterisation of molecular conformations and adsorption geometries within an on-surface reaction and provide insight into the incorporation of metal adatoms within the intermediate structures of the reaction.
... [13] Among the reactions successfully introduced in the field of on-surface synthesis, this coupling reaction occupies a prominent position. It was employed for building polymers on metallic surfaces [14][15][16][17][18][19][20][21] and recently was also observed on the surface of a bulk insulator by thermal activation. [22] However, this reaction has never been demonstrated by atom manipulation up to now. ...
Glaser‐like coupling of terminal alkynes by thermal activation is extensively used in on‐surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser‐like coupling between terminal alkyne carbons within a custom‐synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates, providing insight into the reaction pathway.
... Among the reactions successfully introduced in the field of on-surface synthesis, this coupling reaction occupies a prominent position. It was employed for building polymers on metallic surfaces [12,13,14,15,16,17,18] and recently was also observed on the surface of a bulk insulator by thermal activation [19]. However, this reaction has never been demonstrated by atom manipulation up to now. ...
Glaser-like coupling of terminal alkynes by thermal activation is extensively used in on-surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser-like coupling between terminal alkyne carbons within a custom synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates providing insight into the reaction pathway.
... [13] Among the reactions successfully introduced in the field of on-surface synthesis, this coupling reaction occupies a prominent position. It was employed for building polymers on metallic surfaces [14][15][16][17][18][19][20][21] and recently was also observed on the surface of a bulk insulator by thermal activation. [22] However, this reaction has never been demonstrated by atom manipulation up to now. ...
We demonstrated a Glaser‐like coupling reaction by atom manipulation. The intramolecular reaction was performed within a single molecule, facilitated by the design of the precursor molecule. By using high‐resolution atomic force microscopy, we revealed precursors, intermediates and products. Resolving and manipulating the positions of individual hydrogen atoms in intermediates provided information about the reaction pathway.
Abstract
Glaser‐like coupling of terminal alkynes by thermal activation is extensively used in on‐surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser‐like coupling between terminal alkyne carbons within a custom‐synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates, providing insight into the reaction pathway.
... This polymerisation was investigated using vacuum STM. 92 ...
Self-assembly of ordered molecular structures on surfaces is of great interest in the formation of devices with novel properties. By utilising molecules that form ordered structures, structures can form spontaneously without the need for lithographic processes. The de-halogenation and polymerisation of 1,3,5-tris-(4-iodophenyl)benzene on the hBN surface was investigated with AFM, XPS and SIMS. The molecules were deposited both from solution and in vacuum in minor variations of the previously reported sine-wave structure. Above 100°C, polymerisation was observed with AFM and confirmed with SIMS, which revealed that, after polymerisation, additional iodine was still present suggesting that dehalogenation had not fully taken place prior to polymerisation occurring as suggested by the XPS results. Using a vacuum deposition method, the structures formed by melamine on the hBN surface were investigated with AFM. Thick film triangular islands suggested an alignment of the molecular structure with the hBN substrate. Two main structures were seen: a rhombic lattice 0.87 ± 0.01 nm by 0.89 ± 0.01 nm and a square lattice 2.22 ± 0.03 nm by 2.10 ± 0.10 nm. An additional structure was seen on the top of the thick islands of 2.02 ± 0.06 nm by 0.77 ± 0.03 nm which was deemed to be a variant of the rhombic structure with out-of-plane rows due to the transition into the bulk structure. The rhombic structure was also deemed to be the structure forming an observed moiré pattern with a period of 3.19 ± 0.03 nm. The polymerisation of porphyrin monomers with varying numbers of reactive ethynyl groups on Ag(111) was investigated in vacuum using a novel indirect deposition method and STM. The 2R porphyrin was found to form chains when annealed to 120°C for 15 min with an intra-chain porphyrin separation of 1.33 ± 0.05 nm and an inter-chain distance of 1.90 ± 0.10 nm. The 4R porphyrin was also observed to form a lattice of 1.53 ± 0.04 nm by 1.69 ± 0.02 nm which was rationalised to be partially reacted into chains containing Ag atoms in one direction after heating to 120°C for a 15 min. The 3R porphyrin was observed to form short chains without heating when imaged with a low temperature STM. The 4R porphyrin was also deposited onto HOPG using a solution deposition method and formed lines separated by 1.70 ± 0.20 nm. Finally, mechanical transfer of hBN supported C60 islands was carried out. The C60 islands were found to be undamaged by soft deposit mechanical transfer process from a PPC/PDMS stamp. The transfer process using PPC/PDMS stamps was also used to form hBN/ C60 /hBN heterostructures and to transfer these to silicon nitride TEM grids for imaging although this step involved delaminating the PPC material at high temperature and subsequent cleaning.
