Kinetic Strategies for the Formation of Graphyne Nanowires via

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Kinetic Strategies for the Formation of Graphyne Nanowires via Sonogashira Coupling on Ag(111) Tao Wang, Jianmin Huang, Haifeng Lv, Qitang Fan, Lin Feng, Zhijie Tao, Huanxin Ju, Xiaojun Wu, Steven L. Tait, and Junfa Zhu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08477 • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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Journal of the American Chemical Society

Kinetic Strategies for the Formation of Graphyne Nanowires via Sonogashira Coupling on Ag(111) Tao Wang1, Jianmin Huang1, Haifeng Lv2,3, Qitang Fan1†, Lin Feng1, Zhijie Tao1, Huanxin Ju1, Xiaojun Wu2,3, Steven L. Tait4, and Junfa Zhu1* 1

National Synchrotron Radiation Laboratory and Department of Chemical Physics, University of Science and Technology of China, Hefei 230029, P.R. China 2

Hefei National Laboratory of Physical Sciences at the Microscale, School of Chemistry and Materials Science, CAS Key Laboratory of Materials for Energy Conversion, and CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, P.R. China

3

Synergetic Innovation of Quantum Information & Quantum Technology, University of Science and Technology of China

4

Department of Chemistry, Indiana University, Bloomington, Indiana 47405, U. S. A.

KEYWORDS: on-surface synthesis, kinetic control, Graphdiyne, Graphyne, Sonogashira coupling ABSTRACT: The selection of a reaction pathway with high energy barrier in a multipath on-surface reaction system has been challenging. Herein, we report the successful control of the reaction system of 1,1’-biphenyl, 4-bromo-4’-ethynyl (BPBE) on Ag(111), in which three coupling reactions (Glaser, Ullman, Sonogashira) are involved. Either graphdiyne (GDY) or graphyne (GY) nanowires can be formed by distinct kinetic strategies. As the energetically favorable pathway, the formation of GDY nanowire is achieved by hierarchical activation of Glaser (with lowest energy barrier) and Ullman coupling of BPBE. On the other hand, the formation of GY nanowire originates from the high-selectivity of the high-barrier Sonogashira coupling, whose indispensable kinetic parameters are high surface temperature, low molecular coverage and low precursor evaporation rate, as derived from a series of control experiments. This work achieves the fabrication of GY nanowires via on-surface Sonogashira coupling for the first time and reveals mechanistic control strategies for potential syntheses of other functional nanostructures via cross-couplings on surfaces.

Introduction

on the competition between low-barrier bimolecular and high-barrier unimolecular reactions.33-35 Either high temperature or high dilution condition can enhance the unimolecular process since molecular diffusion is typically a hindrance for bimolecular reaction at these experimental conditions. For more complex reaction systems involving multiple bimolecular reaction pathways, however, to our knowledge, no efficient kinetic approach has been reported.

On-surface synthesis has attracted intense attention in recent years, due to its tremendous potential in fabrication of novel functional molecules1-8 and low-dimensional nanomaterials.9-20 In response to the demand for the synthesis of high-quality nanostructures, many efforts have been made to steer the reaction pathway and improve the chemo-/regio-selectivity, typically by smart precursor design21,22 and surface template effect.23-29 Computational and theoretical insight into details of the reaction kinetics on surfaces have been reported, especially for the popular Ullman coupling reaction.30,31 However, the precise control of complex on-surface reactions with multiple pathways still remains challenging since only very few parameters can be tuned in the UHV environment, in contrast with traditional wet chemistry where various solvents and catalysts can be used.32 In particular, how to pick out an energetically unfavorable reaction pathway from a multipath reaction system has been elusive. In this case, unusual kinetic strategies should be utilized to hinder those energetically favorable pathways. A few studies focused

Scheme 1. The potential formation pathways of graphdiyne (GDY) and graphyne (GY) nanowires from 1,1’-biphenyl,4-bromo-4’-ethynyl (BPBE).

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The reaction of para-halogen substituted terminal alkyne on surface is a complex but intriguing system. On the one hand, there is fascinating complexity due to the three coupling reactions (Glaser, Ullman, Sonogashira) and various alkynyl polymerization products involved. On the other hand, the potential for synthesis of graphdiyne (GDY) and graphyne (GY) nanowires through different pathways makes it intriguing (Scheme 1). Lowdimensional GDY and GY have attracted increasing interests in the last decade, due to their promising applications in molecular electronics, lithium storage, gas purification, and catalysis.36-40 Compared to the relatively widely studied GDY nanowires on surfaces,22,24,29,41 the syntheses of GY nanowires have only been recently reported where complex tribromomethyl and trichloromethyl molecules were used as the precursors.42,43 The fabrication of GY nanowires via Sonogashira coupling (Scheme 1), though a simpler approach, presents a great challenge because Sonogashira coupling usually has a higher reaction barrier than that of Glaser coupling or than the trimerization of alkynyl, according to previous studies.11,44-46

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pling, is explored by a series of well-designed control experiments. Various basic parameters of on-surface reaction kinetics including surface temperature, annealing procedure, reactant concentration, adsorbate coverage, molecular adsorption, desorption, and diffusion all contribute significantly to the mechanism. In solution-based reactions, selection of Glaser/Ullman coupling vs. Sonogashira coupling has been typically realized by using different catalysts.47,48 In contrast, we demonstrate high selectivity with the same Ag(111) surface as a catalyst using kinetic control approach. This work not only reports the first example of GY nanowire via on-surface Sonogashira coupling, but also offers deep insight into the underlying mechanism which might be applied to other on-surface cross-coupling reaction systems. Results and Discussion Formation of GDY nanowire. Figure 1a displays the STM image recorded after depositing 0.8 ML BPBE molecules on Ag(111) held at 360 K. As can be seen, long 1D chains are formed on the sample. The detailed structures of these chains are revealed in the high-resolution STM image shown in Figure 1b. Several small bright dots are observed within the chains (indicated by yellow arrows), implying that these chains should be C-Ag-C bonded organometallics and the bright dots are attributed to Ag adatoms, according to many previous works.17,49-51 Other, dimmer dots fill the regions between organometallic chains (blue arrows). Because the complete dissociation of C-Br bond of BPBE molecule occurs at these conditions, as reported previously,5 these dimmer dots are assigned to chemisorbed Br adatoms on Ag(111). Interestingly, a deeper investigation into Figure 1b shows that the majority of organometallic nanowires have a structural unit composed of two identical rod-like protrusions and one Ag adatom with good periodicity (I, Figure 1b white dashed frame). The detailed configuration of the periodic organometallic chain I is revealed in Figures 1c and 1d. One protrusion is attributed to one BPE unit (debromination product of BPBE) due to their comparable dimensions and aspect ratio. The seamless and smooth connection between two BPE monomers implies the formation of C-C covalent linkage. The covalent structure is most likely 1,3-conjugated diyne generated from Glaser reaction because Sonogashira and Ullman reactions are typically inactive at these conditions, while Glaser coupling can be activated at relatively low temperatures (~330 K on Ag(111)), according to previous works.5,11,45,46,52,53 The corresponding molecular model of structure I is overlaid in Figure 1c, which matches perfectly with the STM image. In Figure 1d, the DFT- simulated STM result of structure I is also in agreement with the experimental one. The center-to-center distance between the two biphenyls as shown in Figure 1c is measured to be about 11.7 ±0.5 Å, further confirming this structure.49 Although product I is predominant on the surface, some byproducts are also observed (labeled II, III, IV, and V in Figure 1b), including other organometallic products and oligomers generated from the trimerization between alkynyls. The statistical analysis of all the products is shown in Figure S1 (Support-

Figure 1. a) Overview STM image of the sample prepared by deposition of BPBE molecules on Ag(111) held at 360 K. Evaporation condition: temperature: 45 °C; time: 15 min. b) Highresolution STM image of an area of as-prepared sample. The typical products are marked as roman numerals. c) Zoom-in STM image of the region marked as dashed rectangle in b), overlaid with corresponding molecular models. d) Experimental and DFT-simulated STM images of a segment of chain I. Color code: C, grey; H, white; Br, red; Ag, silver. Tunneling parameters: a) U=1.2 V, I=0.2 nA; b), c), d) U=0.6 V, I=0.3 nA.

Herein, by utilizing distinct thermal stimuli procedures and kinetic strategies, the reaction of 1,1’-biphenyl,4bromo-4’-ethynyl (BPBE, Scheme 1) on a Ag(111) surface, is efficiently steered. With the interplay of high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT), we confirm that GDY and GY nanowires can be fabricated via different reaction procedures. The mechanism of the formation of GY nanowire which originates from the high-selectivity of Sonogashira cou-


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Journal of the American Chemical Society models are overlaid on c). Color code: C, grey; H, white; Br, red. Tunneling parameters: a), b), c) U=-2.3 V, I=-0.3 nA.

ing information). The periodic organometallic I is the major product with a yield about 78%. Because of the existence of a few separated 1,3-conjugated diynes IV, as indicated by green arrows in Figure 1b, and other diynecontaining units II (Figure S1), it is reasonable to deduce that the dominant reaction process here may follow this order: 1) formation of biradical conjugated diynes via Glaser coupling; 2) connection of these diynes via C-Ag-C bonds. This is further supported by experimental observations: a few close-packed islands composed by diynes IV are observed occasionally on the sample (Figure S2); at the edge of these islands, the evolution from conjugated diyne to organometallic chains I can be directly observed.

Nevertheless, the experiments above unambiguously confirm that GDY nanowires can be fabricated through a hierarchical, stepwise activation of Glaser and Ullman coupling on Ag(111). The GDY nanowire has four phenyls in its structural unit and could potentially be fabricated via a single on-surface coupling reaction, but this would require a large precursor that is difficult to synthesize due to the low solubility. Thus, this on-surface hierarchical coupling strategy provides a significant advance in flexible syntheses of different types of GDY nanowires on surfaces. The quality of the GDY nanowire might be improved by adding protective substituents in the precursor molecule to avoid side reactions.22

Next, an annealing treatment of the sample covered by the organometallic chains was employed to activate further C-C coupling. Figure 2a displays the typical STM image after annealing the sample shown in Figure 1a to 460 K (overview STM image is seen in Figure S3). Seamless and smooth nanowires are observed, accompanied by some separate dimers and “y” type trimers. Figure 2b shows the magnified view of the grey framed region in Figure 2a. The isometric four-dot feature as marked by white arrows in Figure 2b implies the formation of quaterphenyl, which should be generated from the ejection of the Ag adatoms from periodic organometallics I. In other words, the Ullman reaction is activated at these conditions, leading to the formation of GDY type nanowires, as exhibited by the molecular models overlaid in Figure 2c. However, the yield of such GDY nanowires is not very high and the length is short. This might be attributed to two reasons. First, several side reactions are involved, such as the formation of “y” type trimer (mostly cisenediyne, V, yellow circle in Figure 2a)5 and Sonogashira connection (white circle in Figure 2a). This is because a few biradical BPE monomers can be generated after the release of Ag adatoms from organometallic chains (mainly from II and III in Figure S1b). Second, the presence of various species on the surface leads to the formation of irregular dense molecular aggregates, which inhibit the diffusion of biradical diynes IV. The existence of several dispersely distributed diynes IV, as pointed by blue arrows in Figure 2a, supports this point.

Figure 3. a) Overview STM image of the sample prepared by deposition of BPBE on Ag(111) held at 700 K. Evaporation temperature 45 °C; time 15 min. b) Zoom-in STM image of the white framed region in a). The three types of connections are marked as G (Glaser), U (Ullman), and S (Sonogashira). Their magnified views are shown in c), overlaid with the corresponding molecular models and center-to-center distance between biphenyl groups. d) Statistical analysis of product yields by counting more than 300 BPBE monomers. Color code: C, grey; H, white. Tunneling parameters: a), b) U=-2.3 V, I=-0.3 nA.

Formation of GY nanowire. The formation of GDY nanowire on Ag(111) is not surprising because it is an energetically favorable pathway. In the following, a different thermal strategy was utilized. BPBE molecules were directly deposited on a hot Ag(111) surface of 700 K, with the same evaporation conditions as the sample in Figure 1 (evaporation temperature: 45 °C; time: 15 min). Large-area nanowires with smooth connections are formed with these conditions, as shown in Figure 3a. These nanowires should be covalently linked because the C-Ag-C bond cannot exist stably at such high temperature.49,52 A few swinging movements are observed at the terminal of

Figure 2. a) STM image recorded upon annealing the sample in Figure 1 to 460 K. Two insets show the zoom-in STM image of the white framed and yellow circled regions, overlaid with corresponding molecular models. b) Zoom-in STM image of the grey framed region in a). Corresponding molecular



some nanowires (white arrow). This is attributed to either the induction of STM tip as reported previously41 or fast oscillation of the chains at the scanning temperature (~ 85 K). Figure 3b is a separate zoom-in STM image recorded ~20 seconds later in the white framed region in Figure 3a. Interestingly, the dimeric species marked as white dashed circle, not visible in Figure 3a, emerged into the region within this short time. This implies a relatively weak interaction between these organic species and the surface, consistent with the swinging movements of the nanowires. Further investigation of Figure 3b reveals that three different connections are involved in the covalent nanowires, as marked by the white, yellow, and green dotted circles. Note that phenyl groups typically appear in STM with a dot-like morphology and the alkynyl group usually contributes low density of states in the STM image.5,24,25 Using these features in the images, we can distinguish three connection types, most probably originating from Glaser (G), Ullman (U), and Sonogashira (S) coupling reactions, as denoted in Figure 3b. The magnified views of the three connections are shown in Figure 3c. The center-to-center distance between biphenyl groups is measured to be about 13.7, 8.9, and 11.6 Å, respectively (see details in Figure S4), further confirming their alkynyl-alkynyl, phenyl-phenyl, and alkynyl-phenyl structures.44,45,54 The excellent match between STM images and corresponding molecular models also supports this point. In addition to these coupling reactions, some polymerization (P) reaction (branch-like structure in STM image) between alkynyls are observed, as indicated by yellow arrows in Figure 3b. This is because the conjugated diyne structure can easily react with a BPE monomer and form “y” type trimer (mostly cis-enediyne) at an elevated temperature, as reported previously.5 In fact, the majority of the conjugated diynes transform into trimeric structures at these conditions, since few diyne species can be observed on the sample. A statistical analysis towards reaction products is exhibited in Figure 3d. The diyne (G) and “y” type (P) species are classified together in Figure 3d because of their similar coupling. Unlike the lower temperature experiments (Figures 1 and 2), in this case the sample is dominated by a single product: Sonogashira coupling accounts for >70% of the nanowire products. In other words, the high-barrier Sonogashira reaction is preferentially selected from multiple pathways. In addition, some pure GY nanowires are observed on the surface, as denoted in Figure 3b (more detailed analysis about pure GY nanowire is shown below).

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The debromination and dehydrogenation should occur immediately after BPBE molecules are deposited on the hot Ag(111) surface.55,56 Because of the high surface temperature, the majority of the generated biradical BPE monomers should desorb from the surface before they are able to react with another adsorbate. This is evidenced by the sharp contrast between the observed molecular coverage of sample prepared at 360 K (Figure 1a, ~0.8 ML) and at 700 K (Figure S5, ~0.1 ML), using the same evaporation parameters. The strong desorption results in a low concentration of BPE monomer (M) on the surface. Nevertheless, the small amount of BPE monomers that remain on the surface can still react into dimers. Two types of dimers can be formed, via Glaser (D1, Figure 4a) and Sonogashira coupling (D2, Figure 4a), respectively. Note that Ullman coupling is relatively rare due to the steric hindrance between phenyl groups and the instability of an Ag stabilized intermediate at this temperature, which is supported by our previous work (see details in Figure S6).5 In that work, a BPE monomer can react with phenylene of cis-enediyne trimer on Ag(111) at 420 K only via Sonogashira coupling. The proportion of D1 should be higher than that of D2 since Glaser coupling has a lower reaction barrier. Notably, these dimers also have the possibility to desorb from the surface if no additional BPE monomers are supplied to react with them in a short time (Figure 4b). Otherwise, a trimeric structure can be formed via Sonogahsira or Glaser coupling (only Sonogashira coupling for D1) when a new BPE monomer (yellow arrows in Figure 4b) reaches the dimers (the trimerization of alkynyls is not considered here because it has no relevance to the growth mechanism). In contrast, other BPE monomers far away from the dimers (blue arrow in Figure 4b) mostly desorb from the surface. The connection between two dimers via Sonogashira or Ullman reaction during diffusion should be of low probability due to the low concentration and low probability of meeting at appropriate orientations. Consequently, some long molecular chains can be obtained, as typically exhibited by the two examples (2M+D2 or M+D1+D2) in Figure 4c. These chains are stable on the surface against thermal desorption due to the large molecular mass. The continuous deposition of BPBE leads to the further growth of chains (Figure 4d). At low concentration conditions, BPE monomers prefer to react with existing chains on the surface, rather than form dimers via Glaser coupling. This is because, on average, the diffusing monomers are more likely to encounter and react with chains (chain concentration increases with deposition time) rather than another monomer. Because Sonogashra coupling is the dominant reaction during the “one-by-one” growth process (since Ullman coupling is difficult), it occupies the highest yield among the three couplings. The occasionally observed Ullman couplings in experiment might mainly come from the meeting between phenylene ends of two chains on the process of their diffusion, as indicated by blue arrow in Figure 3b (or between chains and cisenediyne trimers as indicated by green arrow in Figure 3b). It is worth noting that the low adsorbate coverage

According to the experimental results above, we conclude that different thermal treatments steer selectivity for GDY or GY nanowires on Ag(111). The high-selectivity of high-barrier Sonogashira coupling on a hot Ag(111) surface should originate from unusual on-surface kinetics. This is because a high-barrier reaction can never prevail in a usual system of competing reactions, even at high temperatures. Discussion of the high-selectivity of Sonogashira coupling. Here, we propose the possible mechanism for the formation of GY nanowire via Sonogashira coupling.


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may also be a critical factor for the formation of highyield nanowires, because otherwise the steric hindrance should inhibit their growth.

In conclusion, low concentration of BPE monomer, low adsorbate coverage, and high surface temperature might

Figure 4. Proposed mechanism of the high-selectivity of Sonogashira coupling and the formation of GY nanowire. Color code: phenyl C, blue; ethynyl C, red; H, white; Ag, silver.

temperature below are based on the samples with coverage of 0.1 ML.

be indispensable kinetic parameters for the highselectivity of Sonogashira coupling in this system. Specifically, high surface temperature should be a necessity because otherwise Sonogashira coupling cannot be activated or has an extremely low reaction rate (more detailed analysis is seen in supporting information). Pure GY nanowires, like the chain 2M+D2 in Figure 4c, can be obtained at these conditions, e.g., chain indicated in Figure 3b.

Eleven samples with different kinetic parameters were prepared, whose STM results are shown in Figure 5a-k, overlaid with the corresponding experimental parameters and yields of Sonogashira connection (overview STM images are shown in Figure S7). They are labeled as sample a-k, respectively, identical to their figure numbers. Surface temperature during deposition. First, BPBE molecules were deposited onto the hot Ag(111) surfaces held at different temperatures, with identical evaporation parameters (temperature: 45 °C; time: 15 min). Figure 5a, Figure 5e and Figure 5i display the results. The molecular coverage decreases gradually with the increase in surface temperature. This is consistent with the mechanism that higher temperature leads to faster molecular desorption. Meanwhile, the proportion of Sonogashira connection increases significantly along with the increase of temperature: 29% at 540 K, 49% at 620 K, and 74% at 700 K. A few pure GY nanowires are observed on the sample of 700 K and the corresponding high-resolution STM image is displayed in Figure 5l. A linescan profile along the white dotted line is derived from the STM image. Accordingly, the average length of the center-to-center (marked as AB in Figure 5l) distance between two biphenyl groups of neighboring BPE monomers, is measured to be about 11.45 Å (Figure 5m), in good agreement with the DFTcalculated value of 11.36 Å (Figure S8).

Control experiments. To corroborate the proposed mechanism above, a series of control experiments were conducted (Figure 5). The three variables used in the experiments are surface temperature during deposition, molecular coverage, and molecular deposition rate. Note that the molecular deposition rate positively correlates with the concentration of BPE monomer on the surface. For the convenience of the following discussion, effective molecular deposition rate is defined as the amount of BPE deposited onto Ag(111) and adsorbing on the surface (without desorption) per unit time. In other words, the same effective deposition rate should lead to approximately the same concentration of adsorbed BPE monomers on the surface (for the same deposition time). The molecules that resist desorption have reacted with other adsorbed molecules on a fast time scale compared to the rate of desorption (a few monomers may remain, but these are usually in close-packed regions). Figures 5c, 5g, and 5i show an example at 0.1 ML coverage, where few excess BPE monomers can be observed. Precise control of effective molecular deposition rate for surfaces of different temperatures (different molecular desorption rate) can be realized by tuning the temperature of the evaporation source to ensure same molecular coverage within same evaporation time. Single control experiments of surface

However, because the concentrations of BPE monomers staying on the surface are different for the three samples, one cannot conclude that the yield of Sonogashira connection increases with surface temperature. Thus we performed another set of control experiments, by



depositing BPBE monomers on Ag(111) held at 540 K, 620 K, or 700 K with identical effective deposition rate (0.0067 ML/min) and molecular coverage (0.1 ML). This ensures surface temperature as the single parameter. Figure 5c, Figure 5g, and Figure 5i, respectively, display the results. Accordingly, the yield of Sonogashira connection increas-

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es gradually from sample c to sample i (60%, 70%, 74%). This is mainly because the ratio of the reaction rate of Glaser coupling of BPE to that of Sonogashira coupling is relatively high at a low surface temperature, thus the BPBE molecule is more

Figure 5. a)-k) Control experiments by changing surface temperature, molecular coverage, and evaporation rate. The experimental conditions and the yield of Sonogashira connection are marked on the corresponding STM images. The yields are estimated by counting more than 200 BPBE monomers for all the samples. The surface temperature is 540 K for a)-d), 620 K for e)h) and 700 K for i)-k). l) Zoom-in STM image of the green framed region in i). m) Linescan profile along the white dotted line in l). Scanning tunneling parameters: a) U= -2.4 V, I=-0.3 nA; b) U=-1.4 V, I=-0.2 nA; c) U=-1.2 V, I=-0.2 nA; d) U=-1.3 V, I=-0.3 nA; e) U=-1.8 V, I=-0.3 nA; f) U=-1.3 V, I=-0.2 nA; g) U= -2.0 V, I=-0.3 nA; h) U=-1.0 V, I=-0.4 nA; i) U=-2.3 V, I=-0.2 nA; j) U=-2.0 V, I=0.3 nA; k) U=-1.4 V, I=-0.3 nA.

likely to take Glaser coupling (qualitative analysis based on Arrhenius equation is given in supporting information).

growth of nanowires. In addition, the existing organics on the surface may accelerate the adsorption of new coming monomers due to the intermolecular interaction. This is supported by the experimental facts: the molecular coverage of sample j is 7 times larger than that of sample i, while its deposition time is only 2.3 times longer than that of sample i. The acceleration of molecular adsorption at a high molecular coverage on the other hand may lead to the increase of concentration of BPE monomers on the surface thus enhancing the occurrence of Glaser coupling.

Molecular coverage. Three groups of control experiments were performed to investigate the influence of molecular coverage on the selectivity of Sonogashira coupling. These groups are sample a vs. sample b, sample e vs. sample f, and sample i vs. sample j. Within each pair, surface temperature and molecular evaporation rate are constant values. It is obvious that at lower coverage, the yield of Sonogashira connection is higher. This is in line with our speculation, as proposed above, that the steric hindrance at high molecular coverage should inhibit the

BPBE evaporation rate. The impact of evaporation rate is studied by changing the evaporation temperature of the molecular source while keeping the molecular cov-


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erage and surface temperature constant. According to the proposed mechanism above, the yield of Sonogashira connection should decrease with an increase in molecular evaporation rate, because of the accompanying increase in concentration of the BPE monomers on the surface. Four comparison groups are presented in Figure 5 to corroborate this idea (sample b vs. sample c, sample f vs. sample g, sample i vs. sample k, and sample a vs. sample d). We attempted to further enhance the Sonogashira reaction using an extremely low molecular evaporation rate to a sample at 620 K (sample h); however, except for some step edge regions, almost no molecules adsorb on the surface. The majority of monomers may desorb from the terrace before interacting with others.

HLL technique, the efficiency of cross-coupling may be enhanced and various functional nanowires and nanoribbons could be fabricated. Precursors with increasing complexity are being employed more broadly in onsurface polymerization studies, so this HLL approach and other methods like it will become increasingly important. Further studies are on-going. Conclusion In conclusion, we report the formation of GDY versus GY nanowires from the BPBE molecule on the Ag(111) surface. The GDY nanowire is formed by hierarchical activation of Glaser and Ullman coupling, as the energetically favorable pathway. On the other hand, the formation of GY nanowire originates from the successful selection of high-barrier Sonogashira coupling from multiple reaction pathways of BPBE. The synergy of high surface temperature, low molecular coverage, and low molecular evaporation rate lead to high-selectivity for Sonogashira coupling. This is well supported by control experiments of surface temperature, molecular coverage, and molecular evaporation rate. The on-surface kinetic strategies reported here for the selection of high-barrier pathway provides guidance for the control of complex reaction systems on surfaces, particularly cross-coupling reactions.

To test the effect of high monomer concentration, we deposited BPBE on Ag(111) held at 93 K (evaporation conditions: 45°C, 15 min) followed by fast annealing (55 K/min) to 700 K. The low surface temperature during deposition was chosen to prevent any initial reaction,5 i.e., to allow loading of a high monomer concentration before annealing to the reaction temperature. Because of the high monomer concentration during the reaction, lowbarrier polymerization between alkynyls (predictably) dominate in this experiment (see Figure S9). This result supports our conclusion that the low-barrier polymerization of alkynyls prevails at high monomer concentration.

ASSOCIATED CONTENT Supporting Information Available. Detailed descriptions of experimental and theoretical procedures, additional STM results, analysis of the relationship between reaction rate and surface temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

Overall, these control experiments efficiently corroborate that high-temperature surface, low molecular coverage, and low molecular evaporation rate are essential elements for the high-selectivity of the Sonogashira coupling of BPBE on Ag(111). Various basic parameters of onsurface kinetics are responsible for the mechanism. High surface temperature is critical to efficiently open the Sonogashira reaction pathway. Strong molecular desorption at high surface temperature and slow molecular deposition rate result in the low concentration of BPE monomers on the surface, thus limiting their collision rate via surface diffusion. As a result, instead of the occurrence of Glaser coupling, these monomers prefer to connect with the existing molecular chains on the surface via Sonoashira coupling.

AUTHOR INFORMATION Corresponding Author * J. F. Zhu, [email protected]

Present Addresses †Fachbereich Chemie, Philipps-Universität Marburg, HansMeerwein-Str., 35032 Marburg, Germany.

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

On the basis of this mechanism, further optimization of the control of Sonogashira coupling can be implemented by introducing hindering functional groups in the precursor. This might completely prohibit Ullman reaction and trimerizations between alkynyls due to sterics.22,57 Computational studies, though beyond the scope of this work, could provide valuable guidance in the interpretation and design of future coupling strategies, as has been shown recently for studies of Ullman coupling.31

Notes The authors declare no conflict of interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21473178, 21773222, 21573204, 215032030) and the National Key R&D Program of China (2017YFA0403402, 2016YFA0200602), CAS Interdisciplinary Innovation Team, and by Supercomputer Center of USTCSCC and SCCAS.

We note that the technique and approach reported here (high surface temperature, low monomer concentration, low surface coverage (HLL)) may be extended to other cross-coupling reaction systems on surfaces. In general, for a molecule containing two different terminal active groups, the homo-coupling of the relatively more active group should be energetically favorable and the occurrence of cross-coupling is always difficult. By using

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SYNOPSIS TOC


Kinetic Strategies for the Formation of Graphyne Nanowires via

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