Direct Formation of C−C Double-BondedStructural Motifs by On-SurfaceDehalogenative Homocoupling of gem-Dibromomethyl MoleculesLiangliang Cai,†,¶ Xin Yu,†,¶ Mengxi Liu,‡ Qiang Sun,† Meiling Bao,† Zeqi Zha,‡,§ Jinliang Pan,‡,§

Honghong Ma,† Huanxin Ju,∥ Shanwei Hu,∥ Liang Xu,† Jiacheng Zou,† Chunxue Yuan,† Timo Jacob,⊥

Jonas Bjork,# Junfa Zhu,∥ Xiaohui Qiu,*,‡,§ and Wei Xu*,†

†Interdisciplinary Materials Research Center, College of Materials Science and Engineering, Tongji University, Caoan Road 4800,Shanghai 201804, People’s Republic of China‡CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience,National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China§University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China∥National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, People’s Republic ofChina⊥Institute of Electrochemistry, Ulm University, Albert Einstein Allee 47, 89069 Ulm, Germany#Department of Physics, Chemistry and Biology, IFM, Linkoping University, 581 83 Linkoping, Sweden

*S Supporting Information

ABSTRACT: Conductive polymers are of great importance in a variety ofchemistry-related disciplines and applications. The recently developed bottom-upon-surface synthesis strategy provides us with opportunities for the fabrication ofvarious nanostructures in a flexible and facile manner, which could be investigatedby high-resolution microscopic techniques in real space. Herein, we designed andsynthesized molecular precursors functionalized with benzal gem-dibromomethylgroups. A combination of scanning tunneling microscopy, noncontact atomic forcemicroscopy, high-resolution synchrotron radiation photoemission spectroscopy,and density functional theory calculations demonstrated that it is feasible toachieve the direct formation of C−C double-bonded structural motifs via on-surface dehalogenative homocoupling reactions on the Au(111) surface.Correspondingly, we convert the sp3-hybridized state to an sp2-hybridized stateof carbon atoms, i.e., from an alkyl group to an alkenyl one. Moreover, by such abottom-up strategy, we have successfully fabricated poly(phenylenevinylene)chains on the surface, which is anticipated to inspire further studies toward understanding the nature of conductivepolymers at the atomic scale.KEYWORDS: dehalogenative homocoupling, gem-dibromomethyl, scanning tunneling microscopy,noncontact atomic force microscopy, surface chemistry

Conductive polymers are intrinsic organic semiconduc-tors involving π electrons, which are delocalized alongthe chain.1 Among others, poly(phenylenevinylene)

(PPV) is one of the most intriguing electroactive polymers,demonstrating high thermal stability, nonlinear optical activity,electroluminescence, and photoluminescence.2−5 Owing to itsunique properties, PPV has been considered for a wide varietyof applications in light-emitting diodes, field-effect transistors,photovoltaic devices, etc.6−9 Numerous approaches have been

developed to synthesize PPV, such as the Wittig-type coupling,Knoevenagel condensation, Heck reaction, and so forth.10−18

In comparison with conventional solution chemistry, on theother hand, the recently developed on-surface synthesisapproach is particularly attractive owing to its following

Received: April 2, 2018Accepted: July 17, 2018Published: July 17, 2018

Artic

lewww.acsnano.orgCite This: ACS Nano 2018, 12, 7959−7966

© 2018 American Chemical Society 7959 DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

Dow

nloa

ded

via

TO

NG

JI U

NIV

on

Sept

embe

r 12

, 201

8 at

08:

18:3

0 (U

TC

).

See

http

s://p

ubs.

acs.

org/

shar

ingg

uide

lines

for

opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

characteristics: (i) it is relatively easy to trigger the reactions(just by heating) from rationally designed molecular precursorson surfaces; (ii) the confinement and catalytic effects ofsurfaces19−30 may largely reduce the reaction energy barriers,resulting in the occurrence of some unexpected reactionsinhibited in solution chemistry.31−43 The on-surface synthesisroute has thus opened a gate to the fabrication of a plethora ofnovel nanostructures/nanomaterials, and, more importantly,the products can be investigated by in situ surface-sensitivecharacterization methods, e.g., scanning tunneling microscopy/spectroscopy (STM/STS), noncontact atomic force micros-copy (nc-AFM), high-resolution synchrotron radiation photo-emission spectroscopy (SRPES), and angle-resolved photo-emission spectroscopy (ARPES), with which real-spaceatomic-scale structural assignments and detailed electronicproperties could be unraveled.44−49 It is therefore of generalinterest to employ such an on-surface synthesis strategy toachieve the bottom-up fabrication of conductive polymers likePPVs.On-surface dehalogenative homocoupling reactions offer a

relatively elegant and efficient route for incorporating variouscarbon scaffolds. To the best of our knowledge, most of theemployed halide precursors have only one halogen attached toa carbon atom. In order to incorporate increasinglycomplicated carbon scaffolds into the formed nanostructures,we recently introduced a molecular precursor functionalizedwith an alkenyl gem-dibromide group on an sp2 carbon, whichresults in the formation of a cumulene moiety by thedehalogenative homocoupling reaction on the Au(111)surface.50 In light of this previous work, to generate conjugatedcarbon backbones with alternate C−C single and double bondsas in a conductive polymer, we now attempt to design amolecular precursor functionalized with a gem-dibromomethylgroup on an sp3 carbon and explore the possibility of directformation of C−C double bonds from sp3-hybridized carbonsby dehalogenative homocoupling reactions on the Au(111)surface. As shown in Figure 1, we have designed and

synthesized a molecule functionalized with the benzal gem-dibromomethyl group (that is, 4-(dibromomethyl)-1,1′-biphenyl, abbreviated DBMBP).51 From the interplay ofSTM/STS, nc-AFM, XPS, and DFT calculations, we haveinvestigated dehalogenative homocoupling of DBMBP mole-cules on the surface. Interestingly, it is found that the

formation of trans-4,4-diphenylstilbene products (i.e., involvingC−C double bonds) occurred at room temperature (RT).Furthermore, we designed a ditopic molecular precursor (thatis, 1,4-bis(dibromomethyl)benzene, named BDBMB) with theaim of forming a one-dimensional chain, that is, PPV withalternate vinylene linkages and phenyl groups. By activating thedehalogenative homocoupling reaction of BDBMB on thesurface, we have successfully fabricated PPV chains (cf. Figure1). This study once more exhibits the versatility of on-surfacedehalogenative C−C homocoupling reactions, and moreimportantly, it provides us with a facile manner for thefabrication of conductive polymers, which would inspirefurther fundamental characterizations toward understandingthe nature of conductive polymers at the atomic scale.

RESULTS AND DISCUSSIONAfter the deposition of DBMBP molecules on Au(111) held atRT, we have observed the formation of ordered islands asshown in Figure 2a, which consist of round protrusions andcurved structures. According to the previous studies,29,52 theround protrusions are attributed to dissociated bromine atomson the surface. The curved structure highlighted by the whitecontour (Figure 2b) is composed of two lobes and a dimcontrast in the center. Moreover, a closer inspection of thedimer structure allowed us to identify that the two bright lobeswere not coaxial and exhibited a staggered arrangement. Tofurther identify the atomic scale structure, we performed DFTcalculations. From a detailed comparison of the experimentaltopology and dimensions with the molecular model and thesimulated STM image (cf. Figure 2c and d), we could identifythat the curved structure should be assigned to a C−C double-bonded dimer. Remarkably, the staggered arrangement of twolobes in a formed dimer together with the characteristic STMcontrast (i.e., the middle part is smooth and seamless andapparently lower than those of the phenyl groups) also impliedthe formation of the C−C double bond. Furthermore, theDBMBP molecules couple to give trans-isomers with a quitehigh yield (>90%), indicating that the trans-isomer isthermodynamically favored over the cis-isomer.To further understand the reaction mechanism on the

formation of the C−C double-bonded dimer structure, wehave calculated the reaction pathway from the DBMBPmolecular precursor to the dimer product through successiveC−Br bond activations and subsequent C−C homocoupling,which is in analogy to the dehalogenative homocouplingreaction of alkenyl gem-dibromides.50 The DBMBP molecule isevaporated from the crucible at RT, so it is unlikely for themolecules to be debrominated in the crucible or in the gasphase during evaporation. As shown in Figure 3a, the barriersfor the successive debromination processes are determined tobe 0.16 and 0.75 eV, respectively. The abstraction of the firstBr atom is nearly spontaneous on the Au(111) surface,reminiscent of the debromination of the bromomethyl group.39

While the activation of the second C−Br bond is significantlylarger than the first one, it is still smaller than thedebromination of aryl halides (∼1 eV).53 We also consideredthe pathway of the subsequent C−C homocoupling, startingfrom a configuration where the debrominated species areadsorbed to adjacent atoms of the Au(111) surface (Figure3b). Here, the energy barrier was calculated to be 0.47 eV, thussmaller than the second debromination barrier, corroboratingwhy no debrominated intermediate structures were observed inthe experiments. Furthermore, the coupling reaction is

Figure 1. Upper: Schematic illustration showing the directformation of a vinylene group through dehalogenative C−Chomocoupling of a gem-dibromomethyl molecule. Lower:Schematic illustration showing dehalogenative homocoupling ofthe ditopic molecular precursor, which results in the formation ofpoly(phenylenevinylene) (PPV).

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7960

exothermic by 3.26 eV, reflecting the irreversibility of the C−Cbond formation. While a detailed discussion of the theoreticalstudies and analysis will be a subject of a separate work, thecalculated pathways presented here well account for theformation of the C−C double-bonded dimer product underthe experimental conditions.In view of the successful formation of the C−C double bond

from the sp3-hybridized carbon, we have further introduced aditopic molecular precursor (i.e., BDBMB) onto the Au(111)surface (as shown in Figure 4a) with the aim of fabricatingpolymer chains, that is, PPVs. As shown in Figure 4a, afterdeposition of BDBMB molecules on Au(111) held at RT andannealing to ∼400 K, we indeed observed the formation ofwell-ordered chain structures on the surface (short chains havealready formed at RT, and postannealing of the sample isneeded to improve the quality of the chain structures). The Br3d core level spectrum of the chain structures (the inset shownin Figure 4a) was fitted with one doublet: 68.6 eV (Br 3d3/2)and 67.6 eV (Br 3d5/2), assigned to detached bromine atoms(from carbon atoms) chemisorbed on the Au(111) sur-face.54−57 The SRPES data unambiguously demonstrate thecomplete debromination of BDBMB molecules on Au(111).From the close-up STM image (Figure 4b), the beaded chainshave a period of 6.9 ± 0.2 Å between adjacent protrusions, andthe detached bromine atoms are observed as dim protrusionsbetween the chains. From a detailed comparison of the equallyscaled high-resolution STM image, STM simulation, nc-AFMimage, nc-AFM simulation, and DFT-optimized model of aPPV chain on Au(111) (as shown in Figure 4c), wedetermined the dimension and topography of the PPV chainand identify that the experimental periodicity corresponds wellto the theoretical value of 6.73 Å. Furthermore, the nc-AFM

images allow us to identify the bonding configurations betweenthe linked phenyl groups. From the close-up high-resolutionSTM image (Figure 4d) and the corresponding nc-AFM image(Figure 4e) of a section of the molecular chain, the staggeredline with crossing angle ∼120° can be observed between twoadjacent phenylenes, which is attributed to the formation of aC−C double bond. Such a staggered line is dramaticallydifferent from the case of the formation of a cumulene moiety(i.e., having three consecutive C−C double bonds) where astraight line with a homogeneous contrast was imaged.50

Therefore, we conclude that the dehalogenative homocouplingof BDBMB molecules as depicted in Figure 1 has been realizedon the Au(111) surface. The nc-AFM image (Figure 4g) alsoproves that the formed C−C double bond in the middle of theadjacent phenylenes could have two trans-configurations(owing to the rotation along a C−C single bond), which isalso reflected from STM images (Figure 4f and Figure S1). Wehave also carried out STS measurements on Au(111)-supported PPV chains. The dI/dV spectrum on the Au(111)substrate shows a typical Shockley surface state at approx-imately −400 mV (curve 5 in Figure 4i).58 In contrast, theintrinsic HOMO and LUMO states of PPV chains were notobserved from dI/dV spectra (curves 1−4 recorded ondifferent sites of a PPV chain), which could be attributed tothe doping effect from the metal surface and the Bradsorbents,59,60 as well as the weak contribution to densityof states (DOS) relative to that from the Au substrate.61

CONCLUSION

In conclusion, from a combination of high-resolution UHV-STM, nc-AFM imaging, SRPES, and DFT calculations, wehave demonstrated the feasibility of direct formation of C−C

Figure 2. (a) Large-scale and (b) close-up STM images showing the formation of an ordered island structure after deposition of DBMBPmolecules on Au(111) held at RT. The STM topography of a dimer product is indicated by the white contour. (c) High-resolution STMimage of the dimer structure. (d) Corresponding simulated STM image overlaid by an equally scaled DFT relaxed structure on Au(111).

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7961

double-bonded structural motifs by dehalogenative homocou-pling of gem-dibromomethyl molecules on the Au(111)surface. Notably, in this case, we have converted the sp3- tothe sp2-hybridized state of carbon atoms, i.e., from an alkylgroup to an alkenyl one. As a consequence, this bottom-up on-surface synthesis strategy allows manufacturing functionalnanostructures with vinylene scaffoldings by rational design ofmolecular precursors, which we exemplify by the fabrication ofPPV chains. It is anticipated that our study will facilitate

further fundamental characterizations toward understandingthe intrinsic nature of conductive polymers or other effects(like doping) at the atomic limit.

METHODS AND MATERIALSSTM/AFM Characterization and Sample Preparation. The

STM experiments were carried out in a UHV chamber with a basepressure of 1 × 10−10 mbar. The whole system was equipped with aSPECS variable-temperature “Aarhus-type” STM,62,63 a molecularevaporator, and standard facilities for sample preparation. The nc-

Figure 3. (a) DFT-calculated reaction pathway for the successive C−Br bond activations of the DBMBP molecule on Au(111). Thestructural models of the initial (IS), transition (TS), intermediate (Int), and final states (FS) along the pathway are also shown. (b) DFT-calculated reaction pathway for the homocoupling from the debrominated intermediates to the dimer product, together with thecorresponding structural models along the pathways.

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7962

AFM experiments were performed with an Omicron low-temperatureAFM, operating at 4.8 K in UHV. The AFM is equipped with a qPlussensor with a tungsten tip mounted on a quartz tuning fork (springconstant: 3600 N/m, resonance frequency of f 0 = 40.7 kHz, Q ≈ 5.6× 104, oscillation amplitude ≈ 100 pm). The Au(111) substrate wasprepared by several cycles of 1.5 keV Ar+ sputtering followed byannealing to 850 K, resulting in clean and flat terraces separated bymonatomic steps. After the system was thoroughly degassed, theDBMBP molecule (synthesized) and BDBMB molecule (purchasedfrom Tokyo Chemical Industry Co., Ltd. with purity >98%) weresublimated from the molecular evaporator onto the substrate. Thesample was thereafter transferred within the UHV chamber to theSPECS STM, where measurements were performed in a typicaltemperature range of 100−150 K, and the typical scanning parameterswere It = 0.5−1.0 nA and Vt = ±1000−2000 mV.DFT Calculations. The calculations were performed within the

framework of DFT using Vienna Ab Initio Simulation Package

(VASP) code,64,65 with the projector augmented wave method todescribe the interaction between ions and electrons66,67 and with aplane wave expanded to an energy cutoff of 400 eV. Exchange−correlation effects were described by the van der Waals densityfunctional (vdW-DF)68 using the version by Hamada denoted as rev-vdWDF2,69 which has been shown to describe the adsorption ofaromatic hydrocarbons on Au(111) accurately.70 The Au(111)surface was modeled as a four-layer slab vertically separated by avacuum region of 15 Å. For the debromination calculations we used ap(7 × 6) surface unit cell, while for the coupling reaction a p(10 × 9)surface unit cell was used, together with a 3×3×1 k-point grid in bothcases. Reaction pathways were calculated with a combination of theclimbing image nudge elastic band (CI-NEB)71 and dimer72 methods,where CI-NEB was used to find an initial guess of a transition state,which was then refined by the dimer method. The atomic structureswere geometrically optimized until the residual forces on all atoms,except the two bottom layers of the Au(111) slab (kept fixed), were

Figure 4. (a) Large-scale and (b) close-up STM images showing the formation of PPV chains after deposition of BDBMB molecules onAu(111) held at RT and annealing to ∼400 K. The XPS data of Br 3d are inserted in (a). (c) Equally scaled STM image, STM simulation, nc-AFM image recorded by CO-functionalized tip/with model overlaid, nc-AFM simulation, and DFT-optimized model of a single PPV chainon Au(111). (d) Close-up STM image of PPV chain and (e) corresponding nc-AFM image highlighting the C−C double-bond configuration.(f) Close-up STM image of the PPV chain with the linkages in two directions and (g) corresponding nc-AFM image. (h) STM image of aPPV chain on the Au(111) surface. (i) dI/dV spectra recorded at different sites (1, PPV edge; 2−4, PPV chain; 5, Au(111) substrate) asmarked in (h). The dI/dV spectra were acquired by a lock-in amplifier while the sample bias was modulated by a 553 Hz, 30 mV (r.m.s)sinusoidal signal under open-feedback conditions.

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7963

smaller than 0.01 eV/Å. The simulated STM image was obtainedusing the Tersoff−Hamann method,73 in which the local density ofstates is used to approximate the tunneling current.Synchrotron Radiation Photoemission Spectroscopy. The

SRPES experiments were performed at the Catalysis and SurfaceScience Endstation at the BL11U beamline in the NationalSynchrotron Radiation Laboratory (NSRL) in Hefei, China. Thesample was transferred from the STM chamber to the endstation by atransport suitcase with a getter pump to keep the pressure under 5 ×10−9 mbar. The core level spectra of Br 3d were recorded with a VGScienta R4000 analyzer with a photon energy of 160 eV, and the peakfitting was performed using the XPSPEAK41 program with Gaussianfunctions after subtraction of a Shirley background. The photonenergies were calibrated and referenced to the binding energy of Au 4fspectrum from a sputter-cleaned Au substrate before every scan of theBr 3d spectra.Synthesis. All commercially available chemicals were purchased

from Adamas-beta, Aldrich, and TCI and used as received withoutfurther purification. 1H NMR spectra were recorded on a BrukerAVANCE 400 spectrometer. The chemical shifts are reported in δppm with reference to residual protons of CDCl3 (7.26 ppm in 1HNMR and 77.16 ppm in 13C NMR). Thin-layer chromatography wasperformed on glass plates coated with a 0.20 mm thickness of silicagel. Column chromatography was performed using neutral silica gelPSQ100B.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.8b02459.

Additional STM images and synthesis data (PDF)

AUTHOR INFORMATIONCorresponding Authors*E-mail: xhqiu@nanoctr.cn.*E-mail: xuwei@tongji.edu.cn.ORCIDQiang Sun: 0000-0003-4903-4570Jonas Bjork: 0000-0002-1345-0006Junfa Zhu: 0000-0003-0888-4261Wei Xu: 0000-0003-0216-794XAuthor Contributions¶L. Cai and X. Yu contributed equally to this work.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSThe authors acknowledge the financial support from theNational Natural Science Foundation of China (21473123,21622307, 21790351, 51403157, 21603045, 21425310,91427301) and the Fundamental Research Funds for theCentral Universities of China and International CooperationTraining Project Funding for Postgraduate of TonggjiUniversity (2018XKJC-009). T.J. and J.B. acknowledgefunding from the Alexander von Humboldt Foundation.Computational resources were allocated by the NationalSupercomputer Centre, Sweden, through SNAC.

REFERENCES(1) Friend, R. H.; Bott, D. C.; Bradley, D. D. C.; Chai, C. K.; Feast,W. J.; Foot, P. J. S.; Giles, J. R. M.; Horton, M. E.; Pereira, C. M.;Townsend, P. D. Electronic Properties of Conjugated Polymers.Philos. Trans. R. Soc., A 1985, 314, 37−49.

(2) Bradley, D. D. C. Precursor-Route Poly(p-phenylenevinylene):Polymer Characterisation and Control of Electronic Properties. J.Phys. D: Appl. Phys. 1987, 20, 1389−1410.(3) Wegner, G. Polymers with Metal-Like Conductivity-A Review oftheir Synthesis, Structure and Properties. Angew. Chem., Int. Ed. Engl.1981, 20, 361−381.(4) Colaneri, N. F.; Bradley, D. D. C.; Friend, R. H.; Burn, P. L.;Holmes, A. B.; Spangler, C. W. Photoexcited States in Poly(p-phenylene vinylene): Comparison with trans,trans-Distyrylbenzene, aModel Oligomer. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 42,11670−11681.(5) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-EmittingDiodes Based on Conjugated Polymers. Nature 1990, 347, 539−541.(6) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.;Holmes, A. B. Synthesis of Light-Emitting Conjugated Polymers forApplications in Electroluminescent Devices. Chem. Rev. 2009, 109,897−1091.(7) Parker, I. D.; Gymer, R. W.; Harrison, M. G.; Friend, R. H.;Ahmend, H. Fabrication of a Novel Electro-Optical IntensityModulator from the Conjugated Polymer, Poly(2,5-dimethoxy-p-phenylene vinylene). Appl. Phys. Lett. 1993, 62, 1519−1521.(8) Antoniadis, H.; Hsieh, B. R.; Abkowitz, M. A.; Jenekhe, S. A.;Stolka, M. Photovoltaic and Photoconductive Properties ofAluminum/poly (p-phenylene vinylene) Interfaces. Synth. Met.1994, 62, 265−271.(9) Hayashi, Y.; Yamada, I.; Takagi, S.; Takasu, A.; Soga, T.; Jimbo,T. Influence of Structure and C60 Composition on Properties ofBlends and Bilayers of Organic Donor-Acceptor Polymer/C60Photovoltaic Devices. Jpn. J. Appl. Phys. 2005, 44, 1296−1300.(10) Blayney, A. J.; Perepichka, I. F.; Wudl, F.; Perepichka, D. F.Advances and Challenges in the Synthesis of Poly (p-phenylenevinylene)-Based Polymers. Isr. J. Chem. 2014, 54, 674−688.(11) Abdellatif, M. M.; Nomura, K. Precise Synthesis of End-Functionalized Oligo(2,5-dialkoxy-1,4-phenylene vinylene)s withControlled Repeat Units via Combined Olefin Metathesis andWittig-Type Coupling. Org. Lett. 2013, 15, 1618−1621.(12) Liao, J.; Wang, Q. Ruthenium-Catalyzed KnoevenagelCondensation: A New Route toward Cyano-Substituted Poly(p-phenylenevinylene)s. Macromolecules 2004, 37, 7061−7063.(13) Gill, R. E.; van Hutten, P. F.; Meetsma, A.; Hadziioannou, G.Synthesis and Crystal Structure of a Cyano-Substituted Oligo(p-phenylenevinylene). Chem. Mater. 1996, 8, 1341−1346.(14) Kim, J. H.; Lee, H. Synthesis, Electrochemistry, andElectroluminescence of Novel Red-Emitting Poly(p-phenyleneviny-lene) Derivative with 2-Pyran-4-ylidene−Malononitrile Obtained bythe Heck Reaction. Chem. Mater. 2002, 14, 2270−2275.(15) Bao, Z.; Chen, Y.; Cai, R.; Yu, L. Conjugated Liquid CrystallinePolymers-Soluble and Fusible Poly(phenyleneviny1ene) by the HeckCoupling Reaction. Macromolecules 1993, 26, 5281−5286.(16) Martelock, H.; Greiner, A.; Heitz, W. Structural Modificationsof Poly(1, 4-phenylenevinylene) to Soluble, Fusible, Liquid-Crystalline Products. Makromol. Chem. 1991, 192, 967−979.(17) Parekh, B. P.; Tangonan, A. A.; Newaz, S. S.; Sanduja, S. K.;Ashraf, A. Q.; Krishnamoorti, R.; Lee, T. R. Use of DMF as SolventAllows for the Facile Synthesis of Soluble MEH-PPV. Macromolecules2004, 37, 8883−8887.(18) Neef, C. J.; Ferraris, J. P. MEH-PPV: Improved SyntheticProcedure and Molecular Weight Control. Macromolecules 2000, 33,2311−2314.(19) Sun, Q.; Cai, L.; Wang, S.; Widmer, R.; Ju, H.; Zhu, J.; Li, L.;He, Y.; Ruffieux, P.; Fasel, R.; Xu, W. Bottom-up Synthesis ofMetalated Carbyne. J. Am. Chem. Soc. 2016, 138, 1106−1109.(20) Zhong, D.; Franke, J.; Podiyanachari, S. K.; Blcmker, T.; Zhang,H.; Kehr, G.; Erker, G.; Fuchs, H.; Chi, L. Linear AlkanePolymerization on a Gold Surface. Science 2011, 334, 213−216.(21) Sun, Q.; Zhang, C.; Kong, H.; Tan, Q.; Xu, W. On-SurfaceAryl−Aryl Coupling via Selective C-H Activation. Chem. Commun.2014, 50, 11825−11828.

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7964

(22) Wiengarten, A.; Seufert, K.; Auwarter, W.; Ecija, D.; Diller, K.;Allegretti, F.; Bischoff, F.; Fischer, S.; Duncan, D. A.; Papageorgiou, A.C.; Klappenberger, F.; Acres, R. G.; Ngo, T. H.; Barth, J. V. Surface-Assisted Dehydrogenative Homocoupling of Porphine Molecules. J.Am. Chem. Soc. 2014, 136, 9346−9354.(23) Kocic, N.; Liu, X.; Chen, S.; Decurtins, S.; Krejcí, O.; Jelínek,P.; Repp, J.; Liu, S. X. Control of Reactivity and Regioselectivity foron-Surface Dehydrogenative Aryl-Aryl Bond Formation. J. Am. Chem.Soc. 2016, 138, 5585−5593.(24) Li, Q.; Yang, B.; Lin, H.; Aghdassi, N.; Miao, K.; Zhang, J.;Zhang, H.; Li, Y.; Duhm, S.; Fan, J.; Chi, L. Surface-ControlledMono/Diselective ortho C−H Bond Activation. J. Am. Chem. Soc.2016, 138, 2809−2814.(25) Han, P.; Akagi, K.; Canova, F. F.; Mutoh, H.; Shiraki, S.; Iwaya,K.; Weiss, P. S.; Asao, N.; Hitosugi, T. Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity.ACS Nano 2014, 8, 9181−9197.(26) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.;Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.;Mullen, K.; Fasel, R. Atomically Precise Bottom-up Fabrication ofGraphene Nanoribbons. Nature 2010, 466, 470−473.(27) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.;Hecht, S. Nano-Architectures by Covalent Assembly of MolecularBuilding Blocks. Nat. Nanotechnol. 2007, 2, 687−691.(28) Lafferentz, L.; Eberhardt, V.; Dri, C.; Africh, C.; Comelli, G.;Esch, F.; Hecht, S.; Grill, L. Controlling on-Surface Polymerization byHierarchical and Substrate-Directed Growth. Nat. Chem. 2012, 4,215−220.(29) Wang, W.; Shi, X.; Wang, S.; Van Hove, M. A.; Lin, N. Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction. J. Am. Chem. Soc. 2011, 133,13264−13267.(30) Fan, Q.; Wang, C.; Han, Y.; Zhu, J.; Hieringer, W.; Kuttner, J.;Hilt, G.; Gottfried, J. M. Surface-Assisted Organic Synthesis ofHyperbenzene Nanotroughs. Angew. Chem., Int. Ed. 2013, 52, 4668−4672.(31) Gao, H. Y.; Held, P. A.; Knor, M.; Muck-Lichtenfeld, C.;Neugebauer, J.; Studer, A.; Fuchs, H. Decarboxylative Polymerizationof 2, 6-Naphthalenedicarboxylic Acid at Surfaces. J. Am. Chem. Soc.2014, 136, 9658−9663.(32) Liu, J.; Chen, Q.; Xiao, L.; Shang, J.; Zhou, X.; Zhang, Y.;Wang, Y.; Shao, X.; Li, J.; Chen, W.; Xu, G. Q.; Tang, H.; Zhao, D.;Wu, K. Lattice-Directed Formation of Covalent and OrganometallicMolecular Wires by Terminal Alkynes on Ag Surfaces. ACS Nano2015, 9, 6305−6314.(33) Sun, Q.; Zhang, C.; Cai, L.; Xie, L.; Tan, Q.; Xu, W. On-SurfaceFormation of Two-Dimensional Polymer via Direct C-H Activation ofMetal Phthalocyanine. Chem. Commun. 2015, 51, 2836−2839.(34) Kawai, S.; Haapasilta, V.; Lindner, B. D.; Tahara, K.; Spijker, P.;Buitendijk, J. A.; Pawlak, R.; Meier, T.; Tobe, Y.; Foster, A. S.; Meyer,E. Thermal Control of Sequential on-Surface Transformation of aHydrocarbon Molecule on a Copper Surface. Nat. Commun. 2016, 7,12711.(35) Zhou, H.; Liu, J.; Du, S.; Zhang, L.; Li, G.; Zhang, Y.; Tang, B.Z.; Gao, H. J. Direct Visualization of Surface-Assisted Two-Dimensional Diyne Polycyclotrimerization. J. Am. Chem. Soc. 2014,136, 5567−5570.(36) Cirera, B.; Gimenez-Agullo, N.; Bjork, J.; Martínez-Pena, F.;Martin-Jimenez, A.; Rodriguez-Fernandez, J.; Pizarro, A. M.; Otero,R.; Gallego, J. M.; Ballester, P.; Galan-Mascaros, J. R.; Ecija, D.Thermal Selectivity of Intermolecular versus Intramolecular Reactionson Surfaces. Nat. Commun. 2016, 7, 11002.(37) Zhang, Y. Q.; Kepcija, N.; Kleinschrodt, M.; Diller, K.; Fischer,S.; Papageorgiou, A. C.; Allegretti, F.; Bjork, J.; Klyatskaya, S.;Klappenberger, F.; Ruben, M.; Barth, J. V. Homo-Coupling ofTerminal Alkynes on a Noble Metal Surface. Nat. Commun. 2012, 3,1286.

(38) Gao, H. Y.; Wagner, H.; Zhong, D.; Franke, J. H.; Studer, A.;Fuchs, H. Glaser Coupling at Metal Surfaces. Angew. Chem., Int. Ed.2013, 52, 4024−4028.(39) Sun, Q.; Cai, L.; Ding, Y.; Ma, H.; Yuan, C.; Xu, W. Single-Molecule Insight into Wurtz Reaction on Metal Surfaces. Phys. Chem.Chem. Phys. 2016, 18, 2730−2735.(40) Oteyza, D. G. de; Gorman, P.; Chen, Y. C.; Wickenburg, S.;Riss, A.; Mowbray, D. J.; Etkin, G.; Pedramrazi, Z.; Tsai, H. Z.; Rubio,A.; Crommie, M. F.; Fischer, F. R. Direct Imaging of Covalent BondStructure in Single-Molecule Chemical Reactions. Science 2013, 340,1434−1437.(41) Sun, Q.; Zhang, C.; Li, Z.; Kong, H.; Tan, Q.; Hu, A.; Xu, W.On-Surface Formation of One-Dimensional Polyphenylene throughBergman Cyclization. J. Am. Chem. Soc. 2013, 135, 8448−8451.(42) Bebensee, F.; Bombis, C.; Vadapoo, S. R.; Cramer, J. R.;Besenbacher, F.; Gothelf, K. V.; Linderoth, T. R. On-Surface Azide-Alkyne Cycloaddition on Cu (111): Does It “Click” in UltrahighVacuum? J. Am. Chem. Soc. 2013, 135, 2136−2139.(43) Díaz Arado, O.; Monig, H.; Wagner, H.; Franke, J. H.;Langewisch, G.; Held, P. A.; Studer, A.; Fuchs, H. On-Surface Azide-Alkyne Cycloaddition on Au (111). ACS Nano 2013, 7, 8509−8515.(44) Franc, G.; Gourdon, A. Covalent Networks through on-SurfaceChemistry in Ultra-High Vacuum: State-of-the-Art and RecentDevelopments. Phys. Chem. Chem. Phys. 2011, 13, 14283−14292.(45) Fan, Q.; Gottfried, J. M.; Zhu, J. Surface-Catalyzed C-CCovalent Coupling Strategies toward the Synthesis of Low-Dimen-sional Carbon-Based Nanostructures. Acc. Chem. Res. 2015, 48,2484−2494.(46) Dong, L.; Liu, P. N.; Lin, N. Surface-Activated CouplingReactions Confined on a Surface. Acc. Chem. Res. 2015, 48, 2765−2774.(47) Held, P. A.; Fuchs, H.; Studer, A. Covalent-Bond Formation viaOn-Surface Chemistry. Chem. - Eur. J. 2017, 23, 5874−5892.(48) Zuzak, R.; Dorel, R.; Krawiec, M.; Such, B.; Kolmer, M.;Szymonski, M.; Echavarren, A. M.; Godlewski, S. NonaceneGenerated by On-Surface Dehydrogenation. ACS Nano 2017, 11,9321−9329.(49) Gao, H. Y.; Held, P. A.; Amirjalayer, S.; Liu, L.; Timmer, A.;Schirmer, B.; Arado, O. D.; Monig, H.; Muck-Lichtenfeld, C.;Neugebauer, J.; Studer, A.; Fuchs, H. Intermolecular On-Surface σ-Bond Metathesis. J. Am. Chem. Soc. 2017, 139, 7012−7019.(50) Sun, Q.; Tran, B. V.; Cai, L.; Ma, H.; Yu, X.; Yuan, C.; Stohr,M.; Xu, W. On-Surface Formation of Cumulene by DehalogenativeHomocoupling of Alkenyl gem-Dibromides. Angew. Chem., Int. Ed.2017, 56, 12165−12169.(51) Xi, H.; Gibb, C. L. D.; Gibb, B. C. Functionalized Deep-CavityCavitands. J. Org. Chem. 1999, 64, 9286−9288.(52) Zhang, C.; Sun, Q.; Chen, H.; Tan, Q.; Xu, W. Formation ofPolyphenyl Chains through Hierarchical Reactions: UllmannCoupling Followed by Cross Dehydrogenative Coupling. Chem.Commun. 2015, 51, 495−498.(53) Bjork, J.; Hanke, F.; Stafstrom, S. Mechanisms of Halogen-Based Covalent Self-Assembly on Metal Surfaces. J. Am. Chem. Soc.2013, 135, 5768−5775.(54) Simonov, K. A.; Vinogradov, N. A.; Vinogradov, A. S.;Generalov, A. V.; Zagrebina, E. M.; Martensson, N.; Cafolla, A. A.;Tomas Carpy, T.; Cunniffe, J. P.; Preobrajenski, A. B. Effect ofSubstrate Chemistry on the Bottom-Up Fabrication of GrapheneNanoribbons: Combined Core-Level Spectroscopy and STM Study. J.Phys. Chem. C 2014, 118, 12532−12540.(55) Basagni, A.; Ferrighi, L.; Cattelan, M.; Nicolas, L.; Handrup, K.;Vaghi, L.; Papagni, A.; Sedona, F.; Valentin, C. D.; Agnoli, S.; Sambi,M. On-Surface Photo-Dissociation of C−Br Bonds: towards RoomTemperature Ullmann Coupling. Chem. Commun. 2015, 51, 12593−12596.(56) Zhang, H.; Lin, H.; Sun, K.; Chen, L.; Zagranyarski, Y.;Aghdassi, N.; Duhm, S.; Li, Q.; Zhong, D.; Li, Y.; Mullen, K.; Fuchs,H.; Chi, L. On-Surface Synthesis of Rylene-Type GrapheneNanoribbons. J. Am. Chem. Soc. 2015, 137, 4022−4025.

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7965

(57) Di Giovannantonio, M.; Urgel, J. I.; Beser, U.; Yakutovich, A.V.; Wilhelm, J.; Pignedoli, C. A.; Ruffieux, P.; Narita, A.; Mullen, K.;Fasel, R. On-Surface Synthesis of Indenofluorene Polymers byOxidative Five-Membered Ring Formation. J. Am. Chem. Soc. 2018,140, 3532−3536.(58) Andreev, T.; Barke, I.; Hovel, H. Adsorbed Rare-Gas Layers onAu(111): Shift of the Shockley Surface State Studied with UltravioletPhotoelectron Spectroscopy and Scanning Tunneling Spectroscopy.Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 205426.(59) Talirz, L.; Sode, H.; Dumslaff, T.; Wang, S.; Sanchez-Valencia,J. R.; Liu, J.; Shinde, P.; Pignedoli, C. A.; Liang, L.; Meunier, V.;Plumb, N. C.; Shi, M.; Feng, X.; Narita, A.; Mullen, K.; Fasel, R.;Ruffieux, P. On-Surface Synthesis and Characterization of 9-AtomWide Armchair Graphene Nanoribbons. ACS Nano 2017, 11, 1380−1388.(60) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G. ConductingPolymers: Halogen Doped Polyacetylene. J. Chem. Phys. 1978, 69,5098−5104.(61) Kawai, S.; Saito, S.; Osumi, S.; Yamaguchi, S.; Foster, A. S.;Spijker, P.; Meyer, E. Atomically Controlled Substitutional Boron-Doping of Graphene Nanoribbons. Nat. Commun. 2015, 6, 8098.(62) Besenbacher, F. Scanning Tunnelling Microscopy Studies ofMetal Surfaces. Rep. Prog. Phys. 1996, 59, 1737−1802.(63) Lægsgaard, E.; Osterlund, L.; Thostrup, P.; Rasmussen, P. B.;Stensgaard, I.; Besenbacher, F. A High-Pressure Scanning TunnelingMicroscope. Rev. Sci. Instrum. 2001, 72, 3537−3542.(64) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Open-Shell Transition Metals. Phys. Rev. B: Condens. Matter Mater. Phys.1993, 48, 13115−13118.(65) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for AbInitio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys.Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186.(66) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B:Condens. Matter Mater. Phys. 1994, 50, 17953−17979.(67) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to theProjector Augmented-Wave Method. Phys. Rev. B: Condens. MatterMater. Phys. 1999, 59, 1758−1775.(68) Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.;Lundqvist, B. I. Van der Waals Density Functional for GeneralGeometries. Phys. Rev. Lett. 2004, 92, 246401−24604.(69) Hamada, I. Van der Waals Density Functional Made Accurate.Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 121103.(70) Bjork, J.; Stafstrom, S. Adsorption of Large Hydrocarbons onCoinage Metals: A van der Waals Density Functional Study.ChemPhysChem 2014, 15, 2851−2858.(71) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A ClimbingImage Nudged Elastic Band Method for Finding Saddle Points andMinimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904.(72) Kastner, J.; Sherwood, P. Superlinearly Converging DimerMethod for Transition State Search. J. Chem. Phys. 2008, 128,014106−014111.(73) Tersoff, J.; Hamann, D. R. Theory of the Scanning TunnelingMicroscope. Phys. Rev. B: Condens. Matter Mater. Phys. 1985, 31,805−813.

ACS Nano Article

DOI: 10.1021/acsnano.8b02459ACS Nano 2018, 12, 7959−7966

7966