On‐Surface Synthesis of Cumulene‐Containing Polymers via Two‐Step Dehalogenative Homocoupling of Dibromomethylene‐Functionalized Tribenzoazulene

Abstract Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on‐surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well‐controlled ultrahigh‐vacuum conditions. Here we report the on‐surface synthesis of a polymer linked by cumulene‐like bonds on a Au(111) surface via sequential thermally activated dehalogenative C−C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene‐like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X‐ray photoelectron spectroscopy, complemented by density functional theory calculations. Our results provide perspectives for the on‐surface synthesis of cumulene‐containing compounds, as well as protocols relevant to the stepwise fabrication of carbon–carbon bonds on surfaces.


Introduction
Over the last century,t he well-known carbon allotropes with sp 2 -( graphite) and sp 3 -( diamond) hybridization have been extensively investigated due to their relevance,r anging from drugs to synthetic materials of interest in many applications because of their high surface area and physicochemical properties. [1][2][3] Only in 1985, fullerenes were observed for the first time, [4] opening an ew era for synthetic carbon allotropes.S ince then, many more allotropes such as graphene, [5] carbon nanotubes, [6] nanobuds, [7] and schwarzites [8] have been discovered, bringing revolutionary developments in diverse fields ranging from nanoelectronics to catalysis and energy devices.N evertheless,m uch less is known about carbon allotropes with sp-hybridization. Carbyne,alinear chain of sp-hybridized carbon atoms, [9] has been the subject of controversy over the last decades. [10] It can exist in two different forms:e ither as polyyne,w here the carbon atoms that compose the linear chain are linked by alternate single and triple bonds (-C C-), or as cumulene with carbon atoms linked by consecutive double bonds ( = C = C = ). [11] While the synthesis of polyynes has been widely explored, [12][13][14] investigations on cumulene compounds have remained scarce as aresult of the drastic increase in reactivity as the number of consecutive cumulene bonds increases. [15][16][17] Recently,traditional solution synthesis has been extended to the study of chemical reactions on single-crystal surfaces under ultrahigh-vacuum (UHV) conditions,w hich provides av ersatile playground for the investigation of novel surfacesupported nanostructures that are difficult to achieve in solution. Theuse of halogen leaving groups at specific sites of the molecules (with low bond energies in comparison to the molecular backbone) has emerged as aparticularly successful approach in the study of on-surface reactions. [15] On metal substrates,m oderate heating readily induces dehalogenation and thus reactive radical sites,w hich, together with surface diffusion of the activated molecules,l eads to the desired formation of covalent (carbon-carbon) bonds.N evertheless, as tepwise pathway to covalent linking reactions on surfaces has been achieved only rarely. [16] Over the last decades, ap lethora of on-surface reactions based on CÀCc oupling such as (dehalogenative) aryl-aryl coupling, [8,15,17,18] and cyclodehydrogenation [19] have been studied via scanning probe microscopy (SPM) and other surface analysis techniques.O nly recently,t he formation of cumulene-containing dimers and polymers by dehalogenative homocoupling reactions of gem-dibromides [20,21] and by enediyne coupling, [22] together with the generation of several intermediates on different surfaces via scanning tunneling microscopy (STM)based manipulation has been reported. [23,24] However,d etailed studies of the on-surface synthesis of cumulenecontaining compounds and their in-depth structural and electronic characterization have remained elusive.H ere,w e report on ac omprehensive STM, scanning tunneling spectroscopy (STS), non-contact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) study of the surface-confined formation of ao ne-dimensional polymer linked by cumulene-like bonds.I mportantly,t he specific adsorption geometry of the molecular precursor 1,5-bis(dibromomethylene)-1,5-dihydrobenzo [5,6]cyclohepta [1,2,3,4-def]fluorene (1)o n Au(111) leads to two coupling steps separated in temperature that result in as elective head-to-head/tail-to-tail monomer sequence in the polymer.

Results and Discussion
Them olecular precursor 1 consists of at ribenzoazulene (TBA) core functionalized with two dibromomethylenes at the pentagon and heptagon ends.I tw as synthesized in four steps:F irst, aSuzuki coupling of 2-biphenylboronic acid and 2-bromoisophthalaldehyde gave aldehyde 4 in ayield of 81 %, which was then quantitatively oxidized to the carboxylic acid 5.I nt he next step,c ompound 5 was cyclized with methanesulfonic acid to ketone 6 with ay ield of 72 %a nd finally precursor 1 was achieved by Ramirez reaction with ayield of 44 %( for synthetic details and characterization, see the Supporting Information). Scheme 1i llustrates the observed stepwise on-surface reactions in the formation of polymers linked by cumulene-like bonds (3)onaAu(111) substrate.A first annealing process leads to the partial dehalogenation of 1 which produces molecular dimers 2 that are selectively connected via three consecutive C À Cdouble bonds linked to the nonbenzenoid pentagonal moieties.Asecond annealing process activates the next dehalogenation step,g iving rise to 1D polymer chains 3 that link the azulene cores via the same cumulene-like connections.
To investigate the on-surface reactions depicted in Scheme 1, asub-monolayer coverage of 1 was sublimed onto the Au(111) substrate held at 325 K. Large-scale STM images show sporadic individual molecular species with al arge apparent height of 3.0 (measured at as ample bias of À0.9 V) coexisting with dimers of as imilar apparent height for both or one of the molecular species (see Figure S1 for more details).
These features suggest that the molecules adopt ah ighly nonplanar conformation on the surface.F igure 1a shows ah igh-resolution STM image of an intact molecule.T he experimental features of 1 are well reproduced by the corresponding STM simulation based on the DFT-optimized geometry on the Au(111) surface (Figure 1b,c), which reveals that the two bromine atoms at the pentagon end of the TBA are much closer to the gold surface than those at the heptagon end (see Figure S2 for details regarding the possible adsorption geometries of 1). Therefore,t his adsorption geometry favors the sequential surface-catalyzed dehalogenation of the molecular precursors,where the dibromomethylenes attached to the pentagonal moiety are debrominated first by virtue of their close proximity to the gold surface.
In order to unravel the reaction mechanism that leads to 3, we have deposited 1 on Au(111) held at 20 Ktoprevent any activation of the molecular precursors,a nd performed temperature-programmed XPS (TP-XPS) measurements. [25] We monitored the Br 3d core level during the annealing of the sample from 230 to 700 Kw ith ac onstant heating rate of 0.2 Ks À1 and extracted kinetic curves that clarify the chemical reactions involving bromine. [28] Figure 1ddisplays the relative amount of the two main components of the bromine signal, that is,B r-C( due to intact bromine functionalities still anchored to the molecule) and Br-Au( associated with bromine detached from the precursor and chemisorbed on the gold substrate). At wo-step sequential dehalogenation process is deduced from these curves,i na greement with the pathway described above (see Figure S3 for details about the TP-XPS map that monitors the reaction pathway and highresolution XPS spectra). Due to the highly nonplanar Scheme 1. On-surfacesynthesis of cumulene-containing polymers.
conformation of 1 described above,w ea ssume that in af irst debromination step (from 270 to 320 K), the bromine atoms at the pentagon end of the TBAc ore dissociate from 1 and bind to the Au(111) surface,which induces the formation of 2, while the second debromination step (from 320 to 390 K) is attributed to the dissociation of the bromine atoms on the heptagon end. At higher temperatures,t he total bromine signal gradually decreases indicating bromine desorption from the Au(111) surface which is completed at 650 K.
Debromination of 2 is completed after annealing the sample to 400 K, as can be seen in Figure 2and Figure 1d.At such temperatures,n ew species appearing as 1D chains with lower apparent height (1.6 )are discerned, which we assign to polymers linked by cumulene-like bonds (3). Figure 2a,b depicts constant-current STM images of 3,e xhibiting meandering chain-like structures composed of molecules with triangular appearance coexisting with asmall fraction (< 7%) of side products (fused molecules), which are tentatively attributed to the formation of pentalene units ( Figure S4). [27] Figure 2c shows an ultrahigh-resolution STM (UHR-STM) image of 3 acquired with aCO-functionalized tip recorded in the Pauli repulsion regime, [28][29][30] where the intramolecular features of the nonbenzenoid molecular backbone in the polymers are clearly discerned. Importantly,aselective C À C coupling,that is,pentagon-pentagon and heptagon-heptagon connections,i nt he formation of 3 is observed. Theg raph depicted in Figure 2d displays the selectivity in the molecular connections of the polymers.S pecifically,m ore than 80 %o f the connections occur between pentagonal-pentagonalo r heptagonal-heptagonal moieties (Scheme 1), while all other connections take place between pentagonal-heptagonalmoieties (statistics out of > 110 molecules). Such selectivity induces ahierarchical CÀCcoupling which is often hampered in covalent linking reactions obtained by one-step on-surface reactions,where only rather simple nanostructures have been achieved.
Theu se of similar functional groups in the on-surface formation of cumulene-like bridged dimers [20] and polymers, [21] and ethynylene-bridged polymers [31] on the Au(111) surface has recently been investigated. In order to confirm the chemical nature of the connections observed in 3,n c-AFM measurements using aC O-functionalized tip were performed. [33] Figure 3a,b depicts the resulting STM and constant-height frequency-shift images where features assigned to the nonbenzenoid molecular backbone linked by as harp line with ah omogeneous contrast are clearly resolved. Importantly,i ntramolecular contrast observed in nc-AFM images results from the short-ranged Pauli repulsion being  maximized in the areas of highest electron density.Therefore, minor variations in electron density assigned to specific bond orders can be recognized. [32,33] Forinstance,C ÀCtriple bonds exhibit in nc-AFM images an enhanced contrast at their central positions. [31,[34][35][36][37] Notably,t he sharp lines observed in the intermolecular connections of 3 (highlighted by the blue arrows in Figure 3b)are well reproduced by the simulated nc-AFM image depicted in Figure 3d,( see Figure S5 for nc-AFM images acquired at different tip-polymer distances). In addition, the DFT-optimized structure (Figure 3c)o f3 on Au(111) reveals an adsorption height of 3.3 ,w ith aC À C distance between cumulene bonds of 3.9 (in agreement with the experimentally measured length of 4.0 AE 0.2 , dashed blue line in Figure 3b). Altogether,t his provides conclusive evidence for the structural assignment of the three consecutive CÀCd ouble bonds, [20,22] that is,c umulene-like connections in the formation of 3 on the Au(111) surface.I n order to further investigate the bond orders of the cumulenelike connections,w ep erformed bond order analysis on the DFT electronic structure based on Badersc harge density partitioning. [39,40] We find that the bond order of the middle bond is 2.11 (CÀCd istance of 1.32 ,c olored red in Figure 3c), while the bond order of the neighboring bonds is 1.44 (C À Cd istance of 1.41 ,c olored blue in Figure 3c), indicating as tronger similarity to ac umulene system, rather than ac onfiguration involving at riple bond. [40] STS measurements on 3 were performed in order to probe its electronic structure.T he positive-and negative-ion resonances (PIR and NIR) that derive from the HOMO and LUMO of the polymer are experimentally detected at À0.7 eV and 0.9 eV,r espectively ( Figure 4). Therefore,t he HOMO-LUMO gap of the polymer on Au(111) is found to be 1.6 eV.I nterestingly,t he voltage-dependent differential conductance spectra (dI/dV vs. V)a cquired at the three different cumulene connections (i.e.p entagon-pentagon, pentagon-heptagon, and heptagon-heptagon) reveal as ystematic upward shift of the frontier orbital energy positions from pentagon-pentagon to heptagon-heptagon connections. This originates from the frontier orbital energy ordering of the singular unit of the polymer,w hose HOMOÀ1a nd LUMO are localized at the pentagon edge,while HOMO and LUMO + 1a re localized at the heptagon, as shown by tight binding (TB) calculations (see Figure S6 for detailed TB calculations and additional dI/dV spectra). Thep airwise coupling of these orbitals form the experimentally observed frontier states of the polymer which are strongly localized on the cumulene connections.D etailed analysis reveals that the frontier orbitals at the pentagon-pentagon connection are the antibonding coupling of two HOMOÀ1a nd the bonding coupling of two LUMO orbitals of the singular unit, which are  T he orange, green, and blue crosses indicate the positions where differential conductanced I/dV spectra were acquired. Open feedback parameters: V b = À5mV, I t = 50 pA. b) dI/dV spectra acquired at the pentagon-cumulene-pentagon (orange line), pentagon-cumuleneheptagon (blue line) and heptagon-cumulene-heptagon (green line) connectionsof3,r evealing aHOMO-LUMO gap of 1.6 eV with the HOMO (LUMO) localized on the heptagon-cumulene-heptagon (pentagon-cumulene-pentagon) connections. The reference spectrum taken on the bare Au(111) surface is shown in pink. The insets show the frontier orbital wave functions of the corresponding molecular dimers (see Figure S6 for details).
both lower in energy than the corresponding orbitals in the pentagon-heptagon connection (antibonding HOMOÀ1a nd HOMO;a nd bonding LUMO and LUMO + 1). Thec orresponding frontier orbitals for the heptagon-heptagon connection (antibonding of two HOMO and bonding of two LUMO + 1) are in turn highest in energy.Asaresult of these couplings,t he HOMO and LUMO of 3 are localized at the pentagon-pentagon and heptagon-heptagon connections, respectively.T his is further corroborated by DFT band structure calculation for the polymer with alternating pentagon-pentagon and heptagon-heptagon connections,w hich shows flat bands ( Figure S7), revealing the nondispersive nature of the polymers,a se xpected from the obtained localized states.

Conclusion
We have demonstrated the on-surface synthesis of apolymer linked via cumulene-like bonds on ac oinage metal surface by ac ombination of high-resolution STM and nc-AFM imaging together with DFT calculations,STS,and XPS. As equential thermally induced dehalogenation is explained by the preferred geometry that the molecular precursor adopts upon absorption on the gold surface,w hich gives rise to selective CÀCc oupling,t hat is,p entagon-pentagon and heptagon-heptagon connections,i nt he formation of 3.T he formation of linear cumulene-like connections between the nonbenzenoid tribenzoazulene backbone units of the polymer 3 is confirmed by Bader analysis and nc-AFM investigations. In addition, STS studies together with theoretical calculations reveal that 3 exhibits an electronic gap of 1.6 eV where as ystematic shift of the frontier orbitals from pentagoncumulene-pentagon to heptagon-cumulene-heptagoni so bserved. We expect that our study is of general relevance for the synthesis and characterization of carbon allotropes with sp-hybridization, as well as for the stepwise formation of carbon-carbon covalent bonds on surfaces,thus opening new avenues in the field of on-surface synthesis with prospects for applications in molecular electronics.

Experimental Section
Sample preparation and STM/nc-AFM measurements.E xperiments were performedu nder ultrahigh-vacuumc onditions (base pressure below 5 10 À10 mbar) with al ow-temperature STM/AFM Scienta Omicron scanning probe microscope.T he Au(111) substrate was prepared by repeated cycleso fA r + sputtering (E = 1keV) and subsequentannealing to 750 Kfor 15 minutes.All STM images shown were recorded in constant-current mode with electrochemically etched tungsten tips at as ample temperature of 5K.S canning parameters are specifiedi ne ach figure caption. Them olecular precursor 1 was thermally deposited onto the clean Au(111) surface held at room temperature with atypical deposition rate of 0.4 min À1 (sublimation temperature % 420 K). Nc-AFM measurements were performed with atungsten tip attached to atuning fork sensor. [41] The tip was ap osteriorif unctionalized by the controlled pick-up of as ingle CO molecule at the tip apex from the previously CO-dosed surface.T he functionalizedt ip enablest he imaging of the intramolecularstructure of organic molecules. [42] Thesensor was driven at its resonance frequency ( 22 350 Hz) with ac onstant amplitude of % 70 pm. Theshift in the resonance frequency of the tuning fork (with the attached CO-functionalized tip) was recorded in constant-height mode (Omicron Matrix electronicsa nd HF2Li PLL by Zurich Instruments). TheS TM and nc-AFM images were analyzedu sing WSxM. [43] XPS measurements were performed at the X03DAb eamline (PEARL endstation) at the SLS synchrotronr adiation facility (Villigen, Switzerland), using linearly (and partially circularly left/ right) polarized radiation with photon energy of 425 eV.XPS spectra were obtained in normal emissiong eometry,u sing ah emispherical electron analyzere quippedw ith am ultichannel plate (MCP) detector.H R-XPS spectra were recorded at the indicated temperature in "swept" mode with 20 eV pass energy,w hile the TP-XPS measurement was performed during the heating of the sample (constant heating rate of 0.2 Ks À1 )using the "fixed" mode (snapshots of the Br 3d core level) acquiringe ach spectrumf or 5swith 100 eV pass energy.T he TP-XPS maps have ar esolution of 3.5 8 8Ci n temperature and 17 si ntime.
DFT calculationsw ere performedw ith the CP2K code (freely available at http://www.cp2k.org/) [44] utilizing the AiiDAplatform. [46] Thee lectronic states were expanded with aT ZV2P Gaussian basis set [47] for Ca nd Hs pecies and aD ZVP basis set for Au species.A cutoff of 600 Ry was used for the plane wave basis set. We used Norm Conserving Goedecker-Teter-Hutter [48] pseudopotentials and the PBE [49] exchange-correlation functionalw ith the D3 dispersion correctionsp roposed by Grimme. [50] Thes urface/adsorbate systems were modelled within the repeated slab Scheme, that is,asimulation cell containing four atomic layers of Au along the [111] direction and alayer of hydrogen atoms to passivate one side of the slab in order to suppress one of the two Au(111) surface states.4 0ngstromso f vacuum were included in the simulation cell to decouple the system from its periodicreplicas in the direction perpendiculartothe surface. We considered supercells of 41.27 40.85 corresponding to 224 surface units.T oo btain the equilibrium geometries,w ek ept the atomic positions of the bottom two layers of the slab fixed to the ideal bulk positions;a ll other atoms were relaxed until forces were lower than 0.005 eV À1 .T oo btain simulated STM images within the Te rsoff-Hamann approximation, [51] we extrapolated the electronic orbitals obtained from CP2K to the vacuum region in order to correct the wrong decay in vacuum of the charge density due to the localized basis.
TheProbe Particle model [50] was used to simulate AFM images.A two-point implementation of the model, where two probe particles represent the carbon and oxygen atoms in the CO molecule,has been employed. Thes tiffness parameters of the Probe Particle as well as the Lennard-Jones parameters of the tip were obtained by fitting to DFT calculations for the polymer 3 depicted in Figure 3c.T he charges of the tip atoms were assigned by the restrained electrostatic potential method. [51] Fort he tip sample electrostatic interactions the Hartree potential obtained from the CP2K slab calculations was used.
Bond order analysis was performedbythe scheme introduced by ngyµnetal. [38] based on the Kohn-Sham orbitals obtained from the CP2K calculation of the whole polymer-slab system. Badersb asins were calculatedwith the Bader Charge Analysis code by Henkelman et al. [52] based on the CP2K valence charge augmented with frozen core charge density.
Band structures were calculated with the Quantum Espresso software package using the PBE exchange-correlationfunctional. The plane wave basis with an energy cutoff of 400 Ry for the charge density was used together with PAWpseudopotentials (SSSP) [53] and aM onkhorst k-mesh of 13 1 1.