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Exploring the Relation Between Intramolecular Conjugation and Band Dispersion in One-Dimensional Polymers C. Garcia-Fernandez, Emil Sierda, Mikel Abadia, Bernhard Eberhard Christian Bugenhagen, Marc Heinrich Prosenc, Roland Wiesendanger, Maciej Bazarnik, Jose Enrique Ortega, Jens Brede, Eduard Matito, and Andrés Arnau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08668 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Exploring the Relation Between Intramolecular Conjugation and Band Dispersion in One-Dimensional Polymers C. García-Fernández,∗,† Emil Sierda,‡,¶ Mikel Abadía,§ Bernhard Bugenhagen,k Marc Heinrich Prosenc,k,⊥ Roland Wiesendanger,‡ Maciej Bazarnik,‡,¶ José Enrique Ortega,#,@,△ Jens Brede ,∗,§ Eduard Matito,∇,††,‡‡ and Andrés Arnau§,¶¶,‡‡ †Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián,Spain ‡Department of Physics, University of Hamburg, Jungiusstrasse 11, D-20355, Hamburg, Germany ¶Institute of Physics, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland §Centro de Física de Materiales CSIC/UPV-EHU-Materials Physics Center, Manuel Lardizabal 5, 20018-San Sebastian, Spain kInstitute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146, Hamburg, Germany ⊥Department of Chemistry, Technical University Kaiserslautern, Erwin-Schroedinger-Str. 52, D-67663, Kaiserslautern, Germany #Centro de Física de Materiales CFM - MPC, Centro Mixto CSIC-UPV/EHU, Paseo Manuel de Lardizabal 5, E-20018, San Sebastián, Spain @Donostia International Physics Center, Paseo Manuel Lardizabal 4, E-20018 San Sebastián, Spain △Departamento Física Aplicada I, Universidad del País Vasco, 20018 San Sebastián, Spain ∇IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain ††Faculty of Chemistry, University of the Basque Country UPV/EHU P.K. 1072, 20080 Donostia, Euskadi, Spain 2 Paseo Manuel de Lardizabal 4, 20018-San ‡‡Donostia International Physics Centre, ACS Paragon Plus Environment Sebastian, Spain

¶¶Departamento Física de Materiales, Universidad del País Vasco, 20018-San Sebastian,


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Abstract Making use of the inherent surface anisotropy of different high index surface planes vicinal to the low index Au(111) orientation, one-dimensional polymers have been synthesized following established procedures from two different precursor molecules. ′

The successful polymerization of both 4, 4′′ -dibromo-p-terphenyl and 5, 5 dibromo(salophenato)Co(II) precursors into poly(p-phenylen) and poly[(salophenato)Co(II)], respectively, has been confirmed by scanning tunneling microscopy and low energy electron diffraction. Angle-resolved photoemission spectroscopy data reveal a highly dispersive band in the case of poly(p-phenylen) while no significant dispersion is resolved for poly[(salophenato)Co(II)]. Based on density functional theory calculations, we explain this observation as a result of a high conjugation along the aromatic phenyl groups in poly(p-phenylen) that is absent in the case of poly[(salophenato)Co(II)], where intramolecular conjugation is interrupted in the salophenatoCo(II) unit. Furthermore, we make use of multicenter and delocalization indexes to characterize the electron mobility (corresponding to a high band dispersion) along different paths associated with individual molecular orbitals.

Introduction One of the recently developed strategies to build graphene nanostructures using a bottomup approach is based on the on-surface dehalogenative homocoupling reaction 1,2 of suitable molecular precursors. Generally, one- and two-dimensional molecular structures can readily be synthesized using this surface-confined variant of the Ullmann reaction. 3 The so obtained structures promise improved thermal and mechanical stability due to covalently linked subunits. 2 A particularly prominent example of one-dimensional molecular wires are graphene nanoribbons 4,5 (GNRs), whose electronic properties are being vastly investigated. 6–8 However, wide and long GNRs with a reduced number of defects have been grown only on metal surfaces, although the construction of electronic devices based on GNRs requires substrates with a high dielectric constant to facilitate gating. How3 ACS Paragon Plus Environment

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ever, the transfer of GNRs from a metal onto a suitable semiconductor 9 or the growth of one-dimensional wires directly on top of a semiconducting surface 10,11 represent real technological challenges. 12 So far, only the narrowest armchair GNR, i.e. poly(p-phenylen), has been synthesized on both metal 13 and semiconductor surfaces, 11 because the current bottleneck in on-surface GNR synthesis is the cyclodehydrogenation reaction which could so-far only be achieved on metal surfaces. The cyclodehydrogenation step follows the polymerization of molecular precursors via the Ullmann coupling reaction to yield the final product for all GNRs besides poly(p-phenylen). Therefore, it would be interesting to consider other precursors which do not require a cyclodehydrogenation reaction, apart from poly(pphenylen), although growing them on a metal surface, as a first step. At the same time, the possibility of inducing spin-polarization in the system could be an additional route to explore, as it might open a new route with potential application in spintronics. One such precursor exploits the incorporation of magnetic Co(II)-ions that induce spin-polarization in molecular bands, as was shown for poly[(salophenato)Co(II)], 14,15 but a full characterization of these bands including their dispersion and magnetic texture has not been addressed until now. Nevertheless, the use of precursor molecules with aromatic character does not necessarily lead to similar electronic properties of the resulting one-dimensional molecular wires after polymerization. In particular, the energy dispersion of molecular bands, intimately related to the electronic delocalization along the wire, is also determined by other factors and not only by the aromatic character of the individual molecular fragments in the precursor molecule. More specifically, the electronic delocalization along the polymeric chain can be reduced due to different reasons: (i) the lack of formation of intermolecular C-C bonds between monomeric units, i.e., the polymeric chain is too short, (ii) the appearance of geometrical distortions in the C-C bonds between units, like twisting angles between planar aromatic units/fragments, (iii) changes in the connections between units from para- to ortho/meta-positions giving rise to cross conjugation, and (iv) the presence 4 ACS Paragon Plus Environment


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of heteroatoms at the intramolecular level in the monomeric unit. These effects, although rather intuitive, deserve being understood. In this work, we study in detail the electronic structure of two different one-dimensional polymers aligned along terraces of high index Au surfaces vicinal to the Au(111) plane. With this aim, we first use scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) data to confirm the formation of sufficiently long polymeric wires, which allow their study by surface averaging angle-resolved photoemission spectroscopy (ARPES). We observe the existence of a highly dispersive band for poly(p-phenylen) on Au(433), which is considered to be a clear finger print of polymerization of poly(pphenylen) chains consisting of more than twenty covalently bonded phenyl units. However, in the case of poly[(salophenato)Co(II)] on Au(433), no significant dispersion is observed in the corresponding ARPES data despite, on average, a covalent linking of about ten salophenatoCo(II) units is deduced from STM data. Then, we address density functional theory calculations of the electronic band structure for the two one-dimensional periodic structures. We find, in nice agreement with the ARPES data, substantial differences in the energy dispersion of the molecular bands for the two systems under study. Additionally, in order to rationalize our findings, we make use of multicenter and delocalization indexes for the monomer units in the polymeric chains. This analysis allows us to identify the main reason for the low electronic delocalization along the poly[(salophenato)Co(II)] chains: already at the intramolecular level, there are no molecular orbitals with a high enough degree of conjugation between the aromatic rings, as compared with poly(p-phenylen). Finally, we introduce a necessary condition for the existence of high electron mobility in polymers by relating the electronic charge delocalization in molecular orbitals with the dispersion of the corresponding molecular band.

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Experimental Section Experiments were carried out in ultrahigh vacuum at base pressures below 2 · 10−10 mbar. The different Au single crystals were prepared by repeated cycles of sputtering (Ar+, 0.8 − 1.2 keV) and annealing to about 750 K. Scanning tunneling microscopy was carried out at room temperature or at about 25 K using an Omicron VT-STM or a home-built VT-STM, 16 respectively. STM images were recorded at constant tunneling current of 50 − 200 pA and constant bias voltages of 1 V (Fig.1 a),0.5 V (Fig.1 b-d) and −0.5 V (Fig.1 e), applied to the sample. Image processing was done with the WSxM software. 17 Angle resolved photoemission measurements were performed using a Phoibos 150 SPECS high-resolution hemispherical electron analyzer while the sample was cooled down to 150K. He-I (hν = 21.2 eV) radiation was provided by a high intensity UVS-300 SPECS discharge lamp coupled to a TMM-302 SPECS monochromator.

Results and Discussion Structural characterization by Scanning Tunneling Microscopy We employed various Au surfaces vicinal to the low index Au(111) surface to align molecular oligomers in one preferential direction exploiting the inherent surface anisotropy. The resulting quasi-one dimensional alignment allows us to maximize the photoemission intensity obtained by spatially averaging techniques. Here, we focus on the results obtained on the Au(433) surface [see SI and references 18 for additional data on other vicinal surfaces]. The Au(433) surface is characterized by mono-atomic steps separating “quasi-infinite one dimensional” terraces extending in [0, 1, 1] direction, where five narrow terraces [with terrace width tn ≈ 1.4 nm] alternate with one wide terrace [terrace width tw ≈ 4.1 nm] along the [2, 1, 1] direction 19,20 as depicted in Fig. 1 (a). The choice of the Au(433) surface was initially motivated by the possibility to study the influence of the terrace width on the self-assembly and polymerization reaction of molecular precursors. 6 ACS Paragon Plus Environment


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However, during the preparation and heat induced dehalogenation (leading to polymerization) the surface undergoes substantial restructuring, as can be seen from the STM images of poly[(salophenato)Co(II)] and poly(p-phenylen) covered surfaces in Fig. 1 (a) and (b), respectively. Adsorbate induced surface restructuring was already observed for benzodiguanamine and naphtalene tetracarboxylic diimide mixtures on Au(455), 21 PTCDA on surfaces vicinal to Ag(111), 22 as well as for metal phthalocyanine on Cu(110), 23 but a detailed study of the phenomenon for 5, 5 dibromo-(salophenato)Co(II) and 4, 4′′ -dibromo′

p-terphenyl is beyond the scope of this work. Importantly, under the given preparation conditions, we find that despite faceting of the surface due to the presence of molecular precursors, molecular chains are still preferentially aligned along the [0, 1, 1] direction. In agreement with previous studies of heat induced polymerization of both 5, 5 dibromo′

(salophenato)Co(II) 14,24,25 and 4, 4′′ -dibromo-p-terphenyl, 11,18 under the current preparation conditions, we find Br atoms embedded next to our molecular chains improving the overall order of the structures. We note that, due to the particular triangular shape of 5, 5 dibromo-(salophenato)Co(II) precursors (see Fig.5 for a structural model), ′

poly[(salophenato)Co(II)] often shows alternating syn (neighboring triangles point in the same directions) and anti (neighboring triangles point in opposite directions) conformations within the same oligomer, as can be seen in the high resolution STM image of Fig. 1 (e).

Angle-Resolved Photoemission Spectroscopy After having confirmed a quasi-one-dimensional alignment of poly[(salophenato)Co(II)] and poly(p-phenylen) oligomers, we proceeded to characterize the valence band by angle resolved photoemission. Measurements were performed parallel to the long oligomer axis, i.e. the kk -axis corresponds to the [0, 1, 1] direction and k⊥ to the [2, 1, 1] direction, respectively. The Fermi surface of the clean Au(433) surface (Fig. 2 (a), top left panel) shows the well-known circular feature due to the Au(111) surface state. 26 The second


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Figure 1: Scanning tunneling microscopy images of the clean, poly[(salophenato)Co(II)] covered and poly(p-phenylen) covered Au(433) surfaces are shown in (a), (b) and (c), respectively. Importantly, quasi one-dimensional molecular wires are roughly aligned along the [0, 1, 1] direction for both poly[(salophenato)Co(II)] (b) and poly(pphenylen) (c) covered surfaces. High resolution images of poly(p-phenylen) (d) and poly[(salophenato)Co(II)] (e) have been partially superimposed with the molecular structure. The images were recorded at 25 K. dominating feature is the sp-band, which crosses the Fermi level close to kk = 1Å−1 . When examining iso-energy surfaces at higher binding energies [Fig. 2 (a), middle and lower left panels] up to a binding energy BE= 1.8 eV, they appear rather featureless. In particular, at wave vectors larger than that corresponding to the sp-band, almost no photoemission intensity is observed. The situation is decisively different for the poly[(salophenato)Co(II)] covered surface: (i) emission from the surface state is quenched at BE=EF , although the more intense sp-band can still be conveniently used to gauge the position within the Fermi surface; (ii) at a BE= 1 eV increased photoemission intensity is found at wave vectors close to 2π/aP P P . Hereafter, this feature will be referred to as PSaCo-band. At BE= 1.3 eV, 8 ACS Paragon Plus Environment


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the PSaCo-band is almost unchanged in wave vector and intensity. Substantially altered is the situation for the poly(p-phenylen) covered surface. At BE= 1.0 eV, similar to the case of poly[(salophenato)Co(II)], photoemission intensity is observed near kk = 2π/aP P P . However, the feature appears sharper, more intense and shows a clear one dimensional character (i.e. the feature appears dispersionless in k⊥ ). The one-dimensional band, hereafter referred to as PPP-band is even more evident at BE= 1.3 eV: two parallel lines with a center of mass at kk = 2π/aP P P . At BE= 1.8 eV the lines are visible at yet higher and lower kk , respectively. In order to further visualize the dispersion, the E(kk ) dependence of the various features and different surfaces is plotted for up to a BE of 3 eV in Fig. 2 (b). As discussed above, it is important to note that the clean surface does not show any photoemission intensity at larger wave vectors than the characteristic, highly dispersive Au sp-band, i.e. at kk > 1Å−1 up to a BE of about 2.5 eV. At higher BE, photoemission from Au d-bands starts to contribute. By comparison, the intensity observed for poly[(salophenato)Co(II)] and poly(p-phenylen) covered surfaces can unambiguously be attributed to originate from the molecular oligomers. However, poly[(salophenato)Co(II)] and poly(p-phenylen) related features appear very different in both intensity and dispersion: the PSaCo-band (blue line) appears almost constant in energy, while the PPP-band disperses rapidly. To quantify the difference we have extracted energy distribution curves (EDCs) which are plotted for kk = 1.45 Å in Fig.2 c. The EDC of the poly[(salophenato)Co(II)] sample (blue trace in Fig.2 c) reveals, albeit clearly distinguishable from the clean Au(433) surface (black trace), only a broad and comparatively weak maximum corresponding to the PSaCo-band. The EDC of the poly(p-phenylen) sample (red trace) shows by comparison a sharp and intense maximum of the PPP-band. The dispersion of both the PSaCo-band and the PPP-band is determined by fitting the corresponding maxima for various values of kk and the results are displayed in Fig. 2 d. The PSaCo-band is essentially flat and within a free electron model an effective mass of mef f = 3 ± 8me (me denotes the free electron mass) is deduced with the top of the band located at E0 = 1.0(6 ± 9) eV. Note that the 9 ACS Paragon Plus Environment

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fit is only a very crude approximation due to the large uncertainties in determining the BE of the PSaCo-band in the corresponding EDCs as is reflected in the large error bars. Experimentally, it can not be excluded that several flat bands with possibly different signs in effective mass contribute to the photoemission intensity. We emphasized this experimental ambiguity by also plotting the best fit with an inverse mef f in Fig. 2 d, which lies well within the experimental error of the fit results from the BE position determined from EDCs at the given kk values. However, we can exclude, that any contribution to the PSaCo-band disperses rapidly. This point becomes very clear when contrasting with the PPP-band fits displayed in red. Here we deduce the top of the band at E0 = 1.0(3 ± 3) eV with mef f = −0.2(3 ± 8)me , typical values for poly(p-phenylen). 11,13,18

Electronic Band Structure Density Functional Theory Calculations In this section, electronic band structures are calculated. Due to the low reactivity of Au surfaces, we define two simplified models consisting of an infinite periodic chain of phenyl rings and salophenatoCo(II) units, respectively, without the inclusion of the substrate. For the poly[(salophenato)Co(II)] chains, spin-polarized calculations were carried out with the standard plane-wave package QUANTUM ESPRESSO, 27 using the generalized gradient approximation (GGA) to the exchange-correlation functional in the parameterization of Perdew, Burke, and Ernzerhof (PBE). 28 Furthermore, to consider the Coulomb on-site repulsion on d-orbitals of the cobalt atom, a rotationally invariant form of the U-Hubbard correction was added through a (U=3 eV) value of the Hubbard parameter. 29 Ultrasoft pseudopotentials with a plane-wave cutoff of 50 Ry for the wave functions and 350 Ry for the charge density were used. Optimization and self-consistent field procedures have been performed through a 9 × 1 × 1 K-point mesh generated according to the MonkhorstPack scheme with a tight energy convergence criterion of 10−8 eV. Moreover, we have also checked the influence of the syn and anti configuration. However, in agreement with previous work, 24 we found only negligible deviations between the two scenarios and focus



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Figure 2: Angle-resolved photoelectron spectroscopy measurements for pristine (left), the poly(p-phenylen) covered (middle) and poly[(salophenato)Co(II)] covered (right) Au(433) surfaces. Iso-energy surfaces taken at different binding energies as indicated in (a) show relevant surface, poly[(salophenato)Co(II)] and poly(p-phenylen)-related features highlighted by yellow, blue and red dotted lines, respectively. The dispersion of these bands in the kx direction (along the long oligomer axis) is conveniently followed in (b). The poly[(salophenato)Co(II)]-related feature (PSaCo-band) is relatively faint while the poly(p-phenylen)-related feature (PPP-band) is very intense as is clearly seen in energy distribution curves (EDCs) plotted in (c). The BE of the PSaCo and PPP-bands (indicated by blue and red arrows, respectively) were deduced by fitting the corresponding EDCs for various values of kx and the results are displayed in (d) with blue and red data points, respectively. Error bars correspond to the fit uncertainty. The resulting dispersions of the PSaCo- and PPP-bands were fit within the free electron model and the results are displayed by the blue and red solid lines, respectively. our discussion on the syn configuration. For the poly(p-phenylen), a simple model is used to simulate the crystal structure of 11 ACS Paragon Plus Environment

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the oligomer chain. It consists of three-phenyl rings with an inter-ring spacing of ≈ 4.3 Å, the corresponding C-C bond lengths are 1.39 Å for atoms inside the phenyl rings and 1.42 Å for carbon atoms connecting the chain. Note that the choice of three phenyl rings in the unit cell is consistent with the size of one salophenatoCo(II) molecule along the chain direction, allowing a direct geometrical comparison between these two unit cells in the first Brillouin zone. However, for the determination of the bandwidth one must consider the folding of the bands in the case of terphenyl chains.

Figure 3: Basic unit cell of the PPP.

Figure 4: DFT calculated band structure for a PPP wire. The dispersive bands (purple) are associated with LUMO/HOMO molecular orbitals having bonding/anti-bonding character (a/b). Flat bands (green) corresponding to MO with non-bonding character (c). The corresponding wave functions at the gamma point (k=0) are shown below. The calculated band structure of poly(p-phenylen) shown in Fig. 4 has a highest occupied molecular orbital (HOMO) band with a bandwidth close to 3.5 eV from gamma (k = 0) to the first Brillouin zone boundary (k= π/a). By fitting a parabola from k= 0 12 ACS Paragon Plus Environment


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to k= (0.15)π/a, i.e., the same range used for the experimental data, we find an effective mass value mef f = −0.24me , in agreement with the experimental result of Fig. 2. This dispersion of the HOMO band is consistent with the molecular symmetry of the PPP oligomers, which reinforce the conjugation between aromatic groups. Indeed, a linear combination of benzene orbitals with major weight at carbon sites connecting the phenyl rings gives rise to the electronic delocalization along the wire length. More precisely, when the molecular wave function of a given molecular orbital has bonding (a) or antibonding (b) character the corresponding molecular band shows dispersion, while orbitals with non-bonding (c) character across the interring bridge give rise to non-dispersive molecular bands [see Fig. 4]. However, the theoretical band structure of the poly[(salophenato)Co(II)] chain [see Fig. 5] does not show any molecular band with significant dispersion as compared to the poly(p-phenylen)-chain. Again, by fitting a parabola from k= 0 to k= (0.15)π/a, we estimate an absolute value of the effective mass mef f = 1.48me , which is 6 times higher than the value of the poly(p-phenylen) . It is worth to mention that, in this case, the optimization procedure leads to an energy minimum that corresponds to a configuration with structural characteristics of aromatic compounds, i.e., a planar geometry, cyclic ring-shaped and carbon resonant bonds. To analyze this feature it should be noted that, because oxygen electronegativity draws electron density away from carbon atoms, the presence of C-O bonds slightly modifies the bond length of nearby carbon atoms, thus perturbing the aromaticity inside the rings by passing from π-like to σ-like carbon bonding. Nevertheless, the fact that these C-O bond lengths of 1.3 Å are close to the C-O carbonyl-like double bond length present in trigonal planar molecules (C-C-O angle of 120◦ ) guarantees that molecular orbitals remain occupied by delocalized electrons over the phenyl rings. Now, the question is: why do these bands do not show any dispersion? As depicted in Fig. 6, one can see that non-dispersive bands (green lines) result from either MOs with electronic charge strongly localized above the Co atom (c), or those with non-bonding character (d) along the chain direction, while dispersive bands (purple 13 ACS Paragon Plus Environment

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Figure 5: Basic unit cell of the salophenatoCo(II) molecule. lines) correspond to MOs with predominantly anti-bonding/bonding character along the chain’s direction (a, b, e), although in this system the bonding/anti-bonding or nonbonding character of molecular orbitals is not so well defined, as illustrated in panels a-e below with the corresponding wave functions at the Gamma point. Indeed, the overlap between the phenyl rings in the unit cell is affected by the presence of the cobalt atom, forcing connection to be established through alternative circuits. Thus, we conclude that even in the presence of a certain degree of aromaticity, the dispersive character of the bands will be defined by the connectivity conjugation degree between its anti-bonding/ bonding orbitals along the wire. Based on these observations, we find that already the salophenatoCo(II) unit has lower conjugation than the terphenyl unit (no matter whether two subunits show aromaticity) and associate the dispersive behavior of molecular bands to conjugation along the whole molecule. In the next section, we use a multicenter and delocalization indexes analysis to relate delocalization of electronic charge in molecular orbitals with dispersion of the corresponding molecular band.



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Figure 6: DFT calculated band structures of the poly[(salophenato)Co(II)] oligomer for the majority (spin up) and minority (spin down) spin channels. Green lines are associated with non-bonding molecular orbitals (c-d) with low dispersion, while purple lines correspond to bonding/anti-bonding MO with higher dispersion (a-b-e).

Multicenter and Delocalization Indexes Analysis The electronic distribution of the monomers of poly(p-phenylen) and poly[(salophenato)Co(II)] is analyzed using electron delocalization measures 30,31 and multicenter indexes. 32 B3LYP/631G* calculations 33,34 have been perfomed with the Gaussian09 package 35 in order to obtain the pertinent Kohn-Sham wavefunctions. The atomic boundaries are defined by the quantum theory of atoms-in-molecules (QTAIM) 36 using these wavefunctions and the AIMAll program. 37 The atomic orbital matrices obtained from AIMAll are put into the ESI-3D program 38–40 to generate the multicenter measures (ING ) and aromaticity descriptors for large conjugated circuits (AV1245). 41 The index ING measures the averaged conjugation per atom along a given ring structure. 32,42 In table 1 we collect ING for the six-member rings of p-terphenyl and H2 salophenatoCo(II). The local aromaticity of the phenyl rings is very similar in both species but the outermost ring of salophenatoCo(II) is slightly less aromatic due to its coordination with cobalt through Co-O bonds. 15 ACS Paragon Plus Environment

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ING and other multicenter indexes cannot be used in large circuits because their cost and numerical accuracy is highly dependent on the number of atoms in the circuit. 41 The AV1245 index was designed to measure aromaticity in large rings and it can also be used to measure the conjugation in large open circuits of atoms. Namely, it measures the average conjugation in consecutive five-atom fragments, permitting the identification of the least-conjugated fragment in the circuit (AVmin). 41 The larger AVmin, the more favorable the electron mobility in the circuit. In order to analyze the existence of highly dispersive bands in poly(p-phenylen) and its absence in poly[(salophenato)Co(II)], we have also implemented the decomposition of AV1245 into Cartesian components and orbital contributions (see supporting information for further details). AV1245 values have been calculated in all possible circuits between the two atoms connecting monomeric structures. We will only discuss the most conjugated circuits of each monomer. AVmin for circuits a and b of p-terphenyl (see Fig. 7) are 0.602 and 0.403, these circuits only having contributions from the space component that corresponds to the direction of the polymer expansion (AVminz ). An orbital contribution analysis of AV1245 in the circuits reveals that the HOMO is the most important orbital in the conjugation of these circuits. The conjugation analysis, based on the values of AVmin and of AVminz , on the circuits of H2 salophenatoCo(II) reveals only two significantly conjugated circuits. All the circuits passing through a Co atom show very weak conjugated paths in the region around Co. The two circuits are depicted in Fig. 8, they show values of 0.143 and 0.228 and are less conjugated around the N atoms. The former circuit is barely conjugated in the direction of the polymer expansion (0.065) whereas the second circuit shows some conjugation in this direction (0.220). The lack of highly conjugated circuits in poly[(salophenato)Co(II)] explains the absence of dispersive bands in this polymer, which can be attributed to the poor delocalization between the C-N fragment connecting the two phenyl rings. However, in the case of poly(p-phenylen), the corresponding AVmin and AVminz are three to five times larger and, thus, poly(p-phenylen) has highly dispersive bands. 16 ACS Paragon Plus Environment


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Table 1: ING for p-terphenyl and H2 salophenatoCo(II). Ring 1 is the outermost phenyl ring in the structure, whereas 2 corresponds to the middle phenyl ring.

p-terphenyl H2 salophenatoCo(II)

ING (1) ING (2) 0.041 0.040 0.037 0.040

Table 2: AVmin and AVminz (i.e., the longitudinal component of AVmin) for the two most conjugated circuits (A and B) for p-terphenyl and H2 salophenatoCo(II). molecule p-terphenyl H2 salophenatoCo(II)

AVmin(A) 0.602 0.228

AVmin(B) 0.403 0.143

AVminz (A) 0.602 0.220

AVminz (B) 0.403 0.065

Figure 7: From top to bottom: A and B; two possible circuits in p-terphenyl. Green, orange and red colors indicate bond contributions to conjugation which are, respectively, 75-100%, 25-75% and 0-25% that of the most contributing bond of each circuit.


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Figure 8: From left to right: A and B; the two most conjugated circuits in H2 salophenatoCo(II). Green, orange and red colors indicate bond contributions to conjugation that are, respectively, 75-100%, 25-75% and 0-25% that of the most contributing bond of each circuit.

Conclusions The use of Au surface planes vicinal to the low index (111) direction permits the synthesis of rather long one-dimensional molecular wires of poly(p-phenylen) and poly[(salophenato)Co(II)], from 4, 4′′ -dibromo-p-terphenyl and 5, 5 dibromo-(salophenato)Co(II) precursors, respec′

tively, via a thermally activated dehalogenative homocoupling reaction. STM images confirm the successful formation of polymers with low defect density which extend into mesoscale dimensions, as confirmed also by LEED. However, despite a similar structural quality, angle-resolved photoemission spectroscopy revealed highly dispersive bands only for poly(p-phenylen) wires and not for poly[(salophenato)Co(II)] . In order to understand this different behavior of the two systems, we perform density functional theory calculations of the electronic bands for the periodic structures finding reasonable agreement with ARPES observations, as well as calculations of multicenter indexes for the monomeric units. These results explain the reasons for the appearance of energy dispersion in the electronic bands of the periodic system: only when conjugation along the fragments in the monomer is large, dispersive bands in the periodic structure can appear. Multicenter indexes can be used to characterize the bonds responsible for lowand high-dispersive character of the band. No significant changes in the non dispersive character of the bands are expected for other 3d transition metal instead of the Co atom. 18 ACS Paragon Plus Environment


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This analysis permits us to arrive at the following conclusion: a necessary condition for the existence of dispersive bands in one-dimensional polymers is the existence of a high degree of electron delocalization between fragments. However, this condition is not sufficient for a dispersive band to appear in the electronic spectrum of the polymer because it is also necessary to have bonding or anti-bonding character in the molecular orbitals leading to conjugated circuits along the fragments, as well as the correct inter-monomer bond formation or, in other words, a reduced number of defects along the chain.

Supporting Information Additional data acquired for poly[(salophenato)Co(II)] on Au(788) and Au(322) surfaces. Description of the Multicenter and Delocalization Indexes.

Acknowledgement The authors thank for technical and human support provided by IZO-SGI SGIker of UPV/EHU and European funding (ERDF and ESF). Financial support comes from Eusko Jaurlaritza (Basque Government) through the projects IT-588-13, IT-756-13, and IT62113 as well as from the Spanish MINECO/FEDER through grants MAT2013-46593-C6-4-P and MAT2016-78293-C6-6-R, as well as projects CTQ2014-52525-P and FIS2016-75862P. ES, MB and RW gratefully acknowledge financial support from the Office of Naval Research via grant No. N00014-16-1-2900 and the Deutsche Forschungsgemeinschaft via SFB668-B4.

Notes and References (1) McCarty, G. S.; Weiss, P. S. Formation and manipulation of protopolymer chains. Journal of the American Chemical Society 2004, 126, 16772–16776.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nanoarchitectures by covalent assembly of molecular building blocks. Nature Nanotech. 2007, 2, 687. (3) Xi, M.; Bent, B. E. Iodobenzene on Cu(111): formation and coupling of adsorbed phenyl groups. Surface Science 1992, 278, 19 – 32. (4) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X. et al. Atomically precise bottom-up fabrication of graphene nanoribbons. Nature 2010, 466, 470–473. (5) Narita, A.; Wang, X.-Y.; Feng, X.; Mullen, K. New advances in nanographene chemistry. Chem. Soc. Rev. 2015, 44, 6616–6643. (6) Ruffieux, P.; Wang, S.; Yang, B.; Sánchez-Sánchez, C.; Liu, J.; Dienel, T.; Talirz, L.; Shinde, P.; Pignedoli, C. A.; Passerone, D. et al. On-surface synthesis of graphene nanoribbons with zigzag edge topology. Nature 2016, 531, 489–492. (7) Carbonell-Sanromá, E.; Brandimarte, P.; Balog, R.; Corso, M.; Kawai, S.; GarciaLekue, A.; Saito, S.; Yamaguchi, S.; Meyer, E.; Sánchez-Portal, D. et al. Quantum dots embedded in graphene nanoribbons by chemical substitution. Nano Lett. 2017, 17, 50–56. (8) Jacobse, P. H.; Kimouche, A.; Gebraad, T.; Ervasti, M. M.; Thijssen, J. M.; Liljeroth, P.; Swart, I. Electronic components embedded in a single graphene nanoribbon. Nature Communications 2017, 8, 119. (9) Chen, Z.; Zhang, W.; Palma, C.-A.; Lodi Rizzini, A.; Liu, B.; Abbas, A.; Richter, N.; Martini, L.; Wang, X.-Y.; Cavani, N. et al. Synthesis of graphene nanoribbons by ambient-pressure chemical vapor deposition and device integration. J. Am. Chem. Soc. 2016, 138, 15488–15496.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

(10) Kolmer, M.; Zuzak, R.; Ahmad Zebari, A. A.; Godlewski, S.; PrauznerBechcicki, J. S.; Piskorz, W.; Zasada, F.; Sojka, Z.; Bleger, D.; Hecht, S. et al. On-surface polymerization on a semiconducting oxide: aryl halide coupling controlled by surface hydroxyl groups on rutile TiO2(011). Chem. Commun. 2015, 51, 11276–11279. (11) Vasseur, G.;

Abadía, M.;

Miccio, L. A.;

Brede, J.;

Garcia-Lekue, A.;

de Oteyza, D. G.; Rogero, C.; Lobo-Checa, J.; Ortega, J. E. Π band dispersion along conjugated organic nanowires synthesized on a metal oxide semiconductor. J. Am. Chem. Soc. 2016, 138, 5685–5692. (12) Engelund, M.; Papior, N.; Brandimarte, P.; Frederiksen, T.; García-Lekue, A.; Sánchez-Portal, D. Search for a metallic dangling-bond wire on n-doped H-passivated semiconductor surfaces. The Journal of Physical Chemistry C 2016, 120, 20303– 20309. (13) Vasseur, G.; Fagot-Revurat, Y.; Sicot, M.; Kierren, B.; Moreau, L.; Malterre, D.; Cardenas, L.; Galeotti, G.; Lipton-Duffin, J.; Rosei, F. et al. Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires. Nat Commun 2016, 7 . (14) DiLullo, A.; Chang, S.-H.; Baadji, N.; Clark, K.; Klöckner, J.-P.; Prosenc, M.-H.; Sanvito, S.; Wiesendanger, R.; Hoffmann, G.; Hla, S.-W. Molecular Kondo chain. Nano Lett. 2012, 12, 3174–3179. (15) Lepicka, K.; Pieta, P.; Shkurenko, A.; Borowicz, P.; Majewska, M.; Rosenkranz, M.; Avdoshenko, S.; Popov, A. A.; Kutner, W. Spectroelectrochemical approaches to mechanistic aspects of charge transport in meso-Nickel(II) schiff base electrochromic polymer. The Journal of Physical Chemistry C 2017, 121, 16710–16720. (16) Kuck, S.; Wienhausen, J.; Hoffmann, G.; Wiesendanger, R. A versatile variable-


Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature scanning tunneling microscope for molecular growth. Rev. Sci. Instrum. 2008, 79, 083903. (17) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (18) Basagni, A.; Vasseur, G.; Pignedoli, C. A.; Vilas-Varela, M.; Peña, D.; Nicolas, L.; Vitali, L.; Lobo-Checa, J.; de Oteyza, D. G.; Sedona, F. et al. Tunable band alignment with unperturbed carrier mobility of on-surface synthesized organic semiconducting wires. ACS Nano 2016, 10, 2644–2651. (19) Rousset, S.; Repain, V.; Baudot, G.; Garreau, Y.; Lecoeur, J. Self-ordering of Au(111) vicinal surfaces and application to nanostructure organized growth. Journal of Physics: Condensed Matter 2003, 15, S3363. (20) Mugarza, A.; Schiller, F.; Kuntze, J.; Cordón, J.; Ruiz-Osés, M.; Ortega, J. E. Modelling nanostructures with vicinal surfaces. Journal of Physics: Condensed Matter 2006, 18, S27. (21) Ruiz-Osés, M.; González-Lakunza, N.; Silanes, I.; Gourdon, A.; Arnau, A.; Ortega, J. E. Self-assembly of heterogeneous supramolecular structures with uniaxial anisotropy. J. Phys. Chem. B 2006, 110, 25573–25577. (22) Schmitt, S.; Schöll, A.; Umbach, E. Multitude of PTCDA superstructures on Ag(111) and vicinal surfaces. J. Phys. Chem. C 2017, (23) Abadía, M.; González-Moreno, R.; Sarasola, A.; Otero-Irurueta, G.; Verdini, A.; Floreano, L.; Garcia-Lekue, A.; Rogero, C. Massive surface reshaping mediated by metal-organic complexes. J. Phys. Chem. C 2014, 118, 29704–29712. (24) Bazarnik, M.; Bugenhagen, B.; Elsebach, M.; Sierda, E.; Frank, A.; Prosenc, M. H.;



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Wiesendanger, R. Toward tailored all-spin molecular devices. Nano Lett. 2016, 16, 577–582. (25) Sierda, E.; Abadia, M.; Brede, J.; Elsebach, M.; Bugenhagen, B.; Prosenc, M. H.; Bazarnik, M.; Wiesendanger, R. On-surface oligomerization of self-terminating molecular chains for the design of spintronic devices. ACS Nano 2017, (26) The surface state is quantized due to confinement in the [2, 1, 1] direction. As a consequence it appears at

π tn

and not at Γ. 20

(27) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I. et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. Journal of Physics: Condensed Matter 2009, 21, 395502 (19pp). (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396–1396. (29) Cococcioni, M.; de Gironcoli, S. Linear response approach to the calculation of the effective interaction parameters in the LDA + U method. Phys. Rev. B 2005, 71, 035105. (30) Bader, R. F. W.; Stephens, M. E. Fluctuation and correlation of electrons in molecular systems. Chem. Phys. Lett. 1974, 26, 445. (31) Fradera, X.; Austen, M. A.; Bader, R. F. W. The Lewis model and beyond. J. Phys. Chem. A 1999, 103, 304–314. (32) Cioslowski, J.; Matito, E.; Solà, M. Properties of aromaticity indices based on the one-electron density matrix. J. Phys. Chem. A 2007, 111, 6521–6525. (33) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648–5652. 23 ACS Paragon Plus Environment

Page 22 of 24

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields. J. Phys. Chem. 1994, 98, 11623–11627. (35) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al. Gaussian 09 ; Gaussian, Inc.: Wallingford, CT, 2009. (36) Bader, R. F. W. Atoms in molecules: A quantum theory; Oxford Univ. Press: Oxford, 1990. (37) Keith, T. A. AIMAll (Version 14.11.23). 2014; TK Gristmill Software, Overland Park KS, USA (aim.tkgristmill.com). (38) Matito, E. Electron sharing indices program for 3D molecular space partitioning; (http://ematito.webs.com/programs.htm) IQC-DIPC, Girona-Donostia (Spain), 2017. (39) Matito, E.; Duran, M.; Solà, M. The aromatic fluctuation index (FLU): A new aromaticity index based on electron delocalization. J. Chem. Phys. 2005, 122, 014109. (40) Matito, E.; Solà, M.; Salvador, P.; Duran, M. Electron sharing indexes at the correlated level. Application to aromaticity measures. Faraday Discuss. 2007, 135, 325– 345. (41) Matito, E. Electronic aromaticity index for large rings. Phys. Chem. Chem. Phys. 2016, 19, 4522. (42) Feixas, F.; Matito, E.; Poater, J.; Solà, M. Quantifying aromaticity with electron delocalisation measures. Chem. Soc. Rev. 2015, 44, 6389–6646.



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