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Exploring the Relation Between Intramolecular Conjugation and Band Dispersion in One-Dimensional Polymers C. García-Fernández,*,† Emil Sierda,‡,§ Mikel Abadía,∥ Bernhard Bugenhagen,⊥ Marc Heinrich Prosenc,⊥,# 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, Poznań, Poland ∥ Centro de Fı ́sica de Materiales CSIC/UPV-EHU-Materials Physics Center, Manuel Lardizabal 5, 20018, San Sebastian, Spain ⊥ Institute 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-Strasse 52, D-67663, Kaiserslautern, Germany ◆ 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 ▲ Departamento Fı ́sica de Materiales, Universidad del Paı ́s Vasco, 20018-San Sebastian, Spain

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‡

S Supporting Information *

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-salophenatoCo(II) precursors into poly(p-phenylene) 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-phenylene) while no significant dispersion is resolved for poly[salophenato-Co(II)]. On the basis of density functional theory calculations, we explain this observation as a result of a high conjugation along the aromatic phenyl groups in poly(p-phenylene) that is absent in the case of poly[salophenato-Co(II)], where intramolecular conjugation is interrupted in the salophenato-Co(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.

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from a metal onto a suitable semiconductor9 or the growth of one-dimensional wires directly on top of a semiconducting surface10,11 represent real technological challenges.12 So far, only the narrowest armchair GNR, that is, poly(pphenylene), has been synthesized on both metal13 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-phenylene). Therefore, it would be interesting to consider other precursors

INTRODUCTION One of the recently developed strategies to build graphene nanostructures using a bottom-up approach is based on the onsurface dehalogenative homocoupling reaction1,2 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 nanoribbons4,5 (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. However, the transfer of GNRs © 2017 American Chemical Society

Received: August 31, 2017 Revised: October 20, 2017 Published: October 23, 2017 27118

DOI: 10.1021/acs.jpcc.7b08668 J. Phys. Chem. C 2017, 121, 27118−27125

The Journal of Physical Chemistry C

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which do not require a cyclodehydrogenation reaction, apart from poly(p-phenylene), 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 explored 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, that is, 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 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-phenylene) on Au(433), which is considered to be a clear fingerprint of polymerization of poly(pphenylene) chains consisting of more than 20 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 10 salophenato-Co(II) units as 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-phenylene). 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.

Article

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 VTSTM,16 respectively. STM images were recorded at constant tunneling current of 50−200 pA and constant bias voltages of 1 V (Figure 1a), 0.5 V (Figure 1 b−d), and −0.5 V (Figure 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 highresolution hemispherical electron analyzer while the sample was cooled down to 150 K. 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.

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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 ref 18 for additional data on other vicinal surfaces]. The Au(433) surface is characterized by monatomic steps separating “quasi-infinite one-dimensional” terraces extending in [0,1̅,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]̅ direction19,20 as depicted in Figure 1a. 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. 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-phenylene) covered surfaces in Figure 1b,c, respectively. Adsorbate-induced surface restructuring was already observed for benzodiguanamine and naphthalene 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 [0,1̅,1] direction. In agreement with previous studies of heat -induced polymerization of both 5,5′-dibromosalophenato-Co(II)14,24,25 and 4,4″-dibromo-p-terphenyl,11,18 we find Br atoms embedded next to the molecular chains under the given preparation conditions. Here, the Br atoms improve the overall order of the structures. We note that due to the particular triangular shape of 5,5′-dibromo-salophenatoCo(II) precursors (see Figure 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) conforma27119


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Figure 1. Scanning tunneling microscopy images of the clean, poly[salophenato-Co(II)] covered and poly(p-phenylene) covered Au(433) surfaces are shown in (a−c), respectively. Importantly, quasi one-dimensional molecular wires are roughly aligned along the [01̅1] direction for both poly[salophenato-Co(II)] (b) and poly(p-phenylene) (c) covered surfaces. High-resolution images of poly(p-phenylene) (d) and poly[salophenatoCo(II)] (e) have been partially superimposed with the molecular structure. The images were recorded at 25 K.

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 k∥ = 2π/aPPP. At BE = 1.8 eV the lines are visible at yet higher and lower k∥, respectively. In order to further visualize the dispersion, the E(k∥) dependence of the various features and different surfaces is plotted for up to a BE of 3 eV in Figure 2b. 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, that is, at k∥ > 1 Å−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[salophenatoCo(II)] and poly(p-phenylene) covered surfaces can unambiguously be attributed to originate from the molecular oligomers. However, poly[salophenato-Co(II)] and poly(p-phenylene) related features appear very different in both intensity and dispersion: the PSaCo-band (blue line) appears almost constant in energy, whereas the PPP-band disperses rapidly. To quantify the difference we have extracted energy distribution curves (EDCs) which are plotted for k∥ = 1.45 Å in Figure 2c. The EDC of the poly[salophenato-Co(II)] sample (blue trace in Figure 2c) reveals, albeit clearly distinguishable from the clean Au(433) surface (black trace), only a broad and comparatively weak maximum corresponding to the PSaCoband. The EDC of the poly(p-phenylene) 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 k∥ and the results are displayed in Figure 2d. The PSaCo-band is essentially flat and within a free electron

tions within the same oligomer, as can be seen in the highresolution STM image of Figure 1e. Angle-Resolved Photoemission Spectroscopy. After having confirmed a quasi-one-dimensional alignment of poly[salophenato-Co(II)] and poly(p-phenylene) oligomers, we proceeded to characterize the valence band by angleresolved photoemission. Measurements were performed parallel to the long oligomer axis, that is, the k∥-axis corresponds to the [0,1̅,1] direction and k⊥ to the [2,1̅,1̅] direction, respectively. The Fermi surface of the clean Au(433) surface (Figure 2a, top left panel) shows the well-known circular feature due to the Au(111) surface state.26 The second dominating feature is the sp-band, which crosses the Fermi level close to k∥ = 1 Å−1. When examining the iso energy surfaces up to binding energies (BEs) of 1.8 eV [Figure 2a, middle and lower left panels], 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/aPPP. Hereafter, this feature will be referred to as PSaCo-band. At BE = 1.3 eV, the PSaCoband is almost unchanged in wave vector and intensity. Substantially altered is the situation for the poly(p-phenylene) covered surface. At BE = 1.0 eV, similar to the case of poly[salophenato-Co(II)], photoemission intensity is observed near k∥ = 2π/aPPP. However, the feature appears sharper, more intense and shows a clear one-dimensional character (i.e., the 27120


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Figure 2. Angle-resolved photoelectron spectroscopy measurements for pristine (left), the poly(p-phenylene) 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-phenylene)-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-phenylene)-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.

model an effective mass of meff = (3 ± 8) me (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 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 meff in Figure 2 d, which lies well within the experimental error of the fit results from the BE position determined from EDCs at the given k∥ 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 meff = −0.2(3 ± 8)me, typical values for poly(p-phenylene).11,13,18

Electronic Band Structure Density Functional Theory Calculations. In this section, electronic band structures are calculated. Because of the low reactivity of Au surfaces, we define two simplified models consisting of an infinite periodic chain of phenyl rings and salophenato-Co(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 parametrization of Perdew, Burke, and Ernzerhof (PBE).28 Furthermore, to consider the Coulomb on-site repulsion on dorbitals 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 selfconsistent field procedures have been performed through a 9 × 27121


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The Journal of Physical Chemistry C 1 × 1 K-point mesh generated according to the Monkhorst− Pack 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 work24 we found only negligible deviations between the two scenarios and focus our discussion on the syn configuration. For the poly(p-phenylene), a simple model is used to simulate the crystal structure of 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 (see Figure 3). Note that the choice of three phenyl rings

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 nonbonding (c) character across the interring bridge give rise to nondispersive molecular bands [see Figure 4]. However, the theoretical band structure of the poly[salophenato-Co(II)] chain [see Figure 5] does not show any

Figure 3. Basic unit cell of poly(p-phenylene) used in our calculations. Figure 5. Basic unit cell of poly[salophenato-Co(II)] used in our calculations.

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. The calculated band structure of poly(p-phenylene) shown in Figure 4 has a highest occupied molecular orbital (HOMO)

molecular band with significant dispersion compared to the poly(p-phenylene). Again, by fitting a parabola from k = 0 to k = (0.15)π/a, we estimate an absolute value of the effective mass meff = 1.48me, which is 6 times higher than the value of the poly(p-phenylene). It is worth mentioning that, in this case, the optimization procedure leads to an energy minimum that corresponds to a configuration with structural characteristics of aromatic compounds, that is, a planar geometry, cyclic ringshaped, 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 Figure 6, one can see that nondispersive bands (green lines) result from either MOs with electronic charge strongly localized above the Co atom (c), or those with nonbonding character (d) along the chain direction, while dispersive bands (purple lines) correspond to MOs with predominantly antibonding/bonding character along the chain’s direction (a, b, e). We note that for poly[salophenatoCo(II)] the bonding/antibonding or nonbonding character of molecular orbitals is not well-defined. This is illustrated in Figure 6a−e, where the wave functions (molecular orbitals) at the Gamma point of the corresponding spin-down bands a−e have been plotted. 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 antibonding/bonding orbitals along the wire. On the basis of these observations, we find that already the salophenato-Co(II) unit has lower conjugation than the

Figure 4. Density functional theory (DFT) calculated band structure for a poly(p-phenylene). The dispersive bands (purple) are associated with LUMO/HOMO molecular orbitals having bonding/antibonding character (a/b). Flat bands (green) corresponding to MO with nonbonding character (c). The corresponding wave functions at the gamma point (k = 0) are shown below.

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 to k = (0.15)π/a, that is, the same range used for the experimental data, we find an effective mass value meff = −0.24me, in agreement with the experimental result of Figure 2. This dispersion of the HOMO band is consistent with the molecular symmetry of the poly(p-phenylene), which reinforce the conjugation between aromatic groups. Indeed, a linear combination of benzene orbitals with major weight at 27122


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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 nonbonding molecular orbitals (c,d) with low dispersion, while purple lines correspond to bonding/ antibonding MO with higher dispersion (a,b,e).

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-phenylene) 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 values (see Table 2) for circuits A and B of pterphenyl (see Figure 7) are 0.602 and 0.403, respectively. The circuits only have contributions from the space component that corresponds to the direction of the polymer expansion (AVminz). An orbital contribution analysis of AV1245 in 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. Multicenter and Delocalization Indexes Analysis. The electronic distribution of the monomers of poly(p-phenylene) and poly[salophenato-Co(II)] is analyzed using electron delocalization measures 30,31 and multicenter indexes.32 B3LYP/6-31G* calculations33,34 have been perfomed with the Gaussian09 package35 in order to obtain the pertinent Kohn− Sham wave functions. The atomic boundaries are defined by the quantum theory of atoms-in-molecules (QTAIM)36 using these wave functions and the AIMAll program.37 The atomic orbital matrices obtained from AIMAll are put into the ESI-3D program38−40 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 Table 1. ING for p-Terphenyl and H2-Salophenato-Co(II)a p-terphenyl H2salophenato-Co(II)

ING(1)

ING(2)

0.041 0.037

0.040 0.040

a

Ring 1 is the outermost phenyl ring in the structure, whereas 2 corresponds to the middle phenyl ring.

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 H2-salophenatoCo(II) is slightly less aromatic due to its coordination with cobalt through CoO bonds. 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

Figure 7. From top to bottom: A and B; the two possible circuits in pterphenyl. 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. 27123


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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 low- and high-dispersive character of the band. No significant changes in the nondispersive character of the bands are expected for other 3d transition metal instead of the Co atom. 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 antibonding character in the molecular orbitals leading to conjugated circuits along the fragments, as well as the correct intermonomer bond formation or, in other words, a reduced number of defects along the chain.

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

AVmin (A)

AVmin (B)

AVminz (A)

AVminz (B)

p-terphenyl H2salophenato-Co(II)

0.602 0.228

0.403 0.143

0.602 0.220

0.403 0.065

circuits reveals that the HOMO is the most important orbital in the conjugation of these circuits. The conjugation analysis of H2salophenato-Co(II), based on the values of AVmin and AVminz (see also Table 2), reveals only the two significantly conjugated circuits which are depicted in Figure 8. They show AVmin values of 0.143 (circuit A) and

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08668. Additional data acquired for poly[salophenato-Co(II)] on Au(788) and Au(322) surfaces. Description of the Multicenter and Delocalization Indexes (PDF)

Figure 8. From left to right: A and B; the two most conjugated circuits in H2-salophenato-Co(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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

0.228 (circuit B) and are less conjugated around the N atoms. Circuit A is barely conjugated in the direction of the polymer expansion (AVminz = 0.065) whereas circuit B shows some conjugation in this direction (AVminz = 0.220). Other circuits (not shown) exhibit very weakly conjugated paths in particular in the vicinity of the Co atom. 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(pphenylene), the corresponding AVmin and AVminz are three to five times larger and, thus, poly(p-phenylene) has highly dispersive bands (see Table 2).

ORCID

C. García-Fernández: 0000-0003-2226-3904 Emil Sierda: 0000-0002-2287-9883 Maciej Bazarnik: 0000-0002-8143-3996 José Enrique Ortega: 0000-0002-6643-806X Jens Brede: 0000-0002-4946-8160 Eduard Matito: 0000-0001-6895-4562 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 IT588-13, IT-756-13, and IT621-13 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 CTQ201452525-P and FIS2016-75862-P. E.S., M.B., and R.W. gratefully acknowledge financial support from the Office of Naval Research via Grant N00014-16-1-2900 and the Deutsche Forschungsgemeinschaft via SFB668-B4.

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-phenylene) and poly[salophenatoCo(II)] from 4,4″-dibromo-p-terphenyl and 5,5′-dibromosalophenato-Co(II) precursors, respectively, 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-phenylene) 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

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REFERENCES

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