On-Surface Synthesis and Characterization of Honeycombene Oligophenylene Macrocycles Min Chen,†,‡,⊥ Jian Shang,†,⊥ Yongfeng Wang,*,§ Kai Wu,*,† Julian Kuttner,‡ Gerhard Hilt,‡ Wolfgang Hieringer,*,∥ and J. Michael Gottfried*,‡ †
BNLMS, College of Chemistry and Molecular Engineering, and §Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, People’s Republic of China ‡ Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35032 Marburg, Germany ∥ Lehrstuhl für Theoretische Chemie and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: We report the on-surface formation and characterization of [30]-honeycombene, a cyclotriacontaphenylene, which consists of 30 phenyl rings (C180H120) and has a diameter of 4.0 nm. This shape-persistent, conjugated, and unsubstituted hexagonal hydrocarbon macrocycle was obtained by solvent-free synthesis on a silver (111) single-crystal surface, making solubilityenhancing alkyl side groups unnecessary. Side products include strained macrocycles with square, pentagonal, and heptagonal shape. The molecules were characterized by scanning tunneling microscopy and density functional theory (DFT) calculations. On the Ag(111) surface, the macrocycles act as molecular quantum corrals and lead to the confinement of surface-state electrons inside the central cavity. The energy of the confined surface state correlates with the size of the macrocycle and is well described by a particle-in-the-box model. Tunneling spectroscopy suggests conjugation within the planar rings and reveals influences of self-assembly on the electronic structure. While the adsorbed molecules appear to be approximately planar, the free molecules have nonplanar conformation, according to DFT. KEYWORDS: on-surface synthesis, Ullmann reaction, macrocycle, quantum corral, conjugation, electron confinement, scanning tunneling microscopy
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There are numerous examples for the synthesis of conjugated macrocycles in solution,21,31 most of which use a combination of arylene and ethynylene building blocks to generate shapepersistent rings.32−35 Macrocycles consisting exclusively of phenylene groups have rarely been made.28,36 A drawback of the solution-based approach is that the large molecules and their precursors carry spacious substituents (typically alkyl chains with 6 to 12 carbon atoms), which are necessary to achieve sufficient solubility during synthesis, but are disadvantageous in other respects,21 because they can prevent efficient πorbital overlap in molecular organic semiconductors, resulting in reduced charge carrier mobility, affect the two- or threedimensional crystallization, and may have adverse effects in molecular machines.37 An illustrative example is the C60wheeled “nanocar” molecule with its 12 C10H21 chains.38 (Note that more recent “nanocars” have less bulky side groups.39,40)
arbon allotropes have been one of the major paradigms in chemistry during the past three decades, with the discovery of fullerenes1 in 1986 setting the direction toward later milestones such as carbon nanotubes2−5 and graphene.6−10 More recently, the focus has shifted toward the chemistry of natural and synthetic carbon allotropes11−14 and their fragments. Such fragments include corannulene (buckybowl)15−17 as a C60 fragment, graphene nanoribbons18,19 and nanosheets20 as graphene fragments, and conjugated macrocycles21,22 as fragments of carbon nanotubes23 or graphene.24 Conductive polymers such as polyacetylene25 also belong to this line of tradition. In this context, conjugated macrocycles have attracted considerable attention. Their unique electronic and optical properties make them interesting candidates for application in molecular and organic electronic devices or photoswitches.26,27 Since conjugated macrocycles are shapepersistent,28 they are potentially interesting as parts of synthetic molecular motors and molecular mechanical machines29 or for the nanostructuring of surfaces.30 © 2016 American Chemical Society
Received: August 24, 2016 Accepted: December 16, 2016 Published: December 16, 2016 134
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(DMQP) react in the presence of metallic silver via scission of the C−Br bond to form the macrocycle. Figure 1b shows a scanning tunneling microscopy (STM) image of the DMQP precursor adsorbed on a Ag(111) single crystal surface at 260 K. As can be seen in Figure 1b, DMQP forms a long-rangeordered supramolecular network, in which the molecules are connected through quartet nodes. The nodes are probably formed by a combination of Br···H hydrogen bonds and Br···Br halogen bonds.55,56 Adsorption of DMQP at room temperature leads to the formation of organometallic polymers with C−Ag− C bonds (see the Supporting Information, Figure S2a), in line with previous observations.57,58 Increasing the temperature to 640 K results in the scission of the C−Br bonds (see the X-ray photoelectron spectroscopy data in the Supporting Information, Figure S3) and formation of C−C bonds. An STM image of the resulting [30]-honeycombene macrocycle (30-[HC]) is shown in Figure 1c. The diameter of this macrocycle (more precisely, the distance between the centers of two opposite edges) is 4.0 nm, which makes it the largest shape-persistent, fully conjugated, and unsubstituted macrocycle known to date. The [30]-HC macrocycles self-assemble on the Ag(111) surface, forming islands with hexagonal symmetry as shown in Figure 1d. The vectors of the near-hexagonal unit cell have a length of 4.9 nm and enclose an angle of 59°. The gap between the edges of two neighboring molecules indicates that there is no formation of covalent bonds between the macrocycles; that is, the self-assembly is apparently driven by van der Waals forces. For large-scale STM images showing side products, see Figure S4 in the Supporting Information. High-resolution images of the competing oligophenylene chains are shown in Figure S5. Insight into the valence electronic structure of the adsorbed macrocycle was obtained by tunneling conductance (dI/dV mapping) experiments, which provide spatial information about the local density of states (LDOS) at different energies. Figure 1e shows a topographic image (left) along with dI/dV maps at bias voltages between 1.5 and 2.5 V. At 1.5 V, the maximum LDOS appears at the edges, while the corners have lower values. With increasing bias voltage, the images show enhanced localization of the density of states around the corners of the macrocycles. At the highest voltage (2.5 V), additional maxima are visible in the centers of the edges. Comparisons with density-functional calculations for an isolated, planar [30]honeycombene molecule (see Figure 2 and the Supporting Information for details) indicate that the features shown in Figure 1e can be qualitatively explained with large-wavelength amplitude modulations in the delocalized π-type molecular orbitals of the conjugated ring. Figure 2a shows summed orbital densities within the specified energy range, which can be viewed as free-molecule dI/dV (or STS) simulations within the Tersoff−Hamann model.59−61 As can be seen, the amplitude modulations obtained extend over several atomic distances and qualitatively resemble the dI/dV patterns shown in Figure 1e. Several orbital densities within the specified energy range add up to yield the observed long-wavelength patterns. The relative energetic ordering observed in the experiments is not reproduced in these simulations, which may partly be explained by the omission of the Ag(111) surface. Note that the underlying molecular orbitals are partly C−C antibonding. Examples of individual MOs are shown in Figure 2b. Such large-wavelength modulations are characteristic for quantummechanical “particle-on-a-ring” type systems.62 As an example for one of the simplest possible model systems, Figure S8c in
If the molecules are to be used in a solid-state-related application, for example for modification of solid surfaces, the solution-based synthesis also represents a detour, because an additional deposition step is required. A logical alternative would be the synthesis in the solid state, preferably directly at the place where the molecules will be used, e.g., on a solid surface. The principal feasibility of the solid-state approach has repeatedly been demonstrated for successful organic syntheses on surfaces. In particular, C−C bond formation using the modified Ullmann41−46 or Glaser47−49 coupling reactions has been employed in attempts to synthesize one- and twodimensional polymers42,44,50,51 on surfaces, whereas syntheses of large molecules with the solid-state approach remain exceptions.24,52−54 Here, we describe the formation of large oligophenylene macrocycles, for which we propose the name [n]-honeycombenes (where n stands for the number of phenylene units), on a metal surface using C−C coupling reactions between small precursor molecules. The adsorbed molecules were characterized with scanning probe techniques. Additional insight into the properties of the free molecules was obtained by density functional theory calculations.
RESULTS AND DISCUSSION [30]-Honeycombene. The reaction scheme for the synthesis of the cyclotriacontaphenylene ([30]-honeycombene, because it consists of 30 phenyl rings) macrocycle is shown in Figure 1a: Six molecules of 4,4⁗-dibromo-meta-quinquephenyl
Figure 1. (a) Formation of the cyclotriacontaphenylene macrocycle [30]-honeycombene ([30]-HC) from 4,4⁗-dibromo-meta-quinquephenyl (DMQP) using a modified Ullmann reaction.41−44 (b) STM image of DMQP on Ag(111) adsorbed at 260 K and imaged at 78 K. Tunneling parameters: U = −0.01 V, I = 3 nA. (c) STM image of a [30]-honeycombene macrocycle on Ag(111) obtained after heating to 640 K, imaged at 4.4 K. Tunneling parameters: U = 0.1 V, I = 0.02 nA. See the text for the meaning of the arrow and the length L. (d) STM image of an island of [30]-honeycombene, imaged at 78 K. The unit cell has dimensions a = b = 4.9 nm, θ = 121°. Tunneling parameters: U = 0.2 V, I = 0.4 nA. (e) Electronic structure of [30]-honeycombene: Topographic image (left, 0.1 V, 0.5 nA) and conductance images (experimental dI/dV maps) at bias voltages between 1.5 and 2.5 V, taken at 4.4 K. 135
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Figure 3. Experimental dI/dV spectra for [30]-honeycombene at the corner (blue) and the edge (red) of the macrocycle. Tunneling current = 0.2 nA.
Figure 2. (a) STS simulation based on DFT orbitals: Summed densities of molecular orbitals (MOs) within the given energy ranges61 (in eV with respect to the Fermi level (EF), taken as centered in the HOMO−LUMO gap) for an isolated, perfectly planar (D6h point group symmetry) [30]-honeycombene from PBE calculations; VASP/p4vasp programs, constant current mode, density contour 6.417 au, corresponding to a tunneling current of 0.5 nA. (b) Illustration of selected molecular orbitals in the unoccupied manifold of planar, isolated [30]-HC. Orbital 9a1u shows nodes at the corner sites; orbitals 8a1u and 17e1g represent orbitals that show diminished amplitude on the ring edges. Within the Tersoff−Hamann model, the densities of several MOs within a specified energy range are summed up to yield STS images. Note that orbital 9a1u does not fall in any of the energy ranges shown in (a), but is included as a representation of an individual orbital with no amplitude at the corners. DFT-PBE/SV(P) calculation (Turbomole) in D6h symmetry. Orbital energies given with respect to EF. Contour value = 0.005.
the Supporting Information shows selected molecular orbitals for an artificial planar hydrogen macrocycle. The six corners of the honeycombene macrocycle consist of meta-substituted phenyl rings. Meta-linkage is known to reduce conjugation in the electronic ground state.63 However, it has been reported for meta-phenylacetylene structures that this does not hold for excited states, i.e., when formerly unoccupied states are involved.63,64 An enhanced density of unoccupied states at the corners is apparent in the tunneling conductance images in Figure 1e, which were taken in the range of unoccupied states. Likewise, the conductance vs bias voltage plots in Figure 3 show that the corners (blue line) possess considerable density of states already at 1.4 eV, which increases further until a maximum is reached at 2.1 eV. The edges show a maximum density of states at 1.4 eV. According to the DFT calculations presented in Figure 2, the enhanced signal at the corner or edge sites can be understood from a superposition of unoccupied orbital densities within the Tersoff−Hamann model.59−61 [18]-Honeycombene. Hyperbenzene, a hexagonal cyclooctadecaphenylene macrocycle recently made by on-surface synthesis on Cu(111),24 is [18]-honeycombene according to the nomenclature used above. Figure 4a shows the synthesis of the macrocycle on Ag(111) by reaction of adsorbed dibromometa-terphenyl (DMTP) molecules at 640 K. (At room temperature, formation of organometallic dimers with C− Ag−C bonds was observed (Figure S2b in the Supporting
Figure 4. (a) Formation of the cyclooctadecaphenylene macrocycle [18]-honeycombene from 4,4″-dibromo-meta-terphenyl (DBMT) using a modified Ullmann reaction.41−43 (b) STM image of an isolated [18]-honeycombene macrocycle on Ag(111), imaged at 4.4 K. Tunneling parameters: U = −0.03 V, I = −0.1 nA. (c) STM image of an island of [18]-honeycombene, imaged at 4.4 K. Tunneling parameters: U = 0.01 V, I = 0.02 nA. The unit cell has the dimensions a = b = 3.3 nm, θ = 118°. (d) Experimental dI/dV spectra for [18]-honeycombene at corner (blue) and edge (red) positions at the perimeter of the macrocycle, for an isolated molecule (top) and a molecule in an island (bottom). (e) Electronic structure of [18]-honeycombene: Topographic image (left) and dI/dV maps at bias voltages between 1.4 and 2.8 V, taken at 4.4 K. (f) As above, but for a macrocycle in an island. The white scale bar corresponds to a length of 1 nm. For large-scale STM images showing side products, see Figure S4 in the Supporting Information. High-resolution images of the competing oligophenylene chains are shown in Figure S6.
Information), in line with previous observations.57,58) STM images of a single macrocycle is shown in Figure 4b. Similar to its larger sibling, [18]-honeycombene forms hexagonal islands (Figure 4c). The two weak protrusions (red) between the 136
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Figure 5. (a) Strained macrocycles formed as side products by on-surface reaction of DMQP. Images of molecular geometries of isolated square “[20]-HC”, pentagonal “[25]-HC”, and heptagonal “[35]-HC” macrocycles in planar configuration as calculated using DFT. (b) STM image of the square cycloicosaphenylene C120H80, tunneling parameters U = 0.1 V, I = 0.02 nA. See the text for the meaning of the arrow and the length L. (c) STM image of the pentagonal cyclopentacosaphenylene C150H100, U = 0.05 V, I = 0.02 nA. (d) STM image of the heptagonal cyclopentatriacontaphenylene C210H140, U = 0.1 V, I = 0.02 nA. (e) Electronic structure of the pentagonal cyclopentacosaphenylene: topographic image (left, 1.4 V, 1 nA) and dI/dV maps at bias voltages between 1.6 and 2.5 V. (f) Electronic structure of the heptagonal cyclopentatriacontaphenylene: topographic image (left, 1.4 V) and experimental dI/dV maps at bias voltages between 1.6 and 2.5 V. The white scale bars correspond to a length of 1 nm. (g, h) Experimental dI/dV spectra for the different macrocycles from square to heptagon, measured at the edges (g) and corners (h) of the macrocycles. All measurements were performed at 4.4 K.
edges of two adjacent molecules are assigned to adatoms, most likely Br atoms. The electronic structure shows similar trends to those found for [30]-honeycombene: Increasing bias voltage shifts the DOS from the edges to the corners (Figure 4e). The behavior is similar for isolated molecules and molecules in an island (Figure 4f), but the maxima appear at higher bias voltages for the latter, as can be seen by comparing the dI/dV spectra in Figure 4d. The electronic structure of the molecule is apparently substantially influenced by the lateral self-assembly. Strained Macrocycles. Besides the regular hexagonal honeycombenes, the DMQP precursor (but not the smaller DMTP precursor) occasionally also forms strained rings with square “[20]-HC”, pentagonal “[25]-HC”, or heptagonal symmetry “[35]-HC”, as shown in Figure 5. The strain-induced deformation of the planar rings can be seen in the STM images: The square cycloicosaphenylene (20 phenyl rings) in Figure 5b, formed by reaction of four DMQP molecules, shows a convex deformation of its edges; that is, they are curved toward the outside. In contrast, the heptagonal cyclopentatriacontaphenylene macrocycle (35 phenylene rings) in Figure 5d shows a concave deformation. The pentagonal cyclopentacosaphenylene (25 phenylenes, Figure 5c) is not visibly deformed in the STM image. The strain is partly induced by adsorption, which forces the macrocycles to assume a planar geometry. In contrast, the free molecules would relieve strain by assuming nonplanar equilibrium conformations, as will be discussed below. The dI/dV maps for the pentagon as shown in Figure 5e reveal enhanced LDOS at the edges for low bias voltage and increasing localization of the LDOS at the corners with increasing bias voltage, again suggesting that the ring is conjugated. At 2.5 V, LDOS maxima appear at both the corners and the edges of the macrocycles. Qualitatively similar changes of the LDOS with the bias voltage were found for the heptagon
(Figure 5f). The dI/dV spectra in Figure 5g, however, reveal subtle differences: With increasing ring size in the series [20]HC, [25]-HC, and [30]-HC, the maxima shift to lower bias voltages, as qualitatively expected from a particle-on-the-ring model for conjugated systems.62,65,66 The observation that the shift is most pronounced between the pentagon and the hexagon possibly reflects effects of ring strain, which should especially affect the square and the heptagon. Note that the heptagon [35]-HC shows a maximum at higher voltage than [30]-HC. Confinement of Surface-State Electrons in SingleMolecule Quantum Corrals. Confinement of surface-state electrons to artificial nanostructures was previously observed for closed metal loops (quantum corrals)67 and for pores in supramolecular networks.68−70 In the following, we will show that the oligophenylene macrocycles act as single-molecule quantum corrals: The spectroscopic dI/dV map in Figure 6a shows the standing electron wave pattern inside the square, pentagonal, and hexagonal macrocycles at the indicated bias voltages. The related dI/dV spectra in Figure 6b, measured in the centers of the macrocycles, reveal that the confined surface state shifts to higher energy when the quantum corrals shrink in size, from 20 mV for the hexagon and 50 mV for the pentagon to 100 mV for the square. Further dI/dV spectra from offcenter positions inside the macrocycles are displayed in the Supporting Information (Figure S7) and show a strong position-dependence of the modulation of the DOS, which on the pristine Ag(111) surface is laterally nearly constant.69 The surface state is described by the energy dispersion ℏ2
E(k) = ε0 + 2m * k 2 with the electron wave number, k, the onset energy, ε0 = −65 meV, the effective mass, m* = 0.42me (where me is the electron mass),71,72 and the reduced Planck constant, ℏ. For the square quantum corral in Figure 6a, the 137
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more detailed model of the quantum corral induced by the square molecule, a geometry optimization using periodic, dispersion-corrected density-functional theory on a five-layer silver slab was performed. Analysis of the resulting density contours parallel to the surface plane reveals an unoccupied state with amplitude inside the macrocycle (Figure 6c) that resembles the experimentally observed feature in Figure 6a. Further related images and computational details are given in the Supporting Information (Figure S9). In contrast to previous related work,71 we did not observe shorter-wavelength features at higher bias voltages. This is attributed to the fact that the standing wave pattern depends not only on the size of the cavity but also on the nature of its “walls”. The organic molecules are expected to be much weaker scatterers (or, in other words, represent a lower potential barrier) than Ag step edges.71 In the related STS spectra, we found only one clear peak, which is in agreement with the fact we did not observe the shorter-wavelength patterns. Within the particle-in-a-box model, it is likely that the higher-energy states (i.e., those with shorter wavelengths) have energies above the wall barrier and therefore do not represent confined electronic states anymore. The elongated, 2-fold symmetric conductance pattern observed inside the pentagon is attributed to an adsorbate/substrate registry effect. Due to the symmetry mismatch between molecule and surface, the local bonding situation will be different for the five edges. Stronger bonding is expected to result in a higher energy barrier for the surface-state electrons, while weaker bonding will lead to a shallower barrier. As a result, the confined surface state can penetrate the potential walls to a different degree, resulting in the observed lower symmetry. The observed mirror-symmetric feature (Cs) is in agreement with the fact that the maximum rotational symmetry obtained from a combination of 6-fold and 5-fold rotational symmetry is a 2-fold mirror symmetry. Properties of the Free Macrocycles. By means of DFT calculations, it is possible to obtain information about the properties of the free molecule. The experimental data shown so far indicate an essentially flat adsorption geometry. However, for the gas-phase molecules, a similarly flat molecular geometry cannot be expected, because even in the simple biphenyl molecule the two phenyl rings are twisted in the minimumenergy conformation, such that the molecule adopts a nonplanar conformation.73 Indeed, geometry optimizations for the isolated [n]-HC molecules (n = 20, 25, 30, 35) in a perfectly planar conformation with the Dmh (m = n/5) symmetry constraint lead to structures that are not local minima on the respective energy surfaces. Rather, calculation of the geometric Hessian shows that a total number of 20, 20, 27, and 32 imaginary frequencies are obtained for n = 20, 25, 30, and 35, respectively, the eigenvectors of which describe both twisting coordinates of adjacent phenyl rings and global out-ofplane deformations that break the Dmh symmetry of the molecule. Nevertheless, the calculated planar structures can be viewed as models for the adsorbed molecules, since the surface acts as a template and forces the molecules into an essentially planar conformation due to van der Waals interactions, as will be discussed shortly. The strain induced by the forced planarity of the ring structures causes additional in-plane deformations, especially in the nonhexagonal molecules. These deformations can clearly be seen in Figure 5a−d as outward (square, pentagon) or inward bending (heptagon) of the sides of the respective polygons. The hexagon-shaped [30]-honeycombene, in contrast, does not
Figure 6. (a) Experimental dI/dV maps at the indicated bias voltages showing the electron standing-wave pattern inside the central cavities of the square [20]-HC, pentagonal [25]-HC, and hexagonal [30]-HC macrocycles (from the left). The overlaid molecular models show the positions of the macrocycles as derived from STM images. The white scale bars correspond to 1 nm. (b) Experimental dI/dV spectra for the three macrocycles showing how the surface-state energy increases with shrinking size of the macrocycle. All measurements were performed at 4.4 K. (c) Density plot of an orbital (number 4955, ε − εF = +0.4 eV) of an essentially planar [20]-HC molecule adsorbed on a five-layer Ag(111) slab from periodic DFT calculations. Displayed is the density in a plane parallel to the surface that lies in between the top layer of the surface and the molecular plane. Bright areas indicate high orbital density at the respective location. The plot resembles the observed experimental feature shown in (a), although at different energy. The plot plane elevation is z = 12 Å; the molecule is located at z ≈ 12.9 Å, and the top Ag(111) surface atom layer is located at z ≈ 9.6 Å. The molecule itself (overlaid) is located on the dark ring. See the Supporting Information for further images and details.
eigenstate energies can easily be estimated by the “particle-in-abox” model66 using the equation h2 (nx 2 + ny 2) 8m*L2 where L is the length of the box and h is Planck’s constant. For the lowest eigenstate (nx = ny = 1), the measured energy of 100 meV corresponds to a length of L = 3.3 nm. This length, which is marked by a white arrow in Figure 5b, is very close to the outer dimensions of the macrocycle and thus indicates substantial penetration of the wave function into the confining potential. This is not surprising considering that the “particlein-a-box” model is derived for a potential with infinitely steep walls. For the hexagonal quantum corral, the eigenvalues are λ described by the equation En = ε0 + m *n A , in which λn are numerically calculated eigenvalues.71 The area, A, of the Enx , ny = ε0 +
( π6 ) with the distance between
hexagon is given by A = L2 cos
two opposite edges L. With these equations, the measured energy of 20 meV corresponds to a length of L = 4.8 nm. As can be seen in Figure 1c, this length (marked by a white arrow) is almost identical to the outer dimension of the macrocycle, again indicating partial penetration of the potential well. For a 138
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anticipated, a large number of local minima can be found by local twisting around the phenyl−phenyl bonds and by global out-of-plane deformations. Some of the lowest-energy local minima found for [30]-HC are collected in Figure 7. (Local minimum structures of [20]-HC, which appears to prefer somewhat different conformations than [30]-HC, can be found in the Supporting Information, Figure S10.) In all conformations of [30]-HC, the phenyl rings are twisted along the C−C bonds with respect to their neighbors, as expected already from the structure of biphenyl73 mentioned above. The phenylene rings can be twisted in an alternating fashion along the straight sides of the ring (Figure 7d), or they can be twisted in a helical fashion (Figure 7c,e). Other, irregular relative orientations are possible as well. Furthermore, the ring structures can globally adopt twisted (Figure 7a), envelope (Figure 7b), planar (Figure 7c,d), or even armchair (Figure 7e) conformations; further conformations may be possible as well. All conformations shown in Figure 7 are very similar in (free) energy (within only 16 (22) kJ/mol, which is likely within the error margins of the present DFT level). The two conformations shown in Figure 6c,d are globally planar and are thus similar to the adsorbed state. To estimate the energy cost for planarization, the energy of the idealized, planar D6h structure discussed above shall be compared with the gas-phase local minimum structures shown in Figure 7. The energy of the fully planar [30]-HC D6h structure is 154 kJ/mol higher than the lowest-energy minimum found in our conformer search (Figure 7a). This value can be compared to the van der Waals attraction to the Ag(111) surface: With estimated 10−11 kJ/mol per carbon atom, based on the adsorption energy of benzene,74 the van der Waals contribution to the adsorption energy is in the range 1800−2000 kJ/mol and thus exceeds the planarization energy by far. Of course, only a part of this van der Waals attraction energy is lost if the molecule adopts a nearly planar structure on a surface rather than a perfectly planar one. Nevertheless, this means that even though the [30]-HC molecule (and other [n]HC molecules) adopts a nonplanar conformation in the gas phase, the van der Waals attraction between the molecule and
show any in-plane deformation from the ideal hexagon structure. To compare the relative stabilities of the macrocycles, it is worthwhile to calculate the gas-phase energies of formation for the planar [n]-HC molecules, starting from the DMQP precursors, according to the equation n/5 DMQP → [n]-HC + n/5 Br2. The calculated data collected in Table 1 show that Table 1. Formation Energies ΔER of Planar [n]-HC Molecules (in Dnh Point Group Symmetry) and n/5 Br2 Molecules from n/5 DMQP Precursor Molecules (in C2v Point Group Symmetry) in the Gas Phase According to DFT Calculations, cf. Figure 1a; kJ/mol, PBE-D3/SV(P) Level of Theory molecule/point group
ΔER
ΔΔER vs [30]-HCa
Strainb
[20]-HC/D4h [25]-HC/D5h [30]-HC/D6h [35]-HC/D7h
+114 +28 +7 +31
+107 +21 0 +25
+27.4 +4.4 0 +3.4
a
Relative formation energies vs [30]-HC. bRelative formation energies per DMQP vs [30]-HC. These numbers are a measure of relative ring strain.
the formation of all rings is endothermic. While formation of the hexagonal [30]-HC is only slightly endothermic, the pentagon, heptagon, and square molecules are increasingly endothermic, reflecting the ring strain effects. The values of ΔΔER in the last column of Table 1 represent the relative ring strain per precursor unit for the different macrocycles. On the surface, the formation energies will be modified due to the adsorption of the molecules and the Br byproduct, which is likely to render the formation of the ring molecules spontaneous. However, the trends regarding the relative ring stability should be the same. Which geometries will the molecules adopt in the gas phase? To approach this question, a search for local minimum structures was performed at the DFT level for [30]-HC (see the Supporting Information for technical details). As may be
Figure 7. Selected optimized structures of [30]-honeycombene identified as local minima on the DFT energy surface, with associated relative energies in kJ/mol (free energies in parentheses for ambient temperature and pressure); PBE-D3/SV(P) level; see the Supporting Information for details. 139
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high vacuum (UHV) with a base pressure of
