Geometric and Electronic Structures of Hydrogen-Bonded Warren

Article pubs.acs.org/JPCC

Geometric and Electronic Structures of Hydrogen-Bonded Warren Truss Networks Comprising Planar Triamide Molecules on Graphite (0001) Shunya Yamazaki,† Yusuke Sueyoshi,† Keiji Matsumoto,† Motoki Tsuboi,† Hiroyuki Ozaki,†,* Osamu Endo,† and Hideyuki Tukada‡ †

Department of Organic and Polymer Materials Chemistry, Faculty of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ‡ Department of Nanosystem Science, Graduate School of Nanobioscience, Yokohama City University, Seto, Kanazawa, Yokohama 236-0027, Japan ABSTRACT: When a very small amount of 1,3,5-benzenetricarboxamide (BTCA) is deposited onto a graphite (0001) surface held at 120 K, an amorphous monolayer is formed owing to the low mobility of the frozen molecules. Upon raising the substrate temperature, they become laid flat and then gradually transformed into peculiar aggregates with slightly tilted orientation, which are completed after annealing at 320 K. Metastable atom electron spectroscopy sensitively probes the changes in the molecular orientation, while ultraviolet photoelectron spectroscopy detects the changes in the electronic structures. Scanning tunneling microscopy affords a clue to the molecular arrangement in the annealed monolayer, for which the firstprinciples calculations are carried out under periodic boundary conditions and the density of states in line with the photoelectron spectra is obtained. It is revealed that hydrogen bonds are introduced among the molecules to provide an anisotropic network of a novel Warren truss structure.

1. INTRODUCTION Organic species physisorbed on a clean inert surface are beneficial to the creation of subnanomaterials with high regularity and stability under ultrahigh vacuum (UHV).1 New covalent bonds can be introduced into a columnar structure of lying chainlike molecules tailored for surface topochemical reactions,1−3 while hydrogen bonds (HBs) are more facilely formed among lying planar molecules with functional groups capable of HB formation.4−8 The geometric structures of HB networks have been mostly studied by scanning tunneling microscopy (STM) for planar molecules on various surfaces,6−17 and it has been found that a certain combination of an organic compound and a substrate surface sometimes produces a few different types of network structures: the arrangements of cyanuric acid (CA) molecules in three different HB networks on graphite (0001) surfaces8 are depicted in Figure 1(a) for example. Such studies did not, however, aim to clarify the electronic structures of the networks. To search for or design the constituent molecules of various subnanosystems, it is of fundamental significance to clarify the conservation or transmutation of the nature of individual molecules when they are arranged with high regularity and united with new bonds or interactions. We demonstrate here that modifications in the electronic structures upon the introduction of hydrogen-bonded periodicity can be probed by electron spectroscopy. From a standpoint of valence electron spectroscopy, inter- or intramolecular HBs were studied for gas-phase organic molecules.18−21 © 2013 American Chemical Society

There have been, however, few attempts on the electronic structures modified by intermolecular HBs on a solid surface.4,5 Our first approach to introduce HBs into a vapor-deposited monolayer was performed for a planar diamide PDB (Figure 1(b)) on graphite (0001).4 A marked difference in stability to sublimation under UHV, found between a monolayer of PDB and that of the related compound incapable of HB formation, suggested the formation of a tapelike network atomic tape. Changes in the electronic structures could not, however, be directly observed because of the complicated structure of PDB. Next we aimed to prepare two-dimensionally bonded networks of melamine molecules;5 though a new distinct band emerged in the metastable atom electron spectra (MAES) of an annealed monolayer, the origin still remains to be clarified due to lack of information on the molecular arrangement. In this study, we try to construct hydrogen-bonded networks of 1,3,5-benzenetricarboxamide (BTCA) (Figure 2(a)) on a graphite (0001) surface and observe changes in both geometric structures (i.e., molecular aggregation) and electronic structures by MAES and ultraviolet photoelectron spectra (UPS), in combination with STM observation and the first-principles calculations under periodic boundary conditions. We have chosen BTCA because it has a relatively simple structure and easily Received: September 30, 2012 Revised: April 13, 2013 Published: April 15, 2013 9652

dx.doi.org/10.1021/jp309705t | J. Phys. Chem. C 2013, 117, 9652−9662

The Journal of Physical Chemistry C

Article

Figure 1. Planar molecules for HB network formation on the graphite (0001) surface: (a) cyanuric acid, (b) 4,4′-[1,4-phenylenethynylene]di(benzamide) (PDB), and (c) melamine. Arrangements of cyanuric acid molecules in three different HB networks8 on graphite are also shown: (a1) hexagonal (Hex), (a2) close-packed (CP), and (a3) flower.

graphite substrate held at 300 K and then heating to 400 K for 10 h to desorb upper layer molecules and leave a monolayer, which was confirmed by the MAES and UPS. Essentially the same procedure was adopted to prepare a monolayer from a multilayer for STM observation. The STM images were obtained in the constant current mode with mechanically polished PtIr tips and calibrated with the images of HOPG. Figure 2. Molecular formula (a) and van der Waals envelope (b) of BTCA.

3. CALCULATIONS All calculations were performed using Gaussian0325 on the basis of density functional theory. The geometry of an isolated BTCA molecule was optimized with Becke’s three-parameter (B3) exchange functional,26 the Lee, Yang, and Parr (LYP) correlation functional,27,28 and the 6-311+G* basis set,29−31 while the HB distance (or intermolecular distance) in a trimer (the fundamental structural unit of the network) was optimized with the B3LYP functional and the 6-31++G basis set.30,33 Kohn− Sham orbital (or molecular orbital (MO)) wave functions for an isolated BTCA were obtained with B32 and LYP functional and the 6-31++G basis set. For the hexagonal (Hex; Figure 1(a1)), the close-packed (CP; Figure 1(a2)), and the Warren truss (WT; see below) arrangement, the wave functions were obtained at the BLYP/6-31++G level under periodic boundary conditions. For the model aggregates of the WT structure comprising eight molecules arranged in manners consistent with the STM images, the total energies were calculated at the same level and compared to discuss the minute arrangement of the molecules.

forms a stable network structure suitable for both electron spectroscopy and STM observation. Moreover, since the molecular structure is reminiscent of the stairlike structure in the crystal of benzamide,22 we also anticipated finding evidence for additional intersheet HBs on other surfaces in the future. Contrary to our expectation to prepare an isotropic sheet spreading twodimensionally (i.e., a hydrogen-bonded network with 3-fold symmetry) and comprising flatly laid molecules, however, it will be shown that novel anisotropic sheets with a peculiar Warren truss arrangement of slightly tilted molecules are obtained for the first time.

2. EXPERIMENTAL SECTION The sample of BTCA was prepared from 1,3,5-benzenetricarboxylic acid chloride and NH4OH and purified by sublimation under vacuum (10−6 Torr) prior to use. Spectral measurements and STM observation were carried out using an electron spectrometer5 and a microscope,23 respectively, for specimens prepared in each sample preparation chamber; both apparatuses were designed for the in situ analysis of organic films under UHV. A piece of highly oriented pyrolytic graphite (HOPG) (NT-MDT Co.; ZYB grade) cleaved in the air was cleaned by heating at 670 K for 48 h in an UHV. An extraordinarily thin monolayer was prepared by depositing BTCA onto the graphite substrate held at 120 K. The amount of deposition was one monolayer equivalence (1 MLE), which contains BTCA molecules necessary to form a monolayer with flat-on orientation.24 As the excitation sources, He* (23S, 19.82 eV) metastable atoms and the He I (21.22 eV) resonance line were used for the MAES and UPS, respectively. The incidence angle of He* or photons and the takeoff angle of electrons were 30° and 60°, respectively. Changes in the MAES and UPS of the monolayer were recorded upon raising the substrate temperature. A multilayer was also obtained by depositing 8 MLE of BTCA onto a

4. RESULTS AND DISCUSSION 4.1. Monolayer Spectra. Figure 3 shows thermally induced changes in the UPS of a BTCA monolayer (curves (ii)−(ix)) prepared on a graphite substrate (curve (i)) held at 120 K. In curve (i), the characteristic bands G and g are assigned to the π valence and the σ conduction bands of graphite, respectively.34,35 Band G and features below 5 eV including band g are also observed in curves (ii)−(ix), indicating that electrons originated in the substrate are detected in the UPS since photons penetrate through the extrathin film to interact with the substrate. Other bands are ascribed to BTCA molecules. Let us assign the BTCA bands on referring to the density of states (DOS) calculated for a single molecule (curve “DOS(BTCA)” in Figure 4); the curve was drawn by superposing Gaussian functions with the same width and height at the calculated 9653



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Figure 3. Thermally induced changes in the He I (21.22 eV) ultraviolet photoelectron spectra (UPS) of a BTCA monolayer prepared by vapor deposition onto a graphite (0001) substrate held at 120 K: (i) the substrate; (ii)−(ix) measured at substrate temperature T. Broken curves (ii) are superimposed on curves (iii)−(ix).

orbital energies ε indicated by bars. Bars labeled with open triangles, circles, and filled triangles correspond to oxygen nonbonding (nO; belonging to the σ-type category), π, and σ MOs, respectively. The values of ε are given in Table 1(a). Bands a−d in curves (ii)−(ix) are ascribed to four bands a−d in DOS(BTCA). We must, however, point out that none of bands a−d (and bands a−d) corresponds to a single MO, but many MOs coexisting in a narrow energy region are responsible for each band: a UPS “band” is composed of many constituent bands overlapping one another, each of which is attributable to a single MO. Since every MO comprising the same type C2p, O2p, N2p, and H1s AOs provides a constituent UPS band with a similar intensity,36 the UPS features reflect the DOS. Taking into account that the BTCA bands are superposed on a smooth background produced by secondary electrons37 as well as the graphite features, the intensities of bands a−d are consistent with those of bands a−d in DOS(BTCA). Since neither orbital energies (that might be determined in gas-phase UPS if conditions permitted) nor their order can be always reproduced well by theoretical (even the first-principles) calculations for a rather complicated molecule such as BTCA, and since many MOs are responsible for an observed (composite) band, it makes little sense to mention the absolute position of apparent peaks in the UPS. But we believe that the tendencies of changes in the overall shapes of composite bands (including their apparent peak positions) should be reflected in the UPS upon network formation accompanied with modifications in the electronic structures and should be reproduced in the calculated DOS as well (provided that the structural model on which the DOS and wave functions for the HB-network are calculated is adequate). At any rate, we basically assign bands a−d to oxygen nonbonding (nO) and π, π and σ, σ and π, and σ MOs, respectively, according to Table 1(a) and DOS(BTCA) in Figure 4. When needed, refinements in the band assignment will be described with the MAES. Upon warming the monolayer, the UPS alter the band shapes. To make it more visible, we have superposed curve (ii) for the as-deposited film (broken lines) on curves (iii)−(ix) measured at various temperatures. At 220 K, bands a and b in curve (iii) become rather intensified and considerably

Figure 4. Difference UPS (ΔUPS) obtained by subtracting curve (ii) from curve (iv) (of Figure 3) (top), calculated density of states for an isolated BTCA molecule (DOS(BTCA)), the Hex (DOS(Hex)), the CP (DOS(CP)), and the WT arrangement (DOS(WT)) of BTCA molecules (bottom), together with the difference DOS (ΔDOS(Hex) = DOS(Hex) − DOS(BTCA); ΔDOS(CP) = DOS(CP) − DOS(BTCA); ΔDOS(WT) = DOS(WT) − DOS(BTCA)) (middle). Each DOS diagram is drawn by superposing Gaussian functions with the same width and height at calculated orbital energies ε given in Table 1. Bars labeled with open triangles, circles, and filled triangles correspond to oxygen nonbonding (nO), π, and σ MOs, respectively. The separation between bands a and b in curve (ii), ΔEka−b = 2.6 eV, is the same as the calculated separation between bands a and b in DOS (BTCA), Δεa−b = 2.6 eV, whereas ΔEkb−c = 1.9 eV and ΔEkc−d = 1.5 eV are different from Δεb−c = 1.3 eV and Δεc−d = 1.0 eV. Therefore, the ε scale is enlarged 1.5 times below −9.2 eV in DOS and ΔDOS curves to correct the positional discrepancy between the experiments and the calculations. The abscissa ε is shifted to place band a in DOS(BTCA) at the peak position of band a in curve (ii).

weakened with a higher- and a lower-Ek shift, respectively, compared with those in curve (ii), and a valley emerges between bands c and d. Further warming to 300 and 320 K continues to intensify band a and makes band c, band d, and the valley between them more distinct in curves (iv) and (v). The decreased intensity of features between bands d and g indicates that the apparent coverage of the substrate is somewhat increased, although the peak height of band g remains almost unchanged due to the increased intensity of the background. Cooling the film to 300 K slightly weakens band a and strengthens bands b−d in curve (vi). Further cooling and warming does not alter the shapes of overall UPS features essentially (curves (vii)−(ix)). These observations must be ascribed to considerable changes in the electronic structures caused by HB formation. We will demonstrate below using MAES and STM data as well that the individuality of a BTCA molecule is maintained at 120 K but gradually modified by the formation and completion of a peculiar network structure. 9654



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Table 1. Orbital Energies and Characters of a Single BTCA Molecule (a) and the WT Structure at the Γ point (b) Obtained by BLYP/6-31++G Calculations no. of MO (a) 54 (HOMO) 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 (b) 108 (HOMO) 107 106 105 104 103 102 101 100 99 98 97

−ε/eV

character

UPS band

no. of MO

−ε/eV

character

UPS band

6.91 6.94 6.98 7.06 9.05 9.08 9.10 9.19 9.23 9.30 10.00 10.12 10.22 10.26 10.31 10.39 10.48 10.59 10.59 10.64 11.02 11.08 11.32 11.45 11.58 11.65 11.84 11.88 12.50 12.51 12.78 12.95 13.39 13.47 13.66 13.74

πB2,3a πB2,3a πB2,3a πN>Oa πB1a σCH>CC,CO πB1a σCH>CC,CO σCH>CC,CO σCH,CC>CO σCO,CH σCO,CH σCC,CH,NH,CO σCO πCONa σCC,NH,CO πCONa πCONa σCO πCONa πCONa πCONa σCC,NH>CH σCC>NH σCC>NH,CO σCC,NH,CO>CH σCC,CO>CH,CO σCC,CH,CO σCH σCH σNH>CC σNH>CC σNH>CC σNH>CC σNH>CC σNH>CC,CH

a a a a b b b b b b c c c c c c c c c c c/d c/d d d d d d d

(b) 6.10 6.10 6.31 6.62 6.62 6.68 6.94 6.94 9.08 9.22 9.22 10.05 10.30 10.30 10.57 10.57 11.01 11.47 11.56 11.56 12.48 13.11 13.70 13.70 5.79 6.10 6.15 6.20 6.22 6.37 6.44 6.50 6.55 6.60 6.70 6.78

nO nO nO πN>Oa πN>Oa πN>Oa πB2,3a πB2,3a πB1a σCH>CC,CO σCH>CC,CO σCO σCO,CH σCO,CH πCONa πCONa πCONa σCC>NH σCC,CH,NH,CO σCC,CH,NH,CO σCH σNH,CC σNH>CC σNH>CC

a a a a a a a a b b b c c c c c c/d d d d

nO nO nO πN>O nO πN>O nO πN>Oa πN>Oa nO πN>Oa πB2,3a

a a a a a a a a a a a a

96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61

Symbols N, O, and C indicate that the MO has distribution on the nitrogen, oxygen, and carbon atoms. MOs labelled as πB1 and πB2,3 correspond to π1 and π2,3 MO of benzene.

a

necessarily the same as that for an apparently corresponding UPS band.39−41 This is true in the BTCA monolayer as well. In curves (ii)−(ix) of Figure 5, there are four BTCA bands with marked variations in the relative intensities. We have superposed curve (ii) for the fresh film maintained at 120 K (broken lines) on curves (iii)−(ix) as in Figure 3 and also curve (iv) after warming the film to 300 K (dashed-dotted line) on curve (vi). To begin with, one may correlate these bands to the UPS bands a−d (i.e., tentatively label them bands a−d) in Figure 3, but it is necessary to examine the consistency of the tentative assignments by means of the characteristics of MAES. Curve (ii) has distinct band g, meaning that there is a large uncovered portion of the substrate surface because metastable atoms He* do not penetrate into the solid and interact with the outermost layer selectively.39 It is noteworthy that band c is most enhanced in curve (ii) unlike in curve (ii) of the UPS in Figure 3, where band a due to the highest DOS in Figure 4 (band a in DOS(BTCA)) is most intensified among the BTCA features. This is an example for the different factors governing

Figure 5 shows changes in the MAES corresponding to those in the UPS of Figure 3. Band g in the substrate spectrum (curve (i)) is due to the σ conduction bands of graphite.38 At first glance one can see that the MAES of the BTCA film (curves (ii)−(ix)) considerably changes the relative band intensities with temperature. Unlike the case of UPS, the intensity of a “constituent” MAES band due to an MO is governed by its local electron distribution at the externally exposed portion of the molecular surface,39 which depends on the type and energy of the MO. Moreover, a certain MO can provide an MAES band with the nominal ionization potential (nomIp = E(He*) − EkMAES; E(He*) is the excitation energy) different from the ionization potential (Ip = hν − EkUPS) determined from the UPS band on account of interaction between colliding partners in Penning ionization.39 From these reasons, MAES and UPS features due to the same set of MOs in various organic films do not always exhibit a very similar appearance with wellcorresponding peaks and valleys. We further point out that the set of MOs mainly responsible for an MAES band is not 9655



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Figure 5. Thermally induced changes in the He* (23S, 19.82 eV) metastable atom electron spectra (MAES) of a BTCA monolayer prepared by vapor deposition onto a graphite (0001) substrate held at 120 K: (i) the substrate; (ii)−(ix) measured at substrate temperature T. Broken curves (ii) are superimposed on curves (iii)−(ix) and dashed-dotted curve (iv) on curve (vi).

Figure 7. Schematic diagrams for molecular orientation in a BTCA monolayer and He*−molecule interactions: (a) random, (b) flat-on, (c) slightly tilted in the WT arrangement.

MOs; the reason why we do not name them bands a−c will be soon disclosed. This is in line with DOS(BTCA) in Figure 4 and Table 1(a) because bands a−c in curve DOS(BTCA) have the contribution of π MOs. Back to curve (ii) in Figure 5, measured at 120 K for the asdeposited film, bands b and c are much more enhanced than bands b′ and c′ in curve (iv), respectively, which is in accordance with the large uncovered portion of the substrate: the graphite band g and the “different” relative intensities of bands a−c indicate that a considerable portion of molecules are overlapped with each other to give tilted and random orientation, so that σ MOs distributed along the molecular framework (such as MOs #43, 33, 42 + 41, and 36 + 35 in Figure 6(a)) as well as π MOs can be effectively probed by He* (see Figure 7(a)). Similar aggregation has been observed for other planar molecules deposited by 1 MLE onto cooled graphite: iron phthalocyanine,42 PDB,4 and melamine5 molecules landed on the substrate are frozen to afford amorphous films and probed by He* at their various portions exposed outside. Thus, it is proper to label these π + σ bands as b and c, as in the UPS curve (ii), to discriminate them from the π bands b′ and c′. Since nO MOs with σ-type electron distribution at the oxygen atoms (MO #52 in Figure 6(a)) as well as π MOs contribute to band a in curve DOS(BTCA), both types of MOs must be responsible for the first band of tilted molecules in curve (ii), and, hence, we can rightly label it as band a. Though band a apparently looks like band a′, band a is slightly less enhanced, and its higher Ek shoulder seems to be scraped off compared with band a′. The latter observation can be related to a possibility: it is not the nO MOs but the π MOs that are responsible for the highest energy portion of band a′, contrary to the prediction of the calculation. We consider that the present calculation might not reproduce the order of actual energies for several highest MOs because we obtained an nO MO as the HOMO and a π MO as the second HOMO for a benzamide molecule using the same functional and basis set as in the case of BTCA, whereas the first and the second UPS bands of gaseous benzamide were assigned to a π and an nO MO, respectively.43 We advance the following discussion, bearing in mind that calculations sometimes fail to reproduce the order of MO energies even around the HOMO,

the relative band intensities in MAES and UPS. Upon warming the film, the intensity of band g is gradually decreased and becomes faint at 300 K (curve (iv)), indicating that BTCA molecules almost cover the substrate. Since the amount of deposition was 1 MLE, they must be oriented with nearly flaton orientation on average, or to say the least, the number of molecules having orientations close to flat-on is considerably increased: the substrate could not be covered with randomly oriented molecules or those arranged with a large inclination angle. Among various MOs of BTCA, π MOs having large electron distribution perpendicular to the molecular plane (such as MO #46 in Figure 6(a)) can be effectively attacked by

Figure 6. Some typical wave functions for a BTCA molecule (a) and the unit cell of the WT arrangement (at the Γ point) (b) (see Figure 10).

He* when the molecules are laid flat (see Figure 7(b)). Therefore, we assign the enhanced bands a′−c′ in curve (iv) to π 9656



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example, if BTCA molecules are arranged with a slight definite tilt so that the edge of every molecule overlaps that of the neighbor (see Figure 7(c)). The relative intensities remain essentially unchanged upon cooling to 220 K (curve (vii)) and 120 K (viii) and further warming to 300 K (ix), as in the case of the UPS, indicating the stability of the aggregation. The peaks of bands a−c and the valleys among them in curve (vi) are lower and shallower, respectively, than those in curve (ii), which strongly suggests the formation of highly organized molecular aggregates. When a molecule physisorbed on a surface is united to an adjacent molecule with a new chemical bond, an MO (of one molecule) engaged in the bond formation interacts with the same type of MO (of another molecule) to produce a pair of new bonding and antibonding MOs, provided that additional interactions between different MOs (with proper symmetry but different energy) are ignored. Similarly, when N molecules on an inert surface are united to form a one- or two-dimensional regular arrangement, the major intermolecular interactions among the MOs afford N MOs with N different energies (provided that there is no degeneracy). One must also realize that the number of molecules having the original MO energy is reduced steadily as the reaction proceeds and the resultant network grows up. If such an MO provides a distinct band in the UPS or MAES, it becomes broader with lower peaks, and if the situation is similar for other MOs, valleys between adjacent bands must become shallower with their increasing widths. We have recently succeeded in observing such changes in both UPS and MAES upon the polymerization of alkadiyne molecules laid flat and arranged to form columnar structures on a graphite surface: the formation (as well as the isomerization) of π-conjugated polymer chains has been sensitively detected by subtracting the monomer spectra from the polymer spectra.41 The monomer molecules provide the well-separated first band attributed to a single π MO (HOMO) in the MAES and that ascribed to two MOs in the UPS, and they are combined with new covalent bonds. On the other hand, BTCA molecules exhibiting “composite” bands due to many overlapping constituent bands in both MAES and UPS are united with HBs. Hence, the situation is rather more complicated in the present case, but the organization of BTCA molecules should be detected within the framework of the MO picture in principle. Can we find evidence for the formation of HB networks in the UPS from the aspect of the electronic structures? A difference UPS (ΔUPS) is drawn at the top of Figure 4 by subtracting curve (ii) from curve (vi) because the above discussion has correlated curve (vi) to a stable network structure and curve (ii) to an amorphous aggregation, in which directional HBs cannot be introduced periodically among molecules to modify the electronic structures in a distinct manner capable of detection: we expect that the electronic structures characteristic of an isolated molecule is essentially maintained in the asdeposited film. In the ΔUPS, we can find peaks P1−P6 at the higher-Ek side of the peak of band a (in curve (ii)), at the valley between bands a and b, at the lower-Ek side of the peak of band b, halfway between bands b and c, and at the peaks of bands c and d. There also exist distinct valleys V1−V3 in the ΔUPS, halfway between band a and the lower-Ek-side valley, halfway between band b and the higher-Ek-side valley, and at the valley between bands c and d. The ΔUPS is compared with two kinds of difference DOS (ΔDOS) obtained by subtracting DOS(BTCA) from DOS calculated for the Hex and the CP arrangement in Figure 1: ΔDOS(Hex) = DOS(Hex) − DOS(BTCA);

which does not, however, alter the following considerations and conclusions. Let us further check the consistency of the above assignments, drawing a picture for the structural transformation upon elevating the substrate temperature. When the film prepared at 120 K is warmed to 300 K, the tilt of molecules is decreased on average to maximize molecule− substrate interactions, which is allowed by an adequate mobility supplied to the molecules. As a result, a majority of molecules become laid flat on the surface, making the π and the nO (σ) MOs more and less easily attacked by He*, respectively. Therefore, the attenuated contribution of the nO MOs is compensated with the intensified one of the π MOs, which results in the intensity of band a′ similar to that of band a. According to Table 1(a), eight MOs are responsible for bands a in the MAES and UPS; the number of π MOs existing in this energy region, Na(π) = 5, is larger than the number of nO MOs, Na(nO) = 3, while Nb(π) = 1 is smaller than Nb(σ) = 2 for band b, and Nc(π) = 2 (or 3, see below) is smaller than (or the same as) Nc(σ) = 3 for band c. This is the reason why band b′ and band c′ are less enhanced than band b and band c, respectively, in the MAES. Since the lowest π MO (#38) is located at the valley between bands c and d in DOS(BTCA), we are uncertain whether or not this MO is responsible for band c/c′. Except for this possibility, there is no π MO contributing to spectral features at Ek lower than the region of band c/c′. Thus the weak band d in curve (ii) is essentially ascribed to σ MOs and, hence, expected to be less enhanced with nearly flat-on orientation at 300 K. Band d seems, however, to be slightly enhanced at 300 K apparently, owing to the increased contribution of smooth features due to very weak σ bands superposed on the upwardsloping background of secondary electrons39 in place of the decreased one of the graphite features between 5 and 3 eV. It was reported that a monolayer of benzene laid flat on cooled graphite provides such subtle σ bands on the MAES.44 Curve (iii) obtained at 220 K exhibits the first three bands with relative intensities similar to those of bands a′−c′ in curve (iv), although curve (iii) is intermediate between curves (ii) and (iv) in the region of bands d and g. These observations suggest that the orientation of molecules is almost flat-on, but some of them might be lapped over others. Since the corresponding UPS in Figure 3 also changes drastically upon warming to 220 K, we feel that some aggregates of BTCA, which will grow up to be a network structure with a certain periodicity, are already formed at 220 K. Further warming the film to 320 K makes the π bands a′−c′ sharper and shifts their positions to slightly higher Ek in curve (v), which suggests the increased number of flat-on molecules or/and the growth of the BTCA aggregates. When the BTCA film warmed to 320 K is cooled to 300 K, the MAES undergoes the second substantial alternation. Since the intensity distributions of the first three bands in curve (vi) are somewhat similar to those of bands a−c rather than those of bands a′−c′, and the fourth band is more enhanced than band d in curves (ii)−(v), we label them bands a−d, which means that the molecules tilt again so that not only π MOs but also σ MOs are effectively probed by He*. However, the degree of molecular tilt is not so large as the averaged tilt in the amorphous film because the intensity of band a relative to the intensity of band b or c in curve (vi) is larger than that in curve (ii); one must remember that N(π)/(N(nO) or N(σ)) is larger for band a than for band b and band c in DOS(BTCA). Nonetheless, since band g is missing, the substrate surface must be completely covered with molecules. Such a situation can be realized, for 9657



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ΔDOS(CP) = DOS(CP) − DOS(BTCA). It is unambiguous that the peaks and the valleys of ΔUPS have little correspondence with those of ΔDOS(Hex) and ΔDOS(CP), meaning that the BTCA aggregates do not have molecular arrangement of these types. Let us seek another BTCA arrangement providing ΔDOS consistent with ΔUPS. 4.2. Multilayer Spectra. Curves (ii) in Figures 8 and 9 show the UPS and MAES of a multilayer prepared by depositing 8

Figure 8. He I UPS of a graphite (0001) substrate (i), a BTCA multilayer (8 MLE) prepared by vapor deposition onto the substrate held at 300 K (ii), and a monolayer left after desorbing molecules in the upper layers of the multilayer by heating to 400 K for 10 h (iii).

Figure 10. STM images for a BTCA monolayer left after desorbing upper layer molecules in a multilayer by heating to 400 K for 10 h; observed with sample bias voltage 1.00 V and tunneling current 13 pA, at T = 300 K (105 Å × 88 Å) (a), 300 K (47 Å × 30 Å) (b), and 80 K (400 Å × 400 Å) (c). In the bottom of (a), the line profiles taken along the blue solid line and the red broken line are shown. The WT arrangement of BTCA molecules is shown in (d).

Figure 9. He* (23S) MAES of a graphite (0001) substrate (i), a BTCA multilayer (8 MLE) prepared by vapor deposition onto the substrate held at 300 K (ii), and a monolayer left after desorbing molecules in the upper layers of the multilayer by heating to 400 K for 10 h (iii).

MLE of BTCA onto a graphite substrate held at 300 K, respectively. Since the UPS does not have bands G and g, and the MAES provides bands a′−c′ with relative intensities similar to those in curve (iv) of Figure 5 for the flat-on monolayer, it is considered that molecules are laid flat and piled up. The multilayer spectra are changed into curves (iii) in Figures 8 and 9 after raising the substrate temperature to 400 K. Since features a−d in both spectra and the intensities of bands G and g in the UPS are very similar to those for the annealed monolayer in Figures 3 and 5 (curves (vi)), molecules laid in the upper layers of the 8 MLE films must be desorbed by heating, but some of the molecules in the inner layers are left on the surface, resulting in the “highly organized aggregates” as in the case of the annealed monolayer. Therefore, we can utilize STM data for the monolayers thus obtained to discuss the details of the geometric and electronic structures of the aggregates, of which formation is detected by UPS and MAES. 4.3. STM Images. Figures 10(a) and 10(b) depict typical STM images for a BTCA monolayer left after desorbing upper layer molecules in a multilayer by heating to 400 K for 10 h. Each bright feature represents a single BTCA molecule. The fast Fourier transform analysis shows a1 = 9.8 ± 0.2 Å, a2 = 16.0 ± 0.2 Å, and γ = 100°. Since 9.8 Å coincides with the optimized

Figure 11. Total energy E of a BTCA trimer (the fundamental structural unit of the hydrogen-bonded network) dependent on the HB distance r(N−H···O).

intermolecular distance in the hydrogen-bonded trimer as shown in Figure 11, we consider that HBs exist along the a1 direction. We focus our attention on triangles formed by three nearest molecules within the two white broken lines. Since they are almost equilateral triangles, HBs must exist between BTCA molecules forming the triangle. Nevertheless, since the distance 9658



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WT in (a) are rotated by 60° around the molecular C3 axis so that the NH groups face the CO groups on the WT boundary to yield arrangement (b) (see light blue circles in Figure 12(b)). Again the interpenetrating of molecules in arrangement (b) is eluded by tilting them to afford arrangement (d). Table 2 shows total energy E calculated for model aggregates in Figure 13 instead of the infinite networks. Aggregates (a)

between the white broken lines 15.8 Å is shorter than 3a1 = 17.0 Å, it is more appropriate to consider that the BTCA network is not two-dimensionally isotropic but anisotropic: the network does not have the 3-fold symmetry. We name this network “Warren truss (WT)” because of the structural similarity to iron bridges of the Warren truss type (see the model structure in Figure 10(d)). Though the shape of each molecule seems subtly different in Figure 10(b), this is attributable to the Moiré pattern based on the commensuration with the graphite, as seen in the large-scale STM image (400 Å × 400 Å) (c): the brightness of lines due to WT running from bottom left to top right changes periodically. This figure indicates that the uniform WT is formed at least over this range. Next, we discuss the positional relationship of adjacent WTs. Figure 12(a) shows a primitive model for the molecular arrangement

Table 2. Energy Ea for Molecular Aggregates (a)−(e) in Figure 13 Calculated Total Energy E for Molecular Aggregates (a)−(e) in Figure 13

a

aggregate

ΔE/kJ·mol−1

(a) (b) (c) (d) (e)

0 −44 +111 +244 −103

E for aggregate (a) is taken as the origin.

and (b) comprise eight molecules placed in the same manner as in the arrangements (c) and (d) in Figure 12, respectively. Aggregate (b) is more stable than aggregate (a) by 44 kJ mol−1 (ΔE(b) = E(b) − E(a) = −44 kJ mol−1). To exert additional (inter-WT) HBs more effectively, the sp2 nitrogens in aggregate (b) are replaced by sp3 nitrogens in aggregate (c). However, the alternation considerably destabilizes the aggregate (ΔE(c) = E(c) − E(a) = 111 kJ mol−1) owing to the destruction of conjugation between the NH2 and the CO group. For a similar reason, aggregate (d) obtained by rotating the NH2 group around the C−N bond becomes further destabilized (ΔE(d) = 244 kJ mol−1). If we twist the whole amide (carbamoyl) group around the ϕ−CONH2 bond, the resultant aggregate (e) is stabilized with ΔE(e) = −103 kJ mol−1. Since aggregate (e) is the most stable among those examined here, it is suggested that the additional HB at the WT boundary or inter-WT HB stabilizes the system at the expense of somewhat reduced interactions of the π electronic systems (conjugation) between the carbamoyl group and the benzene ring. This reminds us of a benzamide crystal in which neighboring dimers are laterally linked with the additional HB to form stairlike structure (the carbamoyl groups are united in a chainlike fashion).22 If aggregate (e) is the partial structure of the real WT network, it seems puzzling that the doubled periodicity along the a2 direction is not reflected on the images. Probably this is explainable by the fact that the images do not exhibit atomic resolution; under such conditions of STM observation it is impossible to discriminate between a WT and the adjacent WT consisting of molecules with a lateral orientation different from that in the former. 4.4. Reexamination of the Spectra. On the basis of the above discussion, let us reexamine the spectra of the annealed monolayer. Upon WT formation, π and nO MOs become interacted with those of the same kind, while σ MOs mainly distributed around the C−H, CO, or/and NH bonds become markedly intermixed, as indicated in Table 1(b). At the WT boundary, tilting molecules in one WT overlap with those in the adjacent WT, exposing the carbamoyl groups arranged linearly. Wave functions for the mixed states such as σCC>NH, σCC>NH,CO, and σCC,NH,CO>CH in Figure 6(b) protrude from the WT edge and are effectively probed by He* and, hence, provide enhanced band d in the MAES (curves (vi)−(ix) of Figure 5). The DOS and ΔDOS for a single infinite WT (corresponding to a doubled row of hydrogen-bonded BTCA molecules

Figure 12. (a) and (b) Primitive models for the arrangement of interpenetrated molecules in the WT structure: (a) neighboring WT is obtained by translation along a2; (b) molecules in every second WT in (a) are rotated by 60° around the molecular C3 axis. (c) and (d) Molecules in (a) and (b) are tilted by ca. 10° to the substrate to avoid interpenetrating. The side view is shown on the bottom of each panel.

produced by simply translating a WT along a2. Since molecules on a WT boundary interpenetrate or overlap one another (in the yellow circles), we consider that the real molecules must be tilted by ca. 10° to the substrate (see Figure 12(c)), which is consistent with different contrasts on the left and the right side of each WT boundary in the STM image (see dark and bright arrows in Figure 10(a)). Since the difference in the contrast is rather ambiguous in Figure 10(a), however, the line profiles taken along the blue solid line and the red broken line are shown at the bottom of Figure 10(a). Alternate higher maxima in the red broken curve corresponding to the molecules on the left side of each WT boundary are always higher than those in the blue solid curve corresponding to the molecules on the right side, which reflects the slight upward slope of the WT from left to right. This observation is in line with the above discussions on the MAES of the annealed monolayer. It seems, however, unnatural that there exists no HB between adjacent molecules on the WT boundary. The situation can be avoided if all the molecules in every second 9659



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Figure 13. Model aggregates of eight molecules for total energy calculation: (a) and (b) arranged in the same manner as arrangements in Figure 12(c) and 12(d); (c) obtained by replacing the sp2 nitrogens of aggregate (b) with sp3 nitrogens; (d) and (e) obtained by rotating the NH2 or the carbamoyl groups of aggregate (b) around the C−N or the ϕ−CONH2 bonds, respectively. Though local structures of (c)−(e) are slightly modified from those of (b) on the WT boundaries, all aggregates are consistent with the STM images in Figure 10.

■

CONCLUSION We tried to obtain direct evidence for modifications in both geometric and electronic structures of molecules upon the formation of a hydrogen-bonded BTCA network. The annealing process of an amorphous monolayer on graphite (0001) was investigated by MAES and UPS. It was found that the relative intensities of the MAES bands are altered markedly, indicating changes in molecular orientation, random → flat-on → slightly tilted, with a monotonous increase in the coverage of the substrate surface. The difference UPS for the slightly tilted molecules does not correspond to the difference DOS calculated for the hexagonal and the close-packed arrangement, which were reported for cyanuric acid monolayers on graphite. The STM images of an annealed monolayer clearly exhibit a peculiar molecular arrangement, for which the WT structure was proposed. The hydrogen-bonded network does not have two-dimensional isotropy but anisotropy. Namely, a doubledrow of BTCA molecules are united by major HBs to form a WT structure; two tilted WTs are laterally linked by additional HBs at partially overlapped carbamoyl groups to afford a WT dimer, and the WT dimers are arranged side by side in the annealed monolayer. Nonetheless, the difference UPS for the annealed monolayer is mostly consistent with the difference DOS calculated for an infinite single WT. Therefore, we consider that the evolution of the electronic structures upon WT formation is essentially governed by the major HB and was successfully detected by UPS. We are now planning calculations for a WT dimer. Modifications in the electronic structures

between the white broken lines in Figures 10(a) and 10(b)) are shown as DOS(WT) and ΔDOS(WT) = DOS(WT) − DOS(BTCA) in Figure 4. One can clearly see that most of the characteristic features of ΔUPS, peaks P1−P3 and P5, as well as valleys V1−V3 are reproduced fairly well in ΔDOS(WT). Furthermore, we expect a possibility that features corresponding to peaks P4 and P6 in ΔUPS can be also reproduced by considering inter-WT interactions in the future. Such good correspondences to ΔUPS cannot be found for ΔDOS(CP) and ΔDOS(Hex) as follows. First, since DOS(WT) places band a at slightly higher ε than DOS(BTCA), peak P1 in ΔDOS(WT) is located on the left shoulder of band a as in ΔUPS; since both DOS(CP) and DOS(Hex) place band a at lower ε, on the contrary, they exhibit a valley and a peak in each ΔDOS on the left and the right side of band a, respectively. Second, the energy difference between the first and the second peak in ΔDOS(CP) is much smaller than that between peak P1 and peak P2 in ΔUPS and ΔDOS(WT), while the corresponding peaks cannot be found in ΔDOS(Hex). Third, in the lower-energy region, both ΔDOS(CP) and ΔDOS(Hex) have no or few peak(s) and/or valley(s) close to the positions of those in ΔUPS except for valley V2 and peak P3 in DOS(Hex); one must be careful not to mistake a peak (valley) for a valley (peak): ΔDOS(Hex) and ΔDOS(CP) have peaks (valleys) close to the position of valley V3 (peak P6). Thus we believe that modifications in the electronic structures of BTCA, caused by HB introduction and WT formation, have been observed by UPS. 9660



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(16) Silly, F.; Shaw, A. Q.; Castell, M. R.; Briggs, G. A. D.; Mura, M.; Martsinovich, N.; Kantorovich, L. Melamine Structures on the Au(111) Surface. J. Phys. Chem. C 2008, 112, 11476−11480. (17) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Molecular and Supramolecular Networks on Surfaces: From Two-Dimensional Crystal Engineering to Reactivity. Angew. Chem., Int. Ed. Engl. 2009, 48, 7298−7332. (18) Carnovale, F.; Gan, T. H.; Peel, J. B. Photoelectron Spectroscopic Studies of the Monomers and Dimers of Acetic and Trifluoracetic Acids. J. Electron Spectrosc. Relat. Phenom. 1980, 20, 53− 67. (19) Tomoda, S.; Achiba, Y.; Nomoto, K.; Sato, K.; Kimura, K. Photoelectron Spectroscopic Study of Simple Hydrogen-Bonded Dimers. I. Supersonic Nozzle Beam Photoelectron Spectrometer and the Formic-Acid Dimer. Chem. Phys. 1983, 74, 113−120. (20) Tomoda, S.; Kimura, K. Photoelectron Spectroscopic Study of Simple Hydrogen-Bonded Dimers. II. The Methanol Dimer. Chem. Phys. 1983, 74, 121−126. (21) Ohno, K.; Imai, K.; Harada, Y. Variations in Reactivity of LonePair Electrons due to Intramolecular Hydrogen Bonding as Observed by Penning Ionization Electron Spectroscopy. J. Am. Chem. Soc. 1985, 107, 8078−8082. (22) Gao, Q.; Jeffrey, G. A.; Ruble, J. R.; McMullan, R. K. A SingleCrystal Neutron Diffraction Refinement of Benzamide at 15 and 123 K. Acta Crystallogr. B 1991, 47, 742−745. (23) Endo, O.; Toda, N.; Ozaki, H.; Mazaki, Y. Domain Structure of 17,19-Hexatriacontadiyne Monolayers Formed on Graphite (0001) Surfaces Studied by UHV-STM. Surf. Sci. 2003, 545, 41−46. (24) Area occupied by a BTCA molecule 80 Å2 was used to estimate 1 MLE. The value is an intermediate between that for CP arrangement 84 Å2 and that for the WT arrangement 74 Å2. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian03, revision C.02; Gaussian, Inc.: Wallingford CT, 2004. (26) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (27) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle− Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (28) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results Obtained with the Correlation Energy Density Functionals of Becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200−206. (29) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650−654. (30) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. Efficient Diffuse Function-Augmented Basis Sets for Anion Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li−F. J. Comput. Chem. 1983, 4, 294−301. (31) Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self-Consistent Molecular Orbital Methods 25. Supplementary Functions for Gaussian Basis Sets. J. Chem. Phys. 1984, 80, 3265−3269. (32) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (33) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. IX. An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724−728. (34) Willis, R. F.; Feuerbacher, B.; Fitton, B. Experimental Investigation of the Band Structure of Graphite. Phys. Rev. B 1971, 4, 2441−2452. (35) Takahashi, T.; Tokailin, H.; Sagawa, T. Electronic Band Structure of Graphite Studied by Highly Angle-Resolved Ultraviolet Photoelectron Spectroscopy. Solid State Commun. 1984, 52, 765−769. (36) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981; pp 27−221.

ascribable to the inter-WT HB will be extracted by comparing differences between the calculated results for a WT dimer and those for a single WT with an appropriate difference spectrum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-42-388-7291. Notes

The authors declare no competing financial interest.

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REFERENCES

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Geometric and Electronic Structures of Hydrogen-Bonded Warren

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