Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron

Article pubs.acs.org/JPCC

Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron Spectroscopy of a Sashlike Polydiacetylene (Atomic Sash) Monolayer: Observation of the π Electronic Structures Peculiar to Two Major Conformers Shunya Yamazaki,† Ryosuke Sasaki,† Keisuke Kato,† Hiroyuki Ozaki,*,† Osamu Endo,† and Yasuhiro Mazaki‡ †

Department of Organic and Polymer Materials Chemistry, Faculty of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ‡ Department of Chemistry, School of Sciences, Kitasato University, Sagamihara, Kanagawa 252-0373, Japan ABSTRACT: Atomic sash (AS) is a single sheet of a sashlike polydiacetylene (PD) made from conjugated alkadiyne molecules laid flat on a graphite (0001) surface. When they are irradiated with UV light, one of the major AS conformers, AS-I, with the PD chain held higher than the alkyl chains in contact with the substrate is formed first but transformed to another conformer AS-II with all of the carbon atoms placed in a common plane. Changes in the π electronic structures upon polymerization and structural transformation are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES), and the spectra are compared with the results of our preceding calculations. The π electronic structures peculiar to each conformer are successfully discriminated: UPS detects differences in the density of states, whereas MAES probes a drastic decrease in the local electron distribution of the π wave functions at the monolayer surface due to the foundering of the PD chain in the lying alkyl chains. The threshold ionization potential 5.0 eV obtained for the PD of the AS-I exposed outside the surface corresponds to the highest HOMO energy in an ideal isolated PD chain.

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INTRODUCTION In an ultrahigh vacuum (UHV), chainlike molecules deposited onto a clean crystallographic surface form extraordinarily thin (0.4 nm) monolayer with flat-on orientation1−14 and characteristic columnar arrangement.1−5,13,14 If the molecules are weakly bound to an unreactive substrate such as a (0001) plane of graphite,8−14 then they can be provided with mobility indispensable for intermolecular reaction.9−14 We do not refer here to severe reactions accompanying the elimination of small chemical species but mild ones taking place “topochemicaly”15 on the surface.10,11 The aggregation and mobility of the chainlike molecules enable us to create subnanomaterials with controlled geometric structures.11 When irradiated with UV light, for example, lying alkyl chains in each column of 17,19-hexatriacontadiyne (HTDY) molecules on graphite are bridged with a polydiacetylene (PD) chain and converted into a single sheet of sashlike macromolecule (atomic sash),10,11 as shown in Figure 1. It is noteworthy that an HTDY monolayer on a Au(111) surface cannot be converted into an AS monolayer because of HTDY−Au interactions stronger than HTDY−graphite interactions: a deficiency of molecular mobility results in the production of small oligomers by UV irradiation.16 If the monolayers of the © 2013 American Chemical Society

Figure 1. (a) Columnar structure of HTDY molecules laid flat in a monolayer with all-trans conformation. (b) Atomic sash (AS) produced by the polymerization of HTDY molecules in panel a. Note that this is one of the AS conformers named AS-II later (see Figure 2).

AS are piled up layer by layer, then a layered organic material with thickness (and chemical structure in the future) controllable at the atomic level can be obtained.17 The AS Received: September 30, 2012 Revised: January 6, 2013 Published: January 10, 2013 2121

dx.doi.org/10.1021/jp309703g | J. Phys. Chem. C 2013, 117, 2121−2128

The Journal of Physical Chemistry C

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formation was first detected by Penning ionization electron spectroscopy (PIES)10 or metastable atom electron spectroscopy (MAES),18 in which the kinetic energy of electrons ejected by collisions between target molecules (M) and metastable helium atoms (He*) (M + He* → M+ + He + e−) is analyzed, and the local electron distribution of individual orbitals is probed at the externally exposed portion of M.19 Later, studies based on STM observations revealed that there exist two major AS conformers depicted in Figure 2: AS-I

characterized in UHV compared with other specimens prepared in the air. From the theoretical aspects, most calculations were limited to unsubstituted PDs.27−29 Probably poly(2,4-hexadiyne) was the only substituted PD27,28 for which the first-principles calculations were carried out prior to our preceding study on the AS-I and AS-II:20 we constructed a simplified model to obtain an optimized geometric structure of the AS-I and compared the electronic structures with those of the AS-II. We revealed almost the same energy for both conformers and explained the origin, found that the HOMO (LUMO) energy is higher (lower) for the AS-II than for the AS-I, and showed that AS-I → AS-II transformation is remotely correlated to the chromatic transition of bulk PDs. The second finding was successfully explained in terms of the orbital shapes dependent on the conformation. Thus the relationship between the electronic and the geometric structures of PDs can be investigated with the AS both experimentally and theoretically, which is not the case for most PDs: the simple n-alkyl groups can link the real system with the theoretical consideration. Using stimulation from an STM tip, we can initiate the polymerization of a (few) HTDY column(s) and induce the structural transformation of a (few) AS in a desired position.30 If such a fabrication is adopted upon constructing molecular devices, then it will be possible that different portions of the device are wired with the AS-I or AS-II. Because the HOMO− LUMO gap is smaller in the AS-II as mentioned above while the PD chain suffers less influence from the underlying surface (and more liable to be attacked by incoming species from the atmosphere) in the AS-I, the selection of the AS-I or AS-II seems meaningful in accordance with the intended use. In this context, using UPS and MAES in combination, we intend here to experimentally reexamine AS formation and isomerization, of which pictures have been modified by recent STM and theoretical studies. Relationships between the valence electronic structures and the geometric structures reflected in the newly measured spectra are compared with the calculations for the AS-I and AS-II.20 In particular, we aim at the selective observation of the π electronic structures peculiar to each conformer. Concretely, the density of states (DOS) in the uppermost π valence region and the local electron distribution of the wave functions are observed during the polymerizaton of an HTDY monolayer prepared on a surface of highly oriented pyrolytic graphite (HOPG) and the transformation of the resultant AS. Although we focus our attention on the pure π region without alkyl MOs, we also present the whole region of spectra because previous spectra published in the 1990s were obtained for mono-9−11,17 and multilayers9,17 on Grafoil substrates. Grafoil consists of graphite crystallites with their (0001) planes (face size 100−2000 Å) oriented parallel to the foil plane, but the crystallites are much smaller than those in HOPG substrates,31 which have been used for STM studies on the HTDY−AS systems.13,14,30

Figure 2. Major conformers of the AS: (a) AS-I, (b) AS-II.

having the PD chain and α-methylenes held higher than other methylenes in contact with the substrate is formed initially and then gradually transformed into another conformer AS-II, with all of the carbon atoms placed in a common plane.14 Such an extrathin polymer monolayer on a crystallographic surface, which comprises PD and n-alkyl chains with definite configuration and conformation, is the most suitable for in situ experimental investigation on the relationship between the geometric and electronic structures of the carbon network.20 Experimental studies on the valence electronic structures of PDs were conducted by X-ray photoelectron spectroscopy (XPS) 21−23 or ultraviolet photoelectron spectroscopy (UPS)23,24 for cleaved single crystals,21,23 thick (100 μm) polycrystalline films,21,22 and Langmuir−Blodgett (LB) multilayered films.24 It was difficult for the single-crystalline and polycrystalline specimens to provide detailed information on the π-conjugated system because they contained the complicated substituents of the backbone that exhibit intense features overlapping the PD bands in the spectra. In the case of ordinary LB films, the PD chains are hardly probed by MAES because they are embedded in the dense forest of standing alkyl chains. For the direct observation of the PD electronic structures, lying PD chains exposed outside the film are indispensable. We can address the issues using an AS monolayer. Although PD monolayers with structural resemblance to the AS-I were prepared on graphite by wet processes and characterized by STM in the air, isomerization to the AS-IIlike structure does not occur in such monolayers.25,26 Because the substituents of the AS, n-C16H33 groups, lack a polar group forming hydrogen bonds, they can be arranged with the “natural” conformation on graphite. In addition, there are much lower risks of contamination for the AS prepared and

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EXPERIMENTAL SECTION The sample of HTDY was synthesized and purified as reported previously.13 The preparation, polymerization, and spectral measurements of HTDY monolayers were carried out with an UHV apparatus designed for the UPS and MAES of organic thin films.32 A piece of HOPG (NT-MDT, ZYB grade) cleaved in the air and cleaned by heating to 670 K for 3 days in the preparation chamber of the spectrometer was used as the substrate. Specimen monolayers were prepared by vapor 2122



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deposition onto the cleaned substrate held at room temperature. The deposited amount of HTDY, monitored with a quartz oscillator calibrated in advance, was 1 monolayer equivalence (MLE), which contains molecules necessary to form a monolayer with the flat-on orientation and the columnar structures in Figure 1a. The specimen was irradiated with UV light (maximum intensity at 190 nm) from a deuterium lamp through a quartz viewing port of the apparatus. The He I (21.22 eV) resonance line and He* (2 3S, 19.82 eV) metastable atoms were used as the excitation sources for UPS and MAES, respectively. The incidence angle of photons or He* atoms and the takeoff angle of electrons are 30 and 60°, respectively. Changes in the UP and the MAE spectra (UPS and MAES) were recorded with increasing time of UV irradiation and after leaving the irradiated monolayer in UHV at room temperature without further irradiation.

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RESULTS AND DISCUSSION In Figures 3 and 4, the He I UPS and the He* (2 3S) MAES of a graphite substrate and an HTDY monolayer are shown as

Figure 4. Changes in the He* (2 3S, 19.82 eV) metastable atom electron spectra (MAES) of an HTDY monolayer prepared by vapor deposition onto a graphite (0001) substrate (a) with increasing time t of UV irradiation: (b) before UV irradiation, (c) t = 10 min, (d) t = 40 min, (e) t = 80 min, and (f) obtained after leaving the irradiated monolayer in UHV for 9 days.

of the monomer with electron distribution perpendicular and parallel to the carbon skeleton plane, whereas intense features A between 13 and 5.5 eV are substantially attributed to alkyl MOs.9 However, band g remains sharply and band G overlaps the higher Ek side of band M, indicating that electrons originated in the graphite substrate are detected in the UPS owing to the thinness of the monolayer and the intrusion of photons into the solid. In the MAES of the monolayer, on the contrary, band g is hardly seen and spectral features are ascribed to HTDY molecules. Because metastable atoms He* do not penetrate into the solid, this observation means that the HTDY molecules cover the graphite surface and are exclusively attacked by He*. Furthermore, the molecules are considered to lie flat and aggregate uniformly because 1 MLE molecules could not cover the surface with other orientations. The flat-on orientation is in line with markedly enhanced bands A3∼A1: band A3 is mainly and band A2 is solely attributed to pseudo-π MOs comprising C2pz and H1s AOs, whereas band A1 is ascribed to σ2s MOs consisting of C2s and H1s AOs; both types of MOs have large distribution perpendicular to the zigzag (xy) plane of the alkyl chains and, therefore, effectively probed by He* in the flat-on orientation (see Figure 5a).8−11,17 Unlike the case of UPS, the first MAES band M is due to the 2π⊥ MO alone because it is effectively but the 2π∥ MO is hardly probed by He* in the flat-on orientation (see Figure 5b).9−11,17 In addition, although the diacetylene 1π⊥ (bonding π⊥; π⊥b) and the 1π∥ (bonding π∥; π∥b) MO contribute to the UPS features around 11.5 eV and the 1π⊥ MO contributes to the MAES

Figure 3. Changes in the He I (21.22 eV) ultraviolet photoelectron spectra (UPS) of an HTDY monolayer prepared by vapor deposition onto a graphite (0001) substrate (a) with increasing time t of UV irradiation: (b) before UV irradiation, (c) t = 10 min, (d) t = 40 min, (e) t = 80 min, and (f) obtained after leaving the irradiated monolayer in UHV for 9 days.

curves a and b, respectively. In the substrate spectra, bands g at 3.3 eV and band G (of UPS) at 14.1 eV are ascribed to the σ conduction bands and the π valence bands of graphite, respectively.33,34 Both UPS and MAES of the monolayer exhibit bands corresponding to those observed for monolayers prepared on Grafoil substrates,9−11,17 although curve b in Figure 3 has much more distinct features probably because a smaller density of domain boundaries in the HOPG substrate provides a higher regularity in molecular aggregation. The first band M of the UPS is assigned to the diacetylene 2π⊥ (antibonding π⊥; π⊥a) and the 2π∥ (antibonding π∥; π∥a) MO 2123



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Figure 5. Schematic diagrams for the moieties of an HTDY molecule lying flat on a substrate and interacting with metastable atoms He* as well as MOs distributed there: (a) a methylene sequence and (b) the conjugated diacetylene and the α-methylenes.

around the peak of band A3, the contributions are almost indiscernible in curves (b) because the DOS for these MOs is tiny compared with the DOS for the alkyl MOs. As will be shown below, the pure π region of each spectrum above the indiscernible 1π⊥ (+ 1π∥) band provides characteristic features sensitive to the evolution of polymerization and isomerization and, hence, is helpful for us to follow the phenomena. Changes in the UPS and MAES with increasing time of UV irradiation are depicted as curves b∼e in Figures 3 and 4, where bands M become weakened while the alkyl and the graphite features remain essentially unchanged. One can confirm that UV irradiation from the deuterium lamp is responsible for the decreased intensities of bands M with Figure 6: in both UPS

Figure 7. Changes in the pure π region of the difference UPS for the irradiated monolayer, obtained by subtracting curve b from curves c∼f in Figure 3.

Figure 8. Difference density of states (DOS) between an infinite AS-I and an HTDY molecule (DOS(AS-I)−DOS(HTDY); (a)) and that between an infinite AS-II and an HTDY molecule (DOS(AS-II)− DOS(HTDY); (b)) obtained by the first-principles calculations.20 The abscissa represents Kohn−Sham orbital energy.

The increasing intensity of minus band −M due to the 2π⊥ + 2π∥ MOs of the HTDY monomer in all of the difference curves is ascribable to the evolution of polymerization. The reaction proceeds even after the irradiation is stopped because band −M is much intensified in curve f−b than in curve e−b, which is due to reactive species surviving after turning off the deuterium lamp: growing PDs temporarily located at unfavorable positions may survive for a long time on the graphite surface under UHV because there is no source of proton to terminate the polymerization. On referring to the difference DOS in Figure 8, band P2 around 13.3 eV in curves c−b, d−b, and e−b is assigned to the DOS around the top of the 1π∥ band (1π∥T), whereas band P1 at 12.2 eV is attributed to the DOS around the bottom of the 2π⊥ band (2π⊥B) and that of the neighboring 1π∥ band (1π∥B). Because these bands become intensified with irradiation time, the “evolution of polymerization” does not only mean a decrease in the number of monomer molecules but also indicates an increased degree of polymerization n (or number of monomer molecules from which a polymer is produced). That is, with elongating π conjugation, the energy of the 2π⊥ MOs of HTDY molecules becomes split and spread to form the 2π⊥ band, whereas the 2π∥ MOs are reorganized into the “isolated” 1π∥ MOs of the PD CC bond (more properly, the 1π∥ band with small dispersion because the 1π∥

Figure 6. Band M of He I UPS for an HTDY monolayer and that of He* (2 3S) MAES for another one after leaving them in UHV for time t without UV irradiation: (a) t = 0 h, (b) t = 6 h, and (c) t = 28 h.

and MAES, bands M are not altered after leaving HTDY monolayers in UHV without UV irradiation. Band M in the UPS (MAES) of the irradiated monolayer becomes further weakened (disappeared) after leaving it for 9 days, as shown in Figure 3f (4f). Owing to the presence of the neighboring band G, however, it is not easy to see the changes around band M in the UPS, so we subtract curve b for the unirradiated monolayer from curves c∼f for the irradiated ones in the pure π region and enlarge the difference UPS, as shown in Figure 7. We also compare in Figure 8 the difference of DOS between an infinite AS-I and an HTDY molecule (DOS(AS-I)−DOS(HTDY)) and that between an infinite AS-II and an HTDY molecule (DOS(AS-II)−DOS(HTDY)), obtained by the first-principles calculations.20 2124



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MOs can interact with the σ electronic system) and σ bands intermixed with the σ2p MOs of the alkyl chains. In addition, it must be noted that band P2 and band P1 in curve f−b are shifted to lower and higher Ek, respectively, compared with other curves in Figure 7. Because similar tendencies are seen in Figure 8 for DOS(AS-I)−DOS(HTDY) and DOS(AS-II)− DOS(HTDY), it is natural to consider that the changes in difference UPS after leaving the irradiated monolayer reflect the structural transformation (isomerization) of the AS, which will be further confirmed by MAES. Because of the extreme sensitivity to the outermost surface as well as the relative band intensities characteristic of the molecular orientation,19 the MAES of Figure 4 provide us with more straightforward information on the polymerization and isomerization than the UPS of Figure 3, as follows. The peak of the 2π⊥ band M becomes lower and lower with irradiation time and completely disappears after leaving the specimen for 9 days. The features ascribable to graphite are, however, still missing, and the alkyl bands A3∼A1 remain markedly enhanced. These observations indicate that the polymerization proceeds without the desorption of molecules and the flat orientation of the alkyl chains is maintained throughout the reaction. To examine modifications in the π electronic and the geometric structure of the monolayer in more detail, the pure π region for the unirradiated monolayer (curve b) is subtracted from that for the irradiated ones (curves c∼f) in Figure 9.

Figure 10. Schematic diagrams for wave functions ψ(π⊥) and ψ(π∥) in the AS-I and the AS-II.

Figure 11. He* approaching the polydiacetylene (PD) chains in the AS-I (a) and the AS-II (b), drawn so that the radius of He* (2.9 Å)35 is proportional to the size of the molecule. An upward hydrogen atom of an α-methylene and a PD carbon with the exposed flank are denoted by Hαup and CPDfl, respectively.

follows. A numerous number of ψ(π∥) or/and ψ(π⊥) distributed at the PD chains are responsible for a UPS “band” and an MAES “band”: both are composed of many overlapping constituent bands, each of which is attributable to a single ψ. Because every ψ comprising the same type C2p AOs provides a constituent UPS band with a similar intensity, the UPS features reflect the DOS. On the other hand, the intensity of a constituent MAES band due to a ψ is governed by its local electron distribution at the externally exposed portion of the molecular surface,8,19 which depends on the type and energy of ψ and the PD length. Furthermore, a certain ψ 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.19 Thus, MAES and UPS features due to the same set of ψ (or MOs in various organic films19) do not always exhibit similar appearance with wellcorresponding peaks and valleys. Bands P2 and P1 in Figure 9 become intensified with irradiation time, meaning that the π MOs of the monomer are recomposed into the elongated π conjugated system. However, the π features of the AS-I almost disappear 9 days after turning off the deuterium lamp. This drastic change in the MAES can be explained by the transformation from the AS-I to AS-II. Both

Figure 9. Changes in the pure π region of the difference MAES for the irradiated monolayer, obtained by subtracting curve b from curves c∼f in Figure 4.

As in the UPS, the minus band −M due to the monomer 2π⊥ MO in curves c−b ∼ f−b means a decreased number of monomer molecules; band P2 and band P1 in curves c−b ∼ e− b are assigned to the high DOS around 1π∥T and that around 2π⊥B and 1π∥B, respectively. Although one may suspect the contribution of the π∥ states to the MAES, the wave functions ψ(π∥) can be probed by He* in the AS-I, where the PD chain is raised higher than the alkyl chains with the twisted methylenes at α and β positions (see Figure 10). It is difficult for ψ(π∥) to interact with He* right above the PD carbons, but the local electron distribution oozing sideward from PD carbons with exposed flanks (CPDfl in Figure 11a) will be effectively detected by He* partly because the LCAO coefficients of ψ(π∥) are larger than those of ψ(π⊥) (Figure 10). In addition, one may wonder how the positions of bands P2 and P1 are unclear compared with those in the UPS; this can be accounted for as 2125



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ψ(π⊥) and ψ(π∥) can be effectively probed by He* for the raised PD chain of the AS-I, whereas they become harder to be attacked by He* if the PD chain is “buried” in the columns of the alkyl chains of the AS-II (see Figure 10). The situation is also understandable, considering the approach of He* to the PD chains, as illustrated in Figure 11. The upward hydrogen atoms of the α-methylenes20 (Hαup) in the AS-I (a) are held at almost the same height from the substrate as the PD carbons, whereas Hαup in the AS-II (b) are maintained higher than the PD carbons. Therefore, the portion of the PD surface accessible by incoming He* atoms with incidence angle θ = 30° is more limited for the AS-II than for the AS-I. To a greater or lesser extent, the limitation is valid unless the lateral direction of He* incidence coincides with the C−CC−C direction (ϕ = 0°) (note that the lateral orientation of graphite crystallites is random). Furthermore, it is probable that He* atoms approaching the PD with θ = 30° and ϕ ≠ 0° become readily quenched by some pseudo-π MOs having a considerable contribution of C2pz and H1s AOs at α-methylenes when the PD chain is sunk to the same height as the alkyl zigzag plane. Thus, we have observed π electronic structures peculiar to the AS-I and AS-II before and after isomerization, respectively. In the difference DOS (Figure 8), there exist slender features extending to the 2π⊥T of each conformer above the valley due to the 2π⊥ + 2π∥ MOs of the monomer, which are observed as band P3 in the difference UPS (Figure 7). The threshold Ek value of band P3 is 16.2 eV for both conformers, resulting in threshold Ip (Ipth) 5.0 (= 21.2 − 16.2) eV. This value is very close to Ipth = 5.1 eV observed for LB multilayers of poly(cadmium tricosa-10,12-diynoate) (PTDA) by UPS.24 These are probably the smallest values so far determined by UPS for PDs in the pristine state. In general, Ip for an organic solid is smaller than that for an isolated gaseous molecule of the same compound: the ionized state is stabilized because the molecular ion left behind upon ionization polarizes surrounding molecules. The difference ΔIp, which is referred to as polarization energy Ep or relaxation shift Sr depending on the definition, is governed by the packing density of the molecules: as solid-phase molecules become more effectively surrounded by neighbors, Ep or Sr becomes larger with smaller Ip (larger Ek).36,37 In an m-layered film (2 ≤ m ≤ 5) of pentacene molecules laid flat and piled up on graphite, for example, Ek for the HOMO band of the outermost surface layer (OSL), in which a molecule lacks neighbors on it, is smaller than that of the inner layers (IL): EkIL(m) − EkOSL(m) = ΔEkI−O(m) = 0.3− 0.35 eV.37,38 The Ek of a monolayer (Ek(1)) is the same as EkOSL(m) but smaller than that for the OSL in a crystalline film (EkOSL(C)): Ek(1) = EkOSL(C) − 0.5 eV.37 Because pentacene molecules in the crystalline film are oriented with the long axis almost perpendicular to the surface, however, the portion of the van der Waals envelope of an OSL molecule exposed outside is small and, hence, EkIL(C) − EkOSL(C) = ΔEkI−O(C) must be considerably smaller than ΔEkI−O(m). Consequently, EkIL(C) = EkOSL(C) + ΔEkI−O(C) = Ek(1) + 0.5 eV + ΔEkI−O(C) becomes larger than Ek(1) by more than 0.5 eV. The relation between pentacene molecules in a crystalline film and those in a monolayer is similar to the relation between a PD chain in the LB films of ref 24 and that in the AS-I. The PD chain of PTDA in the LB multilayer is supposed to be effectively surrounded by alkyl chain forests and neighboring PD chains, whereas that of the AS-I is maintained higher than lying alkyl chains and exposed outside. Taking into account the relationship between EkIL(C) and Ek(1) for pentacene, almost the same Ipth values

observed for the LB film and the AS-I suggest that Ipth would be smaller for the AS-I than for the LB film by 0.5 eV or more if the UPS of “gaseous polymer” with exactly the same conformation as in each film could be obtained. Therefore, we consider that the PD of the AS-I molecule has the highest HOMO energy. This should be attributable to the formation of very long AS molecules with an extreme regularity in the microscopic structures. We obtained STM topographs of 200 × 200 nm2 exhibiting uniform arrays of AS-I molecules with straight PD chains, which means that n is larger than 400 at the lowest estimate.14 Although the calculated 2π⊥T energy of the AS-II is considerably (0.3 eV) higher than that of the AS-I in Figure 8, the obtained Ipth values are the same. We tentatively point out two possibilities. First, n might become smaller upon isomerization on average; that is, the specimen giving curve f in Figures 3 and 4 might contain shorter AS of which n or effective conjugation length (ECL) is not long enough to provide practically the same DOS as an ideal infinite AS. Although fragmentation to pentamers or hexamers was observed by STM for monolayers that underwent more efficient and prolonged UV irradiation,14 we do not consider that the major constituents of the present AS-II monolayer are such oligomers. They have n-dependent HOMO energies at higher IP (lower Ek) compared with the 2π⊥T energy of the infinite AS of course. However, they would also provide intense features below 14 eV in Figure 7 and smear out the polymer bands P2 and P1. This is not the case; the shapes of bands P2 and P1 in curve f−b are similar to those in curves c−b ∼ e−b. Nevertheless, it is supposable that the transformation from the AS-I to AS-II occurring against a tendency to maintain a parallel relation between the alkyl chain direction and the axis of the graphite lattice20 brings about a decrease in ECL, which may cause some lowering of the 2π⊥T energy. Second, because the PD chain is maintained closer to the graphite substrate in the AS-II than in the AS-I, it is possible that the intermixing of the π electronic structures between the AS-II and graphite is responsible for the increased Ipth at least to some extent. Despite a general knowledge that a monolayer of chainlike or planar molecules on graphite belongs to a typical physisorbed system, we empirically have a feeling that such modifications in the electronic structures of the system sometimes affect the electron spectra. It seems rather enigmatic that difference MAES c−b ∼ e−b for the AS-I have almost no discernible features above band −M, unlike band P3 in the difference UPS. This might be ascribed to the character of ψ(π⊥) in this energy region. The number of π⊥ states increases with n or ECL. In each ψ(π⊥), however, the contribution of a certain carbon 2pz AOs in the PD chain is reduced with the progression of delocalization owing to the normalization of the wave function. The reduced LCAO coefficients cause a lowering in the electron distribution of individual ψ(π⊥) outside the van der Waals envelope of the AS, making it difficult to be probed by He*. This effect is expected to be more striking for states around 2π⊥T with larger number of nodes, which is probably responsible for the missing features P3 in the MAES of the AS-I.

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CONCLUSIONS Using UPS and MAES, we investigated the formation of the AS-I from an HTDY monolayer on graphite and the structural transformation of the AS-I to the AS-II. Changes in the pure π bands that reflect the evolution of the conjugated system were 2126



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(3) Ishii, H.; Morikawa, E.; Tang, S. J.; Yoshimura, D.; Ito, E.; Okudaira, K.; Miyamae, T.; Hasegawa, S.; Sprunger, P. T.; Ueno, N.; et al. J. Electron Spectrosc. Relat. Phenom. 1999, 101−103, 559−564. (4) Yoshimura, D.; Ishii, H.; Ouchi, Y.; Ito, E.; Miyamae, T.; Hasegawa, S.; Okudaira, K. K.; Ueno, N.; Seki, K. Phys. Rev. B 1999, 60, 9046−9060. (5) Hosoi, Y.; Niwa, Y.; Sakurai, Y.; Ishii, H.; Ouchi, Y.; Seki, K. Appl. Surf. Sci. 2003, 9805, 1−5. (6) Hostetler, M. J.; Manner, W. L.; Nuzzo, R. G.; Girolami, G. S. J. Phys. Chem. 1995, 99, 15269−15278. (7) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki, K. J. Phys. Chem. B 2000, 104, 7363−7369; 2000, 104, 7370−7376. (8) Ozaki, H.; Harada, Y. J. Am. Chem. Soc. 1990, 112, 5735−5740. (9) Ozaki, H.; Mori, S.; Miyashita, T.; Tsuchiya, T.; Mazaki, Y.; Aoki, M.; Masuda, S.; Harada, Y.; Kobayashi, K. J. Electron Spectrosc. Relat. Phenom. 1994, 68, 531−539. (10) Ozaki, H.; Funaki, T.; Mazaki, Y.; Masuda, S.; Harada, Y. J. Am. Chem. Soc. 1995, 117, 5596−5597. (11) Ozaki, H. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 377− 382. (12) Ozaki, H.; Kasuga, M.; Tsuchiya, T.; Funaki, T.; Mazaki, Y.; Aoki, M.; Masuda, S.; Harada, Y. J. Chem. Phys. 1995, 103, 1226−1228. (13) Endo, O.; Toda, N.; Ozaki, H.; Mazaki, Y. Surf. Sci. 2003, 545, 41−48. (14) Endo, O.; Ootsubo, H.; Toda, N.; Suhara, M.; Ozaki, H.; Mazaki, Y. J. Am. Chem. Soc. 2004, 126, 9894−9895. (15) Wright, J. D. Molecular Crystals; Cambridge, 1987; pp 108−130. (16) Endo, O.; Furuta, T.; Ozaki, H.; Sonoyama, M.; Mazaki, Y. J. Phys. Chem. B 2006, 110, 13100−13106. (17) Ozaki, H.; Magara, T.; Mazaki, Y. J. Electron Spectrosc. Relat. Phenom. 1998, 88−91, 867−873. (18) Abbreviation PIES must be used for electron spectroscopy in which electrons are ejected by a mechanism called Penning ionization.19 This is true when gaseous molecules or ordinary organic surfaces are the targets but not the case for various surfaces of solids including metals or semiconductors. Abbreviation MAES can be used for electron spectroscopy using metastable atoms as the excitation source irrespective of the mechanism of electron emission. We use MAES henceforth following the usage in the Review.19 (19) Harada, Y.; Masuda, S.; Ozaki, H. Chem. Rev. 1997, 97, 1897− 1952. (20) Suhara, M.; Ozaki, H.; Endo, O.; Ishida, T.; Katagiri, H.; Egawa, T.; Katouda, M. J. Phys. Chem. C 2011, 115, 9518−9525. (21) Knecht, J.; Reimer, B.; Bässler, H. Chem. Phys. Lett. 1977, 49, 327−329. (22) Knecht, J.; Bässler, H. Chem. Phys. 1978, 33, 179−183. (23) Stevens, G. C.; Bloor, D.; Williams, P. M. Chem. Phys. 1978, 28, 399−406. (24) Nakahara, H.; Fukuda, K.; Seki, K.; Asada, S.; Inokuchi, H. Chem. Phys. 1987, 118, 123−131. (25) Okawa, Y.; Aono, M. Nature 2001, 409, 683−684. (26) Miura, A.; De Feyter, S.; Abdel-Mottaleb, M. M. S.; Gesquière, A.; Grim, P. C. M.; Moessner, G.; Sieffert, M.; Klapper, M.; Müllen, K.; De Schryver, F. C. Langmuir 2003, 19, 6474−6482. (27) Katagiri, H.; Shimoi, Y.; Abe, S. Chem. Phys. 2004, 306, 191− 200 and references therein. (28) Filhol, J.-S.; Deshamps, J.; Dutremez, S. G.; Boury, B.; Barisien, T.; Legrand, L.; Schott, M. J. Am. Chem. Soc. 2009, 131, 6976−6988. (29) Katagiri, H.; Shimoi, Y.; Abe, S. Phase Transitions 2002, 75, 879−885. (30) Endo, O.; Suhara, M.; Ozaki, H.; Mazaki, Y. e-J. Surf. Sci. Nanotechnol. 2005, 3, 470−472. (31) Niimi, Y.; Matsui, T.; Kambara, H.; Tagami, K.; Tsukada, M.; Fukuyama, H. Phys. Rev. B 2006, 73, 085421. (32) Ozaki, H.; Suhara, M.; Ohashi, T.; Toda, N.; Endo, O.; Tukada, H. J. Electron Spectrosc. Relat. Phenom. 2004, 137−140, 151−154. (33) Takahashi, T.; Tokailin, H.; Sagawa, T. Solid State Commun. 1984, 52, 765−769 and references therein.

examined and compared with the results of preceding calculations, although the alkyl and graphite bands were used to characterize the aggregation of molecules or alkyl chains. Spectral features corresponding to the DOS around the tops and bottoms of the 2π⊥ and the 1π∥ band were clearly observed in the UPS, and some of them were found to be shifted upon isomerization from the AS-I to the AS-II. The local electron distributions of the wave functions around the top of the 1π∥ band and those around the bottoms of the 2π⊥ and the 1π∥ band were probed as enhanced features in the MAES of the ASI, whereas they almost disappeared upon isomerization to the AS-II. Therefore, we succeeded in observing the electronic structures of the π conjugated polymer dependent on the conformation. To our knowledge, this is the first electron spectroscopic report discriminating the π electronic structures peculiar to each conformer. In addition, we obtained the lowest Ipth 5.0 eV ever observed for PDs in the pristine state. Although Nakahara et al. also observed similar value 5.1 eV for the LB multilayer of PTDA, other features characteristic of the π band structures in the lower energy region were not obtained in their UPS.24 It is, moreover, worthy of attention that the AS-I monolayer with PD chains raised and exposed outside the columns of lying alkyl chains provides such a small Ipth value as PD chains buried deep in the forest of standing alkyl chains. Because much smaller polarization energy is expected for the AS-I, the similar Ipth values mean that the HOMO energy for an ideal gas phase molecule is much higher for the AS-I. These observations were attained by the special methodology of AS preparation. The key points are (1) design of the monolayer reaction and the constituent molecule, (2) use of graphite as the substrate, (3) high regularity in the configuration, conformation, and arrangement revealed by recent STM, and (4) UHV environment throughout the experiment. Aspects (1) and (2) are critical for the creation of the AS comprising a PD chain and simple alkyl chains:11 monomers were planned to be laid flat and arranged uniformly under the influence of moderate molecule−substrate interactions and provided with mobility indispensable for polymerization. Aspects (3) and (4) are necessary to avoid the formation of unfavorable products and obtain the AS with a long ECL. It is important that a transfer process that may cause film destruction or chain distortion (reduction of ECL) is not included, unlike the case of LB film preparation. All aspects simplify both molecule− substrate and intermolecular interactions, providing distinct features in the electron spectra. Besides the observation of minute DOS dependent on the conformation by UPS, the application of MAES to this system is of significance because it selectively detects the extrathin monolayer and probes the local electron distribution of wave functions for the elongated π conjugated system. Drastic change due to the foundering of the PD chain was sensitively observed by MAES.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 2901− 2913. (2) Firment, L. E.; Somorjai, G. A. J. Chem. Phys. 1978, 69, 3940− 3952. 2127



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Ultraviolet Photoelectron Spectroscopy and Metastable Atom Electron

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