Scanning tunnelling microscopy analysis of octameric o-phenylenes on Au(111)

Abstract

Oligomeric o-phenylenes are an emerging class of helical molecules that exhibit complex conformational behaviour, which is determined by their terminal groups and the solvent environment. Using scanning tunnelling microscopy (STM), we investigated the conformational behaviour of octameric o-phenylenes (OP₈s) featuring Br or NO₂ terminal groups (OP₈Br and OP₈NO₂). These o-phenylene oligomers are known to adopt a well-defined helical conformation in the solid state, but undergo a rapid helical inversion in solution at ambient temperature. We show that STM microscopy allows the direct observation of perfectly and partially folded helical conformers of OP₈Br and OP₈NO₂, both of which are simultaneously present in their respective molecular films on Au(111). Possible electronic interactions between the Au(111) surface and the exposed π-system of the OP₈ derivatives might result in a thermodynamic bias able to stabilize the metastable, partially folded conformation. Recent studies have revealed that the helical inversion rate of OP₈NO₂ in solution decreases substantially upon oxidation to the corresponding radical cation (OP₈NO₂˙⁺). The OP₈NO₂ and OP₈NO₂˙⁺ film morphologies on Au(111) observed by STM differed substantially with respect to each other, most likely reflecting differences arising from the conformational states of OP₈NO₂ and OP₈NO₂˙⁺ at the molecular level, as well as improved charge-transport properties of the oxidized state.

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Introduction

As helical molecules are ubiquitous in functional biomolecules and have potential applications in chirotechnology,^1–4 they have attracted considerable attention from various sections of the scientific community. Oligomeric and polymeric o-phenylenes, consisting of multiple phenylene units linked together at their ortho-positions, are an emerging class of such helical molecules.^5–19 Due to their heavily angled aromatic connection, oligomeric o-phenylenes are able to adopt a closely packed helical conformation featuring axial chirality.⁹ Neighbouring phenylene units in such π-conjugated backbones comprise a dihedral angle of ∼70°, which results in a cylindrical structure with 3₁-helical geometry (Fig. 1). Recently, Fukushima, Aida, and co-workers reported the synthesis and conformational behaviour of a series of octameric o-phenylenes (OP₈s) derived from veratrole.^9,10 Even though OP₈Br and OP₈NO₂ (Fig. 1a), bearing terminal Br and NO₂ groups, respectively, adopt a virtually perfect 3₁-helical conformation in the solid state, they undergo a rapid helical inversion between their P- and M-helical conformers in solution. Interestingly, upon chemical oxidation of OP₈NO₂, the helical inversion of OP₈NO₂˙⁺ is significantly suppressed.⁹ Time-dependent circular dichroism studies performed on an enantiomerically pure OP₈NO₂˙⁺ sample revealed that the racemization follows first-order kinetics with a rate constant of 2.18 × 10⁻⁶ s⁻¹ at 263 K, which is approximately one order of magnitude lower than that of OP₈NO₂ (9.84 × 10⁻⁴ s⁻¹ at 263 K). Intensive studies by Hartley and coworkers^12–17 indicated that OP₈s exhibit, despite the fact that the perfectly folded helical conformation prevails in the solid state, a complex stereochemistry in solution, involving a large number of partially folded conformers that are similar in energy. Our calculations at the B3LYP/6-31G(d) level of theory showed that the perfectly folded conformers should be ca. 0.2 and 0.4 eV more stable than the partially folded conformers of OP₈Br and OP₈NO₂, respectively (Fig. 1b and c). To gain further insight into the conformational dynamics of o-phenylene oligomers, we focused our attention on investigating metastable OP₈ conformers, including their helical inversion processes. Scanning tunnelling microscopy (STM) is a very powerful technique for this purpose, as it allows the direct observation of surfaces and adsorbed molecules.^23–25 Furthermore, several reports have demonstrated that aromatic molecules are energetically stabilized upon adsorption onto metal surfaces.^20–22 We anticipated that a certain degree of interaction between the π-system of OP₈s and a metal surface would result in an energetic stabilization of the metastable partially folded conformers, as it features a larger number of interaction sites compared to the perfectly folded conformer (Fig. 1b and c). Herein, we describe the STM analysis of OP₈NO₂ and OP₈Br films on Au(111), which provides evidence for the coexistence of both perfectly folded and partially folded conformers. Furthermore, we disclose the STM analysis for films of OP₈NO₂˙⁺, the radical cation salt of OP₈NO₂, for which a slower helical inversion process relative to the neutral form was observed.⁹

Fig. 1 (a) Molecular structures of OP₈Br and OP₈NO₂. (b) Optimized structure for a perfectly folded OP₈Br conformer. Third and sixth rings are perpendicular to the projection plane (c) calculated structure of a partially folded OP₈Br conformer at the B3LYP/6-31G(d) level of theory (see experimental section and ESI† for further details). The stick representations in (b) and (c) illustrate the aromatic backbones and terminal groups in OP₈Br.

Results and discussion

A large-scale STM image (90 × 90 nm) of an OP₈Br film on Au(111), measured at 25 °C in air, exhibited three well-defined regions, which are illustrated by different shades of orange in Fig. 2a. The STM contrast (apparent height) profile displayed three distinct peaks at 0, 0.15, and 0.30 nm (Fig. 2b). The consecutive increase in the apparent height distribution by 0.15 nm suggests that OP₈Br was deposited in a few-layer structure on the metallic surface, whereby the apparent height of the monolayer in the STM imaging is estimated to be 0.15 nm. Time-evolution STM images, which were recorded on the same area of the film by repeated scans, showed that the topmost molecular layer was removed by the STM tip, whereas the deepest molecular layer remained intact (Fig. 3). This observation is indicative of substantial interactions between OP₈Br and Au(111). Notably, there are a number of randomly scattered bright spots that are higher than the surrounding area distributed over the entire observation area. Fig. 2c shows a magnified STM image of a flat monolayer area, defined by the square in Fig. 1a, where the bright spots are coloured in green for clarity. The apparent height distribution (Fig. 2d) shows that the main peak centred at 0 nm tails significantly due to the presence of these bright-spot regions. After waveform separation, the height profile can be best described by taking a small contribution of a broad peak centred at 0.06 nm into account (Fig. 2d). As this value is much smaller than the apparent height of the monolayer of the OP₈Br film (0.15 nm) in the STM imaging, and considering the highly complex stereochemistry of OP₈s, including perfectly and partially folded helical conformers, it is reasonable to assume that the local deviation of the apparent height in the OP₈Br film may originate from the coexistence of different molecular conformers. The perfectly folded OP₈Br conformer adopts a cylindrical geometry, for which theoretical calculations delivered length and diameter values of 1.0 and 0.8 nm, respectively (Fig. 1b). In comparison, the larger molecular axis of partially folded conformers should be longer than that of the perfectly folded one. In the STM images of OP₈Br (Fig. 2a), the averaged area of the bright spots is ∼1.3 nm² (Fig. 2e), which is closer to the cross-section area of the perfectly folded conformer (1.0 × 0.8 nm) than to that of partially folded ones (1.8 × 0.8 nm) (Fig. 1b). Besides the geometrical features, the electronic properties of perfectly and partially folded conformers are also expected to differ with respect to each another, since the partially folded conformers feature an extended geometry, which might be favourable for the delocalization of π-electrons through the backbone (through-bond delocalization). Considering that electronic coupling usually occurs between aromatic compounds and underlying metallic substrates when the π-plane is oriented parallel to the metal surface,²⁷ the partially folded conformers should have better contact with the Au surface and consequently exhibit a better electrical conductance relative to the perfectly folded one. Although the electronic influence exerted by the metallic substrate is likely to be restricted to the first molecular layer, the conformationally altered bottom layer may produce a template effect on the overlying molecular films. These inter-layer π–π interactions are likely to promote the extended geometry of the partially folded OP₈s on the overlying films. In general, apparent heights observed by STM not only reflect the actual size, but also the electrical conductance of the target objects. Given that the perfectly folded conformer is expected to be less conductive, the STM imaging of OP₈Br will underestimate the apparent height of the perfectly folded conformer.

Fig. 2 (a) STM image and (b) STM contrast (apparent height) distribution with a magnification (inset) of OP₈Br on Au(111). Imaging area = 90 × 90 nm; scale bar = 20 nm; tunnelling setpoint (I_t) = 20 pA; sample bias voltage (V_s) = 1.5 V. (c) Magnified STM image and (d) apparent height distribution of a flat monolayer area defined by the square in (a). Imaging area = 29 × 29 nm; scale bar = 10 nm; root mean square (RMS) roughness = 58.96 ± 16.27 pm. (e) Area distribution of the bright spots in (a) (see also Fig. S2†). For (b) and (d), the dotted curve, filled peaks, and blue solid curve indicate raw counts, separated Gaussian distributions, and sums of the Gaussian distributions, respectively.

Fig. 3 Time-evolution STM images for the same area of OP₈Br on Au(111), obtained from the (a) second, (b) fourth, (c) sixth, and (d) eighth scan. Imaging area = 70 × 70 nm; scale bars = 10 nm; I_t = 20 pA; V_s = 1.5 V.

Based on these considerations, we propose a schematic structure for a few-layer film of OP₈Br on Au(111) in Fig. 4a. While partially folded conformers of OP₈Br cover the majority of the Au(111) surface, perfectly folded conformers, responsible for the bright spots in the STM images, are encountered locally. Presumably, the partially folded conformers are arranged on Au(111) in such a way that the longer molecular axis is oriented parallel to the substrate surface so that the interaction between the π-system and Au is maximized. If that is the case, the geometrical difference between the perfect and partially folded conformers should consequently be greater than the apparent height difference (0.06 nm) observed by STM. As previously mentioned, this small difference might be due to a lower conductance of the perfectly folded conformer, which would lead to an underestimation of its actual size.

Fig. 4 Schematic representations of (a) OP₈Br and (b) OP₈NO₂ films on Au(111), characterized by few-layer and monolayer structures, respectively.

OP₈NO₂ on Au(111) essentially behaves like OP₈Br, affording a flat molecular layer with scattered small height deviations. Fig. 5a shows a large-scale STM image (90 × 90 nm) of an OP₈NO₂ film, together with its apparent height profile (Fig. 5b). Here, the height distribution involves two major peaks separated by 0.20 nm, which are not due to molecular-layer spacing, but correspond to two terraces on the Au(111) surface. Hence, unlike OP₈Br, OP₈NO₂ should form a single-layer rather than a few-layer film on Au(111). Consistent with this observation, no degradation of the film surface was observed upon repeated STM scans. Meanwhile, as in the case of the OP₈Br film, a number of bright spots were observed in the STM images of the OP₈NO₂ film. Based on a magnified STM image (Fig. 5c), we analysed the height distribution of a flat region of the film (square region in Fig. 5a), revealing the presence of two distinct regions with a difference in apparent height of ∼0.06 nm (Fig. 5d). This value is identical to that observed for the OP₈Br film, and thus the composition of the OP₈NO₂ film on Au(111) is expected to resemble that of the OP₈Br film containing both partially and perfectly folded conformers. Interestingly, as evident from the number of counts of the bright spots in the histograms of the molecular surface area (Fig. 2e, 5e, S2 and S3†), the surface density of the bright spots in the OP₈NO₂ film, arising from the perfectly folded conformer, is larger than that of the OP₈Br film. This observation is consistent with the fact that polar terminal groups promote the folding of oligomeric o-phenylenes in solution.¹⁰ Fig. 4b illustrates, for clarity, the schematic structure of the single-layer film of OP₈NO₂ on the Au(111) surface. Importantly, the fact that a small deviation (0.06 nm) in the apparent height is commonly observed for the OP₈Br and OP₈NO₂ films might reflect general conformational properties of oligomeric o-phenylenes on metal surfaces.

Fig. 5 (a) STM image and (b) STM contrast (apparent height) distribution of OP₈NO₂ on Au(111). Imaging area = 90 × 90 nm; scale bar = 20 nm; tunnelling setpoint (I_t) = 20 pA; sample bias voltage (V_s) = 1.5 V. The height difference of 0.20 nm originates from a terrace structure of Au(111) (see also Fig. S3†). (c) Magnified STM image and (d) apparent height distribution of the region defined by the square in (a). Imaging area = 29 × 29 nm; scale bar = 10 nm; RMS roughness = 37.72 ± 7.84 pm. (e) Area distribution of the bright spots in (a) (see also Fig. S3†). For (b) and (d), the dotted curve, filled peaks, and blue solid curve indicate raw counts, separated Gaussian distributions, and sums of the Gaussian distributions, respectively.

We were also interested in the conformational behaviour of oxidized oligomeric o-phenylenes on Au(111). In this context, OP₈NO₂ is an interesting target, because its helical inversion in solution can be suppressed upon one-electron oxidation, thus locking the dynamic helix interconversion into one conformation.⁹ The radical cation salt OP₈NO₂˙⁺[SbCl₆⁻] was prepared by one-electron oxidation of OP₈NO₂ with tris(4-bromophenyl)ammoniumyl hexachloroantimonate,⁹ and found to be sufficiently stable under an atmosphere of argon in order to carry out STM imaging. As shown in Fig. 6a, a large-scale STM image of OP₈NO₂˙⁺[SbCl₆⁻] on Au(111) displayed a flat molecular layer, containing a small number of pinhole-like defects, and we could not find any signs for the formation of a multilayer structure. Based on the apparent height profile of each pinhole measured along the lines in Fig. 6a, a film thickness of 0.4–0.6 nm was estimated (Fig. 6b). Due to this large deviation in height, as well as the presence of the counter anion ([SbCl₆]⁻), it is difficult to discuss the conformational state of OP₈NO₂˙⁺ at the molecular level. Nevertheless, the remarkable increase in the apparent height implies that the composition ratio of the perfectly folded conformer in the film increases upon oxidation of OP₈NO₂. It also seems feasible to that the improved charge-transport properties of OP₈NO₂˙⁺ should increase its conductance and thus influence the apparent height in the STM imaging.

Fig. 6 (a) STM image and (b) apparent height profile of OP₈NO₂˙⁺[SbCl₆⁻] on Au(111). Imaging area = 80 × 80 nm; scale bar = 20 nm; I_t = 25 pA; V_s = 1.0 V; RMS roughness = 128.4 ± 20.9 pm. The height profiles were measured along the white lines in (a).

Experimental

The octameric o-phenylenes OP₈Br, OP₈NO₂, and OP₈NO₂˙⁺[SbCl₆⁻] were synthesized according to previously reported procedures.^9,10 Au STM tips were prepared by electrochemical etching of commercially available Au wires (≥99%, ø ∼ 0.25 mm, Nilaco Co.) in aqueous HCl (12 M). Au(111) substrates, obtained by thermal evaporation of Au onto a freshly cleaved mica substrate, were flame-annealed and quenched in ethanol prior to use. Samples for STM imaging were prepared by immersing Au(111) substrates into CH₂Cl₂ solutions (0.5 mM) of OP₈Br or OP₈NO₂ for 1 minute, followed by washing with CH₂Cl₂, and drying in air. OP₈NO₂˙⁺[SbCl₆⁻] films were prepared under an argon atmosphere in a manner similar to those of OP₈Br and OP₈NO₂, except that 1,2-dichlorobenzene was used as the solvent. Constant current-mode STM imaging was carried out on a Nanoscope III STM system (Digital Instruments). All STM measurements were carried out at 25 °C. The STM scanner was calibrated with the Au(111) substrate prior to the experiments. Fig. S1a† shows an STM image (100 × 100 nm) of a flame-annealed Au(111) substrate, which exhibited a typical herringbone pattern on the surface of the entire observation region. The observed STM contrast (apparent height) difference of 0.22 nm (Fig. S1†) is consistent with the well-known interplanar distance of the Au terraces in Au(111). STM images of the OP₈Br and OP₈NO₂ films were recorded in air, while those of the OP₈NO₂˙⁺ film were recorded under argon. The reproducibility of the STM imaging was confirmed by repeated measurements of an OP₈Br film using independently prepared Au(111), as well as independently electro-etched Au tips (Fig. S4†). High-resolution STM images of an OP₈Br film on a single terrace of Au(111) confirmed that the observed bright spots are not due to pollution, but arise from adsorbed individual OP₈Br molecules (Fig. S5†). The bias voltage dependence (−1.0 to +1.5 V) of the STM contrast is relatively small (Fig. S6†), suggesting that the STM imaging should not be affected significantly by the applied measurement conditions. Density functional theory (DFT) calculations were carried out using the Gaussian 09 program package.²⁶ Geometry optimizations of OP₈Br and OP₈NO₂ were performed at the B3LYP/6-31G(d) level of theory, whereby the initial atomic coordinates for the folded and partially folded conformers were obtained from most folded and extended conformers in the literature.¹² The Cartesian coordinates and the energies of the optimized structures of the perfectly and partially folded conformers are listed in the corresponding appendix of the ESI.†

Conclusions

We have reported the first STM analysis of oligomeric o-phenylene films, which represent a fundamental and emerging class of π-conjugated architectures, on Au(111) surfaces. Unlike their p- and m-phenylene counterparts, OP₈s cannot adopt fully-extended conformations, but fold into helical structures. Recent studies on the conformational behaviour of oligomeric o-phenylenes in solution and in the solid state revealed numerous conformational states in response to substituents, the specific environment, and even oxidation states. The present STM analysis of OP₈Br and OP₈NO₂ films on Au(111) demonstrated the coexistence of perfectly and partially folded conformers by virtue of electronic interaction between their molecular π-systems and the Au(111) surface. The apparent film thickness of OP₈NO₂˙⁺, which is the one-electron oxidized form of OP₈NO₂, was much higher than those of the neutral forms (OP₈Br and OP₈NO₂), for which improved charge-transport properties, in combination with the geometrical change of the helical molecule, may be responsible. Considering the fact that the conducting properties of o-phenylene oligomers and polymers have not yet been explored exhaustively, the relationship between conformational states and electrical properties should represent an interesting research target for further STM investigations.

Acknowledgements

This work was supported by a grant-in-aid for Scientific Research on Innovative Areas “π-Figuration” (26102001) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT). SMG gratefully acknowledges an International Research Fellowship from the Japan Society for the Promotion of Science (JSPS).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: STM images of a clean Au substrate and OP₈Br and OP₈NO₂ films on Au(111), analytical data for an STM image of an OP₈Br film on Au(111), as well as Cartesian coordinates and energies of the theoretically optimized structures of the perfectly and partially folded conformers. See DOI: 10.1039/c6ra07173b

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