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Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation

Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled... This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article pubs.acs.org/JACS Cite This: J. Am. Chem. Soc. 2019, 141, 5995−6005 Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation †,# §,# ∥,# † † Fumitaka Ishiwari, Giulia Nascimbeni, Eric Sauter, Hiromu Tago, Yoshiaki Shoji, ⊥ ⊥ ‡ ,∥ ,§ Shintaro Fujii, Manabu Kiguchi, Tomofumi Tada, Michael Zharnikov,* Egbert Zojer,* ,† and Takanori Fukushima* † ‡ Laboratory for Chemistry and Life Science, Institute of Innovative Research, and Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, Graz 8010, Austria Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, Heidelberg 69120, Germany Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan * Supporting Information ABSTRACT: When employing self-assembled monolayers (SAMs) for tuning surface and interface properties, organic molecules that enable strong binding to the substrate, large-area structural uniformity, precise alignment of functional groups, and control of their density are highly desirable. To achieve these goals, tripod systems bearing multiple bonding sites have been developed as an alternative to conventional monodentate systems. Bonding of all three sites has, however, hardly been achieved, with the consequence that structural uniformity and orientational order in tripodal SAMs are usually quite poor. To overcome that problem, we designed 1,8,13-trimercaptomethyl- triptycene (T1) and 1,8,13-trimercaptotriptycene (T2)as potential tripodal SAM precursors and investigated their adsorption behavior on Au(111) combining several advanced experimental techniques and state-of-the-art theoretical simulations. Both SAMs adopt dense, nested hexagonal structures but differ in their adsorption configurations and structural uniformity. While the T2-based SAM exhibits a low degree of order and noticeable deviation from the desired tripodal anchoring, all three anchoring groups of T1 are equally bonded to the surface as thiolates, resulting in an almost upright orientation of the benzene rings and large-area structural uniformity. These superior properties are attributed to the effect of conformationally flexible methylene linkers at the anchoring groups, absent in the case of T2. Both SAMs display interesting electronic properties, and, bearing in mind that the triptycene framework can be functionalized by tail groups in various positions and with high degree of alignment, especially T1 appears as an ideal docking platform for complex and highly functional molecular films. INTRODUCTION context are molecular tripods that usually consist of rigid tetrahedral cores bearing three anchors, such as thiol groups Self-assembled monolayers (SAMs) enable tailoring the 2−22 for binding to Au(111). Examples for such systems include wettability, adhesiveness, and work-functions of solid sub- triarylmethane-based molecular tripods featuring an sp - strates as well as organic/inorganic hybrid interfaces. As 4−8 9−13 hybridized carbon or silicon core (A and B in Figure surfaces and interfaces typically determine the performance of 1a). Also, a methylene thiol-appended adamantane-based devices especially at the nanoscale, the application of SAMs is 14−17 tripod (C in Figure 1a) has been reported to form a of particular technological importance. Besides conventional hierarchical chiral network structure on Au(111). Recently, monodentate SAMs with a single anchor group, several types Mayor et al. investigated the impact of the configuration of of molecular platforms with multiple anchoring sites have anchor groups on the surface adsorption behavior, by recently been developed. This aims at more effectively comparing triarylmethane-based molecular tripods with meta- controlling the orientation, spatial and lateral arrangement, and density of the molecules bonded to solid surfaces. Moreover, multiple anchoring groups help in achieving a Received: January 29, 2019 robust anchoring configuration. Of particular interest in this Published: March 14, 2019 © 2019 American Chemical Society 5995 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article between the sulfur atoms in T2 quite closely fit the lattice structure of Au(111). T1 possesses a certain conformational flexibility due to the possibility of C−C bond-rotation in the thiol-containing functionality (Figure 1b, curved red arrow). Such a rotation allows adjusting the distances between the docking atoms without changing the plane in which the sulfur atoms are arranged. It also does not change the bonding geometry relative to the substrate, which for T1 is different from that in T2 due to the different inclination of the sulfur− carbon bonds. Overall, both T1 and T2 are designed to promote three-point adsorption on a solid surface. A general advantage of triptycenes as anchoring platforms is that they can be substituted with functional units in various ways taking advantage of the four vacant sites per molecule, 25 24 that is, the bridgehead, 4, 5, and 16-positions. To lay the foundations for the further development of triptycene- anchored SAMs, in the present study we focus on the self- assembly behavior of the fundamental triptycene-based tripods Figure 1. (a) Schematic structures of selected examples of reported molecular tripods (A−D). (b) Chemical structures of 1,8,13- T1 and T2 on Au(111). For that, we use a variety of substituted triptycene-based molecular tripods (T1 and T2). Free complementary experimental tools, including scanning tunnel- rotation of the single bonds in A−D, highlighted by the curved red ing microscopy (STM), X-ray photoelectron spectroscopy arrows, might result in anchoring groups pointing away from the (XPS), near-edge X-ray absorption fine structure (NEXAFS) substrate. In contrast, the three sulfur atoms of both T1 and T2 are spectroscopy, and Kelvin probe (KP). The experimental arranged in a plane parallel to the triptycene independent of the findings are rationalized through dispersion-corrected density conformation of the molecule, promoting three-point adsorption on a functional theory (DFT) simulations. We demonstrate that the solid surface. triptycene-based tripods, especially T1,can adoptan adsorption configuration with (nearly) all thiol groups equivalently bonded to the substrate. Moreover, the T1 and para-type substitution patterns (D and D in Figure meta para molecules in the monolayers display an almost upright 1a). They showed that the former can form covalently bonded molecular orientation, an exceptionally high degree of order, monolayers on Au(111), while the latter only grow in and interesting electronic properties. multilayers. This result underlines that careful molecular design of tripods is crucial for developing an optimal molecular RESULTS AND DISCUSSION platform for the controlled assembly on solid surfaces. Such a Synthesis of T1 and T2. The synthesis of T1 is illustrated system should yield an adsorption state with all anchor groups in Scheme 1. The reaction of 1,8,13-trihydroxytriptycene (3) equally bonded to the surface. Ideally, that would also result in a fully vertical molecular orientation. However, most existing Scheme 1. Synthesis of T1 molecular tripods adopt unfavorable conformations, where the anchor groups orient away from the surface due to free bond rotation of the sulfur-containing functionalities (as indicated by curved red arrows in Figure 1a). This leads to significant deviations from the desired tripodal anchoring configuration. Herein, to overcome this problem and to develop an “ideal platform”, we propose novel molecular tripods based on a highly rigid triptycene framework (Figure 1b). We have recently shown that 1,8,13-trisubstituted triptycenes exhibit superb self-assembling abilities to form well-defined, dense Reagents and conditions: (a) Tf O, pyridine, 1,2-dichloroethane, 0− two-dimensional (2D) hexagonal structures through a nested 23−28 60 °C, 97%; (b) MeMgCl, Ni(dppp)Cl , THF, 80 °C, 80%; (c) NBS, packing of the aromatic blades. This suggests that the AIBN, benzene, 50 °C, 59%; (d) AcSK, THF, 25 °C, 79%; (e) AcBr, trisubstituted triptycenes should offer a highly promising MeOH, THF, −78 to 25 °C, 88%. starting point for the development of ideal SAMs featuring well-controlled molecular density and orientation. Notably, due to the tridentate configuration, the present systems differ distinctly from previously reported monodentate triptycene- with triflic anhydride (Tf O) in the presence of pyridine gave based monolayers with a single thiol or selenol group attached tristriflate 4, which was converted into 5 by a Kumada− 29,30 32,33 in the bridgehead position. There, the triptycene moieties Tamao coupling reaction using methylmagnesium chloride are prone to adopting a substantially tilted configuration. and Ni(dppp)Cl (dppp = 1,3-bis(diphenylphosphino)- In the present study, we synthesized two types of molecular propane). Compound 5 was reacted with N-bromosuccinimide tripods (Figure 1b) bearing anchoring thiol groups attached to (NBS) in the presence of azobis(isobutyronitrile) (AIBN), the 1,8,13-positions of the triptycene framework either directly affording 6. Treatment of 6 with potassium thioacetate 5,6 (T2) or via a methylene linker (T1). The former represents a (AcSK) gave 7, whose acetyl groups were hydrolyzed with rigid structure. All anchoring groups are located at fixed HBr, which was generated in situ from acetyl bromide (AcBr) interthiol distances with the sulfur atoms in a plane parallel to and MeOH, to afford 1,8,13-trimercaptomethyltriptycene the triptycene backbone. Notably, the interatomic distances T1. 5996 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article 1,8,13-Trimercaptotriptycene T2 was synthesized from 3 according to Scheme 2. The hydroxyl groups of 3 were Scheme 2. Synthesis of T2 Reagents and conditions: (a) NaH, N,N-dimethylthiocarbamoyl chloride, DMF, 0−70 °C, 83%; (b) Ph O, 260 °C, 84%; (c) KOH, MeOH, THF, 80 °C, 88%. acylated with N,N-dimethylthiocarbamoyl chloride in the presence of NaH to give 8. Upon heating of 8 at 260 °C in diphenyl ether, the Newman−Kwart rearrangement occurred to afford 9. The carbamoyl groups of 9 were hydrolyzed with KOH in a mixture of MeOH and THF, resulting in T2. Compounds T1 and T2 were unambiguously 1 13 characterized by H and C NMR spectroscopy, by FT-IR spectroscopy, and by high-resolution APCI-TOF mass spectrometry (Figures S18−S22 and Figures S30−S33 for T1 and T2, respectively; see the Supporting Information). Successful preparation of single crystals of T1 suitable for X- ray analysis allowed us to further determine the molecular structure of T1 (Figure S1; see the Supporting Information). Preparation of SAMs of T1 and T2 on Au(111). Standard thermally evaporated Au(111) substrates were used. SAMs of T1 (T1/Au) and T2 (T2/Au) were fabricated by simply immersing Au(111) substrates into a degassed THF Figure 2. STM images of (a,c) T1/Au and (b,d) T2/Au acquired at solution of T1 and T2 for 24 h at 25 °C. The samples then 25 °C, and (e) schematic illustration of the proposed molecular were washed with THF, dried under ambient conditions, and arrangement of T1 and T2 on Au(111). annealed at 120 °C. Further details are provided in the Experimental Section.Asareferencesamplefor the spectroscopic analysis, we also prepared benzylthiol (B1) representative S 2p (Figure 3a−c) and C 1s (Figure 3d−f) SAMs on Au(111) using a standard procedure. XP spectra of T1/Au and T2/Au, along with those of B1/Au STM Imaging of T1/Au and T2/Au SAMs. Large-area as a reference. The S 2p spectrum of T1/Au (Figure 3a) is very (50 nm × 50 nm) STM images of T1/Au (Figure 2a) and T2/ similar to that of B1/Au (Figure 3c): It is dominated by a Au (Figure 2b) both show smooth and homogeneous terraces characteristic S 2p doublet of thiolate bound to Au (Figure 3a, with steps of ca. 2.5 Å, which is consistent with the well-known doublet 1) at ∼162.0 eV (S 2p ), with an only small (∼10%) 3/2 interlayer spacing at Au terraces on the surface of Au(111). admixture of an additional feature at 161.0 eV (S 2p ). This 3/2 This observation suggests that T1 and T2 cover the Au(111) suggests that almost all “legs” of the triptycene molecules in surface uniformly. Close-up views (10 nm × 10 nm) of T1/Au T1/Au are bound to the Au substrate as thiolates. This is an (Figure 2c) and T2/Au (Figure 2d), which focus on a terrace, exceptionally good result for tripod-type molecules, which are very similar to one another and display hexagonally aligned usually exhibit multiple bonding geometries with a significant 4,39,40 bright spots at ca. 5 Å separation, indicating that both T1 and portion of unbound and weakly bound anchoring groups. T2 self-assemble on Au(111) to form highly ordered domains. The small feature at 161.0 eV (Figure 3a, doublet 2) is We assume that the bright spots stem from the phenyl rings of frequently observed in high-resolution XP spectra of thiolate- the triptycene units (as their most conductive parts directly based SAMs and is also present in the reference B1/Au SAM linked to the substrate via the anchor groups). Thus, the T1 (Figure 3c). It can be attributed either to an anchoring and T2 molecules on Au(111) likely assemble into a 2D nested configuration differing from a thiolate or, more likely, to hexagonal structure (Figure 2e), which is consistent with the atomic sulfur bound to the substrate, as discussed in detail in packing of 1,8,13-trialkoxytriptycenes observed in X-ray ref 38. Note that a small amount of atomically bound sulfur 23−28 diffraction experiments. Consequently, also the centers should not disturb the molecular packing, as the thiolate of the phenyl groups align hexagonally with a separation of ca. groups are quite loosely packed on the surface (see below). 5 Å. From that, a packing density of the thiolate groups of 4.6 The S 2p spectrum of T2/Au is also dominated by a 14 2 × 10 thiolates/cm can be calculated (Table 1). characteristic S 2p doublet of thiolate bound to Au (Figure 3b, XPS and NEXAFS Analysis of T1/Au and T2/Au SAMs. doublet 1). However, this spectrum contains noticeable By means of XPS and NEXAFS spectroscopy, we further contributions associated with physisorbed/unbound thiols characterized T1/Au and T2/Au SAMs in terms of the sulfur− (Figure 3b, doublet 3; ∼163.4 for S 2p ) and oxidized thiol 3/2 Au bonding state, packing density, orientation, and config- groups (Figure 3b, doublet 4; ∼167.5 for S 2p ). The latter 3/2 uration of the triptycene molecules. Figure 3 shows feature corresponds to sulfonate, which is the most commonly 5997 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article Table 1. Observed and Calculated Effective Thickness, Packing Density of the Thiolate Groups, Average Tilt Angle of the π Plane (α), Average Molecular Tilt Angle (β), Work-Function Changes (ΔΦ), and Position of the Calculated XPS Peaks (Binding Energy) of T1/Au, T2/Au, and B1/Au packing density/10 average tilt angle of average molecular work-function change ΔΦ 2 a a b (thiolate/cm ) π plane α (deg) tilt angle β (deg) (eV) binding energy (eV) system effective thickness (Å) STM XPS calcd NEXAFS calcd NEXAFS calcd Kelvin probe calcd XPS calcd T1/Au 9 4.6 4.6 4.5 81 86.8 7.5 3.4 −0.80 −1.33 284.5 284.47 T2/Au 10.5 4.6 4.1 4.5 67 85.1 36 6.7 −0.75 −1.73 284.1 284.11 B1/Au 7 3.7 4.5 80 77.4 10 14.0 284.1 284.00 The tilt angle refers to the orientation of the phenyl rings with respect to the substrate normal. See text for details. The experimental errors are ±1−1.5 Å for the thickness, ±10% for the packing density, and ±3° for the average tilt angle. In the simulations, work-function changes are reported relative to a calculated work-function of a relaxed Au surface of 5.13 eV. The slightly smaller value of the simulated packing density is a consequence of using the calculated Au lattice constants for reasons discussed in the Experimental Section. of the thiolate groups determined by the XPS analysis of T1/ 14 2 Au (4.6 × 10 thiolate/cm ) agrees perfectly with the estimate from the STM imaging. It corresponds to the ideal value of ca. one S atom per √3 × √3 surface unit cell and is also found for high-quality alkanethiolate SAMs on Au(111). This testifies to the ideal surface coverage in the T1/Au system. For T2/Au, the average coverage derived from the 14 2 XPS data (4.1 × 10 thiolate/cm ) is somewhat smaller. The area-averaging character of the XPS measurements, in combination with the higher local coverage observed for T2/ Au in the STM images, suggest the coexistence of densely packed and more defective (i.e., less densely packed) areas in T2/Au. Notably, all determined packing densities for the triptycene-based SAMs are distinctly higher than that of the 14 2 reference B1 system (3.7 × 10 thiolate/cm ), underlining their superior quality. Consistently, the effective thickness of T1/Au is slightly higher than that of the reference B1/Au SAM (Table 1). The even higher effective thickness of T2/Au, despite the lower density of thiolate groups, is attributed to the presence of some physisorbed molecules. NEXAFS spectroscopy experiments provided further insight Figure 3. (a−c) S 2p and (d−f) C 1s XP spectra of T1/Au (a,d), T2/ into the structural quality of the SAMs and the molecular Au (b,e), and B1/Au (c,f) SAMs. Individual doublets in the S 2p orientation. Representative data in Figure 4 comprise spectra spectra are color-coded and marked by numbers (see text for details); acquired at the so-called magic angle of X-ray incidence (55°). background is shown by gray dashed line. They are independent of the molecular orientation and, thus, exclusively display the electronic structure of the SAMs. 1,38,41−43 Additionally, the differences between the spectra acquired observed oxidized species in thiolate SAMs, that under normal (90°) and grazing (20°) incidence are shown. bonds only weakly to the substrate. For the spectrum They provide information on the molecular orientation. presented in Figure 3b, the portions of the physisorbed/ The 55° spectra of T1/Au (Figure 4a) and T2/Au (Figure unbound thiols and sulfonate sulfur were estimated to be 4c) are similar to one another and also do not significantly ∼15% and ∼20%, respectively. Thus, as compared to T1/Au, deviate from the spectrum of B1/Au (Figure 4e) and from T2/Au exhibits a more heterogeneous bonding structure with reported spectra of oligophenyl SAMs in general. They are some of the “legs” being only weakly bound, not bound, or dominated by the intense π * resonance of phenyl rings oxidized. 1 (Figure 4a, peak 1), which, however, appears at a slightly The C 1s XP spectra of T1/Au (Figure 3d), T2/Au (Figure higher photon energy (∼285.3 eV) than for benzene (∼285.0 3e), and B1/Au (Figure 3f) exhibit only one peak at 284.1, 48 47 eV) or oligophenyl SAMs (285.0−285.1 eV) or even for 284.5, and 284.1 eV, respectively. No contributions related to triptycene SAMs with monodentate bonding configuration contaminations or oxidized species are observed, except for the (∼285 eV). We attribute that shift to a destabilization of the spectrum of T1/Au, in which a very weak signal (asterisk) at lowest unoccupied orbital in the triptycenes due to minor ∼286.5 eV probably due to CO is perceptible. While the distortions of the phenyl rings by the central bridge, but, peak in the spectrum of B1/Au is symmetric, the C 1s peaks obviously, the tridentate bonding configuration is of for T1/Au and T2/Au display some asymmetry, with a higher importance as well. Additional low intensity resonances of intensity at the low binding-energy side for T1/Au and the oligophenyl SAMs, such as the R*/C−S* resonance at ∼287.3 opposite situation for T2/Au. eV and the π * resonance at 288.8−288.9 eV (Figure 4c, peak A quantitative analysis of the XP spectra (for details, see the Experimental Section) provides information on the effective 2), are also resolved in spectra. They are marginally smeared thickness of the SAMs and the packing density of the thiolate out for T1/Au and T2/Au, presumably due to their overlap groups. The results are listed in Table 1. The packing density with the features stemming from the sp carbons at the 5998 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article The average value of β for T1/Au is quite small (∼7.5°), suggesting that the benzene blades of T1 are almost perpendicular to the substrate, which agrees well with the identical adsorption mode of all three anchoring groups (Table 1). The deviation from the fully parallel orientation could be explained by a possible corrugation of the specific anchoring sites of the three thiolate groups. This is, however, not supported by the simulations (see below). Therefore, we rather attribute it to a (small) number of defects, for example, at domain boundaries or step edges, and to the grain structure of the substrate within the macroscopically large area probed by NEXAFS spectroscopy. For T2/Au, the average value of β is noticeably higher (Table 1), reflecting the lower quality of this monolayer as compared to T1/Au. This does not necessarily mean that T2/ Au SAM contains no highly ordered areas of well-aligned molecules (see, e.g., STM experiments). These domains, however, must then coexist with areas of inhomogeneously bound and probably even physisorbed molecules with a strongly inclined or even stochastic orientation. This notion is consistent with the interpretation of the S 2p XP spectra and the derived coverages discussed above. Computational Studies on the Structures of T1/Au Figure 4. C K-edge NEXAFS data for the T1/Au (a,b), T2/Au (c,d), and B1/Au SAMs (e,f). They comprise the spectra acquired at an X- and T2/Au. To gain atomistic insight into the properties of ray incidence angle of 55° (a,c,e) and the difference between the the T1/Au(111) and T2/Au(111) SAMs, we performed spectra acquired at X-ray incidence angles of 90° and 20° (b,d,f). dispersion-corrected density-functional theory (DFT) calcu- Characteristic absorption resonances are marked by numbers (see text lations on periodic, infinitely extended interfaces. To be for details). Horizontal dashed lines in the difference spectra consistent with the experimental situation, we generated correspond to zero. densely packed SAMs by choosing a 3 × 3 Au surface unit cell containing one molecule. This results in a hexagonal arrangement of triptycene molecules (Figure 5a,b) with a 14 2 bridgehead positions. In addition, the spectra exhibited a packing density of 4.45 × 10 thiolate/cm consistent with the variety of σ*-like resonances (Figure 4a, peaks 3 and 4) at experimental values. The length of the resulting surface unit- higher excitation energies. The 90°−20° NEXAFS spectra of T1/Au, T2/Au, and B1/ Au exhibit pronounced linear dichroism (Figure 4b,d,f) with the effect being particularly strong for the π * resonances of the phenyl rings (Figure 4a, peak 1). In view of the specific orientation of the respective orbitals (perpendicular to the ring plane), a positive sign of the π * difference peaks suggests upright molecular orientation of the phenyl rings relative to the substrate. This geometry corresponds to a predominantly downward orientation of the anchoring groups, allowing efficient anchoring of the triptycene tripods to the substrate, in full agreement with the conclusions from the XPS data. A quantitative analysis of the NEXAFS data was performed within the commonly applied theoretical framework, relying on the most prominent π * resonance. To that aim, we correlated the dependence of its intensity on the incidence angle of the X-ray beams (θ) with a theoretical expression for a vector-like orbital, using the average tilt angle of the π * orbitals (α) as the sole fitting parameter. The resulting values of α are 81°,67°, and 80° for the T1/Au, T2/Au, and B1/Au, respectively (Table 1). Because of the 3-fold symmetry of T1 and T2, the average value of the molecular tilt angle (β) can be directly obtained from the dependence of the intensity of the π * resonance on cos θ. The resulting values of β are shown in Table 1, along with the value for the B1/Au SAM. The latter Figure 5. DFT-optimized structures of T1/Au (a and e; top and side can, however, only be considered as a lower limit of the views, respectively) and T2/Au (b and f; top and side views, average tilt angle in that system due to the lower molecular respectively) on a 5-layer Au(111) slab and anchoring positions of the symmetry, which results in a dependence of the calculated thiolate groups of T1 (c) and T2 (d). Only the S atoms and the Au value of β on the molecular twist (here set to 0° yielding the slab are shown. The black rectangles represent the unit cell of the minimum value of β for a given α). interfaces. 5999 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article cell vectors is 8.82 Å, which is somewhat larger than the unit- investigations of the electronic properties of the “parent” cell vector in the bulk assemblies of tripodal triptycenes, such interfaces T1/Au and T2/Au. 23−28 as 1,8,13-tridodecyloxytriptycene (8.1 Å). This difference Kelvin-probe experiments on T1/Au and T2/Au yield work- arises from the fact that the dimensions of the surface unit cell functions (Φ) of 4.40 and 4.45 eV, respectively. With a Φ are determined by the periodicity of the Au substrate, while the value of a bare, freshly sputtered Au(111) substrate of 5.20 periodicity in the bulk reflects the optimum intrinsic distance eV, this results in work-function modifications (ΔΦ)of for a hexagonal assembly of triptycene molecules. Con- −0.80 eV (for T1) and −0.75 eV (for T2). These values are sequently, one can expect some strain in the adsorbate layer, comparable to those obtained for biphenylthiolate monolayers which might be one of the reasons for the structural on Au(111) (Φ = 4.35−4.42 eV). imperfections found particularly for T2 (without flexible As Kelvin probe is an area-averaging technique, the similarity methyl linkers). in the final work-function of T1/Au and T2/Au might seem A screening of possible anchoring sites for the densely surprising considering the much higher degree of disorder in packed monolayers yields S atoms located on the bridge sites the T2/Au films. Disorder ought to result in much less ideally shifted toward fcc hollow positions in the case of T1/Au and S aligned dipoles and, consequently, a distinctly reduced work- atoms at fcc-hollow sites in T2/Au (Figure 5a,b). This is function modification. As this is not observed, we conclude consistent with the computational results for isolated adsorbed that for an ideally arranged T2/Au interface, much larger work- molecules on Au(111). The difference in anchoring sites is function changes than for T1/Au should be observed. clearly visible in Figure 5c,d, where only the S atoms on the To test this hypothesis, we resorted to the simulations, Au(111) surface are shown. The site in T1/Au corresponds to which describe the situation of two perfectly ordered the ideal anchoring position typically found when simulating monolayers: The calculated work-function modification for thiolate-bonded SAMs on Au(111) using a methodology T1/Au (ΔΦ = −1.33 eV) somewhat overestimates the 53,54 similar to the present one. The occurrence of a supposedly experimental value. This is in line with what we typically less ideal anchoring site in T2/Au is attributed to the structural observe for polar SAMs and can partly be attributed to the rigidity of T2. It enforces an unusual arrangement of the S−C residual disorder in the experiments caused by step edges and bonds nearly perpendicular to the Au surface, with the actual grain boundaries. Additionally, the calculated molecular values of the angles between the bonds and the surface normal dipoles and bond dipoles are influenced by the employed varying between 0.7° and 3.4°. The unusual thiolate bonding computational methodology (see the Supporting Information). geometry results in some distortions of the molecular structure In line with the value for T1/Au, we calculate a work-function of T2 upon adsorption, with the distance between neighboring change of −1.38 eV for the biphenylthiolate SAM. In sharp S atoms increasing by 0.2 Å as compared to an isolated contrast to those two cases, for a perfectly ordered T2/Au molecule. For T2/Au also the heights of the three docking interface a much larger value of ΔΦ = −1.73 eV is obtained, as groups vary quite significantly (between 0.61 and 1.03 Å expected on the basis of the arguments in the previous relative to the topmost Au layer), while they are essentially the paragraph. same (1.16 Å) for all S atoms in T1/Au (see Figure 5e and f). What remains to be explained is why the intrinsic work- Consistent with the less ideal bonding configuration of T2/Au, function change for a T2/Au interface is by ca. 0.4 eV larger than that for T1/Au. To clarify that, we performed the the binding energy per molecule (representative of breaking the bond between the substrate and the adsorbate) is following test: We modeled benzylthiolate (B1)and significantly smaller than that for T1/Au (5.43 eV vs 7.16 benzenethiolate (B2) SAMs, which differ only in the presence eV). A similar trend is observed for the adsorption energy of a methyl linker between the phenyl and the thiolate in the characteristic of bond formation (1.62 eV vs 2.67 eV). former system. A full geometry optimization for both systems Simulated structural parameters for the absorbed molecules results in structures with the S atoms in bridge position shifted are summarized in Table 1. The tilt angle of the π * orbitals toward fcc-hollow sites (i.e., consistent with the situation for (α) and the molecular tilt angles (β) for T1/Au are 86.8° and T1/Au). This yields a slightly larger work-function change of 3.4°, respectively, which is in good agreement with the −1.44 eV for B2/Au as compared to −1.33 eV for B1/Au. NEXAFS results (α =81° and β = 7.5°). Conversely, the When the S atom of the B2 molecule is fixed at the fcc-hollow simulated values for well-ordered T2/Au (α = 85.1° and β = position (i.e., the favorable position for T2), the ΔΦ value for 6.7°)differ significantly from the NEXAFS values (α =67° and B2/Au increased to −1.52 eV. When additionally fixing the β =36°). As indicated already earlier, we attribute that to the position of the C atom bonded to the thiolate to the position it coexistence of ordered and disordered domains in T2/Au, with assumes in T2/Au, ΔΦ rises further to −1.65 eV. This shows essentially upright-standing molecules (β =7.5°) in the that the difference in ΔΦ between T2/Au and T1/Au arises ordered regions separated by severely disordered structures from the different hybridization states of the C atom bonded to 2 3 in between (see discussion of S 2p XP spectra). thethiolate(sp vs sp hybridized) and, even more Electronic Properties of the Interface. Functionalization importantly, from differences in the C−S−Au bonding of metal surfaces with SAMs is useful for tailoring the geometries. 55−59 electronic properties of metal substrates. Here, the A more local view of the electrostatics of the SAMs can be 62,63 triptycene-based SAM systems have a particularly high gained from an in-depth analysis of the XPS data. The potential as surface modifier, because of the following: (i) calculated C 1s XP spectra of T1/Au and T2/Au at a photon They form dense and ordered monolayers with, in the case of energy of 350 eV are reported in Figure 6. The energies scale is 61,62 T1, essentially upright-standing molecules. (ii) They can be shifted by 18.88 eV in both systems to align the efficiently chemically modified with various (polar) functional experimental and calculated maxima for T1/Au. Fully groups at the SAM−ambient interface at the 4,5,16- and consistent with the experiments, the positions of the peak bridgehead positions. To establish the basis for future maxima in the calculations differ by 0.4 eV between T1/Au applications, we here discuss experimental and theoretical (Figure 6a, 284.1 eV) and T2/Au (Figure 6b, 284.5 eV). The 6000 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article adopt nested 2D hexagonal structures, which promotes the self-assembly process. The synthesis of T1 and T2 is achieved by sequential organic transformations from 1,8,13-trihydroxytriptycene in good overall yields. STM imaging of T1 and T2 assembled on Au(111) suggests the formation of uniform self-assembled monolayers (SAM) with an ordered 2D hexagonal arrange- ment of the triptycenes. On the basis of our XPS data, we conclude that (nearly) all of the S atoms of T1 bind to Au(111). This results in an upright orientation of the molecules, as confirmed by NEXAFS measurements and quantum-mechanical simulations with a measured (calculated) tilt angle of 7.5° (3.4°). Conversely, the SAM of T2 may contain significant amounts of unbound or weakly bound thiol groups, which causes partial oxidation of the thiol functionality. The large average tilt angle of 36° of T2 on Au(111) determined by NEXAFS spectroscopy in combination with the STM, XPS, and modeling results suggests the coexistence of well-ordered domains with essentially upright standing molecules and highly disordered regions. The lower structural quality of the T2/Au interface can be traced back to a less favorable bonding arrangement in the immediate interface region, which also results in lower binding energies. Interestingly, despite the significantly different degrees of order in the T1 and T2 SAMs, the changes in the area- Figure 6. Simulated C 1s XP spectra of T1/Au (a) and T2/Au (b) for averaged work-function caused by the SAMs are essentially the a primary photon energy of 350 eV. The contributions of the different same for both interfaces (ca. −0.8 eV). On the basis of the groups of chemically equivalent C atoms are also shown, where the simulations and the XPS experiments, this can be rationalized vertical position represents their z coordinates with respect to the image plane position (0.9 Å above the average z position of the by a significantly larger change in the well-ordered regions of topmost Au layer). T2/Au caused mostly be the different bonding geometry, which is eventually diminished by smaller values for the disordered parts of the film. magnitude of that difference is close to the shift in ΔΦ The results presented in this study establish a new type of between the simulated T1/Au and T2/Au interfaces. As shifts tripodal SAM, whose architecture is distinctly different from in the electrostatic energy directly impact core-level binding conventional monolayers of molecular tripods. The advantages energies, this further supports the notion that for perfectly of the triptycene system, particularly T1, are the reliable ordered SAMs the interfacial dipoles are larger in the T2/Au tripodal adsorption configuration, the efficient large-area case. The reason why the electrostatic shift is resolved in the uniform 2D self-assembly, and an almost ideal upright XPS experiments despite the disordered regions is that binding orientation of the benzene rings, projected to the attached energies are impacted by the local electrostatic potential at the functional groups. Importantly, the triptycene tripods can be position of the excited atom such that variations of the readily decorated using the bridgehead or the 4,5,16- electrostatic potential do not average out. Figure 6 also positions. As either one or three functional groups per shows the energetic positions of the C 1s core levels of the tripod can then be substituted, their density and separation can individual C atoms in the SAMs, which allows a direct readily be varied. Thus, the presented systems can serve as comparison between T1/Au and T2/Au on an atom by atom stable and conformationally rigid anchors, for example, for level. Obviously, beyond the global shift between the spectra, polar entities modifying sample work-functions, for recognition the differences in binding energies between T1/Au and T2/Au functionalities in combination with biomolecules, or for are small for electrons from equivalent C atoms, except for CB receptor groups in sensing applications. This makes them and C1 carbons (see insets in Figure 6a,b). This confirms the highly promising building blocks for applications in organic earlier conclusion that differences in electrostatic energies and and molecular circuits, biomedical devices, optical and work-functions in the two SAMs originate from the immediate chemical sensors, solid catalyst, and many more. anchoring region. CONCLUSIONS EXPERIMENTAL SECTION ■ ■ Combining experimental and computational studies, we have Materials. Unless otherwise stated, all commercial reagents were used as received. Benzylthiol (B1) and hexadecanethiol (HDT) were demonstrated that triptycene-based molecular tripods (T1 and purchased from Sigma-Aldrich. Compound 3 was prepared according T2) with thiol-containing functionalities at the 1,8,13-positions to previously reported procedures and unambiguously characterized self-assemble into dense, uniform, and ordered monolayers on by nuclear magnetic resonance (NMR) spectroscopy and atmospheric a metal surface with an upright orientation of the benzene pressure chemical ionization time-of-flight (APCI-TOF) mass planes. The key of the molecular design of T1 and T2 is that spectrometry. For long-term storage of T1 and T2, these compounds the three thiol groups are attached to a rigid triptycene were stored under an argon or nitrogen atmosphere in the freezer to framework in a way that they can efficiently bond to a surface, avoid oxidation of the thiol groups. irrespective of possible conformational states. Moreover, General. NMR spectroscopy measurements were carried out on a 1,8,13-substituted triptycenes have a strong tendency to Bruker AVANCE-500 spectrometer (500 MHz for H and 125 MHz 6001 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article 13 1 for C) or AVANCE-400 spectrometer (400 MHz for H and 100 energy analyzer in normal emission geometry. The photon energy MHz for C). Chemical shifts (δ) are expressed relative to the (PE) was set to either 350 or 580 eV, depending on the BE range. 1 44 resonances of the residual nondeuterated solvents for H [CDCl , The BE scale was referenced to the Au 4f peak at 84.0 eV. The 7/2 1 1 13 H(δ) = 7.26 ppm; acetone-d , H(δ) = 2.05 ppm] and C [CDCl , energy resolution was ∼0.3 eV at a PE of 350 eV and ∼0.5 at 580 eV. 6 3 13 13 The XPS data were used to calculate the effective thickness and C(δ) = 77.16 ppm; acetone-d , C(δ) = 29.8 and 206.3 ppm]. packing density of the SAMs, relying on the C 1s/Au 4f and S 2p/Au Absolute values of the coupling constants are given in Hertz (Hz), 66,67 4f intensity ratios using standard procedures. For the thickness regardless of their sign. Multiplicities are abbreviated as singlet (s), evaluation, a standard expression for the attenuation of the doublet (d), triplet (t), multiplet (m), and broad (br) (see the photoemission signal was assumed together with literature values Supporting Information). Infrared (IR) spectra were recorded at 25 for the attenuation lengths. The spectrometer-specific coefficients °C on a JASCO FT/IR-6600ST Fourier-transform infrared were determined with the help of the reference HDT SAM with a spectrometer. High-resolution mass spectrometry measurements known thickness (18.9 ± 0.1 Å) and packing density (4.63 × 10 were carried out on a Bruker micrOTOF II mass spectrometer 2 37 molecules/cm ; √3 × √3 structure). equipped with an atmospheric pressure chemical ionization (APCI) NEXAFS spectroscopy measurements were performed at the same probe or an electrospray ionization (ESI) probe. beamline. The spectra were collected at the C K-edge in the partial STM Measurements. STM tips were mechanically cut from a electron yield mode with a retarding voltage of −150 V. The tungsten wire (diameter 0.25 mm; Nilaco). Au(111) substrates, polarization factor of the X-ray’s was estimated as ∼88%; the energy obtained by thermal evaporation of Au onto a freshly cleaved mica resolution was ∼0.30 eV. The incidence angle of the light was varied substrate, were flame-annealed and quenched in ethanol prior to use. from 90° (normal incidence geometry; E-vector in surface plane) to Samples for STM imaging were prepared by immersing an Au(111) 20° (grazing incidence geometry; E-vector near surface normal) in substrate into a degassed THF solution (2.0 μ mol/L) of T1 or T2 for steps of 10°−20°, which is a standard approach enabling the 24 h, and the resultant substrate was washed with THF, dried in air, determination of the molecular orientation from NEXAFS data. and then thermally annealed (120 °C, 1 h) under reduced pressure. Raw spectra were normalized to the incident photon flux by division Constant current-mode STM imaging was carried out on a through a spectrum of a clean, freshly sputtered gold sample. The PE Nanoscope III STM system (Digital Instruments). All STM scale was referenced to the pronounced π* resonance of highly measurements were performed at 25 °C in air. The STM scanner oriented pyrolytic graphite at 285.38 eV. was calibrated with an Au(111) substrate prior to the experiments. Kelvin Probe Measurements. Work-function measurements The observed STM contrast (apparent height) difference of 2.5 Å was were carried out using a UHV Kelvin Probe 2001 system (KP consistent with the well-known interlayer separation at Au terraces on −9 technology Ltd., UK). The pressure in the UHV chamber was ∼10 Au(111). mbar. As reference, we used HDT/Au with the work-function value Preparation of SAMs for the Spectroscopy and Kelvin set to 4.30 eV according to the literature. The latter value was Probe Measurements. The SAMs for these experiments were additionally verified by referencing it to the work-function of freshly prepared on commercial Au substrates (Georg Albert PVD, Silz, sputtered gold set to 5.20 eV. The accuracy of the WF values is ca. Germany). These substrates were prepared by thermal evaporation of ±0.05 eV. 30 nm of Au (99.99% purity) onto a polished single-crystal silicon Computational Methodology. The calculations were performed (100) wafer (Silicon Sense) that had been precoated with a 5 nm 73 74 using the FHI-aims code and employing the PBE functional in titanium adhesion layer. The resulting Au films are polycrystalline, combination with the surface parametrization of the Tkatchenko− having a grain size of 20−50 nm and predominantly exhibiting a Scheffler dispersion correction. The latter were turned off between (111) orientation. The SAMs were prepared by immersion of a fresh the bulk Au atoms. Periodic boundary conditions and the repeated substrate in a degassed THF solution (2 μM−1 mM) of T1 or T2 for slab approach including a vacuum region of at least 20 Å in the z 24 h at 25 °C. After immersion, the films were washed with THF and direction were employed to represent the interface. To compensate dried by blowing argon. Finally, some of the samples were annealed at for the electrostatic asymmetry of the slab, a self-consistently 100 °C for 1 h either under inert gas atmosphere or under ultrahigh calculated dipole layer was inserted in the vacuum. To sample the vacuum (UHV) conditions. In addition, several reference SAMs, that reciprocal space, a nonorthogonal 6 × 6 × 1 Γ-centered k-point grid is, those of B1 and HDT on Au(111), were prepared using standard was used. The dimensions of the unit cells in the x and y directions procedures. HDT/Au was used as a reference system for the XPS were defined according to the calculated Au nearest neighbor distance and work-function measurements (see below). B1 can be regarded as (2.940 Å), to avoid spurious surface relaxations. The metal was a partial structure of T1, making it a suitable monothiol reference. modeled using 5 layers of Au, with the bottom 3 layers fixed at their Because of the presence of the methylene linker between the benzene bulk positions during the optimization. The presented results were ring and thiol group, a sufficiently good quality of this monolayer can obtained using the default FHI-aims “tight” basis set and setting the be expected, similar to the analogous nitrile-substituted system. At −6 total energy criterion for the self-consistency cycle to 10 eV. The the same time, we refrained from studying benzenethiol as the optimizations were performed until the maximum residual force monothiol reference to T2, as it has been shown to form SAMs of component per atom was below 0.01 eV/Å. For the initial screening only limited quality when employing the standard immersion 47,65 of different docking sites, less accurate settings were adopted, using procedure. the default FHI-aims “light” basis set and stopping the optimization XPS and NEXAFS Spectroscopy Measurements. The XPS, when the maximum residual force component per atom was below NEXAFS spectroscopy, and work-function measurements were −9 0.05 eV/Å. performed under UHV conditions (1.5 × 10 mbar) at 25 °C. Binding energies, E , are defined such that they reflect the energy bind Laboratory XPS measurements were carried out with a MAX200 needed to break the bond between the molecule and the substrate and (Leybold-Heraeus) spectrometer equipped with an Mg Kα X-ray to remove the molecules from the SAM: source (200 W) and a hemispherical analyzer. The spectra were corrected for the spectrometer transmission, and the binding energy EE=−E −E bind Trip/Au Au Trip (1) (BE) scale was referenced to the Au 4f peak at 84.0 eV. Because 7/2 the quality of the laboratory spectra in terms of statistics and energy Here, E is the energy per unit cell of the SAM adsorbed to the Trip/Au resolution was inferior to the synchrotron data, they were mostly used surface, E is the energy of the optimized pristine Au slab, and E is Au Trip to verify the film thickness and packing density. the energy of the optimized gas-phase molecular radical. Conversely, Synchrotron-based XPS measurements were carried out at the adsorption energies, E ,reflect the energetics of forming the ads bending magnet HE-SGM beamline of the synchrotron storage ring monolayers and at the same time replacing the molecular S−H BESSY II in Berlin, Germany. This beamline provides a moderate X- bonds by bonds to the Au surface. They are, thus, defined as ray intensity helping to avoid X-ray damage during the spectra EE=−E −E + 3/2E ads Trip/Au Au Trip ‐H H2 (2) acquisition. The spectra were collected with a Scienta R3000 electron 6002 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article E in this equation represents the energy of the optimized gas- Research Foundation (Deutsche Forschungsgemeinschaft; Trip‑H phase triptycene molecule in which all S atoms are saturated with DFG, grant ZH 63/22-1 for E.S. and M.Z.), and the Austrian hydrogens, and E is the energy of an isolated H molecule. H2 2 Science Fund (FWF, I2081-N20 for G. N. and E.Z.). T.F. The XP spectra were simulated within the initial state approach to acknowledges support from the Dynamic Alliance for Open avoid artifacts arising from a combination of periodic boundary 62 Innovation Bridging Human, Environment and Materials from conditions and explicit excitations in each unit cell. For obtaining MEXT, Japan. We thank Dr. Valiparambil Sanjayan Sajisha and the spectra, the 1s core level energies for every C atom were taken Ms. K. Takenouchi for their assistance in the synthesis of T1 from the atom projected density of states output files. Subsequently, and T2. We thank Suzukakedai Materials Analysis Division, they were shifted considering the screening of the core hole by the 78,79 metal substrate via an electrostatic image charge model assuming Technical Department, Tokyo Institute of Technology, for a dielectric constant of the SAM of 3.9. To model the spectra, the their support with the NMR measurement and single-crystal X- individual resonances were broadened using Gaussian functions with a ray analysis. E.S. and M.Z. thank the Helmholtz Zentrum variance of 0.15 eV and an intensity scaled using an exponential Berlin for the allocation of synchrotron radiation beamtime at attenuation function to account for the finite escape depth of the BESSY II and A. Nefedov and Ch. Wöll for the technical photoelectrons. Additionally, the energy scales for both interfaces cooperation during the experiments there. 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Self-Assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols: Specific Characteristics and Odd−Even Effects. Phys. Chem. Chem. Phys. 2010, 12, 12123−12127. 6005 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the American Chemical Society Pubmed Central

Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation

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This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article pubs.acs.org/JACS Cite This: J. Am. Chem. Soc. 2019, 141, 5995−6005 Triptycene Tripods for the Formation of Highly Uniform and Densely Packed Self-Assembled Monolayers with Controlled Molecular Orientation †,# §,# ∥,# † † Fumitaka Ishiwari, Giulia Nascimbeni, Eric Sauter, Hiromu Tago, Yoshiaki Shoji, ⊥ ⊥ ‡ ,∥ ,§ Shintaro Fujii, Manabu Kiguchi, Tomofumi Tada, Michael Zharnikov,* Egbert Zojer,* ,† and Takanori Fukushima* † ‡ Laboratory for Chemistry and Life Science, Institute of Innovative Research, and Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, Graz 8010, Austria Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, Heidelberg 69120, Germany Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan * Supporting Information ABSTRACT: When employing self-assembled monolayers (SAMs) for tuning surface and interface properties, organic molecules that enable strong binding to the substrate, large-area structural uniformity, precise alignment of functional groups, and control of their density are highly desirable. To achieve these goals, tripod systems bearing multiple bonding sites have been developed as an alternative to conventional monodentate systems. Bonding of all three sites has, however, hardly been achieved, with the consequence that structural uniformity and orientational order in tripodal SAMs are usually quite poor. To overcome that problem, we designed 1,8,13-trimercaptomethyl- triptycene (T1) and 1,8,13-trimercaptotriptycene (T2)as potential tripodal SAM precursors and investigated their adsorption behavior on Au(111) combining several advanced experimental techniques and state-of-the-art theoretical simulations. Both SAMs adopt dense, nested hexagonal structures but differ in their adsorption configurations and structural uniformity. While the T2-based SAM exhibits a low degree of order and noticeable deviation from the desired tripodal anchoring, all three anchoring groups of T1 are equally bonded to the surface as thiolates, resulting in an almost upright orientation of the benzene rings and large-area structural uniformity. These superior properties are attributed to the effect of conformationally flexible methylene linkers at the anchoring groups, absent in the case of T2. Both SAMs display interesting electronic properties, and, bearing in mind that the triptycene framework can be functionalized by tail groups in various positions and with high degree of alignment, especially T1 appears as an ideal docking platform for complex and highly functional molecular films. INTRODUCTION context are molecular tripods that usually consist of rigid tetrahedral cores bearing three anchors, such as thiol groups Self-assembled monolayers (SAMs) enable tailoring the 2−22 for binding to Au(111). Examples for such systems include wettability, adhesiveness, and work-functions of solid sub- triarylmethane-based molecular tripods featuring an sp - strates as well as organic/inorganic hybrid interfaces. As 4−8 9−13 hybridized carbon or silicon core (A and B in Figure surfaces and interfaces typically determine the performance of 1a). Also, a methylene thiol-appended adamantane-based devices especially at the nanoscale, the application of SAMs is 14−17 tripod (C in Figure 1a) has been reported to form a of particular technological importance. Besides conventional hierarchical chiral network structure on Au(111). Recently, monodentate SAMs with a single anchor group, several types Mayor et al. investigated the impact of the configuration of of molecular platforms with multiple anchoring sites have anchor groups on the surface adsorption behavior, by recently been developed. This aims at more effectively comparing triarylmethane-based molecular tripods with meta- controlling the orientation, spatial and lateral arrangement, and density of the molecules bonded to solid surfaces. Moreover, multiple anchoring groups help in achieving a Received: January 29, 2019 robust anchoring configuration. Of particular interest in this Published: March 14, 2019 © 2019 American Chemical Society 5995 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article between the sulfur atoms in T2 quite closely fit the lattice structure of Au(111). T1 possesses a certain conformational flexibility due to the possibility of C−C bond-rotation in the thiol-containing functionality (Figure 1b, curved red arrow). Such a rotation allows adjusting the distances between the docking atoms without changing the plane in which the sulfur atoms are arranged. It also does not change the bonding geometry relative to the substrate, which for T1 is different from that in T2 due to the different inclination of the sulfur− carbon bonds. Overall, both T1 and T2 are designed to promote three-point adsorption on a solid surface. A general advantage of triptycenes as anchoring platforms is that they can be substituted with functional units in various ways taking advantage of the four vacant sites per molecule, 25 24 that is, the bridgehead, 4, 5, and 16-positions. To lay the foundations for the further development of triptycene- anchored SAMs, in the present study we focus on the self- assembly behavior of the fundamental triptycene-based tripods Figure 1. (a) Schematic structures of selected examples of reported molecular tripods (A−D). (b) Chemical structures of 1,8,13- T1 and T2 on Au(111). For that, we use a variety of substituted triptycene-based molecular tripods (T1 and T2). Free complementary experimental tools, including scanning tunnel- rotation of the single bonds in A−D, highlighted by the curved red ing microscopy (STM), X-ray photoelectron spectroscopy arrows, might result in anchoring groups pointing away from the (XPS), near-edge X-ray absorption fine structure (NEXAFS) substrate. In contrast, the three sulfur atoms of both T1 and T2 are spectroscopy, and Kelvin probe (KP). The experimental arranged in a plane parallel to the triptycene independent of the findings are rationalized through dispersion-corrected density conformation of the molecule, promoting three-point adsorption on a functional theory (DFT) simulations. We demonstrate that the solid surface. triptycene-based tripods, especially T1,can adoptan adsorption configuration with (nearly) all thiol groups equivalently bonded to the substrate. Moreover, the T1 and para-type substitution patterns (D and D in Figure meta para molecules in the monolayers display an almost upright 1a). They showed that the former can form covalently bonded molecular orientation, an exceptionally high degree of order, monolayers on Au(111), while the latter only grow in and interesting electronic properties. multilayers. This result underlines that careful molecular design of tripods is crucial for developing an optimal molecular RESULTS AND DISCUSSION platform for the controlled assembly on solid surfaces. Such a Synthesis of T1 and T2. The synthesis of T1 is illustrated system should yield an adsorption state with all anchor groups in Scheme 1. The reaction of 1,8,13-trihydroxytriptycene (3) equally bonded to the surface. Ideally, that would also result in a fully vertical molecular orientation. However, most existing Scheme 1. Synthesis of T1 molecular tripods adopt unfavorable conformations, where the anchor groups orient away from the surface due to free bond rotation of the sulfur-containing functionalities (as indicated by curved red arrows in Figure 1a). This leads to significant deviations from the desired tripodal anchoring configuration. Herein, to overcome this problem and to develop an “ideal platform”, we propose novel molecular tripods based on a highly rigid triptycene framework (Figure 1b). We have recently shown that 1,8,13-trisubstituted triptycenes exhibit superb self-assembling abilities to form well-defined, dense Reagents and conditions: (a) Tf O, pyridine, 1,2-dichloroethane, 0− two-dimensional (2D) hexagonal structures through a nested 23−28 60 °C, 97%; (b) MeMgCl, Ni(dppp)Cl , THF, 80 °C, 80%; (c) NBS, packing of the aromatic blades. This suggests that the AIBN, benzene, 50 °C, 59%; (d) AcSK, THF, 25 °C, 79%; (e) AcBr, trisubstituted triptycenes should offer a highly promising MeOH, THF, −78 to 25 °C, 88%. starting point for the development of ideal SAMs featuring well-controlled molecular density and orientation. Notably, due to the tridentate configuration, the present systems differ distinctly from previously reported monodentate triptycene- with triflic anhydride (Tf O) in the presence of pyridine gave based monolayers with a single thiol or selenol group attached tristriflate 4, which was converted into 5 by a Kumada− 29,30 32,33 in the bridgehead position. There, the triptycene moieties Tamao coupling reaction using methylmagnesium chloride are prone to adopting a substantially tilted configuration. and Ni(dppp)Cl (dppp = 1,3-bis(diphenylphosphino)- In the present study, we synthesized two types of molecular propane). Compound 5 was reacted with N-bromosuccinimide tripods (Figure 1b) bearing anchoring thiol groups attached to (NBS) in the presence of azobis(isobutyronitrile) (AIBN), the 1,8,13-positions of the triptycene framework either directly affording 6. Treatment of 6 with potassium thioacetate 5,6 (T2) or via a methylene linker (T1). The former represents a (AcSK) gave 7, whose acetyl groups were hydrolyzed with rigid structure. All anchoring groups are located at fixed HBr, which was generated in situ from acetyl bromide (AcBr) interthiol distances with the sulfur atoms in a plane parallel to and MeOH, to afford 1,8,13-trimercaptomethyltriptycene the triptycene backbone. Notably, the interatomic distances T1. 5996 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article 1,8,13-Trimercaptotriptycene T2 was synthesized from 3 according to Scheme 2. The hydroxyl groups of 3 were Scheme 2. Synthesis of T2 Reagents and conditions: (a) NaH, N,N-dimethylthiocarbamoyl chloride, DMF, 0−70 °C, 83%; (b) Ph O, 260 °C, 84%; (c) KOH, MeOH, THF, 80 °C, 88%. acylated with N,N-dimethylthiocarbamoyl chloride in the presence of NaH to give 8. Upon heating of 8 at 260 °C in diphenyl ether, the Newman−Kwart rearrangement occurred to afford 9. The carbamoyl groups of 9 were hydrolyzed with KOH in a mixture of MeOH and THF, resulting in T2. Compounds T1 and T2 were unambiguously 1 13 characterized by H and C NMR spectroscopy, by FT-IR spectroscopy, and by high-resolution APCI-TOF mass spectrometry (Figures S18−S22 and Figures S30−S33 for T1 and T2, respectively; see the Supporting Information). Successful preparation of single crystals of T1 suitable for X- ray analysis allowed us to further determine the molecular structure of T1 (Figure S1; see the Supporting Information). Preparation of SAMs of T1 and T2 on Au(111). Standard thermally evaporated Au(111) substrates were used. SAMs of T1 (T1/Au) and T2 (T2/Au) were fabricated by simply immersing Au(111) substrates into a degassed THF Figure 2. STM images of (a,c) T1/Au and (b,d) T2/Au acquired at solution of T1 and T2 for 24 h at 25 °C. The samples then 25 °C, and (e) schematic illustration of the proposed molecular were washed with THF, dried under ambient conditions, and arrangement of T1 and T2 on Au(111). annealed at 120 °C. Further details are provided in the Experimental Section.Asareferencesamplefor the spectroscopic analysis, we also prepared benzylthiol (B1) representative S 2p (Figure 3a−c) and C 1s (Figure 3d−f) SAMs on Au(111) using a standard procedure. XP spectra of T1/Au and T2/Au, along with those of B1/Au STM Imaging of T1/Au and T2/Au SAMs. Large-area as a reference. The S 2p spectrum of T1/Au (Figure 3a) is very (50 nm × 50 nm) STM images of T1/Au (Figure 2a) and T2/ similar to that of B1/Au (Figure 3c): It is dominated by a Au (Figure 2b) both show smooth and homogeneous terraces characteristic S 2p doublet of thiolate bound to Au (Figure 3a, with steps of ca. 2.5 Å, which is consistent with the well-known doublet 1) at ∼162.0 eV (S 2p ), with an only small (∼10%) 3/2 interlayer spacing at Au terraces on the surface of Au(111). admixture of an additional feature at 161.0 eV (S 2p ). This 3/2 This observation suggests that T1 and T2 cover the Au(111) suggests that almost all “legs” of the triptycene molecules in surface uniformly. Close-up views (10 nm × 10 nm) of T1/Au T1/Au are bound to the Au substrate as thiolates. This is an (Figure 2c) and T2/Au (Figure 2d), which focus on a terrace, exceptionally good result for tripod-type molecules, which are very similar to one another and display hexagonally aligned usually exhibit multiple bonding geometries with a significant 4,39,40 bright spots at ca. 5 Å separation, indicating that both T1 and portion of unbound and weakly bound anchoring groups. T2 self-assemble on Au(111) to form highly ordered domains. The small feature at 161.0 eV (Figure 3a, doublet 2) is We assume that the bright spots stem from the phenyl rings of frequently observed in high-resolution XP spectra of thiolate- the triptycene units (as their most conductive parts directly based SAMs and is also present in the reference B1/Au SAM linked to the substrate via the anchor groups). Thus, the T1 (Figure 3c). It can be attributed either to an anchoring and T2 molecules on Au(111) likely assemble into a 2D nested configuration differing from a thiolate or, more likely, to hexagonal structure (Figure 2e), which is consistent with the atomic sulfur bound to the substrate, as discussed in detail in packing of 1,8,13-trialkoxytriptycenes observed in X-ray ref 38. Note that a small amount of atomically bound sulfur 23−28 diffraction experiments. Consequently, also the centers should not disturb the molecular packing, as the thiolate of the phenyl groups align hexagonally with a separation of ca. groups are quite loosely packed on the surface (see below). 5 Å. From that, a packing density of the thiolate groups of 4.6 The S 2p spectrum of T2/Au is also dominated by a 14 2 × 10 thiolates/cm can be calculated (Table 1). characteristic S 2p doublet of thiolate bound to Au (Figure 3b, XPS and NEXAFS Analysis of T1/Au and T2/Au SAMs. doublet 1). However, this spectrum contains noticeable By means of XPS and NEXAFS spectroscopy, we further contributions associated with physisorbed/unbound thiols characterized T1/Au and T2/Au SAMs in terms of the sulfur− (Figure 3b, doublet 3; ∼163.4 for S 2p ) and oxidized thiol 3/2 Au bonding state, packing density, orientation, and config- groups (Figure 3b, doublet 4; ∼167.5 for S 2p ). The latter 3/2 uration of the triptycene molecules. Figure 3 shows feature corresponds to sulfonate, which is the most commonly 5997 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article Table 1. Observed and Calculated Effective Thickness, Packing Density of the Thiolate Groups, Average Tilt Angle of the π Plane (α), Average Molecular Tilt Angle (β), Work-Function Changes (ΔΦ), and Position of the Calculated XPS Peaks (Binding Energy) of T1/Au, T2/Au, and B1/Au packing density/10 average tilt angle of average molecular work-function change ΔΦ 2 a a b (thiolate/cm ) π plane α (deg) tilt angle β (deg) (eV) binding energy (eV) system effective thickness (Å) STM XPS calcd NEXAFS calcd NEXAFS calcd Kelvin probe calcd XPS calcd T1/Au 9 4.6 4.6 4.5 81 86.8 7.5 3.4 −0.80 −1.33 284.5 284.47 T2/Au 10.5 4.6 4.1 4.5 67 85.1 36 6.7 −0.75 −1.73 284.1 284.11 B1/Au 7 3.7 4.5 80 77.4 10 14.0 284.1 284.00 The tilt angle refers to the orientation of the phenyl rings with respect to the substrate normal. See text for details. The experimental errors are ±1−1.5 Å for the thickness, ±10% for the packing density, and ±3° for the average tilt angle. In the simulations, work-function changes are reported relative to a calculated work-function of a relaxed Au surface of 5.13 eV. The slightly smaller value of the simulated packing density is a consequence of using the calculated Au lattice constants for reasons discussed in the Experimental Section. of the thiolate groups determined by the XPS analysis of T1/ 14 2 Au (4.6 × 10 thiolate/cm ) agrees perfectly with the estimate from the STM imaging. It corresponds to the ideal value of ca. one S atom per √3 × √3 surface unit cell and is also found for high-quality alkanethiolate SAMs on Au(111). This testifies to the ideal surface coverage in the T1/Au system. For T2/Au, the average coverage derived from the 14 2 XPS data (4.1 × 10 thiolate/cm ) is somewhat smaller. The area-averaging character of the XPS measurements, in combination with the higher local coverage observed for T2/ Au in the STM images, suggest the coexistence of densely packed and more defective (i.e., less densely packed) areas in T2/Au. Notably, all determined packing densities for the triptycene-based SAMs are distinctly higher than that of the 14 2 reference B1 system (3.7 × 10 thiolate/cm ), underlining their superior quality. Consistently, the effective thickness of T1/Au is slightly higher than that of the reference B1/Au SAM (Table 1). The even higher effective thickness of T2/Au, despite the lower density of thiolate groups, is attributed to the presence of some physisorbed molecules. NEXAFS spectroscopy experiments provided further insight Figure 3. (a−c) S 2p and (d−f) C 1s XP spectra of T1/Au (a,d), T2/ into the structural quality of the SAMs and the molecular Au (b,e), and B1/Au (c,f) SAMs. Individual doublets in the S 2p orientation. Representative data in Figure 4 comprise spectra spectra are color-coded and marked by numbers (see text for details); acquired at the so-called magic angle of X-ray incidence (55°). background is shown by gray dashed line. They are independent of the molecular orientation and, thus, exclusively display the electronic structure of the SAMs. 1,38,41−43 Additionally, the differences between the spectra acquired observed oxidized species in thiolate SAMs, that under normal (90°) and grazing (20°) incidence are shown. bonds only weakly to the substrate. For the spectrum They provide information on the molecular orientation. presented in Figure 3b, the portions of the physisorbed/ The 55° spectra of T1/Au (Figure 4a) and T2/Au (Figure unbound thiols and sulfonate sulfur were estimated to be 4c) are similar to one another and also do not significantly ∼15% and ∼20%, respectively. Thus, as compared to T1/Au, deviate from the spectrum of B1/Au (Figure 4e) and from T2/Au exhibits a more heterogeneous bonding structure with reported spectra of oligophenyl SAMs in general. They are some of the “legs” being only weakly bound, not bound, or dominated by the intense π * resonance of phenyl rings oxidized. 1 (Figure 4a, peak 1), which, however, appears at a slightly The C 1s XP spectra of T1/Au (Figure 3d), T2/Au (Figure higher photon energy (∼285.3 eV) than for benzene (∼285.0 3e), and B1/Au (Figure 3f) exhibit only one peak at 284.1, 48 47 eV) or oligophenyl SAMs (285.0−285.1 eV) or even for 284.5, and 284.1 eV, respectively. No contributions related to triptycene SAMs with monodentate bonding configuration contaminations or oxidized species are observed, except for the (∼285 eV). We attribute that shift to a destabilization of the spectrum of T1/Au, in which a very weak signal (asterisk) at lowest unoccupied orbital in the triptycenes due to minor ∼286.5 eV probably due to CO is perceptible. While the distortions of the phenyl rings by the central bridge, but, peak in the spectrum of B1/Au is symmetric, the C 1s peaks obviously, the tridentate bonding configuration is of for T1/Au and T2/Au display some asymmetry, with a higher importance as well. Additional low intensity resonances of intensity at the low binding-energy side for T1/Au and the oligophenyl SAMs, such as the R*/C−S* resonance at ∼287.3 opposite situation for T2/Au. eV and the π * resonance at 288.8−288.9 eV (Figure 4c, peak A quantitative analysis of the XP spectra (for details, see the Experimental Section) provides information on the effective 2), are also resolved in spectra. They are marginally smeared thickness of the SAMs and the packing density of the thiolate out for T1/Au and T2/Au, presumably due to their overlap groups. The results are listed in Table 1. The packing density with the features stemming from the sp carbons at the 5998 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article The average value of β for T1/Au is quite small (∼7.5°), suggesting that the benzene blades of T1 are almost perpendicular to the substrate, which agrees well with the identical adsorption mode of all three anchoring groups (Table 1). The deviation from the fully parallel orientation could be explained by a possible corrugation of the specific anchoring sites of the three thiolate groups. This is, however, not supported by the simulations (see below). Therefore, we rather attribute it to a (small) number of defects, for example, at domain boundaries or step edges, and to the grain structure of the substrate within the macroscopically large area probed by NEXAFS spectroscopy. For T2/Au, the average value of β is noticeably higher (Table 1), reflecting the lower quality of this monolayer as compared to T1/Au. This does not necessarily mean that T2/ Au SAM contains no highly ordered areas of well-aligned molecules (see, e.g., STM experiments). These domains, however, must then coexist with areas of inhomogeneously bound and probably even physisorbed molecules with a strongly inclined or even stochastic orientation. This notion is consistent with the interpretation of the S 2p XP spectra and the derived coverages discussed above. Computational Studies on the Structures of T1/Au Figure 4. C K-edge NEXAFS data for the T1/Au (a,b), T2/Au (c,d), and B1/Au SAMs (e,f). They comprise the spectra acquired at an X- and T2/Au. To gain atomistic insight into the properties of ray incidence angle of 55° (a,c,e) and the difference between the the T1/Au(111) and T2/Au(111) SAMs, we performed spectra acquired at X-ray incidence angles of 90° and 20° (b,d,f). dispersion-corrected density-functional theory (DFT) calcu- Characteristic absorption resonances are marked by numbers (see text lations on periodic, infinitely extended interfaces. To be for details). Horizontal dashed lines in the difference spectra consistent with the experimental situation, we generated correspond to zero. densely packed SAMs by choosing a 3 × 3 Au surface unit cell containing one molecule. This results in a hexagonal arrangement of triptycene molecules (Figure 5a,b) with a 14 2 bridgehead positions. In addition, the spectra exhibited a packing density of 4.45 × 10 thiolate/cm consistent with the variety of σ*-like resonances (Figure 4a, peaks 3 and 4) at experimental values. The length of the resulting surface unit- higher excitation energies. The 90°−20° NEXAFS spectra of T1/Au, T2/Au, and B1/ Au exhibit pronounced linear dichroism (Figure 4b,d,f) with the effect being particularly strong for the π * resonances of the phenyl rings (Figure 4a, peak 1). In view of the specific orientation of the respective orbitals (perpendicular to the ring plane), a positive sign of the π * difference peaks suggests upright molecular orientation of the phenyl rings relative to the substrate. This geometry corresponds to a predominantly downward orientation of the anchoring groups, allowing efficient anchoring of the triptycene tripods to the substrate, in full agreement with the conclusions from the XPS data. A quantitative analysis of the NEXAFS data was performed within the commonly applied theoretical framework, relying on the most prominent π * resonance. To that aim, we correlated the dependence of its intensity on the incidence angle of the X-ray beams (θ) with a theoretical expression for a vector-like orbital, using the average tilt angle of the π * orbitals (α) as the sole fitting parameter. The resulting values of α are 81°,67°, and 80° for the T1/Au, T2/Au, and B1/Au, respectively (Table 1). Because of the 3-fold symmetry of T1 and T2, the average value of the molecular tilt angle (β) can be directly obtained from the dependence of the intensity of the π * resonance on cos θ. The resulting values of β are shown in Table 1, along with the value for the B1/Au SAM. The latter Figure 5. DFT-optimized structures of T1/Au (a and e; top and side can, however, only be considered as a lower limit of the views, respectively) and T2/Au (b and f; top and side views, average tilt angle in that system due to the lower molecular respectively) on a 5-layer Au(111) slab and anchoring positions of the symmetry, which results in a dependence of the calculated thiolate groups of T1 (c) and T2 (d). Only the S atoms and the Au value of β on the molecular twist (here set to 0° yielding the slab are shown. The black rectangles represent the unit cell of the minimum value of β for a given α). interfaces. 5999 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article cell vectors is 8.82 Å, which is somewhat larger than the unit- investigations of the electronic properties of the “parent” cell vector in the bulk assemblies of tripodal triptycenes, such interfaces T1/Au and T2/Au. 23−28 as 1,8,13-tridodecyloxytriptycene (8.1 Å). This difference Kelvin-probe experiments on T1/Au and T2/Au yield work- arises from the fact that the dimensions of the surface unit cell functions (Φ) of 4.40 and 4.45 eV, respectively. With a Φ are determined by the periodicity of the Au substrate, while the value of a bare, freshly sputtered Au(111) substrate of 5.20 periodicity in the bulk reflects the optimum intrinsic distance eV, this results in work-function modifications (ΔΦ)of for a hexagonal assembly of triptycene molecules. Con- −0.80 eV (for T1) and −0.75 eV (for T2). These values are sequently, one can expect some strain in the adsorbate layer, comparable to those obtained for biphenylthiolate monolayers which might be one of the reasons for the structural on Au(111) (Φ = 4.35−4.42 eV). imperfections found particularly for T2 (without flexible As Kelvin probe is an area-averaging technique, the similarity methyl linkers). in the final work-function of T1/Au and T2/Au might seem A screening of possible anchoring sites for the densely surprising considering the much higher degree of disorder in packed monolayers yields S atoms located on the bridge sites the T2/Au films. Disorder ought to result in much less ideally shifted toward fcc hollow positions in the case of T1/Au and S aligned dipoles and, consequently, a distinctly reduced work- atoms at fcc-hollow sites in T2/Au (Figure 5a,b). This is function modification. As this is not observed, we conclude consistent with the computational results for isolated adsorbed that for an ideally arranged T2/Au interface, much larger work- molecules on Au(111). The difference in anchoring sites is function changes than for T1/Au should be observed. clearly visible in Figure 5c,d, where only the S atoms on the To test this hypothesis, we resorted to the simulations, Au(111) surface are shown. The site in T1/Au corresponds to which describe the situation of two perfectly ordered the ideal anchoring position typically found when simulating monolayers: The calculated work-function modification for thiolate-bonded SAMs on Au(111) using a methodology T1/Au (ΔΦ = −1.33 eV) somewhat overestimates the 53,54 similar to the present one. The occurrence of a supposedly experimental value. This is in line with what we typically less ideal anchoring site in T2/Au is attributed to the structural observe for polar SAMs and can partly be attributed to the rigidity of T2. It enforces an unusual arrangement of the S−C residual disorder in the experiments caused by step edges and bonds nearly perpendicular to the Au surface, with the actual grain boundaries. Additionally, the calculated molecular values of the angles between the bonds and the surface normal dipoles and bond dipoles are influenced by the employed varying between 0.7° and 3.4°. The unusual thiolate bonding computational methodology (see the Supporting Information). geometry results in some distortions of the molecular structure In line with the value for T1/Au, we calculate a work-function of T2 upon adsorption, with the distance between neighboring change of −1.38 eV for the biphenylthiolate SAM. In sharp S atoms increasing by 0.2 Å as compared to an isolated contrast to those two cases, for a perfectly ordered T2/Au molecule. For T2/Au also the heights of the three docking interface a much larger value of ΔΦ = −1.73 eV is obtained, as groups vary quite significantly (between 0.61 and 1.03 Å expected on the basis of the arguments in the previous relative to the topmost Au layer), while they are essentially the paragraph. same (1.16 Å) for all S atoms in T1/Au (see Figure 5e and f). What remains to be explained is why the intrinsic work- Consistent with the less ideal bonding configuration of T2/Au, function change for a T2/Au interface is by ca. 0.4 eV larger than that for T1/Au. To clarify that, we performed the the binding energy per molecule (representative of breaking the bond between the substrate and the adsorbate) is following test: We modeled benzylthiolate (B1)and significantly smaller than that for T1/Au (5.43 eV vs 7.16 benzenethiolate (B2) SAMs, which differ only in the presence eV). A similar trend is observed for the adsorption energy of a methyl linker between the phenyl and the thiolate in the characteristic of bond formation (1.62 eV vs 2.67 eV). former system. A full geometry optimization for both systems Simulated structural parameters for the absorbed molecules results in structures with the S atoms in bridge position shifted are summarized in Table 1. The tilt angle of the π * orbitals toward fcc-hollow sites (i.e., consistent with the situation for (α) and the molecular tilt angles (β) for T1/Au are 86.8° and T1/Au). This yields a slightly larger work-function change of 3.4°, respectively, which is in good agreement with the −1.44 eV for B2/Au as compared to −1.33 eV for B1/Au. NEXAFS results (α =81° and β = 7.5°). Conversely, the When the S atom of the B2 molecule is fixed at the fcc-hollow simulated values for well-ordered T2/Au (α = 85.1° and β = position (i.e., the favorable position for T2), the ΔΦ value for 6.7°)differ significantly from the NEXAFS values (α =67° and B2/Au increased to −1.52 eV. When additionally fixing the β =36°). As indicated already earlier, we attribute that to the position of the C atom bonded to the thiolate to the position it coexistence of ordered and disordered domains in T2/Au, with assumes in T2/Au, ΔΦ rises further to −1.65 eV. This shows essentially upright-standing molecules (β =7.5°) in the that the difference in ΔΦ between T2/Au and T1/Au arises ordered regions separated by severely disordered structures from the different hybridization states of the C atom bonded to 2 3 in between (see discussion of S 2p XP spectra). thethiolate(sp vs sp hybridized) and, even more Electronic Properties of the Interface. Functionalization importantly, from differences in the C−S−Au bonding of metal surfaces with SAMs is useful for tailoring the geometries. 55−59 electronic properties of metal substrates. Here, the A more local view of the electrostatics of the SAMs can be 62,63 triptycene-based SAM systems have a particularly high gained from an in-depth analysis of the XPS data. The potential as surface modifier, because of the following: (i) calculated C 1s XP spectra of T1/Au and T2/Au at a photon They form dense and ordered monolayers with, in the case of energy of 350 eV are reported in Figure 6. The energies scale is 61,62 T1, essentially upright-standing molecules. (ii) They can be shifted by 18.88 eV in both systems to align the efficiently chemically modified with various (polar) functional experimental and calculated maxima for T1/Au. Fully groups at the SAM−ambient interface at the 4,5,16- and consistent with the experiments, the positions of the peak bridgehead positions. To establish the basis for future maxima in the calculations differ by 0.4 eV between T1/Au applications, we here discuss experimental and theoretical (Figure 6a, 284.1 eV) and T2/Au (Figure 6b, 284.5 eV). The 6000 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article adopt nested 2D hexagonal structures, which promotes the self-assembly process. The synthesis of T1 and T2 is achieved by sequential organic transformations from 1,8,13-trihydroxytriptycene in good overall yields. STM imaging of T1 and T2 assembled on Au(111) suggests the formation of uniform self-assembled monolayers (SAM) with an ordered 2D hexagonal arrange- ment of the triptycenes. On the basis of our XPS data, we conclude that (nearly) all of the S atoms of T1 bind to Au(111). This results in an upright orientation of the molecules, as confirmed by NEXAFS measurements and quantum-mechanical simulations with a measured (calculated) tilt angle of 7.5° (3.4°). Conversely, the SAM of T2 may contain significant amounts of unbound or weakly bound thiol groups, which causes partial oxidation of the thiol functionality. The large average tilt angle of 36° of T2 on Au(111) determined by NEXAFS spectroscopy in combination with the STM, XPS, and modeling results suggests the coexistence of well-ordered domains with essentially upright standing molecules and highly disordered regions. The lower structural quality of the T2/Au interface can be traced back to a less favorable bonding arrangement in the immediate interface region, which also results in lower binding energies. Interestingly, despite the significantly different degrees of order in the T1 and T2 SAMs, the changes in the area- Figure 6. Simulated C 1s XP spectra of T1/Au (a) and T2/Au (b) for averaged work-function caused by the SAMs are essentially the a primary photon energy of 350 eV. The contributions of the different same for both interfaces (ca. −0.8 eV). On the basis of the groups of chemically equivalent C atoms are also shown, where the simulations and the XPS experiments, this can be rationalized vertical position represents their z coordinates with respect to the image plane position (0.9 Å above the average z position of the by a significantly larger change in the well-ordered regions of topmost Au layer). T2/Au caused mostly be the different bonding geometry, which is eventually diminished by smaller values for the disordered parts of the film. magnitude of that difference is close to the shift in ΔΦ The results presented in this study establish a new type of between the simulated T1/Au and T2/Au interfaces. As shifts tripodal SAM, whose architecture is distinctly different from in the electrostatic energy directly impact core-level binding conventional monolayers of molecular tripods. The advantages energies, this further supports the notion that for perfectly of the triptycene system, particularly T1, are the reliable ordered SAMs the interfacial dipoles are larger in the T2/Au tripodal adsorption configuration, the efficient large-area case. The reason why the electrostatic shift is resolved in the uniform 2D self-assembly, and an almost ideal upright XPS experiments despite the disordered regions is that binding orientation of the benzene rings, projected to the attached energies are impacted by the local electrostatic potential at the functional groups. Importantly, the triptycene tripods can be position of the excited atom such that variations of the readily decorated using the bridgehead or the 4,5,16- electrostatic potential do not average out. Figure 6 also positions. As either one or three functional groups per shows the energetic positions of the C 1s core levels of the tripod can then be substituted, their density and separation can individual C atoms in the SAMs, which allows a direct readily be varied. Thus, the presented systems can serve as comparison between T1/Au and T2/Au on an atom by atom stable and conformationally rigid anchors, for example, for level. Obviously, beyond the global shift between the spectra, polar entities modifying sample work-functions, for recognition the differences in binding energies between T1/Au and T2/Au functionalities in combination with biomolecules, or for are small for electrons from equivalent C atoms, except for CB receptor groups in sensing applications. This makes them and C1 carbons (see insets in Figure 6a,b). This confirms the highly promising building blocks for applications in organic earlier conclusion that differences in electrostatic energies and and molecular circuits, biomedical devices, optical and work-functions in the two SAMs originate from the immediate chemical sensors, solid catalyst, and many more. anchoring region. CONCLUSIONS EXPERIMENTAL SECTION ■ ■ Combining experimental and computational studies, we have Materials. Unless otherwise stated, all commercial reagents were used as received. Benzylthiol (B1) and hexadecanethiol (HDT) were demonstrated that triptycene-based molecular tripods (T1 and purchased from Sigma-Aldrich. Compound 3 was prepared according T2) with thiol-containing functionalities at the 1,8,13-positions to previously reported procedures and unambiguously characterized self-assemble into dense, uniform, and ordered monolayers on by nuclear magnetic resonance (NMR) spectroscopy and atmospheric a metal surface with an upright orientation of the benzene pressure chemical ionization time-of-flight (APCI-TOF) mass planes. The key of the molecular design of T1 and T2 is that spectrometry. For long-term storage of T1 and T2, these compounds the three thiol groups are attached to a rigid triptycene were stored under an argon or nitrogen atmosphere in the freezer to framework in a way that they can efficiently bond to a surface, avoid oxidation of the thiol groups. irrespective of possible conformational states. Moreover, General. NMR spectroscopy measurements were carried out on a 1,8,13-substituted triptycenes have a strong tendency to Bruker AVANCE-500 spectrometer (500 MHz for H and 125 MHz 6001 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article 13 1 for C) or AVANCE-400 spectrometer (400 MHz for H and 100 energy analyzer in normal emission geometry. The photon energy MHz for C). Chemical shifts (δ) are expressed relative to the (PE) was set to either 350 or 580 eV, depending on the BE range. 1 44 resonances of the residual nondeuterated solvents for H [CDCl , The BE scale was referenced to the Au 4f peak at 84.0 eV. The 7/2 1 1 13 H(δ) = 7.26 ppm; acetone-d , H(δ) = 2.05 ppm] and C [CDCl , energy resolution was ∼0.3 eV at a PE of 350 eV and ∼0.5 at 580 eV. 6 3 13 13 The XPS data were used to calculate the effective thickness and C(δ) = 77.16 ppm; acetone-d , C(δ) = 29.8 and 206.3 ppm]. packing density of the SAMs, relying on the C 1s/Au 4f and S 2p/Au Absolute values of the coupling constants are given in Hertz (Hz), 66,67 4f intensity ratios using standard procedures. For the thickness regardless of their sign. Multiplicities are abbreviated as singlet (s), evaluation, a standard expression for the attenuation of the doublet (d), triplet (t), multiplet (m), and broad (br) (see the photoemission signal was assumed together with literature values Supporting Information). Infrared (IR) spectra were recorded at 25 for the attenuation lengths. The spectrometer-specific coefficients °C on a JASCO FT/IR-6600ST Fourier-transform infrared were determined with the help of the reference HDT SAM with a spectrometer. High-resolution mass spectrometry measurements known thickness (18.9 ± 0.1 Å) and packing density (4.63 × 10 were carried out on a Bruker micrOTOF II mass spectrometer 2 37 molecules/cm ; √3 × √3 structure). equipped with an atmospheric pressure chemical ionization (APCI) NEXAFS spectroscopy measurements were performed at the same probe or an electrospray ionization (ESI) probe. beamline. The spectra were collected at the C K-edge in the partial STM Measurements. STM tips were mechanically cut from a electron yield mode with a retarding voltage of −150 V. The tungsten wire (diameter 0.25 mm; Nilaco). Au(111) substrates, polarization factor of the X-ray’s was estimated as ∼88%; the energy obtained by thermal evaporation of Au onto a freshly cleaved mica resolution was ∼0.30 eV. The incidence angle of the light was varied substrate, were flame-annealed and quenched in ethanol prior to use. from 90° (normal incidence geometry; E-vector in surface plane) to Samples for STM imaging were prepared by immersing an Au(111) 20° (grazing incidence geometry; E-vector near surface normal) in substrate into a degassed THF solution (2.0 μ mol/L) of T1 or T2 for steps of 10°−20°, which is a standard approach enabling the 24 h, and the resultant substrate was washed with THF, dried in air, determination of the molecular orientation from NEXAFS data. and then thermally annealed (120 °C, 1 h) under reduced pressure. Raw spectra were normalized to the incident photon flux by division Constant current-mode STM imaging was carried out on a through a spectrum of a clean, freshly sputtered gold sample. The PE Nanoscope III STM system (Digital Instruments). All STM scale was referenced to the pronounced π* resonance of highly measurements were performed at 25 °C in air. The STM scanner oriented pyrolytic graphite at 285.38 eV. was calibrated with an Au(111) substrate prior to the experiments. Kelvin Probe Measurements. Work-function measurements The observed STM contrast (apparent height) difference of 2.5 Å was were carried out using a UHV Kelvin Probe 2001 system (KP consistent with the well-known interlayer separation at Au terraces on −9 technology Ltd., UK). The pressure in the UHV chamber was ∼10 Au(111). mbar. As reference, we used HDT/Au with the work-function value Preparation of SAMs for the Spectroscopy and Kelvin set to 4.30 eV according to the literature. The latter value was Probe Measurements. The SAMs for these experiments were additionally verified by referencing it to the work-function of freshly prepared on commercial Au substrates (Georg Albert PVD, Silz, sputtered gold set to 5.20 eV. The accuracy of the WF values is ca. Germany). These substrates were prepared by thermal evaporation of ±0.05 eV. 30 nm of Au (99.99% purity) onto a polished single-crystal silicon Computational Methodology. The calculations were performed (100) wafer (Silicon Sense) that had been precoated with a 5 nm 73 74 using the FHI-aims code and employing the PBE functional in titanium adhesion layer. The resulting Au films are polycrystalline, combination with the surface parametrization of the Tkatchenko− having a grain size of 20−50 nm and predominantly exhibiting a Scheffler dispersion correction. The latter were turned off between (111) orientation. The SAMs were prepared by immersion of a fresh the bulk Au atoms. Periodic boundary conditions and the repeated substrate in a degassed THF solution (2 μM−1 mM) of T1 or T2 for slab approach including a vacuum region of at least 20 Å in the z 24 h at 25 °C. After immersion, the films were washed with THF and direction were employed to represent the interface. To compensate dried by blowing argon. Finally, some of the samples were annealed at for the electrostatic asymmetry of the slab, a self-consistently 100 °C for 1 h either under inert gas atmosphere or under ultrahigh calculated dipole layer was inserted in the vacuum. To sample the vacuum (UHV) conditions. In addition, several reference SAMs, that reciprocal space, a nonorthogonal 6 × 6 × 1 Γ-centered k-point grid is, those of B1 and HDT on Au(111), were prepared using standard was used. The dimensions of the unit cells in the x and y directions procedures. HDT/Au was used as a reference system for the XPS were defined according to the calculated Au nearest neighbor distance and work-function measurements (see below). B1 can be regarded as (2.940 Å), to avoid spurious surface relaxations. The metal was a partial structure of T1, making it a suitable monothiol reference. modeled using 5 layers of Au, with the bottom 3 layers fixed at their Because of the presence of the methylene linker between the benzene bulk positions during the optimization. The presented results were ring and thiol group, a sufficiently good quality of this monolayer can obtained using the default FHI-aims “tight” basis set and setting the be expected, similar to the analogous nitrile-substituted system. At −6 total energy criterion for the self-consistency cycle to 10 eV. The the same time, we refrained from studying benzenethiol as the optimizations were performed until the maximum residual force monothiol reference to T2, as it has been shown to form SAMs of component per atom was below 0.01 eV/Å. For the initial screening only limited quality when employing the standard immersion 47,65 of different docking sites, less accurate settings were adopted, using procedure. the default FHI-aims “light” basis set and stopping the optimization XPS and NEXAFS Spectroscopy Measurements. The XPS, when the maximum residual force component per atom was below NEXAFS spectroscopy, and work-function measurements were −9 0.05 eV/Å. performed under UHV conditions (1.5 × 10 mbar) at 25 °C. Binding energies, E , are defined such that they reflect the energy bind Laboratory XPS measurements were carried out with a MAX200 needed to break the bond between the molecule and the substrate and (Leybold-Heraeus) spectrometer equipped with an Mg Kα X-ray to remove the molecules from the SAM: source (200 W) and a hemispherical analyzer. The spectra were corrected for the spectrometer transmission, and the binding energy EE=−E −E bind Trip/Au Au Trip (1) (BE) scale was referenced to the Au 4f peak at 84.0 eV. Because 7/2 the quality of the laboratory spectra in terms of statistics and energy Here, E is the energy per unit cell of the SAM adsorbed to the Trip/Au resolution was inferior to the synchrotron data, they were mostly used surface, E is the energy of the optimized pristine Au slab, and E is Au Trip to verify the film thickness and packing density. the energy of the optimized gas-phase molecular radical. Conversely, Synchrotron-based XPS measurements were carried out at the adsorption energies, E ,reflect the energetics of forming the ads bending magnet HE-SGM beamline of the synchrotron storage ring monolayers and at the same time replacing the molecular S−H BESSY II in Berlin, Germany. This beamline provides a moderate X- bonds by bonds to the Au surface. They are, thus, defined as ray intensity helping to avoid X-ray damage during the spectra EE=−E −E + 3/2E ads Trip/Au Au Trip ‐H H2 (2) acquisition. The spectra were collected with a Scienta R3000 electron 6002 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005 Journal of the American Chemical Society Article E in this equation represents the energy of the optimized gas- Research Foundation (Deutsche Forschungsgemeinschaft; Trip‑H phase triptycene molecule in which all S atoms are saturated with DFG, grant ZH 63/22-1 for E.S. and M.Z.), and the Austrian hydrogens, and E is the energy of an isolated H molecule. H2 2 Science Fund (FWF, I2081-N20 for G. N. and E.Z.). T.F. The XP spectra were simulated within the initial state approach to acknowledges support from the Dynamic Alliance for Open avoid artifacts arising from a combination of periodic boundary 62 Innovation Bridging Human, Environment and Materials from conditions and explicit excitations in each unit cell. For obtaining MEXT, Japan. We thank Dr. Valiparambil Sanjayan Sajisha and the spectra, the 1s core level energies for every C atom were taken Ms. K. Takenouchi for their assistance in the synthesis of T1 from the atom projected density of states output files. Subsequently, and T2. We thank Suzukakedai Materials Analysis Division, they were shifted considering the screening of the core hole by the 78,79 metal substrate via an electrostatic image charge model assuming Technical Department, Tokyo Institute of Technology, for a dielectric constant of the SAM of 3.9. To model the spectra, the their support with the NMR measurement and single-crystal X- individual resonances were broadened using Gaussian functions with a ray analysis. E.S. and M.Z. thank the Helmholtz Zentrum variance of 0.15 eV and an intensity scaled using an exponential Berlin for the allocation of synchrotron radiation beamtime at attenuation function to account for the finite escape depth of the BESSY II and A. Nefedov and Ch. Wöll for the technical photoelectrons. Additionally, the energy scales for both interfaces cooperation during the experiments there. 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Self-Assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols: Specific Characteristics and Odd−Even Effects. Phys. Chem. Chem. Phys. 2010, 12, 12123−12127. 6005 DOI: 10.1021/jacs.9b00950 J. Am. Chem. Soc. 2019, 141, 5995−6005

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