Reestablishing OddâEven Effects in Anthracene-Derived Monolayers

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Reestablishing Odd-Even Effects in Anthracene-Derived Monolayers by Introduction of a Pseudo-C Symmetry 2v

Christoph Partes, Eric Sauter, Michael Gärtner, Martin Kind, Andika Asyuda, Michael Bolte, Michael Zharnikov, and Andreas Terfort J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05299 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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The Journal of Physical Chemistry

Reestablishing Odd-Even Effects in Anthracene-Derived Monolayers by Introduction of a Pseudo-C2v Symmetry

Christoph Partes,1# Eric Sauter,2# Michael Gärtner,1 Martin Kind,1 Andika Asyuda,2 Michael Bolte,1 Michael Zharnikov,2,* and Andreas Terfort1,* 1Institute

of Inorganic and Analytical Chemistry, Frankfurt University, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany

2Applied

Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

ACS Paragon Plus Environment

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ABSTRACT: A series of anthracene[2,3-d]oxazolyl-2-alkylthioacetates (AOxCnSAc) with n = 2-6 methylene groups in the alkyl chain were designed and synthesized to investigate the influence of the substitution along the long axis of the molecule on the structural behavior of the respective self-assembled monolayers (SAMs) on Au(111). While in previous work anthraceneterminated alkanethiols, in which the alkyl group was attached to the off-axis 2-position of the acene, showed an exceptionally small influence of the number of methylene groups (n) in the aliphatic linker, the new system exhibits a strong dependence of almost all monolayer properties on the length of the aliphatic linker, with the parity of n being the decisive parameter. These socalled odd-even effects are a result of the bond angle at the anchoring S atom as well as the alltrans conformation of the alkyl linkers, which in particular affect the packing density and the orientation of the aromatic part. Thus, using a variety of complementary experimental techniques, distinct odd-even variation of molecular inclination (by 10-11°) and the packing density (by 20%) were found, with the packing density of the odd-numbered systems being even as high as the ones of the well-established alkanethiolate systems. The high quality and welldefined character of these SAMs, along with a low band gap of only 3.0 eV, make them relevant for application in organic and molecular electronics.


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1. INTRODUCTION As part of the advent of organic and molecular electronics, the design and optimization of the involved interfaces becomes increasingly important. Self-assembled monolayers (SAMs) of tailored organic molecules are the most versatile tool for precise adjustment of a variety of surface and interfacial properties as well as for even performing specific functions in electronic devices.1 In particular, SAMs have been used as charge carrier injection layers,2-5 as templates for the directed growth of semiconductor layers,6,7 or as ultra-thin insulating layers.8,9 For the smallest SAM-based electronic devices, the so-called SAMFETs, the molecules even contain parts fulfilling several of the roles, e.g., carrying simultaneously a semiconducting end-group for the formation of the charge transporting channel as well as an aliphatic part to act as gate insulator.10,11,12 In such systems, the length/extent of the respective parts is decisive for their performance, e.g. more extended -systems typically show better charge carrier mobilities due to lower band gaps and better coupling between the molecules.13,14 Apart from the molecular structure, the relative orientation and, therefore, the packing of the -systems is a factor which affects the electronic coupling within the semiconducting layer.13-16 This orientation can be finetuned by the introduction of the alkyl linker between the -system and the anchoring atom (docking group) of the SAM constituents, relying on the interplay of the bond-angle at this atom and the parity of the number of methylene (CH2) groups in the linker, termed as n.17-28 This behavior is generally described as odd-even effect in monomolecular self-assembly.24 In case of the frequently used thiolate and selenolate SAMs on Au(111), a bond-angle of ~112° at the anchoring atom and a thermodynamically optimal all-trans configuration of the aliphatic spacer should lead to a more upright orientation (a smaller tilt angle) of the aromatic moiety for an odd number of the CH2 units in the spacer, while higher tilt angles are predicted for araliphatic SAMs


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with an even number of the methylene groups.17 These tilt angles, in turn, should lead to higher and lower packing densities, respectively, in the resulting SAMs. Indeed, these odd-even effects have been observed for a variety of different symmetrically anchored aromatic systems, such as biphenyl-17,18,20 and terphenyl-21,22 substituted alkanethiolate SAMs and biphenyl-substituted alkaneselenolate monolayers23,25,26. Nevertheless, these oligophenyl systems are not optimally suited for the application in molecule-based electronics because of their relatively large band gap of > 4.2 eV.29,30 In contrast, acenes show significantly smaller band gaps, resulting in particular in a variety of application-relevant electronic studies employing anthracene and its derivatives with band gaps of ~3.2 eV.31,32 However, when implemented into araliphatic SAMs, the anthracene-derived molecules show the odd-even effects with the extent being noticeably smaller than for all other systems.33 This specific behavior was tentatively explained by an orientational ambiguity resulting from the inherent off-axis attachment of the aliphatic spacer (see Figure 1a), what resulted in a much poorer controllability of the packing parameters in the semiconducting part of the SAMs.33 Because of this limitation, we were looking for a way to “symmetrize” the system, while basically leaving the electronic properties unchanged. A useful approach in this context is the annellation of acene systems with an oxazole ring, as was demonstrated in a previous publication where we reported the preparation of purely aromatic SAMs with an anchoring group on the long axis of the acene backbone.34


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Figure 1. (a) Nonsymmetric attachment of anthracene:33 the anthracene moiety can adopt a multitude of positions at the given predetermined orientation, related to the optimal binding angle and the parity of n, as signified by the cones. The flexibility permits the adoption of almost upright orientations of the aromatic part independently of the parity of n, what basically reduces the controllability of the system. (b) (Pseudo)symmetric attachment of anthracene (the present study): at the given predetermined orientation, the anthracene moieties can only move within a tight conformational ‘cylinder’ without any change of the molecular tilt; consequently, the full extent of the odd-even effects can be expected.

On this basis, in the present study, we designed and synthesized a series of (pseudo)symmetrical anthracene[2,3-d]oxazolyl-2-(CH2)n-thioacetates (AOxCnSAc) with n = 26 (general structure shown in Figure 2) serving as precursors for the SAMs with the almost symmetric attachment of anthracene to the adjacent alkyl linker. The respective SAMs were


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prepared on Au(111) substrates and characterized in detail by ellipsometry, X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and infrared (IR) spectroscopy to test the effect of the symmetric attachment of anthracene on the structural properties of the monolayers, making them promising candidates for application in organic semiconductor devices.

Figure 2. Schematic representation of the target molecules, serving as the SAM precursors and abbreviated as AOxCnSAc, with A signifying anthracene, Ox oxazole, Ac acetyl, and n the number of the methylene units (C) in the aliphatic linker. Since the acetyl group should be cleaved upon the adsorption and S is common for all the molecules, we will use acronyms AOxCn for the SAM notation. The red dashed line represents the main molecular (pseudo) axis.

2. EXPERIMENTAL SECTION Syntheses of the SAM Precursors. A detailed description of the synthesis procedures can be found in the Supporting Information. UV-VIS Spectroscopy and Optical Band Gap. UV-vis absorption spectra in solutions were recorded at room temperature with a Varian Cary 50 Scan UV-vis spectrophotometer. Band gaps were estimated by an established extrapolation method.35 Monolayer Preparation. The formation of the anthracene[2,3-d]oxazolyl-2-alkanethiolate (AOxCn) SAMs was carried out by immersion of Au(111) substrates into the solutions of the


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respective precursor (c ≤ 1 mM) in degassed ethanol for 20-24 h under an atmosphere of argon. The substrates for the ellipsometry and IRRAS experiments were prepared by thermal evaporation of gold (200 nm thickness) onto Si(100) wafers (Wacker, Germany) with an intermediate Cr layer (5 nm thickness) as an adhesion promoter. If the substrates were older than one day, they were treated in hydrogen plasma prior to SAM formation.36 The substrates for the X-ray spectroscopic experiments were purchased from Georg Albert PVD-Beschichtungen and used as received. They were prepared by thermal evaporation of gold (30 nm thickness, 99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. SAMs of perdeuterated dodecanethiolate and hexadecanethiolate (HDT) were prepared as reference layers on similar gold substrates as used for the respective experiments. The monolayer samples were either characterized immediately after the preparation or kept in nitrogen-filled containers until advanced spectroscopic characterization. Ellipsometry. The ellipsometric characterization of the monolayers, in context of the effective thickness, was carried out using a Sentech SE 400 ellipsometer with He-Ne laser (632.8 nm) and an incidence angle of the primary beam of 70° with respect to the sample surface normal. For each sample, effective thickness was measured at four different preselected positions and average values were calculated. The complex refractive index of the Au substrates was measured at these positions before SAM formation. The extinction coefficients and the refractive indices of the monolayers were assumed to be 0 and 1.55, respectively. All measurements were performed at room temperature X-ray Spectroscopy. The SAMs were characterized by synchrotron-based XPS and angle resolved NEXAFS spectroscopy. The experiments were carried out at the HE-SGM beamline


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(bending magnet) of the synchrotron storage ring BESSY II in Berlin. The experimental station was equipped with a Scienta R3000 electron energy analyzer and a custom-designed partial electron yield (PEY) detector.37 The measurements were performed at room temperature and under ultra-high vacuum conditions. The acquisition time was kept reasonably short to avoid any noticeable damage induced by X-rays.38,39 For comparison, we also measured the spectra of the AOx SAM, which differs from the AOxCn monolayers by the lack of the aliphatic linker.34 The measurements were repeated several times to achieve consistent data over the entire series. XPS. The XP spectra were acquired in normal emission geometry. Photon energy (PE) was selected at either 350 or 580 eV depending on the particular region. The binding energy (BE) scale of the spectra was calibrated to the Au 4f7/2 emission of the underlying substrate at 84.0 eV.40 The energy resolution was ∼0.3 eV at a PE of 350 eV, and somewhat lower at a PE of 580 eV. The spectra were fitted by symmetric Voigt functions, representing individual peaks and a linear background. The S 2p3/2,1/2 doublets were fitted by a combination of two peaks with the same fwhm, the standard branching ratio (2), and a standard spin-orbit splitting of ~1.18 eV,40 which was additionally verified by fit. The same peak parameters were used for identical spectral regions. Using the XPS data, the effective thickness and packing density of the AOxCn SAMs were evaluated, following the approach of refs 41 and 42. The thickness was calculated on the basis of the C1s/Au4f intensity ratio and the literature values for attenuation lengths of the photoemission signal at the given kinetic energies.43 The spectrometer-specific coefficients were determined by using the HDT SAM with the well-known thickness (1.89  0.02 nm)44, as the reference. The packing density was calculated on the basis of the S2p/Au4f intensity ratio using the HDT SAM


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with the well-known packing density (4.631014 molecules/cm2; corresponding to the (3  3)R30° molecular lattice)44 as the reference. NEXAFS Spectroscopy. The spectra were acquired at the C K-edge in the PEY acquisition mode. The retarding voltage was set at 150 V. The primary X-ray beam was linearly polarized with a polarization factor of ~91%. The energy resolution was ∼0.3 eV. To monitor molecular orientations in the SAMs, the incidence angle of X-rays was varied from 90° with respect to the surface plane (E-vector in the surface plane) to 20° (E-vector nearly normal to the surface) in steps of 10°-20°. Accordingly, the orientation of the E-vector varied from being in the surface plane to nearly normal to the surface. This is a standard approach in NEXAFS spectroscopy, relying on the so called linear dichroism in X-ray absorption, i.e. strong dependence of the crosssection of the resonant photoexcitation process on the orientation of the E-vector of the linearly polarized light with respect to the molecular orbital of interest.45 The raw spectra were normalized to the incident photon flux by division by a spectrum of a clean, freshly sputtered gold sample.45 Afterwards, the spectra were reduced to the standard form by subtracting linear pre-edge background and normalizing to the unity edge jump (determined by a nearly horizontal plateau 40-50 eV above the respective absorption edges). The energy scale was calibrated to the most intense π* resonance of highly oriented pyrolytic graphite at 285.38 eV.46 IR Spectroscopy. IR spectra of the neat substances and the SAMs were recorded using a Thermo Nicolet 6700 Fourier transform IR spectrometer with a narrow band mercury cadmium telluride semiconductor detector. During the measurements, the optical path was purged with dried and CO2-free air. All spectra were recorded at a resolution of 4 cm-1. Spectra of the neat substances were obtained using a single-reflection diamond attenuated total reflection unit.


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Infrared reflection absorption spectra of the SAMs were recorded with p-polarized light, with an incidence angle of 80° with respect to the sample surface normal. Perdeuterated dodecanethiolate SAMs on gold served as a reference for the IRRAS measurements. To aid the assignment of vibrational modes and to identify the direction of their transition dipole moments (TDMs), density functional theory (DFT) calculations of IR spectra of isolated molecules were carried out (see the Supporting Information for details).

3. RESULTS 3.1. Syntheses of AOxCnSAc. The anthracene[2,3-d]oxazolyl alkanethioacetates, AOxCnSAc, were synthesized by a ring closure strategy starting from 2-amino-3-hydroxyanthracene (1)34 (Scheme 1). Our initial idea was to use the ω–bromo derivatives of alkylcarbonic acid ortho esters for the cyclisation, which in turn can be synthesized from the respective ω–bromo alkyl cyanides 2 via the respective Pinner salts 3. Unfortunately, neither the ortho esters nor the Pinner salts47 led to satisfactory cyclisation results. Only after transformation of 1 into its corresponding trimethylsilyl ether 4, the respective oxazoles could be obtained. These were then converted into the corresponding ω–thioacetates by nucleophilic substitution. All ring-closing reactions gave yields of 25-52%, the yields of the substitution reactions from 5a-d to AOxCnSAc (n = 3-6) were 55-99%. A remarkably difficult task was the synthesis of the molecules in the series with n = 2, as all the derivatives, starting from the ω–bromo cyanides to the oxazole thioacetates, are very prone to elimination reactions forming the respective vinyl derivatives. To avoid the application of 3-bromopropionimidate hydrochloride, which already has been reported to be very sensitive,48 the thioacetate group was introduced in the molecule already before performing all the other steps. This was possible by addition of thioacetic acid to acrylonitrile. But even for this molecule


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(6), maintaining temperatures below 20 °C were absolutely necessary during formation of the respective Pinner salt (7) to avoid deacetylation reactions. The final ring-closure reaction with silyl derivative 4 yielded 30% of the thioacetate (AOxC2SAc), the long-term stability of which in solution was poor (CDCl3, very low, DMSO-d6, acceptable, as indicated by significant signals of the elimination product 2-vinylanthraceneoxazole in the 1H NMR). The deprotection of the thioacetates could be accomplished using K2CO3 in methanol (see Supporting Information), but the thiols turned out to be unstable upon storage. As it is known that thioacetates form similar SAMs as the respective thiols,49-51 we compared the quality of the monolayers obtained from both precursors and could not find any differences, what led to the decision to only use the stable thioacetates for further work.

Br

Br ( )n

(i)

Br

N

NH2+Cl-

(ii)

Br ( ) n

( )n

O

NH2

5a-d (n = 3 - 6) NH2

(iv)

OH 4

1 (vi)

AcS

N ( )n

6 (n = 2)

(vii)

NH2+ Cl AcS

Br ( )n

O

3a-d (n = 3 - 6)

2a-d (n = 3 - 6)

N

N

(iii)

( )n

OSiMe3

-

O

7 (n = 2)

(viii)

(v)

N O

SAc ( )n

AOxCnSAc (n = 2 - 6)

Scheme 1. Syntheses of the title compounds. (i) NaCN, DMF, (ii) MeOH, 1.3 equiv. sat. HCl/Et2O, (iii) MeOH, 40 °C, 2-52%, (iv) HMDS, 3.0 equiv., H3CNO2, RT, 71%, (v) KSAc, DMF, 55-99%, (vi) AcSH, cat. n-Bu4NOH, 66%, (vii) AcCl, MeOH, -21 °C, 79%, (viii) MeOH, 40 °C, 30%.


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3.2. UV-vis Spectroscopy. In order to determine the influence of an aliphatic linker on the band gap of the arene system, UV-vis spectra of the AOxCnSAc molecules were recorded (see Figures S18 and S19 in the Supporting Information). The resulting band gaps for all AOxCnSAc were determined to be 3.03 eV, what is slightly below the value of 3.06 eV for anthracenoxazolinethione,34 a comparable aromatic system, and significantly lower than the band gap of anthracene (3.2 eV),32,52,53 what can be explained by the somewhat extended -system. Due

to

the

structural

and

electronic

similarity

of

the

AOxCnS

family

with

anthracenoxazolinethione, for which a CV study determined a HOMO position of -5.53 eV, we can assume a similar electronic situation for the compounds studied here.54

3.3. Ellipsometry. The effective thicknesses of the AOxCn SAMs derived on the basis of the ellipsometry data are presented in Figure 3 and additionally compiled in Table 1, for comparison with the other relevant values and data. Instead of a nearly linear increase with the number of the methylene unts, as was observed for the ACn monolayers (non-symmetric attachment of the anthracene moiety), a clear zig-zag behavior is observed. Such a behavior is a fingerprint of the odd-even effects, reflecting the respective variation of the packing density depending on the parity of n, with comparably higher thicknesses for the odd n (n = 3, 5) and lower ones for the even n (n = 2, 4, 6). Interestingly, the values for the odd n are only somewhat smaller than the ultimate limit of the film thickness, given by the molecular length (1.64 and 1.9 nm for n = 3 and 5, respectively) and the SAu distance (0.24 nm).55,56 This suggests a nearly upright molecular orientation in these SAMs.


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Effective thickness (nm)

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2.0

Ellipsometry

1.8 1.6 1.4 1.2 1.0 0.8

0

1

2

3

4

5

6

Number of the CH2 units

Figure 3. Effective thicknesses of the AOxCn SAMs derived on the basis of the ellipsometry data. Table 1. Ellipsometry- and XPS-derived Effective Thicknesses and XPS-derived Packing Densities of the AOxCnS SAMs along with the Molecular Lengths of the Respective Precursors Effective Effective thickness Monolayer Molecular Packing density / thickness XPS / nm (n) length / nm molecules cm-2 ellipsometry / nm 2 1.55 1.13 ± 0.06 1.17 ± 0.01 3.7  1014 3 1.64 1.56 ± 0.03 1.63 ± 0.02 4.6  1014 4 1.78 1.53 ± 0.05 1.56 ± 0.02 3.3  1014 5 1.90 4.8  1014 1.91 ± 0.05 1.88 ± 0.02 6 2.03 4.3  1014 1.87 ± 0.05 1.78 ± 0.02

3.3. XPS. The S 2p and C 1s XP spectra of the AOx and AOxCn films are presented in Figure 4. The S 2p spectra of all these films in Figure 4a are dominated by an intense S 2p3/2,1/2 doublet (1) at a BE of ~162.0 eV (S 2p3/2) characteristic of the thiolate-gold bonding.39 This suggests a well-defined SAM character of the AOx and AOxCn films, with nearly all molecules being attached to the substrate via the sulfur atom. No traces of other sulfur-derived species, including physisorbed or oxidized ones, are observed, except for a weak doublet (2) at a BE of 161.0 eV (S 2p3/2), exhibited as a low BE shoulder of the thiolate signal. This doublet is frequently observed in the XP spectra of thiolate SAMs as far as the energy resolution and statistics are high


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enough.39 It is usually ascribed to atomic sulfur, appearing presumably after the cleavage of the Au-thiolate bond (see discussion in refs 39 and 57). An alternative assignment is a distinctly different bonding configuration of the S atom as compared to thiolate,57 but the atomic sulfur description is more realistic in our opinion. Interestingly, the doublet 2 has a comparably lower intensity for the odd n, which suggests a higher quality of these SAMs as compared to those with the even n. The C 1s spectra of AOx and AOxCn films in Figure 4b are dominated by a strong peak at a BE of 284.05 - 284.3 eV (1), accompanied by few weak features at the high BE side (2-4). The dominant peak is assigned to the combined contributions of the aliphatic linker and the anthracene moiety, with a dominance of the latter because of the strong attenuation of the photoemission signal at the given kinetic energy of electrons.58 Interestingly, the exact BE position of this peak exhibits the characteristic variations, typical for the SAMs showing oddeven behavior,59 with lower BE values for the odd n (284.05-284.1 eV) and higher BE values for the even n (284.25-284.3 eV). As to the weak features at the high BE side (2-4), they stem from the carbon atoms in the oxazole ring; the exact assignment can be found in ref 34.


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a

XPS: S 2p

b

C 1s AOxC6

AOxC6 AOxC5

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AOxC5

AOxC4

AOxC4

AOxC3 1

AOxC3 2

AOxC2

AOxC2 1

AOx

AOx 166

164

162

160

2 4 3

292 290 288 286 284 282

Binding energy (eV)

Figure 4. S 2p (a) and C 1s (b) XP spectra of the AOx and AOxCn SAMs acquired at a photon energy of 350 eV. The individual doublets (a) and peaks (b) are depicted by different colors and marked by numbers; see text for details. The vertical dashed lines in panel (b) highlight the BE positions of the main peak (1).

The N 1s and O 1s spectra of the AOxCn SAMs (not shown) exhibit single emissions at 398.7398.8 eV (N 1s) and 533.6 eV (O 1s) assigned, respectively, to the nitrogen and oxygen atoms in the oxazole ring.34 These BEs are slightly higher than those for the AOx SAM (398.5 and 533.3 eV, respectively), because of the weaker screening of the photoemission hole by the Au substrate. The O 1s spectra of the AOxCn SAMs exhibit additionally a weak peak at a BE of ~531.7 eV, related presumably to a minor contamination. The intensity of this peak is somewhat stronger for the odd n as compared to the even n, once more underlining a better quality of the


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former SAMs. The intensities of the C 1s signal in Figure 4b exhibit an odd-even variation with higher values for the odd n and lower values for the even n. Combining these intensities with those of the Au 4f signal, stemming from the substrate, the effective thickness of the AOx and AOxCn SAMs could be evaluated (see section 2 for details). The results are presented in Figure 5 and are additionally compiled in Table 1. Similarly to the ellipsometry-derived values (Figure 3), the effective thicknesses of the AOxCn monolayers exhibit clear zig-zag behavior, suggesting oddeven variation of the packing density and molecular inclination. The relation between the zig-zag behavior and the parity on n is the same in Figures 3 and 5 and even the absolute values of the effective thicknesses in these figures agree well with each other (see also Table 1).

Effective thickness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0

XPS

1.8 1.6 1.4 1.2 1.0 0.8

0

1

2

3

4

5

6


Figure 5. Effective thicknesses of the AOx and AOxCn SAMs derived on the basis of the XPS data. A direct estimate of the packing density could be made as well on the basis of the S2p/Au4f intensity ratios (see section 2 for details). The respective data are presented in Figure 6 and compiled in Table 1. The packing densities exhibit clear odd-even behavior, with higher values for the odd n and lower values the even n, which agrees with the variation of the effective


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thickness (see Figures 3 and 5). The value for n = 6 is somewhat high, compared to the general behavior, but this is probably related to the limited accuracy of the experiments and the given packing density evaluation approach. Interestingly, the packing density values for the AOxC3 and AOxC5 SAMs are noticeable higher than that of the AOx monolayer and are very close to the 4.6  1014 molecules/cm2 value characteristic of the most typical (3  3)R30° and (23× 3)rect structures of thiolate SAMs on Au(111).44,60 Thus, the introduction of a proper aliphatic linker improves the packing of the anthracene-bearing molecules to the ultimate limit.

Packing density (mol./cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.0x1014 4.5x1014 4.0x1014 3.5x1014 3.0x1014

0

1

2

3

4

5

6


Figure 6. Packing density of the AOx and AOxCn SAMs derived on the basis of the XPS data.

3.4. NEXAFS Spectroscopy. The C K-edge NEXAFS spectra of the AOxCn SAMs acquired at an X-ray incident angle of 55° ("magic angle") are shown in Figure 7. These spectra are exclusively representative of the electronic structure of the studied film and are not affected by orientation effects.45 Accordingly, in view of the similar molecular structure, the spectra of all SAMs in Figure 7 are similar to each other, with the dominant contribution associated with anthracene - the common building block, whereas the contributions related to the oxazole ring


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and, especially, to the aliphatic linker are strongly suppressed. The spectra exhibit rich patterns of the absorption resonances with the PE positions and tentative assignments compiled in Table 2 for the AOxCn SAMs (the analogous assignments for the AOx monolayer can be found in ref 34). The most prominent feature, characteristic of acenes and anthracene in particular,61-63 is the "double" π* resonance (1a and 1b). It originates from the splitting of the characteristic single π1* resonance of benzene45,64 and poly-p-phenylenes,61 due to the chemical shift of symmetry independent carbon atoms within anthracene with strong influence of excitonic effects.61-63 Significantly, the intensity relation between the π1a* and π1b* features agrees well with both theoretical predictions62 and experimental observations33,34,63,65. A further important feature is the R* resonance of the aliphatic linker (2)66,67, which increases in intensity with increasing n, in accordance with the molecular compositions. The characteristic * resonance of oxazole at a PE of ~287.7 eV (8; see ref 34 as well) is strongly suppressed in the spectra of the AOxCn SAMs.


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NEXAFS: C K-edge 1b 1a 34 2

55° AOxC6 5 6 7

AOxC5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AOxC4

AOxC3

AOxC2

8

280

290

AOx

300

310

320

Photon energy (eV)

Figure 7. C K-edge NEXAFS spectra of the AOx and AOxCn SAMs acquired at an X-ray incident angle of 55°. Some characteristic absorption resonances are marked by numbers (see text and Table 2 for details).

Along with the above analysis, providing, complementary to the XPS results, information on the identity and chemical composition of the thiolate oxazole SAMs, molecular orientation in these films was studied. This study was based on the linear dichroism effects in X-ray absorption (see the Experimental Part), taking into account the entire sets of the NEXAFS spectra for all monolayers studied. A convenient way to monitor these effects is to plot the difference between the spectra taken at the normal (90°) and grazing (5-30°) X-ray incidence. Such difference curves, based on the spectra acquired at 90° and 20°, are presented in Figure 8 for all SAMs


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studied. All these curves exhibit pronounced positive and negative peaks at the positions of the π*-type and σ*-type resonances, respectively, which, in view of the orientation of the TDMs associated with these orbitals (perpendicular and parallel to the molecular backbone, respectively), suggest high orientational order with the aromatic systems oriented nearly perpendicularly to the surface in all AOxCn films.

Table 2. PE Positions (eV) of the Absorption Resonances in the C K-edge NEXAFS Spectra of the AOxCn SAMs, along with the Respective Tentative Assignments According to refs 61 and 62 for Anthracene, ref 34 for Oxazole, and refs 66 and 67 for Alkyl Linker The accuracy of the PE values was estimated at  0.05 eV for the most-pronounced 1a, 1b, and 4 resonances and somewhat lower for the other features. position assignment

1a 285.4 π1a*

1b 285.95 π1b*

2 287.3 R*

3 288.5 auπ*

4 290.0 b3gπ*

5 293.6 *

6 300.4 *

7 306.0 *

The above qualitative considerations were complemented by a quantitative analysis of the NEXAFS data. For this analysis we used the most prominent resonances, π1a* and π1b*, evaluating the dependence of their intensity on the X-ray incidence angle and comparing it with the theoretical expression for a vector-type orbital. The above analysis was performed within the standard theoretical framework,45 with few minor modifications simplifying the evaluation and fitting procedure. The resulting average tilt angles (α) of the respective TDMs with respect to the surface normal are presented in Figure 9. These angles are between 67° and 80° and show a zigzag behavior with lower values for the even n and higher values for the odd n.


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NEXAFS: C K-edge 1a 1b

90°-20° 4

AOxC6 5

23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AOxC5 6

7

AOxC4

AOxC3

AOxC2 8

AOx

280

290

300

310

320

Photon energy (eV)

Figure 8. Difference between the NEXAFS spectra of AOxCn SAMs collected under the normal (90°) and grazing (20°) incidence geometry. Most prominent difference peaks are marked by numbers (see text and Table 1 for details). The horizontal dashed lines correspond to zero.

Considering that the TDMs associated with the considered π* orbitals are oriented perpendicular to the molecular backbone, such a behavior suggests smaller molecular inclination for the odd n and larger inclination for the even n. The exact values of the average tilt angle of the molecular backbone (β) can, however, be only calculated as far as the so called twist angle () of this backbone is known. This angle describes rotation of the molecular backbone around its axis and is usually defined with respect to the plane spanned by this axis and the surface normal.68 The relevant angles are related by the formula cos(α) = sin(β)cos().18 In some studies,


21


combined evaluation of NEXAFS and IR spectra yielded exact information on , enabling the unambiguous determination of β.5,69 There are also specifically functionalized SAMs reported, which allow for an exact determination of both  and β solely by NEXAFS spectroscopy.68 Yet, in the case of the SAMs investigated in this study, the value of  is not known and could not be determined experimentally, so that we are only left with reasonable assumptions. Setting  = 0° und applying the values of the α angles (63° to 76°) to the formula given above yields the smallest possible values of β, i.e. 10-16° for n = odd and ~22-23° for n = even. However, under the assumption that the AOxCn molecules in the SAMs adopt twist angles analogous to the ones found in molecular crystals of polyacenes, such as anthracene,70 a γ value of ~26° can assumed. The resulting β values would then be 11-18° for n = odd and 25-26° for n = even.

85

Average tilt angle (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 75 70 65 0

1

2

3

4

5

6


Figure 9. Average tilt angle of the π* molecular orbitals of the anthraceneoxazole moiety in the AOx and AOxCn SAMs with respect to the surface normal derived from the NEXAFS data. 3.5. IR Spectroscopy. Complementary to the above XPS and NEXAFS spectroscopy data, IR spectroscopy experiments were performed. They allow to monitor the molecular identity and the chemical state as well as to assess molecular orientation in the SAMs. As reference, spectra of


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the bulk substances were taken by ATR IR spectroscopy. Exemplary, spectra of the AOxC2S and AOxC3S samples (calculated spectra of the thioacetates, labeled "DFT", the ATR spectra of the bulk substances, labeled "neat", and IRRA spectra of the respective monolayers, labeled "SAM") are displayed in Figure 10. Note that the spectra of the other AOxCnS species are very similar to the ones displayed in Figure 10. The IRRA spectra of the SAMs mimic those of the respective bulk materials, showing however few distinct differences. The most prominent difference is that the strong band at 1684-1690 cm-1 in the ATR spectra is absent in the SAM spectra. As this band is representative of the thioacetate group CO stretch mode, its absence is a hint on the cleavage of the acetyl group during formation of thiolates that bind to the gold surface. Compared to the bulk spectra, some IR bands in the SAM spectra are weakened due to the surface selection rule on metals (vide infra).71 Generally, the magnitude of the signal strengths in the IRRA spectra is typical of monolayers pointing out the formation of AOxCn SAMs on the gold surface under abstraction of the acetyl groups.


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Figure 10. IR spectra of an even-numbered AOxC2 system (left panel) and an odd-numbered AOxC3 system (right panel). From top to bottom, the traces show DFT calculated spectra of the isolated AOxCnSAc molecules, experimental spectra of the neat AOxCnSAc molecules recorded with an ATR unit, and experimental reflection-absorption spectra of the AOxCn SAMs on gold. For the spectra of the monolayers, absorbance scale bars are given. Assignments of the most prominent bands can be found in Table 3.

Table 3. Assigment of Bands in the IR Spectra to Vibrational Modes in the AOxCnS Species Wavenumbers are given in cm-1. For some modes, the direction of the transition dipole moment (TDM) is given. mode1

TDM2

 CO acetyl  COC oxazole  C2N oxazole  CC anthracenyl  CC anthracenyl  CH anthracenyl  CH anthracenyl  CH anthracenyl

\ \ || || || oop oop

DFT3 1745 1620 1572 1428 1424 1189 895 736

n=2 neat4 1684 1614 1565 1429 1413 1184 895 735

SAM5

DFT3

1614 1665 1433 1415 1187 895 738

1749 1622 1573 1430 1422 1189 893 734

n=3 neat4 1690 1617 1566 1429 1412 1184 895 735

SAM5 1617 1566 1433 1415 1188 888 738

1) : stretch, : in plane bending, : out of plane bending, C2: atom between N and O atom in oxazole ring 2) ||: parallel to main molecular axis (compare red line in Fig. 2), oop: (out of plane) perpendicular to the aromatic plane, \: in the aromatic plane but inclined (not parallel to main molecular axis) 3) calculated spectra 4) spectra of bulk material, recorded with ATR unit 5) IRRA spectra of SAMs The IR data also provide some qualitative information on the orientation of the molecules in the SAMs. In vicinity of metal surfaces, only the components of the TDMs that are parallel to the surface normal contribute to the signal strength of vibrational modes in IR spectra (surface selection rule on metals).71 From the relative attenuation of the out-of-plane bands in the SAM spectra (around 735 and 890 cm-1), it can be inferred that the anthracene moieties must be rather


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upright, i.e. the main molecular axis (red dashed line in Figure 2) is almost parallel to the surface normal. The || bands that belong to vibrational modes with TDMs parallel to the main axis (around 1188, 1415 and 1430 cm-1) consequently are not attenuated. The TDMs of the \ bands at around 1570 and 1620 cm-1 are parallel to the aromatic plane but do not directly point into the same direction as the || bands. Thus, they show only a slight attenuation in the SAM spectra.

4. DISCUSSION The objectives of this project were (i) to obtain araliphatic SAMs with pseudo-C2v-symmetric attachment of the aromatic backbone to the docking group and (ii) to investigate whether such SAMs exhibit odd-even effects, in contrast to the analogous monolayers with the non-symmetric attachment33. In order to ‘symmetrize’ the araliphatic anthracene-based SAM precursors, a fivemembered oxazole ring was introduced in the molecular structure. As no literature exists for the synthesis of those compounds, a route via ring-closure reaction between a Pinner salt and OTMS-activated 2-amino-3-hydroxyanthracene was established. Hitherto, no such reaction has been reported, therefore a new route to alkyl-substituted areneoxazoles could be demonstrated. The annellation of the oxazole ring to the anthracene system did somewhat lower the band gap making these molecules and SAMs good candidates for applications in semiconducting devices. For the fabrication of the AOxCn films, a standard preparation protocol could be utilized despite the presence of the acetyl protecting groups, demonstrating again that thioacetates are viable precursors for thiolate monolayers. This notion was supported by S 2p XPS data, which show the typical doublets for chemical bound thiolates, with only trace signals of presumably atomic sulfur, which is also characteristic of thiolate monolayers.39,57 Already at these spectra a


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certain odd-even effect becomes visible, as the SAMs with an even number of the methylene groups shower higher signals for the latter species. As ellipsometric and XPS data demonstrate, thickness of the SAMs generally increases with increasing length of the alkyl linker, but in a staircase-like manner. The derivatives with an odd number of methylene groups consistently show a higher film thickness than the ones with an even number. This goes in line with the expected odd-even effect, which has been reported for other systems with attachment of the alkyl chain along the long axis of an otherwise rigid molecular system.17-28 As mentioned in section 1, within this concept, assuming a bond angle of the substrateSC "joint" of ~112° at Au(111) and an all-trans conformation of the alkyl linker, an even parity is expected to lead to a more tilted aromatic part with higher steric demand and lower packing densities. Subsequently, an odd parity should result in less tilted aromatic heads groups and higher packings densities. Exactly this behavior was observed for the AOxCn SAMs. As consistently shown by IRRAS and NEXAFS spectroscopy, the molecular orientation in these monolayers indeed show pronounced odd-even variation, with smaller inclination for n = odd and larger for n = even. Whereas the IRRAS data only allow a qualitative conclusion, the evaluation of the NEXAFS data makes possible an estimate for the average tilt angle of the anthraceneoxazole moieties in the AOxCn SAMs, viz. ~15° for n = odd (the average value for n = 3 and 5) and 25-26° for n = even. The lower values of the tilt angle suggest a higher effective thickness and a higher packing density in the monolayers. Indeed, the effective thicknesses of the AOxCn SAMs, determined on the basis of both ellipsometry and XPS data (see Figures 3 and 5, respectively), consistently exhibited a characteristic zig-zag behavior, with higher values for n = odd and lower values for n = even rather than a continuous, linear growth of the effective thicknesses with increasing n. The packing density of the AOxC3 and AOxC5 SAMs was found


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to be close to that of non-substituted alkane monolayers (4.63  1014 molecules/cm2) and by ~20% higher than that of the AOxC2, AOxC4, and AOxC6 monolayers (3.7  1014 molecules/cm2, which is the average value for these three SAMs; see Table 1). This basically means that the re-introduction of the possibility to attach an alkyl linker along the main axis of the aromatic system (see Figure 1) regains the opportunity for a fine-tuning of the structural quality of the respective monolayer system.

5. CONCLUSIONS By annellation of an oxazole ring to the anthracene system, we could introduce an attachment site for alkyl chains along the long axis of the aromatic system. Using aliphatic linkers of variable length, we could demonstrate that this strategy (re-)introduces the structural odd-even effect that became lost in the previous work, where the linkers were attached to the 2-position of the unmodified anthracene system. All the SAMs of the series were well-defined and attached to the Au(111) substrates via thiolate anchoring groups, although the respective thioacetates were employed as starting materials. The SAMs were studied by a combination of several complementary experimental techniques, which provided reliable and consistent results. The most important finding are distinct odd-effects, with clear and consistent correlation of the molecular orientation (determined by NEXAFS spectroscopy and supported by IRRAS), effective thickness (from ellipsometry and XPS), and packing density (from XPS) with the length of the aliphatic linker in the target molecules, with the parity of n being the decisive parameter. This symmetric anchoring of the anthraceneoxazole backbones and the respective odd-even effects allow an additional control over the packing and orientation of the anthracene moieties in the monomolecular assembly. Significantly, with a band gap of these molecules of


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about 3.0 eV, which is slightly lower than that of anthracene, these monolayers might become relevant for application in organic and molecular electronics, as low band gaps have been reported to be advantageous not only for field-effect transistors, but also for SAM-based photovoltaics,72 sensors,73 and charge-carrier injection layers.74

ASSOCIATED CONTENT SUPPORTING INFORMATION The supporting information is available free of charge on the ACS Publication website at DOI: 1H

and 13C NMR spectra of all title compounds AOxCnSAc (n = 2-6), the solid state structure of

2-amino-3-hydroxyanthracene, details on the DFT calculation of vibrational spectra of isolated molecules and the determination of optical band gap of AntOxC2SAc. The X-ray crystallographic data of 2-amino-3-hydroxyanthracene in CIF format (CCDC 1920292) are provided in a separate file.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.Z.) *E-mail: [email protected] (A.T.)

ORCID Michael Zharnikov: 0000-0002-3708-7571 Andreas Terfort: 0000-0003-2369-5151 Michael Bolte: 0000-0001-5296-6251


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Author Contributions #

C.P. and E.S. have provided equivalent contributions.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT E.S. and M.Z thank the Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime at BESSY II, A. Nefedov and Ch. Wöll for the technical cooperation during the experiments there, and T. Wächter for the assistance during some of these experiments. This work has been supported financially by the Beilstein-Institut, Frankfurt/Main, Germany, within the research collaboration NanoBiC (C.P. and A.T.), and by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG), within the grant ZH 63/22-1 (E.S. and M.Z.). A.A. acknowledges the financial support by the DAAD-Aceh Scholarship of Excellence.


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Reestablishing OddâEven Effects in Anthracene-Derived Monolayers

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