Compensation of the OddâEven Effects in Araliphatic Self-Assembled

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Compensation of the Odd-Even Effects in Araliphatic Self-Assembled Monolayers by Nonsymmetric Attachment of the Aromatic Part John Dauselt,† Jianli Zhao,‡ Martin Kind,† Robert Binder,† Asif Bashir,§ Andreas Terfort,*,† and Michael Zharnikov*,‡ †

Institut f€ur Anorganische und Analytische Chemie, Universit€at Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany Angewandte Physikalische Chemie, Universit€at Heidelberg, 69120 Heidelberg, Germany § Max-Planck-Institut f€ur Eisenforschung GmbH, Max-Planck-Strasse 1, 40237 D€usseldorf, Germany ‡

bS Supporting Information ABSTRACT: The integrity, chemical identity, packing density, and molecular orientation in SAMs formed by anthracene-substituted alkanethiols (Ant-n) with a variable number of methylene groups (n) in the aliphatic linker on Au(111) and Ag(111) substrates were studied by a combination of several complementary experimental techniques. The Ant-n molecules were found to form well-defined and highly ordered SAMs on these substrates, with very small inclination of the anthracene backbone. In addition, the Ant-n SAMs exhibited odd-even effects, that is, dependence of the molecular orientation and packing density on the length of the aliphatic linker, with the parity of n being the decisive parameter. While the direction (sign) of this odd-even behavior on gold and silver is the same as for SAMs of biphenyl- and terphenyl-substituted alkanethiolates and -alkaneselenolates, suggesting a common reason behind this phenomenon, the presence of a strong substrate-headgroup-C bending potential, the extent of the odd-even effects in the Ant-n films is noticeably smaller than in the latter systems. This behavior can be explained by the additional rotational degree of freedom of the anthracene unit in the case of the Ant-n SAMs due to its nonsymmetrical attachment to the adjacent alkyl linker. This permits the adoption of an almost upright orientation of the aromatic part regardless of the parity of n. The above behavior is an instructive example that even seemingly subtle changes of the molecular structure may have noticeable implications for the properties of the resulting assemblies.

1. INTRODUCTION Self-assembled monolayers (SAMs) are 2D polycrystalline films of semirigid molecules that are chemically anchored to a substrate by a suitable headgroup and carrying, at the other end of the molecular chain, a specific tail group, which determines the chemical and physical properties of the entire system. For more than two decades, these films have attracted noticeable attention from the scientific community.1-7 The reasons for this attention are multiple. First, SAMs provide means to control properties of surfaces and interfaces by redefining their physical and chemical identity. Second, these films can serve as a basis for numerous nanotechnology applications,7 such as, for example, chemical lithography8,9 or biosensor fabrication. Third, because of their well-defined identity, SAMs represent important model systems to monitor the behavior of macromolecular and biological systems.10 Finally, SAMs as ordered 2D assemblies of different molecules are of interest on their own. An important issue in SAM-related research and applications is the control over the SAM structure. At the given substrate, this can be partly achieved by the selection of the most suitable headgroup and molecular spacer. In particular, for technologically important aromatic SAMs on coinage metal substrates, an essential improvement of the film quality can be achieved by the introduction of the selenium headgroup instead of the most r 2011 American Chemical Society

frequently used thiol (or disulfide) one (selenium has the same valence electron configuration as sulfur and is its neighbor in the 16th column of the periodic table).11-13 The former group is characterized by a comparably small corrugation of the headgroup-substrate binding potential that decouples to some extent the optimal arrangement of the aromatic matrix and the structural template provided by the substrate.11-13 In contrast, such a decoupling does not occur for the thiol headgroup, resulting in structural polymorphism, formation of small domains, and appearance of dislocations and other defects.14-16 An alternative way to decouple the aromatic part and the structural template provided by the substrate is the introduction of a short alkyl linker between the aromatic backbone and the headgroup, transforming the molecule in substituted alkanethiol (AT) or alkaneselenol (ASe).12,16-30 Additional structural flexibility associated with such a modification allows one to form SAMs with very high packing density and orientational order.12,17-19,21,22,24,26,27,31 Yet what is probably even more important, this modification provides a practical tool to control the packing density and the molecular orientation in the aromatic SAMs. This is possible due to linker-mediated “transfer” of the Received: December 14, 2010 Published: January 21, 2011 2841

dx.doi.org/10.1021/jp1118647 | J. Phys. Chem. C 2011, 115, 2841–2854

The Journal of Physical Chemistry C

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Figure 1. Schematic representation of the target molecules; n signifies the number of methylene groups in the alkane chain. The two axes A and B are used in the discussion.

predetermined molecular orientation, given by the so-called bending potential, associated with the substrate-headgroupCR “joint”, to the aromatic matrix. The respective contribution is that strong that it can prevent a dense, thermodynamically optimal (from the viewpoint of the aromatic part) molecular packing. Whether it takes place or not depends on the optimal geometry of the substrate-headgroup-CR joint (which itself is a function of the substrate metal) and the parity of the number of methylene groups (n) in the linker.17,18,22,25,28,30 At the given optimal binding angle at the SAM-substrate interface, which is close to ∼112° and ∼180° for Au(111) and Ag(111),17,32 respectively, this parity predefines the orientation of the CH2-CH2 bond adjacent to the aromatic moiety, affecting in the odd-even way the packing density and orientation of these moieties. In particular, a more tilted orientation for the last CH2-CH2 segment of the alkane chain occurs at n = odd for Au(111) and n = even for Ag(111). This results in a more upright orientation of the adjacent aromatic moieties (a smaller tilt angle) and, consequently, in a denser molecular packing. Similarly, for an even number of the CH2 units in the SAMs on Au(111) or for an odd number of these entities in the case of Ag(111), a less tilted orientation of the last CH2-CH2 segment of the alkane chain is predicted, resulting in a larger tilt of the adjacent aromatic moieties and thus in a reduced density. The above odd-even effects have so far been observed in a variety of systems, with the aromatic moiety being biphenyl,12,17-19,21,26,27 thiophene,25 p-terphenyl,22,24 or perfluorinated terphenyl.31 As an extension and continuation of this activity, we decided to check and monitor possible odd-even behavior for another aromatic entity, anthracene, combining it with AT chains (headgroup þ linker) of variable length. Our further interest was a possible improvement of the quality of the anthracene-based SAMs.13,15,33,34 Because of the specific transport properties of anthracene, these films can be considered as potential 2D organic semiconductors and, what is probably even more important, serve as surfactants for the growth of high-quality 3D films of other organic semiconductors.35-37 In the following, we will present and shortly discuss the results obtained using different experimental techniques. A thorough discussion and interpretation is given in section 3 followed by a summary in section 4. The experimental details can be found in section 5.

2. RESULTS 2.1. XPS and HRXPS. XPS and HRXPS give information about identity, integrity, chemical composition, and effective thickness of the target films. The S 2p and C 1s HRXPS spectra of Ant2, Ant3, and Ant12 on Au are shown in Figures 2 and 3, respectively, representative for the entire series of the target films both on Au and on Ag. The S 2p spectra of the target SAMs (see Figure 2) are dominated by a characteristic doublet at a binding

Figure 2. S 2p HRXPS spectra of Ant2, Ant3, and Ant12 on Au (O) acquired at a photon energy of 350 eV, along with the corresponding fits (solid black lines) by the individual S 2p3/2,1/2 doublets related to thiolate and unbound S (solid gray lines) and a background (dashed line). The spectra are representative for the entire series.

Figure 3. C 1s HRXPS spectra of Ant3, Ant4, and Ant12 on Au acquired at a photon energy of 350 eV. The spectra are representative for the entire series. The small intensity difference between the individual systems is related to a saturation of the emission signal due to a strong self-attenuation at the given kinetic energy (ca. 62 eV).

energy (BE) of 162.0-162.05 eV (S 2p3/2) for Au and 161.90161.95 eV for Ag. This doublet can be clearly assigned to thiolate species bonded to the surfaces of gold or silver.38-41 The fwhm of the S 2p3/2 and S 2p1/2 components is quite similar for all the targets films; it varies slightly in a nonsystematic way in a range of 2842

dx.doi.org/10.1021/jp1118647 |J. Phys. Chem. C 2011, 115, 2841–2854


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Figure 4. BE position (S 2p3/2) of the thiolate-related S 2p doublet for the target films on Au (b) and Ag (O).

0.61-0.65 eV as a function of n, which is noticeably higher than the value (∼0.45-0.50 eV) expected for the case of equivalent adsorption sites for all SAM constituents.41,42 The high fwhm values in the present case can be associated with a superposition of several S 2p signals with slightly different BEs, which implies the presence of several different adsorption sites in the target films. The BE position of the thiolate-related doublet in the target films exhibited a systematic odd-even variation as a function of n for 1 e n e 6, as is demonstrated in Figure 4. In the given n range, the direction of the odd-even modulation on Au is opposite to that on Ag. The behavior at 10 e n e 12 is not systematic in the sense of the odd-even effects. Considering that the odd-even modulation of the BE position is not accompanied by the analogous behavior of the S 2p3/2 and S 2p1/2 fwhm, this modulation cannot be associated with a redistribution of the adsorption sites but rather with an appearance of additional strain on the substrate-S-C joint at either even (Au) or odd (Ag) n. A similar phenomenon has been observed for SAMs of biphenylsubstituted alkanethiolates (BPn) on (111) Au and Ag.42 In addition to the thiolate-related doublet, a further, low intense doublet at a BE of ∼161.0 eV (S 2p3/2) was observed for some of the target films, Ant1/Ag, Ant2/Ag, and Ant3/Ag. It had low intensity (few % of the total value) for the two latter films but was quite intense for the former SAM. We ascribe this feature to a thiolate-type bound sulfur with a different binding chemistry and/or geometry as compared to the “standard” thiolate-type bond observed in thiol-derived SAMs on coinage metal substrates (see refs 39,43,44 and discussion in refs 45 and 46). This doublet is a general phenomenon in thiolate-related SAMs and can be clearly recognized in many cases when performing synchrotron-based HRXPS.41,42,45,47 Further, there are weak traces of an additional doublet at ∼163.5 eV (S 2p3/2) for some of the target films, Ant3/Au (see Figure 2), Ant4/Au, and Ant6/ Au. This doublet is associated with a small amount of physisorbed molecules (caught presumably in the hydrocarbon matrix), which we have not succeeded to remove for few of the investigated SAMs by the washing step. Note that no systematic behavior in this regard was observed. The C 1s HRXPS spectra of the target SAMs (see Figure 3) exhibit a single emission peak at a BE of 284.05-284.3 eV assigned to the molecular backbone. The BE position of this emission exhibits weak odd-even variation overimposed on a continuous increase with increasing molecular length (not shown).

Figure 5. Effective thickness of the target films on Au (b) and Ag (O) derived on the basis of the XPS (a) and ellipsometry (b) data, along with the theoretical thickness of these films (c) assuming the vertical orientation of the SAM constituents. The accuracy of the thickness values is 1-2 Å, apart from a possible systematic error.

A similar behavior was observed previously for AT SAMs and attributed to a reduced screening of the photoemission hole by the substrate electrons with increasing separation between this hole (in the topmost part of the films probed at the given PE) and the substrate.48 At a PE of 350 eV (highest energy resolution), the fwhm of the emission peak is 0.73-0.76 eV for the films on Au and 0.76-0.81 eV for the films on Ag. Such values are typical for the well-defined and -ordered SAMs of long-chain alkanethiolates (ATs) on (111) Au and Ag,48 which implies a high structural homogeneity of the hydrocarbon matrix in the target films. Interestingly, no splitting of the emission into the component characteristic of the aliphatic and anthracene parts of the molecular backbone was observed, similar to the cases of biphenyl-42 and terphenyl-substituted22 ATs. This behavior is presumably related to the limited applicability of the chemical shift concept to AT SAMs.49-51 Apart from the above analysis of the spectra, we used the XPS and HRXPS data to monitor the effective thickness of the target films. The respective values, derived on the basis of the XPS-data, are presented in Figure 5, along with the analogous ellipsometryderived values (see next section) and the “theoretical” thickness of the films assuming the vertical orientation of the target 2843



Figure 6. Normalized S2p/Au4f intensity ratios for the target films on Au (based on the data for a PE of 350 eV). The ratios were normalized to the analogous parameter for DDT/Au.

molecules. Whereas the latter estimate suggests the expected monotonous increase of the film thickness with increasing n, an additional odd-even modulation of this growth behavior at a level of (3-4% is observed at 1 e n e 6, which for Ag is opposite to that for Au. For 10 e n e 12, the odd-even modulation persists to some extent, but the inverse character of this modulation for Au and Ag is not observed any more. Interestingly, the absolute values of the XPS-derived effective thickness are comparable or even slightly higher (an effect of probable systematic error) than the “theoretical” thickness for the film of the vertically oriented molecules, which assumes a dense molecular packing. The packing density of the target films on Au cannot only be monitored by the effective thickness, but also by using the S 2p/ Au 4f intensity ratio. This ratio can be normalized to the analogous parameter for a well-defined reference film, DDT/ Au, which is characterized by a molecular density of 4.63 1014 cm-2 associated with an area per molecule of 21.6 Å2.4 The normalized S 2p/Au 4f intensity ratios for the target films on Au are presented in Figure 5a. Whereas the absolute values of these parameters can deviate to some extent from the real packing density ratios, especially at 10 e n e 12 (much thicker than the reference DDT/Au), due to a particular strong attenuation of the photoemission signals and possible material dependence of the attenuation length at the given PE (350 eV), their relative variation within a continuous thickness range (1 e n e 6 or 10 e n e 12; see below) is fully representative for the packing density variation as a function of n. As seen in Figure 6, the molecular density exhibits pronounced odd-even behavior, which is opposite to the odd-even modulation of the effective thickness for the films on Au in Figure 5a. Presumably, the effect of attenuation of the S 2p signal by more dense film (n = odd) is stronger than the effect of the packing density, as was previously observed in the BPn SAMs on (111) Au and Ag.42 The somewhat lower values at 10 e n e 12 as compared to those at 1 e n e 6 can be related to the attenuation effects and do not necessarily manifest a lower packing density of the target films. 2.2. Ellipsometry. The effective thickness of the target films derived on the basis of the ellipsometry data is presented in Figure 5b. The observed behavior mimics that of the XPSderived thickness (cf., Figure 5a) in terms of both the absolute values and the relative variation. Once more, there is an expected monotonous increase of the film thickness with increasing n,

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along with an additional odd-even modulation at a level of few percents at 1 e n e 6, which for Ag is opposite to that for Au. For 10 e n e 12, the odd-even modulation persists to some extent, but the inverse character of this modulation for Au and Ag is not observed any more. Further, similar to the case of XPS, the absolute values of the ellipsometry-derived effective thickness are comparable to the “theoretical” thicknesses of the films of vertically oriented molecules, indicating dense molecular packing. 2.3. NEXAFS Spectroscopy. Sampling the electronic structure of unoccupied molecular orbitals, NEXAFS spectroscopy provides information about the chemical identity of the adsorbed film. Further, relying on the angular dependence of the transition matrix elements for resonant excitations,52 the average orientation of the constituents can be probed. A fingerprint of such an orientation is the linear dichroism, that is, the dependence of the absorption resonance intensity on the orientation of the electric field vector of the synchrotron light with respect to the molecular orbital of interest. An efficient way to monitor the linear dichroism is to plot the difference of the NEXAFS spectra acquired at normal (90°) and grazing (20°) angles of X-ray incidence. In contrast, a spectrum acquired at the so-called magic angle of X-ray incidence (∼55°) is not affected by any effects related to molecular orientation and gives only an information on the chemical identity of investigated samples.52 Carbon K-edge NEXAFS spectra of Ant2/Au, Ant3/Au, and Ant12/Au acquired at an X-ray incident angle of 55° are shown in Figure 7a representative of the entire series both on Au and on Ag. The spectra are dominated by the pronounced π1a* and π1b* resonances at ca. 284.5 and 285.9 eV with no pronounced relative intensity, form, or PE position variation as a function of n. Such double π* resonance structure is typical for polyacenes and characteristic of anthracene;53,54 it was previously observed for SAMs containing the latter functional unit.15,34 It originates from the splitting of the characteristic singular π1* resonance of polyp-phenylenes due to the chemical shift of the two symmetry independent carbon atoms of anthracene with strong influence of excitonic effects.53,54 Further, the spectra exhibit weaker R*/ C-S* resonances at 287.65 eV, π2* resonance at 289.65 eV, and several σ* resonances at 293.5, 300.4, and 305.8 eV, respectively (the assignments are performed in accordance with ref 34). At high n, the R*/C-S* resonance has a dominating R* character and is representative of the aliphatic part of the Ant-n molecules.55-58 The intensity of this resonance increases with increasing n. The σ1* resonance at 293.5 eV is presumably comprised of several individual features because the corresponding difference peak in the 90°-20° spectra (Figure 7b; see below) is located at 295.5 eV. Along with the above analysis of the NEXAFS spectra acquired at the magic angle, the linear dichroism effects in the target films were monitored. As mentioned above, a convenient way to follow the linear dichroism effects is to plot the difference between the NEXAFS spectra acquired at the normal and grazing incidence of the primary X-ray beam. Such difference curves (90°-20°) are presented in Figure 7b for Ant2/Au, Ant3/Au, and Ant12/Au, representative of the entire series both on Au and on Ag. The pronounced difference peaks at the PE positions of the characteristic absorption resonances suggest high orientational order in the target films. In addition, the signs of the observed difference peaks, the positive sign for the π*-like resonances and the negative sign for the σ*-like ones, imply an upright molecular orientation in the target films. 2844



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Figure 8. Average tilt angle of the LUMO-π* molecular orbital of the anthracene moiety in the target films on Au (b) and Ag (O) with respect to the surface normal.

Figure 7. (a) Carbon K-edge NEXAFS spectra of Ant2, Ant3, and Ant12 on Au acquired at an X-ray incident angle of 55° and (b) difference between the spectra of the above films acquired at X-ray incident angles of 90° and 20°. The most prominent absorption resonances are marked. See text for details.

The above qualitative considerations were complemented by a quantitative analysis of the entire set of the C K-edge NEXAFS spectra acquired at different X-ray incidence angles. For this analysis, we used the prominent π1a* and π1b* resonances (see Figure 7) related to the same LUMO-π* molecular orbital of the anthracene moiety.54 The joint intensity I of these resonances was monitored as a function of X-ray incidence angle Θ and evaluated according to the theoretical expression for a vectortype orbital:52 1 1 IðR , ΘÞ ¼ A P 1 þ ð3 cos2 Θ - 1Þð3 cos2 R - 1Þ 3 2 2 þ ð1 - PÞð1=2Þ sin R ð1Þ where A is a constant, P is the polarization factor of the synchrotron light, and R is the average tilt angle of the molecular orbital. The derived average tilt angles (R) of the LUMO-π* molecular orbital of the anthracene moiety in the target films with respect to the surface normal are compiled in Figure 8; the accuracy of these values is (2-3°. The values are quite high, suggesting, in accordance with the XPS and ellipsometry data, that the Ant-n molecules in the target films are only slightly inclined. Significantly, at 1 e n e 6, the tilt angles of the LUMO-π* orbital exhibit small (1-2°) but systematic oddeven variation, with higher values at odd n for Au and even n for Ag. For 10 e n e 12, the odd-even modulation persists to some extent, but does not correlate with those at 1 e n e 6.

Taking into account that the transition dipole moments (TDMs) for the C1s f π1,2* transitions are oriented perpendicular to the plane of the anthracene unit, the larger tilt of the LUMO-π* orbital suggests a smaller molecular inclination and the smaller tilt a larger molecular inclination. Note, however, that the exact values of the average tilt angle of the anthracene backbone (β) can only be obtained after taking into account the twist angle (γ) of this backbone with respect to the plane spanned by the surface normal and the molecular axis (γ = 0 corresponds to the TDMπ laying in this plane; see schematic drawing in ref 59). The relevant angles are related by the formula:18,34,59 cosðRÞ ¼ sinðβÞ cosðγÞ ð2Þ Because the twist angles for the entire series are not exactly known, the respective evaluation cannot be performed absolutely strictly. However, one can tentatively assume that the twist angles in Ant-n films are similar to one another and close to the value characteristic of the respective molecular crystals, that is, 26° (both þ26° and -26° twist occur, which is of no importance here because cos is an even function).60 Introducing the respective correction, we get average tilt angles of the anthracene backbones in the Ant-n films (1 e n e 6) on Au and Ag around 12.3° and 14.7°, respectively, with the analogous, but inverse odd-even variation as a function of n. This means smaller molecular inclination at odd n for Au and even n for Ag as compared to their counterparts with different parity. For 10 e n e 12, the average tilt angles of the anthracene backbones are even smaller than at 1 e n e 6, being around 10° and 12.3° for Au and Ag, respectively. Of course, there is no guarantee that the twist angles of the anthracene backbones do not vary along the series and are somewhat different for Au and Ag. This can affect the extent of the odd-even variation of the molecular inclination but does not change the principal result regarding the presence of the odd-even effects in molecular inclination. 2.4. IR Spectroscopy. IR spectroscopy is a powerful tool to identify molecular species chemisorbed on metal surfaces and to obtain information on their chemical state, their degree of order, and their orientation. For the latter, the TDMs of the respective vibrations have to be assigned using theoretical methods. Although we previously used DFT to determine peak frequencies and the respective TDMs,15,61,62 we figured it appropriate to reevaluate the functionals and basis sets for optimal match with the experimental data. Figure 9 shows several calculated spectra of an 2845



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isolated Ant4 molecule as compared to the experimental bulk spectrum of Ant4. This comparison of experimental and calculated spectra reveals that for all used functionals and basis sets, the energies of the CH stretching modes are slightly overestimated, while in the fingerprint region, the calculated wavenumber positions of many bands closely resemble the experimental ones (for the assignment of the bands, see Table 1 and, in more detail, Table S1 in the Supporting Information). The calculated intensities of the aromatic CH stretching modes are markedly stronger than in the experimental spectrum.

Figure 9. Comparison of theoretical IR spectra of Ant4 (calculated using different DFT functionals and different basis sets) with an experimental ATR spectrum of Ant4. On the basis of this comparison, a suitable combination (BP86/svp) was chosen for all further evaluations.

Moreover, in none of the calculated spectra are the fingerprint region band intensities in perfect accordance with the experimental data. Nevertheless, the most important bands can be related to the measured data and therefore used for an estimation of the orientation. Because the consistency between experimental and calculated data was best with BP86/svp, we decided to use this combination to calculate a whole set of spectra for Ant1 to Ant12. These calculations were used for assignment of the bands in the experimental ATR and IRRA spectra. For this, a full set of bulk and IRRA spectra of all anthracenesubstituted alkanethiols was recorded and evaluated. All spectra and tables with detailed assignments of the bands can be found in the Supporting Information (Figures S1-S4 and Tables S1S4). A summary of the assignments is given in Table 1. Figure 10 shows as an example a set of spectra for one anthracenesubstituted alkanethiol, Ant10. Comparison of bulk and monolayer spectra clearly indicates the formation of ordered Ant10 layers on gold and on silver. The absorbance signal intensities are in accordance with the assumption of monolayer formation. A careful comparison of the spectra on Au and Ag (Figures S3 and S4) reveals that the latter spectra are somewhat noisier and also have tendency to show artifacts, which leads us to believe that the SAMs on Au are of better quality (e.g., form larger domains) than the ones on Ag. Apart from the bands that are assigned to the aliphatic chain (particularly bands 2 and 3), the IRRA spectra in Figure 10 resemble data published for anthracenethiolate (AntS) SAMs on gold15 and anthraceneselenolate (Ant-Se) SAMs on gold.13 A striking difference between the bulk and the monolayer spectra is the almost entire absence of the aromatic out-of-plane bands in the IRRA spectra (bands 12-16), while the in-plane bands (most notably 4-6 and 10) are still visible and even look enhanced in intensity. The extinction of certain bands in IRRAS

Table 1. Assignment of the Most Intense Bands in the Calculated, Bulk, and Monolayer IR Spectra of the Anthracene-Substituted Alkanethiolsa band

TDM

calculation

ATR

IRRAS Au

IRRAS Ag

ν CH arom

3106-3110

3047-3053

3055-3057

3055-3057

νas CH2

2993-3044

2916-2937

2918-2933

2918-2927

3 4

νs CH2 ν CC arom δ CH arom

ip

2941-2978 1634-1638

2845-2854 1626-1633

2848-2860 1626-1633

2850-2856 1626-1633

5

ν CC arom δ CH arom

ip A

1541-1559

1529-1535

1531-1535

1531-1535

6


ip B

1451-1478

1448-1470

1458-1460

1458-1460

7


ip

1291-1308

1295-1311

8


ip

1259-1273

1269-1273

1257-1282

1257-1282

9

γ CH2

1184-1222

1186-1201

1192-1213

1190-1209

1158-1174

1151-1174

1165-1168

1165-1168

949-958 906-919

949-960 906-912

) )

1 2


ip B

11 12

δ CH arom τ CH aliph δ CH arom

op

)

10

13

δ CH arom

op

889-893

887-895

14

δ CH arom

op

862-870

864-874

15

δ CH arom

op

801-817

795-814

16

δ CH arom

op

735-737

733-741

737-741

737-739

17

δrock CH2

719-727

710-721

715-723

704-721

)

For Ant1-Ant12, ranges of band positions of the respective modes are given in cm-1. Orientations of the transition dipole moment (TDM) of some modes are listed in the third column. Explanation of the abbreviations and symbols: vibrational modes: ν, stretching mode; as, asymmetric; s, symmetric; arom, aromatic; δ, bending mode; γ, wagging mode; τ, torsion mode; aliph, aliphatic; δrock, rocking mode. Orientation of the TDM: op, TDM is mainly or fully perpendicular to the plane of the aromatic rings; ip, TDM is fully or almost in plane of the aromatic rings; ip A, TDMs of all molecules are mainly or fully parallel to the A axis as depicted in Figure 1; ip B, TDMs of all molecules are mainly or fully parallel to the B axis as depicted in Figure 1. Where no information on the direction is given, the directions of the TDMs are not uniform throughout Ant1-Ant12, and/or the bands consist of more than one vibrational mode with different directions of the TDMs. )

a

assignment

2846



Figure 10. Spectra of Ant10 as a typical example for the IR data of the anthracene-substituted alkanethiols. (a) Bulk spectrum recorded with an ATR-unit, (b) theoretically calculated spectrum of the isolated molecule, (c) IRRAS of a thiolate monolayer on Au(111), and (d) IRRAS of a thiolate monolayer on Ag(111). The numbers in panel a indicate the assignment of some vibrational modes. See text for further explanation.

can be rationalized by the surface selection rules on metal surfaces, which state that intensities of IR bands with a TDM oriented parallel to a metal surface are strongly attenuated.63,64 Thereof, it can be concluded that the anthracene moieties of the chemisorbed thiolate molecules must be oriented perpendicular to the substrate surface. The strong in-plane bands 5, 6, and 10 have TDMs oriented parallel or almost parallel to the anthracene framework (i.e., along the directions A and B, respectively, as depicted in Figure 1). The relatively strong signals of these bands give rise to the assumption that the anthracene moieties are quite upright. Unfortunately, this surmise cannot be further confirmed, because in the spectra of the anthracene-substituted alkanethiols there are no sufficiently intense in-plane bands with TDMs perpendicular to directions A or B; hence, a comparison of enhancement of bands 5, 6, and 10 with such bands in IRRAS is not possible. The IRRAS data can be used to deduce information not only on the orientation, but also on the degree of order of the chemisorbed thiolate molecules, especially regarding the aliphatic chain. The band positions of the symmetric and asymmetric CH2 stretching modes are dependent on the number of gauche defects in the aliphatic chain.65 In particular, the band positions of the asymmetric stretching modes as obtained from the IRRA spectra of the Ant-n monolayers show the same tendency for the SAMs on both gold and silver: the higher is the number n of methylene units, the lower is the band position (starting from values around 2930 cm-1 for low n and ending with 2918 cm-1 for n = 12; the exact values can be found in the Supporting Information, Tables S3 and S4). From this finding, it can be deduced that the degree of order in the alkane linker increases with the chain length; that is, the amount of gauche defects decreases and the chains tend to be all-trans. The tendency of increasing order with increasing chain length is a common phenomenon that has often been observed for alkane SAMs and is part of the self-assembly process of alkane chains.4,7,65 Obviously, the anthracene group in the investigated SAMs does not inhibit this process. Besides the positions of the CH stretching bands, their intensities are also meaningful. Because the number of aromatic

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Figure 11. Relative intensities of the aliphatic and aromatic CH stretching modes in the IRRA spectra of Ant1-Ant12 on Au(111) and Ag(111). Observe that the aromatic signals basically stay constant while the aliphatic signals increase steadily.

C-atoms is equal for all investigated anthracene-substituted alkanethiols, one would expect constant aromatic CH stretching signal intensities as far as all Ant-n molecules form monolayers of comparable density and orientation. On the other hand, the intensity of the aliphatic CH stretching bands should increase with increasing n. In Figure 11, relative integrated intensities of the aromatic and aliphatic CH stretching modes of the IRRA spectra of the Ant-n SAMs on Au and Ag are plotted against n. In general, the data show the expected behavior, although the aliphatic intensities are substantially scattered. In particular, on silver, the intensity of these signals for Ant2, Ant3, and especially Ant6 is too high as compared to the ones on Au, a telltale for the difficulties of proper monolayer formation on Ag. This might be due to aliphatic contaminations, which are more likely in shortchained SAMs. Nevertheless, the signals of the aromatic C-H stretching vibrations remain almost constant for all the films showing the formation of densely packed monolayers. A possible way to reveal odd-even effects in the IRRAS data is to plot the intensity of a band that is mainly due to the vibration of the anthracene unit against the number of the CH2 units. An adequate candidate for such a representation is band 10, the TDM of which is parallel to axis B as shown in Figure 1. Depending on the Au-S-C and Ag-S-C angles and the number of CH2 units, one would expect an alternating tilt angle of the TDM and thus an alternating intensity of band 10. As can be seen in Figure 12, the intensity of band 10 does not exhibit a pronounced dependence on the parity of n. A slight odd-even effect becomes visible for the layers on Au, while for Ag the data again are quite scattered. This goes in line with the previous observations regarding a low extent of the odd-even effects in this new class of monomolecular films. 2.5. Contact Angle Goniometry. Static water contact angles (θst) of the target films on Au and Ag are presented in Figure 13. The absolute values of these angles are comparable with the ones of other well-ordered aromatic systems22 and exceed the ones for less-ordered systems such as 4-mercaptobiphenyl on Au (advancing water contact angle θadv = 73°),66 suggesting quite a dense molecular packing in the target systems. On closer inspection, the θst values of the target films are somewhat lower than the analogous values for SAMs of terphenyl-substituted ATs on Au and Ag, which exhibit advancing (θadv) and receding (θrec) water contact angles of ca. 90° and 79°, respectively,22 because 2847



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properties of the SAM-ambient interface of the anthracenebased SAMs are affected much stronger by the exact orientation and packing density of the terminal rings as compared to their oligophenyl-based counterparts. We suggest that this again may be explained by the polarizability of the anthracene moiety, with the exact vector of maximal charge shiftability being decisive for the surface energy. Thus, even very small changes in the inclination can cause the relatively large odd-even effect for the contact angles as observed here.

Figure 12. Relative intensities of mode 10 plotted versus the number of CH2 units. See the text for an explanation.

Figure 13. Static water contact angles of the target films on Au (b) and Ag (O).

typically θst can be considered closer to θadv than to θrec (for a detailed discussion, see ref 67). The reason for this presumably is the better polarizability of the anthracene moieties due to their large, uninterrupted π-systems as compared to the oligophenyl systems, because the surface energy (and thus the contact angles) does not only depend on permanent dipole moments but on dispersive forces, too. In fact, in the systems under discussion (only aromatic and aliphatic parts without any polar groups), the latter contribution will be the major one, making polarizability a major factor. An interesting observation is that at 1 e n e 6, the static contact angles, θst, of the target films exhibit a systematic odd-even variation with higher values at odd n for the films on Au and even n for the films on Ag as compared to their counterparts with different parity. For 10 e n e 12, the odd-even modulation persists to some extent, but does not correlate completely with that at 1 e n e 6 with respect to the parity of n. The extent of the odd-even variation of θst is 2-4°, which is much larger than that for the SAMs of terphenylsubstituted ATs (0.5-1°)22 and comparable with those for the SAMs of methyl-biphenyl-substituted ATs (3-5°)18 and selenolates (2-6°).30 Taking into account the much smaller extent of the odd-even variation of the packing density and molecular inclination in the Ant-n SAMs as compared to all the above systems, these relations are quite surprising. Presumably, the

3. DISCUSSION The results of the HRXPS, XPS, ellipsometry, NEXAFS spectroscopy, IRRAS, and contact angle goniometry measurements are quite consistent, suggesting the formation of highquality Ant-n SAMs on both Au(111) and Ag(111). The NEXAFS and IRRA spectra exhibit characteristic absorption resonances (NEXAFS) and absorption bands (IRRAS) of anthracene, implying adsorption and assembly of the target molecules on both gold and silver substrates. According to the S 2p XP spectra, either all or the vast majority of these molecules are bound to the surface in the thiolate-like fashion. In accordance with this adsorption mode and as follows from the NEXAFS spectroscopy and IRRAS data, the aromatic parts in the SAMs have an upright orientation, with a surprisingly small molecular inclination given by an average tilt angle of 10-15°. Such an orientation can be associated with dense molecular packing, which was complementarily evidenced by XPS, HRXPS, IRRAS, ellipsometry, and contact angle goniometry. In addition, the IR spectra also reveal an enhancing order of the alkane chains with increasing n as becomes visible by the position of the CH2 stretch modes, indicating that the alignment of the alkane chains goes cooperative with the packing of the anthracene units (or at least is not disturbed by it). Significantly, the molecular inclination of the anthracene units in the Ant-n SAMs is noticeably smaller than in the analogous films of anthracenethiolate (Ant-S)15,34 or anthraceneselenolate (Ant-Se),13,33 in which no alkyl linker between the anthracene moiety and the headgroup is used. In particular, for Ant-S/Au, average tilt angles of 23°34 and 43°15 were reported on the basis of the NEXAFS data, while for Ant-S/Ag the reported value was 14°.34 The analogous value for Ant-Se/Au is 33°.13,33 Thus, at least for Au(111), the introduction of the aliphatic linker between the anthracene moiety and headgroup resulted in significant reduction in the molecular inclination and, presumably, higher molecular density. Indeed, the area per molecule for Ant-S/Au was reported to be 28.7 Å2.15,34 Taking into account the analogous value of 21.6 Å2 for AT SAMs on Au(111)4 and referring to Figure 6, the area per molecule in the Ant-n films of this study can be tentatively estimated at 24 Å2. Apart from the high quality of the Ant-n SAMs, at 1 e n e 6 we observe small but systematic odd-even effects with respect to the parity of n, which is the number of the methylene units in the aliphatic linker of the Ant-n molecules. As follows from the NEXAFS and IRRAS data, the Ant-n molecules on Au have a smaller molecular inclination at n = odd and a larger molecular inclination at n = even. On Ag, the opposite situation, that is, a smaller molecular inclination at n = even and larger at n = odd, is observed. The above odd-even behavior is accompanied by the respective changes in the packing density of the SAM constituents monitored directly by XPS and ellipsometry and indirectly by water contact angle goniometry: a higher packing density is 2848


The Journal of Physical Chemistry C observed on Au at n = odd and a lower packing density at n = even. The opposite situation occurs on Ag, with a higher packing density at n = even and a lower one at n = odd. The direction of the odd-even effects (i.e., the relation between the system behavior and the parity of n) in the Ant-n SAMs on Au(111) and Ag(111) correlates precisely with the odd-even behavior observed previously in the analogous films of biphenyl-, thiophene-, and terphenyl-substituted ATs (BPn, Th-n, and TPn, respectively) and biphenyl-substituted alkaneselenolates (BPnSe),12,17-19,21-25,28,30 manifesting a generality of the odd-even effects and suggesting a common reason behind these effects in substituted AT SAMs. As mentioned in section 1, this reason is presumably a quite “rigid” bending potential associated with the substrate-headgroup-CR “joint”. This potential favors a certain bending of this joint (different for Au(111) and Ag(111) substrates), predetermining the molecular orientation, which, as far as the adjacent alkyl linker has some rigidity, can be transferred to the aromatic substituent, such as biphenyl, terphenyl, or anthracene, in two different ways, depending on the parity of the alkyl linker length (n). Whereas these moieties tend to form a thermodynamically optimal structural arrangement, associated usually with dense molecular packing and small inclination, the respective orientation can either fit to or deviate from the orientation predetermined by the bending potential and the parity of n. In the former case, the bending potential enters cooperatively into the balance of the structure-building interactions, and the resulting structure is characterized by dense molecular packing and small inclination. In most cases, the latter parameters of the substituted ATs SAMs are even superior as compared to the films in which the respective substitution (e.g., biphenyl, terphenyl, or anthracene) is directly attached to the headgroup. Presumably, as mentioned in section 1, the introduction of a short aliphatic linker between the headgroup and the substitution moiety decouples to some extent the moiety lattice from the structural template provided by the substrate,68 which gives these moieties an additional freedom to achieve optimal structural arrangement. In the case that the optimal molecular arrangement deviates from the predetermined one, the bending potential enters competitively into the balance of the structure-building interactions, and the resulting structure depends on the relative strength of the individual contributions. In the case of biphenyl and even terphenyl substitutions, the contribution of the bending potential prevails over the intermolecular interaction, which results in a significant extent of the odd-even effects. In particular, the packing densities of the BPn, BPnSe, and TPn films on Au and Ag exhibit an odd-even variation (peak-topeak) of 10-15%,17 12-15%,30 and 10-15%,22 respectively. In agreement, the average tilt angles of the biphenyl and terphenyl moieties in these films reveal an odd-even variation (peak-topeak) of 12-17° for BPn,18 6-14% for BPnSe,30 and 7-14° for TPn.22 These linkers exhibited close to all-trans conformation in the case of cooperative balance between the bending potential and structure-building interactions, but were partially disordered in the case of the competitive balance. The structure of the films then represents a compromise between the competitive interactions, associated with the presence of significant stress, which is released to some extent through a partial deviation from the optimal value of the substrate-headgroup-CR angle and partial disordering of the aliphatic linkers.30 As seen from the experimental data, the extent of odd-even effects in the Ant-n SAMs is noticeably smaller than that in the

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analogous films with biphenyl and terphenyl substitutions. The odd-even variation of the packing density is only 3-6% (see sections 2.1 and 2.2), while the respective variation of the molecular inclination is only 1-2° (see section 2.3). Note that both these effects are within the experimental error and only their systematic character over the experimental data by the different techniques for the entire Ant-n series at 1 e n e 6 suggests that they are real. This means that either the balance of the bending potential and the structure-building interactions in the Ant-n SAMs shifts in favor of the latter or the bending potential enters differently into this balance. Several reasons can be responsible for such a behavior. First, whereas dihedral rotation in biphenyl or terphenyl derivatives around the bond (or bonds) between the benzene rings is possible, anthracene has a fixed planar conformation. Note, however, that the dihedral rotation in biphenyl or terphenyl derivatives only occurs in the molecular state with characteristic torsion angles of 39° (biphenyl69,70) and 37.5° (terphenyl71). In the bulk crystals, these moieties are almost planar due to the intermolecular interaction.69-71 Because the packing densities in the biphenyl- and terphenyl-based SAMs are close to that in the bulk crystals, such a planar conformation can be expected in the respective SAMs as well, at least when the cooperative balance of the bending potential and intermolecular interaction results in densely packed SAMs. There are also several indirect experimental indications that the conformation is indeed planar,59,72,73 even in the case of heteroaromatic systems.62 In the case of the competitive balance, associated with a looser molecular packing, deviations from the planar confirmation are possible,74 but, even if this is the case, they are rather not the reason but only a possible consequence of the odd-even effects. So, it is quite unlikely that the molecular torsion is the reason behind the different extent of the odd-even effects in the biphenyl-/terphenyl- and anthracene-substituted AT SAMs. A second hypothesis is that the optimal molecular lattices for oligophenyls and anthracene are so different that the intermolecular interaction is much stronger in the latter case. According to the crystallographic data,60,69,71 the 3D lattices for the above molecules have the same space group (P2a1/a) with, however, somewhat different unit cell parameters. Extracting a planeconfined, “free” molecular layer from these structures, which should represent a SAM-like film with the substrate-SAM interaction equal to zero, one gets herringbone arrangements with tilt angles of 35°, 20°, and 12-13° and twist angles of 26°, 32°, and 32° for anthracene, biphenyl, and terphenyl, respectively. The in-plane intermolecular spacing is 5.16, 4.93, and 4.93 Å for anthracene, biphenyl, and terphenyl, respectively, which is close (but not identical) to the structural templates provided by the Au(111) and Ag(111) substrates. The chain-chain spacing, measured perpendicular to the chain axis, is 4.23, 4.63, and 4.83 Å for anthracene, biphenyl, and terphenyl, respectively. The introduction of the strongly interacting substrate as a well-defined structural template will affect the molecular packing and exact molecular structure, however maintaining the herringbone arrangement with an angle close to the bulk value, as has been recently shown for anthraceneselenolate SAMs on Au(111).13,33 Even though the selenolate headgroup provides better conditions for the formation of the optimal molecular lattice of an aromatic SAMs than the thiolate headgroup,11,13 the introduction of the short alkyl linker in the latter case results in a similar effect. Considering the different values of molecular tilt and 2849



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well as a very weakly pronounced odd-even effect. That an odd-even effect can be observed at all is presumably due to a small deviation from the surface normal due to the twist angles of the anthracene units, which are adopted to permit the formation of the preferred herringbone pattern.

Figure 14. Because of the substitution in the 2-position, the anthracene units can adopt a multitude of positions at the given predetermined orientation even if a very rigid conformation of the alkyl chain is assumed, as signified by the cones (left). The flexibility permits the adoption of almost upright orientations of the aromatic part independently from the number of methylene groups, which blurs out the odd-even effect, that can be observed much stronger for the oligophenylalkanethiols, which can only move within a tight conformational “cylinder” (right) without any change of the molecular tilt. The latter change is only possible at the cost of the bending potential and alkyl chain conformation.

packing densities in BPn, BPnSe, and TPn SAMs, one can assume that the molecular packing in these films possesses a broad flexibility within the general structural motif given by the bulk structure. It is then logical to expect a similar flexibility in the case of anthracene (similar structural motif in bulk), which presumably excludes the exact structural differences as the reason behind the lower extent of odd-even effects in the anthracene-based films. So, we are only left with the final special feature of the anthracene-based films, the special symmetry of the SAM constituents. In contrast to the biphenyl and terphenyl moieties, the attachment of the anthracene backbone to the aliphatic linker occurs off-centered of the long C2 axis of the anthracene molecule (compare Figure 1, axis A), because this axis does not contain any atoms. The substitution at the closest available atom, position 2 in the anthracene system, reduces the symmetry to C1v. This “symmetry breaking” can result in structural polymorphism as, for example, observed in 2-naphthalenethiolate SAMs on Au(111), for which several different orientations of the naphthalene backbone were recorded.75 Considering the small molecular inclination in the Ant-n films, such an orientational polymorphism is hardly possible in the present case. However, nonsymmetric attachment of the anthracene moiety provides an additional flexibility in the molecular arrangement by rotation of the anthracene moiety around the anthracene-alkyl bond as schematically shown in Figure 14, left side (this axis is identical to axis B in Figure 1). The resulting “conformational cone” results in an always upright orientation for the anthracene unit in Ant-n SAMs with n = odd, which we consider to be the case of the cooperative balance between the bending potential and structurebuilding interactions. If n = even, the axis of the cone is tilted about 60° with respect to the surface normal, but still permits the upright orientation of the anthracene unit at an dihedral angle of 180° at the respective anthracene-alkyl bond. It should be emphasized that this rotation can happen without any significant distortion neither of the optimal binding geometry at the headgroup-substrate interface nor of the all-trans conformation of the alkane chain, which is impossible for the symmetrically attached biphenyl and terphenyl moieties (Figure 14, right side). This conformational adjustment fully explains the observations described above, which is the almost indistinguishable, upright orientation of the anthracene units regardless of the parity of n as

4. CONCLUSIONS The integrity, chemical identity, packing density, and molecular orientation in SAMs formed by anthracene-substituted alkanethiols with variable length of the aliphatic linker on Au(111) and Ag(111) were studied by a combination of several complementary experimental techniques. The Ant-n molecules were found to form well-defined and highly ordered SAMs on these substrates, with very small inclination of the anthracene backbone, which is distinctly superior as compared to the cases when anthracene is directly attached to the headgroup. Beyond the high quality, the Ant-n SAMs exhibit odd-even effects, that is, dependence of the molecular orientation and packing density on the length of the aliphatic linker in the target molecules, with the parity of n being the decisive parameter. The relation between the properties of the Ant-n films and the parity of n is on Au opposite to that on Ag. Whereas the sign of the odd-even behavior of the Ant-n SAMs correlates precisely with that of SAMs of biphenyl- and terphenyl-substituted alkanethiolates and alkaneselenolates, suggesting the common reason behind this phenomenon, the presence of a strong substrate-headgroup-C bending potential, the extent of the odd-even effects in the Ant-n films is noticeably smaller than that in the latter systems. This behavior was tentatively explained by the nonsymmetrical attachment of the anthracene moiety to the adjacent alkyl linker: The substitution in 2-position reduces the symmetry of the nearby C2 axis to C1. Within such an architecture, a rotation of the anthracene moiety along the anthracene-alkyl bond results in considerable change in its tilt with respect to the substrate, giving the system an additional degree of freedom for the case of competitive balance between the bending potential and structure-building interactions. Because of this additional freedom, an almost upright orientation of the anthracene unit and subsequently tight molecular packing can be achieved even in the case that the bending potential forces a significant inclination of the anthracene moieties. The present study is an instructive example that even seemingly subtle changes of the molecular structure, such as nonsymmetric attachment of the aliphatic linker in the present case, may have noticeable implications for the properties of the resulting assemblies. On the practical side, this partial “compensation” of the odd-even effects in the Ant-n SAMs is not only an interesting and important phenomenon, but also permits the formation of a series of monolayers with continuously increasing thickness (useful, e.g., for the electronic decoupling of the aromatic part from the metal surface) without changing significantly either the molecular packing or the orientation in the aromatic part. It has still to be investigated if this persistence is maintained when these films are used as surfactants for the growth of organic semiconductors. The effect of further changes, for example, the use of a selenium headgroup or introduction of additional “symmetrizing” linker between the aliphatic chain and anthracene backbone, might bring more information about the force and degree of freedom balance within this interesting and practically important system. 2850



5. EXPERIMENTAL SECTION The anthracen-2-yl alkanethiols Ant-n were synthesized as described in the Supporting Information. In short, all the substances were derived from 2-bromoanthracene,76 which after lithiation was reacted with the appropriate carbon electrophiles. Thus, for the introduction of the C1 unit, dimethylformamide was used, while the C2 chain became introduced by reaction with oxirane. For all the higher alkyl chains, copper-mediated cross couplings with the 1,ω-dibromoalkanes have been used. After the couplings, the thiol groups became introduced using either the thio-variant of the Mitsunobu reaction or the two-step reaction with thiourea. The gold and silver substrates were prepared by thermal evaporation of 100 nm of gold or 100 nm of silver (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. The resulting metal films were polycrystalline, with the predominant (111) orientation of the individual grains and a grain size of 20-50 nm. The SAMs were prepared by immersion of the freshly prepared substrates into a 1 mM solution of the target compounds in ethanol at room temperature for 24-36 h. After immersion, the samples were carefully rinsed with pure solvent and blown dry with argon. For Ant-n SAMs on silver, sometimes sonication in THF was necessary to obtain neat films. They were either characterized immediately or stored under inert gas atmosphere in glass containers until the experiments at the synchrotrons (see below). The Ant-n SAMs were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray photoelectron spectroscopy (HRXPS), ellipsometry, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, infrared reflection absorption spectroscopy (IRRAS), and contact angle goniometry. All experiments were performed at room temperature. The XPS, HRXPS, and NEXAFS spectroscopy measurements were carried out under UHV conditions at a base pressure

Compensation of the OddâEven Effects in Araliphatic Self-Assembled

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