Molecular Rotors on Au(111): Rotator Orientation from IR

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0125, and Institute of Organic Chemistry and Biochemistry, A...
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Molecular Rotors on Au(111): Rotator Orientation from IR Spectroscopy Mary E. Mulcahy,† Thomas F. Magnera,† and Josef Michl*,†,‡ Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0125, and Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague, Czech Republic ReceiVed: July 18, 2009; ReVised Manuscript ReceiVed: October 15, 2009

Gold surfaces carrying altitudinal molecular rotors firmly attached through sulfur and/or mercury atoms have been examined by IR spectroscopy. The presence of an intact rotator has been confirmed and its average orientation with respect to the gold surface determined by single reflection thin-layer attenuated total reflection (ATR) spectroscopy, using metal surface selection rules. The requisite IR polarization directions were obtained from IR linear dichroism of a model rotator oriented in stretched polyethylene. The results are compatible with those of prior differential barrier height imaging measurements and molecular mechanics calculations. Introduction Molecular Rotors. Central to artificial “molecular machinery”, molecular rotors have long been a source of fascination.1-4 Our interest has been centered on the synthesis of molecular rotors consisting of a part (“rotator”) that can turn around an axle to which it is rigidly covalently bonded, and on the sturdy mounting of the axle-bearing “stator” on a surface.5-12 We have already reported that the dipolar rotor 1 has been synthesized and mounted successfully on an Au(111) surface at submonolayer densities. It was characterized by several techniques, primarily ellipsometry, scanning tunneling microscopy (STM), infrared (IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS), and about a third of the mounted rotors were shown to be free to rotate by differential barrier height imaging (BHI). The details of the synthesis were included in the Supporting Information that accompanied the communication. The details of the ellipsometric, STM, and XPS measurements are being reported separately.13 Presently, we provide the full details of the IR measurements. These were important for two reasons: (i) to confirm the presence of the chemically intact rotator on the gold surface and (ii) to deduce its average orientation relative to the surface, which was found to be compatible with the differential BHI results. Rotors 1-3 (Chart 1) are of the altitudinal type. They consist of two stators with an affinity for gold provided by the attached -HgSCH2CH2SCH3 or -HgOOCCF3 functionalities. The stators support a rotator (4) carrying axle, designed to lie parallel to the surface. The rotator is also available separately. The orientation of the rotator relative to the surface is specified by R, the angle that the mean plane of the rotator makes with the surface normal z (Figure 1). Our choice of rotator structure was influenced not only by the large C-F bond dipole moment but also by the large IR intensities of the characteristic vibrations that the presence of four C-F bonds is expected to introduce. STM13 measurements of rotor samples made in the same manner as those used for IR experiments in this paper suggested a coverage of about 80% of a monolayer. Since the rotator represents only a small fraction of the rotor molecule, maximum sensitivity was required and the spectra were still quite noisy. † ‡

University of Colorado. Academy of Sciences of the Czech Republic.

CHART 1: Molecular Rotors 1-3 and the Dipolar Rotator 4

Nevertheless, both objectives, i and ii, were reached. We relied on a thin-layer version of the attenuated total reflection (ATR) technique that achieves a considerable sensitivity enhancement by relatively simple means, such that a single reflection is

10.1021/jp906809b CCC: $40.75  2009 American Chemical Society Published on Web 11/06/2009

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Figure 1. Cartoon of 1 or 2 bonded to a gold surface. Left: the average plane of the rotator is parallel to the gold surface normal. Right: the plane is rotated by an angle R.

Figure 3. Reflectivity as a function of incident angle θi for a thinlayer system (Figure 2), with n2 ) 1 (20 Å). Solid: n1 ) 4, n3 ) 2.4 - 40i. Dashed: n1 ) 1, n3 ) 2.4 - 40i. Dotted: n1 ) 4, n3 ) 3.45. Inset: a more detailed view of the dotted line below the critical angle.

Figure 2. A thin-layer ATR setup (schematic).

sufficient. This is important, since we found it difficult to establish perfect contact between our Au surface and the Ge surface of a conventional ATR crystal over the large area that would be required for a multiple reflection measurement. Since the thin-layer ATR method is relatively rarely used, we provide below a brief introduction to it. We then describe the application of surface selection rules when one of the layers is a conductor. The determination of IR transition moment directions in the surface-mounted sample, which permits the determination of the average orientation adopted by the rotator 4, required a knowledge of transition moment directions in 4 itself. This was obtained from IR linear dichroism (LD) in stretched polyethylene by standard procedures.14 Thin-Layer ATR Spectroscopy. In thin-layer ATR spectroscopy (Figure 2), IR radiation passes through a high index of refraction ATR crystal (optical medium 1) to a sample (optical medium 2) deposited onto a substrate (optical medium 3) that is pressed against the ATR crystal. The angle of incidence is larger than critical and an evanescent wave is established in the sample, allowing an absorbance spectrum to be collected.15,16 Medium 2 is much thinner than the penetration depth of the evanescent wave, and the reflection of the light is dictated by n1 and n3, and not n1 and n2. This is exemplified by the Fresnel17,18 reflection coefficient Rp for p-polarized light for a 20 Å thin layer of air (n2 ) 1) sandwiched between a germanium ATR crystal (n1 ) 4) and a silicon substrate (n3 ) 3.4, dotted line in Figure 3). The critical angle for light traveling from germanium to air is ∼14°, but total internal reflection is observed only at incidence angles larger than the critical angle for a germanium/silicon interface, ∼60°. If the air in the preceding example is replaced by an absorbing organic film, the observed absorption signal is enhanced over traditional grazing angle specular reflection or transmission when silicon or gold is used as the third substrate.19-21 An absorption spectrum of the film can then be obtained in a single reflection. In the case of gold, this may be especially surprising, since the calculated reflectivity of a clean gold substrate pressed against a germanium ATR crystal as a function of incident angle closely resembles that

Figure 4. Relative mean square electric field strengths 〈EZ22〉/〈Ep12〉 in a thin organic sample (n2 ) 1.5 - 0.5i) coated on a Ge ATR crystal (n1 ) 4) and covered with silicon (n3 ) 3.4, dotted), gold (n3 ) 2 40i, dashed), and air (n3 ) 1, solid).

for a gold surface in air (solid and dashed lines in Figure 3). Nonetheless, observed signal peaks from a self-assembled monolayer of n-octadecane-1-thiol-d37 on gold were almost an order of magnitude larger using a thin-layer ATR than those obtained from grazing angle reflectance.21 Signal enhancement in ATR spectroscopy is ordinarily obtained through the use of multiple reflections within the ATR crystal.16,22,23 The origin of the enhancement observed in the thin-layer system using just a single reflection has been the subject of several papers.19,20,24-26 Early on, when the thin film was backed by a metal, it was hypothesized that the enhancement resulted from the amplification of the electric field in the sample either due to the excitation of surface metal plasmons25 or multiple reflections of the IR radiation within the sample.26 The more recent publications focus on nonmetal substrates and the ratio of the indices of refraction n1 and n2, but the arguments remain valid when the backing substrate is a metal.19,20,24 Rowell et al.19 also point out the importance of the phase shift experienced by the impinging radiation upon reflection. The relative mean square electric field strength along Z normal to the medium boundary in the thin layer 2 of a thin-layer system was calculated using Hansen’s formula.19,27 It has been plotted in Figure 4 as a function of incident angle θi for an organic film supported by a gold or a silicon substrate pressed against a Ge ATR crystal. The maximum electric field intensities are

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similar in the two cases but are reached at very different angles of incidence. The relative mean square electric field strength 〈EZ22〉/〈Ep12〉 for an organic film on a germanium ATR crystal backed by air is an order of magnitude smaller (Figure 4). Here, EZ2 and Ep1 are the electric field in medium 2 (the Z component) and in medium 1 (p-polarized), respectively. For the gold substrate, any incident angle between 60 and 80° will result in a large electric field component along Z with maximum intensities occurring close to 70-75°. For a silicon substrate, an incident angle of ∼59° is ideal. The use of the fixed 65° incident angle of the commercial GATR unit results in a 〈EZ2〉/〈Ep12〉 value that is only about half of its maximum. We believe that the constraint to 65° is responsible for our failure to obtain reliable single-reflection spectra of submonolayers of molecular rotors deposited on SiO2/Si wafers even though we were able to obtain them on nearly identical samples deposited on gold. IR Transition Moment Directions.14a In our thin-layer ATR measurements, the third medium is gold, and the selection rules for samples located on a conductor surface28,29 apply to the thin layer of medium 2. Because image charges cancel dipoles parallel to the surface, only the component of the fth transition moment Mf along the surface normal Z is active and the probability of absorption by a molecule is proportional to |Z · Mf|2. Averaged over the sample, the probability is |Mf|2Kf, where Kf ) 〈cos2 fZ〉. In the definition of the orientation factor Kf of the fth transition moment, fZ is the angle that Mf makes with Z and pointed brackets indicate an ensemble average. The orientation factor Kf reflects, first, the average alignment of the molecular framework xyz in the sample with respect to the normal Z and, second, the orientation of the transition moment within the molecular framework. The alignment of the molecules is sufficiently characterized by two of the three orientation factors Ku ) 〈cos2 uZ〉 of the three principal molecular orientation axes (u ) x, y, z), since ∑u Ku ) 1 (uZ is the angle that the molecular axis u makes with Z). The principal orientation axes are those that diagonalize the orientation tensor 〈cos u cos V〉, and in high-symmetry molecules, they coincide with the molecular symmetry axes. The orientation of the transition moment Mf in the molecular frame is characterized by its direction cosines cos φuf. The relative weight with which the absorbance |Z · Mf|2 of the fth transition enters into the spectrum obtained in the thin-layer ATR experiment can therefore be written as |Mf|2∑u Ku cos2 φuf, whereas for an ordinary isotropic sample it is |Mf|2/3. A comparison of relative peak intensities in each of the two spectra therefore gives information on the molecular orientation factors Ku and the direction of Mf in the molecular frame. Experimental Section Materials. The syntheses of the molecular rotors 1-38 and the rotator 48,30 (Chart 1) have been described. Au(111) goldcoated mica substrates were purchased from Molecular Imaging (Tempe, AZ), and were flame annealed just prior to use with a hydrogen torch for 30 s as directed by Molecular Imaging. All solvents were spectroscopic grade (Aldrich). Single Reflection Thin-Layer ATR Spectroscopy. Rotors 1 (in CH2Cl2) and 2 (in THF) were allowed to adsorb on gold substrates from ∼10-5 M solutions under ambient conditions for times varying from 1 to 5 min. Upon removal from the deposition solutions, samples were rinsed with copious amounts of the solvents. FTIR spectra (Figures 5-10) of the samples were recorded (10 000 scans, 2 cm-1 resolution) with a Nicolet Nexus 670 FTIR spectrometer using Harrick’s GATR, a 65°

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Figure 5. IR spectrum of 4 in KBr (solid line) and DFT predicted peak positions and intensities (bars). Asterisks mark transitions correlated with observed peaks, cf. Table 1.

fixed angle single reflection ATR accessory. The background was a clean gold substrate in contact with the Ge crystal of the GATR attachment. The GATR accessory is equipped with a pressure applicator to optimize contact between the sample and the crystal by using a torque screwdriver (0.35 Nm). In the spectra of 1 and 2 self-assembled on gold, the choice of baseline could greatly affect the measured peak heights. In an effort to account for this, the spectra were fitted using three different baselines. First, the KBr spectrum of 1 was fitted with a series of Gaussians. The spectra of 1 and 2 adsorbed on gold were then also fitted by a series of Gaussians, five of which had the same positions and full widths at half-height that were found for peaks 2, 4, 5, 8, and 13 in the KBr spectrum of 1. The Gaussians not associated with the rotator were used to create a composite baseline that was then subtracted from the spectrum. This was done twice independently, using distinct starting points, leading to somewhat different baselines. A third baseline was determined using the automatic correction function provided in the Omnic software31 that is used to run the FTIR. Peaks were then fitted with Gaussians using Mathematica’s leastsquares fitting algorithm function NonLinearFit,32 using the positions and peak widths used for the KBr spectrum, but allowing the amplitudes to vary. LD-IR Spectroscopy. A polyethylene sheet was made by placing polyethylene pellets in a single layer between two metal blocks lined with Teflon and then placing the metal block into an oven for ∼30 min at 165 °C. Upon removal from the oven, the block system was pressed together to a pressure of ∼4000 psi for 5 min. This produced a polyethylene sheet about 1 mm thick. The sheet was then placed in methylene chloride for a day to remove contaminants and was air-dried.14b After the polyethylene sheet was mechanically stretched, background spectra were taken with light polarized parallel and perpendicular to the stretching direction using a Nicolet Nexus 670 FTIR spectrometer and a Molectron IGP-225 polarizer (1000 scans, 2 cm-1 resolution). A saturated solution of 4 in CH2Cl2 was prepared, and a few drops were placed on the stretched sheet in a covered dish with a small amount of CH2Cl2 overnight. After removal from the covered dish, the sheet was rinsed with CH2Cl2 briefly and allowed to dry, at which point it was examined for any residual crystals of 4. If crystals were found, the sheet was rinsed again. The process was repeated until no crystals were present, and sample spectra with light polarized parallel and perpendicular to the stretching direction were taken in the same manner as the baselines. The baselines were subtracted from the sample spectra and a dichroic ratio df was determined for each reliably observed peak using the standard trial-and-error method,14c and converted to the orientation factor Kf using Kf ) df/(df + 2).

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TABLE 1: IR Transitions of 4 in KBr Correlated with (i) DFT Predicted Transitions and (ii) Peaks in the IR Spectrum of 1 in KBr

a

peak

ν (cm-1), 4 obsd.

ν (cm-1), 4 calcd.

rel. int.

sym

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15a 16 17 18a 19a

536 1047 766 1072 1091 1230 1607 1006 1260 1273 1574 957 1151 1595 1941 1610 924 1974 1830

565 1077 784 1108 1138 1253 1657 1028 1294 1320 1624 968 1185 1645

0.011 0.022 0.24 0.17 0.87 0.064 0.019 0.36 1.00 0.061 0.028 0.024 0.18 0.0081

1668 936

0.035 0.21

b b b b b a a a a b a b b b b b b b b

ν (cm-1), 1 obsd. 1050 1076 1100 1229 1008 1259 951 1157

Assigned as overtones or combination bands.

TABLE 2: Kf and |φxexp| Values of 4 from LD-IR in Stretched Polyethylene and |φxcalcd| Values from DFT Calculations peak

symmetry

Kf

|φexp x | (deg)

|φcalcd | (deg) x

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

b b b b b a a a a b a b b b b b b b b

0.15 0.17 0.17 0.20 0.26 0.27 0.29 0.31 0.31 0.32 0.32 0.35 0.35 0.37 0.39 0.41 0.42 0.50 0.50

14 18 18 24 33

16 2 6 29 10

41

76

45 45 47 50 53 54 66 66

32 36 76 69 72

Calculations. The B3LYP/6-31G(d,p) method was employed for both the geometry optimization and frequency calculations, using the Gaussian98 program package.33 Results Transition Moment Directions in 4. Table 1 lists all of the peaks of 4 reproducibly observed in KBr and labels them in the far left column by numbers arranged in the order of increasing Kf values. These labels will be used throughout the paper. DFT predicted peak positions, intensities, and symmetries (Table 1) and dipole moment directions within the molecular framework (Table 2) are listed for comparison with the experimental results. An illustration is provided by Figure 5, which displays both experimental and calculated results in the spectral range 1350-950 cm-1. A comparison of positions and intensities resulted in the assignments given in Table 1. The C2 symmetry of 4 implies that the transition dipole moments either are polarized along the axis of rotational symmetry, labeled y in Chart 1 (irreducible representation a),

or are polarized in the xz plane (irreducible representation b).34 The orientation factors Kf in Table 2 therefore fall into one of two groups, Kay and Kbf .14d All Kay values should be identical, and they nearly are, with an average Ky value of 0.30. The Kbf values are expected to satisfy Kx e Kbf e Kz. While it is possible that none of the transitions of b symmetry actually lie along the x or z axis, and hence none of the observed Kf values actually equal Kx or Kz, the requirement that Kx + Ky + Kz ) 1 leaves little uncertainty as to what Kx and Kz must be. If we took the largest value of Kf (0.50) to equal Kz and the smallest (0.15) to equal Kx, the three molecular orientation factors would add up to 0.95. Arbitrarily, we divide the 0.05 difference equally between the values for Kx and Kz, making the assumption Kx ) 0.125, Ky ) 0.30, and Kz ) 0.575. Given these values, the transition dipole moment directions that lie in the xz plane can be determined from cot2 φx ) (Kz - Kf)/(Kf - Kx), where φx is the angle between the transition moment direction and the x axis (Table 2). The remaining task is the determination of the location of the molecular orientation axes x and z in the molecular frame. From molecular shape, we expect z to lie close to the direction indicated in Chart 1, which connects the para positions in the biphenyl substructure.14d To estimate the deviation of z from this line, we assign the transitions to those calculated in the DFT procedure (Table 2), and then use a least-squares fit to determine the angle by which the z axis has to be rotated about the y axis away from this line to obtain the best agreement between the φexp values deduced from the measurements and x the φcalcd values obtained from the DFT calculations. The best x agreement (Table 2) was obtained when the angle of rotation was 8° in the direction away from the axial fluorine atoms. Transition Assignment in 1. Spectra of 1 and 4 in KBr (Figure 6) were compared to determine which peaks in the IR spectrum of 1 are attributable to the rotator. These were assumed to be the intense peaks in the skeletal and C-F stretching region that appeared at approximately the same positions in the spectra of 1 and 4, and have been identified as such in Table 1. The identification of peak 8 at 1006 cm-1 in 4 in the spectrum of 1 is complicated because the latter contains three closely spaced peaks at 1008, 1015, and 1020 cm-1. To determine the correct assignment, KBr spectra of 1 and 4 were compared with the

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Figure 6. IR spectra of 1 (top) and 4 (bottom) in KBr, cf. Table 1.

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Figure 9. IR spectrum of 1 (Figure 8) after subtraction of three different baselines (see text). Red and green: baseline fitted with Gaussians. Blue: baseline fitted with the Omnic software.

Figure 7. IR spectra of 1 (dashed), 3 (dash-dot), and 4 (solid) in KBr. Figure 10. IR spectrum of 2 (Figure 8) after subtraction of three different baselines (see text). For the color scheme, see the caption to Figure 9.

Figure 8. IR spectrum of 1 in KBr (top) and single reflection thinlayer ATR spectra of submonolayers of 1 (bottom) and 2 (middle) on gold, cf. Table 1.

KBr spectrum of a nonpolar version of the molecular rotor 3 (Figure 7). Since the peaks at ∼1020 and ∼1015 cm-1 are present in both the spectra of 1 and 3, albeit the latter only as a shoulder in the spectrum of 3, they cannot be assigned to the rotator. Peak 8 (at 1008 cm-1 in 1), which is absent in 3, has been assigned to the rotator. An FTIR spectrum of 2 in KBr revealed that the -OOCCF3 functionality obscures the region between 1350 and 850 cm-1 where the majority of peaks originating from the rotator occur. As also briefly reported elsewhere,9,13 when 2 adsorbs onto gold, the -OOCCF3 groups are lost and ultimately will not interfere with data analysis. This is observed in the IR spectra as well, and the same peaks were used to determine the orientation of the rotator in the adsorbed submonolayers of 1 and 2 (Figure 6). Transition Moment Directions in 1 and 2 Mounted on Gold. Figure 8 presents a part of the spectra of 1 in KBr and submonolayers of 1 and 2 on gold measured using the thinlayer ATR. The GATR accessory was needed to detect the small signal intensities from the submonolayers, but it resulted in poor baselines, which compounded problems due to peak overlap. The spectra seen in Figures 9 and 10 are the baseline subtraction results from the original spectra given in Figure 8. In the spectral region 950-1350 cm-1, only three different peaks potentially associated with transition dipole moments

having a symmetry could be identified (6, 8, and 9), and the weak transition 7 was not seen. Unfortunately, peaks 6 and 9 are obscured by a large feature in the baseline, leaving peak 8 as the only definitive a symmetry dipole moment for which the ratio of absorbances in KBr and on gold can be determined. As described in detail in the Experimental Section, great attention was paid to the subtraction of the baseline when relative peak intensities were measured. A comparison of relative peak intensities in the spectra of isotropic samples of 1 and 2 in KBr and in the spectra of oriented samples adsorbed on gold permits a determination of the average angle that the y axis of the rotator forms with the surface normal Z. In the following, we assume that the rotor molecules are adsorbed perfectly on the Au(111) surface, with the stator axes normal to the surface and the axle parallel to it (Figure 1), such that for R ) 0 the molecular y axis of each molecule is lined up with Z and Ky ) 1, Kx ) Kz ) 0. Under this assumption, the angle between y and Z becomes equal to the angle R that defines the molecular conformation. For all a-symmetry vibrations of the rotator, polarized along y, the intensity ratio should be the same in both spectra. A measurement of the relative intensity of any one b-symmetry vibration f for which cos2 φx(f) is known yields a value for R: gold ΚBr gold tan2 R ) [ΙΚBr /Ιf ) cos2 φexp y /Ιy ]/[(Ιf x (f)]

(1)

where the superscripts on the intensities I indicate the nature of the sample and the subscripts indicate the transition (y stands for any transition of symmetry a). In reality, the values of R obtained from different peaks f are not all the same, and the variation observed in Table 3 provides an idea of the error associated with the determination. A summary of peak intensities from Figures 9 and 10 is given in Table 3. The last two rows contain results of measurements on two additional distinct samples of 2 on gold and demonstrate

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TABLE 3: Peak Absorbances for 1 in KBr and 1 and 2 on Gold from the Spectra of Figure 9 and the Resulting Values of r (eq 1) peak

2

4

5

8

KBr (1)

0.0424

0.0852

0.0931

Au red green blue

0.00133 0.000460 0.00229

0.00142 0.000331 0.00256

0.00203 0.000600 0.00749

Au red green blue blue blue

0.00105 0.00108 0.000499 0.00138 0.00066

0.00257 0.00305 0.00227 0.00244 0.00089

0.00236 0.00199 0.0016 0.00159 0.00083

13

R

0.0322

0.0107

0.000739 0.000860 0.000721

0.000953 0.000721 0.000734

53 ( 12 38 ( 19 53 ( 14

0.000499 0.000903 0.000339 0.0014 0.00065

0.000603 0.0000498 0.000225 0.00241 0.0004

59 ( 7 43 ( 9 57 ( 6 48 ( 16 46 ( 12

1

2

the reproducibility of the results. These values and the experimentally determined values of φx given in Table 2 were used in eq 1 to calculate the R values reported in Table 3. The results show that the choice of baseline does not have a large effect on the value found for R. There is no difference between results for 1 and 2. Considering all the sources of error, we estimate the final value of R to be 50 ( 25° in both, clearly distinct from 0 and 90°. While specific values and errors are given for each baseline in Table 3, we are cautious of their accuracy because they were determined using data from only a single a symmetry transition moment. Lacking better control of the somewhat erratic baseline imposed by the GATR and lacking confirming measurements for additional a transition moments, we will basically only claim to know for certain that the rotator does not lie parallel nor perpendicular to the gold surface normal. Relative to these uncertainties, the assumption that the alignment of the molecular rotor with the gold surface is that shown in Figure 1, allowing us to identify R with the angle between y and Z that is being measured, appears justified, even if it is fulfilled only approximately in reality. Discussion Molecular Rotor Adsorption. The first of our objectives was structural, to prove the presence of the molecular rotors on the gold surface, with their rotators intact. This has been accomplished. In spite of the weakness of the IR signals observed, they provide good evidence for the claim that the molecules of 1 and 2 have been attached and the rotator stayed intact. Molecular Rotor Conformation. Our second objective was conformational, to examine the mode in which the molecular rotors are attached, and, specifically, to obtain information about the average angle that the rotator makes with the surface normal. This is an even more demanding measurement to perform on a submonolayer, and the conclusions are necessarily more tentative. The molecular rotors 1 and 2 were designed to attach to the gold surface with all of 10 mercury atoms, and in the case of 1, also the 20 sulfur atoms, holding the rotation axle parallel to the surface. The presence of a single Hg doublet in the XPS spectra strongly suggests that this has been accomplished. The XPS spectra13 also demonstrate that the -OCOCF3 groups are lost in the adsorption process, and this agrees with the presently observed IR spectra. These results, and the firm adhesion of 2 to the gold surface after intense rinsing with solvent, suggest that organometallic salts such as alkylmercury trifluoroacetates could be useful for attaching organic molecules to metal surfaces and might offer certain advantages relative to the usually used thiols, such as improved resistance to aerial oxidation. Experiments designed to test this notion have given encouraging initial results.35

The finding that the average orientation of the rotator is the same in 1 and 2 provides additional indirect evidence that the molecular rotors are attached to the gold surface in the orientation in which they were intended to be (Figure 1). The preferred orientation of the rotator in the mounted molecular rotor is determined by its interactions with the substrate and with the stators. The electrostatic interaction of its dipole with the metal favors R ) 0 and 180°. Interactions of the rotator with the stators are similar to those in biphenyl, where H-H repulsions and π-conjugation compromise to yield a twist angle close to 35°. There is one such interaction at each side of the molecular axle. The helical conformations of the aryl substituents at the cyclobutadiene rings of the two stators determine whether the two rotator-stator interactions act in concert or whether they approximately cancel. If they act in concert, they will tend to affect the otherwise electrostatically favored value of R. Semiquantitative information on the conformational situation on the gold surface was obtained from molecular mechanics calculations for 1 on a Au(111) surface, which included the electrostatic effect of the metal.8,11 Due to the helical arrangement of the aryl substituents imposed by their crowding on the cyclobutadiene ring, rotors 1 and 2 are expected to exist in one of three pairs of conformational enantiomers, PPP/MMM, PMP/ MPM, and MMP/PPM, where the outside letters indicate the chirality of the tetraarylcyclobutadiene and the inside letter that of the rotator. The calculated energies of the three conformations are similar, and in each, the favored orientation of the rotator is different. Inspection shows that in PPP/MMM the effects of the two stators on the orientation of the rotator tend to cancel, whereas in MMP/PPM they act in concert. The situation in PMP/ MPM is less clear-cut. In these calculations, the -HgSCH2CH2SCH3 functionalities were treated with one of two constraints. Either one of them was located beneath the rotor axle, inhibiting rotational motion, or they were all located outside of the rotator’s path.11 When one of the -HgSCH2CH2SCH3 chains is located under the rotator, the orientation of the latter is constrained to angles of R ) ∼90°. When the -HgSCH2CH2SCH3 groups do not interfere with the motion of the rotator, R is calculated to be 0 (or 180), 30 (or 210), and (45 (or (135°) in the MMM, MMP, and MPM conformations, respectively. The accuracy of the molecular mechanics calculations was not sufficient to permit a reliable evaluation of the average angle R at room temperature, but they make it clear that in the absence of interference with a -HgSCH2CH2SCH3 chain the y axis will be inclined toward the surface normal Z rather than perpendicular to it. The differential BHI results for rotor 1 showed that roughly a third of the rotor molecules adsorbed onto the gold surface

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are capable of rotating their dipole direction from pointing approximately toward the surface to pointing approximately away from it, and that many of them can slowly switch spontaneously from the nonrotating to the rotating state and back. The simplest interpretation of these observations is that about two-thirds of the time one of the -HgSCH2CH2SCH3 chains moves under the rotator and blocks its motion. In this state, R must be close to 90°. In the rotors that are free to rotate, R will be much closer to 0°. The average value of R that we find, 50°, is thus compatible with data deduced from the differential BHI measurement and with the results of molecular mechanics calculations. Conclusion The presence of an intact rotator and its approximate average orientation with respect to the surface normal in submonolayers of molecular rotors 1 and 2 on a gold surface were determined by single reflection thin-layer ATR. The requisite IR transition moment directions were determined by LD-IR spectroscopy of the rotator 4 in stretched polyethylene. The use of the thinlayer ATR provided enhanced signal-to-noise ratios, allowing an observation of signals from the rotator that were too weak to observe with traditional grazing angle specular reflection. Because only IR active vibrational modes perpendicular to a metal surface can absorb radiation, the relative attenuation of specific signal peaks after adsorption onto gold allowed the determination of the rotator orientation by comparison with an isotropic KBr sample. On average, the mean plane of the rotator lies neither parallel nor perpendicular to the gold surface but at an angle of 50 ( 25°, the same in samples prepared from 1 and 2. This is compatible with the results of molecular mechanics calculations and differential BHI measurements. We believe that the present report has significance that goes beyond the specific results obtained for 1-4, in that it also (i) illustrates the sensitivity of single-reflection thin-layer ATR spectroscopy for submonomolecular samples, (ii) documents the power of a combined LD-IR measurement in stretched polyethylene with relative IR peak intensity measurements on a metal surface for determination of adsorbate conformation, and (iii) draws attention to the possibility that organometallic salts such as alkylmercury trifluoroacetates could be of general utility for attaching organic molecules to metal surfaces, such as gold. Acknowledgment. We are grateful to Dr. Paweł Rempala for performing the DFT calculations and to Dr. Xiaolai Zheng for preparing a sample of 4. Support from the NSF (CHE 0446688 and 0848477) and from the Grant Agency of the ASCR (A 400 550 616) is gratefully acknowledged.

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