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Effects of Substituents on the Structure and Bonding of Thiophene on Cu(111) P. K. Milligan,† B. Murphy,‡ D. Lennon,† B. C. C. Cowie,‡ and M. Kadodwala*,† Department of Chemistry, UniVersity of Glasgow, Glasgow, G12 8QQ UK, and CLRC Daresbury Laboratory, Warrington, WA4 4AD UK ReceiVed: December 4, 2000; In Final Form: March 12, 2001
The local registry and orientation of chemisorbed 3-methylthiophene and 3-methoxythiophene on Cu(111) has been determined using normal incidence X-ray standing wavefield absorption and near edge X-ray absorption fine structure spectroscopy, respectively. 3-methylthiohene and 3-methoxythiophene have, within experimental error, identical local registries (atop), Cu-S separations (2.57 ( 0.03 and 2.55 ( 0.05 Å), and orientations (24 ( 4° and 26 ( 4°). These results are placed in context with previous studies of thiophene and 3-chlorothiophene and provide quantitative information on how simple substituents affect the adsorption structure of the thiophene ring.
Introduction A cornerstone of synthetic chemistry is the use of substituents to modify the chemical activity of functional groups in a controlled manner. A rationale of the role of the substituent in synthetic reactions can be found in any undergraduate textbook; substituents influence reactivity either through steric means or by altering the electronic structure of the functional group. Although there is a highly developed understanding of the effects of substituents in synthetic chemistry, this is not the case in surface chemistry, where an understanding of the influence of substituents is still developing. The present study is part of a wider program where the objective is to discover the effects of simple substituents on the bonding of a model adsorbate/ substrate system, specifically thiophene/Cu(111). In combination with previous studies of thiophene1 and 3-chlorothiophene2 on Cu(111) from this laboratory, the present work forms the first systematic study, with quantitative synchrotron-based structural techniques, of the influence of substituents on the bonding of an aromatic heterocyclic molecule to a metal surface. The choice of small substituent groups, Me-, MeO-, and Cl-, was motivated by a desire to reduce the possibility of steric effects and, hence, discover if purely electronic effects of the groups would significantly affect bonding. In principle, it may be argued that MeO, a relatively big group, could have some steric effects in addition to influences electronic properties. The Me-, MeO-, and Cl- groups were chosen because they would be expected to have differing effects on electron structure because the first two are electron donating, whereas the latter is electron withdrawing. There has been growing interest within surface science in studying the effects of substituents on both the chemistry and structure of adsorbates. For instance, Gellman3 has played a leading role in the development of the use of substituents to probe the nature of transition states in surface reactions. There have been some studies on the influence of substituents on the structure of the adsorption complex. However, there have been relatively few studies using synchrotron-based structural tech* To whom correspondence should be addressed. E-mail: malcolmk@ chem.gla.ac.uk. Fax: + 44 141 330 4888. Phone: +44 141 330 4380. † University of Glasgow. ‡ CLRC Daresbury Laboratory.
niques where the effect of substituents on local registry and orientation has been studied quantitatively. One such study by Woodruff and co-workers4 investigated the effects (using energy scanned photoelectron diffraction (PhD)) on the orientation and local registry of pyridine on Cu(110) of a methyl group at the 2 position. This work shows that like pyridine 2-methylpyridine bonds with its aromatic ring almost perpendicular to the surface. The methyl group causes a twist of the pyridine ring, so that it is pointing away from the surface. Zhou and White5 have studied the orientation of chlorobenzene on Ag(111) using NEXAFS. The benzene ring of chlorobenzene was found to be inclined by 45 ( 6° to the surface. Although there are individual studies of the structure of derivatives using a variety of techniques, there have been no systematic investigations from which trends in bonding could be established. The work presented here is therefore unique, because we have studied the bonding of a molecule and some of its derivatives using the same techniques, which facilitates comparisons of structures. From the data we have collected, we have surprisingly found that the substituents did not cause a change, within experimental error, in the orientation or local registry of the adsorbed thiophene ring. This would imply that the bonding of the thiophene is dominated by the S-Cu interaction and that any perturbation of the aromatic system does not affect bonding significantly. Although substituents do not have a gross effect of the adsorption structure of the thiophene ring, they do have rather subtle effects on bonding, namely, the inhibition of a phase transition, which will be discussed. Experimental Section Experiments reported in this paper were performed in two separate UHV systems. Initial characterization experiments were performed in a UHV system at Glasgow. This system is equipped with a retarding field analyzer (RFA) which was used both to collect electron induced Auger (AES) spectra and to obtain low energy electron diffraction (LEED) patterns. Residual gas analysis and temperature programmed desorption (TPD) were performed with a triple filter (0-300 amu) quadrupole mass spectrometer (QMS; Hiden Ltd). The QMS was attached to a linear drive, which allowed it to be reproducibly positioned
10.1021/jp004366+ CCC: $20.00 © 2001 American Chemical Society Published on Web 05/15/2001
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within 1 mm of the face of a crystal during TPD. To prevent the detection of gas molecules which may have desorbed from the sample holder and not the crystal surface, a stainless shield with a 3 mm diameter entrance aperture and pumping holes was placed over the head of the QMS. A heating rate of 0.5 K/s was used to collect TPD spectra. NEXAFS and NIXSW experiments were performed at the Central Laboratories of the Research Councils laboratory at Daresbury, at station 6.3 of the Synchrotron Radiation Source (SRS). This station has been described in detail elsewhere;6 it is equipped with the usual sample heating and argon ion bombardment facilities in addition to LEED optics. A double pass cylindrical mirror analyzer is used for electron stimulated Auger spectroscopy, and a concentric hemispherical analyzer (CHA) is used for photon stimulated AES and X-ray photoelectron spectroscopy (XPS). The same sample preparation procedures were used in both characterization and synchrotron-based experiments. The Cu(111) sample crystal was cleaned by cycles of Ar+ bombardment followed by annealing to 773 K. After Ar+ sputtering and annealing, the cleanliness and surface quality of the crystal were verified with AES and LEED. An Introduction to NIXSW NIXSW and NEXAFS are the two structural probes that have been used in this study. NEXAFS is a widely used technique, and Stohr7 has given a comprehensive description of the technique and its applications. In contrast to NEXAFS, the NIXSW technique is less commonly used. A comprehensive review of NIXSW is given by Woodruff et al.;8 however, a brief description of the technique is now given to familiarize readers with its basic concepts. In an NIXSW experiment, X-ray radiation with the required energy to generate a Bragg reflection at normal incidence strikes a surface of a solid. A standing wavefield is generated within the solid by the interference of the incident and backscattered waves. The nodal planes of the standing wavefield lie parallel to the Bragg scattering planes, and their separation is equal to that of the scattering planes. When the photon energy is scanned through a range of reflectivity associated with the standing wavefield, its phase changes in a way that can be modeled. If the X-ray absorption of an atom is monitored as the photon energy is scanned, the resulting profile can be used to determine its location relative to the bulk scattering planes. The coherent position (D) and coherent fraction (fco) are the adjustable parameters determined when a experimental profile is modeled. The D is the distance of a atom from the nearest scattering plane, whereas the fco is a measure of the distribution of positions that an atom has with respect to the scattering plane. The fco can take values between 0 and 1. A low fco value represents a broad distribution of positions with respect to the scattering plane, which would arise from either the occupation of multiple adsorption sites or vibrational motion, whereas a high fco, a value close to 1, represents an atom with a well defined position with respect to the scattering plane. The local registry of an element can be unambiguously determined by triangulating its position with respect to two different sets of reflecting planes, (111) and (1h11) in the present case. Because of their inherent surface sensitivity, Auger electrons and photoelectrons are usually used to monitor X-ray absorption profiles. Results Initial Characterization. (i) 3-Methylthiophene. TPD spectra showing the desorption of 3-methylthiophene from overlayers
Figure 1. TPD spectra which show the desorption of 3-methylthiophene from a Cu(111) surface which had been dosed at cryogenic temperatures with sequentially increasing exposures.
prepared at cryogenic temperature (ca. 120 K) with gradually increasing coverage are shown in Figure 1. Apart from 3-methylthiophene, no other species were observed in TPD experiments. No evidence for the decomposition of 3-methylthiophene on Cu(111), such as the presence of sulfur and carbon, was observed in AES spectra taken after a series of adsorption and desorption cycles. Three desorption features are observed in the spectra. The first feature occurs at 318 K and is rapidly saturated with increasing coverage; we have assigned this feature to desorption from defect sites. With the saturation of the defect state, a second feature develops, which initially has a Tmax of 265 K. This desorption peak increases in intensity with coverage and both broadens and shifts to lower temperatures; at saturation, the Tmax is 235 K. This state is assigned to desorption from a chemisorbed monolayer. With further exposure, a feature develops with a Tmax of 168 K, which does not saturate with increasing exposure; this state is assigned to desorption from condensed mutlilayers of 3-methylthiophene. The absolute coverage of 3-methylthiophene in terms of monolayers (ML) of sulfur (where 1 ML ) 1 adsorbate atom per substrate atom) within the chemisorbed overlayer was determined experimentally. This was achieved by annealing a multilayer surface to a temperature sufficient to desorb the condensed layers. Subsequently, AES spectra [S(LVV), Cu(LMM), and C (KLL)] were taken of the resulting chemisorbed overlayer. The absolute sulfur coverage was then determined by calibrating the sulfur Auger intensity against a pure sulfur overlayer with a known surface coverage. A Cu(111)/(x7×x7)R19°-S overlayer prepared by room-temperature adsorption of H2S, which has a coverage of 0.43 ML,9 was the surface used for calibration purposes. Coverages of 3-methoxythiophene were calibrated in a similar way. Using the outlined procedure, the coverage of the 3-methylthiophene chemisorbed overlayer is found to be 0.11 ( 0.03 ML. LEED showed that 3-methylthiophene did not form any structures with long range order. (ii) 3-Methoxythiophene. In Figure 2 are TPD spectra showing the desorption of 3-methoxythiophene from surfaces which have
Structure and Bonding of Thiophene
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5233
Figure 3. Copper NIXSW profile (dotted line) collected using the (111) planes and by monitoring the intensity Cu(LVV) Auger. The fit to the experimental profile is also shown (solid line); the parameters used to generate it are D ) 0.00 Å, fco ) 0.90, an energy spread (∆E) ) 0.90 eV, and a mosaic width ) 0.1°. Figure 2. (a) TPD spectra which show the development of the desorption from the chemisorbed state. (b) TPD spectra collected from condensed layers of 3-methoxythiophene which show the development of the three distinct multilayer desorption states at 180, 186, and 196 K.
been dosed with sequentially higher exposures at cryogenic temperatures (ca. 120 K). With initial exposure, desorption is observed from a single state which saturates and has a Tmax ) 251 K; this peak is assigned to desorption from the chemisorbed state. It should be noted that this desorption feature does not exhibit the same broadening and shift to lower temperature observed for 3-methythiophene. Also, unlike 3-methylthiophene, there is no observed desorption from defect sites. The absence of a desorption feature associated with defects sites is attributed to the dissociation of 3-methoxythiophene at these sites. This is supported by post-TPD AES analysis, which shows carbon and 0.02 ML of sulfur present on the surface. A similar amount of dissociation has been observed for 3-chlorothiophene, and this was also associated with defect sites. The amount of thiophene within the saturated chemisorbed state was found to be 0.12 ( 0.03 ML. After the saturation of the chemisorbed monolayer state, a new feature develops with a Tmax of 181 K. With increasing exposure, this peak does not saturate, but it does split into three distinct peaks with Tmax’s at 180, 186, and 196 K. We speculate that these three distinct peaks are due to the presence of three conformers of 3-methoxythiophene: s-cis, gauche, and s-trans,10 within condensed multilayers at the various temperatures. The three conformers are likely to display differing intermolecular interactions and, hence, have different desorption temperatures. The relative amounts of each conformer within condensed layers will be highly dependent upon temperature. We must stress that the assignment of these three desorption features to the different conformations of 3-methoxythiophene is speculation. For an unambiguous assignment, a technique, such as a vibrational spectroscopy, which is sensitive to conformation should be used to study the temperature dependent structure of the multilayers. Structural Measurements. The objective of the structural measurements was to determine the local registry and orientation
of 3-methyl and 3-methoxythiophene within chemisorbed layers using NIXSW and NEXAFS. Overlayers used in NEXAFS experiments were prepared by dosing an amount of either 3-methyl or 3-methoxythiophene which was known to be close to that required for a saturated layer from initial characterization experiments. In contrast in NIXSW experiments, overlayers were prepared by annealing surfaces with multilayers of 3-methyl or 3-methoxythiophene to temperatures sufficient to desorb condensed layers. In both NIXSW and NEXAFS experiments, absolute coverages were determined by calibration against a (x7 × x7)R19°-S surface using AES. Sulfur K edge NEXAFS spectra were collected at two X-ray incident angles, 90° (normal) and 19.5° (grazing), with respect to the surface. The NEXAFS spectra were collected by monitoring the intensity of the KLL sulfur Auger electron signal. NIXSW experiments were performed using two sets of planes, the (111) and (1h11). Substrate NIXSW profiles were obtained by monitoring the intensity of the LVV copper Auger electron, whereas the sulfur 1s photoelectron was monitored to obtain the adsorbate profile. NIXSW. The first phase of any NIXSW study is to obtain and fit a substrate profile from which values for the energy spread (∆E) of the incident X-rays and the mosaic spread of the Cu(111) surface are determined. These two values are then used as nonadjustable parameters in fitting the sulfur NIXSW profiles and only fco and D are variables. A copper profile collected using the (111) planes which is representative of those collected during the study is displayed in Figure 3. As expected for a well ordered Cu(111) surface, all copper NIXSW profiles collected during this work were fitted with a D value of 0.00 Å and with fco values that fell in the range of 0.85-0.90. A typical set of nonadjustable parameters used to fit a Cu NIXSW profile are a ∆E of 0.8 eV and a mosaic spread of 0.1°. (i) 3-Methylthiophene. In NIXSW experiments on 3-methylthiophene, the lowest coverage surface studied was 0.07 ( 0.02 ML, whereas the maximum coverage was 0.09 ( 0.02 ML. This compares with a saturation coverage of 011 ( 0.03 ML determined in initial characterization experiments. Clearly, all overlayers studied had the same coverages within experimental error, and the coverages were close to that expected for a
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Figure 4. Sulfur (111) and (1h11) NIXSW profiles (dotted line) collected from a chemisorbed overlayer of 3-methylthiophene. The fits for the profiles are also shown (solid lines).
TABLE 1: Average Values of D and fCo Determined from (111) and (1h11) Profiles Collected from Chemisorbed Overlayers of 3-Methylthiophene and 3-Methoxythiophene 3-methylthiophene 3-methoxythiophene
plane
fco
D/Å
(111) (1h11) (111) (1h11)
0.90 ( 0.05 0.83 ( 0.05 0.87 ( 0.05 0.68 ( 0.05
0.49 ( 0.03 0.79 ( 0.03 0.47 ( 0.03 0.77 ( 0.03
saturated overlayer. The average values of fco and D obtained using the (111) and (1h11) planes are given in Table 1, and representative (111) and (1h11) sulfur profiles are displayed in Figure 4. The (111) sulfur profile was fitted with D ) 0.49 ( 0.03 Å and fco ) 0.90 ( 0.05. The fco value is within experimental error of that obtained for the copper substrate which indicates that the sulfur atom adopts a single well-defined adsorption site on the surface. The D value (the distance of the atom from the nearest (111) plane) is too small to be a feasible distance from the sulfur atom to the (111) plane which passes through the ideal (nonrelaxed) surface plane. This would indicate that the observed D value represents the distance of the sulfur atom from the next hypothetical (111) plane and that the atom is 2.57 ( 0.03 Å [D + 1 lattice spacing (2.08 Å)] above the (111) surface plane. The adsorption site for 3-methythiophene is determined unambiguously from (1h11) data. Given that the sulfur atom is 2.57 Å above the surface, expected (1h11) D and fco values for atop, bridging, and three-fold sites (both FCC and HCP) can be calculated; these are then compared with the experimentally observed values. In Table 2, a comparison is made between the experimental observed (1h11) fco and D values and those calculated from the observed (111) data. The best agreement between experimental (fco ) 0.83, D ) 0.79 Å) and calculated values is for the occupancy of an atop site (fco ) 0.90, D ) 0.86 Å). To summarize, the NIXSW data is consistent with the sulfur atom of 3-methylthiophene being located in an atop site, with a Cu-S separation (assuming no relaxation of the surface copper atoms) of 2.57 ( 0.03 Å (ii) 3-Methoxythiophene. Identical experimental and analysis/ interpretation methodologies to those outlined for 3-methyl-
Figure 5. Sulfur (111) and (1h11) NIXSW profiles (dotted line) collected from a chemisorbed overlayer of 3-methoxylthiophene. The fits for the profiles are also shown (solid lines).
TABLE 2: Comparison Made between the Experimentally Observed (1h11) D and fco Valves Obtained Experimentally for 3-Methylthiophene and 3-Metyhoxythiophene Overlayers and Those that Would Be Expected for Atop, Bridging, and Three-Fold Sites for Both Molecules, (FCC and HCP) Given the Observed (111) Values 3-methylthiophene
3-methoxythiophene
site
D (1h11)/Å
fco
D (1h11)/Å
fco
atop bridge F.C.C. H.C.P. expt
0.86 1.90 2.24 (0.16) 1.55 0.79
0.90 0.30 0.90 0.90 0.83
0.85 1.89 2.24 (0.16) 1.54 0.77
0.87 0.29 0.87 0.87 0.68
thiophene were used for 3-methoxythiophene. Surfaces studied had coverages of 3-methoxythiophene which ranged from 0.05 ( 0.02 to 0.08 ( 0.02 ML; these compare with a saturation coverage of 0.12 ( 0.03 ML determined in laboratory-based characterization experiments. The fco and D values obtained from NIXSW profiles collected using the (111) and (1h11) planes are listed in Table 1, and representative (111) and (1h11) sulfur profiles are shown in Figure 5. The data for 3-methoxythiophene is very similar to that obtained for 3-methylthiophene, which indicates that both molecules adopt similar adsorption sites. Using a similar argument to those used previously, the observed (111) D of 0.47 ( 0.03 Å implies a sulfur-surface plane separation of 2.55 ( 0.05 Å. The (1h11) data (Table 2) is consistent once again with an atop adsorption site for sulfur. The one slight difference between the 3-methyl and 3-methoxythiophene data is that in the latter case a (1h11) fco is observed which is slightly lower than expected, 0.69 ( 0.05 instead of 0.87 ( 0.05. This lower (1h11) fco arises from a larger than expected degree of uncertainty in the position of the sulfur atom with respect to the (1h11) plane. This uncertainty in position can originate either from vibrational motion parallel to the surface or a static displacement of the molecule. Unfortunately, from the current data, one cannot definitely say which of the two effects contribute to the lowering of the (1h11) fco. We could, however, on the basis of the fact that 3-methoxythiophene has a lower (1h11) fco than 3-methylthiophene speculate, that a static
Structure and Bonding of Thiophene
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5235 TABLE 3: Areas and Positions of the Six Guassian Peaks Used To Fit Representative NEXAFS Spectra of 3-Methoxythiophene and 3-Methythiophene Chemisorbed Overlayers Shown in Figures 6 and 7 3-methylthiophene resonance pre-π p s a b c
Figure 6. Grazing and normal incidence spectra (dotted lines) collected from a chemisorbed layer of 3-methylthiophene. Also shown are fits (solid lines) which were derived from the addition of six Gaussians and an ionization step (dashed lines).
displacement of the molecule, caused by the larger methoxy substituent is the origin of the reduced fco. In summary, the NIXSW data is consistent with the sulfur atom of the 3-methoxythiophene being located in an atop site and having a Cu-S separation of 2.55 ( 0.05 Å. NEXAFS. (i) 3-Methylthiophene. NEXAFS measurements were collected from surfaces which had 3-methylthiophene coverages ranging from 0.03 ( 0.02 to 0.07 ( 0.02 ML. In Figure 6 are two representative NEXAFS spectra collected with grazing and normal X-ray incidence. The grazing angle spectrum is dominated by a feature at 2467.9 eV, whereas the normal spectrum has a feature centered around 2468.7 eV. These spectra are very similar to those obtained in our previous studies of 3-chlorothiophene and thiophene. Assuming a “building block” approach7 to NEXAFS spectra, thiophene and 3-methylthiophene would be expected to share many of same resonances. The validity of this assumption in the case of thiophene derivatives is supported by previous work by ourselves and other groups2,11,12. We have shown that 3-chlorothiophene NEXAFS spectra can be modeled with the same resonances used to fit thiophene spectra. It has also been shown that thiophene related compounds such as bithiophene11 and polythiophenes12 give NEXAFS spectra very similar to that of thiophene. Applying the assignments made for thiophene to 3-methylthiophene, we can say that the feature at 2467.9 eV in the grazing spectrum has a significant contribution from a resonance arising from a S(1s) f π* transition. In contrast, the feature at 2468.7 eV in the normal incidence spectrum has a dominant contribution from a resonance due to a S(1s) f σ*(C-S) transition. The polarization dependence of both the π* and σ* resonances indicate that 3-methylthiophene is oriented with its ring parallel, or nearly parallel, to the surface. A more quantitative measure of the orientation of the thiophene ring can be obtained from the polarization dependence of the π* resonance. The intensity of the π* resonance was determined by fitting the 3-methylthiophene NEXAFS spectra with symmetric Gaussian peaks, and a Gaussian broadened step was used to model the absorption threshold or edge jump. The
3-methoxythiophene
position/ grazing/ normal/ position/ grazing/ normal/ eV area area eV area area 2465.9 2467.9 2468.8 2470.4 2471.9 2471.6
0.26 4.41 0.63 1.37 0.03 0.08
0.17 0.47 2.20 1.21 0.26 0.03
2465.1 2467.4 2468.1 2469.8 2470.6 2471.5
0.24 3.15 1.72 0.91 0.36 0.01
0.10 0.30 2.46 1.44 0.26 0.01
degree of broadening of the step was determined from 3-methylthiophene multilayer NEXAFS spectra, where the step edge was more discernible because of the relatively weak intensities of the resonances. Prior to curve fitting, all NEXAFS spectra were normalized to the edge jump. In the curve-fitting procedure, six peaks were used, which have been labeled pre-π*, π*, σ*, a, b, and c. The areas and positions of the six peaks used to fit the spectra are given in Table 3. A comparison between the peak and step positions used to fit the current data and those used by us in an earlier NEXAFS study of thiophene on Cu(111) is given in Table 4. We should state that the fitting parameters used in our earlier thiophene study were in good agreement with those determined in two previous NEXAFS studies involving thiophene by Stohr and co-workers13 and Ohta and co-workers14 If we consider the fitting of the 3-methylthiophene spectra, it should be noted that peak a is significantly more intense than both peaks b and c; however, the presence of these two small components did improve the quality of the fits. The intensity of the π* resonance for 3-methoxythiophene adsorbed on Cu(111) can be expressed as
I(θ) ∝ 1/3P[1 + 1/2(3 cos2 θ - 1)(3 cos2 R - 1)] + 1/2(1 - P) sin2 R (1) where P denotes the degree of polarization (P ) 0.9 in this case), θ is the angle of X-ray incidence, and R is the angle of the transition moment with respect to the surface.7 The orientation of the thiophene ring determined from the polarization dependence of the π* resonance was found to be 24 ( 4°. Although the polarization dependence of the σ* could have been used to determine orientation, this approach was rejected because the intensity of this resonance is sensitive to the absolute position of the step edge and the resonances a, b, and c. Clearly, the intensity of the σ* resonance could have a systematic error due to the positioning of the step edge. (ii) 3-Methoxythiophene. Identical experimental methodologies to those used for 3-methylthiophene were used for 3-methoxythiophene. NEXAFS data was collected from overlayers that had coverages of 3-methoxythiophene ranging from 0.06 ( 0.02 to 0.09 ( 0.02 ML. In Figure 7 are displayed representative grazing and normal incidence spectra collected from a 3-methoxythiophene overlayer. It is readily apparent that the 3-methoxythiophene spectra are very similar to those obtained for 3-methylthiophene. The grazing incidence spectrum is dominated by a feature at 2467.4 eV which has a major contribution from a π* resonance. The dominant peak in the normal spectrum is at 2468.2 eV and has a major contribution from the σ* resonance. Analysis of the spectra was done using identical procedures to those applied to 3-methylthiophene spectra. Once again the NEXAFS spectra were fitted with six
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TABLE 4: Positions of the Six Gaussians and the Edge Jump Used in the Fitting of 3-Methylthiophene and 3-Methoxythiophene Compared with Those Used To Fit Thiophene Spectra in a Previous Study (Reference 1) pre-π* π* σ* a b c ionization threshold
thiophene
3-chlorothiophene
3-methylthiophene
3-methoxythiophene
2466.5 2467.8 2468.7 2470.7 2471.7 2472.8 2472.4
2465.0 2467.1 2468.0 2469.1 2470.9 2471.5 2472.4
2465.9 2467.9 2468.8 2470.4 2471.9 2471.6 2472.4
2465.1 2467.4 2468.1 2469.8 2470.6 2471.5 2472.4
Figure 7. Grazing and normal incidence spectra (dotted lines) collected from a chemisorbed layer of 3-methylthiophene. Also shown are fits (solid lines) which were derived from the addition of six Gaussians and an ionization step (dashed lines).
Gaussian peaks and a Gaussian broadened step. The Gaussians used to fit both 3-methylthiophene and 3-methoxythiophene data had the same width (1.4 eV fwhm). An orientation for 3-methoxythiophene of 26 ( 4° was determined from the polarization dependence of the π* state. To summarize, NEXAFS data shows that the orientations of the thiophene ring in both 3-methylthiophene and 3-methoxythiophene are the same within experimental error. Discussion (i) Structure of 3-Methylthiophene and 3-Methoxythiophene. We will first discuss the current results on 3-methylthiophene and 3-methoxythiophene before placing them in the context of our previous work on thiophene and 3-chlorothiophene. Within experimental error, the local registries and orientations of the thiophene rings in 3-methylthiophene and 3-methoxythiophene are identical. From our NIXSW and NEXAFS results, there are a few details about the adsorption structure of both molecules which remain unknown. The first most obvious uncertainty is which conformation 3-methoxythiophene adopts in the adsorbed state. Distefano and coworkers,10 using a combination of photoelectron spectroscopy and ab initio calculations, determined that the s-cis conformer was the most stable form of 3-methoxythiophene in the gas phase with a 96% population. In the condensed phase, and in particular in a chemisorbed state, the relative stabilities of the conformers could be completely different to that observed in
the gas phase, and the s-cis conformer may not be the most stable form. Indeed, if, as we postulate, the three multilayer desorption states are due to the presence of the s-cis, s-trans, and gauche conformers in the condensed state, this would imply differing relative stabilities in the condensed phase than in the gas phase. Another interesting aspect of the TPD data is that the relative intensities of the three desorption peaks change with increasing exposure of 3-methoxythiophene. With initial multilayer formation, desorption predominantly occurs at 186 K, and with increasing exposure, two features at 180 and 196 K become more prominent. The origin of this phenomenon could be that chemisorbed molecules in a particular conformation could preferentially stabilize a conformation in the condensed layers. With increasing thickness of the condensed layer, the influence of the chemisorbed layer diminishes. Whatever conformation 3-methoxythiophene adopts on Cu(111), the similarity between the data obtained for methoxy and methyl substituted thiophenes shows that the choice of substituent does not significantly affect adsorption structure. A second aspect of the adsorption geometry which remains ambiguous is whether in the chemisorbed state the thiophene ring undergoes a small twist about an axis which runs through the sulfur atom and bisects the aromatic ring. This axis would be the C2 axis of a free thiophene molecule, and in discussions of substituted thiophenes, will be referred to as a psuedo C2 axis. In a previous study on the structure of 3-chlorothiophene on Cu(111), we were able to determine the heights of both the sulfur and chlorine atoms above the surface. From a comparison of the heights of the sulfur and chlorine atoms with the orientation of the thiophene ring determined from NEXAFS, we were able to speculate that the thiophene ring had twisted about its pseudo C2 axis in such a way as to bring the chlorine atom closer to the surface. Such a twist of the thiophene ring would not be detected by NEXAFS because its transition moment is parallel to the pseudo C2 axis. In the case of 3-chlorothiophene, we proposed that the driving force for such a twist was to bring the chlorine atom closer to the surface and, hence, maximize it interaction. A similar argument would be also valid in the cases of 3-methyl and 3-methoxythiophene. In summary, it is clear from the available data that the thiophene rings in both molecules have very similar adsorption structures with (within experimental error) identical ring orientations, adsorption site, and Cu-S separation. However, there are some aspects of the adsorption geometry of both 3-methyl and 3-methoxythiophene which cannot be discovered unambiguously with the current NEXAFS and NIXSW data, such as twists in the thiophene ring and the conformation of 3-methoxythiophene. (ii) Influence of Substituents. Finally, the results of this current study should be considered in the context of our previous work on the structure of thiophene and 3-chlorothiophene on Cu(111). This combined body of work represents the first systematic study of the quantitative effects of substituents on the bonding of an aromatic/heterocyclic species to a metal surface.
Structure and Bonding of Thiophene
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TABLE 5: Adsorption Site, Cu-S Separation, and Orientation of the Thiophene Ring for r and β Forms of Thiophene, 3-Chlorothiophene, 3-Methylthiophene, and 3-Methoxythiophene
R-thiophene β-thiophene 3-chlorothiophene 3-methylthiophene 3-methoxythiophene
adsorption site
Cu-S separation/ Å
ring orientation
atop atop atop atop atop
2.62 ( 0.03 2.83 ( 0.03 2.63 ( 0.05 2.57 ( 0.03 2.55 ( 0.05
26 ( 5° 44 ( 6° 23 ( 8° 24 ( 4° 26 ( 4°
The most significant effect of substituents on the bonding of thiophene is most clearly demonstrated in the initial TPD characterization data. As in the case of 3-chlorothiophene, only desorption from a single chemisorbed monolayer state is observed in TPD experiments of 3-methylthiophene and 3-methoxythiophene overlayers. This behavior contrasts with thiophene, where desorption occurs from two monolayer states (R and β) which are populated sequentially with increasing coverage. In our previous study, we were able to demonstrate that the origin of the second desorption state was due to a coverage-induced phase transition of thiophene, which involved an increase in the orientation of molecules within the layer and a decrease in the strength of the S-Cu bond. The absence of a second monolayer desorption state for the substituted thiophenes is strong evidence for the lack of a coverage driven phase transition. NIXSW and NEXAFS data also confirms that a phase transition does not occur. The local registry and orientation of the thiophene rings does not change significantly within chemisorbed overlayers of 3-chlorothiophene, 3-methylthiophene, and 3-methoxythiophene with increasing coverages up to saturation. Apart from inhibiting the reorientation of the thiophene ring, the next question to be considered is whether the substituents, both electron donating (MeO- and Me-) and withdrawing (Cl-), influence the bonding of the ring to the surface. Listed in Table 5 are the adsorption site, Cu-S bond separation, and orientation of the thiophene ring for the three derivatives studied. All of the substituted thiophenes and R- and β-state thiophene adsorb with the sulfur in an atop site. Within experimental error, R-state thiophene and the three thiophene derivatives have both the same Cu-S separation and orientation of the thiophene ring. Clearly, the only effect that substituents placed at the 3 position have on the bonding of the thiophene ring is to inhibit a coverage-induced phase transition. The inhibition of the phase transition for derivatives is not due to any significant change in the bonding of the thiophene ring to the surface. The substituents must have a more subtle influence on the bonding of the thiophene ring, which although inhibiting the phase transition does not significantly change the bonding geometry of the ring itself. We believe that such a subtle effect could be caused by a weak attractive interaction between the substituent group and the surface. This interaction would effectively pin the thiophene derivative to the surface preventing any reorientation of the ring with increasing coverage but would not be strong enough to change the bonding of the ring. On the basis of chemical intuition, one could imagine that both the Cl- and MeO- would be weakly attracted to the surface. In both cases, lone pair electrons on the chlorine and oxygen atoms would form an attractive interaction with the surface. The weakness of the interaction between the chlorine atom of the 3-chlorothiophene and the metal surface is demonstrated experimentally in our previous NIXSW study. The observed Cu-Cl separation of 3.51 Å is longer than the expected value of 3.03 Å derived from van der Waal radii.
Unlike MeO- and Cl- there are no lone pair electrons associated with the CH3 group which could form a weak bond with the surface. However, this does not mean that a CH3 group could not interact with a metal substrate. This is illustrated in previous RAIRS experiments15 which have shown a weakening of C-H bonds within hydrocarbons adsorbed on Cu(111) because of a C-H...Cu interaction. To summarize, the general conclusion that can be drawn from this and our previous work is that substituents do not significantly affect the bonding of a thiophene ring to Cu(111). This can be rationalized if the interaction between the sulfur atom and the surface was the most dominant in bonding. If a π interaction only had a minor role in the bonding of thiophene to Cu(111), then any change in the electronic structure of the aromatic ring brought about by substituents would not significantly affect the overall molecule-surface interaction and adsorption structure would not be affected. There is evidence in the literature to support the suggestion that the bonding of thiophene to Cu(111) is dominated by a sulfur-copper interaction. In a theoretical study, Rodriguez16 found that the bonding of thiophene to Cu(100) was dominated by the sulfur atom, with >95% of the bonding being due to this interaction. Although substituents do not have a gross effect of the bonding of the thiophene ring, they do inhibit a coverage-driven phase transition of the thiophene ring. Clearly, the inhibiting properties of the substituent do not originate from any significant change in the structure of the thiophene ring of the adsorbed derivatives. We do believe, however, that the substituents interact strongly enough with the surface to effectively pin the thiophene derivatives in a geometry with their thiophene rings almost parallel to the surface but not strongly enough to perturb the bonding of the ring to the surface. Acknowledgment. M.K. and D.L. gratefully acknowledge the support of the Royal Society for the award of an equipment grant. The EPSRC are thanked for the provision of a research studentship to P.K.M. D.L. thanks Imperial Chemical Industries for the award of an ICI Lectureship in Heterogeneous Catalysis. References and Notes (1) Milligan, P. K.; Murphy, B.; Lennon, D.; Cowie, B. C. C.; Kadodwala, M. F. J. Phys. Chem. B. 2001, 105, 140. (2) Milligan, P. K.; Murphy, B.; Lennon, D.; Cowie, B. C. C.; Kadodwala, M. F. 1999, 430, 45. (3) Gellman, A. J. Acc. Chem. Res. 2000, 33, 19 and reference therein. (4) Terborg, R.; Pocik, M.; Hoeft, J.-T.; Kittel, M.; Pascal, M.; Jang, J. H.; Lamont, C. L. A.; Bradshaw, A. M.; Woodruff, D. P. Surf. Sci. 2000, 457, 1. (5) Zhou, X.-L.; White, J. M. J. Phys. Chem. 1990, 92, 5612. (6) McDowell, A. A.; Norman, D.; West, J. B.; Campuzano, J. C.; Jones, R. G. Nucl. Instrum. Methods A 1986, 246, 131. (7) Stohr, J. NEXAFS Spectroscopy; Springer: Berlin, 1992. (8) Woodruff, D. P.; Cowie, B. C. C.; Ettema, A. R. H. F. J. Phys. Condens. Matter. 1994, 6, 10633. (9) Domange, J. L.; Oudar, J. Surf. Sci. 1968, 11, 124. (10) Distefano, G.; de Palo, M.; Dal Colle, M.; Modelli, A.; Jones, D.; Favaretto, L. J. Mol. Struct (THEOCHEM) 1997, 418, 99. (11) Ramsey, M. G.; Netzer, F. P.; Steinmuller, D.; Steinmuller-Nethl, D.; Lloyd, D. R. J. Chem. Phys. 1992, 97, 4489. (12) Tourillon, G.; Mahatsekake, C.; Andrieu, C.; Williams, G. P.; Garrett, P. R.; Braun, W. Surf. Sci. 1988, 201, 171. (13) Hitchcock, A. P.; Horsley, J. A.; Stohr, J. J. Chem. Phys. 1986, 85, 4835. (14) Terada, S.; Yokoyama, T.; Sakano, M.; Imanishi, A.; Kitajima, Y.; Kiguchi, M.; Okamoto, Y.; Ohta, T. Surf. Sci. 1998, 414, 107. (15) Raval, R.; Parker, S. F.; Chesters, M. A. Surf. Sci. 1993, 289, 227. (16) Rodriguez, J. A. Surf. Sci. 1990, 234, 421.
