J. Phys. Chem. B 2006, 110, 1083-1090
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Lifting the Mirror Symmetry of Metal Surfaces: Decoupling the Electronic and Physical Manifestations of Surface Chirality Andrew Mulligan,† Ian Lane,‡ Gilles B. D. Rousseau,† Shona M. Johnston,† David Lennon,† and Malcolm Kadodwala*,† Department of Chemistry, Joseph Black Building, UniVersity of Glasgow, Glasgow G12 8QQ, U.K., and Department of Chemistry, DaVid Keir Building, Queen’s UniVersity of Belfast, Belfast, BT9 5AG, U.K. ReceiVed: September 28, 2005; In Final Form: NoVember 7, 2005
Naturally occurring metal surfaces possess planes of mirror symmetry on the nanometer-length scale. This mirror symmetry can be lifted and chirality “physically” conveyed onto a surface by adsorbing a chiral molecule. Until now, it has not been known whether the conveying of chirality is limited to just the physical structure or whether it goes deeper and permeates the electronic structure of the underlying surface. By using optically active second harmonic generation (OA-SHG), it is demonstrated that the adsorption of some, but not all, chiral molecules can reversibly, and without significant structural rearrangement, measurably lift the mirror symmetry of the surface electronic structure of a metal. It is proposed that the ability of a chiral molecule to place a significant “chiral perturbation” on the electronic structure of a surface is correlated to its adsorption geometry. The microscopic origins of the observed optical activity are also discussed in terms of classical models of chirality. The results of the study challenge current models of how chiral adsorbates induce enantioselectivity in the chemical/physical behavior of heterogeneous systems, which are based on geometric/ stereochemical arguments, by suggesting that chiral electronic perturbations could play a role.
Introduction The importance of chiral technologies,1 such as biosensing, chiral separation, and heterogeneous asymmetric catalysis, has driven the recent growth in the number of studies of surface chirality. In many of the chiral technologies, an achiral surface is modified by the adsorption of a chiral molecule into displaying enantiospecific chemical and physical properties. Current understanding of the action of chiral modifiers is based on geometric arguments. For instance, in heterogeneous asymmetric catalysis, the chiral modifier interacts with the achiral reactants, controlling the stereochemistry of the surface transition state, and consequently, of the reaction products. In chiral recognition, the adsorbed chiral molecule preferentially forms a diastereomeric complex with a particular enantiomer. The majority of fundamental studies have consequently concentrated on the physical manifestations of chirality at interfaces.2-18 There have been no previous experimental studies to determine whether chiral adsorbates are not only able to lift the mirror symmetry of the physical surface, but can also instill chirality on the underlying surface electronic structure. The instilling of chirality onto the electronic structure by an adsorbate is a possible mechanism by which enantiospecific chemical and physical properties are conveyed onto an initially achiral surface. By the application of optically active second harmonic generation (OA-SHG), evidence is provided that some, but not all, chiral adsorbates can both lift the mirror symmetry of a physical surface and place a significant chiral perturbation on the surface electronic structure. In this paper, we present OASHG data from two chiral adsorbate substrate systems (R)- and (S)-2-butanol/Cu(111) and (R)- and (S)-1-(1-naphthyl)ethylamine *
[email protected]. † University of Glasgow. ‡ Queen’s University of Belfast.
(NEA)/Cu(111), along with analysis based on established theory. The data are further interpreted to provide information on the microscopic origin of the observed optical activity. This work builds upon the foundations laid by our previous work.19 Experimental Section Experiments were performed in two ultrahigh vacuum (UHV) systems. Temperature-programmed desorption (TPD) data were collected in a system that has been described in detail previously.20 SHG, UV photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS) measurements were performed in a second system, which will be briefly described. The system was equipped with the usual sample preparation facilities and low-energy electron diffraction (LEED) optics for confirming crystal quality. In addition, it has a discharge lamp (VG Ltd) which provides He(I) radiation and a concentric hemispherical analyzer (CHA) (CLAM 2 VG Ltd). All UPS spectra were collected in normal emission and with radiation incident at 47° to the surface normal. In both UHV systems, the Cu(111) surface was cleaned by cycles of Ar+ bombardment (1 keV, 40 min, ca. 16 µA) followed by annealing to 900 K. Surface cleanliness was monitored by electron beam atomic emission spectroscopy (AES, collected using an RFA) in the TPD system, while XPS was used in the second system. In both cases, LEED was used to monitor surface quality. All SHG measurements were performed with the light incident along the [011h] direction of the Cu(111) surface. A schematic of the optical setup is shown in Figure 1. SHG measurements were performed using 8-12 ns pulses of fundamental Nd:YAG laser (Spectra Physics Quanta Ray) 1064 nm radiation at a repetition rate of 10 Hz and ∼3.6 mJ/pulse. The beam was defocused (10 mm diameter) and was incident on the crystal at 60° with respect to the surface normal; the
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[
1 eee 1 eee 3 eee x3 I/II 2 - 2 χxxz + 4 χzxx + 4 χzzz fp ) (t ) eem mee 2 p + (χeem xzy - χzxy + χxyz ) × nII
[
]
1 eem eee eem x3 I/II I/II χxyz + 2(χxxz + χxzx ) × nII + hp ) t t 2 s p 3(χeem - χeem) × n + 2χmeen zzz II xxz II 2 zxx
(2e)
]
(2f)
In this formalism, fs, gs and hp involve only chiral tensor elements of the second-order susceptibility χ, which are zero for an achiral surface (C∞V) and change sign for interfaces of opposite handedness, while fp, gp and hs involve only achiral elements. Using eq 2a-f, the following expression for the SH intensity expected for a polin measurement can be derived: Figure 1. Schematic diagram of the optical setup used to obtain SHG data.
incident polarization was varied using a λ/2 plate. The SH signal (532 nm) was monitored at 60° with respect to the surface normal: it first passed through a polarizer before being focused into a spectrograph and then detected on an intensified CCD camera. Two types of SHG measurements were performed in this study. In the first, the intensity of p-polarized SH radiation is monitored as a function of the polarization of the incident radiation. In the second, p-polarized radiation is incident on the surface, and the SH intensity is monitored as a function of its outgoing polarization. For brevity, these two experimental methodologies will subsequently be referred to as polin and polout. Theory A theory for optically active SHG that includes both the dipole and “magnetic” (magnetic dipole and electric quadrupolar) contributions was initially developed by Maki and coworkers21 and then further refined by Schanne-Klein and coworkers.22,23 In the formalism developed by these groups, the field of the reflected SH radiation can be expressed as a function of the p- and s-polarized (Ep(ω) and Es(ω)) components of the incident beam
Ers,p(2ω) )
i4πω/ctII/I s,p [f E 2(ω) + gs,pEs2(ω) + hs,pEsEp(ω)] nII cos θII s,p p (1)
where ts,pI/II are the Fresnel coefficients for s- and p-polarized radiation, nII is the refractive index of the medium, θII is the angle of incidence, and
[
eee eem x3 I/II 2 χxyz + 2χxzx nII + (t ) 1 mee fs ) 1 3 2 p χ n - χmeen - χmeen 2 xxz II 4 zxx II 4 zzz II
hs )
]
(2a)
x3 I/II I/II eee eem eem t t [2χxxz sin θII - (χmee xyz + χxzy + χxyz ) × nII] 2 s p (2b) x3 I/II 2 eem (t ) [2χxxz + χmee zxx ]nII 2 s
(2c)
x3 I/II 2 eee eem (t ) [χzxx - (χeem xyz + χzxy ] × nII] 2 s
(2d)
gs ) gp )
I2ω(θ) k2LD
[
(fp′ + g′p + (f′p - g′p) cos 2θ + h′p sin 2θ)2 + (fp′′+ g′′p + (f′′p - g′′p) cos 2θ + h′′p sin 2θ)2
]
(3)
where KLD is a proportionality constant, fs,p ) fs,p ′ + ifs,p ′ , hs,p ) h′s,p + ih′′s,p, gs,p ) g′s,p + ig′′s,p, and θ is the angle between the incident polarization and the p-direction. In a similar way, the following expression for the SH intensities expected for a polout measurement can be derived:
I2ω(δ) ) (|fp|2 cos2 δ + |fs|2 sin2 δ + 2(fp′fs′ + fp′′fs′′) sin δ cos δ)|Ep(ω)|4 (4) δ is the angle between the detection polarization and the p-direction. Results A. Preparation and Characterization of Layers. 1. 2-Butanol. The adsorption behavior of 2-butanol on Cu(111) was characterized using a combination of TPD, XPS, and UPS. Data from all three techniques are consistent with 2-butanol undergoing reversible molecular adsorption and only being weakly perturbed by adsorption. In Figure 2, nested TPD data collected for sequentially higher coverages of racemic 2-butanol are displayed; identical spectra are obtained for (R)- and (S)-2butanol. Three states are observed in the TPD data at 170, 210, and 270 K, which are attributed to desorption from condensed multilayers, the monolayer, and defect sites, respectively. On the basis of these results, saturated monolayers of racemic, (R)-, and (S)-2-butanol were routinely produced by annealing multilayers to a temperature just sufficient to desorb the condensed layers. By using C 1s XPS spectra (Figure 3a), it was determined that the monolayers of racemic, (R)-, and (S)-2-butanol formed by this procedure had the same absolute coverage within experimental error. C 1s spectra collected after annealing the monolayers to room temperature showed an absence of carbon, indicating that no dissociation had taken place. UPS spectra were collected from the adsorbed monolayers to determine the extent to which the electronic structure of the 2-butanol was perturbed from the gas phase. The UPS spectrum collected from the clean Cu(111) surface, Figure 4b, is in agreement with those taken by Westphal and Goldmann24 in a previous study. The clean Cu(111) spectrum is dominated by a highly structured d-band emission between 2.2 and 4.4 eV. In Figure 3b are UPS (hν ) 21.2 eV, He (I)) collected from monolayers of (R)- and (S)-2-
Mirror Symmetry of Metal Surfaces
Figure 2. TPD spectra showing the desorption of 2-butanol (fraction 59 amu) from surfaces which had been dosed with sequentially higher exposures (0.5, 1, 2, 4, 5 × 10-6 mbar‚s) at ca. 100 K. All spectra were collected with a heating rate of 0.5 K s-1. Desorption is observed from defect (270 K), chemisorbed (210 K), and multilayer (170 K) states.
butanol and a multilayer of (R)-2-butanol. In the monolayer spectra, the sharp d-band structure is completely quenched; this is consistent with previous work which has found that translationally disordered saturated monolayers of molecular adsorbates causes a complete quenching of this structure. Both (R)- and (S)-2-butanol monolayers display adsorbate-induced bands at 10.5, 8.8, 7.6, and 6.2 eV, the relative intensities of which are
J. Phys. Chem. B, Vol. 110, No. 2, 2006 1085 the same for both layers. The strong similarity between the relative intensities of the bands in the two spectra indicates that the R and S enantiomers adopt similar adsorption geometries within the adsorbed layers. The relative position of the adsorbate-induced bands from the monolayer is identical to that in the multilayer spectrum, indicating that the electronic structure of 2-butanol is not significantly perturbed by adsorption. This observation illustrates the weak interaction between adsorbate and surface, which is also supported by the relatively low desorption temperature of the monolayer and the lack of dissociation. Neither (R)- nor (S)-2-butanol display long-range translational order as evidenced by LEED. 2. NEA. NEA overlayers with sub-monolayer coverage were prepared by exposing the crystal at 110 K to an appropriate amount of vapor. By using this procedure, overlayers of R- and S-NEA with the same absolute coverage, as monitored by C 1s XPS, could be produced. The C 1s signal from these NEA layers is 30 ( 5% of that obtained from the saturated 2-butanol monolayers, which clearly points to a sub-monolayer coverage. Figure 4a presents C 1s spectra collected from a sub-monolayer of NEA adsorbed at 110 K and after subsequent annealing to 223 and 273 K; within experimental error, the amount of carbon on the surface remains constant up to 223 K. However, further annealing to 273 K results in the disappearance of all carbon, which is consistent with the complete desorption of NEA and the lack of any dissociation, which would leave a residual C 1s signal from chemisorbed moieties. UPS data from R- and S-NEA layers are displayed in Figure 4b; in contrast to 2-butanol, the sharp d-band structure is only partially quenched, which is consistent with a sub-monolayer coverage. Adsorbateinduced bands are observed at 10.7, 8.5, and 6.3 eV, with peaks in both the R- and S-NEA spectra displaying the same relative intensities. The relative positions of the adsorbate-induced bands are similar to those observed for the multilayer, indicating once again a weak adsorbate-substrate interaction. The adsorbed layers of NEA display no long-range order as evidenced by LEED. SHG Measurements. Figure 5 depicts profiles obtained from polout and polin experiments from a clean surface. The polin
Figure 3. (a) C1s XP spectra of clean Cu(111) (crosses), (R)-2-butanol (squares), (S)-2-butanol (down-triangles), and monolayer racemic (uptriangle) 2-butanol. Also shown is an XP spectrum recorded after completion of experiments (diamonds). (b) Normal emission UP spectrum of condensed multilayers (×0.5); racemic, R, and S monolayers; 2-butanol adsorbed on Cu(111). All spectra are plotted relative to EF.
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Figure 4. (a) 1s XPS spectra collected from an adsorbed NEA layer at 173 K (diamonds) and after subsequent desorption of that layer. The XPS data show the absence of residual carbon on the surface after desorption (squares), indicating that no dissociation had taken place, which is indicative of a weak interaction. (b) Normal emission UP spectrum of clean Cu(111) (×0.5), after adsorption of S-NEA and R-NEA, and subsequent annealing to 173 K. All spectra are plotted relative to EF.
Figure 5. (a) Polin profile of SH intensity vs λ/2 angle (squares) is displayed with the best-fit cos4 θ curve. The λ/2 angles for incident p- and s-polarizations are labeled. (b) Polout profile of SH intensity vs polarizer angle (squares) is displayed with the best-fit cos2 δ curve. The polarizer angles for outgoing s- and p-polarizations are labeled.
profile is symmetrical, as would be expected for an achiral surface, with a maximum for incident p-polarization and an SH signal below a detectable limit, as the polarization tends to s. The polin profile is fitted well with an I ∝ cos4 θ relationship. The clean Cu(111) surface has C3V symmetry; consequently, only four tensor elements of the second-order susceptibility are nonvanishing: xxx, xxy, zxx, and zzz.25 The cos4 θ dependence of the SH signal reflects a quadratic dependence on the z-component of the intensity of the incident radiation. Such a dependence implies that χzzz is significantly larger than the three other nonvanishing elements of the (111) surface. The polout profile from the clean Cu(111) is once again symmetric, but
displays a reasonable fit with a cos2 δ relationship. Such a cos2 δ dependence is expected from a C3V (111) where the zzz is solely dominant. The dominance of the χzzz element for Cu(111) follows the behavior of other metal surface surfaces. Previous studies on Pd(111),26 Au(111),27 Ag(111),27 and Cu(110)28 have shown that the χzzz elements are at least twenty times larger than the next largest nonvanishing element. In Figure 6 are shown the polout and polin profiles from layers of (R)-, (S)-, and racemic 2-butanol. The shapes of the profiles are identical, with the same absolute SH intensity, to that found for the clean surface, clearly showing that no measurable optical response is observed from the 2-butanol/Cu(111) interface.
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J. Phys. Chem. B, Vol. 110, No. 2, 2006 1087
Figure 6. (a) Polout and (b) polin profiles obtained from racemic (squares) and layers of (R)- (up-triangle) and (S)- (down-triangle) 2-butanol are displayed. The position of in/outgoing p- and s-polarizations are labeled along with median points ((45°).
Figure 7. Polin profiles obtained after desorption (squares) and layers of R- (up-triangle) and S- (down-triangle) NEA are displayed. The position of in/outgoing p- and s-polarizations are labeled along with median points ((45°).
Similarly, polout and polin profiles for R- and S-NEA are displayed in Figures 7 and 8. However, the polin profiles (Figure 7) display no asymmetry and are very similar to the clean surface, with no change in the absolute SH upon adsorption. The polout profiles (Figure 8) do display pronounced asymmetries about the p-polarization direction, with the R-NEA and S-NEA profiles displaying maxima at 45° and -45°, respectively. The asymmetry of the two profiles has been parametrized in two ways: the first method uses the I45/I-45 ratio, while the
Figure 8. Polout profiles for obtained after desorption of NEA layer (squares) and layers of R- (up-triangle) and S- (down-triangle) NEA are displayed. An R - S difference profile (circles) is also displayed along with the best-fit sin2 R curve. The polarizer angle has been defined such that 0° is outgoing p-polarization. The data in the R and S profiles display a greater noise level than that collected for the postdesorption profile.
second method involves taking the ratio of the integrated area at negative (I-) and positive (I+) angles. All these ratios are listed in Table 1. Both types of ratio show that the R and S profiles are asymmetric, and they are those expected for an optically active measurement: I45/I-45 for one enantiomer is equal to the I-45/I45 ratio of the other. Similar polout measure-
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TABLE 1: Parameters Showing the Asymmetry of SH Profiles Obtained from S- and R-NEA Monolayersa R-NEA S-NEA
I45/I-45
I-45/I45
I+/I-
I-/I+
1.50 ( 0.10 0.75 ( 0.07
0.67 ( 0.07 1.33 + 0.13
1.14 ( 0.06 0.90 ( 0.06
0.88 ( 0.05 1.11 ( 0.07
a The tabulated values are averages obtained from five freshly prepared surfaces. The quoted errors correspond to one standard deviation in ratios.
ments have been collected after the NEA layers had been removed from the surface through annealing. The obtained profiles are once again symmetric about the p-polarization direction. Before pursuing a more detailed analysis of OA-SHG data, the influence that the adsorbate layer and metal surface have on the SH response must first be considered. The second-order susceptibility (χint) of the adsorbate-covered Cu(111) surface has two contributions, one from the metal-adsorbate interface (χmetal), which is governed by the electronic structure of the metal and a second from the adsorbate-vacuum interface (χads). The results of previous SHG studies of adsorbate-covered singlecrystal metal surfaces have been interpreted on the basis that χmetal . χads,29 consequently any changes in SH signal upon adsorption are the result of a perturbation of the surface electronic structure, rather than a direct contribution by the adsorbate. The validity of χmetal . χads is supported by our own experimental results. The absence of any significant change in the shapes of the polout and polin profiles on going from clean Cu(111) to 2-butanol/Cu(111) indicates that relative sizes of the χ tensor elements remain the same and that the zzz element still dominates the χint of the adsorbate-covered surface. This would not be expected if χads made a significant contribution to the χint, particularly since the polar OH bond of 2-butanol is expected to be close to parallel to the surface. A similar argument can be applied to NEA, where no difference is observed in the polin data. Since the highly polarizable aromatic ring of NEA is close to the plane of the surface (x-y plane), one would expect that, if χads made a significant contribution to χint, then the zzz element would not be solely dominant. Apart from being consistent with our experimental observation, the validity of χmetal . χads is also supported by the known properties of the metal. The χCu value for Cu(111) at a fundamental wavelength of 1064 nm is ≈ 10-12 esu,30 which is 2 orders of magnitude greater than the value expected from a monolayer of an organic molecule with intrinsically large χads, such as a charge-transfer system. In the case of NEA and 2-butanol, which do not have a charge-transfer band at either the fundamental or SH wavelength, the disparity between χCu and χads would be considerably larger. Although there is no available experimental data for χ(2) of alcohols, there is considerable literature on aromatic compounds, which provide a valuable comparison for NEA. In this previous work, the hyperpolarizabilities (β) of aromatic compounds were all found to be e10-30 esu.31,32 Assuming this as an upper limit for the β value of a NEA molecule, and using the surface density of NEA, a χ(2) of the adsorbed layer can be calculated. Bonello and co-workers9 found that a saturated monolayer of NEA contains ∼8.2 × 1013 molecule cm-2, which would indicate that our submonolayer surfaces have an average density of ∼2.5 × 1013 molecule cm-2. With this surface density, the χ(2) of the sub-monolayer NEA layer would be e2.5 × 10-17 esu, a value which is at least 4 orders of magnitude smaller than that of the Cu substrate. Analysis of SHG Measurements. The assumption that χmetal . χads has a profound affect on the interpretation of an optical
activity SH response from an adsorbate-metal interface. Under this assumption, an optically active SH response would only be observed if a chiral adsorbate perturbs a surface in such a way that the intrinsic mirror symmetry of some/all of the surface electronic states within the vicinity of EF is lifted. The polin and polout profiles from chiral adsorbate-covered metal surface will have a general form of
I2ω(θ) ) Iachiral + k2LD
[
(fp′ + g′p + (fp′ - g′p) cos 2θ + h′p sin 2θ)2 + ... ...(fp′′+ g′′p + (fp′′- g′′p) cos 2θ + h′′p sin 2θ)2
]
(5)
I2ω(δ) ) Iachiral + (|fp|2 cos2 δ + |fs|2 sin2 δ + 2(fp′fs′ + fp′′fs′′) sin δ cos δ)|Ep(ω)|4 (6) Iachiral is the SH response which originates from achiral electronic states of the metal. These achiral electronic states could either be associated with regions of clean Cu(111) surface or metal states which retain mirror symmetry in the presence of the chiral adsorbates. It is clear from the data that only NEA lifts the mirror symmetry of the surface electronic structure of Cu(111). However, only the polout profile, Figure 8, displays an observable optically active response, and the polin profiles retain a cos4 θ dependence. I2ω(θ) will only display a cos4 θ dependence if fp > hp and gp. The fact that fp > gp is consistent with the discussed behavior of metallic surfaces, where χzzz is at least 2 orders of magnitude greater than other nonvanishing elements and only fp is dependent upon χzzz. For a polin measurement, an optically active response arises from the hp term (eq 5), while for polout, the fs term is responsible (eq 6). It would initially appear surprising that an optically active response is only observed in polout measurements when both fs and hp contain chiral tensor elements of second-order susceptibility. Indeed, both hp and fs have the same dependence on the dipole “chiral” tensor element χeee xyz , which must be the dominant term if an asymmetry is observed in a polout measurement,23 consequently
fs I/II 2 (tp )
≈
hp I/II I/II tp ts
≈ 2χeee xyz sin θII cos θII
(7)
However, the lack of an observable optically active response in the polin data is consistent with the SH signal originating from the Cu surface and can be rationalized in terms of the I/II relative size of the Fresnel coefficients tI/II p and ts . Using bulk I/II 33 optical data for copper, we can calculate that tI/II p /ts ) 7.5, thus fs/hp ) 7.5. So, since we can neglect gp, in eq 5 hp is relatively smaller in comparison to the achiral component fp than fs to fp in eq 6. Consequently, for a chiral response dominated by the Cu surface, chiral effects should be more pronounced in polout rather than in polin measurements. We now go on to determine whether the forms of the polout profiles are consistent with that expected from eq 6. Since the R- and S-NEA overlayers have the same coverage, it is valid to assume that the Iachiral contributions to the polout profiles for these two surfaces would be identical. The chiral term fs reverses sign on going from the R- to S-NEA overlayer, while |fs|2 and
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|fp|2 are identical for each layer. Consequently, the following expression can be derived for the form of the difference profile:
I2ω(δ)L-R ) [2(fp′fs′ + fp′′fs′′) sin 2δ]|Ep(ω)|4
(8)
In Figure 8 is the polout difference spectrum obtained from the R- and S-NEA data; it is well-fitted to a sin 2δ dependence. Clearly, the form of the polout data can be readily reproduced using the established formalisms of OA-SHG. Discussion Our experimental data clearly shows that, while all chiral molecules when adsorbed lift the inherent mirror symmetry of the physical surface, not all can then further go on to instill a measurable chiral perturbation onto the surface electronic structure of the metal. In the present study, NEA reversibly instills chirality onto the electronic structure, while 2-butanol does not. Several previous studies have shown that a chiral adsorbate can restructure (etch) achiral metal surfaces, producing homochiral facets. Although they have not been detected experimentally, it is reasonable to assume that these facets have chiral electronic states associated with them. Consequently, through the etching process, parts of the surface have the mirror symmetry of the electronic structure lifted. We believe that our data is inconsistent with a chiral etching process for the following reasons. NEA interacts weakly with Cu(111), adsorbing reversibly without forming strong chemical bonds, which are required for etching. The instilling of chirality is observed at cyrogenic temperatures, which also reduces the likelihood of a thermally activated etching process. Finally, chiral facets remain on the surface after the removal of the adsorbate and are stable up to the annealing temperature of the metal (ca. 900 K for Cu); this is at odds with the reversible nature of the instilling process observed by us. Clearly, an alternative explanation, which does not require a gross chiral restructuring of the metal surface, must be developed to rationalize the observed reversible instilling of chirality. A useful starting point for understanding how chirality can be conveyed onto the electronic structure of a metal surface is to draw a comparison with chiral metal complexes.34 The electronic structure of a metal ion is chirally perturbed in a chiral ligand environment, as evidenced by circular dichroisms observed in the absorption spectroscopy of metal-metal transitions. The electronic orbitals of the metal ion are symmetry-adapted to the crystal or ligand field of the directly ligating atoms, and overlap between metal and ligand electronic states can be ignored. This situation can be thought to be strongly analogous with present chiral adsorbate-subtrate systems. Both 2-butanol and NEA interact only very weakly with Cu(111), which indicates that there is negligible mixing of the electronic states of the adsorbates and those of the metal surface. Consequently, the lifting of the mirror symmetry of the electronic structure must be due to the exertion of a chiral perturbation on the surface by the adsorbate. To explain the differing behavior of 2-butanol and NEA, we suggest that the ability of an adsorbate to exert a chiral perturbation on a surface is intimately connected with the adsorption geometry adopted. 2-Butanol, consistent with other alcohols, bonds to Cu(111) through the lone pair of electrons on the oxygen35 and with the oxygen likely to be directly above a Cu atom. In this bonding geometry, the three other inequivalent groups attached to the chiral center are effectively pointing away from the metal surface, and 2-butanol only has a “single point of contact” with the surface. In the case of NEA, a geometry is adopted where the aromatic ring is
Figure 9. (a) adsorption geometry of NEA9 on Cu(111) and (b) adsorption geometry of 2-butanol.5
tilted toward the surface,9 which results in three inequivalent groups attached to the chiral center in close proximity to the surface, Figure 9. Three inequivalent noncollinear points of contact are a minimum requirement to lift the mirror symmetry of a surface. Consequently, NEA can place a chiral perturbation on the surface, while 2-butanol cannot. Finally, the microscopic origins of the observed optical activity will be discussed in terms of two established (Kuhn36 and Kauzmann37) models used to describe chiral molecular systems. In the Kuhn model, a chiral system is described in terms of coupled oscillators in a chiral arrangement. While in the Kauzmann model, a chiral system is described by two electrons which oscillate along two identical helices having a common axis, the motion of the electrons is such that they are always diametrically opposite each other across the common axis. Hache and co-workers have applied these two classical models of chirality to rationalize optically active SHG measurements.23,38 It was established that optically active SHG responses in polout measurements are obtained from chiral systems that can be described by the classical Kuhn model, with the optical activity in these systems arising from the electric terms. This indicates that the instilled chirality of the surface electronic structure can be best described by the Kuhn model. We believe that such a description of the chiral nature of the electronic structure is realistic for the following reasons. The electronic structure of the clean Cu(111) surface has intrinsic threefold
1090 J. Phys. Chem. B, Vol. 110, No. 2, 2006 symmetry and belongs to the C3V point group. Although the SHG data reveals that the mirror symmetry of the electronic structure is lifted by the adsorption of NEA, it does not allow the actual point group to be determined. However, since the Cu(111) surface is not physically reconstructed by the adsorption of NEA, we suggest that the electronic structure retains the threefold symmetry and hence belongs to the C3 point group. A surface electronic structure with C3 symmetry is analogous to a chiral molecule with three inequivalent coupled oscillators. Summary In this study, we present data indicating that the physical and electronic aspects of surface chirality can be decoupled experimentally. It has been demonstrated that, while an achiral metal surface will have its “physical” mirror symmetry lifted by the adsorption of any chiral molecule, this is not a sufficient condition to instill a measurable chiral perturbation onto the underlying electronic structure. This is a highly significant observation, as it shows that the point group symmetry of the surface electronic structure may not directly reflect that of the adsorbate. We propose that the ability of an adsorbate to convey chirality onto the electronic structure of a metal surface is related to its adsorption geometry. Specifically, an adsorbate can only chirally perturb the electronic of a metal if it adopts an adsorption geometry where at least three (noncollinear) inequivalent groups are in close proximity to the surface. The observation that some but not all chiral adsorbates can instill chirality onto the electronic structure of a surface has implications for the understanding of how the introduction of a chiral adsorbate can induce enantiospecific physical and chemical properties in heterogeneous systems. For instance, current models for how chiral adsorbates induce asymmetry in surface reactions are based on solely geometric arguments,39-41 where the adsorbed chiral modifier interacts with the achiral reactants, controlling the stereochemistry of the surface transition state and consequently of the reaction products. The models can explain some but not all of the features of enantioselective reactions, and in particular, they cannot predict in a meaningful way the effect of structural variations of the modifier on enantioselectivity.42 We suggest that, to fully model the action of a chiral modifier, its ability to instill chirality on the surface electronic structure should be taken into account. In the case where a modifier instils chirality onto the electronic structure, the absolute configurations of a chiral transition state and chiral product would be expected to govern how strongly they interact with the metal surface, which will influence directly the thermodynamics of the reaction and consequently enantioselectivity. Our observations offer an alternative explanation for the intense circular dichroism observed in metal transitions of Au nanoparticles that are surrounded by an overlayer of chiral molecules.43 It has been suggested that chirality is induced in the Au nanoparticles through an adsorbate-induced reconstruction. Clearly, our results would suggest that the electronic structure of the Au particle could be conveyed with chirality without significant reconstruction. Acknowledgment. Dr. L. Hecht is thanked for his help with the design of the laser system. A.M., S.M.J., and G.B.D.R. would like to thank the EPSRC for the provision of studentships. M.K. would like to thank the EPSRC and the University of Glasgow for funding. References and Notes (1) Sheldon, R. A. Chirotechnology; Marcel Dekker: New York, 1993.
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