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Reflection FTIR Studies of the Conformation of 2,2′-Bipyridine Adsorbed at the Au(111) Electrode/ Electrolyte Interface Manjali Hoon-Khosla,†,‡ W. Ronald Fawcett,‡ John D. Goddard,† Wei-Quan Tian,† and Jacek Lipkowski*,† Guelph-Waterloo Center for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, N1G 2W1 Canada, and Department of Chemistry, University of California, Davis, California 95616 Received August 11, 1999. In Final Form: November 10, 1999 Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been employed to study the conformation of 2,2′-bipyridine (22BPY) adsorbed at the Au(111) electrode surface. In addition, ab initio Hartree-Fock calculations were carried out to calculate the IR spectra for the transplanar, cis-planar, and twisted conformations both for an isolated 22BPY molecule and for the molecule attached to a gold atom or ion. The spectra for the trans- and the cis-conformations were predicted to be distinctly different. The differences between the predicted spectra and the differences for most structural parameters between the cis-planar conformer and the torsional conformations with a twist of 0° to 40° between the two rings were small. SNIFTIRS spectra with a good signal-to-noise ratio were measured only for those potentials where the 22BPY molecules associate into stacks. Efficient packing into stacks requires a planar or very nearly planar conformation for the 22BPY molecules. For these potentials, the SNIFTIRS spectra provided direct evidence that 22BPY molecules coordinated to the surface gold atoms assume a cis- or nearly cis-conformation.
Introduction The 2,2′-bipyridine (22BPY) molecule is an interesting compound for surface coordination studies. This molecule has two pyridine rings connected via the C2-C2′ bond, which has been estimated to have approximately 10% double-bond character.1 In principle, the molecule may assume different conformations when adsorbed on a metal surface. The first is the coplanar trans-conformation (0° angle of rotation around the C-C axis) with zero dipole moment and C2h symmetry for an isolated 22BPY molecule. The second conformation is a coplanar cis-form (180° angle of rotation) with a dipole moment equal to 3.8 D and C2v symmetry.2 In addition, the planes of the two pyridine rings may be rotated around the C2-C2′ axis by an angle intermediate between 0° and 180°, resulting in a geometry with C2 symmetry. These geometric isomers are displayed in Figure 1. The coplanar trans-conformation is known to be the most stable form in solution or in the crystalline state.1,2 The 22BPY molecule is well-known as a chelating ligand, and in complexes it usually assumes a coplanar or nearly coplanar cis-, or cisoid-conformation.3 A conformation with an 80° torsional angle was estimated from average molecular packing densities for 22BPY coordinated to a platinum electrode surface.4 Two models of the binding of a 22BPY molecule to a metal surface may be envisaged. The 22BPY molecule may coordinate to a metal surface either through the two nonbonding orbitals on the nitrogen atoms or via the π orbitals of the aromatic rings. Consequently, the adsorbed 22BPY molecule might assume different surface geom* To whom correspondence should be addressed. † University of Guelph. ‡ University of California, Davis. (1) Merritt L. L.; Schroeder, E. D. Acta Crystallogr. 1965, 195, 801. (2) Jaime, C.; Font, J. J. Mol. Struct. 1989, 195, 103. (3) Strukl, J. S.; Walter, J. L. Spectrochim. Acta 1971, 27A, 223-238 (4) Chaffins, S. A.; Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. T.; Elliott, C. M. Langmuir 1990, 6, 957.
Figure 1. Diagram showing the geometric conformers of an isolated 22BPY molecule: (a) Coplanar trans-conformation with C2h symmetry; (b) coplanar cis-conformation with C2v symmetry; (c) a conformer with C2 symmetry due to a twist of 40°.
etries and its adsorption may have multiple states. Recently, the adsorption of 22BPY at gold single-crystal electrodes has been investigated using chronocoulometry and second-harmonics generation (SHG),5 scanning tunneling microscopy (STM),6-9 and surface X-ray scatter(5) Yang, D. F.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. J. Phys. Chem. 1994, 98, 7083. (6) (a) Cunha, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376. (b)Cunha, F.; Tao, N. J.; Wang, X. M.; Jin, Q.; Duong, B.; D’Agnesse, J. Langmuir 1996, 12, 6410.
10.1021/la9910733 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/12/2000
Conformation of Adsorbed 2,2′-Bipyridine
ing.10,11 The results of the electrochemical experiments demonstrated the effect of surface charge density on the orientation geometry of 22BPY. At negative charge densities, the 22BPY molecule assumes a flat (π-bonded) state; however, the adsorbate was shown to be N-bonded to the surface at positive charge densities. This conclusion has been supported by STM studies by Tao and his group6,7 and by Wandlowski and co-workers.8,9 For a positively charged surface, the STM images showed stacks of vertically standing 22BPY molecules reminiscent of “rolls of coins”. These images suggest the 22BPY molecules in the stacks assume a cis-coplanar or cisoid nearly coplanar conformation and form a complex with gold atoms on the surface.6-9 Although this hypothesis is easily envisioned, neither the electrochemical data nor the STM images can provide a unique distinction between the trans- and cisconformations of an adsorbed organic molecule. The objective of the present work is to apply subtractively normalized interfacial Fourier transform infrared (SNIFTIRS) spectroscopy to determine the conformation of the 22BPY molecules adsorbed at the Au(111) electrode surface. The character of the IR spectra is determined by the symmetry of the molecule. For molecules adsorbed at metal surfaces, surface selection rules restrict the number of allowed vibrations to those that have a nonzero component of the transition dipole in the direction normal to the surface.12,13 The combination of symmetry rules and surface selection rules makes IR spectroscopy a powerful tool for surface coordination studies. We have previously used this technique to study surface coordination of pyridine,14 benzonitrile,15 and 4-cyanopyridine16 to the Au(111) electrode and of acetonitrile to polycrystalline gold.17 Below, we will combine SNIFTIRS with ab initio Hartree-Fock calculations to demonstrate that the IR spectrum has a distinctly different fingerprint for the cisand trans-conformations of the 22BPY molecule adsorbed at a gold electrode surface. We will demonstrate that, using SNIFTIRS, one can uniquely determine the conformation of an adsorbed 22BPY molecule.
Langmuir, Vol. 16, No. 5, 2000 2357 flame annealed prior to each experiment to ensure a clean surface. A cylindrical platinum foil was used as the counter electrode, and the reference electrode was a AgCl (3 M KCl saturated with AgCl) electrode connected to the cell through a salt bridge. For the sake of comparison with previous electrochemical results, all potentials measured with respect to Ag/AgCl were converted to the saturated calomel electrode (SCE) scale. Apparatus. The SNIFTIRS experimental procedures and instrumentation were described in refs 14 and 15. A syringetype IR cell with a 60° CaF2 prism window was used in this study. For this prism, normal incidence angles at each of the two side faces resulted in an incidence angle of about 80° at the electrode/solution interface. Such a configuration ensured maximum enhancement of the incident photon beam.19 During the IR experiment, the working electrode was pushed against the flat part of the CaF2 window to form a thin layer. The electrolyte solution was deaerated by purging with pure argon for about 20 min before the measurements, and argon was allowed to flow over the solution at all times during the experiment. A transmission spectrum was obtained from a KBr pellet with crystalline 22BPY. The spectra were measured using a Nicolet 20SX/C FTIR apparatus equipped with a MCT-B detector cooled by liquid nitrogen with a 4 cm-1 resolution. The sample compartment of the FTIR apparatus was purged throughout the experiment using CO2- and H2O-free air provided by a Puregas heatless dryer. The electrode potential was controlled with a PAR 173 potentiostat. Spectroscopic Procedure. The SNIFTIR spectra were determined using a multiple potential step (MPS) procedure in which the electrode potential was stepped m times between the reference and the sample potentials, E1 and E2, respectively. During each step, n interferograms were acquired at both potentials. Data acquisition was delayed for 10 s after each potential change to allow the interface to reach thermodynamic equilibrium at the imposed polarization. The change of the electrode potential was synchronized with the acquisition of the interferograms by connecting the external trigger port of the PAR 173 potentiostat to the communication port of the DX 486 computer of the FTIR instrument. This procedure was repeated m times until a total number of N ) n × m interferograms was acquired for each of the two potentials. Typical values of n and m employed in this study were 100 and 20, respectively. The interferograms were added, Fourier transformed, and used to calculate a relative change of the electrode reflectivity, which is defined as
Experimental Section Reagents and Electrodes. KClO4 (ACS Certified, Fisher) was purified as described elsewhere.18 22BPY (Aldrich) was twice sublimed under vacuum. All solutions were prepared from D2O (99.9%, Cambridge Isotope Laboratories). All measurements were performed using a 1 × 10-3 M 22BPY solution in D2O containing 5 × 10-2 M KClO4 as the supporting electrolyte at 20 ( 2 °C. The IR spectra were recorded with D2O as the solvent to avoid interference from water bands in our region of interest. The working electrode was a Au(111) single crystal, grown, cut, and polished in our laboratory. The top layer perturbed by mechanical polishing was removed by electropolishing.18 The electrode was (7) Tao, N. J. Imaging Surfaces and Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH-Wiley: New York, 1999; Chapter 5. (8) Dretshchkow, Th.; Lampner, D.; Wandlowski, Th. J. Electroanal. Chem. 1998, 458, 121. (9) Dretshchkow, Th.; Wandlowski, Th. J. Electroanal. Chem. 1999, 467, 207. (10) Wandlowski, Th.; Ocko, B. M.; Magnussen, O. M.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 155. (11) Wu, S.; Lipkowski, J.; Magnussen, O. M.; Ocko, B. M.; Wandlowski, Th. J. Electroanal. Chem. 1998, 446, 67. (12) Moskovits, M. J. Chem. Phys. 1982, 77, 4408. (13) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (14) Hoon-Khosla, M.; Lipkowski, J.; Fawcett, W. R.; Chen, A. Electrochim. Acta, in press. (15) Chen, A.; Richer, J.; Roscoe, S. G.; Lipkowski, J. Langmuir 1997, 13, 4747. (16) Chen, A.; Sun, S. G.; Yang, D. F.; Pettinger, B.; Lipkowski, J. Can. J. Chem. 1996, 74, 2321. (17) Faguy, P. W.; Fawcett, W. R.; Liu, G.; Motheo, A. J. J. Electroanal. Chem. 1992, 339, 339. (18) Richer, J.; Lipkowski, J. J. Electrochem. Soc. 1986, 133, 121.
∆R/R ) (R(E2) - R(E1))/R(E1)
(1)
where R(E1) and R(E2) are the electrode reflectivities at the reference and sample potentials E1 and E2, respectively. The reference potential was always equal to -0.75 V (SCE), where 22BPY molecules are totally desorbed from the electrode surface. Under these conditions, the measured changes of the reflectivity ∆R/R represent the difference between the absorption spectrum of Γ 22BPY molecules that are in solution at potential E1 and the spectrum of the Γ molecules adsorbed at the electrode surface at potential E2. When linearly polarized light is used, Beer’s law is given by IR(Ei)/IO ) exp{-2.3(Ei)Γ cos2 θ(Ei)},20 where IR and IO are the intensities of the reflected and incident radiation. Noting that R(Ei) ≡ IR(Ei) and that IR(Ei)/IO ≈ 1 - 2.3(Ei)Γ cos2 θ(Ei) for 2.3(Ei)Γ cos2 θ(Ei) , 1, the measured ∆R/R can be expressed as
∆R/R ) 2.3Γ[cos2θ(E1)(E1) - cos2θ(E2)(E2)]
(2)
where θ is the angle between the direction of the electric field of the photon and the direction in which the dipole moment of the molecule changes, is the molar absorption coefficient, and Γ is the surface concentration of the adsorbed species. Using s-polarized light, the electric field of the photon at the surface is effectively equal to zero and hence (E2) is equal to zero as well. Equation 2 then simplifies to (19) Faguy, P. W.; Fawcett, W. R. Appl. Spectrosc. 1990, 44, 1309. (20) Liptay, W. Angew. Chem. 1969, 81, 195.
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∆R/R ) 2.3Γdes(E1)
(3)
and
∫(∆R/R) dν ) 2.3Γ∫
des(E1)
dν
(4)
We introduced the subscript “des” to the extinction coefficient for E1 to emphasize that at this potential the molecules are desorbed into the thin layer cavity and therefore may absorb s-polarized radiation. In this case, the absorption band is positive and the band intensity is proportional to the product of the surface concentration of 22BPY adsorbed at potential E2 and the integrated molar absorption coefficient of 22BPY in the bulk of the thin layer cavity. When p-polarized radiation is employed, both the adsorbed and solution species are IR active. In that case the ∆R/R spectrum displays bipolar features. The positive bands on this spectrum correspond to absorption by molecules desorbed into the thin layer cavity at E1, and the negative bands correspond to the absorption of the IR radiation by molecules adsorbed at the electrode surface at potential E2. Ab Initio Calculations. The harmonic vibrational spectra of molecules were computed using ab initio molecular orbital techniques that analytically determine the second derivative of the energy with respect to the nuclear positions.21,22 These second derivatives along with the reduced masses yield the force constants and thus the vibrational frequencies. In this study we used the Hartree-Fock (HF) theoretical method in combination with the 6-31G* polarized basis set on carbon and nitrogen and a LANL2DZ set on gold. This is often the smallest basis set that gives satisfactory results for vibrational frequency calculations on organic molecules. For gold, inner shell electrons are treated via effective core potentials (ECPs). This treatment includes some relativistic effects, which can be important in an atom such as gold. The LANL2DZ basis set is one of the most frequently used ECPs. We have optimized the geometry of the organic molecules attached to a single gold atom or cation at the HF level, using the 6-31G* basis set for the heterocycle and the LANL2DZ basis set for the gold atom. Harmonic frequency values computed at the HF level are known to systematically overestimate vibrational frequencies by 10% to 12% relative to the experimentally observed anharmonic frequencies. Thus, it is usual to scale frequencies predicted at the HF level by an empirical factor of 0.8929 prior to comparison with experiment.22 In addition to the frequencies of the spectral lines, the program reports the normal modes. Thus, the direction and magnitude of the nuclear displacements that occur when a system absorbs a quantum of energy are predicted. It is also possible to animate these vibrational modes in various graphical packages to yield further insight into the nuclear motions. This information can be used to determine the direction of the transition dipole.
Results and Discussion Electrochemical Studies. The energetics of 22BPY adsorption at a Au(111) surface has been investigated in our laboratory using electrochemical techniques.5 To facilitate the discussion of our spectroscopic data, we will briefly review the main points of these electrochemical studies. Figure 2 shows the change of the differential capacity, the charge density, and the surface concentration of 22BPY as a function of the electrode potential. Differential capacity and charge density curves for the 22BPY solution merge with the curves for the supporting electrolyte, and the surface concentration drops to zero at E (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision B.1; Gaussian, Inc.: Pittsburgh, PA, 1995. (22) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996.
Figure 2. (a) Differential capacity curves recorded at the sweep rate 5 mV s-1 and the an ac modulation frequency 25 Hz for the Au(111) electrode in the presence of 0.01 M KClO4 (dotted line) and 0.1 M KClO4 + 0.001 M 22BPY aqueous solution (solid line). (b) Charge density plots for 0.1 M KClO4 (dotted line) and 0.1 M KClO4 + 0.001 M 22BPY aqueous solution (solid line). (c) Adsorption isotherm (Γ vs E plot) recorded for the Au(111) electrode in 0.1 M KClO4 + 0.001 M 22BPY aqueous solution.
< -0.65 V. The 22BPY is apparently desorbed from the Au(111) electrode surface at these negative potentials and is adsorbed when E > -0.65 V. A large negative shift in the value of the potential of zero charge (pzc) was detected. This indicates that at low absolute values of the surface charge density the adsorbed 22BPY molecules assume a surface coordination in which the negative pole of their permanent dipole moment faces the metal surface and the component of the dipole moment in the direction normal to the surface is large. This may correspond to either an N-bonded coplanar cis-orientation or an Nbonded cisoid torsional orientation. In addition, the surface concentration plot displays two steps. It has been suggested5 that adsorbed 22BPY molecules assume a flat (πbonded) orientation at the negatively charged surface and a tilted N-bonded state at the positively charged surface. The data in Figure 2 indicate that the transition from the π-bonded to the cis-N-bonded state occurs around the pzc of the Au(111) electrode, measured in the presence of 22BPY in the solution. At positive potentials, the differential capacity curve drops significantly below the values for the pure supporting electrolyte, and this feature suggests that 22BPY molecules form a condensed monolayer. For these potentials, recent STM studies by Tao et al.6,7 and Wandlowski and Dretschkow8,9 show that 22BPY molecules aggregate into surface assemblies similar in appearance to rolls of coins. Spectroscopic Studies. Interpretation of IR Spectra. We will now discuss how FTIR spectroscopy can be used to determine the conformation of the 22BPY molecules coordinated to the Au(111) electrode surface at positive potentials, where they aggregate into molecular stacks. Figure 3 compares the (I) the transmission IR spectrum of solid Zn(22BPY)Cl2 in KBr (taken from ref 23) and (II) the transmission spectrum of solid 22BPY in KBr (mea(23) Bartlett, J. R.; Cooney, R. P. Spectrochim. Acta 1987, 43A, 1543.
Conformation of Adsorbed 2,2′-Bipyridine
Figure 3. Comparison of transmission and SNIFTIRS spectra for 22BPY. Spectrum I is a transmission spectrum of Zn(22BPY)Cl2 taken from ref 23. Spectrum II is a transmission spectrum measured for a sample of crystalline 22BPY in KBr. SNIFTIRS spectra III and IV were obtained using (III) s-polarized and (IV) p-polarized light for a 0.001 M 22BPY + 0.05 M KClO4 solution and potential modulated between E1 ) - 0.75 V and E2 ) 0.35 V.
sured in this work) to two SNIFTIR spectra of 22BPY adsorbed at E2 ) 0.35 V from a D2O solution, recorded using either s- (III) or p-polarized light (IV). In the Zn(22BPY)Cl2 complex, the 22BPY ligand assumes a cisplanar conformation that is characterized by C2v symmetry.3,23 In contrast, 22BPY molecules have a transplanar conformation and C2h symmetry, in the pure crystalline state.2,24-26 Therefore, spectra I and II may be used as fingerprints of the two planar conformations of the 22BPY molecule. In the frequency range 1600-1200 cm-1, all the IR bands in spectra I and II correspond to the in-plane deformations. The bands in spectrum I have either a1 or b1 symmetry.3,23 The direction of the transition dipole moment is parallel to the C2v axis (z-coordinate) for a1 bands and is perpendicular to the C2v axis for b1 bands (parallel to the x-coordinate). All bands in spectrum II have bu symmetry, and the transition dipole moments of these bands have nonzero z- and x-components.24-26 We have performed ab initio calculations to determine the directions of the transition dipole for the bu bands. The motion of atoms corresponding to the bu bands and the directions of the transition dipole moments are shown schematically in Figure 4. With the exception of the band at 1558 cm-1, where the transition dipole moment has a strong component in the z-direction, the changes of the dipole moment for all bu bands have a predominant component in the x-direction. Spectra I and II show that a change from cis- to transconformation of the 22BPY molecule has significant impact on the IR spectrum of the molecule. The band frequencies and the number of bands change with the conformation of the molecule. However, for the 22BPY molecule (24) Strukl, J. S.; Walter, J. L. Spectrochim. Acta 1971, 27A, 209. (25) Neto, N.; Muniz-Miranda, M.; Angeloni, L.; Castellucci, E. Spectrochim. Acta 1983, 39A, 97. (26) Muniz-Miranda, M.; Castellucci, E.; Neto, N.; Sbrana, G. Spectrochim. Acta 1983, 39A, 107.
Langmuir, Vol. 16, No. 5, 2000 2359
coordinated to the metal surface, only those vibrations that have a nonzero component of the transition dipole in the direction normal to the surface will be allowed.12,13 The IR spectra for adsorbed molecules depend on the orientation of the molecule at the metal surface. Consistent with the electrochemical and STM experiments, we assume that at positive potentials the z-x plane of the planar 22BPY molecule is oriented in the direction normal to the metal surface (with the z-axis perpendicular and the x-axis parallel to the surface). If the conformation of the molecule is trans, only the transition dipole for the band at ∼1558 cm-1 would have a strong component in the direction normal to the metal surface. The transition moments for other bu bands would be directed predominantly in the direction parallel to the metal surface. For the cis-conformation, the transition dipoles of the three a1 bands should be oriented in the direction normal to the surface and the transition dipoles for the b1 bands should be parallel to the surface. Consequently, if the 22BPY molecule is adsorbed with the plane of the molecule normal (or tilted) with respect to the gold surface, only one strong band at ∼1558 cm-1 should be observed when it has the trans-conformation. In contrast, three strong bands at ∼1600, 1473, and 1319 cm-1 should be observed for the cis-bonded molecule. For the vertically adsorbed molecule, the IR spectra for cis- and trans-conformers would have significantly different fingerprints. Following the above discussion, we will interpret the SNIFTIR spectra that were recorded for positive potentials corresponding to the condensed state of adsorbed 22BPY. We have already mentioned that the electric field of the s-polarized photon is close to zero at the gold electrode surface. Therefore, molecules adsorbed at the gold surface do not absorb s-polarized IR radiation and this SNIFTIR spectrum plots the absorption bands of desorbed molecules in the thin layer cavity at the reference potential. This should be the identical spectrum for molecules dissolved in the D2O solution, where 22BPY molecules assume a nearly planar trans-conformation.25,26 Therefore, for an s-polarized photon the SNIFTIR spectrum resembles the transmission spectrum for crystalline 22BPY in a KBr pellet. Small differences between band frequencies in spectra II and III may be explained by the effect of the solvent. Spectrum IV was acquired using p-polarized light. For molecules adsorbed at a metal surface, the modes that have a strong component of the transition dipole in the direction normal to the surface absorb p-polarized photons. In this case, SNIFTIRS plots the difference between spectra of molecules adsorbed at the electrode surface at E2 and those of molecules desorbed into the thin layer cavity at E1. In the differential spectrum, positive bands correspond to the absorption of IR photons by molecules desorbed into the thin layer cavity at E1 and negative bands correspond to absorption by molecules adsorbed at the electrode surface at E2. Consistent with this interpretation, the positive bands in spectrum IV are essentially mirror images of the bands in spectrum III. The three negative bands at 1591, 1483, and 1305 cm-1 correspond to the absorption of p-polarized photons by molecules adsorbed at the electrode surface. The frequencies of these bands are close to the frequencies of the corresponding a1 bands in spectrum I for the Zn complex of 22BPY. The small difference between the frequencies of a1 bands in spectrum I and those of negative bands in spectrum IV may well be explained by the different strengths of the coordination bond for 22BPY attached to the gold atom on the surface and to the zinc ion in the complex. Indeed, extensive IR studies of 22BPY complexes with various metal ions indicate that the vibrational IR frequencies of
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Figure 4. (top panel) IR transmission spectrum of crystalline 22BPY in a KBr pellet. Bars are the calculated bu vibrational intensities and frequencies for the trans-conformer of 22BPY. (bottom panel) Schematic diagrams of the nuclear displacements for the bu bands of the trans-conformer of 22BPY. For the sake of clarity the hydrogen atoms are not shown in this diagram. The arrow shows the direction of the transition dipole moment. These nuclear displacements and transitional dipole moments are calculated from the distorted geometry along the vibrational mode of each vibration of interest.
the ligand change with the nature of the central metal ion.3 One can also note that the amplitudes of the a1 bands in spectrum I and the amplitudes of the negative bands in spectrum IV change with frequency in a very similar
manner. Therefore, the negative bands in spectrum IV are fingerprints of the cis-conformation. IR spectroscopy provides direct evidence that at positive potentials the adsorbed 22BPY molecules assume the cis-conformation.
Conformation of Adsorbed 2,2′-Bipyridine
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Table 1. Comparison of the ab Initio Calculations Performed for the cis-22BPY Molecule Coordinated to a Gold Cation (Labeled as C2v) with a cisoid-22BPY Molecule (Twist Angle of 40°) also Coordinated to the Gold Cation in the Range 1600-1200 cm-1
Figure 5. Series of SNIFTIR spectra recorded for 22BPY adsorbed at the Au(111) electrode acquired using s-polarized light for 0.001 M 22BPY + 0.05 M KClO4 in D2O. For each spectrum, the reference potential E1 was equal to -0.75 V and the value of E2 varied as indicated in the figure. All potentials are referred to the SCE scale.
The IR spectrum for the cisoid-22BPY only has bands of a and b symmetry (labeled as C2). The integrated IR intensities I have units of kilometers per mole.
It is interesting to note that, after desorption, the conformation of the 22BPY molecules is changed. In spectrum IV, the positive bands correspond to the trans-conformer and the negative bands correspond to the cis-conformer. It is more difficult to distinguish between a cis-planar and a torsional conformation. When the two pyridine rings are twisted along the 2C-2C′ axis, out of the plane normal to the electrode surface, the symmetry of the molecule is changed to C2.27 The cis-planar conformation of the adsorbed 22BPY molecules results in the 2C-2C′ axis being parallel to the gold surface. Nearly free rotation about this 2C-2C′ axis could result in a torsional angle σ and lowering of the molecular symmetry to C2. Our ab initio21 calculations performed for 22BPY coordinated to the Au+ ion show that in the range 16001200 cm-1 the IR spectrum only has bands of a and b symmetry (labeled as C2). The transition dipole of the a bands has a strong component in the direction normal to the surface while the transition dipole of the b bands is parallel to the surface. The a bands for the torsional conformation appear at frequencies that are basically equal to the frequencies of the a1 bands of the planar cisconformation. The results of our calculations, presented in Table 1, did not show any significant shift of a band frequencies or band intensities when the angle of rotation between the two pyridine rings varied between 180° (a planar cis-conformation) and a minimum of 140°. Using SNIFTIRS, the distinction between the cis-planar and a torsional conformation is rather difficult to discern. However, at positive potentials where the molecules stack into polymeric-like chains, the 22BPY molecule has to (27) Vincent, A. Molecular symmetry and group theory; John Wiley & Sons: Brisbane, 1990; p 141.
Figure 6. Comparison of the potential dependence of the integrated absorptivity of the b1 band at 1465 cm-1, acquired using s-polarized radiation, and the surface concentration of 22BPY determined from chronocoulometric data. The scale of the integrated absorptivities was adjusted to give the best fit to the surface concentration data.
assume a planar or nearly planar conformation. For this range of potentials, the possibility of a torsional conformation with a significant twist angle may be excluded. Dependence on the Electrode Potential. Figure 5 shows a series of subtractively normalized s-polarized IR spectra, acquired by stepping the potential between the reference potential (El ) -0.75 V) and a sample potential (-0.35 < E2 < 0.35 V). These spectra correspond to molecules that are desorbed from the electrode surface at E1 and remain randomly oriented in the thin layer cavity. The spectra essentially display two pairs of strong bands in the frequency regions 1450 and 1600 cm-1 that are characteristic for the trans-conformer. According to eq 4, the integrated intensity of these bands should be proportional to the surface concentration of 22BPY adsorbed initially
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only their amplitude changes slightly with E2 in proportion to the change of the surface concentration. The negative bands correspond to the adsorbed species. Their frequencies display a negligible dependence on the electrode potential. Recently, we have observed similar behavior for a1 bands of adsorbed pyridine molecules.14 The amplitude of a1 bands changes in proportion to the surface concentration of adsorbed pyridine. This behavior indicates that the orientation of 22BPY molecules does not change within this range of potentials. However, this is the potential range of the condensed phase of the adsorbed film only. An insufficient signal-to-noise ratio prevented us from investigating the potential-controlled reorientation of the adsorbed molecules taking place at the rising section of the isotherm in the potential range between -500 and -50 mV. We are presently developing a photoelastic photon polarization modulation IR spectroscopic technique that should be able to study the potentialcontrolled reorientation of 22BPY molecules soon.
Figure 7. Series of SNIFTIR spectra recorded for 22BPY adsorbed at the Au(111) electrode acquired using p-polarized light for a 0.001 M 22BPY + 0.05 M KClO4 solution in D2O. For each spectrum, the reference potential E1 was equal to -0.75 V and the value of E2 varied as indicated in the figure. All potentials are referred to the SCE scale.
at E2. Figure 6 shows the correlation between the integrated intensity of the band at 1465 cm-1 and the surface concentration of adsorbed 22BPY determined from chronocoulometric experiments.5 For the purpose of this comparison, the scale of the integrated intensities was adjusted so that the integrated intensities for E2 > 0 V gave a good fit to the surface concentration data. The signal-to-noise ratio in the SNIFTIR spectra was very poor for E2 < -0.35 V, and hence, the spectra could be recorded only for potentials corresponding to surface concentrations >30% of the limiting value for 22BPY adsorption. Within this range of higher surface concentrations, the two sets of data display the same dependence on the electrode potential, consistent with the predictions from eq 4. Figure 7 shows a series of subtractively normalized p-polarized IR spectra, acquired by stepping the potential between the reference potential (El ) -0.75 V) and a variable sample potential E2. The throughput for the p-polarized light was smaller than that for the s-polarized light. Therefore, for p-polarized light the spectra were measured with a satisfactory signal-to-noise ratio only for potentials >-50 mV, where the surface concentration was >60% of the limiting value. These SNIFTIRS spectra change little with the applied potential. Consistent with the previous discussion, the positive bands originate from the species desorbed into the solution at E1, and hence
Summary and Conclusions We have employed SNIFTIRS to determine the conformation of 22BPY adsorbed at the Au(111) electrode surface. In addition, we have performed ab initio HartreeFock calculations of the IR spectra for trans-planar, cisplanar, and torsional conformations of an isolated molecule and for the molecule coordinated to a gold atom or ion. The spectra for the trans-planar and cis-planar conformations are distinctly different. The differences between the spectra for the cis-planar conformation and a torisonal conformation with the twist angle e40° are small. Such small spectral differences could not be observed with SNIFTIRS. However, SNIFTIRS spectra with a good signal-to-noise ratio were measured only for positive potentials where 22BPY molecules associate into stacks similar in appearance to rolls of coins. A planar or nearly planar conformation is required for an efficient stacking of 22BPY molecules, and hence, the torsional conformation with a significant twist angle may be excluded. For these potentials, we have provided direct spectroscopic evidence that the 22BPY molecules coordinated to surface gold atoms assume the cis-conformation. Finally, at the revision stage of this paper we learned that similar IR studies of 22BPY adsorption at the Au(111) surface were submitted for publication to another journal.28 By and large, the results of this work and that of ref 28 agree well. Acknowledgment. Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. LA9910733 (28) Noda, H.; Minoha, T.; Wan, L.-J.; Osawa, M. J. Electroanal. Chem., submitted.
