Contribution of the Charge Transfer Mechanism to the Surface

Pemberton et al.17-20 have used the SERS effect to determine the adsorption ..... The experimental Vmax values obtained for the three solvents and exc...
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Langmuir 1999, 15, 2500-2507

Contribution of the Charge Transfer Mechanism to the Surface-Enhanced Raman Scattering of the Binuclear Ion Complex [Fe2((Bpe)(CN)10]6- Adsorbed on a Silver Electrode in Different Solvents Paola Corio, Ma´rcia L. A. Temperini, and Paulo S. Santos Instituto de Quı´mica da Universidade de Sa˜ o Paulo, C.P. 26077, CEP 05599-970 Sa˜ o Paulo, SP, Brazil

Joel C. Rubim* Instituto de Quı´mica da Universidade de Brası´lia, C.P.04478, CEP 70919-970 Brası´lia, DF, Brazil Received August 21, 1998. In Final Form: January 21, 1999 A SERS (surface-enhanced Raman scattering) study of the binuclear ion complex [Fe2BPE(CN)10]6(BPE ) trans-1,2-bis (4-pyridyl) ethylene) adsorbed on a silver electrode in different solvents is presented. The cycle voltammogram of the complex, in the region of the FeII/FeIII redox process, shows two oxidation and two reduction waves separated by 0.15 V, indicating that the two iron centers are electronically coupled via the bridging ligand. The SERS measurements have shown that both SERS intensity and frequency position of the bridging ligand modes present strong dependence on the applied potential. Remarkable changes in the structure of the ligand are observed for applied potentials more negative than -1.0 V, where the complex is reduced. The CdC ethylenic inter ring stretching mode shifts from 1637 to 1555 cm-1, indicating a decrease in this bond order for the reduced molecule. The chemical interaction of the complex with the silver surface also involves one or more CN ligands as evidenced by an upward frequency shift of the CN stretching mode in the adsorbed complex. Upon reduction, the ν(CN) frequency shifts to lower energies, indicating that the electron transferred in the faradaic process is delocalized over the complex. On the basis of the SERS excitation profiles and their dependence on the exciting radiation, two potential modulated photon assisted charge-transfer processes have been characterized: an adsorbate to metal (HOMO(CN) f Ag) and a metal to adsorbate (Ag f LUMO(BPE)), responsible for the enhancement of the ν(CN) and BPE modes, respectively. Resonance between the energy of the exciting radiation and the metal/adsorbate charge-transfer transitions is achieved at different applied potentials for different solvents, thus indicating that the position of the energy levels of the adsorbed complex relative to the Fermi level (EF) changes according to the chemical nature of the solvent and the solvent/adsorbate interaction. Energy diagrams showing the relative positions of the donor and acceptor states of the surface complex formed by the binuclear complex and the silver electrode surface in different solvents have also been proposed.

Introduction Surface-enhanced Raman scattering (SERS) has proved to be a powerful technique to characterize products and intermediates of oxidation-reduction reactions occurring at electrode-solution interfaces1-6 as well as to determine the position of the energy levels of an adsorbate in relation to the Fermi level of the metal electrode.4,6-12 In this * To whom correspondence should be addressed. Telephone: (061)-307-2155. Fax: (061)-273-4149. E-mail: [email protected]. (1) Rubim, J. C. J. Electroanal. Chem. Interfacial Electrochem. 1987, 220, 339 and references therein. (2) Otto, A. Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys. Condens. Matter 1992, 4, 1143 and references therein. (3) Hosten, C. M., Birke, R. L., Lombardi, J. E. J. Phys. Chem. 1992, 96, 6585. (4) Corio, P.; Rubim, J. C. J. Phys. Chem. 1995, 99, 13217. (5) Shi C. T., Zhang W., Birke, R. L., Lombardi, J. R. J. Electroanal. Chem. 1997, 423, 67. (6) Corio, P.; Rubim, J. C. J. Raman Spectrosc. 1997, 28, 235. (7) Thietke, J.; Billmann, J.; Otto, A. In Dynamics on Surfaces; Pullmann, B., Jortner, J., Nitzan, A., Gerber, B., Eds.: Reidel: Dordrecht, The Netherlands, 1984; p 345. (8) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A. Bernard, I.; Sun, S. C. Chem. Phys. Lett. 1984, 104, 240. (9) Furtak, T. E.; Roy, D. Surf. Sci. 1985, 158, 126. (10) Lombardi, J. R., Birke, R. L., Lu, T. H., Xu, J. J. Chem. Phys. 1986, 84, 4174.

context, attention has been given to systems in which the charge transfer (CT) mechanism is contributing to the enhancement of the Raman scattering of species adsorbed on metal surfaces in order to address the participation of a resonance Raman effect on this part of the total enhancement.6,10,12 The effects of the applied potential, energy of exciting radiation, and supporting electrolyte have already been described.10,12 More recently the contribution of the Herzberg-Teller mechanism to the charge-transfer mechanism on the enhancement of Raman-forbidden modes of phthalocyanines adsorbed on a silver electrode has also been addressed.13 Since the discovery of SERS,14,15 little attention has been given to the role of the nature of the solvent to the SERS effect. Regarding to this subject, we call attention to the work of Mineo and Itoh,16 who have studied the SERRS (11) Rubim, J. C.; Temperini, M. L. A., Corio, P.; Sala, O.; Jubert, A. H.; Chacon-Villalba, M. E.; Aymonino, P. J. J. Phys. Chem. 1995, 99, 345. (12) Rubim, J. C.; Corio, P.; Ribeiro, M. C. C.; Matz, M. J. Phys. Chem. 1995, 99, 15765. (13) Corio, P.; Rubim, J. C.; Aroca, R. Langmuir 1998, 14, 4162. (14) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1. (15) Albrecht, M. G.; Creighton. J. A. J. Am. Chem. Soc. 1977, 99, 5215.

10.1021/la981082w CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999

Surface-Enhanced Raman Scattering

effect of a merocyanine dye adsorbed on a silver electrode in water and acetonitrile solutions. Pemberton et al.17-20 have used the SERS effect to determine the adsorption geometry of solvents on the electrode surface, and Chang and Chen21 have investigated the influence of different solvents on the SERS effect of water adsorbed on a silver electrode. It is worth of notice that all above-mentioned studies were performed in working solutions containing halide ions which strongly adsorb on silver electrodes. Very recently we addressed the contribution of different solvents (acetonitrile, ethanol, and water) and cations (Na+, Li+, and Mg2+) of supporting electrolytes (perchlorates that do not present specific adsorption) to the SERS intensities of the [Fe(phen)2(CN)2] complex (phen ) 1,10-phenanthroline) adsorbed on a silver electrode.22 Due to the strong interaction between the solvent and the complex that led to a remarkable solvatochromic effect and to the adsorption of different chemical species at the electrode in the different solvents, it was not possible to precise the effective contribution of the solvent to the SERS effect. Therefore, one of our goals in this work is to provide a more detailed description of the charge-transfer mechanism of enhancement working in the SERS effect of adsorbed molecules, including the role of the nature of the solvent/adsorbate interaction. For this purpose, the binuclear ion complex [Fe2BPE(CN)10]6- (BPE ) trans1,2-bis (4-pyridyl) ethylene) was chosen as the model adsorbate. Recently the SERS and SEIR (surface-enhanced infrared spectroscopy) spectra of the ion complex [Fe2BPE(CN)10]6- deposited on silver island films have been investigated and the observed enhancement was discussed based on the electromagnetic mechanism of enhancement.23 It is well-known that binuclear complexes of the type [Fe2(CN)10L],6- where L is the bridging ligand, form an interesting class of compounds since they can serve as models for the super exchange effect, i.e., electron-transfer processes involving donor and acceptor states separated by large distances as in proteins. Furthermore, it is wellknown that the nature of the solvent has marked effects on the reactivity and spectroscopic properties of cyanoferrate complexes in solution.24-26 In this context SERS of species adsorbed on electrode surfaces has proved to be a powerful tool in the study of the [Fe2(CN)10L]6- ion complexes where the bridging ligand were 4-picolylamine,27 pyrazine,11 and 4,4′-bipyridine.11 Therefore, a second aim of this work is to study the SERS effect of the binuclear ion complex [Fe2BPE(CN)10]6- adsorbed on a silver electrode in order to obtain information upon the electron charge-transfer processes within the complex in different solvents. To achieve the above proposed goals the electrochemical behavior as well as the SERS intensities of this complex adsorbed on a silver electrode in perchlorate solutions as a function of the applied potential and exciting radiation (16) Mineo, Y.; Itoh, K. J. Phys. Chem. 1991, 95, 2451. (17) Sobocinski, R. L.; Pemberton, J. E. Langmuir 1992, 8, 2049. (18) Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 2301. (19) Pemberton, J. E.; Joa, S. L. J. Electroanal. Chem. 1994, 378, 149. (20) Pemberton, J. E.; Joa, S. L.; Shen, A. J.; Woelfel, K. J. J. Chem. Soc., Faraday Trans. 1996, 92, 3683. (21) Chang, Y. C.; Chen, T. T. Chin. J. Phys. 1996, 34, 1352. (22) Corio, P. Temperini, M. L. A.; Rubim, J. C.; Santos, P. S. J. Phys. Chem., submitted for publication. (23) Aroca, R.; Corio, P.; Rubim, J. C. Ann. Chim. 1997, 87, 1. (24) Toma, H. E.; Takasugi, M. S. J. Solution Chem. 1983, 12, 547. (25) Toma, H. E.; Takasugi, M. S. J. Solution Chem. 1989, 18, 575. (26) Toma, H. E.; Takasugi, M. S. Polyhedron 1989, 8, 941. (27) Temperini, M. L. A.; Rubim, J. C.; Sala, O.; Jubert, A. H.; Villalba, M. E. C.; Aymonino, P. J. J. Raman. Spec. 1991, 22, 301.

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Figure 1. Cycle voltammograms of (a) a silver electrode in a 0.1 mol‚L-1 [Fe2BPE(CN)10]6- and 0.5 mol‚L-1 NaClO4 aqueous solution (the upper voltammogram was recorded in current scale that made possible the observation of the oxidationreduction waves associated with the formation of an insoluble salt of Ag(I) and the binuclear anion complex) and (b) a gold electrode in a 0.01 mol‚L-1 [Fe2BPE(CN)10]6- and 0.5 mol‚L-1 NaClO4 aqueous solution.

will be investigated in three different solvents: water, ethanol (C2H5OH) and acetonitrile (CH3CN). Experimental Section The Raman and SERS spectra were acquired on a Renishaw Raman System 3000 equipped with an Olympus microscope (BTH2) with an 80× objective to focus the laser beam on the sample. As exciting radiation, the 488.0 and 514.5 nm lines from an air cooled Ar+ laser (Ohmnichrom), the 632.8 nm line from an air cooled He-Ne (Spectra Physics) laser, and the 782.0 nm line of a solid-state Al-doped GaAs laser (Renishaw) were used. The laser power near the sample was ca. 1 mW. The spectral resolution were 4, 1, and better than 1 cm-1 for the 514.5, 632.8, and 782.0 nm excitations, respectively. The electronic spectra were recorded on a Beckman DU-70 spectrometer. Some of the SERS spectra presented in Figure 1S (see paragraph on Supporting Information at the end of the paper) were acquired on a SPEX triplemate equipped with an OMA III (PAR-EG&G) detector and using Ar+ and Kr+ water-cooled lasers. The SERS substrate was a silver electrode with 0.2 cm2 of geometrical area activated by performing successive oxidation reduction cycles in the working solution at 0.1 V‚s-1. The number of cycles were different for the three solvents: 5 for water, 8 for ethanol, and 10 for acetonitrile. The SERS spectra were then recorded varying the applied potential in steps of 0.2 V from 0.0 to -1.4 V. It is worth of mention that there is no difference in the results (in the SERS excitation profiles) whether the activation of the silver electrode is performed in the absence or in the presence of the adsorbate. The reason for the use of oxidation reduction cycles in the presence of the ion complex was a much better signal-to-noise ratio, which gave better accuracy to the SERS intensity measurements. The spectroelectrochemical cell used in this work has been described elsewhere.26 All the potentials are referred to an Ag/AgCl reference electrode. The electrochemical system used was a PAR 263 potentiostat/ galvanostat from EG&G. The Na6[Fe2(BPE)(CN)10] complex was prepared according to the procedure described by Felix and Ludi29 and references (28) Nicolai, S. H. A., di Maschio, P.; Rubim, J. C. J. Electroanal. Chem., to be submitted for publication.

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Corio et al.

therein. The purity of the binuclear complex was confirmed by elemental analysis and UV-vis, infrared, and Raman spectroscopies. The chemicals and organic solvents were supplied by Aldrich and Merck. All chemicals were analytical grade. The BPE ligand was recrystalized from water/ethanol solution. The electrolyte solutions were prepared using analytical grade chemicals and doubly distilled water. The organic solvents were spectroscopic grade. The SERS and UV-vis spectra were obtained using 10-4 mol‚L-1 solutions of [Fe2BPE(CN)10]6- in water or ethanol. Since the complex is almost insoluble in CH3CN, solutions of the complex in this solvent were prepared from saturated solutions.

Results and Discussion Electrochemical Measurements. The cyclic voltammogram (CV) of a silver electrode in a 0.1 mol‚L-1 [Fe2BPE(CN)10]6- aqueous solution is displayed in Figure 1a. One can observe three well-resolved oxidation processes, at 0.06, 0.33, and 0.5 V. The first process (peak Ia) is assigned to the formation of an insoluble salt involving Ag(I) and the [Fe2BPE(CN)10]6- anion, indicating that this ion complex has a pseudo halide behavior. The reduction of this silver salt occurs at potentials more negative than -0.06 V (peak Ic). Other pentacyanoferrate complexes present similar voltammetric behavior on silver electrodes.11,27 The formation of an insoluble species involving Ag(I) and the anion complex can be visualized by the addition of some drops of a silver nitrate solution to a water solution of the binuclear complex. The oxidation wave at 0.33 V (peak IIa) corresponds to the oxidation of the FeII centers to FeIII. Its respective reduction occurs at 0.13 V with a shoulder at 0.07 V (peak IIc). The increase in the cathodic current at potentials more positive than 0.5 V (peak IIIa) is due to the silver dissolution in the working solution. In the absence of the complex, the silver(I) in the electrolyte solution gives origin to the peak IIIc. According to Flanagan et al.30 the cyclic voltammogram of systems presenting more than one electroactive center should present different characteristics depending on whether the centers are interacting (delocalized) or noninteracting (localized). For two noninteracting centers the voltammogram should present only one pair of oxidation-reduction waves while for two interacting centers two pairs of waves should be observed. Since on the silver electrode there is the formation of the Ag(I) insoluble salt before the oxidation of FeII to FeIII, we decided to investigate the electrochemical behavior of [Fe2(CN)10BPE]6- on a gold electrode, as presented in Figure 1b. In the voltammogram of Figure 1b one observes two oxidation and two reduction waves, at 0.26 and 0.41 V and at 0.32 and 0.17 V, respectively. The two-wave voltammetric response suggests that the two iron centers are electronically coupled via the BPE bridging ligand. Therefore, the first one electron transfer causes a change in the electronic density on both FeII centers, displacing the half-wave potential for a second one-electron transfer. Accordingly, one can conclude that the two iron centers are interacting; i.e., the electronic π system is delocalized over the entire complex. Our conclusion is different from that claimed by Felix and Ludi.29 These authors claim that they have not observed two reduction and two oxidation waves in the voltammogram of the binuclear complex. It is not possible to present any tentative explanation regarding this difference since in ref 29 the voltammograms of the binuclear complex were not presented. (29) Felix, F.; Ludi, A. Inorg. Chem. 1978, 17, 1782. (30) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248.

Figure 2. Raman and IR (KBr pellet) spectra of the complex Na6[Fe2BPE(CN)10] as a solid.

Figure 3. Structural formula of the BPE bridging ligand.

It is worth it to mention that this two-wave voltammetric response observed on the gold electrode was not present in the voltammogram using a silver electrode. This different response is due to the formation of surface film (precipitate) involving a silver(I) salt of the anion complex. Therefore, the oxidation-reduction waves IIa and IIc are associated with the oxidation and reduction of the iron centers within the silver salt. For potentials more negative than 0.0 V, any silver salt formed has been reduced. Therefore, we can say that the adsorbed species that will be further investigated by SERS is the [Fe2(CN)10BPE]6ion complex, which has two Fe(II) interacting centers, and this conclusion is going to gain support by the SERS results. SERS Measurements. The Raman and infrared spectra of the Na6[Fe2BPE(CN)10] complex in the solid state are presented in Figure 2. Table 1 presents a tentative assignment for the observed vibrational frequencies. Figure 3 presents the structural formula of the BPE ligand indicating the numbers of each atom in the molecule for a better understanding of the tentative assignment. From the Raman and infrared spectra one can see that almost all lines observed in the Raman spectrum are not observed in the infrared and vice versa, indicating the presence of a center of symmetry in the complex; i.e., the symmetry of the complex remains that of the BPE ligand, C2h (see Figure 3). Before the effect of the solvent on the SERS intensities of the [Fe2BPE(CN)10]6- ion complex adsorbed on a silver electrode is discussed, let us first investigate the SERS spectra in ethanol solution and its dependence on the applied potential as shown in Figure 4. These results show that both SERS intensity and frequency position of the bridging ligand modes present strong dependence on the applied potential. For potentials less negative than -0.9 V, the SERS spectra resemble the solid-state Raman spectrum, except for changes in the relative intensities.

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Table 1. Vibrational Wavenumbers Observed in the Raman and Infrared Spectra of the Na6Fe2BPE(CN)10 Complex in the Solid State and Those Observed in the SERS Spectra of the Complex Adsorbed on Silver in 0.5 mol‚L-1 NaClO4 Ethanolic Solution at the Indicated Applied Potentials with the Respective Tentative Assignment (λexc ) 632.8 nm) solid Raman

SERS (C2H5OH) IR

2090 (m)a 2088 (w) 2046 (vvs) 2035 (vs)

E ) -0.4 V

E ) -1.3 V

2114 (s) 2097 (s)

2094 (m)

2032 (w) 2024 (s) 1998 (s)

1691 (m) 1660 (w) 1633 (m)

1637 (vs)

1633 (w)

1605 (s)

1604 (m) 1555 (vs)

1628 (m) 1605 (m) 1601 (vs) 1537 (w) 1491 (w)

tentative assignt symmetry (C2h)32,35 approximate mode description νCtN νCtN νCtN νCtN νCtNb νCtNb 883+826 Comb(?) νC(1)dC(2) Comb(?) νCdC ring νCdC ring νC(1)dC(2)b Py (19a)

1422 (m) 1339 (w) 1314 (w)

1337 (m)

1327 (m)

in p. CH bend (olef.)

1277 (m)

νsC(3)-C(2)b νaC(3)-C(2) in p. CH bend (Py) + in p. CH bend (olef.)b

1263 (s) 1227 (m)

1243 (w)

1198 (s) 1064 (w)

1199 (s)

1015 (m) 883 (w)

1010 (m)

1242 (s) 1217 (sh) 1067 1040 1005 (m)

556 (w)

654 (w) 570 (m) 552 (m)

Ag Bu Ag Ag Ag Bu Ag Ag Bu Ag Ag Ag

Py ring breathing Py o.p (10a)

Ag Bg Bu Ag Au Bu

Py i.p (6a) 554 (m)

Bu

νsC(3)-C(2) Py (18a)

826 (m) 654 (w)

symmetry (C2h)

Symbols in parentheses are relative intensities: vw ) very weak, w ) weak, m ) medium, s ) strong, vs ) very strong, vvs ) very very strong, and sh ) shoulder. In p. CH bend (olef) means in-plane CH bending mode of the CdC moiety; in p. CH bend (Py) means in-plane CH bending mode of the Py ring; o.p. means out of plane mode. b Vibrational mode associated with the complex in the reduced form. a

Figure 4. SERS spectra of the ion complex [Fe2BPE(CN)10]6adsorbed on a silver electrode in a 0.5 mol‚L-1 NaClO4/C2H5OH solution at the indicated applied potentials. λexc ) 632.8 nm.

Remarkable changes in the SERS spectra are observed for applied potentials more negative than -1.0 V, where the complex is reduced. According to molecular orbital calculations31 the first excited state of the BPE is C(1)-C(2) antibonding and C(3)-C(2) and C(3′)-C(1) bonding in character. Therefore, one would expect that the transfer of one electron to the LUMO of BPE would cause a decrease on the C(1)-C(2) bond order and an increase in the C(3)-C(2) and C(3′)C(1) bond orders. The strong SERS feature at 1555 cm-1 is therefore assigned to the C(1)dC(2) ethylenic interring stretching mode of the reduced species, which represents a downshift of ca. 82 cm-1 in relation to the (31) Abu-Eittah, R.; Hamed, M. M.; Mohamed, A. A. Int. J. Quantum Chem. 1991, 39, 211.

same vibrational mode in the unreduced species, where it appears at 1637 cm-1.32 This shift is similar to the one observed for the parent molecule trans-stilbene,33 when one compares the ground and first excited electronic state Raman spectra. Another reduction sensitive mode is the symmetric C(3)-C(2) stretching mode (symmetric means the in-phase C(3)-C(2), C(3′)-C(1) stretching vibrations), which shifts from 1198 to 1277 cm-1 as a consequence of C(3)-C(2) (or C(3′)-C(1)) bond order increase upon reduction. According to Hamaguchi et al.34 the olefinic CH bending mode of S° trans-stilbene shifts from 1318 to 1241 cm-1 in the S1 first excited state. On the basis of this assignment we tentatively assign the 1337 cm-1 SERS band (observed at -0.4 V) to the olefinic in-plane CH bending mode of BPE, which shifts to 1242 cm-1 upon reduction of the complex. The chemical interaction of the complex with the silver surface also involves one or more CN ligands as evidenced by the frequency position of the CN stretching mode in the adsorbed complex. In the solid state the CN stretching mode is observed at 2090 cm-1 while in the SERS spectrum at -0.4 V it is observed at 2114 cm-1. This upward shift on the ν(CN) mode indicates that at least one of the CN ligands is interacting with the silver surface via the electron lone pair of the N atom. This behavior has already been observed for other cyanoferrate complexes.11,21 Sup(32) McMahon, J. J.; Babcock, G. T. Spectrochim. Acta 1982, 38A, 1115. (33) Hester, R. E., Matousek, P., Moore, J. N.; Parker, A. W.; Toner, W. T.; Towrie, M. Chem. Phys. Lett. 1993, 208, 471. (34) Hamagushi, H.; Urano, T.; Tasumi, M. Chem. Phys. Lett. 1984, 106, 153. (35) Yang, W.; Hulteen, J.; Schatz, G. C.; Van Duyne, R. P. J. Chem. Phys. 1996, 104, 4313.

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Figure 5. SERS spectra of the ion complex [Fe2BPE(CN)10]6adsorbed on a silver electrode in a 0.5 mol‚L-1 NaClO4 aqueous solution at the indicated applied potentials. λexc ) 632.8 nm.

Figure 6. SERS spectra of the ion complex [Fe2BPE(CN)10]6adsorbed on a silver electrode in a 0.5 mol‚L-1 NaClO4/CH3CN solution at the indicated applied potentials. λexc ) 632.8 nm.

porting evidence for this hypothesis is the observation of a SERS signal at ca. 230 cm-1, attributed to a νAg-N stretching mode from the chemisorbed complex (in Figure 6 one can see that the dependence for the SERS intensity of this mode on the applied potential follows that of the ν(CN) mode). The ν(CN) mode seems to be also sensitive to the electrochemical reduction, since at negative potentials a well-defined band appears at ca. 2042 cm-1. It has been observed, for a variety of cyanoferrate complexes, that the frequency of the ν(CN) mode changes according to the oxidation state of the metal center.11 This shift in the ν(CN) frequency in then indicative of a partial reduction of the iron centers. Given that both ν(CN) and BPE vibrational modes are sensitive to reduction, one can conclude that the electron being transferred is delocalized over the entire complex, including the BPE and iron centers. Here again the experimental results point out to the delocalization of the electronic system over all the complex. The Solvent Effect. Figures 5 and 6 present the SERS spectra of the ion complex [Fe2BPE(CN)10]6- adsorbed on a silver electrode in 0.5 mol‚L-1 NaClO4 aqueous solution and 0.5 mol‚L-1 NaClO4/CH3CN solution at the indicated applied potentials. For potentials less negative than -0.5 V, the SERS spectra in water and acetonitrile are very similar. As the potential becomes more negative than -0.5 V, one observes a remarkable difference between the SERS spectra in these two solvents, i.e., while in acetonitrile the BPE vibrational modes decrease their intensities and the band assigned to the CN stretching mode increases, the opposite behavior is observed in water. Note also that in water the CN stretching is not observed at potentials where the complex is reduced while in acetonitrile the CN

Corio et al.

Figure 7. SERS excitation profiles for ν(CN) stretching and indicated BPE modes obtained for the [Fe2BPE(CN)10]6- adsorbed on a silver electrode in a 0.5 mol‚L-1 C2H5OH solution. Laser excitation ) 632.8 nm.

stretching frequency of the reduced complex is well defined. On the other hand, the SERS features at 1555, 1277, and 1245 cm-1, characteristic of the reduced ligand (BPE), are strong in the SERS spectra in water but completely absent in the SERS spectra in acetonitrile. Comparing the above results with those of Figure 4, one can see that the spectra in ethanol as the solvent lies as an intermediate case, since both CN stretching and BPE vibrational modes are enhanced over a wide range of applied potentials. The excitation of the SERS spectra of the complex adsorbed on the silver electrode in ACN solution using different laser excitations has shown (Figure 1S of the Supporting Information) that the characteristic modes of the reduced ligand at 1555, 1277, and 1242 cm-1 are only observed at 782.0 nm excitation. On the other hand, the CN stretching modes of the reduced complex, at 2047 and 1990 cm-1, can be seen only at 632.8 nm excitation. These results suggest the presence of a resonance Raman effect on the reduction product involving two different states: one at the ligand BPE, resonantly excited at 782.0 nm, and a second state involving the CN ligands, which is in resonance at 632.8 nm. For a better understanding of the solvent effect on the SERS intensities of the adsorbed complex it is necessary to look at the SERS excitation profiles (SERS intensity vs applied potential) of some vibrational modes of the adsorbate in the presence of the three solvents and at different laser excitations. Figure 7 displays the SERS intensity vs Vapp curves for the ν(CN) and BPE modes obtained from the SERS spectra of [Fe2BPE(CN)10].6- According to the charge-transfer model for SERS,12 it is assumed that the energies of donors or acceptorselectronic states of the SERS substrate depend on the applied potential. The fundamental observation on which this model is based is that the Raman intensity reaches a maximum when the energy of the exciting radiation is in resonance with a metal/adsorbate charge-transfer transition. Thus, the SERS excitation profiles in Figure 7 suggest the presence of two independent potential modulated photon assisted charge-transfer transitions. In fact, while the BPE modes are enhanced by a metal to adsorbate (Ag f π*(BPE)) transition, the enhancement of the ν(CN) mode involves an adsorbate to metal (HOMO(CN) f Ag) charge-transfer process. The Dependence of the SERS Excitation Profiles of the ν(CN) Stretching Mode on the Solvent and Energy of the Exciting Radiation. Let us first investigate the HOMO(CN) f Ag charge-transfer process. Figure 8 presents the SERS excitation profiles for the

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Langmuir, Vol. 15, No. 7, 1999 2505

Figure 9. Diagram of the charge-transfer transitions involved in the SERS effect of [Fe2BPE(CN)10]6- adsorbed on a silver electrode. The relative position of the donor states (HOMO(CN)) of the complex in H2O, C2H5OH and CH3CN and the dependence of the local density of acceptor states of the silver electrode (LDS(a)-Ag) near the Fermi level on the applied potential are displayed in the diagram. The vertical bars, continuum and dotted, represent the 632.8 nm (1.96 eV) and 782.0 nm (1.58 eV) laser excitations, respectively. Table 2. Potentials of Maximum SERS Intensity (Vmax) for the ν(CN) Stretching Mode in Na6Fe2Bpe(CN)10 in Water, Ethanol, and Acetonitrile at the Indicated Exciting Radiations Vmax/V

Figure 8. SERS excitation profiles of the ν(CN) stretching mode for the three different solvents and three different laser excitations.

ν(CN) mode in the three solvents at three different laser excitations. The experimental Vmax values obtained for the three solvents and exciting radiations are displayed in Table 2. In all solvents one observes that the potential of maximum SERS intensity (Vmax) shifts to more negative potentials as the energy of the laser excitation increases. This behavior is characteristic of an adsorbate to metal charge-transfer process. It is interesting to note that, for the same laser excitation energy, the position of Vmax shifts to more negative values as the solvent is changed from water to ethanol and acetonitrile. To rationalize these results, one needs to consider that the position of the donor (HOMO(CN)) energy

solvent

λexc ) 514.5 nm (pωL ) 2.41 eV)

λexc ) 632.8 nm (pωL ) 1.96 eV)

λexc ) 782.0 nm (pωL ) 1.58 eV)

H2O C2H5OH CH3CN

-0.9 -1.0 < -1.1

-0.4 -0.8 -1.0

-0.6 -0.7

levels of the adsorbed complex relative to the Fermi level (EF) changes according to the chemical nature of the solvent and the solvent/adsorbate interaction. As a consequence, a resonance condition between the energy of the exciting radiation and the metal/adsorbate potential modulated charge-transfer transition is achieved at different applied potentials. Since the energy of the acceptor states located near the Fermi level of the silver electrode increases as the applied potential is made more negative, the values of Vmax obtained suggest that the HOMO(CN) in water lies at lower energy than in acetonitrile, ethanol being the intermediate case. This can be understood in terms of the hydrogen bonding interaction expected to occur between the CN groups (Lewis base) and the acidic protons of C2H5OH or H2O (strong Lewis acid), thus causing a decrease in the energy of the HOMO(CN). The hydrogen bonding interaction takes place less efficiently in ethanol, which is a weaker acid than water. Figure 9 displays an energy diagram showing the relative positions of the donor (HOMO(CN)) and acceptor states (LDS(a)Ag) of the surface complex formed by the binuclear complex on the silver surface as probed by SERS. Note that, when the applied potential equals Vmax, the resonance condition for each solvent is achieved. The relative positions of the energy levels in this diagram were scaled by the SERS results.

2506 Langmuir, Vol. 15, No. 7, 1999

Corio et al.

According to the charge-transfer model for the SERS effect, the Vmax dependence on the laser excitation (pωL) can be described as12

( a1)pω

Vmax ) -

L

+ Vox

(1)

where Vox is the FeII/FeIII half-wave potential and the parameter a is a constant. The numerical value of the parameter a can also be evaluated from the following relation:12

a)

EMLCT Vred - Vox

(2)

Here Vred is the reduction potential of the ligand (BPE in this case), Vox is the half-wave potential for the FeII/FeIII one-electron process and EMLCT is the energy of the metal to ligand charge-transfer transition as obtained by UVvis absorption spectroscopy. A straight line has been obtained by plotting the potential of maximum SERS intensity vs the energy of the exciting radiation for the ν(CN) stretching mode for the data obtained in ethanol (see Figure 2S of Supporting Information). The data in ethanol have been chosen because the SERS intensity of the CN stretching mode in water is very low at 782.0 nm excitation and, in acetonitrile, the Vmax value for the 514.5 nm laser excitation lies at potentials more negative than the reduction of the complex. Fitting the obtained straight line to eq 1, one finds a Vox value of 0.16V and a ) 2.1 eV/V. It is interesting to note that the Vox value obtained according to the SERS excitation profiles is in good agreement to the observed FeII/FeIII half-wave potential (see Figure 1). To further verify the proposed model for the SERS enhancement, the parameter a shall also be evaluated according to eq 2. The UV-vis absorption spectra of the [Fe2BPE(CN)10]6- ion complex in water and ethanol solutions (see Figure 3S of Supporting Information) show absorption maxima at 458 and 480 nm, respectively. These absorptions are assigned to the metal to ligand chargetransfer process (MLCT). This red shift is interpreted as a solvatochromic effect. The MLCT transition decreases in energy as the acidic character of the solvent decreases, which means that the lowest unoccupied molecular orbital (LUMO) of the system is stabilized in more aprotic solvents. We have not been successful in observing the reduction wave of the free bridging ligand (BPE) in C2H5OH solution or of the [Fe2BPE(CN)10]6- complex in aqueous solution using gold or silver electrodes due to the reduction of the solvent (H2 evolution reaction). We have then investigated the electrochemical reduction of the BPE in a 0.5 mol‚L-1 NaClO4/C2H5OH solution and that of the BPE in the [Fe2BPE(CN)10]6- ion complex in a 0.5 mol‚L-1 NaClO4 aqueous solution using a copper electrode covered by a mercury drop. The redox process observed in both cases are irreversible. The BPE free ligand presents a reduction wave with E1/2 at -1.32 V in ethanol, while the complex in water solution presents two reduction waves with E1/2 values of -0.98 and -1.23 V. These values are in good agreement with the reduction of the complex observed in the SERS spectra. Upon substitution of the values of EMLCT ) 2.58 eV (or 480 nm), Vox ) 0.22 V (as obtained from the voltammogram of Figure 1) and Vred ) -1.0 V (as estimated from the SERS spectra in ethanol) in eq 2, one obtains a ) 2.11 eV/V. This result is in excellent agreement with the value

Figure 10. SERS excitation profiles for the BPE vibrational modes for the complex [Fe2BPE(CN)10]6- in a 0.5 mol‚L-1 NaClO4/C2H5OH solution at the indicated exciting wavelengths.

obtained from the experimental SERS excitation profiles (see Figure 9). The Dependence of the SERS Excitation Profiles of the BPE Vibrational Modes on the Solvent and Exciting Radiation. Figure 10 displays the SERS excitation profiles for two characteristic Raman bands of the BPE ligand in the [Fe2BPE(CN)10]6- complex. Note that as the energy of the excitation source decreases, the potential of maximum SERS intensity (Vmax) shifts to more negative values. This result clearly shows that the chargetransfer mechanism causing the extra enhancement due to the applied potential involves donor states located near the Fermi level of the silver electrode (LDS(d)-Ag) and acceptor states located at the BPE ligand, i.e., the lowest unoccupied molecular orbital of the BPE ligand (LUMO(BPE)). Figure 11 illustrates the effect of the solvent on the position of the Vmax values. Depending on the solvent, the Vmax values for the BPE modes follow the decreasing order (more negative): H2O < C2H5OH < CH3CN. These results can be rationalized considering that in solutions of nonhydroxylic solvents, like CH3CN, preferential solvation of the hydrophobic moiety of the complexsthe BPE ligandsoccurs. As a consequence, the LUMO of the BPE ligand is stabilized by this solvent. The behavior of the SERS spectra of the [Fe2BPE(CN)10]6- ion complex in different solvents reflects therefore different solvation sites of the complex for hydroxylic and nonhydroxylic solvents. In fact, preferential and asymmetric solvation in substituted cyanoiron(II) complexes have already been proposed.24-26 Using the above SERS results for the BPE modes, Figure 12 presents an energy diagram showing the positions of the LUMO of BPE in relation to the donor state (LDS(d)-Ag) for the three different solvents. The UV-vis results show that the metal to ligand charge-transfer transition within the binuclear complex depends on the solvent. The above SERS results have shown that the charge-transfer process operative on the SERS effect also depends on the nature of the solvent. Albeit the UV-vis absorption spectrum in acetonitrile could not be obtained due to the limited solubility of the complex in this solvent, one could predict, based on the SERS results, that the absorption maximum in this solvent should occur at lower energy (larger wavelength) in comparison to water and ethanol as solvents. The effect of the acetonitrile as stabilizing the acceptor states of the bridging ligand (BPE) can also account for the fact that the SERS features characteristic of the reduced BPE ligand are not observed at 514.5 nm excitation in this solvent (Figure 1S); i.e., in this solvent and at this excitation energy, the resonance condition for the reduced species is not achieved.

Surface-Enhanced Raman Scattering

Langmuir, Vol. 15, No. 7, 1999 2507

Figure 12. Energy diagram displaying the relative position of the local density of donor states (LDS(d)-Ag) near the Fermi level and the acceptor states (LUMO(BPE)) for the adsorbed complex in the three investigated solvents.

(ii) The electronic system in the complex is well delocalized, and the electrochemical reduction promotes the occupation of electron acceptor states delocalized over the BPE bridging ligand and iron centers. (iii) Two photon-assisted potential modulated chargetransfer processes between the substrate and the metal surface have been identified: (a) a metal to adsorbate CT, characterized as a charge-transfer process from LDS(Ag) donor states near the Fermi level to the LUMO(BPE) (Ag f π*(BPE) and (b) an adsorbate to metal CT, characterized as a charge-transfer process from donor states located at the HOMO(CN) to the LDS(Ag) acceptor states near the Fermi level (HOMO (CN) f Ag). (iv) The potential of maximum SERS intensity for the BPE and CN modes depends on the solvent since the relative position (in relation to EF) of the electronic states involved in the photon assisted charge-transfer processes depends on the nature of the solvent-adsorbate interaction. While hydroxilic solvents such as water have the effect of stabilizing the HOMO(CN), nonhydroxilic solvents such as acetonitrile have the effect of stabilizing the LUMO(BPE). (v) The results here presented also confirm the proposed model10,12 for the charge-transfer mechanism operating in the SERS effect at electrode surfaces as being a resonance Raman effect modulated by the applied potential.

Figure 11. SERS excitation profiles of the indicated BPE vibrational modes for the complex [Fe2BPE(CN)10]6- adsorbed on a silver electrode in the three investigated solvents.

Conclusions From the results presented in this work we can draw the following conclusions. (i) The ion complex [Fe2BPE(CN)10]6- adsorbs on the silver surface via electronic interaction between the CN groups and electron acceptor states of the metallic surface.

Acknowledgment. The authors would like to acknowledge the support of FAPESP and CNPq. P.C. thanks FAPESP for the grant of a doctoral fellowship (94/29970). Supporting Information Available: Figures showing SERS spectra of [Fe2BPE(CN)10]6-, a plot of the dependence of Vmax on pωL, and electronic spectra of [Fe2BPE(CN)10]6-. This material is available free of charge via the Internet at http://pubs.acs.org. LA981082W