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Contribution of the Herzberg-Teller Mechanism to the Surface-Enhanced Raman Scattering of Iron Phthalocyanine Adsorbed on a Silver Electrode Paola Corio,*,† Joel C. Rubim,† and Ricardo Aroca‡ Instituto de Quı´mica, Universidade de Sa˜ o Paulo, Sa˜ o Paulo, SP, Brazil, C.P. 26077, 05599-970, and Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Canada, N9B 3P4 Received January 13, 1998. In Final Form: April 20, 1998 The SERS (surface-enhanced Raman scattering) and SERRS (surface-enhanced resonant Raman scattering) spectra of iron(II) phthalocyanine (FePc) adsorbed onto a silver electrode have been investigated. The electrochemical SERRS has shown that the reduction of FePc molecule is reversible at negative potentials below -1.0 V vs Ag/AgCl. The SERS spectra obtained by excitation off resonance contain mainly a1g and a2g vibrational modes. The excitation profiles of these modes show a maximum of SERS intensity that shifts to more positive values as the laser energy is decreased. We discuss the results of the a2g modes as being enhanced by a potential modulated adsorbate to metal charge-transfer mechanism, in which two electronic states of the MPc are coupled by a Herzberg-Teller term. In this case the acceptor state is localized on the Ag-SERS active site and the MPc coupling electronic excited states are of A2u and A1u symmetry, resulting in the enhancement of a2g vibrational modes.

Introduction In the last few years attention has been given to experiments in which the charge-transfer mechanism is contributing to the enhancement of the Raman scattering of species adsorbed on metal surfaces1-8 with the aim to address the participation of a resonance Raman effect on this part of the total enhancement. In a recent work,5 we have proposed a formalism derived from the time-domain description of the resonance Raman effect that describes the dependence of the SER intensities on the electrode potential. This approach accounts for the enhancement of totally symmetric modes via a Franck-Condon mechanism, and only one electronic excited state of the adsorbate/metal system is considered. The aim of 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 multiple excited electronic states, vibronic coupling, and symmetry selection rules. In this context, the SERS effect of iron phthalocyanine adsorbed on a silver electrode is discussed. In the resonance Raman (RR) spectra of metal phthalocyanines (MPc) two mechanisms of enhancement occur * Fax: 55 11 815 55 79, phone: 55 11 818 3853, e-mail: [email protected]. † Universidade de Sa ˜ o Paulo. ‡ University of Windsor. (1) 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. (2) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A. Bernard, I.; Sun, S. C. Chem. Phys. Lett. 1984, 104, 240. (3) Rubim, J. C.; Temperini, M. L. A., Corio, P.; Sala, O.; Jubert, A. Chacon-Villalba, M. E.; Aymonino, P. J. J. Phys. Chem. 1995, 99, 345. (4) Corio, P.; Rubim, J. C. J. Phys. Chem. 1995, 99, 13217. (5) Rubim, J. C.; Corio, P.; Ribeiro, M. C. C.; Matz, M. J. Phys. Chem. 1995, 99, 15765. (6) Corio, P.; Rubim, J. C. J. Raman Spectrosc. 1997, 28, 235. (7) Arenas, J. F., Wolley, M. S., Otero, J. C., Marcos, J. M. J. Phys. Chem. 1996, 100, 9. (8) Kambhampati, P., Child, C. M., Campion, A. J. Chem. Soc., Faraday Trans. 1996, 92(23), 4775.

simultaneously:9 an enhancement via Albrecht’s term A (the Franck-Condon mechanism), that works only on the totally symmetric modes, and an enhancement via the Albrecht’s term B (the Herzberg-Teller mechanism), that allows for non totally symmetric modes to be enhanced. The Herzberg-Teller mechanism is due to the interaction of two different electronic excited states and allows the enhancement of vibrational modes that couple these two electronic states. The symmetry selection rule requires that the symmetry of the vibrational mode coupling the two excited states is contained in the direct symmetry product of the two electronic states involved in the process. In the case of the MPc, these transitions would be the Q and Soret bands, which have Eu symmetry in the D4h symmetry point group. The direct product (Eu × Eu) contains the following representation: a1g, a2g, b1g, and b2g. Therefore, the vibrational modes of the MPc having any of these symmetries can be intensified in the RR spectra excited, for instance, with a laser line close to the Q-band. This work presents the RR and NR spectra of FePc in the solid state as well as its SER spectra at different applied potentials and exciting radiation wavelengths. The potential modulated charge transfer between adsorbate and metal electronic levels and the contribution of the Herzberg-Teller mechanism to the enhancement of non totally symmetric modes at the electrode surface are discussed. Experimental Part The Raman and SER 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, the 632.8 nm line of an air-cooled He-Ne laser and the 782.0 nm line of a solid-state Al doped GaAs laser from Renishaw were used. The SER substrate was a silver electrode with 0.2 cm2 of geometrical area activated according to the following procedure: (9) Palys, B. J.; van den Ham, D. M. W.; Briels, W.; Feil, D. J. Raman Spectrosc. 1995, 26, 63.

S0743-7463(98)00062-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/30/1998

Raman Scattering of Iron Phthalocyanine

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Table 1. Vibrational Modes Observed in the Normal Raman, Resonant Raman and SER Spectra of FePc in the Indicated Wavelength and Tentative Assignment of Symmetry Speciesa solid λexc ) 632.8 nm

SERS λexc ) 514.5 nm

λexc) 632.8 nm E ) -0.4V

1609 (m) 1523 (s) 1516 (s)

1608 (w) 1587 (m) 1526 (vs)

1456 (s) 1438 (m) 1400 (m)

1451 (m)

1338 (s) 1306 (s) 1215 (s) 1192 (s) 1140 (s)

1339 (s)

1107 (s) 1036 (w) 1007 (w) 955 (s) 850 (m) 831 (m) 777 (w) 748 (s) 721 (w) 679 (vs) 636 (w) 593 (m)

1387 (m)

1135 (w)

E ) -0.4V 1603 (s) 1585 (s) 1531 (s)

symmetry

1600 (m)

a1g

1581 1531 (vs)

1528 (s)

a1g

1440

1477 (s) 1448 (s) 1434 (s)

1451 (s)

1340 (s) 1306 (s)

1337 (s) 1302 (w)

1328 (vs) 1300(s)

1355 (s) 1333 (s) 1300 (s)

1192 (m) 1140 (m)

1188(w)

1180

1182(m)

1124 (m)

1120(s) 1100(s) 1032 1007 (m) 954 (m)

1116 (s) 1100 (s) 1030 (w) 1015 (w) 954 (w)

1387 (w)

1014 (m) 955 (m) 830 (s)

λexc)632.8 nm E ) -1.0V

E ) -0.8V

1452 (s) 1432 (w) 1398 (w)

1108 (m)

831 (m)

830 (s)

750 (s)

830(m)

a1g a1g eg a1g a2g a2g a2g a1g a1g a2g a2g a2g a1g

749(m)

743 (m)

a2g

679 (m)

680 (s)

680 (s)

683(s)

680 (m)

a1g

593 (w)

593 (m)

593 (m)

593(s)

585 (s) 511 (w) 482 (w)

a1g

483 (m) a

1528 (vs)

λexc )514.5 nm

482 (m)

482(w)

a2g

Key: w ) weak, m ) medium, s ) strong, and vs ) very strong.

initially the silver electrode is activated performing successive oxidation reduction cycles in the -0.6 to +0.2 V potential range in a 0.1 mol/L KCl solution. The reflectance absorption spectrum of the electrode surface was recorded, showing a broad plasmon absorption centered at 520 nm. Then the electrode is removed from the spectroeletrochemical cell and a drop (ca. 0.05 × 10-3 L) of a 10-4 mol/L solution of FePc in DMF (dimethyformamide) is put on the top of the silver electrode. This drop is maintained on the electrode surface for 60 s, and then the electrode is thoroughly washed with doubly distilled water. After this treatment the silver electrode is transferred back to the spectroelectrochemical cell containing a 0.1 mol/L Na2SO4 in water solution as the supporting electrolyte, and SER spectra are recorded by varying the applied potential in steps of 0.1 V from 0.0 to -1.2 V. The spectroelectrochemical cell used in this work was described elsewhere.10 All the potentials are referred to an Ag/AgCl reference electrode. The electrochemical system used to monitor the potential applied to the electrode was a PAR 263 potentiostate/ galvanostate from EG&G. The FePc was supplied by Eastman Kodak and was used as received without any further purification. The electrolyte solutions were prepared using analytical grade chemicals and doubly distilled water.

Results and Discussion Raman Spectra of the FePc in the Solid State and Vibrational Assignment. The normal Raman (NR) and resonant Raman (RR) spectra of FePc as a solid and excited at different wavelengths are shown in Figure 1. Table 1 presents a list of the observed Raman frequencies with their respective tentative symmetry assignment. The RR spectrum, excited at 632.8 nm, within the Q absorption band, shows a larger contribution from the non totally symmetric modes than the spectrum excited at 514.5 nm. (10) Nicolai, S. H. A.; Di Mascio, P.; Rubim, J. C. J. Electroanal. Chem. to be submitted.

Figure 1. Raman spectra of FePc as a solid at λL ) 514.5 nm (NR) and λL ) 632.8 nm (RR).

For the NR spectrum, vibrational modes of a1g, b1g, b2g and eg symmetries are expected to be active.9 In the RR spectrum, vibrational modes of a2g symmetry are enhanced

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Figure 2. SER spectra FePc adsorbed on a silver electrode in 0.1 M Na2SO4 at the indicated potentials. Laser power ≈ 0.5 mW. λL ) 632.8 nm.

due to vibronic coupling between two excited electronic states of the FePc.11 According to symmetry considerations within the D4h symmetry point group, the a2g modes are those that appear only in the RR spectrum: 1306, 1215, 1192, 1105, 955, 750, and 483 cm-1. The eg modes are those that appear only in the NR spectrum. In the NR spectrum we found only one feature that could be assigned to an eg mode, the band at 1397 cm-1. Raman signals that appear in the NR as well as in the RR spectra can be assigned to a1g, b1g or b2g modes: 1603, 1534, 1337, 1140, 830, 684, and 594 cm-1. The assignment of the a1g modes was based on ref 12. SERS Results. Figures 2 and 3 display the SERS spectra of the FePc adsorbed on a silver electrode for different applied potentials and for two exciting radiation, 632.8 and 514.5 nm, respectively. In Figure 2, at 632.8 (11) Hamaguchi, H. Adv. Infrared Raman Spectrosc. 1985, 12, 273.

nm excitation, a strong contribution of resonance Raman effect is observed, while at 514.5 nm excitation the signal observed can be attributed almost entirely to the SERS effect, since the exciting radiation is no longer in resonance with the MPc absorption band and no resonant Raman enhancement is operative. The results of Figures 2 and 3 show that the SERS spectra at -0.4 V are similar to the solid state spectra, i.e., at 514.5 nm excitation the SERS spectrum resembles the NR spectrum where mainly a1g modes are observed, and at 632.8 nm the SERRS spectrum is similar to the RR spectrum where totally symmetric modes are also observed. The resemblance between SER and solid state Raman spectra is an indication that the adsorption of the molecule on the metal surface does not disturb significantly the electronic structure of the Pc macrocycle. This is an expected behavior, since the 18 π electron system formed


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by the phthalocyanine is a very well delocalized system; hence, the adsorption of FePc on the electrode surface does not cause any significant change in the Raman frequencies or on their relative intensities. On the other hand, it is interesting to compare the response of the SER spectra obtained in and out of resonant condition when the applied potential is made more negative. It is seen in the data obtained at 514.5 nm (Figure 3) that at -0.8 V modes with a2g symmetry are strongly enhanced, e.g. the modes at 1300, 955, 748, and 482 cm-1. This spectrum, at -0.8 V, shows basically a1g and a2g vibrational modes. For potentials more negative than -1.0 V, significant frequency shifts and new Raman bands are observed for the resonant case as well as for the nonresonant case. In cyclic voltammetry studies performed on FePC molecule adsorbed on the silver electrode in the same electrolyte solution used in the SERS measurements, we

have not been able to observe any reduction wave in the 0.0 to -0.9 V potential range. We only observe a reduction wave close to -1.0 V. In fact, Scherson et al. and Tanaka et al.13,14 have also investigated the voltammetry of FePc and have only observed a reduction wave close to -0.95 V vs Hg/HgO/OH- (or -1.1 V vs SCE). Therefore, the changes in the spectra at -1.0 V are assigned to the reduction of the molecule. The reduced compound presents weaker bonds due to the occupation of antibonding orbitals (π*). Consequently, most of the bands are displaced to smaller wavenumbers. New bands, characteristic of the reduced species, are also observed at 1476 (12) Aroca, R.; Zeng, Z. Q.; Mink, J. J. Phys. Chem. Solids 1990, 51, 2, 135. (13) Scherson, D. A.; Fierro, C. A.; Tryk, D.; Gupta, S. L.; Yeager, E. B.; Eldridge, J.; Hoffman, R. W. J. Electroanal Chem. 1985, 184, 419. (14) Tanaka, A. A.; Fierro, C.; Scherson, D.; Yeager, E. B. J. Phys. Chem. 1987, 91, 3799.

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and 1430 cm-1. These new Raman features observed for all exciting radiations at -1.0 V in the reduction product are not observed in the NR or RR spectra of the solid state and cannot be assigned to vibrational modes of the FePc molecule. Therefore, they are assigned to the radical anion of the macrocycle. It is worth mentioning that this spectroelectrochemical behavior is reversible. Figure 4 displays the SER spectra at different applied potentials excited at 782.0 nm. Notably, some of the a2g modes that are present at 0.0 V are also observed in the solid-state spectrum excited at 782.0 nm. This behavior is due to a preresonance effect at this wavelength. On the other hand, if one considers, for example, the a2g modes at 1104, 1196 and 1303 cm-1, it can be seen that these modes are enhanced for potentials less negative than those observed for 514.5 nm excitation. It is important to stress that all the observed wavenumbers in the SERS spectrum at -0.8V with λexc ) 514.5 nm (Figure 3) and at -0.4V

with λexc ) 782.0 nm (Figure 4) correspond to vibrational modes of the FePc molecule. Therefore, the significant changes in relative intensities observed for the a2g modes cannot be the result of any faradaic charge-transfer process: either the reduction of the Pc macrocycle or its metal center. The fact the SERS spectra excited at wavelengths far from resonance with the Q-band, give rise to the enhancement of the a2g vibrational modes (nonallowed modes at normal Raman condition), at different applied potentials for different excitations, suggests the participation of a potential modulated charge-transfer mechanism of enhancement. The SERS Enhancement Mechanism. The results presented above can be rationalized in terms of the different symmetries of the excited electronic states in the resonant Raman and SERS effects. In the case of the SERS process, the electronic transi-


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Figure 6. Schematic representation of electronic transitions excited in resonant Raman and SERS processes. In the resonance Raman case, the a2g modes are enhanced at 632.8 nm excitation which is in resonance with the Q-band since the Eu r A1g (Q-band) transition is borrowing intensity from the Eu r A1g (Soret band) via vibrational coupling. In the SERS case, the a2g modes are enhanced at 782.0 or 514.5 nm excitation since the potential modulated A1u r A1g CT transition (FePc f Ag) borrows intensity from the A2u r A1g transition (FePc f Ag) via a Herzberg-Teller mechanism.

Figure 5. Electronic configurations for the ground and excited electronic states involved in resonant Raman and SERS processes. The figure presents electronic states of the FePc molecule involved in the RR process and the electronic states of the Ag-FePc surface complex involved in the SERS process.

tions to be considered involve electronic states of the electrode as well as of the adsorbed molecule. The enhancement of inactive modes in the normal Raman spectra observed in the SERS results with exciting radiation out of resonance can be explained considering an electronic charge-transfer transition between the occupied orbitals of the MPcs and an acceptor level localized in the electrode (FePc f Ag transition). As a result, the excited states involved in this charge-transfer process have a different number of electrons than the excited electronic states of the resonance Raman process, and therefore belong to different symmetry representations. The main difference to be considered is that in the electronic transitions corresponding to the Q and Soret bands, the LUMO (eg) would not be occupied, since the electron moves into an electronic level localized in the electrode, near the Fermi level. For the transition from the HOMO (a1u) to the acceptor state in the silver electrode surface (which is equivalent to the Q-band) the electronic configuration of the excited electronic state would involve the occupation, by only one electron, of the a1u orbital,

resulting in a state of A1u symmetry. Considering the transition from the a2u orbital to the metal acceptor level (which is equivalent to the Soret band), the excited electronic state is of A2u symmetry. These two excited states can be coupled by vibrational modes with a2g symmetry, since A1u × A2u ) A2g. Therefore, the SERS results obtained can be described by a potential modulated adsorbate to metal charge-transfer mechanism, in which two electronic states of the FePc-Ag system are coupled by a Herzberg-Teller term. The above description of the electronic configurations and the symmetry species of the excited states involved in SERS and resonance Raman processes are represented schematically in Figures 5 and 6. The SERS Excitation Profiles. The SERS excitation profiles (normalized intensity vs applied potential) for different vibrational modes of FePc adsorbed on silver electrode are reported in Figure 7 a-d. The profiles were obtained for two different wavelengths, both of them out of resonance with the Q-band of the Pc molecule, minimizing the contribution of resonant Raman effect. The profiles obtained for λexc ) 632.8 nm are not considered, since the observed Raman signal is due mainly to the RR effect. It can be seen that the potential of maximum SERS intensity is displaced to more negative values as the energy of exciting radiation is increased for a1g as well as for a2g modes. According to the charge-transfer model proposed for the SERS effect, this is the expected behavior when the charge-transfer involved in the SERS enhancement is from donor states in the adsorbed molecule to acceptor states in the silver electrode.1,2,5 The observations are rationalized by proposing that one of the enhancement mechanism operating for the FePc complex is an electronic charge transfer from an occupied energy level localized in the FePc to unoccupied energy levels localized in the electrode. This mechanism is responsible for the enhancement of a1g and a2g modes. Since the FePc has a low-lying π* orbital there is also the possibility of an Ag f FePc charge-transfer transition. This additional CT process (Franck-Condon mechanismsonly totally symmetric modes are enhanced) could be responsible for the shoulders at less negative potentials observed in the SERS excitation profiles of the a1g modes at 514.5 nm.

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Figure 7. SERS excitation profiles (normalized SERS intensity vs applied potential) FePc vibrational modes at 514.5 and 782.0 nm laser excitation.

Conclusions The results presented here for the SERS effect of FePc adsorbed on a silver electrode support the following conclusions. (i) The FePc molecule adsorbs on the silver electrode, this adsorption process does not cause significant changes on the bond orders of the macrocycle, and the structure of the molecule is preserved. (ii) The FePc molecule is reduced at potentials more negative than -1.0 V. Since new Raman features are observed at these potentials in the region of the macrocycle vibration modes, it is believed that the reduction takes place at the macrocycle. However, since this is a welldelocalized 18 π* electron system and delocalization can be extended up to the metal center, we cannot disregard a partial reduction of the Fe(II) central ion. (iii) Both mechanisms of enhancement, the electromagnetic (EM) and the potential modulated charge transfer (CT), are operating in this system. At 632.8 nm excitation, the contribution of the resonance Raman effect is very strong, and the CT contribution is hardly observed. On the other hand, the contribution of the CT mechanism of enhancement is evident when the SER spectra are excited far from the resonance condition within the Q-band.

(iv) The CT mechanism of enhancement is a potential modulated charge transfer process involving two donor states at the FePc and an acceptor state at the silver electrode surface. The totally symmetric modes, a1g, are enhanced via a Franck-Condon mechanism, and the a2g modes are enhanced via a Herzberg-Teller mechanism. However, only the a2g vibrational modes have the proper symmetry to couple the two excited electronic states of A1u and A2u symmetry. Note that these symmetries are different from those of the excited states involved in the unperturbed RR effect of the FePc. (v) Excitation of the SER spectra at wavelengths off resonance with the Q-band (e.g. at 514.5 nm) may enhance of the a2g vibrational modes (nonallowed modes at normal Raman condition), if the applied potential is close to the resonance condition for a potential modulated chargetransfer process from donor states at the FePc to acceptor states on the silver electrode. Acknowledgment. P.C. thanks the FAPESP for the grant of a fellowship (94/2997-0). J.C.R. thanks the FAPESP for research grants (FAPESP 94/5629-5 and 94/ 4440-2) and the CNPq for a research fellowship. R.A. acknowledges the support of the NSERC of Canada. LA980062R

Contribution of the HerzbergâTeller Mechanism to the Surface

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