Raman spectroscopy of cobalt phthalocyanine adsorbed on a silver electrode

J. Electroanal. Chem., 113 (1980) 113--125

113

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

RAMAN SPECTROSCOPY OF COBALT PHTHALOCYANINE ADSORBED ON A SILVER ELECTRODE

R. KOTZ and E. YEAGER Case laboratories for Electrochemical Studies and the Chemistry Departments, Case Western Reserve University, Cleveland, OH 44106 (U.S.A.)

(Received 12th March 1980; in revised form 21st April 1980)

ABSTRACT Raman spectra of cobalt tetrasulfonated phthalocyanine adsorbed on a silver electrode in aqueous electrolytes have been recorded in situ. It is shown that the intensity of the Raman bands is directly related to the amount of charge transfered during the electrochemical activation of the silver. The strong potential dependence of distinct Raman bands is discussed with respect to the resonance properties of the adsorbate, taking into account the orientation of the molecule on the surface.

(I) INTRODUCTION The transition metal phthalocyanines have attracted much attention among e l e c t r o c h e m i s t s in r e c e n t y e a r s (see, e.g. refs. 1 - - 1 1 ) . V a r i o u s w o r k e r s have d e m o n s t r a t e d t h a t t h e e l e c t r o c h e m i c a l r e d u c t i o n o f 02 is c a t a l y z e d b y t h e s e c o m p l e x e s w h e n p r e s e n t as e i t h e r m o n o l a y e r s o r m u c h t h i c k e r layers o n c a r b o n a n d m e t a l e l e c t r o d e surfaces [ 1 - - 7 ] . In a d d i t i o n , several a t t e m p t s h a v e b e e n m a d e to sensitize s e m i c o n d u c t o r e l e c t r o d e s f o r p h o t o e l e c t r o c h e m i c a l p r o c e s s e s b y a d s o r b i n g m o n o - o r m u l t i l a y e r s o f such m a c r o c y c l i c s o n t h e surface [ 8 - - 1 1 ] . R e c e n t l y , Nikolic et al. [12] have a t t e m p t e d t o gain insight i n t o t h e s t a t e o f t h e w a t e r - s o l u b l e t e t r a s u l f o n a t e d iron a n d c o b a l t p h t h a l o c y a n i n e s (TSPc) a d s o r b e d o n e l e c t r o d e surfaces b y using s p e c u l a r r e f l e c t a n c e s p e c t r o s c o p y (visible). T h e s e t w o c o m p l e x e s are o f special i n t e r e s t b e c a u s e t h e C o T S P c is a g o o d c a t a l y s t f o r t h e 02 r e d u c t i o n p r i n c i p a l l y t o h y d r o g e n p e r o x i d e , while t h e F e T S P c c a t a l y z e s t h e f o u r - e l e c t r o n r e d u c t i o n t o w a t e r w i t h v e r y little or n o p e r o x i d e p r o d u c e d o v e r a wide p H r a n g e [ 7]. T h e in-situ visible r e f l e c t a n c e s p e c t r a o f b o t h o f t h e s e a d s o r b e d c o m p l e x e s are v e r y similar t o t h e transmission a b s o r p t i o n s p e c t r a o f t h e c o r r e s p o n d i n g b u l k s o l u t i o n p h a s e species. O n t h e basis o f this similarity, Nikolic et al. h a v e p r o p o s e d t h a t t h e essential p l a n a r p h t h a l o c y a n i n e ligands in t h e a d s o r b e d C o T S P c a n d F e T S P c c o m p l e x e s are surr o u n d e d b y w a t e r o n b o t h sides a n d t h e r e f o r e t h a t t h e s e c o m p l e x e s are a d s o r b e d w i t h t h e p l a n e o f t h e p h t h a l o c y a n i n e p e r p e n d i c u l a r to t h e s u r f a c e w i t h t w o o f t h e sulfonic acid g r o u p s i n t e r a c t i n g w i t h t h e surface. T h e s e w o r k e r s also

114

found optical evidence for the formation of an 02 adduct with the adsorbed CoTSPc and FeTSPc. On the other hand, Somorjai et al. [ 13], using low-energy electron diffraction, concluded that copper phthalocyanine was adsorbed on single-crystal copper and platinum at the vacuum interface with the plane of the phthalocyanine ligand parallel to the surface and the transition metal interacting directly with the substrate. In the present work, in-situ Raman spectroscopy has been used to examine CoTSPc adsorbed from aqueous solution on polycrystalline silver. This metal has been used as the substrate because of the large surface enhancement of the Raman signal for virtually all molecules adsorbed on this metal [14], even in the absence of an intrinsic resonant Raman effect in the adsorbed species. Emphasis has been placed in the present study on obtaining information concerning the adsorbed CoTSPc rather than on the explanation for the surface enhancement of the Raman signal. (II) EXPERIMENTAL The Raman spectra of CoTPSc in solution and on the electrode surface were measured using a Spex spectrometer (Ramalog) equipped with holographically ruled gratings. The light source was a argon ion laser (Coherent Radiation CR-8) and the photon-counting system contained a GaAs photomultiplier (RCA C31034) showing flat spectra response in the region of the available Arlaser lines. The electrochemical cell had two Pyrex optical windows for the exciting laser beam and for the detection of the Raman scattered light perpendicular to the incident beam (Fig. 1). The fixed angle of incidence of the laser radiation onto the Ag working electrode was ~ 6 0 °. The potential of the working electrode was controlled with a potentiostat (P.A.R. 173) and a waveform generator (P.A.R. 175). All potentials are quoted with respect to the saturated calomel electrode (SCE) which was located in an external c o m p a r t m e n t separated by a stopcock from the bulk electrolyte. A platinum plate was used as the counter electrode. An Ag disc (~0.25 cm 2) embedded in a Teflon rod served as the working electrode. Before each electro-

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115

chemical experiment the working electrode was polished with AlzO3 to yield a mirror-like finish and then carefully rinsed in distilled water. The 0.05 M H:SO4 electrolyte was prepared from ultrapure grade sulfuric acid (Baker Ultrex) and triply distilled water and was deaerated by bubbling with nitrogen. The CoTSPc was synthesized and purified according to the m e t h o d of Weber and Busch [15]. During the surface Raman experiments the concentration of CoTSPc in the electrolyte was 10 -s M. For activation of the Ag electrode a single potential sweep (100 mV s -1) from --0.20 to +0.45 V vs. SCE was performed, holding the potential at +0.45 V until the charge passed through the working electrode was ~ 2 0 mC cm -2 and then sweeping back to --0.20 V, where part of the dissolved Ag was redeposited. The solution-phase Raman spectra were obtained for a concentration of 10 -2 M CoTSPc in 0.05 M H2SO4, using a standard rotating-cell technique to minimize local heating effects in the strongly absorbing solution. The high absorption of the solution also required that the Raman scattered light be collected from a region immediately adjacent to the front window of the cell and this resulted in a significant contribution of the window to the spectra below 500 a m -1. (III) R E S U L T S

The solution-phase Raman spectrum is not available in the literature for the tetrasulfonated species and is needed for comparison with the surface spectra. The Raman spectrum of CoTSPc in the nitrogen-saturated solution is shown in Fig. 2 (spectrum A). Despite the high complexity of the molecule, relatively few Raman lines are clearly detectable, the frequencies of which are indicated I

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116

in the figure. However, the signal strength of the principal peaks is considerably stronger than would normally be expected for a 10 -2 M solution. The 514.5 nm Ar-laser line used for the excitation does n o t fall on any of the principal absorption bands in the transmission spectrum (see insert in Fig. 8) and consequently probably a pre-resonant enhancement is involved. The considerable background is probably due to fluorescence. The effect of 02 saturation of the solution is shown in Fig. 2 (spectrum B). The intensities of at least four Raman bands are modified drastically. The vibration peaks at 1609 and 1597 cm -1 are strongly enhanced, whereas the band at 694 cm -1 decreases. A new peak appears at 1153 cm -1. The voltammetry curves of a polycrystalline Ag electrode in 0.05 M H2SO4 with 10 -4 M CoTSPc present in the electrolyte are reproduced in Fig. 3. No change is evident due to the addition of the CoTSPc if the anodic sweep limit is kept below +300 mV. However, after activation of the Ag electrode the current--potential curve changes significantly in the potential region negative of --0.40 V. The hydrogen evolution is shifted to more positive potentials by ~0.1 V and peaks occur in the anodic and cathodic sweeps at ~ 0 . 5 5 V. These peaks have a charge under them of 2--3 pC cm -2 and are probably associated with a change in oxidation state (one electron) in the adsorbed macrocyclic complex. Just a simple electrode area increment associated with surface roughening does n o t appear to be the principal explanation for activation since the flat region of the voltammetry curve between --0.4 V and +0.1 V shows no appreciable increm e n t in the effective capacitance. In Fig. 4 the in-situ Raman spectra of CoTSPc adsorbed on the Ag electrode are shown for different electrode potentials. A strong dependence o f the relative peak intensities on the applied potential is evident. While the intensities of

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the bands at 691 and 1545 cm-' decrease due to a potential change from +0.20 to --0.20 V, the bands at 368, 500, 603, 1105, 1130, 1153, 1336, 1595, and 1609 cm-' increase. At a potential o f - - 0 . 2 0 V the spectra features are most pronounced and numerous before the intensity of the entire spectrum decreases at --0.40 V. No significant shifts of bands were observed within experimental accuracy. However, at a potential o f - - 0 . 2 0 the band at 1545 cm -~ splits into two (1540 and 1555 cm-~). In order to verify whether the observed Raman spectra are really due to a surface effect, the intensity of the Raman band at 1336 cm -1 was monitored as a function of the charge transferred during a series of activation cycles. The plot in Fig. 5 represents a superposition of the results of the three different activation procedures: repetitive activation at (1) constant p otential (+0.45 V ), transferring constant amounts of charge (10 mC cm-2), (2) constant potential (+0.45 V) transferring increasing amounts of charge and (3) increasing potentials transferring constant amounts of charge (10 mC cm-2); all yielded essentially the same results. Figure 5 shows an almost linear increase of the relative Raman intensity at 1336 cm -1 with the transferred charge up to 80 mC cm -2. Beyond this point the curve levels o f f and further application of activation cycles had no influence on the signal strength. As is already evident in Fig. 4, the variation of the electrode potential influences the distinct vibrations bands differently. This behavior is further exam-

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ined in Fig. 6a, b where the intensities of certain Raman bands were recorded during a complete potential sweep. Changing the electrode potential from +0.20 to --0.20 V causes the bands plotted in Fig. 6a to increase in intensity whereas those bands in Fig. 6b decrease. Beyond - 0 . 2 0 V all peaks decrease in intensity. In addition, a strong hysteresis is noticed for all bands, indicating

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120 irreversible changes in the vibrational properties of the adsorbate. If the potential sweep is limited to the range between +0.20 and --0.20 V, the hysteresis is almost negligible. Holding the potential for about 2 min at ~-0.60 V gave rise to a completely different Raman spectrum shown in Fig. 7. Although most of the strong vibrational bands evident in Fig. 7 are already indicated in the Raman spectra of Fig. 4, the relative intensities have changed completely. The prominent bands of the previous spectra (Fig. 4) at 692, 1336, 1545, 1595 and 1609 cm -~ are strongly reduced or virtually absent, whereas the bands at 1144, 1268 and 1495 c m - ' , which were only weak before, become the d o m i n a n t features in Fig. 7. In view of the strong absorption bands of CoTSPc in the solution phase between 600 and 700 nm and below 400 nm, a resonance of pre-resonance enhancement contribution to the Raman spectra of the adsorbed species is likely. Figure 7 shows three in-situ Raman spectra of CoTSPc adsorbed on Ag at a potential of --0.20 V excited at different wavelengths available with the argon ion laser. Although the three wavelengths lie in the phthalocyanine transmission window [10,12], several bands show deviation from the ~o4 law. The arrows in Fig. 8 indicate the intensities of the bands for k0 = 488.0 and 456.9 nm calculated from those for k0 = 514.5 nm, taking into account the different scaling factors, the laser power and the co4 law. The bands at 1153, 1304, 1336, and 1595 cm-1 are strongly enhanced when using shorter excitation wavelength, whereas the band at 603 cm -~ is clearly inhibited. However, owing to the uncertainty of the underlying baseline these calculations can serve only as a guideline. Finally, the influence of different electrolytes on the results should be mentioned. Similar spectral features to those presented for the adsorbed CoTSPc were obtained in 0.05 M Na:SO4, 0.1 M NaC104 and 0.1 M NaOH with lower band intensities. Surprisingly, 0.1 M KCI turned out to be unfavourable. No Raman signal for the adsorbed CoTSPc was detected with this electrolyte. In addition, no change in the Raman spectrum of the adsorbed CoTSPc in 0.05 M H:SO4 was obtained with 02 saturation compared to N2 saturation in the 02 reduction region (--0.2 V). (IV) DISCUSSION AND CONCLUSIONS The experimental results demonstrate clearly that the observed Raman spectra with the Ag electrode after activation are associated with the adsorbed CoTSPc rather than the solution phase species. The solution-phase concentration (e.g. 10 -s M) during these experiments was t o o low to lead to observable Raman signals under the conditions prevailing in these studies. Further evidence for a surface effect is the dependence of the Raman peak intensities on the electrode potential and also the charge transferred during the activation of the Ag electrode. The nearly linear increment in the Raman peak intensities with increasing charge during activation is similar to that observed with pyridine on Ag [16] and indicates t h a t the charge during activation is an important parameter controlling the intensity of the surface-enhanced Raman signal. The activation

121 procedure probably results in both a cleaning up of the Ag electrode surface as well as surface roughening. With the minimal purification procedures used in the present study and also all surface Raman electrochemical studies to date, the adsorption of various impurities, including underpotential deposited species, is likely to be a problem. The anodic t r e a t m e n t should strip off such impurities and facilitate the adsorption of the CoTSPc. The high sensitivity of the roughness parameters to the a m o u n t o f dissolved and redeposited Ag during anodic cycling in H2SO4 has been shown recently in electroreflectance studies [ 17]. Undoubtedly, substantial surface morphology changes including roughening occur during activation in H2SO4, although there is some uncertainty as to how pronounced such effects are for Ag in KC1 [16]. The surface roughening appears to be intrinsically necessary for the realization of large surface enhancement with adsorbed species on Ag and not just a matter of increased surface area. The levelling off the curve in Fig. 5 implies that the roughening has reached a quasi-steady state and does not increase with further anodic activation. The frequencies observed in the Raman spectra of the CoTSPc in the aqueous solution are all between 600 and 1600 cm -~ and generally are in good agreement with those observed in the Raman spectra of solid phthalocyanine [18,19]. The Raman spectra for the CoTSPc adsorbed on Ag at potentials cathodic to --0.20 V also showed weak rather broad peaks at 368 and 500 cm -1, which were n o t evident for the solution-phase species, perhaps because of the strong interference from the window at frequencies below 500 cm -1. On the basis of vibrational bandassignments which have been carried out for porphyrins and hemes, the vibrations above 600 cm -1 can be attributed to a variety of stretching and deformation modes involving the CoTSPc ring structure [20--22]. In infrared studies of negative ions of metal phthalocyanines, lines near 700 cm -1 were assigned to the non-planar deformation vibrations of C--H bonds of the benzene rings [23]. Those bands below 600 cm -1 may be attributed to Co--N or Co--ligand vibrations [20,22]. Raman spectra of phthalocyanines with various central metal atoms, recorded by Aleksandrov et al. [18] and in our laboratory, indicate that the band at 1545 cm -1 is sensitive to the central atom. Therefore, this vibration can be attributed to inner parts of the planar ligand. Upon tetrasulfonation, the spectra features of the macrocyclic remain essentially the same with the exception of one new band at 1275 cm -~ for the CoTSPc which can thus be correlated to the SO~ groups. Because of the high degree of internal coordinate mixing such assignments are very approximate. The close similarity between the Raman spectrum of CoTSPc adsorbed on the Ag electrode at +0.20 V {Fig. 4) and the solution-phase spectrum (Fig. 2a) implies t h a t the interaction between surface and adsorbate is only weak. The few strong vibrational bands evidenced in both spectra are hardly modified by the presence of the metal surface. Thus, the molecule is adsorbed on the surface with the TSPc ligand not interacting strongly with the surface. One possible configuration is with the plane of the ligand perpendicular to the surface and with one or two sulfonate groups interacting directly with the electrode surface (Fig. 9a). This configuration was proposed by Nikolic et al. [12] on the basis of the visible reflectance spectrum but it is not clear why the adsorption

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Fig. 9. Possible c o n f i g u r a t i o n s f o r CoTSPc a d s o r b e d o n a n e l e c t r o d e surface.

on the surface in this configuration should be particularly strong. An alternative is shown in Fig. 9b with the CoTSPc coupled to the surface S through an S--O--Co bond which keeps the ligand at a sufficient distance from the surface to have a layer of water molecules interposed between the plane of the ligand and the metal surface. However, the S--O--Co linkage would n o t be expected to have a bond angle of 180 ° , and hence there could be some steric problems on a fiat metal surface because of the tilt of the plane of the ligand. On the other hand, the surface sites for the bonding might be corner or edge metal atoms or defect sites of such a nature as to keep the plane of the ligand out from the surface. The surface concentration of such sites could easily be sufficient to account for near-monolayer coverage with CoTSPc in view of the large size of the ligand. A third possibility is t h a t an oxygen-linked dimeric species such as TSPcCo-O--O--CoTSPc is adsorbed with one of the CoTSPc lying fiat on the surface and the other parallel or nearly parallel to it out in the electrolyte phase (see Fig. 9c). Such species have been proposed to exist in aqueous solutions in the presence of 02 [24]. Nikolic et al. [12] proposed that such dimeric species may be adsorbed edge-on in the presence of 02 (see Fig. 9d). Even in N2-saturated solutions, there would still be sufficient trace 02 present to permit monolayer coverage with such --O--O-- bridged species, since only ~ 10 -11 moles are required per cm 2. If the adsorption involves such a dimeric species with the configuration in Fig. 9c, the direct interaction of the one CoTSPc unit with the metal surface would probably broaden its Raman bands to the point where t h e y would not show up well in the observed Raman spectrum. However, the CoTSPc unit out in the solution would exhibit essentially the normal solution phase Raman as well as UV-visible spectrum. On the basis of the present Raman data, it is n o t possible to distinguish between these models. It is indeed likely that more than one configuration for the adsorbed CoTSPc is involved.

123 Changing the electrode potential to more negative values introduces significant modulation of the relative band intensities. If the CoTSPc is assumed to be parallel to the surface, a strong electrochromic effect can be expected due to the interaction of the metal surface with the delocalized ~ orbital system of the ligand leading to shifts of the electronic absorption bands, thus modulating the resonance or pre-resonance enhancement of certain Raman bands. The strong enhancement of the bands at 1153, 1336 and ~ 1 6 0 0 cm -1 with decreasing excitation wavelength (Fig. 8) supports the above assumption. Furthermore, these enhanced lines show a c o m m o n potential dependence (Fig. 6a), whereas the potential dependence of the lines at 1542 and 691 cm -1 is quite different (Fig. 6b). However, the potential dependence of the 603 cm -~ line does not fit into this scheme. In addition, structural changes in the adsorbed molecule arising from interactions with the metal surface can change the resonance enhancement of particular vibrational modes. The enhancement of the vibrational modes at 1589 and 1638 cm -~ in hemoglobin has been explained by an out-of-plane movement of the central metal atom introducing a certain degree of doming and puckering of the porphyrin ring [ 22,25]. Applying similar arguments to the system under investigation, the enhancement of the vibrational modes at 1595 and 1609 cm -1 in the adsorbed species can be explained by an out-of-plane movement of the Co atom due to the interaction of the molecule with the metal surface, accompanied by a slight tilt of the pyrrole and benzene rings with respect to the molecular plane. The main contributions to these modes are probably the valence vibrations of the C=C conjugated bonds of the benzene rings and the periphery of the conjugated ring [ 25,26]. A comparison of the changes in the Raman spectra of the solution phase due to 02 saturation (Fig. 2b) with the changes introduced in the surface spectra due to more negative potentials (Fig. 4) reveals a surprising similarity. In both cases the bands at 1609, 1595 and 1153 cm -~ are enhanced and the band at 694 cm -1 is reduced. The introduction of 02 modifies the UV-visible absorption spectrum [ 24,27,28] and hence probably also changes the resonant or pre-resonant enhancement of certain Raman lines. Several workers [24,27,28] have attributed the changes in the electronic absorption spectra produced by 02 to the formation of the TSPcCo--O--O--CoTSPc species, while others [29] favor a species of the form TSPcCo--O--CoTSPc--O2H. The possibility exists for substantial ring-current coupling between the two phthalocyanine ring systems in either form. Apparently biasing the potential to more negative values produces a similar effect on the Ag surface by favoring a configuration such as that in Fig. 9b, c or d in which the n orbitals of two PC ligands interact with each other or the lr orbitals of one PC ligand interact with the conduction bands of the metal electrode. The splitting of the 1545 cm -~ band at more negative potentials (e.g.--0.2 V in Fig. 4) provides some evidence for such an interaction of the ~ orbitals of the PC with the electrode surface. In a recent resonant Raman study of a silane coupling agent consisting of SiPc, a strong resonance-enhanced band has been observed at 1545 cm -~ using UV excitation [19]. This band was assigned to a mixed contribution of C=N and Si--N vibrations. Coupling this species to glass through an Si--O--SiPc linkage results in a shift of this band to 1555 cm -1. This

124

was explained on the basis of the interaction of the PC ligand with SiOH groups of the surface. The significant spectral differences between +0.2 V and --0.2 V in Fig. 4 might also be due to the formation of a partial O adsorbed layer on the Ag surface at the more positive potentials. The voltammetry curve, however, gives little evidence of such. In the UV-visible reflectance studies spectra changes were observed at a potential of +0.45 V SCE when 02 was introduced [12], while in the present study no change was observed in the Raman spectrum with introduction of 02 at --0.20 V. One possible explanation for this apparent effect is t h a t at the quite negative potential used in the present study, the O2 was reduced to water under limiting current conditions and the concentration of 02 in solution at the electrode surface as well as the surface concentration of any 02 adduct were reduced effectively to zero. Alternatively, it is possible that the residual 02 in solution, even after prolonged N2 saturation, was sufficient to saturate the surface completely with respect to the formation of the 02 adduct. The large changes in the Raman spectrum of the adsorbed CoTSPc at potentials in the hydrogen evolution region indicate a major change in the adsorbed species. These potentials are sufficiently negative that the cobalt may be pulled out of the adsorbed complex. The free acid TSPcH2 then might remain adsorbed on the surface. The Raman spectrum of PcH2, however, is similar to that of the corresponding transition metal complex [18]. Therefore, an additional modification of the macrocyclic seems necessary, perhaps a breaking up of the ring structure. Similar changes have been observed for CuTSPc adsorbed on Ag under the same conditions. This suggests that the redox potential of the central metal atom is n o t critical. The present investigation shows that in-situ Raman spectroscopy can provide fundamental information about the adsorption of transition metal phthalocyanines on a metal electrode. However, a more conclusive interpretation and the extension o f the results obtained on Ag to other electrode materials await detailed vibrational assignments and a deeper understanding of the surface enhancement. The extension of the present studies to include UV laser excitation should yield further information concerning the interaction of the Co with various species in the polar positions. Such work is planned. ACKNOWLEDGEMENTS

This research has been supported by the U.S. Department of Energy. The authors are also pleased to acknowledge helpful discussion with Drs. B. Nikolic and R. Adzic of Belgrade and S. Simic-Glavaski of Case Western Reserve University. One of us (R.K.) is grateful to the Deutsche Forschungsgemeinschaft for supplementary financial support for his stay in Cleveland. REFERENCES 1 R. J a s i n k i , J. E l e c t r o c h e m . S o c . , 1 1 2 ( 1 9 6 5 ) 5 2 6 . 2 M. S a v y , C. B e r n a r d a n d G. M a g n e t , E l e c t r o c h i m . Acta.~ 2 0 ( 1 9 7 5 ) 3 8 3 . 3 A . J . A p p l e b y a n d M. S a v y , E l e c t r o c h i m . A c t a , 21 ( 1 9 7 6 ) 5 6 7 .

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4 5 6 7

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

M. B e h r e t , W. C a u b e r g a n d G. S a n d s t e d e , Bet. B u n s e n g e s . P h y s . C h e m . , 81 ( 1 9 7 7 ) 54. H. Meier, U. T s c h i r w i t z , E. Z i m m e r h a c k l , W. A l b r e c h t a n d G. Zeitler, J. P h y s . C h e m . , 81 ( 1 9 7 7 ) 7 1 2 . J. Z a g a l , R . K . S e n a n d E. Y e a g e r , J. E l e c t r o a n a l . C h e m . , 8 3 ( 1 9 7 7 ) 2 0 7 . E. Y e a g e r , J. Zagal, B.Z. N i k o l i c , a n d R . R . A d z i c in E. Y e a g e r , J . D . E . M c I n t y r e , B. Miller a n d S. B r u c k e n s t e i n (Eds.), P r o c e e d i n g s o f t h e T h i r d S y m p o s i u m in E l e c t r o d e P r o c e s s e s , T h e E l e c t r o c h e m i cal S o c i e t y , P r i n c e t o n , N . J . , 1 9 8 0 . H. T a c h i k a w a a n d L . R . F a u l k n e r , J. A m . C h e m . S o c . , 1 0 1 ( 1 9 7 9 ) 4 7 7 9 . F. F o n a n d L . R . F a u l k n e r , J. A m . C h e m . S o c . , 1 0 1 ( 1 9 7 9 ) 4 7 7 9 . S. M e s h i t s u k a a n d K. T a m a r u , J. C h e m . S o c . , F a r a d a y T r a n s . I, 7 3 ( 1 9 7 7 ) 2 3 6 a n d 7 6 0 . N. M i n a m i , T. W a t a n a b e , A. F u j i s h i m a a n d K . H o n d a , Ber. B u n s e n g e s . P h y s . C h e m . , 8 3 ( 1 9 7 9 ) 4 7 6 . B.Z. N i k o l i c , R . R . A d z i c a n d E. Y e a g e r , J. E l e c t r o a n a l . C h e m . , 1 0 3 ( 1 9 7 9 ) 2 8 1 . J.C. B u c h h o l z a n d G . A . S o m o r j a i , J. C h e m . P h y s . , 6 6 ( 1 9 7 7 ) 5 7 3 . R.P. V a n D u y n e , in C.B. M o o r e ( E d . ) , C h e m i c a l a n d B i o c h e m i c a l A p p l i c a t i o n s o f Lasers, Vol. 4, A c a d e m i c Press, N e w Y o r k , 1 9 7 8 , Ch. 4 a n d r e f e r e n c e s t h e r e i n . J. W e b e r a n d D. B u s c h , I n o r g . C h e m . , 4 ( 1 9 6 5 ) 4 6 9 . B. P e t t i n g e r , U. W e n n i n g a n d D.M. K o l b , B e t B u n s e n g e s . P h y s . C h e m . , 8 2 ( 1 9 7 8 ) 1 3 2 6 . R. K S t z , H . J . L e w e r e n z a n d E. K r e t z s c h m a n n , P h y s . L e t t . , A 7 0 ( 1 9 7 9 ) 4 5 2 . I.V. A l e k s a n d r o v , Y.S. B o b o v i c h , V . G . M a s l o v a n d A . N . S i d o r o v , O p t . S p e c t r o s c . 3 7 ( 1 9 7 4 ) 2 6 5 . H. I s h i d a , J . L . K o e n i g , B. A s u m o t o a n d M.E. K e n n e y , P o l y m . Sci. E n g . , in press. P. T h o a i , J . P . L e i c k n a m , M. M o m e n t e a u a n d B. L o o c k , S p e c . L e t t . , 1 2 ( 1 9 7 9 ) 3 3 3 . A . L . V e r m a n , R. M e n d e l s o h n a n d H . J . B e r n s t e i n , J. C h e m . P h y s . , 61 ( 1 9 7 4 ) 3 8 3 . T . G . S p i r o a n d R . M . L o e h r i n R . J . H . C l a r k a n d R . F . H e s t e r , (eds.), A d v a n c e s in I R a n d R a m a n S p e c t r o s c o p y . H e y d e n , N e w Y o r k , 1 9 7 7 , V o l . 1, C h . 3, p. 9 8 . A.N. Sidorov, Opt. Spectrosc., 40 (1976) 280. L . C . G r u e n a n d B . J . B l a g r o v e , A u s t . J. C h e m . , 2 6 ( 1 9 7 3 ) 3 1 9 . T.C. S t r e k a s a n d T . G . S p i r o , B i o c h i m . B i o p h y s . A c t a , 2 6 3 ( 1 9 7 2 ) 8 3 0 . A . N . S i d o r o v a n d I.P. K o t l y a r , O p t . S p e c t r o s c . , 1 1 ( 1 9 6 1 ) 9 2 . J. V e p r e k - S i s k a , E. S c h w e r t n e r o v a a n d D.M. W a g n e r o v a , C h e m i c a , 2 6 ( 1 9 7 2 ) 7 5 . D . M . W a g n e r o v a , E. S c h w e r t n e r o v a a n d J . V e p r e k - S i s k a , C o l l e c t . C z e c h . C h e m . C o m m u n . , 3 9 ( 1 9 7 4 ) 1980. K. F e n k a r t a n d C . H . B r u b a k e r , J. I n o r g . N u c l . C h e m . , 3 0 ( 1 9 6 8 ) 3 2 4 5 .