J. Phys. Chem. 1985,89, 763-768 We have made a redetermination of E0,,(C102/C10~) at 25
"C. We measured the potential of a platinum electrode against the SCE electrode at the temperatures 1 1.O, 22.0, 23.0, and 29.0 "C in unbuffered C102/C102- solutions containing 2 X mol < [C102-] < mol dm-3. E" a. dm-3 C102 and 4 X (C102/C102-) was then calculated from the Nernst equation in which the activity coefficient of C102 was set equal unity and that of C10, was calculated from the Debye-Huckel law. Figure 4 shows a plot of E0,,(C102/C102-) calculated from the present measurements against the temperature together with those determined p r e v i ~ u s l y .By ~ ~ interpolation we find at 25 "C E" ,, (C1O2/C1O2-) = 0.934 f 0.002 V. Using this value and a value for AfCoao(03)= 174.9 kJ mol-' calculated from the NBS value of AfG0g(03)4and a recently determined value of Henry's law constant for ozone,25we find AfC",,(OH) = 26.8 f 1.0 kJ mol-' and E",,(OH/OH-) at 25 OC = 1.91 V in agreement with the (24) Zolutukhin, V. M.; Flis, I. E.; Mishenko, K. P. Zh. Prikl. Khim. (Leningrad) 1965, 38, 359; Chem. Abstr. 1965, 62, 12494a. (25) Roth, J. A,; Sullivan, D. E. Ind. Eng. Chem. Fundam. 1981,20, 137.
763
values determined by Schwarz and Dodson' (AfC",,(OH) = 25.1 kJ mol-' and E",,(OH/OH-) = 1.89 V). The standard Gibbs energies of formation, A,Goaorfor OH, 0-,and 03-are 26.8,94.7, and 77.1 kJ mol-', and the standard oxidation potentials, E O , , (OH), of the couples OH/OH-, OH/H20, O-/OH-, and 03/03are 1.91, 2.74, 1.78, and 1.01 V.
Acknowledgment. The authors are indebted to H. Schwarz, BNL, for, after the submission of this paper, bringing to their attention a discrepancy between the NBS's values of standard Gibbs energy of formation and standard oxidation potential of C102/C102-. We are grateful to E. Bjergbakke for performing the computations. We thank R. Brodersen and N. Jmgensen, Institute of Biochemistry, Aarhus University for making the stopped-flow apparatus available to us. We thank H. Corfitzen for technical assistance and T. Johansen for skillful operation of the Rispr Linac. U.K.K. thanks the Danish Natural Science Research Council for financial support. Registry NO. C102, 10049-04-4; 03,10028-15-6; 02,7782-44-7; OH, 14280-30-9; 03-, 12596-80-4; C102-, 14998-27-7.
Surface-Enhanced Raman Spectroscopy as a Monitor of Iron( I I I ) Protoporphyrin Reduction at a Silver Electrode in Aqueous and Acetonitrile Solutions: Vibronic Resonance Enhancement Amplified by Surface Enhancement Luis A. Sanchez and Thomas G. Spiro* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: July 9, 1984)
Raman spectra are reported for anodized silver electrodes held at various potentials in contact with hemin (iron(II1) protoporphyrin IX) chloride dissolved in aqueous base (pH 10.5) or with the dimethyl ester dissolved in acetonitrile. Good quality spectra were obtained with 406.7-nm excitation, showing features characteristic of Raman spectra which are resonant with the porphyrin B band. At -0.3 V vs. SCE, the frequencies of the Raman bands correspond to those of a 5-coordinate high-spin Fe"' heme; there are slight frequency shifts attributable to the surface interaction, which is suggested to involve the peripheral vinyl groups for the dimethyl ester, and the proprionate groups for aqueous hemin. As the electrode potential is made more negative, the Fe"' spectrum is replaced by another, which is characteristic of a high-spin Fe" heme. The midpoint of this transition for aqueous hemin, - 4 . 6 5 V (vs. SCE) corresponds to the average of the anodic and cathodic peak potentials (-0.72 and -0.58 V) of the Ag electrode cyclic voltammogram; the cathodic peak is 0.40 V more negative than at at'F electrode, suggesting strong adsorption of the hemin at the silver surface. The Raman spectral transition is quite gradual; the fraction of reduced hemin varies approximately linearly with the potential instead of in a Nernstian fashion. This behavior may reflect differential rates of adsorption and desorption of the oxidized and reduced species. Raman spectra at increasing wavelengths (488.0, 514.5, 647.1, 676.4 nm) show a gradual transition to a set of bands which arise from vibronically active nontotally symmetricvibrations. The enhancementpattern is quite similar to that observed in resonance Raman spectra of metalloporphyrins in solution, implying that the molecular electronic states are not significantly altered by the interaction with the surface. The data are therefore consistent with straightforward electromagnetic enhancement of the molecular resonances by the Ag surface.
Introduction The discovery that the Raman scattering from molecular vibrations can be enhanced by several orders of magnitude when the molecules are adsorbed on roughened silver electrodes' or silver colloids2 has led to intense interest in the applicability of surface-enhanced Raman (SER) spectroscopy to the characterization of molecules at surface^.^ When the laser wavelength lies near (1) (a) Jeanmaire, D. L.; van Duyne, R. P. J . Elecrroanal. Chem. 1977, 84, 1-20. (b) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. SOC.1977, 99, 5215-8. (2) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. SOC., Faraday Trans. 2 1979, 75, 790-8. (3) (a) Van Duyne, R. P. In 'Chemical and Biochemical Application of Lasers";Moore, C. B., Ed.;Academic Press: New York, 1979; Vol. 4, Chapter 5. (b) Chang, R. K.; Furtak, T. E., Eds. "Surface Enhanced Raman Scattering"; Plenum Press: New York, 1982. (c) Birke, R. L.; Lombardi, J. R.; Sanchez, L. A. In "Electrochemical Studies of Biological Redox Components";Kadish, K. M., Ed.;American Chemical Society, Washington, DC, 1982; Adv. Chem. Ser. No. 4.
electronic absorption bands of the molecule, then the metal surface can further amplify the resonance-enhanced Raman spectrum;'q4g5 an additional advantage is that fluorescence can be effectively quenched by the metal surfaces4 It was recently shown, for example, that fluorescence-free resonance Raman spectra can be obtained at exceedingly low (submicromolar) concentrations for flavoproteins adsorbed on silver colloids;6 ordinarily, flavin R R spectra are quite difficult to obtain because of the intense natural fluorescence of the chromophore. In the case of glucose oxidase, it was shown that the activity of the enzyme remained nearly intact, while it was adsorbed on the colloid, and was fully recovered upon desorption from the colloid following laser irradiatione6 This result is reassuring with respect to the possibility of applying SER ~~
~
~
~~~
~~
(4) Lippitsch, M. E. Chem. Phys. Lett. 1981, 79, 2. (5) Weitz, D. A.; Garoff, S.; Gertsten, J. I.; Nitzan, A. J . Chem. Phys. 1983, 78, 5324-38. ( 6 ) Copeland, R. A.; Fodor, S. P. A,; Spiro, T. G. J . Am. Chem. Soc., in press.
0022-3654/85/2089-0763$01.50/00 1985 American Chemical Society
764
Sanchez and Spiro
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 Counter e l e c t r o d e
H3Cr__lCH H3 COOCCHpCH,
CH-CHp
Working electrode
H,COOCCYCH,
Figure 2. Structural diagram for a metalloprotoporphyrin dimethyl ester.
Reference e I ec t r o d e
Figure 1. Diagram of the apparatus for obtaining SER spectra.
spectroscopy to complex molecular structures without disrupting them by the adsorption and laser excitation. Work in this laboratory on metalloporphyrin electrode films,' and their electrochemical properties,8 has drawn our attention to the promise which S E R spectroscopy holds for studying interactions of metalloporphyrins with surfaces. While showing good electroactivity, these films were found to be readily inactivated by electrochemical cycling in contact with water, even though the bulk of the porphyrin chromophores remained largely intact, as judged by their absorption spectra.8 Thus we would like to examine the nature of the molecules directly in contact with the electrode. These are exactly the molecules that are expected to dominate the S E R signal, and the extensive literature on metalloporphyrin resonance Raman spectra9 can be brought to bear in making structural inferences from the S E R spectra. Recent studies have demonstrated that S E R spectra of porphyrins adsorbed on to silver electrodes are readily obtained.1° In this work we examine SER spectra for iron-protoporphyrin (hemin) complexes adsorbed on silver electrodes, since the resonance Raman spectra of this complex and its adducts have been analyzed in some detail." We have been able to obtain good quality SER spectra for anodized silver electrodes in contact with hemin chloride dissolved in aqueous base (where it is present as the p-oxo dimer) and also for the dimethyl ester in acetonitrile. These spectra resemble those of the solution species, with slight frequency shifts. At sufficiently negative potentials, the spectra change from those characteristic of Fe"' to those characteristic of Fe". Thus S E R spectroscopy can be used to monitor the oxidation state of the hemin adsorbed on the electrode. The redox ratio at the surface is found to be decidedly non-Nernstian, as might be expected from the importance of adsorption processes. Much of the activity in S E R spectroscopy has been directed toward elucidating the fundamental mechanisms of surface enh a n ~ e m e n t . ~This . ~ has proved to be a difficult and contentious matter, because of the multiplicity of effects that can be envisaged and the difficulty of distinguishing among them on the basis of the available experimental evidence. An advantage of porphyrins in this regard is that the resonance enhancement pattern is quite specific with respect to the nature of the porphyrin excited states, so that perturbations brought about by interactions with the surface should be easily detectable. An important qualitative feature is that excitation of porphyrins near their intense B bands (-400 nm) enhances totally symmetric vibrations of the ring via Franck-Condon (A term) scattering, while excitation near the much weaker Q bands (-530 nm) enhances nontotally symmetric (7) Macor, K. A,; Spiro, T. G. J . Am. Cbem. SOC.1983, 105, 5601. (8) Macor, K. A.; Spiro, T. G. J . Electroanal. Cbem. 1984, 163, 223. (9),Spiro, T. G. In "Iron Porphyrins, Part 11; Physical Bioinorganic Chemistry Series"; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1982; pp 89-1 59. (10) (a) Cotton, T. M.; Schultz, S . G.; Van Duyne, R. P. J . Am. Chem. SOC.1982, 104, 6528. (b) Itabashi, M.; Kato, K.; Itoh, K. Chem. Pbys. Lett. 1983, 97,528. (11) Choi, S.; Spiro, T. G.; Langrey, K. C.; Smith, K. M.; Budd, D. L.; LaMar, G. N. J . Am. Cbem. SOC.1982, 104, 4345.
modes which are effective in Q-B mixing, via vibronic (B term) In this study we find that excitation near the B band of the solution absorption spectrum likewise enhances totally symmetric modes in the SER spectrum, whereas excitation on the red side of the Q band enhances nontotally symmetric vibrations. This observation implies that the character of the Raman scattering is established by the molecules themselves, with the surface acting as an amplifier, although careful study of the excitation profiles may reveal some shifts in the electronic energy levels. Experimental Section Figure 1 is a diagram of the electrochemical cell used to obtain Raman spectra from the surface of a silver rod (99.9% purity) electrode in contact with an electrolyte solution containing the species of interest. The light scattered from the laser beam incident on the electrode surface was collected at an angle of 135'. The electrode was held close to the front window (- 1 mm) to minimize the absorption losses and background luminescence from the solution. The electrode was pretreated by polishing with emery paper, washing with a 50/50 mixture of hydrogen peroxide in ammonia, and polishing to brightness with alumina powder, followed by a distilled water rinse. The electrode was placed in the cell containing the electrolyte and substrate, which was purged with dry N2. The electrode was a n o d i ~ e d l -by ~ stepping the potential from -0.3 to +0.25 V (vs. SCE), holding it at that potential for 5 s, and stepping it to the potential at which the Raman spectrum was recorded. The potential was controlled with a PAR 173 potentiostat/l75 universal programmer. Reference spectra for species in solution were obtained via backscattering from a spinning N M R sample tube. Absorption spectra were recorded with a Hewlitt Packard spectrophotometer. Various lines of the Kr+ and Ar+ lasers (Spectra Physics 170, 171) were used for excitation. The Raman photons were collected and focussed into a computer-controlled Spex 1401 double monochromator, equipped with a cooled photomultiplier (RCA) and photon-counting electronics. Hemin chloride (Sigma) was dissolved in dilute aqueous NaOH (pH 10.5) containing 0.1 M KC1 as supporting electrolyte. The dimethyl ester of hemin chloride was prepared according to ref 13. It was dissolved in analytical reagent grade acetonitrile (Mallinckrodt) containing 0.1 M tetrabutylammonium perchlorate (Baker). Results Figure 2 is a structural diagram for a metal complex of protoporphyrin IX dimethyl ester (PPDME). Figure 3 shows visible absorption spectra for hemin chloride (ClFePP) in dilute aqueous base and for its dimethyl ester in acetonitrile. Arrows mark the position of the laser lines used for Raman excitation. The chloro complex remains intact in acetonitrile, but dissolution of hemin chloride in aqueous base is accompanied by formation of the p-oxo dimer." Both complexes contain high-spin five-coordinate Fe"' and have similar absorption spectra (and Raman spectra, see below). The absorption bands of the p-oxo dimer are somewhat broadened, presumably due to interaction between the porphyrin transition dipoles, and the B band (-400 nm) is split. (12) Spiro, T. G.; Stein, P. Annu. Rev. Pbys. Cbem. 1977, 28, 501. (13) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Cbem. 1970, 32, 2443.
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 765
Protoporphyrin Reduction a t a Ag Electrode
100
75
25
Figure 5. Fraction of the hemin adsorbed from aqueous base remaining as Fe"' vs. the electrode potential, as estimated from the intensities of the v4 SER bands at 1370 cm-l (Fe"') and 1360 cm-l (Fe"). The dotted curve is the expected equilibrium (Nernstian) behavior for a redox couple with a formal potential of -0.65 V vs. SCE.
-0.73
400
700
600
500
Wavelength (nm)
0.0
Figure 3. Absorption spectra for hemin chloride in aqueous base (pH 10.5) (B) and of the dimethyl esther in acetonitrile (A). -0.57 0
n
SERS e
A,=
W
406.7 nm Figure 6. Cyclic voltammogram for a roughened silver electrode in aqueous (pH 10.5) hemin chloride (0.5 mM) containing 0.1 M KCI as supporting electrolyte. Scan rate, 100 mV/s.
-1.OV n
I
I050
I
1
I250
I
I45C
I
I
I650
A cm-' Figure 4. 406.7-nm-excited Raman spectra of hemin chloride (1 5 mM)
in aqueous base (bottom spectrum, a) and of a roughened silver electrode surface in contact with hemin chloride (0.15 mM) in aqueous base and held at the indicated potentials vs. SCE (b-e). Experimental conditions: laser power, -30 mW, spectral slit width, 5 cm-I; accumulation time, 1 s; scan rate, 1 cm-'/s.
Figure 4 shows the 406.7-nm-excited R R spectrum of hemin chloride in dilute aqueous base (bottom) and compares it with
spectra recorded from the surface of the electrode held at various potentials. These are certainly SER spectra since (a) the solution in contact with the electrode was 100-fold more dilute than the solution from which the RR spectrum was recorded (15 mM) and (b) slight frequency shifts are observed, even at -0.3 V, where the predominant surface species is the same as that in solution. At more negative potentials the SER spectrum of a reduced species is observed. Figure 5 is a plot of the fractional reduction of the surface species, as estimated from the Raman band intensities, as a function of the electrode potential. An essentially linear relationship is observed, which deviates markedly from the Nernstian behavior (dotted line) that would be expected for a redox couple at equilibrium with the electrode. Figure 6 gives a cyclic voltammogram of the silver electrode in contact with the aqueous hemin solution. In Figure 7 SER spectra are shown for the silver electrode in contact with hemin chloride dimethyl esther dissolved in acetonitrile. At -0.9 V the spectrum of a reduced species is again observed. Figure 8 compares SER spectra of the electrode in contact with aqueous hemin, recorded at several excitation frequencies ranging from the violet to the deep red region of the spectrum. Marked changes in the enhancement pattern are observed. The Raman bands are identified in the figure via assignments determined previously for iron protoporphyrin comp1exes.l
Discussion Adsorption ofZron(ZZZ) Protoporphyrin. Table I lists the R R frequencies and assignments of (Fe"'PPDME)Cl as determined by Choi et al." Similar frequencies are observed for all fivecoordinate high-spin iron(II1) protoporphyrin complexes. The 406.7-nm SER spectrum (Ag electrode, CH3CN solution, Figure 6) shows the enhancement pattern expected for B-band excitation. In the spectral region recorded, all of the AI, modes appear (v2.
Sanchez and Spiro
766 The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 CI(FemPPDME)
(Fe' PP)O on A g 2
ID
c! 676.4 nm
/n/
"4"'
1125 Y22
514.5
I200
1400
L
1600
Raman s h i f t Figure 7. 406.7-nm-excited Raman spectra of the surface of a roughened silver electrode, at the indicated applied potentials, in contact with
(Fe"*PPDME)CIin acetonitrile, with 0.1 M tetrabutylammonium perchlorate supporting electrolyte. Experimental conditions as in Figure 4. TABLE I: Mode Assignments and Frequencies (em-') for Iron(II1) Protoporphyrin Complexes in Solution and Adsorbed on Ag
YC< V l O Bl,
u37 Eu u19 A2g
Al,
y2
BI, Eu
VI1
U38 y3
AI,
y20 B2,
&(=CHI)
(1)
6,(=CHz)
(2)
Y2I A28
b(CH2) y 5 + y9 A,, y13
BI,
y30 B2g u6
+ u0
y22 A2g
CIFe"' RR" SER
aqueous hemin RR SERb
1626 1626 1591 1571 1570 1553 1533 1491 1453 1435 1403 1403 1373 1340 1309 1308 1260 1228 1167 1130 1125
1624 1619 1590
1591
1570
1569
1535 1493
1532 1491
1373
1624 1616 1585 1561 1570 1550 1530 1490
1429
1434 1400
1375 1339
1370
1307 1225 1170 1127
1306 1310 1225 1247 1171 1129 1125
"From ref 11; (Fe"'PPDME)CI is protoporphyrin IX dimethyl ester. bCombined list of bands from all excitation wavelengths (Figure 8). v3, v4), as do the E,-type modes ( v ~ v~ ~, ~which ) , are activated by the asymmetric disposition of the vinyl substituents," as well as the vinyl C=C stretch (vCec). The v3 and v4 modes stand out particularly clearly and are a t the same frequencies, 1493 and 1375 cm-I, as observed in the solution RR spectrum. The 1500-1600-~m-~region is poorly resolved, but bands can be seen at positions corresponding to v3,, v2, and v38. A point of considerable interest is that vcd, which is at 1626 cm-I in the solution RR spectrum, shifts down to 1619 cm-' and appears broadened in the SER spectrum. This suggests interaction of the vinyl groups with the Ag surface; such interaction would not be surprising in view of the well-known propensity of Ag+ to bind to double bonds.I4 It cannot be excluded, however, that adsorption of (Fe"'PPDME)Cl may also involve bridging of the C1- ligand to the surface and/or weak interactions with the propionate ester oxygen atoms. (14) Muhs, M. A.; Weiss,
F. T.J . Am. Chem. SOC.1962, 84, 4691.
+4
4T7 I125
I
,
I
I
I
1250
1400
I
1
I
1550
A
CM-I Figure 8. Raman spectra at the indicated excitation wavelengths from
the surface of a roughened silver electrode in contact with aqueous (pH 10.5) hemin chloride 0.15 mM), containing 0.1 M KCI. The electrode was held at -0.3 V vs. SCE. The bands are assigned to various modes of the porphyrin ring, as discussed in the text. Experimental conditions as in Figure 4. The 406.7-nm spectra of (Fe111PP)20(Figure 4) likewise show the enhancement pattern expected for B-band excitation, and the frequencies are close to those of (FeII'PPDME)Cl (Table I). The spectral shifts on adsorption, however, are different from those observed for (Fe"'PPDME)CI. The 1626-cm-l vC4 band is unaffected in the SER spectrum, but the skeletal modes v4 (1375 cm-I) and vj7 (1591 cm-I) shift down by 5 cm-'. A plausible explanation is that (Fe111PP)20has an alternative interaction mode, namely, via the proprionate carboxylate groups; their interaction with the silver surface might be especially effective since they are attached to neighboring positions on the porphyrin ring (see Figure 2) and could readily bind to the surface simultaneously. Carboxylates are known to be attracted to silver surfaces; poly(viny1 acetate), for example, is an effective stabilizer of silver colloids.'* Binding to Ag through peripheral carboxylate groups has also been suggested for tetrakis-p(carboxylatopheny1)porphinefrom SER evidence.Ioa Carboxylate interactions are precluded for the dimethyl ester complex, however, leaving the vinyl groups as the site for adsorption for (Fe"IPPDME)Cl. The origin of the (Fe111PP)20frequency shifts for v4 and v37 is uncertain. v4 is a breathing mode of the C-N pyrrole bonds16 and is a marker frequency for the oxidation state of the central ~ metal ion, as well as for alterations in the ring K b ~ n d i n g .Its 5-cm-' downshift relative to the solution species suggests partial transfer of charge from the electrode to the heme complex. v3, is a marker for the porphyrin core size," but the other skeletal modes above 1450 cm-* are also dependent on core size, and they are unshifted in the adsorbed species. The reason for a specific effect on u37 is unclear. (15) Lee, P. C.; Meisel, D. Chem. Phys. Lett. 1983, 99, 262-5. (16) Abe, M.; Kitagawa, T.; Kyogoku, Y . J. Chem. Phys. 1978,69,4526.
Protoporphyrin Reduction at a Ag Electrode
Reduction to Fe". When the potential of the silver electrode in contact with the aqueous hemin solution is stepped to increasingly negative potentials (Figure 4) the SER spectrum of the Fe"' complex is gradually replaced by one characteristic of a high-spin Fe" complex." In particular, the Fe"' v4 band at 1370 cm-' is replaced by one at 1360 cm-', while the vj band at 1490 cm-' is replaced by one at 1473 cm-'. These frequencies are close to those observed" for the 2-methylimidazole (high-spin fivecoordinate) complex of iron(I1) protoporphyrin, at 1357 and 1471 cm-I. The actual coordination number of the Fe" species is uncertain, since the vibrational pattern of high-spin six-coordinate Fe" species has yet to be fully characterized." For a low-spin Fe" complex, however, vj would be at a substantially higher frequency ( 1495 cm-I) because of the smaller core size." In the (FenlPPDME)C1 SER spectra (Figure 7) bands corresponding to v4 and v3 likewise grow in at lower frequencies when the potential is switched to -0.9 V. The new frequencies are a few cm-I higher than for the protoporphyrin complex adsorbed from water, plausibly due to the different mode of adsorption. As far as we are aware, this is the first report of SER spectra from both partners of a redox couple coadsorbed on an electrode, although SER bands associated with tetrathiofulvalene molecules oxidized to different extents when adsorbed on Ag or Au particles have recently been reported.'* A feature of particular interest is the wide potential span over which the reduced spectrum gradually replaces the oxidized spectrum (Figure 4). In Figure 5 we plot the fraction of the heme adsorbed from the aqueous solution which remains oxidized at the silver surface. This fraction was estimated from the relative intensities of the v4 bands at 1370 and 1360 cm-l for the oxidized and reduced species, respectively. The shallow and apparently linear dependence of the redox fraction on the applied potential is far from what would be expected if the Nernst equation governed the concentrations of the redox partners. In that case the concentration ratio would change by a factor of 10 for each 0.06 V of potential difference, as indicated by the dotted curve in Figure 5; this is the expected concentration profile for a Nernstian couple with a formal potential at the midpoint potential of the experimental data, -0.65 V vs. SCE. The wide deviation of the experimental points from the Nernstian curve implies that the redox partners are not in equilibrium at the electrode surface. This lack of equilibration is no doubt associated with adsorption and desorption processes which are slow on the electrochemical time scale. The importance of such processes is confirmed by the cyclic voltammogram of the silver electrode in contact with the aqueous hemin solution, shown in Figure 6. Although well-developed cathodic (-0.73 V) and anodic (-0.57 V) waves are seen, they are broad, and the peak separation is over twice that expected for a reversible redox couple (0.06 V). Moreover, the cathodic peak potential is substantially more negative than that observed (-0.30 V) for a platinum electrode with the same solution. (The Pt wave was also broad and no anodic peak was observed.) Thus the silver electrode displays substantial irreversibility and a large shift to negative potentials, implying strong adsorption of the oxidized form. (This might be due to anchoring by all four propionate groups of the pox0 dimer, which probably dissociates upon reduction.) The midpoint potential, -0.65 V, is nevertheless the same for the cyclic voltammogram as for the Raman titration (Figure 6), so that the same processes are evidently being sampled by the S E R signal as by the electrochemistry. Wavelength Dependence and Enhancement Mechanism. Striking changes are seen in the SER spectrum when the excitation wavelength is tuned from the violet through the red region, as shown in Figure 8. At 406.7 nm, the SERS spectrum is very similar to the R R spectrum of the complex in solution, as discussed above. This was found to be the case at 514.5 nm as well. At longer wavelengths, well-defined R R spectra proved difficult to
-
(17) Terner, J.; Stong, J. D.; Nagumo, M.; Nicol, M. S.; El-Sayed, M. A,; Spiro, T. G . Proc. Natl. Acad. Sci. U.S.A. 1981,78, 1313. (18) Sandroff, C. J.; Weitz, D. A.; Chung, J. C.; Herschbach, D. R. J . Phys. Chem. 1983.87,2127.
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 767 obtain, due to background luminescence. The observed spectral pattern can be interpreted, however, in relation to the well-established R R enhancement mechanisms which have been observed for various metalloporphyrin complexes? The electronic spectra of metalloporphyrins are dominated by a very strong B (or Soret) band near 400 nm and a pair of weaker bands at longer wavelengths, -500 and 530 nm for protoporphyrins, called Ql and Qo (or @ and a). In Gouterman's four-orbital model,Ig the B and Qo bands are explained as arising from in- and out-of-phase combinations of interacting transitions between the (nearly degenerate) filled orbitals, al, and a2,, and the (degenerate) empty orbitals, e,*. The Ql band is an envelope of vibronic transitions, associated with vibrational modes that are effective in mixing the Q and B excited states. R R spectra obtained by excitation in the strongly allowed B band are dominated by totally symmetric modes of the porphyrin ring which have appreciable Franck-Condon overlaps with the excited state and are enhanced via A term (Franck-Condon) scattering, which is the dominant resonance effect for allowed transitions. When the excitation is in resonance with the quasi-forbidden Q band, however, the Franck-Condon scattering (which depends on the square of the transition moment) is much weaker and the Raman spectrum is dominated by vibronic scattering (B term), which enhances the modes that are effective in Q-B mixing. These modes are non-totally symmetric. In the idealized D4*symmetry of the porphyrin skeleton, they are of symmetry B,,, B2,, and A,,, with depolarization ratios of three-fourths (BI, and Bo) or greater (A2,h9 In SER spectra, the bands are all depolarizedja (presumably due to multiple scattering effects at the roughened surface) and polarization measurements cannot be used to establish mode symmetries. Since, however, the R R spectra of heme complexes have been assigned in detail, the modes can be identified from their frequencies. As already noted, the SER spectrum with 406.7-nm excitation is very similar to that of the R R spectrum at the same wavelength. The bands which are enhanced are associated with totally symmetric modes, and also with E,-type (infrared) modes, whose Raman activity is induced by the asymmetric disposition of the peripheral vinyl groups, which are in conjugation with the porphyrin ?r system." In addition the C = C stretching mode of the vinyl groups is enhanced with B band excitation. All of these features can be discerned in the 406.7-nm SER spectrum. With 647.1- and 676.4-nm excitation, a different set of bands is observed, which can be identified with nontotally symmetric modes of the five-coordinate high-spin Fe"' heme, as seen by the correspondences in Table I with the assigned modes of (Fe"'PPDME)Cl. Thus strong bands are observed at 1125, 1306, and 1560 cm-l which are identifiable with the A2, modes vZ2,vZ1, and ~ 1 9 while , the B, mode vI1 is found at 1550 cm-'. A broad band at 1400 cm- B, where the A,, and B2, modes v20 and ~ 2 9 are known to be superimposed, likewise becomes prominent. Two of the bands deviate somewhat from the (Fe"'PPDME)Cl assignments. The highest frequency band, found at 1616 cm-I, must be the B,, mode vl0. It is at a distincly lower frequency than the 1626-cm-' band seen with violet excitation and assigned to the vinyl C=C stretch. In (Fe"'PPDME)Cl both modes were assignedI2 at 1626 cm-'. It is not clear whether this difference is associated with the adsorption of the complex on the electrode or reflects an improved resolution of the two modes in the variable excitation SER spectra. A prominent SER band appears at 1247 , the neighboring cm-' and is assigned to the B,, mode ~ 1 3 while band at 1226 cm-I, seen also at 406.7-nm excitation, is assigned to the A,, combination mode u5 + u9. In (Fe"'PPDME)CI these assignments were reversed;" again it is not clear whether this difference is associated with the adsorption or with the larger range of wavelengths used in the SER spectra. Another feature of interest in the spectra excited at long wavelengths is a broad band at 1340 cm-I. This is the frequency expected for one component
-
-
(19) Gouterman, M. In "The Porphyrins"; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111, Part A, pp 1-156.
J . Phys. Chem. 1985, 89, 768-770
768
of the vinyl CH2 scissors mode which is anomolously polarized via coupling to skeletal A2 modes.'2 In heme protein spectra this band has a normal width?621 The broadening in the SER spectra may reflect a heterogeneity among the vinyl groups at the surface. The observation of totally symmetric modes with blue excitation and nontotally symmetric modes with red excitation demonstrates that the SER spectra display the enhancement pattern expected for resonance Raman scattering of porphyrins. Thus the spectral pattern is consistent with a simple electromagnetic enhancement of the R R spectra. Moreover, the spectra are quite similar to t h a e observed at the same wavelengths in solution except for the small frequency shifts already noted. At 514.5 as well as 406.7 nm, the R R spectra of (FePP)20 in aqueous solution were found to resemble the S E R spectra shown in Figure 8 very closely. We were unable to obtain solution spectra at 647.1 or 676.4 nm due to high backgrounds. However, Asher and Schuster2' have reported enhancement of nontotally symmetric porphyrin skeletal modes in R R spectra of high-spin FeIII-containing complexes of methemoglobin obtained with excitation in the 600-nmregion. The absorption band near 600 nm which is seen for many high-spin Fe"' hemes (see Figure 2) has been assigned'* to a charge transfer excitation (azu d r ) , which gains intensity by mixing with the Q transition. It is therefore expected to produce resonance enhancements similar to those seen for the Q bands. The spectra actually observed with 488.0 and 514.5 nm (Figure 8) which are within the (FelllPP)zO Q bands show a mixed pattern, with some totally symmetric modes (vz and v4, but not v3) remaining prominent among the emerging nontotally symmetric modes.
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(20) Spiro, T. G.; Strekas, T. C. Proc. Natl. Acad. Sci. U.S.A. 1972, 69, 2622. (21) Asher, S. A.; Schuster, T. N. Biochemistry 1979, 18, 5377.
The similarity of the S E R and R R spectra over a range of excitation wavelengths spanning the B, Q, and charge-transfer transitions establishes that adsorption on the Ag surface does not markedly influence the excited states of (Fe111PP)20.Slight shifts in the energy levels cannot be precluded, and it will be of interest to measure detailed excitation profiles in order to gauge the magnitude of such shifts. Had there been substantial mixing of porphyrin and silver electronic states, however, then the SER and solution R R spectra should have shown noticeable deviations in the relative band intensities. Thus, the data are consistent with a straightforward electromagnetic explanation of the SER effect in this case. Weitz et aL5 have estimated a factor of lo3 for the electromagnetic enhancement via Ag particles of Raman bands that are already resonantly enhanced, and a similar estimate was obtained experimentally for flavoproteins adsorbed on Ag colloids.6 We have not determined enhancement factors in this study, but the observation of SER spectra at 647.1 and 676.4 nm, where no solution R R spectra were obtainable, suggests that the SER enhancement increases appreciably in the red region. This would be consistent with nonresonant Ag electrode excitation profiles22 which have shown increased enhancements in the red region.
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Acknowledgment. This work was supported by Grant DOEAC02-81ER 10861 from the U.S. Department of Energy. We thank Lisa Miller for her assistance. Registry No. (Fe"'PPDME)CI, 15741-03-4; hemin chloride, 1600913-5; silver, 7440-22-4. (22) (a) Creighton, J. A.; Albrecht, N. G.; Hester, R.E.; Matthew, J. A. D. Chem. Phys. Lett. 1978,55, 55. (b) Pettinger, B.; Wenning, U.; Kolb, D. M. Ber. Bunsenges. Phys. Chem. 1978,82, 1326. (c) Girlando, A,; Gordon, J. G.; Heitman, D.; Philpott, M. R.; Seki, H.; Swalan, J. D. Surf. Sci. 1980, 101, 417.
A Comparison of the Effects of Perfluoromethylcyclohexane on the Photoconductivity of Tetramethylsilane and on Photoionlzatlon Quantum Yields of N,N,N',N'-Tetramethyl-p-phenylenediamine Dissolved in the Same Solvent J. Casanovas,* J. P. Guelfucci, R. Laou Si0 Hoi, LA 277, Centre de Physique Atomique, UniversitC Paul Sabatier, 31 062 Toulouse Cedex. France
and R. Grob LA 277, Laboratoire de Chimie Analytique Ecole Nationale SupCrieure de Chimie, 31077 Toulouse Cedex, France (Received: August 1 , 1984)
The quenching of the ionization current of tetramethylsilane (Mesi) by perfluoromethylcyclohexane (C,F,,)has been studied, 5 c (M) 5 2.5 X lo-]), of the applied electric field at room temperature, as a function of the scavenger concentration ( strength (2 IE (kV cm-I) 5 15), and of the photon energy (vacuum UV photons: 8.05 Iku (eV) I 10.2; high-energy photons: 5;; = 1.25 MeV). The corresponding results are compared to that of Lee and Lipsky on the effect of C7F,4on the photoionization quantum yield of TMPD dissolved in Me4Si. The Io/Ic =f(c) curves ( I o and I, being respectively the ionization current of Me4Si or TMPD without scavenger and with a scavenger concentration c ) deduced from each of these experiments are very different. These differences are attributed to distinct mechanisms of action for C7FI4.
Introduction In the course of the past years Lipsky and co-workers have published a series of papers'-3 on N,N,N',N'-tetramethyl-pphenylenediamine (TMPD) fluorescence and photocurrent ( I ) Wu, K. C.; Lipsky, S. J . Chem. Phys. 1977, 66, 5614. (2) Lee, K.;Lipsky, S. Radiat. Phys. Chem. 1980, 15, 305. (3) Lee, K.; Lipsky, S. J . Phys. Chem. 1982, 86, 1985.
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quenching by some electron scavengers particularly perfluorocarbons. In order to explain their results Lipsky et al.'x2 postulated as a first step the existence Of a metastable excited state of TMPD which would be the precursor of the ion pair and which would interact with the added scavenger. However, recently, on the basis of more extended experiments Lee and Lipsky3 suggested that fluorocarbons quench the TMPD photocurrent by reacting with 0 1985 American Chemical Society
