J. Phys. Chem. 1995,99, 13217-13223
13217
Surface-Enhanced Raman Spectroscopic Study of the Adsorption and Reduction of [M(bipy)3I2+ Complexes (M = Co, Ni) on a Silver Electrode Paola Corio and Joel C. Rubim* Instituto de Quimica da Universidade de Siio Paulo, C. P.26077, 05599-970, Siio Paulo, S.P., Brazil Received: March 1, 1995; In Final Form: June 26, 1995@
The spectroelectrochemical behwior of the [M(bipy)3I2+complexes (M = Co, Ni and bipy = 2,2'-bipyridine) adsorbed on a silver electrode has been investigated by surface-enhanced Raman spectroscopy (SERS). The voltammetric measurements have shown that in the case of the [Co(bipy)3I2+ complex two redox processes can occur. The first one, reversible, occurs near 0.2 V (SCE) and has been assigned to the Co(II)/Co(III) redox process. The SERS results suggest that the transferred electron on going from Co(II1) to Co(I1) is localized on a molecular orbital with a main contribution of an atomic orbital of the central ion. The Ni(III)/Ni(II) redox process could not be monitored by SERS. The second redox process, less reversible, is observed for potentials more negative than -1.0 V, for both complexes. The dependence of the Raman frequencies on the applied potentials supports the conclusion that the electron transferred in this process is localized in the bipy ligand. Further evidence for the localization of the transferred electron on the ligand is given by reflectance measurements for both complexes. On the basis of the charge-transfer model for SERS and on the data given by the dependence of SERS excitation profiles on the laser excitation energy, an energy diagram for the adsorbed complexes is presented.
Introduction
The surface-enhanced Raman scattering effect of molecules adsorbed on silver electrodes has already proved to be a powerful technique in the characterization of chemical species formed during electrochemical reduction or oxidation processes. As examples, one can mention the oxidation or reduction of mononuclear complexes of the type M11L/M111L'-4 and binuclear complexes as [Fe2(CN)loLI6- 5-7 as well as the oxidation or reduction of molecular species as methylviologen,8 ethylene? CO,Io iodide,] I 4,4'-bipyridine,I2 trans-a~abenzene,'~ nitrobenzene,14 nitrate,I5 and cyanopyridines.I6 In some cases the careful analyses of the dependence of the SERS frequencies and intensities on the applied potential made possible to establish the structure of reduction product precursors.' The aim of this work is to investigate the electrochemical behavior of [M(bipy)3I2+complexes (M = Co, Ni) adsorbed at a silver electrode-solution interface by SERS. One of the motivations for this study lies in the fact that the literature data regarding the localization of the electrons transferred in the following reactions are controversial:
+
[ C ~ ( b i p y ) ~ ] ~e+ [Co(bipy),I2+
= [Co(bipy),I2+
+ e- = [Co(bipy),]+
(1) (2)
The literature data regarding reaction 2 point out in the direction that the transferred electron is populating a molecular orbital with more pronounced contribution of an atomic orbital of the central ion, Le., Co(I1) is reduced to Co(1) in reaction 2. In this sense one can mention the works of Prassad and ScaifeI7 and the review article of Creutz and Sutin.I8 It is worth mentioning that Szalda et al.I9 have also proposed the formation of [Co(bipy)# species in reaction 2 based on pulse radiolysis measurements.
* Author to whom correspondence @
should be sent. Abstract published in Advance ACS Abstracts, August 1, 1995.
0022-3654/95/2099- 13217$09.0010
On the other hand, Richert et aL20 based on voltammetric measurements of [M(bipy)312+complexes (M = Mn, Fe, Co), have concluded that the electron transferred in reaction 2 is localized on a molecular orbital of the ligand bipy. Since SERS can distinguish reduction products at a molecular level, it will be used to monitor the reduction of the tris bipy complexes of Co(I1) and Ni(I1). The results will be discussed in conjunction with the reflectance and electrochemical data obtained for both complexes. Experimental Section
The [M(bipy)312+complexes have been prepared according to literature indications2I in the form of perchlorate salts. The stoichiometry was checked by elemental analysis of H, C, and N content. The results have shown that the observed results presented a deviation of the expected values of less than 2%. The instrument used to obtain the SER spectra was described elsewhere,22and a PAR 273 potentiostatfgalvanostat was used in the electrochemical measurements. The working electrode was made of a silver (99.99% purity) rod of 0.20 cm2 of geometrical area. A Pt foil was used as the auxiliary electrode, and all potentials are quoted in reference to the saturated calomel electrode. Three different laser lines have been used to excite the SER spectra. The 647.1 nm line from a Kr+ laser (Spectra Physics 2020) operated at 100 mW and the 514.5 and 488.0 nm lines of an Ar+ laser (Spectra Physics 2025) operated at 60 and 40 mW, respectively. The electrolyte solution, 0.1 M K2SO4, was prepared using doubly distilled water and analytical grade K2S04. All the reactants used in the preparation of the complexes were also of analytical grade. Before each SERS measurement the silver electrode was activated in a deaerated electrolyte solution containing an amount of the complex to give a concentration of M. The activation consisted of applying several oxidation-reduction cycles (ORC) between -0.6 and 0.5 V at 100 mV/s.
0 1995 American Chemical Society
13218 J. Phys. Chem., Vol. 99, No. 35, 1995
Corio and Rubim This is evidenced by the decrease in the current peaks and peak potential shifts by successive cycling. It has been observed for both complexes that on applying potentials more negative than -1.3 V, a second oxidation wave appears. In Figure 1, these two oxidation waves are labeled I and 11. Note that a third broad oxidation wave, labeled 111, is observed only in the CV of the Ni complex. According to Richert et a1.,20 the CV of the [M(bipy)3I2+ complexes (M = Fe, Co, Mn) in nonaqueous media show similar oxidation waves. On the basis of their interpretation the results of Figure 1 can be explained as follows: for potentials more negative than - 1.1 V the [M(bipy)3I2+complex is reduced and gives the [M1*(bipy)2bipy'-]species (M = Co, Ni). When this species is oxidized it gives the oxidation wave (I). In the presence of water the following reaction would occur:2o
+
[C~(II)(bipy)~bipy*-]H20 == -1 2
-0 B
-0 4
00
04
A
+
z
w
a a 3 0
-1.0 -0.8 -0.6-0.4 -0.2 00 0.2
-1 2
-08
- 0.4
0:o
0.4
0.6
0.4
C.E
Figure 1. Cyclic voltammograms (CV) of a silver electrode in (a) 0.1 M KzS04 and M [Co(bipy)3I2+aqueous solution, u = 100 mV/s M [Ni(bipy)#+ aqueous solution at the and (b) 0.1 M KzS04 and indicated scan rates, . The insert shows the CV curve for a gold electrode in the same solution of Figure lb.
The cell and instrumentation used to obtain the reflectance spectra were described elsewhere.23 Results and Discussion
I. Electrochemical Measurements. The electrochemical behavior of the [M(bipy)3I2+ complexes in a 0.1 M K2S04 solutions is presented in Figure 1. The cycle voltammograms (CV) of Figure la, obtained on a silver electrode, shows that two different redox processes occur for the [Co(bipy)312+ complex. The first one, at positive potentials, has reversible characteristics and is the Co(III)/Co(II) redox process, Le., reaction 1 described in the Introduction. The second, occurring at negative potentials corresponds to the reaction 2 and is less reversible. The same behavior is observed for the [Ni(bipy)312+complex (Figure lb), with the exception that the Ni(III)/Ni(II) redox process is not observed since it occurs at potentials more positive than the threshold for silver dissolution. Measurements on gold electrodes (see the inset of Figure lb) show that the oxidation of Ni(I1) to Ni(II1) has an E112 at ca. 0.5 V. In the case of the Ni complex, the Ni(II)/Ni(III) redox process is less reversible.
[Co"(bipy),OH]
+ Hbipy (3)
If the potential is made more negative, [CoI1(bipy)20H] is further reduced in a two-electron process to give [ C ~ " b i p y ~ - ( O H ) ]Probably .~~ this is the species giving rise to the oxidation wave (11). The corresponding reduction wave is not observed because it occurs simultaneously to the hydrogen evolution reaction. Unfortunately we do not have an explanation for the broad oxidation wave (111) of Figure lb. It has been observed only when the potential was made negative enough to cause the reduction of the ligand in the Ni complex. It is not dependent on the kind of electrode material used since it has been observed for silver (roughened and not roughened), gold and glassy carbon electrodes. Probably, the reduction products formed at potentials more negative than -1.1 V have undergone some chemical reaction giving rise to some products that are oxidized for potentials less negative than -0.6 V. 11. SEW Measurements. Figure 2a displays the SER spectra (647.1 nm excitation) of the [Co(bipy)3I2+complex adsorbed on a silver electrode in the potential region where reaction 1 occurs. Note that from 0.2 V, where the complex is in the form of [Co(bipy)3l3+,to -0.2 V, where the complex is in the form of [Co(bipy)312+,no shift on the Raman frequencies of the adsorbed complex is observed. Just a progressive increase in the SERS intensities is observed as the potential becomes more negative. These results suggest that the ligands are not affected by the electron-transfer reaction, Le., the transferred charge is mainly localized on a molecular orbital with a main contribution of an atomic orbital of the Co(II1) central ion. Figure 2b displays the SER spectra covering the potential region where reaction 2 occurs. Note that for potentials more negative than -0.9 V, the SER spectra present marked changes with significant shifts on the Raman frequencies and changes on the relative intensities. Figure 2 c shows the SER spectra of the adsorbed [Ni(bipy)3I2+complex for the potential region where the following reaction occurs: [Ni(bipy),12+
+ e-
[Ni(bi~y)~l+
(4)
It is worth mentioning that for both complexes the SERS signal remained observable even after removing the electrode from the working solution, vigorous washing, and subsequent acquisition of an ex situ spectrum. For instance, by removing the electrode at the potential of -0.6 V a weak signal is observed. But if the electrode is removed at - 1.2 V an ex situ spectrum with a good signal-to-noise ratio is observed. No
Adsorption and Reduction of [M(bipy)3I2+Complexes
J. Phys. Chem., Vol. 99,No. 35, 1995 13219 anion25326and the Raman frequencies of the [Ru(bipy)#+ complex in ,the ground and excited state^^^,^^ are also presented. The excited state of the [Ru(bipy)3I2+ complex has been characterized by time resolved resonance Raman scattering as The numbers in the last row of Table [Ru"'(bipy)2bipy'-I2+ 1 refer to the potential energy distribution (PED) presented in Table 2. A careful analyses of the above results suggest that the electron transferred in reaction 2 or 4 is localized in the ligand, causing the pronounced changes on the relative intensities and shifts in the Raman frequencies. This means that reactions 2 and 4 could be better described by the following general reaction: 27928
- 0.2 v -0.1
8000
v
7000
0.0 v
BODO
"
5000
Y
5 0"
*O.l
4000
v
MOO
'0.2 v
2000
IS00
l%O
1400
1300
1200
1100
1000
+
[ M ( b i ~ y ) ~ ] * +e- ==[M"(bipy),bipy
R.nm Ohltt,'cm-l
( b> -I1
v
-IOV 4000 3500
- 0.9 v
3000
4
5
2500
-08V
2000
1500
-J lib0
1soo
lib0
moo
1 i O
rlbo
moo
Roman snift/cn-l
(c) -1.25 V
-1.15
v
-1.05 V
0'
-0.95 V
, 1600
1500
1400
1300
1ZW
1100
1000
Raman S h i f t l c m - '
Figure 2. SER spectra of a silver electrode, at the indicated potentials, in 0.1 M K2S04 and M [M(bipy)#+ aqueous solution: ,(a) and (b) [Co(bipy)#+ and (c) [Ni(bipy)#+. Laser excitation at 647.1 nm.
Raman signal was observed by removing the electrode at 0.2 V where the Co(II1) species is present at the electrode surface. These results represent strong evidences for a chemical interaction between the adsorbate, at least for M(I1) and their reduced forms, and the electrode surface. It is worth mentioning that even if the complex is added to the solution after the electrode roughening, the ex situ spectra of M(I1) complexes and their reduced forms are observed. Table 1 lists the vibrational frequencies observed in the SER spectra excited at 647.1 nm for the potentials of -0.2 and - 1.1 V, i.e., before and after the reduction of the [Co(bipy)3I2+ complex. For comparison purposes the IR and Raman vibrational frequencies of some reference compounds as the free ligand,24 the lithium and sodium salts of the bipy'- radical
.-3+ ]
(5)
According to reaction 5, the modes that are preferentially enhanced for potentials more negative than -0.9 V are due to the formation of a bipy'- radical anion in the complex. The effect of the bipy ligand reduction can be visualized through the diagram of Figure 3a. This is a schematic picture of the molecular orbital energies for bipy and its radical anion. See also Figure 3b for a display of the HOMO (highest occupied molecular orbital-@ and LUMO (lowest unoccupied molecular orbital-n7) of bipy calculated by the MNDO method.26 The addition of an electron to the LUMO n7, which has a bonding character for the C-C inter-ring bond, should cause an increase in its bond order. Therefore, the Raman frequency that has a contribution from the c 2 - C ~bond stretching in the radical anion should present a higher energy in comparison to that of the neutral molecule or for the molecule coordinated to the metal ion. According to the normal-coordinate analyses performed by Strommen et aLZ8 for the [Ru(bipy)3I2+complex in the ground state, the vibrational mode numbered 9 in Tables 1 and 2 has a significant contribution of the C-C inter-ring stretching. This mode is assigned to the Raman frequency observed at 1320 cm-I. For the radical anion and for the [Ru(bipy)#+ complex in the excited state it is observed at 1357 and 1365 cm-', respectively. In the complexes here studied this mode is observed at ca. 1304 cm-' (at -0.2 V) having its relative intensity strongly decreased for potentials near - 1.1 V. Unfortunately this mode is not observed in the SER spectrum of the reduced complex (at - 1.1 V) since no Raman feature is observed in the 1350-1380 cm-' spectral region at -1.1 v. However, several other Raman frequencies observed in the SER spectrum at -1.1 V present frequency values close to vibrational frequencies observed in the IR or Raman spectra of compounds having bipy in the reduced form. Some Raman frequencies shift to lower energies as the ligand is reduced. This is because these frequencies are associated to vibrational modes with strong contribution from CZ-c3, C4-C5, c5-c6 and CZ-N bond stretching, e.g., modes 5 and 6 (see Tables 1 and 2). Note that the 7c7 state has an antibonding character for these bonds (see Figure 3b). According to Noble and Peacock,26the bipy'- radical anion presents a broad absorption band near 550 nm due to the n7 nlo transition. Therefore, using the 514.5 nm laser excitation, one could obtain the resonance Raman spectrum of the bipy'radical anion. Compare now the SER spectra of Figure 2b, excitedat 647.1 nm, with those of Figure 4, obtained at 514.5 nm excitation, both for the tris bipy cobalt complex. (The respective SER spectra for the nickel complex are presented in Figure 1 SM.) It is ease to see that the Raman signals of the ligand in the reduced form are stronger when excited in the red. At 514.5 nm excitation the SERS features of the reduced
-
13220 J. Phys. Chem., Vol. 99, No. 35, 1995
Corio and Rubim
TABLE 1: Vibrational Frequencies (em-') Observed in the Infrared (IR) and Raman Spectra of bipy2"and Li+[bipyY-t5 in the [Ru(bipy)gI2+,and [R~~~~(bipy)2bipy'-]~~,~* in Comparison to the Raman Frequencies Observed Raman Spectra of Na+[bi~y]*-,2~ in the SER Spectra of the rCo(bips),l2+Complex Adsorbed on a Silver Electrode at -0.2 and -1.1 V IR bipy Li' bipy'1591 1586 1574 1563 1482 1457 1446 1421 1311
1147 1094
Li+ bipy'-
Na+bipy'-
1554
1558
1563 1588 1495 1459 1480 1410 1421
5
1565
1544 1582 1467
1482
n.0.
7
1430
8
n.0.
9 10
1608
1548
1595
1563
1506
1478
1491
1495
1429
1450
1427
1350 1276
1357 1273
1320 1276
1365 1285
1304 1286
1205 1164
1205 1151
1264 1176
1212 1164
1268 1173
1033
1110 106.7 1043
1100 1028 1014
1100 1068 1040 1021
1486 1412
1272 1265 1200 1165 1148 1107
Raman SERS assignment [Ru(bipy)3I2+ [R~(bipy)3]~+~ [Co(bipy)3I2+-0.2 V [Co(bipy)3I2+- 1.1 V u (according to Table 2)
6
1260
1044 995
I
11 12
1165 1153 1100
13 14 15 16
Only those frequencies related to the reduction product, the [Co(bipy)2bipy'-]+ complex, are displayed.
TABLE 2: Potential Energy Distribution (PED) for the Vibrational Modes of A1 Species Obtained by Normal-Coordinate Analyses for the [Ru(bipy)3I2+ComplexB mode
Raman freq (cm-I)
5 6
1608 1563
7
1491
A
PED
- 1.1 v
$
8
1450
9 10
1320 1276
11 12 13
1264 1176 1110
14
1067
15
1043
B
24000 22000 20000 18000 16000 14000 12000 10000 eo00 6000
-
1.0 v
-09v
-08V
l i 0
15bo
1400
1300
1200
1100
lobo
Reman Shitt/ca-i
Figure 4. SER spectra of a silver electrode in 0.1 M K2S04 and M [Co(bipy)3I2+ aqueous solution at the indicate potentials. Laser excitation at 514.5 nm. 0.24
1
L
t = 6.95min
The numbers after the motion description indicates the percentage of the respective motion to the normal mode of vibration.
b 0.0
'
1
I
I
1
I
500
600
700
800
900
1
1000
A / nm Figure 3. (a) Energy diagram for the molecular orbitals of bipy and the radical anion bipy'-. (b) Molecular orbitals for bipy calculated by the MNDO method.26
bipy at 1544 and 1467 cm-' appear only as weak shoulders at the potential of -1.1 V. These results suggest that the n7 n10transition has shifted to lower energies due to the coordination of bipy to the central metal ion. In fact, the reflectance spectra of the [M(bipy)3I2+ complexes in the potential region where they are reduced (see +
Figure 5. In situ reflectance spectra of a silver electrode in 0.1 M &SO4 and M [Co(bipy)3I2+aqueous solution. I& is the reflectance spectrum at -0.6 V and R is the reflectance spectrum obtained after holding the electrode potential at -1.3 V for 1 min. The insert shows reflectance spectra for the silver electrode in the same electrolyte solution containing M [Ni(bipy)3I2+complex. For the insert R is the reflectance spectrum at -0.6 V after staying for 3 min at -1.3 V. t refers to the total time developed after the application of -1.3 V.
Figure 5 ) show an absorption feature near 630 nm. Therefore, the SER spectrum excited at 647.1 nm for the potential of - 1.1
J. Phys. Chem., Vol. 99, No. 35, 1995 13221
Adsorption and Reduction of [M(bipy)3I2+Complexes V corresponds to a surface-enhancedresonance Raman spectrum of the reduction product of the [M(bipy)3I2+complex. In the reflectance spectra of Figure 5 there is also another feature at 820 nm which is assigned to the n7 n8.9 transition in the bipy radical anion.29 It is worth mentioning that none of these absorptions can be assigned to changes in the reflectivity of the silver surface since no absorption feature was observed by changing the applied potential from -0.6 to - 1.3 V in the absence of the complexes. Another evidence for this is that the absorption features observed in the presence of the complexes increase in intensity by staying at -1.3 V for long periods of time. The features disappear completely if the applied potential is -0.6 V. 111. Mechanism of Enhancement. A criterion to establish the type of mechanism of enhancement is to obtain the dependence of the SERS intensity on the applied potential for different laser excitation^.^^^^-^^ Figure 6 displays the SERS intensity vs Vapp curves for different vibrational modes and different laser energies obtained from the SER spectra of the [Ni(bipy)3I2+complex. (For the [Co(bipy)3I2+complex similar curves were obtained but with not well-resolved profile. See Figure 2SM.) The profiles of Figure 6 show two remarkable aspects: (i) The majority of the SERS profiles present more than one potential of maximum SERS intensity (Vmax). Note that the profiles for data obtained using the 488 and 5 14.5 nm excitation have three maxima (Vmaxl,Vmax2, and Vmax3) and the profiles for the 647.1 nm radiation have only two. (ii) As the laser energy decreases, all V,, shift to more negative values. According to the charge-transfer models for SERS,30-36there should be a linear relationship between the laser excitation energy and Vmax. In ref 36, the corresponding relation for a photon-assisted charge transfer is given as
-
ECT
= hwL = a(vred - vmax)
-
I
1.0-
-
0.9
0.8 0.7 -
0.6-
-
0.5
-
0.4
1597
-0-
0.3-
-e1472
0.2
-v-
0.1 0.0 -
1314 1163 1014
-m-A-
. I -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
-0.1
EIV
:I
1.1
0.8
.-0
0.70.6-
.o
0.5
.-
0.41
2 3
1514.5 nm
o.zi .;,;,
2
. ,
' I
0.1
,
,
,
,
.
7-v- 1314 ,
,
,
I
,
~,..... ..... , 1163 -
*0;
0.0
I
/A/
-
3 0.3-
I
1070 770
-0.'-
-A-
-0.1
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
EIV 1.1
-
1.0-
0.7-
I
0.9 0.8
(6)
where ECTis the charge-transfer energy being resonantly excited at the laser energy h w ~ .A negative value for parameter a means that the SERS charge-transfer mechanism working in this case is from a donor level near EF (the Fermi level) to an acceptor level of the adsorbate (the LUMO). The linear coefficient of the straight line corresponding to eq 6 is the potential for which RUL is zero. According to the charge-transfer model described in ref 36, this is the reduction potential of the species displaying SERS. Figure 7 shows the curves of ~ W vs L Vmax1.2.3 for the [M(bipy)3I2+complexes. All curves show a behavior that can be described by an equation similar to eq 6. The parameter Vred for the tris bipy Co and Ni complexes are -1.20 and -1.23 V, respectively. Observe that in both cases the Vred values are very close to the half wave potentials in the CV curves of Figure lb,c. Note also that the angular coefficients (a)of the straight lines in Figure 7 are positive, Le., the parameter a is negative. Therefore, one can conclude that the excited states involved in the charge-transfer processes being resonantly excited in the SERS measurements are molecular orbitals of the bipy ligand. The electronic absorption spectra of the [M(bipy)#+ complexes show three transitions in the UV region, assigned to ligand internal ~t z* transitions that are displayed in Table 3. For potentials where the reduced form is present on the surface (1-1.2 V) one can not disregard that part of the enhancement at 647.1 nm excitation is due to a resonance Raman effect presented by the reduced complex since it absorbs
-
1.1
.-0
-8
;.
3 .-
647.1 nm
1597 1472 - - v - - - 1314 -0-
0.6-
o
0.5-
0.4-
3 0.3-
B
0.2: 0.1
-
0'
'." 0
00 -01
,
-1.2
,
,
-1.0
,
,
-0.8
,
,
-0.6
,
--0
,
-0.4
...._m ...._1163
.'. .-a
,
,
-0.2
,
,
0.0
~- -A-. ,
1014
~
I
0.2
EIV
Figure 6. SERS excitation profiles, at three laser excitations, for the [Ni(bipy)3l2+complex adsorbed on a silver electrode in the same solution as in Figure 2c. The SERS intensities (integrated areas) were obtained from spectra recorded during scanning the applied potential in the negative direction at 10 mV s-' . Each spectrum was acquired during a total integration time of 10 s. See text for more details.
at 630 nm. Therefore one can say that at these potentials the overall enhancement is due to a surface-enhanced resonance Raman scattering (SERRS). The presence of a resonance enhancement inherent to the adsorbate itself can explain why in the ex situ experiments the Raman signal of the reduced form of the M(I1) complexes are stronger than that observed for the M(I1) complexes. The fact that no signal was observed for the Co(II1) species in the ex situ experiment can be explained as follows. The removal of the electrode from the working solution leads the surface to be exposed to oxygen. As already shown 22 the exposure of the electrode surface to oxygen causes the oxidation of the SERS-active sites involved in the photon-assisted charge-
Corio and Rubim
13222 J. Phys. Chem., Vol. 99, No. 35, 1995 E-E,/eV
A -2.0 -1.5
a 1 2 05eVN. p=2.53eV, Vred=-1 23V 0 Vmax? NI a =?.74eVN. p3.3%V, Vred=-I.O5V X Vmaxl Co a =2.14eVN, p 2 57eV. Vred=-?.2QV 0 Vmax? Co a =1.24eVN, p=3WeV, Vred=-Z.UV
2.0 1.9
-1 0
-0.8
-0.6
-0.4
-0.2
0.2
0.0
VmaxN
Figure 7. Laser excitation energy vs V,,, plots for the [M(bipy)3I2+complexes. The parameters a and p are the angular and linear coefficients, respectively, obtained by linear regression. Note that a = -a and (-P/a) = Vred and that some points are coincident.
-
TABLE 3: Ligand (bipy) Internal R fi Electronic Transitions Observed in the Absorption Spectra of the [M(bipy)#+ Complexes with the Respective Assignment Lahm energyIeV assignment 203 244 302
6.11 5.08 4.11
n6 n6
n.5
--
nIO*
na*, n9* n7*
transfer mechanism of enhancement, Le., this mechanism of enhancement is quenched. In this case, only the electromagnetic mechanism of enhancement plays a role. If the enhancement is not large enough to bring the signal above the detection limit of the instrumentation, no signal will be observed. On the other hand, if the adsorbate alone presents resonance Raman effect at the exciting radiation, then the occurrence of both phenomena can bring the signal above the detection limit, and indeed this is what occurs with the reduced forms of the Co(I1) and Ni(1I) complexes. The fact that a very weak signal is observed for the M(I1) species or none for the Co(II1) species is not by itself evidence that these species are not adsorbed on the electrode surface. Note that the reduced form may also be enhanced by a photon-assisted charge-transfermechanism. Albeit the n7 orbital has already an electron in it, the n8,9 orbitals are still unoccupied and are ca. 1 eV above n7. Therefore, depending on the value of parameter a for this species, a photon-assisted charge transfer would be feasible. Unfortunately, due to surface concentration changes at these negative potentials and to the hydrogen evolution reaction, SERS excitation profiles would have no meaning, making difficult the evaluation of this parameter. As mentioned above, the bipy ligand has at least three unoccupied orbitals of low energy that can act as acceptor levels in charge-transfer transitions between the electrode and the adsorbate. Therefore, the three different values of Vmax for a given excitation are assigned to three potential modulated photon-assisted charge-transfer processes. So, the first maximum in the SERS profiles, Vmaxl,can be assigned to the Ag(EF) n7* transition. Similarly, Vmax2and Vmax3are assigned to Ag(EF) ng9* and Ag(EF) nlo* transitions, respectively. The parameter a in eq 6 is related to the way the energy of the electrode donor or acceptor states depends on the applied potential. This parameter transforms a potential difference in an energy difference felt by the a d ~ o r b a t e . ~For ~ the [Ni(bipy)312+ complex, a is -2.05 sVN as obtained from Figure 7. The difference between Vmax2and Vmaxl is ca. -0.55 V, meaning that the two acceptor levels are separated by 1.13 eV
- -
-
0.5
1.o
d-d = 2.39eV
t Figure 8. Left-side diagram shows the energy levels position of the [Ni(bipy)3I2+complex relative to EF. Right side of the diagram shows the dependence of the silver electrode donor levels on the applied potential. To illustrate this effect, a vertical bar corresponding to the laser excitation at 488.0 nm is displayed. Note that as V,, equals the different V,,,, the resonant condition for each corresponding charge transfer is achieved. See text for details.
(=2.05 e V N x 0.55 V). The third maxima (Vmax3)is 1 V more negative than Vmax1. Therefore, the third acceptor level is separated from the first one by nearly 2 eV. So, the differences in energy between the three charge-transfer states are ca. 1 eV, very close to the differences observed in the electronic absorption spectra of the complexes (see Table 3). Taking into consideration the above results and discussion, an energy diagram for the adsorbed [Ni(bipy)#+ complex is presented on the left side of Figure 8. It displays the relative positions of the energy levels involved in the charge-transfer transitions between the silver electrode and the adsorbate. On the right side of Figure 8 the energy values were converted to potentials by using the parameter a = -2.05 eVN. The right side diagram is showing the dependence of the silver electrode donor levels on the applied potential. A similar diagram was also obtained for the tris-bipy cobalt(I1) complex (see Figure 3SM). The alignment of the energy levels in relation to the Fermi level was done assuming that at the reduction potential of the adsorbate (Vred) the electron energy of the metal side is equal to the energy of the first adsorbate’s acceptor level (n7). Let us consider that at Vapp = Vox the corresponding energy is -0.92 eV ( a = -2.05 e V N and Vox= 0.45 V were used). This should be the position of the HOMO (eg), Le., the donor level of the adsorbed [Ni(bipy)312+complex. The adsorbate acceptor level (777) stays at a v r e d , Le., 2.58 eV above EF. Therefore, the energy difference between these two levels is 3.50 eV. Note that this value is less than 10% larger than the EMLCT, the metal-to-ligand charge-transfer energy obtained for this complex by UV-vis spectro~copy.~~ A similar correlation for the tris-bipy cobalt complex could not be done since the EMLCT for the [Co(bipy)312+is not known and during the reduction of Co(II1) to Co(I1) there is a change in the spin configuration.
J. Phys. Chem., Vol. 99, No. 35, 1995 13223
Adsorption and Reduction of [M(bipy)312+Complexes Conclusion
Considering the results presented here, the following conclusions can be drawn: (i) The [M(bipy)3]*+complexes adsorb on the silver electrode by a x interaction that involves donor states of the silver and x* orbitals of the bipy ligand. (ii) At potentials more negative than -1.0 V, where the trisbipy M(I1) complexes are reduced, the SER spectrum resembles that of the bipy’- radical anion. Therefore, the electron transferred in this reaction is from a silver donor level to the x7 acceptor level of bipy. No indication for the formation of Co(1) or Ni(1) complexes was found. (iii) The SERS excitation profiles (ZSERS vs V,,,) have shown three potentials of maximum SERS intensity, assigned to three different photon-assisted charge-transfer processes from a donor level near EF to the n7,xg,9, and x10antibonding orbitals of the bipy ligand. (iv) Using the data obtained on the laser excitation energy vs V,,, plot, it was possible to map the position of the energy levels of the adsorbed complexes as well as the silver donor levels relative to EF. The above conclusions could be achieved because the SERS effect of these complexes has a strong contribution from the charge-transfer mechanism of enhancement. As shown in ref 36 the charge-transfer mechanism of enhancement is very similar to the resonance Raman effect, Le., a vibronic effect. This work shows, as proposed b e f ~ r e that , ~ the SERS effect can be used as a technical tool to get information about the excited states of the adsorbed molecules. Acknowledgment. P.C. thanks FAPESP (FundagZo de Amparo k Pesquisa do Estado de Siio Paulo) for the grant of a fellowship. J.C.R. acknowledges FAPESP for a research grant (91/4313-2) and CNPq (Conselho Nacional de Pesquisas) for financial support. Supporting Information Available: Figures of SER spectra (6 pages). Ordering information is given on any current masthead page. References and Notes (1) Farquharson, S.; Weaver, M. J.; Lay, P. A,; Magnuson, R. H.; Taube, R. H. J. Am. Chem. SOC.1983, 105, 3350. (2) Simic-Glavaski, B.; Zecevic, S.; Yeager, E. J. Phys. Chem. 1983, 87, 4555. (3) Farquharson, S.; Guyer, K. L.; Lay, P. A.; Magnuson, R. H.; Weaver, M. J. J. Am. Chem. SOC. 1984,106, 5123.
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