Mixed-Valent States in the Redox Chemistry of Double

uptake for all dimers assuming, as seems very likely, that they all have the same diffusion coefficient. With the H2-Hz dimer, the waves are too close...
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J. Phys. Chem. 1993,97, 6090-6095

6090

Mixed-Valent States in the Redox Chemistry of Double- and Triple-Decker-Sandwich Free Base and Copper Porphyrins Asma El-Kasmi,IPDoris Lexa,lnPhilippe Maillard,IbMichel Momenteau,lband Jean-Michel Sav6ant'JP Laboratoire d'Electrochimie Mol6culaire de I'UniversitC de Paris 7, Unit6 Associ6e au CNRS No. 438, 2 Place Jussieu. 75251 Paris Cedex 05, France, and Section de Biologie, The Institut Curie, Unit6 Inserm 219, 91405 Orsay, France Received: December 14, 1992; In Final Form: March 25, 1993

Double- and triple-decker-sandwich free base and copper porphyrins, where tetraphenylporphyrin rings are linked together by two urea bridges, have been studied by cyclic voltammetry and UV-vis spectroelectrochemistry. Mixed-valent states appear in all cases, even with dimers and trimers containing twice or thrice the same porphyrin. The separations between the standard potentials in the Hz-Hz, Cu-Cu, Hz-Cu dimers as well as in the H r H r H z , Cu-Cu-Cu, Hz-CU-HZ trimers are interpreted in terms of Coulombic repulsion and interaction with the counter cations.

Molecules containing several porphyrins or phthalocyanine rings, bound together by metal-metal bonds,24 by bidentate axial ligands,s-' or by peripheral covalent linking,12-*7have attracted continuous attention because of their potential implications in the fieldsof biological systems,conductingmaterials, and catalysis of the transformation of small molecules such as dioxygen. In the work reported below we investigated the appearance of mixed-valent states in the reduction of the dimeric and trimeric porphyrins shown in Figure 1, that derive from tetraphenylporphyrin by urea-linking of the ortho-positions of the phenyl rings and bear on the outside faces a basket-handle strap linked to the remaining phenyl-ortho-positions by secondary amide groups. In an attempt to unambiguously assess the nature of the interactions between the porphyrin moieties that may appear upon reduction, we purposely limited our study to free bases and copper complexes. One reason for this choice is that these compounds are expected to give rise to T* anion radicals upon injection of an electron, rather than to a change of the oxidation state of the central metal. The second reason is that the redox chemistry of the monomers, dimers, and trimers are then not complicated by associated ligand exchange reactions. These two problems have indeed hampered a clear identification of theeffects underlying the appearance and stability of mixed-valent studies in such systems in past studies.

Results and Discussion Cyclic Voltammetry. With the exceptionof one test experiment, the solvent was 1,2-dichloroethane with 0.1 M n-BudNPF6 as supportingelectrolyte. The working electrodewas a glassy carbon disc and the temperature 17 OC. All potentials are referred in the following report to the aqueous SCE, but their precise determinationwas made by means of an internal standard, namely, bis(pentamethyl)ferrocene, the standard potential of which is -0.06 V vs aqueous SCE. Each of the two monomers (which contains one of the same basket-handlestrap as in theoutside face of thedimers and trimers on each side of the porphyrin in cross position one to the other) gives rise (Figure 2) to a reversiblecyclic voltammetric reduction wave (63-mV cathodic-to-anodic peak separation). It is seen that the formal potential of the free base, determined as the midpoint between the cathodic and anodic peaks, -1.038 V vs SCE, is positive to that of the copper complex: -1.156 V vs SCE. An overall view of the voltammograms obtained with three dimers, H2-H2, Cu-Cu, and H2-Cu and the three trimers H2H2-H2, Cu-Cu-Cu, and H2-cu-H~ is shown in Figure 3. 0022-365419312097-6090$04.00/0

All three dimers exhibit two successive reversible one-electron waves, pointing to the formation, between the first and thesecond wave, of a mixed-valent state. The potential separation in the symmetrical dimers is of the order of 100 mV and of 200 mV in the H2-Cu dimer. The overall location of thedouble wavesystem along the potential axis is less negative with the H2-H2 dimer than with the Cu-Cu dimer. A likely explanation for the appearance of a mixed-valent state in the symmetrical dimers, that will be ellaborated in more detail later on, is the existence of a repulsive interaction between the two negatively charged porphyrin rings when the dimer is fully reduced. The same type of repulsive interaction also appears in the mixed dimer since the potential separation is then larger than the difference in formal potentials of the H2 and Cu monomers. The cyclic voltammetry of the trimers exhibits what seems to be a two-electron wave followed by a one-electron wave. The separation between these two waves is bigger for the H2-Cu-H2 trimer than for the H2-HrH2 and Cu-Cu-Cu trimers. This indicates that the last wave of the H2-Cu-H2 trimer involves the reduction of the central porphyrin, whereas the first two-electron wave would correspond to the injection of one electron in each of the outside porphyrin free bases. Because of the overlapping of the waves in most of the investigated dimers and trimers, a deconvolution procedure is needed for determining precisely the formal potentials characterizing each electron uptake. In the case of the Cu-H2 dimer, the waves are sufficiently separated for the potentials to be obtaineddirectly from the cyclic voltammogram (Figure 2) as the midpoint between the cathodic and anodic peak potentials after magnification of the potential scale. In addition, the height of the first cathodic wave may be taken as a standard for the current correspondingto a one-electron uptake for all dimers assuming, as seems very likely, that they all have the same diffusion coefficient. With the H2-Hz dimer, the waves are too close one from the other for this procedure to be applied. The deconvolution of the two waves was then made as follows. The single wave obtained with the monomers is corrected in height using the first wave of the H2-Cu dimer at the same concentration so as to simulate the height and shape of the first wave of the H2-H2 dimer. The second wave is then reconstructed by difference between the experimental H2-H2 cathodic trace and the simulation of its first wave, as illustrated in Figure 4. The same deconvolution procedure was used with the Cu-Cu dimer. The formal potentials thus obtained for the three dimers are listed in Table I. 0 1993 American Chemical Society

Redox Chemistry of Free Base and Copper Porphyrins

The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 6091

Figure 1. Porphyrin monomers dimers and trimers investigated in this work.

a cu-cu

E ( V n SCE) 0

-1

Figure 2. Cyclic voltammetry of the free base (a, top) and copper (b, bottom) monomer complex in 1,2-dichloroethane + 0.1 M n-Bu4NPF6 at a glass carbon electrode. Scan rate: 0.1 V/s. Porphyrin concentration: 0.5 mM. Temperature: 17 OC.

In the case of the trimers, we start with the H2-Cu-H2 compound. The first wave that appears to be a two-electron wave is in fact the superposition of two one-electron waves: the peak width, E,p- E, (difference between the half-peak potential, E,12, and the peak potential, E,, corresponds to a difference of the two formal potentials of 56 mV.I8 Wemay thus deconvolutethe whole cathodiccurrent-potential curve, as shown in Figure 5. The first two-electron wave is reconstructed by addition of two one-electron waves taken from the monomer, shifted by 56 mV one from the other, and adapted in height so as to reproduce the peak height of the trimer twoelectron wave. The third wave is then reconstructed from the difference between the experimental trace and the reconstructed two-electron wave. The same procedure was applied to the H2-HrH2 trimer (Figure 6 ) and to the Cu-Cu-Cu trimer. The peak width of the first two-electron wave corresponds to a difference of the two formal potentials of 64 mV in the first case and 70 mV in the second case. The ensuing formal potentials for the three trimers are listed in Table I. We now analyze in more detail the nature and magnitude of the repulsive interactions, responsible for the appearance of the

2 cu -cu-cu

E( V WSCE )

E(VnSCE 4

0

0

-1

Figure 3. Cyclic voltammetry of the dimeric and trimeric free base and copper complexes in 1,2-dichloroethane + 0.1 M n-Bu4NPF6at a glassy carbon electrode. Scan rate: 0.1 V/s. Porphyrin concentration: 0.5 mM. Temperature: 17 OC.

mixed-valent state, in the dimers as well as the location of each formal potential as compared to the formal potentials of the monomer. For the symmetrical dimers one must first correct the formal potentials previously determined for a statistical effect due to the symmetry of the m o l e ~ u l ealong ~ ~ the following lines. Consider the two-step reduction of a molecule A-B. Let PA and $ be the formal potentials for the intrinsic reduction of the A and B moieties, respectively:

A

+ e- F? A - ( e ) , B + e- e B-(I$)

Injection of one electron in the molecule A-B creates a mixture of -AB and AB- molecules where the electron is located in the

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The Journal of Physical Chemistry, Vol. 97, No. 22, 1993

I

I

TABLE Ik Formal Potentials. of the Dimers Corrected from the Symmetry Effect

e -1 P E(VvrSCE)

-1

-0,8

Figure 4. Deconvolution of the reduction waves of the H2-H2 dimer (full line). Dashed line: simulation of the first wave from the wave of the monomer corrected in intensity according to the first wave of the H2-Cu dimer (see text). Dotted line: reconstruction of the second wave by difference between the full and dashed lines.

TABLE I: Formal Potentials' in the Reduction*of the Monomer, Dimer, and Trimer Porphyrins porphyrin

1st reduction

2nd reduction

-1.038 -1.156 -1.034 -1.121 -1,047 -1.018 -1.102 -1.024

-1.132 (98) -1.232 (1 Io) -1 .223 (1 80) -1.082 (64) -1.172 (70) -1.080 (56)

H2 cu H242 cu-cu H2-c~ H*-H2-H2 cu-cu-cu H~-CU-H~

3rd reduction

-1.192 (111) -1.270 (98) -1.280 (200)

+

E(

-1

V~CE)

-v

Figure 5. Deconvolution of the reduction waves of the H2-cu-H~ trimer (full line). Starred lines: two one-electron waves of the monomer shifted by 56 mV and corrected in height so as to reproduce the peak height of the first two-electron wave of the trimer (dashed line). Dotted line: reconstruction of the third wave by difference between the full and the dashed lines.

E(VHSCE

-0.8

-1

4 2

Figure 6. Deconvolution of the reduction waves of the Hz-Hz-H~trimer (full line). Starred lines: two one-electron waves of the monomer shifted by 64 mV and corrected in height so as to reproduce the peak height of the first two-electron wave of the trimer (dashed line). Dotted line: reconstruction of the third wave by difference between the full and the dashed lines.

A and B moieties, respectively. The equilibrium concentration ratios characterizing this distribution are given by the following equations

where E is the electrode potential. The overall formal potential, being defined by E=

porphyrin

1st reduction

2nd reduction

I32442 cu-cu H2-c~

-1.051 -1.138 -1.047

-1.1 1 4 (63) -1.214 (75) -1.223 (180)

In 1,2-dichloroethane, in volts vs aqueous S C E at 17 OC. Between parentheses: separation between the first and second formal potentials.

is thus related to

and E: according to

Thus, in thecase whereA and B are the same, asin the symmetrical dimers, PA = = Eo

e

RT = Eo + -In 2 F Similarly when the injection of a second electron is considered, neglecting for the moment the attending repulsive interactions, @,AB

In volts vs aqueous SCE. Between parentheses: separation between the first and second formal potentials or the second and third formal 0.1 M n-Bu4NPF6. Porphyrins potentials. In 1,2-dichloroethane concentration: 0.5 mM. Temperature: 17 OC.

-0,8

El-Kasmi et al.

~ y + y , l n~[-AB]~ [AB1 + [AB-]

= E O -RT Tln 2 In other words, the fact that, starting from the neutral dimer, the electron may be located in one or the other moiety results in a (RT/F)In 2 positive shift of the formal potential. Conversely, the fact that starting from the dianion, one electron may be removed from one or the other moiety results in a ( R T / F )In 2 negative shift of the second formal potential. If no other effect would interfere, these shifts would result in a two-electron wave being exactly the double of the monomer wave with no change in shape.I9 The values of the formal potentials corrected from this symmetry effect (17.4 mV at 17 "C) are summarized in Table 11. In the case of the H&u dimer, the symmetry effect vanishes and the correction becomes accordingly negligible. Since the formal potentials of the first waves are close to those of the monomer, the appearance of a mixed-valent state may be attributed to Coulombic repulsion between the two negative charges in the fully reduced product. The magnitude of the repulsive interactions (Table 11) is, however, much weaker than predicted for tv(ro unit charges separated by a distance d = 7.3 A (distance between the two porphyrin rings estimated from molecular models) in vacuum:

-eo2 - 14.4 eV/%, head

(Le., 1.97 eV)

If solvent molecules were able to fill the portion of space between the two porphyrin rings while keeping their mobility, the coulombic interaction would be divided by the static dielectric constant of thesolvent. With 1,2-dichloroethane, e, = 10 20andtheCoulombic interaction would then be of the order of 0.2 eV. A test experiment carried out with the Cu-Cu dimer in N,N'-dimethylformamide ( E , = 36.7 2O) revealed that the distance between the two cyclic voltammetric waves is not very different from what it is in 1,l'dichloroethane. As a matter of fact, it increases slightly by about 30 mV, opposite to what is expected from the increase in the static dielectric constant. We may therefore conclude that the weakening of the Coulombic repulsion is not primarily due to the solvent. Either the solvent molecules are unable to penetrate the gap between the two porphyrin rings or, if they do, they loose a large part of their mobility. If immobile solvent molecules are located between the porphyrins rings, the optical dielectric constant, cop would replace E , in the estimate of the Coulombic repulsion. copbeing of the order of 2, the presence of the solvent would be unable to explain why the experimental repulsive interaction is so weak.

The Journal of Physical Chemistry, Vol. 97,No. 22, 1993 6093

Redox Chemistry of Free Base and Copper Porphyrins

W

B 0 -

0 + e-+ 8:

0

n

0 or

Figure 7. Schematic representation of the interactions between the negative charges in the porphyrin rings and the counter cation. A: no cation in the gap between the porphyrins. B: one cation in the gap.

NHCO dipoles are present between the porphyrin rings from the very manner in which the dimers were synthesized. It has been shown, in monomeric porphyrins, that the presence of such NHCO dipoles attached to the ortho-position of the phenyl rings exerts, through their interactions with the negative charges borne by the porphyrin complex, a very significant influence on the values of formal potentials,2i in the manner of a “local” solvent. However, the difference with a true solvent resides in the lack of mobility of the NHCO dipoles, which therefore react to the presence of electric charges in their vicinity’through electronic polarization, but not through fluctuational polarization as a true solvent does.21d We thus expect the existence of interactions between the NHCO dipoles and the negative charges on the porphyrin rings that may contribute to the value of the formal potentials, as it does in monomeric porphyrins. These interactions should not, however, be able to cause the large decrease of the Coulombic repulsion that we observe because the fixed, or almost fixed, NHCO dipoles behave like a polarized solvent, Le., like a medium of very low dielectric constant (c = 2). In addition to the presence of the solvent, one has to take into account that of counter cations which accompanies the injection of negative charges in the porphyrin rings. In the case of an electrochemical investigation as reported here, the counter cations are part of the supporting electrolyte, here n-Bu4NPF6, that is present in large excess (0.1 M for a porphyrin concentration of 0.5 mM). However, even if the reduction of the dimers would be carried out in another fashion, the presence of the counter cations should be taken into account as well, even though under somewhat different conditions. Rough estimates of the effect that the presence of counter cations may have on the Coulombic repulsion between the negatively charged porphyrin rings may be obtained from the simplified schemes shown in Figure 7. The Debye-Huckel distance for a 0.1 M solution of a fully dissociated electrolyte in a solvent of dielectric constant equal to 10 is 3.4 A, i.e., about the same as the hard-sphere radius of a n-Bu4N+ion and also about the same as half the distance between the two porphyrin rings. With a configuration of charges such as that represented in Figure 7A, the Coulombic interactions would make the second formal potential more negative than the first by 300 mV. In the much less probable configuration shown in Figure 7B, the Coulombicinteractions would render the second formal potential positive to the first by ca. 1300 mV. It thus suffices that a very small percentage of the charge configurations be of the B-type, while most of them would be of the A-type, to further decrease the repulsive interaction. Another possibility is that, the charge configuration being of the A-type, ion pairing

would take place, thus reinforcing the attractive interaction between the negative-positive pairs, and would therefore globally decrease the repulsive interaction. Such a phenomenon is likely to occur in a solvent of such low dielectric constant as 1,2dichloroethane. This would explain why the repulsive interaction appears as somewhat larger in DMF than in 1,2-dichloroethane since ion pairing is expected to be stronger in the second case than in the first. The global Coulombic repulsive interaction may thus be estimated as 63 and 75 meV in the Hz-Hz and Cu-Cu dimers, respectively. In the mixed dimer, the first wave formal potential is very close to that of H2-Cu corrected from the symmetry effect. The difference between the second formal potential and the corrected first standard potential of the Cu-Cu dimer, 87 mV, is a measure of the Coulombic repulsion which is thus seen to be of the same order of magnitude as in the symmetrical dimers. In the A-A-A trimers (A = H2, Cu), the first two electrons are injected at potentials that are close one to the other, whereas the uptake of the third electron requires a significantly more negative potential. This observation indicates that, in the doubly reduced molecule, the species that contains one electron in each of the outside porphyrin rings predominates over the two species where the electrons are located in adjacent rings because Coulombic repulsions is less in the first case than in the second. Under these conditions, the symmetry factors that should be used for correcting the formal potentials may be handled as follows. The first injected electron may sit with equal probability in any of the three porphyrin rings. It follows that the corrected formal potential for the first reduction is obtained from the experimental formal potential by subtraction of (RTIF) In 3 = 27.5 mV, i.e., in the case of the l42-Hz-H~ trimer, -1.045 V vs aqueous SCE. For the second reduction, if it is assumed that the -A-A-Aspecies totally prevails over tbe two other possible forms (-A-A--A and A-A--A-), then the Coulombic repulsion would be equal to (1.0824.0275) - 1.045 = 0.010 eV. We may however obtain a more accurate estimate of the Coulombicrepulsion by considering that the equilibrium concentrations of -A-A--A and A-A--Aare not completely negligible vis-a-vis that of -A-A-A-. The observed formal potential, for the second reduction is related to the formal potential of the first wave corrected, as described above from the symmetry effect, (-1.045 V vs aqueous SCE), through - I$

+ CRTZ =

F In11 + 2 e ~ pRT [ ~ ( c R T- 2CRD)]] -

1x1 3 (1)

where CRTZ and CRD are the Coulombic repulsion energies in the -A-A-A- trimer and the -A-A- dimer, respectively (63 meV in the H2case). Iterative resolution of the above equation for the H2-Hz-Hz trimer leads to a value of CRTZ = 16 meV for the Coulombic repulsion between the two negative charges located each in the outside porphyrin rings. The enlarged distance between the charges and their possible shielding by the central uncharged porphyrin ring are responsible for the drastic decrease of the Coulombic repulsion vis-&-visthat observed in the H2-H2 dimer. The symmetry effect on the third reduction is a negative shift of

F ln(1

+ 2 exp[&(CRT2

- CRD)])

(2)

i.e., 7 mV in the case of the HZ-HZ-H~trimer. The corrected formal potential for the third electron uptake is therefore -1.1 84 V vs aqueous SCE. Thus, the Coulombic repulsion against the introduction of the third electron is 1.184 - (1.045 + 0.016) = 0.123 eV, Le., about twice the Coulombic repulsion in the H2-Hz dimer as expected from the fact that, in major part, the reaction

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The Journal of Physical Chemistry, Vol. 97, No. 22, 1993

consists in the injection of one electron in the central porphyrin ring in a molecule where one negative charge is already present in the outside porphyrin rings. Application of the same analysis to the Cu-Cu-Cu trimer leads to a corrected value of -1.129 V vs aqueous SCE for the first formal potential. If the two charges were located on the outside rings after the second reduction, -1.17 15,the Coulombicrepulsion wouldthen be-1.129 1.172-0.0275 = 0.015 meV. Themore refined procedure (eq 1) used above leads here to a Coulombic repulsion of 0.020 eV, Le., almost the same as that for the H2H2-H2 trimer. At the level of the third reduction, the symmetry effect entails a negative shift that can be estimated from eq 2 to be equal to 5 mV. The corrected formal potential for the third reduction is therefore -1.265 V vs aqueous SCE, and thus the Coulombic repulsion for the injection of the third electron is 1.265-(1.129+0.020) =O.l16eV,closetowhatwasfoundwith the H2-H2-H2 trimer and to twice the Coulombic repulsion in the symmetrical dimer. With H2-Cu-H2, the two first waves also very close one to the other as in the H2-H2-H2 and Cu-Cu-Cu trimers but the third wave is much more separated from the first waves. This observation is consistent with the location of the first electron in one of the two H2-porphyrins, of the second in the heretofore unoccupied H2-porphyrin and of the third in the central Cuporphyrin. The symmetry factor for the first electron uptake is 2 instead of 3 in the preceding cases because the Cu-porphyrin is much more difficult to reduce than the free base (by 118 mV in, e.g., themonomers, as seen inTable I). Practically,the product of the first electron uptake thus consists only of two species where the electron is located in one or the other outside H2-porphyrin. The corrected first formal potential of H2-cu-H~ is thus -1.024 - 0.017 = -1,041 V vs SCE, practically the same as in the H2H2-H2 trimer. After the second reduction, there is one electron in each of the outside H2-porphyrins. The corresponding corrected formal potential is thus -1.080 0.017 = -1.062 V vs aqueous SCE. The Coulombic repulsion at this level is therefore 21 meV, Le., practically the same as those between the outside porphyrins in the H2-H2-H2 and Cu-Cu-Cu trimers. In the third reduction there is no symmetry effect since the reaction consists in the injection of one electron into a single isomer, -H2-Cu-H2-. By reference to the corrected third formal potential of the Cu-Cu-Cu trimer, the Coulombic repulsion for the introduction of the third electron may thus be estimated as 131 meV, about the same as it is with H ~ - H ~ - H zand Cu-Cu-Cu and, as expected, twice what it is in the three dimers. It may be noted that the interactions between the negative charges in the H2- and Cu-porphyrin rings, attributed to Coulombic repulsion, are not exactly the same in all investigated compounds. The differences that appear might be rationalized in terms of interactions between the charges and the polarizable NHCO dipoles that are present in theurea bridges and the outside basket-handle straps. They are, however, too small to warrant a detailed analysis. Spectroelectrochemistry. The Cu-Cu dimer was taken as an example for a detailed analysis of thin-layer spectrcelectrochemical behavior in the visiblenear-infrared region (up to 2000 nm) in an attempt to see whether or not the mixed-valent compound displays an intervalence-transferabsorption band as previously observed in the near-infrared region with other intervalent compounds.22Electrolysis at successive potentials allowed a clear observation, upon reduction of the Cu-Cu dimer as well as reoxidation of its dianion, of the spectrum of the mixed-valent complex. No band characteristic of an intervalence-transfer absorption appeared in the near-infrared region, in agreement with the fact, revealed by the cyclic voltammetric study, that the interactions between the cofacial porphyrins are weak. The UV-visible spectra of the starting dimer and trimer porphyrins revealed some changes as compared to those of the

+

+

El-Kasmi et al.

TABLE III: Soret Bands in the Monomers, Dimers, and Trimers ~~~~

~

porphyrin

cu cu-cu cu-cu-cu

~

~

~

X,,,(Soret) (nm)

porphyrin

X,,,(Soret) (nm)

417 412 409

H2 HrH2 Hz-H2-H2

422 417 410

monomers, namely, a small blue shift of the Soret band (Table 111) consistent with weak exitonii interactions also revealed by recent fluorescence and phosphorescence m e a s ~ r e m e n t on s ~ the ~ same free base dimers and trimers as those investigated here.

Experimental Section Chemicals. Solvents and Supporting Electrolytes. 1,2Dichloroethane (Carlo Erba) was distilled over P205 before use. DMF (Merck) was vacuum distilled before use. NBu4PFs was used as supporting electrolyte throughout the present work. It was from commercialorigin (Fluka purum) and was recrystallized twice in 1,Zdichloroethane and dried in vacuum at 50 ‘C before use. Porphyrins. The monomers were prepared and characterized according to previously described procedure^.^^ The dimers and trimers were synthesized as described in ref 25. Instrumentation. The cells, instruments, and procedures used for cyclic voltammetry and thin-layer UV-vis spectroelectrochemistry were the same as those previously described.26 The UV-vis-near-IR spectra were recorded on a Varian 2300 spectrophotometer. The working electrodes were a 3-mmdiameter glassy carbon disk in cyclic voltammetry and a 2.4-cm2 platinum grid in spectroelectrochemistry. ConclWioM The main conclusion that emerges from the above results is that the mixed-valent states that appear in all dimers and trimers that we have investigated are related to Coulombic repulsion between the negativecharges introduced successivelyin thevarious parallel porphyrin rings. The magnitude of the Coulombic repulsion is greatly reduced by the electrostatic interaction between the negativelycharged porphyrins and thecounter cations. The remaining Coulombic repulsion is rather weak, of the order of 6&80 meV between adjacent porphyrin rings in the dimers and trimers and of 15-20 meV between the outside rings in the trimers. That the electrons introduced successively in the porphyrin rings upon reduction of the substratesare strictly bound to each porphyrin ring is confirmed by the absence of intervalencetransfer absorption band in the mixed-valent dimers.

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