Electrosynthesis and characterization of symmetrical and unsymmetrical linear porphyrin dimers and their precursor monomers

Article abstract of DOI:10.1021/ja9523956

The electrochemical synthesis of linear symmetrical octaethylporphyrin-octaethylporphyrin and unsymmetrical tetraphenylporphyrin-octaethylporphyrin dimeric porphyrins with one viologen as a spacer and of their precursor porphyrin monomers is reported. The mechanism of the electrochemical substitution leading to the dimeric porphyrins is discussed. These new compounds have been characterized by ¹H NMR, UV-vis spectroscopy, FAB mass spectroscopy, and microanalysis. Electrochemical data are reported, and the redox behavior is analyzed for the monomers and the dimers in terms of π-π interactions between the two porphyrin centers. ESR measurements were carried out on the unsymmetrical Cu-Zn and Cu-Cu dimeric metalloporphyrins which displayed intermolecular metal interactions.

Full text of DOI:10.1021/ja9523956

J. Am. Chem. Soc. 1996, 118, 2969-2979
2969
Electrosynthesis and Characterization of Symmetrical and
Unsymmetrical Linear Porphyrin Dimers and Their Precursor
Monomers
A. Giraudeau, L. Ruhlmann, L. El Kahef, and M. Gross*
Contribution from the Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, URA
au CNRS No. 405, UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, F-67000 Strasbourg, France
ReceiVed July 19, 1995. ReVised Manuscript ReceiVed January 29, 1996X
Abstract: The electrochemical synthesis of linear symmetrical octaethylporphyrin-octaethylporphyrin and unsym-
metrical tetraphenylporphyrin-octaethylporphyrin dimeric porphyrins with one viologen as a spacer and of their
precursor porphyrin monomers is reported. The mechanism of the electrochemical substitution leading to the dimeric
porphyrins is discussed. These new compounds have been characterized by 1H NMR, UV-vis spectroscopy, FAB
mass spectroscopy, and microanalysis. Electrochemical data are reported, and the redox behavior is analyzed for
the monomers and the dimers in terms of π-π interactions between the two porphyrin centers. ESR measurements
were carried out on the unsymmetrical Cu-Zn and Cu-Cu dimeric metalloporphyrins which displayed intermolecular
metal interactions.
Introduction
acceptor were covalently linked through a rigid spacer, the
electron transfer rates were enhanced via through-bond coupling.
Second, the decrease of the electron transfer rate with the
distance between the donor and the acceptor was more important
with aliphatic than aromatic spacers. For instance, poly-
(phenylene)-bridged bisporphyrin adducts5 exhibited slower
intramolecular electron transfer rates which depended little on
the increasing distance between the donor-acceptor pairs but
which reflected, instead, disruption of electronic coupling by
the twist angles at each phenyl-porphyrin connection.6 Im-
portant information on the electronic interactions between donor
and acceptor moieties in polyporphyrin molecules7,8 emerged
from correlations between their redox and optical properties.
L’Her, Collman, et al.9 demonstrated that the coupling of two
porphyrins in a cofacial configuration induced a “cofacial effect”
which originated in strong interactions between the two
π-electron ring systems. These authors were able to modulate
the extent of this “cofacial effect” by modifying the chemical
Many chemical models have been built in the past years
aimed at mimicking natural photosynthetic systems.1 Among
these, polynuclear porphyrins and phthalocyanines attracted
considerable interest owing to the wide diversity of their electron
transfer characteristics in relation with their chemical structure.2
Of these characteristics, electronic coupling between the mono-
meric subunits in supramolecules is of major interest because
of its relevance to intramolecular electron transfer in donor-
acceptor couples.3 The effects of the intervening matrix on this
electronic coupling have been scrutinized4 in molecules incor-
porating both electron donor and electron acceptor sites, and
consistent trends emerged. First, when the donor and the
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
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0002-7863/96/1518-2969$12.00/0 © 1996 American Chemical Society

2970 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Giraudeau et al.
Figure 1. Porphyrins and bisporphyrins investigated in this study.
nature and length of the linking bridge. This modulation was
monitored via the gradual uncoupling of the oxidation and
reduction steps characteristic of the porphyrins in the dimer and
also through changes in the UV-vis absorption spectra observed
upon generation of cation radicals. In other porphyrin dimers,
Sessler et al.3d ascribed shifts and/or splittings of the UV-vis
bands to excitonic interactions between the porphyrin π
systems: the effects being more pronounced in gabled than in
linear dimers. The general trend among the various models
studied was that interactions between two porphyrins subunits
of a given dimer induced shifts and/or splittings of the UV-
vis absorption bands.
Our interest in the electrochemical synthesis of porphyrin
dimers was prompted by studies on the reactivity10,11 of
porphyrin π-cation radicals with nucleophiles. It was observed12ab
that electrochemical oxidation of zinc tetraphenylporphyrin in
the presence of nucleophiles generated stable â-substituted
porphyrins. The yields of this simple one-pot reaction were
quite good (>60%). It was therefore reasonable to expect that
this method might also be effective for the synthesis of
bisporphyrins from porphyrin monomers and an appropriate
nucleophile as spacer. A Lewis base containing two distinct
nucleophilic sites was considered as a good spacer, on the
assumption that each of the two sites might react with one
porphyrin. Therefore, the electrochemical oxidation of meso-
tetraphenylporphyrin (TPP) or octaethylporphyrin (OEP) was
carried out in the presence of 4,4′-bipyridine.
This reaction produced first â- or meso-substituted porphyrins
for TPPs and OEPs, respectively. These resulting monomeric
porphyrins, bearing 4,4′-bipyridinium as substituent, were, in
turn, the precursors in the synthesis of the porphyrin dimers. In
the next phase, the precursors also reacted as nucleophiles with
electrogenerated oxidized porphyrins to generate the expected
bisporphyrins.
This paper reports the electrosynthesis of symmetric (OEP-
OEP) and asymmetric (OEP-TPP) linear porphyrin dimers. The
mechanism of this reaction is discussed. The redox character-
istics of the porphyrin dimers and their UV-vis absorption
spectra have been analyzed to identify the interactions between
the two porphyrin moieties in the dimer. Proton NMR spectra
allowed unambiguous assignment of the different substitutions
and showed rotation between the two porphyrin rings in the
bisporphyrins. An asymmetric Cu-Cu dimer was also prepared
to provide a good ESR sensor for detecting metal-metal
interactions. The porphyrins investigated are illustrated in
Figure 1.
(10) (a) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; Vol. 5, Chapter 3. (b) Dolphin, D.; Felton, R. H.
Acc. Chem. Res. 1974, 7, 26-32. (c) Stanienda, A. Z. Phys. Chem. 1964,
229, 259-266. (d) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92,
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103-107. (f) Spaulding, L. O.; Eller, P. G.; Bertrand, J. A.; Felton, R. H.
J. Am. Chem. Soc. 1974, 96, 982-987.
Electrosynthesis
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Felton, R. H. Ann. N. Y. Acad. Sci. 1973, 206, 117-119. (b) Barnett, G.
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4012. (d) Padilla, A. G.; Wu, S. M.; Shine, H. J. J. Chem. Soc., Chem.
Commun. 1976, 236-237. (e) Shine, H. J.; Padilla, A. G.; Wu, S. M. J.
Org. Chem. 1979, 44, 4069-4075. (f) Rachlewicz, K.; Latos-Grazynski,
L. Inorg. Chem. 1995, 34, 718-727.
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Soc., Chem. Commun. 1986, 621-622. (b) Giraudeau, A.; El Kahef, L.
Can. J. Chem. 1991, 69, 1161-1165. (c) El Kahef, L.; Gross, M.; Giraudeau,
A. J. Chem. Soc., Chem. Commun. 1989, 963.
Precursor Monomers. 1. (H2TPP-â-4,4′-bpy)+ClO4- (3-
H2). The first oxidation step of H2TPP (1-H2) was a reversible13
one-electron reaction in CH3CN:1,2-C2H4Cl2 (1:4) (Figure 2),
and it became an irreversible two-electron process in the
presence of the nucleophile pyridine. Under the latter condi-
tions, exhaustive electrooxidation generated the corresponding
â-substituted porphyrin 2-H2.12c
(13) Stanienda, A. Z. Naturforsch. 1968, 23b, 147-152.

Linear Porphyrin Dimers
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2971
of ZnOEP.14 The product of the electrolysis was isolated and
characterized as the meso-substituted zinc porphyrin (ZnOEP-
meso-py)+ClO4-(5-Zn), with 71% yield.
3. (ZnOEP-meso-4,4′-bpy)+ClO4- (6-Zn). Stationary and
cyclic voltammetry revealed identical characteristics for the
ZnOEP oxidation in the presence of 4,4′-bipyridine and pyridine.
After electrolyzing for 48 h, the initial pink solution turned violet
and finally yellow-brown. The purified (ZnOEP-meso-4,4′-
-
bpy)+ClO4 (6-Zn) was obtained with 78% yield, the global
reaction being as follows:
(2)
In all cases, protons are liberated during the course of the
reaction and should be trapped by the added Lewis base to
prevent demetalation of the metalloporphyrin. The minimum
required is 2 equiv of the Lewis base. When oxidized the free
base H2TPP protonated easily, and therefore the Lewis base was
added in a large excess to prevent formation of inactive H4-
TPP2+. When the porphyrin P is oxidized in the presence of
4,4′-bipyridine, the reaction is limited to the formation of the
monomeric P-bpy+ because the 4,4′-bipyridine present in
solution acted as a stronger nucleophile than the generated
P-bpy+.
Figure 2. Cyclic voltammetry changes observed in the first-oxidation
step of H2TPP after addition of pyridine. CH3CN + 1,2-C2H4Cl2 (1/4)
+ 0.1 M TEAP. V ) 100 mV s-1. Working electrode: Pt. Relative
concentrations [H2TPP]/[pyridine]: 1/0-1/4.
Similar results were obtained in the oxidation of H2TPP in
the presence of 4,4′-bipyridine as nucleophile. In a typical
experiment, 30 mg (0.049 mmol) of H2TPP (1-H2) and 150 mg
(0.960 mmol) of 4,4′-bipyridine were introduced in 25 mL of
the mixed solvent CH3CN:CHCl3 (1:4) containing 0.1 M TEAP
(tetraethylammonium perchlorate). The working electrode
potential was held constant at 1.05 V/SCE, and the initial violet
solution turned increasingly yellow-brown. During the elec-
trolysis, the solution was monitored by UV-vis spectropho-
tometry. The decrease of the oxidation current was not
exponential with time. When this current reached the residual
current measured in the absence of electroactive material,
typically after 3 h, in the above conditions, the number of
electrons transferred was 2.2/molecule of H2TPP. After extrac-
tion and purification (see the Experimental Section), the
â-substituted porphyrin (H2TPP-â-4,4′-bpy)+ClO4- (3-H2) was
characterized, and the yield was 94% for the following global
reaction:
-
Dimers. 1. (H2TPP-â-4,4′-bpy-meso-ZnOEP)2+2ClO4
(7-H2-Zn). The â-substituted free base porphyrin 3-H2 reacted
as nucleophile toward electrochemically generated ZnOEP•+
cation radicals. ZnOEP (4-Zn) was choosen here because its
oxidation potential is lower (E1/2 ) 0.68 V/SCE) than that of
the â-substituted free base 3-H2 (E1/2 ) 1.25 V/SCE), and
therefore the electrogeneration of ZnOEP•+ was possible without
oxidation of 3-H2. After electrolyzing 4-Zn for 48 h in the
presence of 3-H2, the dimer 7-H2-Zn, namely, (H2TPP-â-4,4′-
bpy-meso-ZnOEP)2+2ClO4-, was obtained with 73% yield,
according to the following global reaction scheme:
(3)
2. (ZnOEP-meso-4,4′-bpy-meso-ZnOEP)2+2ClO4- (8-Zn-
Zn). Because the difference between the oxidation potentials
of 4-Zn and of 6-Zn is 230 mV (Table 1), the electrolysis
potential had to be held strictly constant at 0.70 V/SCE, which
is as low as possible to avoid the first oxidation of the
nucleophile 6-Zn (E1/2 ) 0.91 V/SCE) and nevertheless to
generate ZnOEP•+ at an appreciable rate from 4-Zn (E1/2 ) 0.68
V/SCE, Table 1). After 72 h of electrolysis, the dimer 8-Zn-
Zn was obtained with 68% yield, according to the following
reaction scheme:
(1)
-
2. (ZnOEP-meso-py)+ClO4 (5-Zn). In a first set of
experiments, the electrochemical oxidation of ZnOEP was
monitored in the presence of the nucleophile pyridine. The
purpose was to determine whether the experimental conditions
used for the electrooxidative â-substitution on ZnTPP in the
presence of pyridine12a,b were also valid for meso-substitution
(14) (a) Callot, H. J.; Louati, A.; Gross, M. Tetrahedron Lett. 1980, 21,
3281-3284. (b) Callot, H. J.; Louati, A.; Gross, M. Bull. Soc. Chim. Fr.
1983, 317-320.

2972 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Giraudeau et al.
Table 1. Electrochemical Data for the Studied Porphyrins and Bisporphyrinsa
ring oxidation
ring reduction
III
II
I
I
II
III
porphyrin
E1/2
E1/2
E1/2
E1/2
E1/2
E1/2
H2TPP
ZnTPP
CuTPP
H2OEPd
ZnOEP
CuOEPd
1.34
1.16
1.19
1.30
0.94
1.19
1.10
0.80
0.79
0.81
0.68
0.79
-1.10
-1.37
-1.46
-1.46
-1.60
-1.46
-1.48
-1.75
-1.86
-1.40
py+ or 4,4′-bpy+ Reduction
2-H2
2-Zn
5-Zn
3-H2
3-Zn
6-Zn
1.19b,c
0.96
-0.96
-1.14c
-1.38
-1.60
-1.12
-1.26
-1.31
1.29
1.27
-1.06c
0.94
-1.10
1.25c
0.96
-0.73
1.25
-0.72
1.20c
0.91
-0.65
Viologen Reduction
7-H2-H2
7-H2-Zn
7-Zn-Zn
7-Cu-Zn
7-Cu-Cu
8-Zn-Zn
1.34c
1.22b
1.27b
1.18b
1.16
1.20b
0.93
0.95b
0.92
1.07
0.90b
-0.05
-0.06
-0.10
-0.08
-0.05
-0.06
-0.50
-0.59
-0.64
-0.63
-0.58
-0.63
-1.15
-1.18
-1.35
-1.32
-1.31
-1.65
-1.32b
-1.41
1.27c
-1.60
1.43
-1.67b
-1.47
1.42b
-1.66b
1.22b
a All potentials in V/SCE obtained from stationary (RDE) voltammetry in CH3CN/1,2-C2H4Cl2 (1:4) and 0.1 M TEAP. Working electrode: Pt.
Potential values in italics were observed for TPP; normal characters for OEP and underlined potentials were observed in both OEP and TPP
systems. All steps were reversible one-electron transfers except those with b and c superscripts. b Two-electron transfer. c Irreversible transfer.
d Reference 34b.
result from an ECNECB process, and side-chain substitutions
result from an ECBECN scheme.16 With the other substrates
the distinction is not as readily apparent.
In the presence of nucleophiles like pyridine, ZnOEP and
ZnTPP exhibited similar first-oxidation curves in stationary and
cyclic voltammetry. The free base H2TPP exhibited a specific,
(4)
distinct behavior. Therefore the first oxidation of H2TPP and
ZnTPP was studied, and pyridine was selected as nucleophile
because of the easy availability of its derivatives. The only
effect of added pyridine on the oxidation of ZnTPP was that
the first-oxidation potential, which corresponded to the genera-
tion of the radical monocation, was shifted toward more negative
potentials,17 as expected from the axial coordination of pyridine
on the central metal. In order to establish the reaction
mechanism of the oxidative nucleophilic substitution, a series
of exhaustive electrolytic oxidations were therefore carried out
on ZnTPP (first oxidation) in the presence of several substituted
pyridines18 displaying a wide range of Lewis basicities. In the
presence of 3-picoline (3-pic) and 3,5-lutidine (3,5-lut), the
oxidation products were â-substituted porphyrins (ZnTPP-â-3-
pic)+ and (ZnTPP-â-3,5-lut)+ and required 2 Faradays/mol of
substituted porphyrin. In the presence of 2-picoline, 2,6-lutidine,
3-bromopyridine, or 4-cyanopyridine, the cation radical ZnT-
PP•+ 19 was the sole product of the oxidation. These experi-
ments indicated clearly that, with nucleophiles whose pKa was
close to that of the pyridine, the nucleophilic substitution did
not occur when the ortho carbons (2 or 6) of the pyridine were
sterically crowded or when the pKa of the nucleophile was low.
Selective Metalation of the Unsymmetrical Dimer TPP-
OEP (7-H2-H2). This dimer was obtained by demetalation of
7-H2-Zn with HCl/acetone. Zn was incorporated into the
â-substituted free base 3-H2 by the usual reaction of zinc acetate
with free base in methanol/chloroform solution to yield 3-Zn.
Similarly, symmetrical (homo-dimetalated) 7-Zn-Zn was ob-
tained by reaction of 7-H2-H2 free base with excess zinc acetate,
and 7-Cu-Cu with excess copper acetate. As well, the hetero-
dimetalated 7-CuII-Zn bisporphyrin was obtained by reaction
of 7-H2-Zn with 1.1 equiv of CuII(CH3COO)2‚2H2O in CH2Cl2/
CH3OH. The acetate counterions of the metalloporphyrins were
converted to perchlorate by treatment with tetraethylammonium
perchlorate in order to have a homogeneous series of complexes.
Reaction Mechanisms
Previous results12b on TPP have established that the substitu-
tion of a nucleophile (Nu) on a porphyrin (P1) was achieved in
a two-electron reaction involving two chemical steps, a depro-
tonation and a nucleophilic substitution:
(15) Adams, R. N. Electrochemistry at Solid Electrodes; M. Dekker:
New York, 1969; Chapter 10.
Eo1x
P1 + 2Nu 8 P1-â-Nu+ + 2e- + NuH+
(16) Mechanism is ECNECB when the initially formed cation radical
R
•+ is captured by a nucleophile, and ring substitution is the ultimate result.
Mechanism is ECBECN when R•+ transfers first a proton to a base, and the
ultimate result is side-chain substitution.
In the present electrosynthesis of the precursor monomers and
dimers, this reaction was validated as a global reaction scheme.
Because a proper understanding of the mechanism of these
oxidations allows the reactions conditions to be tailored so as
to obtain specific products in both a useful and practical manner,
we attempted to understand the mechanism in detail. With the
alkyl-substituted benzenes, the mechanism15 is apparent from
the nature of the products obtained. Nucleophilic substitutions
(17) (a) Manassen, J. Isr. J. Chem. 1974, 12, 1059-1067. (b) Giraudeau,
A.; Gross, M.; Callot, H. J. Electrochim. Acta 1981, 26, 1839-1843. (c)
Kadish, K. M.; Shine, L. R.; Rhodes, R. K.; Bottomley, L. A. Inorg. Chem.
1981, 20, 1274-1277. (d) Kadish, K. M.; Lin, X. Q.; Han, B. C. Inorg.
Chem. 1987, 26, 4161-4167.
(18) (a) Schoefield, K. S. Hetero-Aromatic Nitrogen Compounds; Plenum
Press: New York, 1967; p 146. (b) Handbook of Chemistry and Physics,
67th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1986-1987; D.
159, 160.

Linear Porphyrin Dimers
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2973
They suggested therefore that the first chemical reaction was a
nucleophilic attack rather than a deprotonation.
At variance with ZnTPP, the first-electrochemical oxidation
curves of H2TPP were modified in the presence of Lewis base
nucleophiles.20 When increasing amounts of pyridine were
added to a solution of H2TPP, the first-oxidation wave, observed
by stationary voltammetry, increased at the expense of the
second-oxidation wave. Ultimately, the first-oxidation wave
exhibited a limiting current that was 2 times the current in the
absence of pyridine, while the second-oxidation wave disap-
peared, at a pyridine concentration 4 times that of H2TPP.
Consistent with this result, the first-oxidation peak in cyclic
voltammetry changed from a one-electron reversible signal to
an irreversible one, while the peak current doubled and the
second-oxidation peak disappeared (Figure 2). The method
allowing this discrimination between an ECN ECB or ECB ECN
scheme was described by Eberson and Parker.21 It was based
on monitoring the disappearance of the reduction peak of the
cation radical P•+ upon addition of several pyridines having
similar basicities but differing by the size and the positions of
the substituents. The experimental scheme was as follows: All
other experimental parameters being held constant, a first set
of experiments established the concentration of pyridine (Cpy)
that was necessary to eliminate the reduction peak of H2TPP•+.
Under the same experimental conditions, elimination of this
reduction peak required 4Cpy with 2,6-lutidine, 2.5Cpy with 2,4-
lutidine, and only 2Cpy with 3,5-lutidine. These results clearly
show that a nucleophilic attack is the primary chemical step
and not an acid-base reaction.
Figure 3. Effects of adding excess pyridine on the cyclic voltammo-
gram of H2TPP. CH3CN + 1,2-C2H4Cl2 (1/4) + 0.1 M TEAP. (1)
H2TPP, (2) H2TPP + excess pyridine, after several cycles. V ) 100
1
mV s-1. Working electrode: Pt. (a) py-H+ + e- a py + /2H2, (b)
1/2H2 a H+ + e-, (c) Reference 25.
the presence of pyridine. Two types of π-cation radicals have
been reported for metalloporphyrins.10b,f Large spin density at
the bridging meso-carbon atoms and small spin density at the
pyrrole carbon atoms are characteristic of a2u radicals, while
a1u radicals have nodes at the meso-carbons. Hence â-substitu-
tion is expected with a2u radicals and meso-substitution with
a1u radicals as observed, respectively, with TPP and OEP
π-cation radicals. Dolphin et al.24 have also performed such
â-substitutions with the cation radical ZnTPP•+ and the dication
ZnTPP2+. In the experiments reported here, electrogeneration
of the dication from ZnTPP in the presence of pyridine did not
produce such substitutions: Only an isoporphyrin was obtained,
as previously reported by Kadish et al.23 This isoporphyrin was
characterized by its UV-vis absorption spectrum.25 Results by
Shine11e indicated that nitrite anions (NO2-) reacted on both
the â- and the meso-positions of cation radical ZnTPP•+. Recent
work11f demonstrated that pyridine â-substitutions occurred in
iron tetraphenylporphyrin π-cation radical to give the stable
â-substituted porphyrin and also the unstable isoporphyrin
generated by attack of the pyridine at the meso- carbon. In our
experiments, the presence of peak c on the CV curves (Figure
3) could be explained by reduction of such an isoporphyrin.
Similar reductions steps25b,c characterized the redox chemistry
of isoporphyrins. In these electrochemical substitutions carried
out on tetraphenylporphyrins, the two pathways (â- and meso-
attack) would occur, but only the â-attack leads to a stable
configuration. This diverse reactivity of pyridine nucleophiles
with porphyrin cation radicals may be rationalized on the basis
of the views developed by Abrahamovitch et al.,26 on the effects
of the nucleophilic character of the reactant on to the reaction
orientation. On this basis, it was expected that, in the presence
of strong nucleophiles like pyridine, the electron densities at
each carbon of the porphyrin cation radical determined the
substitution site. In contrast, with weak nucleophiles, the
These results led to the following proposed mechanism:
MTPP f MTPP•+ + e-
(E1)
MTPP•+ + Nu f (MTPP-â-Nu)•+
(CN)
(E2)
(MTPP-â-Nu)•+ f (MTPP-â-Nu)2+ + e-
(MTPP-â-Nu)2+ f (MTPP-â-Nu)+ + H+ (CB)
When cyclic voltammetry was carried out on H2TPP, new peaks
were observed after addition of pyridine (Figure 3). Peak a at
-0.38 V/SCE was ascribed22 to the reduction of the cation py-
H+ to py and 1/2H2, while peak b at -0.25 V/SCE was assumed
to correspond to the oxidation of the molecular hydrogen
generated through peak a in the presence of a Lewis base. This
proposed interpretation of peaks a and b implied that the pyridine
nucleophile not only reacted with the porphyrin radical cation
but that it was also acting as a Bro¨nsted base in solution. When
the rate of the nucleophilic reaction CN was slow enough, only
the first electron transfer was observed23 on the CV or SV
curves, as, for instance, with ZnTPP. The faster nucleophilic
reaction (CN) on the free base cation radical H2TPP•+ may be
explained by the higher reactivity of this radical. With ZnOEP,
the reaction mechanism was the same (E1CNE2CB) as with
ZnTPP, the only difference being that the substitution occurred
at the meso-carbons of the porphyrin.
(24) Dolphin, D.; Halko, D. J.; Johnson, E. C.; Rousseau, K. In Porphyrin
Chemistry AdVances; Longo, F. R., Ed.; Ann Arbor Science: MI, 1979;
Chapter 10.
These results were consistent with those observed by Shine11d,e
and Smith11b,c on the cation radicals ZnTPP•+ and ZnOEP•+ in
(25) (a) Dolphin, D.; Felton, R. H.; Borg, D. C.; Fajer, J. J. Am. Chem.
Soc. 1970, 92, 743-745. (b) Hinman, A. S.; Pavelich, B. J.; Pons, S.; Kondo,
A. E. J. Electroanal. Chem. 1987, 234, 145-162. (c) El-Kasmi, A.; Lexa,
D.; Maillard, P.; Momenteau, M.; Saveant, J. M. J. Am. Chem. Soc. 1991,
113, 1586-1595.
(26) Abrahamovitch, R. A.; Sara, J. G. In AdVances in Heterocyclic
Chemistry; Katritzky, A. R., Boulton, A. J., Eds.; Academic Press: New
York, 1966; Vol. 6.
(19) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J.
Am. Chem. Soc. 1970, 92, 3451-3459.
(20) Lexa, D.; Reix, M. J. Chem. Phys. 1974, 71, 511-516.
(21) Parker, V. D.; Eberson, L. Tetrahedron Lett. 1969, 33, 2839-2842.
(22) Cauquis, G.; Cros, J. L.; Genies, M. Bull. Soc. Chim. Fr. 1971,
3765-3772.
(23) Kadish, K. M.; Rhodes, R. K. Inorg. Chem. 1981, 20, 2961-2966.

2974 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Giraudeau et al.
Figure 5. UV-vis absorption spectra of the bisporphyrin 8-Zn-Zn
and the corresponding subunit 6-Zn in CH2Cl2.
Proton NMR Spectra
Figure 4. UV-vis absorption spectra of the bisporphyrin 7-H2-Zn and
its monomeric subunits 3-H2 and 6-Zn in CH2Cl2.
Monomers. In the monomeric porphyrin 3-H2, the pyrrolic
protons all appeared down field from those of the phenyl ring,
thereby indicating that the aromaticity of the porphyrin ring was
not interrupted.11e One of the â-proton signals appeared as a
singlet at field 9.43 ppm. It corresponded to the proton adjacent
to the bipyridinium substituent. The other â-proton signals
appeared as two singlets at 9.08 (2H) and 8.96 (1H) ppm plus
a multiplet (3H) in the region 8.83-8.77 ppm.
The signals for the phenyl protons were identifiable as ortho,
meta, or para on the basis of earlier results11c,e or reports on
pyridinium-substituted cations. As expected, the internal N-H
protons appeared as a singlet at higher field, -2.67 ppm.
The bipyridinium protons appeared at low field, respectively,
as two doublets and doubled doublets. With the corresponding
zinc-metalated monomer 3-Zn, the typical adjacent â-H proton
signal appeared again at low field (9.30 ppm) as a signature
characterizing the mono-â-substitution. The other â-H and
bipyridinium proton signals were not as clearly separated as
with the unmetalated monomer. All the phenyl protons appeared
here as a complex multiplet in the range of 7.99-7.08 ppm.
substitution site may be determined by the localization energies,
and therefore the meso-positions are favored as compared with
the â-sites.
UV-Vis Absorption Spectra. Spectra for some of the
porphyrins studied here are shown in Figures 4 and 5. Complete
UV-vis data are reported in the Experimental Section for the
electrosynthesized monomeric and dimeric porphyrins.
Monomers. All the porphyrins synthesized exhibited a red
shift of the Soret (B) and visible (Q) bands compared to the
corresponding unsubstituted porphyrins. This bathochromic
shift resulted from the electron-withdrawing effects27,28 of the
meso- or â-substituent on the porphyrin. The shifts of the Soret
bands were comparable (about 15 nm) for â- and meso-
substitutions. However, the bathochromic shift of the Q bands
was slightly larger with 4,4′-bpy+ than with py+ as expected
from the slightly more electron-withdrawing character of 4,4′-
bpy+.
Dimers. Their UV-vis absorption spectra (Figure 4) were
the mere addition of the spectra of the corresponding monomeric
subunits. However, the Soret band of the symmetrical dimer
8-Zn-Zn showed significant broadening (Figure 5), and its
extinction coefficient was ca. one-half of that expected. This
result suggested excitonic interactions between the two por-
phyrin rings in this dimer. Similar spectral effects in dimeric
or trimeric porphyrins29 have been previously reported and
analyzed3d as dependent on the interporphyrin angles, the
direction of the transition dipole moments in the monomer
subunits,30 and also the distances between porphyrins.
With the monomers 5-Zn and 6-Zn prepared from ZnOEP,
1H NMR observation of two different types of meso-H signals
in the range of 10.44-10.34 ppm was fully consistent with the
(29) (a) Sessler, J. L.; Capuano, V. L. Tetrahedron Lett. 1993, 34, 2287-
2290. (b) Osuka, A.; Ikawa, Y.; Maruyama, K. Bull. Chem. Soc. Jpn. 1992,
65, 3322-3330. (c) Wagner, R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994,
116, 9759-9760. (d) Prathapan, S.; Johnson, T. E.; Lindsey, J. S. J. Am.
Chem. Soc. 1993, 115, 7519-7520. (e) Seth, J.; Palaniappan, V.; Johnson,
T. E.; Prathapan, S.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1994,
116, 10578-10592.
(30) (a) Tabushi, I.; Sasaki, T. J. Am. Chem. Soc. 1983, 105, 2901-
2902. (b) Tabushi, I.; Kugimiya, S. I.; Kimmaird, M. G.; Sasaki, T. J. Am.
Chem. Soc. 1983, 107, 4192-4199. (c) Sessler, J. L.; Hugdahl, J.; Johnson,
M. R. J. Org. Chem. 1986, 51, 2838-2840. (d) Won, Y.; Friesner, R. A.;
Johnson, M. R.; Sessler, J. L. Photosynth. Res. 1989, 22, 201-210. (e)
Sessler, J. L.; Capuano, V. L.; Harriman, A. J. Am. Chem. Soc. 1993, 115,
4618-4628.
(27) Hirota, J.; Okura, I. J. Phys. Chem. 1993, 97, 6867-6870.
(28) Giraudeau, A.; Callot, H. J.; Jordan, J.; Ezahr, I.; Gross, M. J. Am.
Chem. Soc. 1979, 101, 3857-3862.

Linear Porphyrin Dimers
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2975
Figure 6. 1H NMR of the bisporphyrin 7-H2-Zn (400 MHz, (CD3)2CO), aromatic region of the spectrum.
proposed meso-substitution.31 These two singlets which inte-
grated in the ratio 1:2 or 2:1 appear to be diagnostic of
transannular monosubstitution on a meso-position. The ethyl
proton signals gave a well-separated set of three distinct quartets
and triplets in the range 4.21-1.39 ppm. At variance with the
expected effect of the electron-withdrawing bpy+ or py+
substituents, the protons belonging to the two ethyl groups
adjacent to these substituents appeared at higher field than the
protons of the other ethyl groups. This result was obtained with
all octaethylporphyrins substituted by bpy+ and py+. It might
be explained by assuming that the protons of the two adjacent
ethyl groups underwent a significant shielding. This shielding
originated in the positions of these protons, which were located
inside the shielding cone of the pyridinic substituent. Thus, in
every case the proton nearest the substituent resonated at higher
field than the other ones. The chemical shifts induced by the
positive charge were similar with the bipyridinium and pyri-
1
Figure 7. N-H proton signals in the H NMR spectra of 3-H2, 7-H2-
H2, and 7-H2-Zn (400 MHz, (CD3)2CO).
dinium substituents; their proton signals appeared at low field
as in the monomer 3-H2.
Dimers. The three unsymmetrical dimers 7-H2-Zn (Figure
equivalent of twice the corresponding signal of the monomer.
This reflected the complete longitudinal symmetry in the
molecule. As expected, the four viologen proton doublets of
the monomer merged now into two doublets at low field.
1
6), 7-H2-H2 and 7-Zn-Zn gave very well-resolved H NMR
1
spectra. The observed H NMR signals could be analyzed in
two sets: The first one corresponded to the porphyrinic protons
and the second one to the viologen protons. As shown with
7-H2-Zn and 7-Zn-Zn complexes, the porphyrinic proton signals
could be fairly well described as the addition of those of the
corresponding monomers 3-H2, 3-Zn, and 6-Zn. Only slight
chemical shifts were observed. As shown in Figure 7 with the
dimers 7-H2-Zn and 7-H2-H2, the inner N-H proton signals
appeared as a singlet for the TPP subunit, and they split into a
well-resolved pair for the OEP subunit. This result suggested
that N-H tautomerization was slow in OEP and faster in TPP
on the NMR time scale. Such observations have been well
documented previously.32 As seen with the monomers, the
viologen proton signals appeared again as four doublets.
However, the doublets observed at the higher fields with the
singly charged bipyridinium in the monomers appeared now at
lower fields due to the double positive charge on the spacer.
1
Surprisingly, the H NMR dimer spectra recorded at 300 K
with dilute solutions (10-4-10-3 M) did not show any viologen
proton signals nor any signal of the â-H proton adjacent to the
viologen. Only in concentrated (10-3-10-2 M) or saturated
solutions are their resonances observed. This lack of signals
was examined by variable temperature experiments recorded
on diluted solutions and illustrated for the dimer 7-H2-Zn in
1
Figure 8. At low temperature, the H NMR spectrum was
consistent with the spectrum recorded at room temperature in
concentrated solutions. All the viologen protons appeared as
four doublets. When the sample was warmed, complete
flattening was observed in the temperature range 270-280 K.
The viologen subunit was conjugated with only one conforma-
tional degree of freedom, and the geometry of the molecule
could be described, as a first approximation, by the angle
1
As clearly demonstrated with the meso-proton signals, the H
(32) (a) Nagata, T. Bull. Chem. Soc. Jpn. 1991, 64, 3005-3016. (b)
Abraham, R. J.; Hawkes, G. E.; Smith, K. M. Tetrahedron Lett. 1974, 16,
1483. (c) Hobbs, J. D.; Majumder, S. A.; Luo, L.; Sickelsmith, G. A.; Quirke,
J. M. E.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc.
1994, 116, 3261-3270.
NMR spectrum of the symmetrical dimer 8-Zn-Zn was the
(31) Smith, K. M.; Bobe, F. W.; Minnetian, O. M.; Abraham, R. J.
Tetrahedron 1984, 40, 3263-3272.

2976 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Giraudeau et al.
Figure 8. VT-1H NMR study of 7-H2-Zn at 400 MHz. Solvent:
(CD3)2CO; 300-210 K.
between the two porphyrin planes. Recent results6,33 have
shown that, in 4,4′-bipyridine complexes, rotation from a planar
to a perpendicular conformation causes a decrease of the π-π-
electronic coupling between the two porphyrin subunits. Thus,
the observed pseudocoalescence might result from a thermally
activated rotation between the two porphyrin rings. It is
reasonable to speculate that the rotation would also be slowed
or hindered by intermolecular Coulombic interactions, which
are strongly dependent on the concentration in solution.
Electrochemistry
The electrochemical behavior of free base porphyrins and
metalloporphyrins involving nonelectroactive metals is well
documented.19,34 In the potential range of 2 to -2 V, the
oxidation and the reduction of the π ring proceed each via two
reversible one-electron steps generating radical cations and
dications or radical anions and dianions, respectively. The redox
behavior of pyridinium35 and viologen36 has also been studied
in many reports which consistently demonstrated that they were
reduced, respectively, in one or two distinct one-electron steps.
Electrochemical results are gathered in Table 1. The number
of exchanged electrons for each system was determined by
exhaustive Coulometry or comparison between the limiting
currents of the different waves characterizing a given system.
Figure 9. Cyclic voltammograms of 7-Zn-Zn, 7-H2-Zn, and 7-H2-H2
in CH3CN + 1,2-C2H4Cl2 (1/4) + 0.1 M TEAP. Scan rate: V ) 100
mV s-1. Working electrode: Pt.
(33) Gourdon, A. New J. Chem. 1992, 16, 953-957.
The assignment of the sites undergoing the electron transfers
was achieved by spectrophotoelectrochemical experiments or
comparison of the recorded voltammograms with those of well-
known parent compounds and by use of the usual criteria34b
relative to the redox behavior of the porphyrinic systems.
Typical cyclic voltammograms are presented in Figure 9.
Monomers. With the substituted monomers, the known
oxidations and reductions of the porphyrin were observed, plus
an additional one-electron transfer, corresponding to the reduc-
tion of the 4,4′-bpy+ or py+ substituent. The measured reduction
(34) (a) Fuhrhop, J. H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91,
4174-4181. (b) Fuhrhop, J. H.; Kadish, K. M.; Davis, D. G. J. Am. Chem.
Soc. 1973, 95, 5140-5147. (c) Wolberg, A.; Manassen, J. J. Am. Chem.
Soc. 1970, 92, 2982-2991. (d) Antipas, A.; Dolphin, D.; Gouterman, M.;
Johnson, E. C. J. Am. Chem. Soc. 1978, 100, 7705-7709.
(35) (a) Grimshaw, J.; Moore, S.; Thompson, N.; Trocha-Grimshaw, J.
J. Chem. Soc., Chem. Commun. 1983, 783-784. (b) Volke, J.; Urban, J.;
Volkeova, V. Electrochim. Acta 1992, 37, 2481-2490.
(36) (a) Elafsen, R. M.; Edsberg, R. L. Can. J. Chem. 1957, 35, 646-
650. (b) Osa, T.; Kuwana, T. J. Electroanal. Chem. 1969, 22, 389-406.
(c) Bard, A. J. AdV. Phys. Org. Chem. 1976, 13, 155-278. (d) Wu¨m, G.
S.; Berneth, H. Top. Curr. Chem. 1980, 92, 1-44.

Linear Porphyrin Dimers
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2977
potentials for these reductions, -1.1 V/SCE for 4,4′-bpy+ and
-0.65 V/SCE for py+, matched the usual reduction potentials
of these cationic species.
The presence of a substituting group induced some changes
in the parameters characterizing the electron transfers to or from
the porphyrin ring. The oxidation potentials shifted about 150
mV more positive than the oxidation of the corresponding
unsubstituted porphyrin. This shift may be ascribed to the
electron-withdrawing effect37 of the positively charged sub-
stituent on the π system of the porphyrin. The irreversible two-
electron transfer observed with the free bases 2-H2 and 3-H2
may be explained by the instability and the high reactivity of
the radical, resulting from the first electron transfer, which
therefore facilitated a second electron transfer.
Dimers. They exhibited three distinct electroactive compo-
nents, namely, the viologen spacer and the two lateral porphyrins
P1 and P2. The viologen (4,4′-bpy)2+ was reduced in two
distinct steps, each being a reversible one-electron reaction as
is well known for the viologens.36
As expected for viologen bearing electron-rich substituents,
these reductions occurred easily at low potentials. Meanwhile,
the porphyrins P1 and P2 remained unaffected by these electron
transfers. The measured potentials of the viologens in the series
of dimers also indicated that their first and second reductions
became more difficult when the net ring charge34b increased on
the substituting porphyrins P1 and P2: The reduction potentials
of the viologen became more negative in the sequence 7-H2-H2
< 7-Cu-Cu < 7-H2-Zn < 7-Cu-Zn < 7-Zn-Zn. This result
matched the sequence proposed by Fuhrhop and Kadish34b for
the electronegativity of porphyrins.
The electrochemical oxidations and reductions of the por-
phyrins P1 and P2 occurred on both sides of the electroactivity
range of the spacer. As a rule, these charge transfers on P1
and P2 were one-electron reversible processes. Some of them,
which appeared as reversible two-electron transfers, actually
resulted from the merging of two distinct one-electron steps.
In these porphyrins P1 and P2, the observed electron transfers
were easily assigned to the π systems of the OEP or TPP, based
on the potentials measured for the corresponding bipyridinium-
substituted monoporphyrins. This assignment was also achieved
by use of known general redox characteristics34,37-40 of the
porphyrins. The electrochemical oxidation of the porphyrin
dimers was monitored by spectroelectrochemistry (Figure 10).
The first oxidation revealed a decrease in the intensity of the
Soret band, as usually observed19,34,41 in the generation of
porphyrin cation radicals. The oxidized porphyrin reverted
quantitatively to the initial dimer on the reverse potential scan
(Figure 10a). The reduction reactions were more complex.
Although cyclic voltammetry indicated that the first reduction
was reversible, the electrogenerated species were not stable on
longer time scales. Spectroelectrochemistry indicated that the
first reduction produced radical anions and the expected
weakening of the Soret band. However, the return scan
Figure 10. (a) UV-vis absorption spectra recorded during the
oxidation of 7-Zn-Zn in CH3CN/1,2-C2H4Cl2 (1/4) and 0.1 M TEAP
at -1.1 V/SCE. (b) UV-vis absorption spectra of 7-Zn-Zn: (s)
unoxidized form, (- - -) two-electron-oxidized species (first wave,
0.95 V), and (‚‚‚) four-electron-oxidized species (second wave, 1.27
V).
generated only a fraction of the initial dimer. Thus, the dimers
seemed more stable in the oxidized state than in the reduced
state, as already observed for the monomers. The strikingly
close values observed between the reduction potentials of the
dimers and those of the monomers or the initial porphyrin
components could be expected because the reduction of the
dimers actually occurred on neutral molecules, the 4,4′-bpy2+
becoming neutral (via two one-electron reductions) before the
reductions of the porphyrins P1 and P2. The symmetrical dimer
8-Zn-Zn was oxidized via two steps each involving two electrons
and could be described as noninteracting redox centers42 under
the conditions of the experiment.
1
ESR. As reported above, H NMR results on concentrated
solutions of dimers indicated that hindered rotation was possible
for one porphyrin with respect to the second one in the dimers.
Hence, the existence of possible inter- or intramolecular
interactions in these dimers should be considered. We have
therefore synthesized porphyrin dimers containing one or two
divalent copper ions, namely, 7-Cu-Zn and 7-Cu-Cu. In such
molecules, the paramagnetic copper served as a good sensor
for the existence of metal-metal interactions. With this aim,
the magnetic susceptibility (ø ) f(T)) of the dimeric 7-Cu-Cu
solid was studied. The values of ø were obtained by double
integration of the ESR signal characteristics of Cu(II) in the
porphyrin dimer. Recent ESR results43 for a planar biscopperII
phthalocyanine yielded a 96 G peak-to-peak signal, which
(37) (a) Kadish, K. M.; Morrison, M. M. Inorg. Chem. 1976, 15, 980-
987. (b) Giraudeau, A.; Louati, A.; Gross, M.; Callot, H. J.; Hanson, L. K.;
Rhodes, R. K.; Kadish, K. M. Inorg. Chem. 1982, 21, 1581-1586.
(38) (a) Felton, R. H.; Linschitz, H. J. Am. Chem. Soc. 1966, 88, 1113-
1116. (b) Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2260-2266.
(39) (a) Kadish, K. M.; Davis, D. G. Ann. N. Y. Acad. Sci. 1973, 206,
495-502. (b) Kadish, K. M.; Morrison, M. M. J. Am. Chem. Soc. 1976,
98, 3326-3328.
(40) Giraudeau, A.; Callot, H. J.; Gross, M. Inorg. Chem. 1979, 18, 201-
206.
(41) (a) Malinski, T.; Chang, D.; Latour, J. M.; Marchon, J. C.; Gross,
M.; Giraudeau, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 3947-3944. (b)
Kadish, K. M.; Dubois, D.; Barbe, J. M.; Guilard, R. Inorg. Chem. 1991,
30, 4498-4501. (c) Kadish, K. M.; Han, B. C.; Endo, A. Inorg. Chem.
1991, 30, 4502-4506.
(42) Hammel, D.; Erk, P.; Schuler, B.; Heinze, J.; Mu¨llen, K. AdV. Mater.
1992, 11, 737-739.
(43) Lelie`vre, D.; Bosio, L.; Simon, J.; Andre´, J. J.; Bensebaa, F. J. Am.
Chem. Soc. 1992, 114, 4475-4479.

2978 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Giraudeau et al.
indicated a significant dipolar coupling. On the other hand,
the magnetic susceptibilities (ø) measured at various tempera-
tures revealed the existence of both ferromagnetic intermolecular
and antiferromagnetic intramolecular interactions.
recorded on the studied molecules consistently indicated that
their subunits (i.e., porphyrin(s) and spacer) were almost
noninteracting under the present experimental conditions.
The synthetic method reported here is the first method
allowing electrochemical synthesis of porphyrin dimers. It
opens a new avenue to the preparation of diverse porphyrin
derivatives, through appropriate selection of the porphyrin
subunits and the spacer. Hence, further studies now become
possible on ad-hoc assemblies of chromophoric subunits. Such
studies are of major interest for further understanding the
interactions and the charge transfers between these subunits and
ultimately for effectively replicating photosynthetic pathways
or creating new materials.
In the porphyrin dimer 7-Cu-Cu, the ESR spectra exhibited
a peak-to-peak Wpp ) 95 G, thus indicating the occurrence of
strong intermolecular dipolar coupling. The low value obtained
for g ()2.066) indicated that the spin-orbit coupling was higher
in 7-Cu-Cu than in the above-cited biscopperII phthalocyanine
(g ) 2.048). Hence, the unpaired electron was more localized
on the copper in 7-Cu-Cu than in the biscopper phthalocyanine.
The plot ø(T) showed that in the dimer 7-Cu-Cu the susceptibil-
ity obeyed a Curie-Weiss relationship (ø ) C/(T - Tc)) in
which Tc ) 7.1 K (Tc ) coupling temperature). Although the
peak-to-peak field broadened slowly with temperature (from 96
G at 300 K to 110 G at 4 K), this broadening indicated that a
ferromagnetic intermolecular interaction existed, although weak.
Actually, the exchange interaction energy was estimated as 0.6
meV (J ) kBTc for an assumed one-dimension cubic stacking).
The dimer 7-Cu-Zn was also studied by ESR under the same
conditions as 7-Cu-Cu. The obtained spectrum exhibited a
hyperfine structure due to the copper(II) (S ) 3/2). This
hyperfine structure resulted from a dilution effect of the copper
porphyrins, as the second metal (Zn) was diamagnetic in the
dimer. In this dimer, 7-Cu-Zn, the plot ø(T) revealed, as in the
dimer 7-Cu-Cu, a ferromagnetic interaction with Tc ) 4.8 K.
These ESR results obtained on solid dimers therefore
indicated the absence of intramolecular copper-copper interac-
tions (in 7-Cu-Cu) and the occurrence of a copper-copper
ferromagnetic coupling in both dimers 7-Cu-Zn and 7-Cu-Cu.
ESR experiments carried out with solutions of these dimers did
not produce additional information.
Experimental Section
Materials. All solvents and chemicals were of reagent grade quality,
purchased commercially, and used without further purification except
as noted below. CH2Cl2 and CHCl3, for use in exhaustive electrolysis,
were heated at reflux with and distilled from CaH2. The supporting
electrolyte tetraethylammonium perchlorate (TEAP) was purified as
previously described.28 Complex 4-Zn was purchased from Aldrich
Chemical Co. Thin layer chromatography (TLC) was performed on
commercially prepared alumina or silica gel plates purchased from Roth
Sochiel.
Apparatus. All electrochemical measurements were carried out
under argon. Voltammetric data were obtained with a standard three-
electrode system using a Bruker E 130 M potentiostat and a high-
impedance millivoltmeter (Minisis 6000, Tacussel). Current-potential
curves were obtained from an Ifelec If 3802 X-Y recorder. The working
electrode was a platinum disk (EDI type, Solea Tacussel) of 3.14 mm2
surface area. A platinum wire was used as the auxiliary electrode.
The reference electrode was a saturated calomel electrode (SCE) that
was electrically connected to the studied solution by a junction bridge
filled with the corresponding solvent-supporting electrolyte solution.
Coulometric measurements and quantitative electrochemical synthesis
were performed in either a standard 50 mL cell or a large 850 mL cell.
In the standard cell, the working electrode was a platinum wire (o.d.
) 0.8 mm) of 60 cm length. In the large cell, the working electrode
was a cylindric platinum grid (o.d. ) 5 cm, height ) 7 cm). For the
controlled-potential electrolysis, the anodic and cathodic compartments
were separated by a fritted glass disk to prevent diffusion of the
electrogenerated species.
Conclusion
A new method has been reported allowing the synthesis of
porphyrin dimers by controlled potential electrolysis. The
generated species have been identified. This method was based
on a nucleophilic substitution carried out on two electrooxidized
porphyrins. It allowed the two-step synthesis of symmetric
dimers (OEP-OEP) and asymmetric dimers (TPP-OEP) with
good yield (>50%). It was also demonstrated that each of the
two substitution steps was accounted for by a two-electron
E1CNE2CB mechanism. In the 1H NMR spectra of the 7-H2-Zn
porphyrin dimer, the lack of signals typical of the bridging
viologen protons and the â-H protons adjacent to the viologen
provided evidence of rotation at the juncture of the TPP unit
with the viologen spacer and also between the two pyridinium
subunits of the spacer. UV-vis spectrum of the symmetric
dimer 8-Zn-Zn showed the presence of excitonic interactions
between the porphyrin subunits. The ESR analysis indicated
that an intermolecular ferromagnetic coupling occurred between
copper atoms.
The redox characteristics of the studied monomers and dimers
revealed, in all these molecules, that the oxidation and reduction
potentials of the porphyrin subunits flanked both sides of the
electroactive range characterizing the spacer. The electron-
withdrawing character of the latter was quantitated through the
anodic shift of the first-oxidation potential of the porphyrin
component(s). The monomers provided experimental evidence
that their first oxidation produced stable porphyrin radical
cations only when metalated. Also their reduction mechanism
was complex, probably because the reduced bipyridinium cation
was not stable. The dimers exhibited an electrochemical
behavior of their porphyrin components that was close to that
of the corresponding monomers. All the redox parameters
The spectrophotometric analyses were performed with a Hewlett-
Packard 8452 A diode array spectrometer. The thin layer spectroelec-
trochemical cell has been previously described.44
UV-vis spectra were recorded on a Shimazu UV-260 spectropho-
tometer. 1H NMR spectra were obtained in CDCl3 or (CD3)2CO on
Bruker AM-400 (400 MHz), AC 300 (300 MHz), and WP 200 (200
MHz) spectrometers. Fast atom bombardment mass spectra (FAB-
MS) were obtained using a 3-nitrobenzyl alcohol (NBA) matrix on a
FAB-HF mass spectrometer. Elementary analyses were performed by
the microanalysis services of Institut Charles Sadron or Chemical
Department of IUT Sud (Strasbourg). ESR spectra were recorded at
X-band frequencies on a Bruker ESP 300 spectrometer equipped with
a variable temperature accessory.
Synthesis. Compounds 1-H2 and 1-Zn were prepared and purified
by known procedures.45,46 Compounds 2-H2 and 2-Zn have been
previously reported.12,14
Electrochemical Synthesis: General Procedure. Prior to elec-
trolysis, the corresponding mixtures were stirred and degassed by
bubbling argon through the solution for 10 min. Then, the desired
working potential was applied. During anodic oxidation, the electro-
lyzed solution was continuously stirred and maintained under argon.
(44) Bernard, C.; Gisselbrecht, J. P.; Gross, M.; Vogel, E.; Lausmann,
M. Inorg. Chem. 1994, 33, 2393-2401.
(45) (a) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.;
Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476. (b) Barnett, G. H.;
Hudson, M. F.; Smith, K. M. Tetrahedron Lett. 1973, 30, 2887-2888.
(46) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, K. J. Inorg. Nucl.
Chem. 1970, 32, 2443-2445.

Linear Porphyrin Dimers
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2979
After electrolysis, solvents were removed by rotary evaporator. The
residue was dissolved in a minimum of CH2Cl2; then the mixture was
poured into water, and the organic layer was washed twice. Typical
preparation and characterization of a substituted monomer (3-H2) and
a dimer (7-H2-Zn) are reported hereafter.
3-H2. H2TPP (30 mg, 0.049 mmol) and 150 mg of 4,4′-bipyridine
(0.960 mmol) were dissolved in 25 mL of a CHCl3/CH3CN (4/1) and
0.1 M TEAP. solution. The electrolysis was carried out during 3 h at
1.05 V/SCE. After treatment, the organic extract was concentrated
(∼10 mL) and chromatographed on alumina. The first fraction (eluted
with CH2Cl2) was unreacted H2TPP. The desired product was eluted
with a mixture of CH2Cl2/CH3OH (98/2). After evaporation of the
solvent, 3-H2 was recrystallized from CH2Cl2/n-hexane to give violet-
blue crystals (40 mg, yield 94%). 3-H2 was dried under vacuum at
120 °C during 24 h.
By the same procedure, anodic oxidation of 980 mg of H2TPP (1.6
mmol) in the presence of 5 g of 4,4′-bipyridine (32 mmol) in a 850
mL solution of the supporting electrolyte gave 3-H2 with a yield of
75% after 48 h of electrolysis: UV-vis (CH2Cl2) λmax (nm) (ꢀ, M-1
cm-1) 422 (258 700), 526 (15 200), 564 (2400), 602 (4300), 658 (9200);
1H NMR (200 MHz, (CD3)2CO) δ 9.58 (d, Jo ) 7 Hz, 2 H, 4,4′-bpy+),
9.43 (s, 1 H, â-H adjacent to 4,4′-bpy+), 9.08 (s, 2 H, â-H), 8.96 (s,
1H, â-H), 8.98 (dd, Jo ) 4.4 Hz, Jm ) 1.7 Hz, 2H, 4,4′-bpy+), 8.83-
8.77 (m, 3 H, â-H), 8.48 (d, Jo ) 7 Hz, 2 H, 4,4′-bpy+), 8.34-8.22
(m, 6 H, o-H), 8.13 (dd, Jo ) 4.7 Hz, Jm ) 1.6 Hz, 2 H, o-H of Ph
adjacent to 4,4′-bpy+), 8.05 (dd, Jo ) 4.4 Hz, Jm ) 1.7 Hz, 2 H, 4,4′-
bpy+), 7.90-7.81 (m, 9 H, m- and p-H), 7.52-7.49 (m, 3 H, m- and
p-H of Ph adjacent to 4,4′-bpy+), -2.67 (s, 2 H, NH); FAB-MS (NBA)
m/z ) 769.2 (C54H37N6)+. Anal. Calcd. for C54H37N6ClO4: C, 74.60;
H, 4.29; N, 9.67; Cl, 4.08. Found: C, 74.31; H, 4.34; N, 9.49; Cl,
4.25.
7-H2-Zn. ZnOEP (220 mg, 0.37 mmol) and 643 mg of 3-H2 (0.74
mmol) were dissolved in 850 mL of a 1,2-C2H4Cl2/CH3CN (4/1) and
0.1 M TEAP mixture. The electrolysis was carried out at 0.70 V/SCE
during 48 h. After treatment the organic extract was chromatographed
on a silica gel column (Kieselgel, 230-400 mesh). The first fraction
(elution with CH2Cl2) gave unoxidized ZnOEP. The second bluish-
purple fraction (unreacted 3-H2) was collected by elution with CH2Cl2/
CH3OH (98/2). Further elution with CH2Cl2/CH3OH (96/4) gave the
dimer 7-H2-Zn. After evaporation of the solvent, 7-H2-Zn was
recrystallized from CH2Cl2/n-hexane to give black-violet crystals (428
mg, 0.27 mmol, yield 73%): UV-vis (CH2Cl2) λmax (nm) (ꢀ, M-1 cm-1
)
406 (176 200), 424 (416 600), 536 (24 000), 577 (20 800), 656 (8000);
1H NMR (400 MHz, (CD3)2CO) δ 10.83 (d, Jo ) 6.4 Hz, 2 H, 4,4′-
bpy2+), 10.44 (s, 2 H, meso-H), 10.37 (s, 1 H, meso-H), 9.97 (d, Jo )
6.6 Hz, 2 H, 4,4′-bpy2+), 9.60 (d, Jo ) 6.4 Hz, 2 H, 4,4′-bpy2+), 9.60
(s, 1 H, â-H adjacent to 4,4′-bpy2+), 9.22 (d, Jo ) 6.6 Hz, 2 H, 4,4′-
bpy2+), 9.11 (d, Jcis ) 5 Hz, 1 H, â-H), 9.10 (d, Jcis ) 5 Hz, 1 H, â-H),
9.03 (d, Jcis ) 5 Hz, 1 H, â-H), 8.91 (d, Jcis ) 5 Hz, 1 H, â-H), 8.84
(d, Jcis ) 4.7 Hz, 1 H, â-H), 8.81 (d, Jcis ) 4.7 Hz, 1 H, â-H), 8.39 (dt,
Jo ) 7.6 Hz, Jm ) 2.1 Hz, 2 H, o-H of Ph adjacent to 4,4′-bpy2+),
8.32-8.27 (m, 6 H, o-H of Ph), 7.91-7.75 (m, 12 H, m- and p-H),
4.20 (q, 3J ) 7.5 Hz, 12 H, 6 CH2 of -CH2-CH3), 2.79 (q, 3J ) 7.4 Hz,
4 H, 2 CH2 of -CH2-CH3), 1.97 (t, 3J ) 7.5 Hz, 18 H, 6 CH3 of -CH2-
3
CH3), 1.59 (t, J ) 7.4 Hz, 6 H, 2 CH3 of -CH2-CH3 adjacent to 4,4′-
bpy2+), -2.59 (s, 2 H, NH of TPP); FAB-MS (NBA) m/z ) 1564.5
(C90H80N10Zn(ClO4)2+), 1465.5 (C90H80N10Zn(ClO4)+), 1366.6 (C90H80N10-
Zn+), 683.2 (C90H80N10Zn2+). Anal. Calcd. for C90H80N10Zn(ClO4)2:
C, 69.03; H, 5.15; N, 8.94; Cl, 4.53; Zn, 4.18. Found: C, 68.90; H,
4.99; N, 8.90; Cl, 4.75; Zn, 4.20.
Acknowledgment. We warmly thank the Centre National
de la Recherche Scientifique for financial support. We express
our gratitude to Professor J. Fajer for helpful discussions and
critical comments. We are also grateful to Drs. J. J. Andre and
C. Haubtmann, Institut Charles Sadron (UP au CNRS No. 0022),
for the ESR studies.
Supporting Information Available: Procedures for the
preparation of and selected physical data for compounds 3-Zn,
5-Zn, 6-Zn, 7-H2-H2, 7-Zn-Zn, 7-Cu-Cu, 7-Cu-Zn, and 8-Zn-
Zn. Also lists of ESR spectra and UV-vis spectra (10 pages).
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immediately follows this article in the microfilm version of the
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