Greatly enhanced intermolecular π-dimer formation of a porphyrin trimer radical trications through multiple πbonds

Article abstract of DOI:10.1002/chem.201002822

A trefoil-like porphyrin trimer linked by triphenylamine (TPA-TPZn ₃) was synthesized. A three-electron oxidation of TPA-TPZn ₃ forms a radical trication (TPA-TPZn₃³⁺), in which each porphyrin ring undergoes a o

Full text of DOI:10.1002/chem.201002822

DOI: 10.1002/chem.201002822
Greatly Enhanced Intermolecular p-Dimer Formation of a Porphyrin Trimer
Radical Trications through Multiple p Bonds
Atsuro Takai,[a] Claude P. Gros,[b] Jean-Michel Barbe,*[b] and Shunichi Fukuzumi*[a, c]
6+
Abstract:
A
trefoil-like porphyrin
region. The initial oxidation potential
of TPA-TPZn3 is negatively shifted rel-
ative to that of the corresponding mo-
nomer porphyrin, which results from
the stabilization of the oxidized state
of TPA-TPZn3 associated with the di-
merization. The thermodynamic pa-
rameters (i.e., DH, DS, and DG) for
the formation of (TPA-TPZn3)2 were
determined by measuring Vis/NIR
spectra at various temperatures. The
trimer linked by triphenylamine (TPA-
TPZn3) was synthesized. A three-elec-
tron oxidation of TPA-TPZn3 forms a
radical trication (TPA-TPZn33+), in
which each porphyrin ring undergoes a
one-electron oxidation. The radical tri-
cation TPA-TPZn3 spontaneously di-
merizes to afford (TPA-TPZn3)2
CH2Cl2. The characteristic charge-reso-
nance band due to the charge delocali-
6+
formation constant of (TPA-TPZn3)2
is significantly larger than that of the
radical cation dimer of the correspond-
ing monomer porphyrin (e.g., over
2000-fold at 233 K). The electronic
states were investigated using EPR
spectroscopic analysis. The greatly en-
hanced dimerization of TPA-TPZn3
results from multiple p-bond formation
between the porphyrin radical cations.
3+
6+
in
Keywords: dimerization · electron
transfer · electronic structure · mul-
tiple bonds · pi interactions
3+
zation over the p system of (TPA-
6+
TPZn3)2
was observed in the NIR
Introduction
dimer and p-radical anion dimer of aromatic compounds are
widely known as a result of p bonding between radical cat-
ions and radical anions, respectively.[7,8]
Electronic communication between p-conjugated molecules
plays a crucial role in efficient energy transfer and electron
transfer not only in biological systems, such as the photosyn-
thetic process,[1,2] but also in organic electroconductive net-
works, such as polythiophene and phthalocyanine.[3] Elec-
tron-transfer reactions in these systems are often governed
by electronic interactions (e.g., geometry and expansion of
p conjugation) and electrostatic interactions (e.g., p–p inter-
actions)[4] between adjacent chromophores. Even though p-
conjugated molecules are similarly charged, a strong elec-
tronic interaction which fully compensates the electrostatic
repulsion can exist through mediating counterions[5] and p-
bond formation.[6–8] Formation of a stable p-radical cation
Such an electronic interaction between aromatic radical
ions is expected to manipulate the electron-transfer pro-
AHCTUNGTRENNUNG
cess.[5,6a,9] In particular, a significant electronic interaction
exists between dimeric bacteriochlorophylls in the photosyn-
thetic reaction center, the so-called special pair, when it un-
dergoes a one-electron oxidation, thereby leading to effi-
cient and unidirectional electron transfer in the natural pho-
tosystem.[9] Thus, much attention has been focused on the
electronic interactions between two or more porphyrins in
the oxidized state from the point of view of bio-inspired
chemistry[6,10–15] in addition to supramolecular chemistry in
the design of highly organized assemblies that consist of por-
phyrin p systems.[6b,15,16] In this context, we have reported
the effect of electronic interactions in cofacial porphyrin p-
dimer radical cations on the dynamics of photoinduced elec-
tron transfer.[6a]
A porphyrin p-radical cation dimer gains larger stabiliza-
tion energy in the p orbital than a p-dimer radical cation be-
cause of the lack of an electron in the antibonding p orbital.
Electronic interactions between porphyrin p-radical cations,
therefore, lead to distinctly different spectroscopic and
structural properties from those in porphyrin p-dimer radi-
cal cations.[6a,11,13a] Recently, we reported the intramolecular
electronic interactions between porphyrin radical cations in
a multi-porphyrin system.[6b] However, there has so far been
no report on intermolecular electronic interactions of exten-
sive (multiple) p-conjugated macrocycles. If multiple
p bonds are formed between multiple porphyrin radical cat-
ions, the intermolecular electronic interactions would be sig-
[a] A. Takai, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
Osaka University
2–1 Yamada-oka, Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[b] Dr. C. P. Gros, Dr. J.-M. Barbe
ICMUB, UMR CNRS 5260
Universitꢀ de Bourgogne
9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex (France)
E-mail: jean-michel.barbe@u-bourgogne.fr
[c] Prof. Dr. S. Fukuzumi
Department of Bioinspired Science
Ewha Womans University
Seoul 120-750 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002822.
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Chem. Eur. J. 2011, 17, 3420 – 3428

FULL PAPER
nificantly enhanced relative to the case of a monomeric por-
phyrin radical cation, in contrast to the case of electron (or
hole) hopping through weak electronic interactions over
multiple electron donors.[14,17]
We report herein, for the first time, the effects of multiple
p bonds on the dimerization equilibria of p-conjugated
macrocycle radical cations. We employed a novel porphyrin
Strong p bonding between porphyrin radical cations re-
sults in charge delocalization and spin coupling, which can
be experimentally recognized by characteristic NIR absorp-
tion bands and distinctive changes in EPR spectra.[7] Thus,
the electron-transfer properties of TPA-TPZn3, TPA-MPZn,
and Ph-MPZn were evaluated based on Vis/NIR and EPR
spectroscopic analysis and electrochemical measurements.
As a result, a remarkably large formation constant of the p-
trimer that consists of
a trefoil scaffold TPA-TPZn3
3+
(Scheme 1). The corresponding monomer porphyrin involv-
ing triphenylamine and phenyl groups (TPA-MPZn and
Ph-MPZn, respectively) were also investigated for compare-
dimer complex of the radical trication TPA-TPZn3 was
obtained relative to those of the monomer radical cations
+
+
C
C
.
TPA-MPZn and Ph-MPZn
ACHTUNGTRENNUNGison. The TPA moiety is a reasonable linker to enhance the
nucleophilicity of porphyrins and to link three porphyrin
units independently.[18]
Results and Discussion
Synthesis and characterization: A new type of efficient one-
step synthesis of a C3v-symmetrical porphyrin trimer is de-
scribed herein (Scheme 2). Following reported methodolo-
gies,[19,20] a,c-biladiene dibromide was dissolved in ethanol
and mixed with tris(aminophenylaldehyde) in the presence
of para-toluenesulfonic acid (PTSA), followed by metalation
with zinc(II) ions. Classical purification of the crude reaction
mixture with column chromatography on silica gel with di-
chloromethane as an eluent gave the tris(porphyrin) tris-
AHCTUNTGRENGN(UN zinc) complex (TPA-TPZn3) in 33% yield. A similar por-
phyrin preparation was carried out by the condensation of
a,c-biladiene and 4-(diphenylamino)benzaldehyde to synthe-
size the corresponding monomer porphyrin TPA-MPZn in
57% yield. The detailed experiments and spectroscopic data
of TPA-TPZn3 and TPA-MPZn are given in the Experimen-
tal Section and Supporting Information.
Intermolecular p-bond formation between porphyrin radical
cations: Figure 1 shows the UV/Vis spectra of TPA-TPZn3,
TPA-MPZn, and Ph-MPZn. The Soret and Q bands of TPA-
TPZn3 are slightly blue shifted relative to those of TPA-
MPZn and Ph-MPZn, contrary to the case of the cofacial
porphyrin dimer[6a,21] and trimer,[6b] for which red shifts are
observed relative to the corresponding monomer porphyr-
ins.
Scheme 1. Structure of TPA-TPZn3, TPA-MPZn, and Ph-MPZn.
Scheme 2. Synthesis of TPA-TPZn3.
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3421

S. Fukuzumi, J.-M. Barbe et al.
(bpy=2,2ꢂ-bipyridine).[22] Upon addition of three equiva-
lents of [RuACTHNUTRGNEUNG
(bpy)3]3+ ions to a solution of TPA-TPZn3 in
CH2Cl2 at 298 K, new absorption bands at l=655 and
960 nm appear. The absorbance at l=960 nm slightly in-
creases until the addition of approximately one equivalent
of [Ru
largest value at a ratio of 3:1 for the concentration of [Ru-
AHCTUNGTRENNUNG
(bpy)3]3+ to the concentration of TPZn33+, at which point
(bpy)3]3+ ions followed by a large increase to give the
ACHTUNGTRENNUNG
the porphyrin units had undergone a one-electron oxidation
(inset, Figure 2). No increase in absorbance is observed by
the addition of over three equivalents of [RuACTHNUTRGNEUNG
(bpy)3]3+ ions
(Figure S2 in the Supporting Information). The absorption
band at l=655 nm is typically assigned to the monomer por-
phyrin radical cation.[23] This outcome indicates that the ini-
tial three-electron oxidation of TPA-TPZn3 forms TPA-
TPZn33+, in which each porphyrin ring has undergone a
one-electron oxidation.
Figure 1. UV/Vis spectra of TPA-TPZn3 (dashed line), TPA-MPZn
(dotted line), and Ph-MPZn (solid line) at 3.0ꢃ10ꢀ6 m in CH2Cl2 at
298 K.
Meanwhile, there are two possibilities to explain the NIR
absorption band at l=960 nm: 1) an intervalence charge-
transfer (IVCT) band between the porphyrin radical cations
through the TPA linker and 2) a charge-resonance band due
to the intermolecular p-bond formation between the por-
phyrin radical cations. To clarify this point, concentration
The optimized structure of TPA-TPZn3 obtained by DFT
calculations (B3LYP/3-21G) reveals that each porphyrin is
geometrically independent and is nearly orthogonal to the
TPA moiety (838; Figure S1 in the Supporting Information).
These results indicate that there are little electronic interac-
tions among the porphyrin units of TPA-TPZn3 in the neu-
tral form.
3+
dependence of the Vis/NIR spectrum of TPA-TPZn3 were
measured. Figure 3a shows the spectral changes of TPA-
3+
TPZn3 at various concentrations. The ratio of absorbances
at l=960 and 655 nm (A960/A655) increases with an increase
3+
in the concentration of TPA-TPZn3 (inset, Figure 3a). If
the NIR absorption was assigned to an IVCT band, the A960
/
The Vis/NIR spectral changes upon electron-transfer oxi-
dation of the porphyrin units were recorded to investigate
possible electronic interactions between porphyrin moieties
in the oxidized state. Figure 2 shows the Vis/NIR titration of
A655 values would be the same irrespective of the concentra-
3+ [24]
tion of TPA-TPZn3
.
Thus, the NIR absorption is as-
signed to the charge-resonance band due to the dimerization
3+ [25]
of TPA-TPZn3
.
Equation (1) is an equilibrium of the di-
TPA-TPZn3 with the one-electron oxidant [Ru
A
merization of TPA-TPZn33+, in which K is the formation
constant of (TPA-TPZn3)26+, as described below in detail.
The radical cation of Ph-MPZn (Ph-MPZn +) exhibits a
C
small NIR absorption (lmax =1020 nm) under more concen-
3+
trated conditions relative to the case of TPA-TPZn3 at
low temperatures (Figure S3 in the Supporting Information).
+
C
This result indicates that TPA-MPZn hardly dimerizes to
form a radical cation dimer. When two equivalents of [Ru-
AHCTUNGTRENNUNG
(bpy)3]3+ ions are added to a solution of TPA-MPZn in
CH2Cl2, new absorption bands are observed (Figure S4a in
the Supporting Information). The absorption bands are in
accordance with that observed for pristine TPA in the pres-
ence of one equivalent of [RuACHTNUGRTNEUNG
(bpy)3]3+ ions (Figure S4b in
the Supporting Information).[24,26] The second oxidation of
TPA-MPZn, therefore, occurs at the TPA moiety.
GK
H
3þ
6þ
2
TPA-TPZn33þ þ TPA-TPZn3
ðTPA-TPZn Þ
ð1Þ
3
In contrast, the Vis/NIR spectra of TPA-MPZn in the
presence of one equivalent of [Ru
(bpy)3]3+ ions only shows
AHCTUNGTRENNUNG
an absorption at lmax =655 nm due to the monomer porphy-
rin radical cation and little change in the NIR region (Fig-
Figure 2. Vis/NIR spectral changes upon the addition of [RuACTHNUTRGNEUNG
(bpy)3]3+
ions (0–4.3ꢃ10ꢀ5 m) to a solution of TPA-TPZn3 in CH2Cl2 (1.4ꢃ10ꢀ5 m)
at 298 K. Inset: Plot of DA value at l=960 nm versus the concentration
ure 3b). The A920/A655 value is small (ca. 0.05) and constant
+
C
ratio of added [Ru
A
throughout the change in the concentration of TPA-MPZn
.
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Chem. Eur. J. 2011, 17, 3420 – 3428

Three-Electron Oxidation of a Porphyrin Trimer
FULL PAPER
3+
Figure 3. a) Vis/NIR spectral changes of TPA-TPZn3
(solid line), 1.9ꢃ10ꢀ5 m (dashed line), and 9.4ꢃ10ꢀ6 m (dotted line) in
at 3.7ꢃ10ꢀ5
m
CH2Cl2 at 298 K. Inset: The ratio of absorbances at l=960/655 nm (A960
/
A655) versus concentration of TPA-TPZn33+. b) Vis/NIR spectral changes
Figure 4. Cyclic voltammograms (100 mVsꢀ1) of a) Ph-MPZn, b) TPA-
MPZn, and c) TPA-TPZn3 (4ꢃ10ꢀ4 m) in CH2Cl2 containing 0.1m
TBAPF6. d) Differential pulse voltammogram of TPA-TPZn3 under the
same conditions as the cyclic voltammetry. The dashed line is the simula-
tion curve from using two discrete Gaussian-shape oxidation waves
(dotted lines).
+
of TPA-MPZn at 8.8ꢃ10ꢀ5 m (solid line), 4.4ꢃ10ꢀ5 m (dashed line), and
C
1.7ꢃ10ꢀ6 m (dotted line) in CH2Cl2 at 298 K. Inset: The ratio of absorb-
ance at l=920/655 nm (A920/A655) versus [TPA-MPZn]·+
.
The differences in the electron-transfer oxidation process-
es of the porphyrin units were also investigated by using
cyclic voltammetry (CV) and differential pulse voltammetry
(DPV; Figure 4). Ph-MPZn (Figure 4a) exhibits two reversi-
(Figure S5 in the Supporting Information). The third redox
waves of TPA-MPZn (1.16 V) is, thus, assigned to the
second oxidation of the porphyrin ring.
ble redox waves, which are typical for the stepwise oxidation
The redox behavior of TPA-TPZn3 is shown in Fig-
ure 4c,d. The initial three-electron oxidation process was at
0.66 V, and the second three-electron oxidation process oc-
curred at 1.11 V[27] with a shoulder at 1.02 V, thus indicating
the presence of another redox process. The DPV of TPA-
TPZn3 can be simulated by two Gaussian-shape oxidation
waves: two oxidation potentials at 1.02 and 1.11 V were ob-
tained with a ratio of current intensity of 1:3. This outcome
indicates that the TPA moiety is oxidized at 1.02 V, whereas
the second oxidation of the porphyrin rings in TPA-TPZn3
+
C
processes of the porphyrin rings: Ph-MPZn/Ph-MPZn and
Ph-MPZn +/Ph-MPZn2+ (0.69 and 0.99 V, respectively).
C
Similarly the first oxidation of TPA-MPZn (Figure 4b)
occurs at the porphyrin ring (0.68 V). The second oxidation
(0.88 V) is attributed to the oxidation of the TPA moiety, on
the basis of the Vis/NIR spectroscopic analysis. Differential
pulse voltammograms of the free-base porphyrins Ph-MPH2
and TPA-MPH2 also indicate that the TPA moiety is oxi-
dized after the formation of the porphyrin radical cation
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3423

S. Fukuzumi, J.-M. Barbe et al.
takes place at 1.11 V. The positive shifts of the second oxida-
tion of the porphyrin rings in TPA-MPZn and TPA-TPZn3
with respect to Ph-MPZn result from the electrophilicity of
the oxidized TPA moiety. In contrast, the negative shift of
the first oxidation potential of TPA-TPZn3 follows from the
stabilization of the oxidized state of TPA-TPZn3 associated
with the dimerization,[28] although the electrostatic repulsion
between porphyrin radical cations induces the destabiliza-
tion. The electron donation from the TPA to porphyrin
moiety may also contribute to the negative shift of the first
oxidation potential of TPA-TPZn3.
Enhanced dimerization of the TPA-TPZn3 radical cation
through multiple p-bond formation: The formation constant
6+
K of (TPA-TPZn3)2 in Equation (1) was determined from
the absorbance change at l=960 nm by using Equation (2),
in which A960 is the absorbance at l=960 nm, eD is the
6+
molar absorption coefficient of (TPA-TPZn3)2
at l=
3+
960 nm, and [C] is the total concentration of TPA-TPZn3
.
1=2
1=2
ð2Þ
ðA960=eDÞ ¼ ðKÞ ð½Cꢁꢀ2 A960=eDÞ
The eD value is determined to be 4.6ꢃ104 mꢀ1 cmꢀ1 from
the temperature-modulated absorption changes at the mo-
nomer and dimer band maxima (Figure 5).[6a,7d] The change
in absorbance at l=960 nm at various concentrations of
3+
TPA-TPZn3
at 298 K is shown in Figure 6a. From the
linear plot of (A960/eD)1/2 versus [C]ꢀ2A960/eD, the K value is
determined to be 1.0ꢃ104 mꢀ1 at 298 K (inset, Figure 6a).
The K values were similarly determined at various tempera-
tures to show a linear correlation between lnACTHNUTRGNEUNG
(K/mꢀ1) and
Tꢀ1 (Figure 6b). This vanꢂt Hoff plot gives thermodynamic
parameters for the formation of (TPA-TPZn3)26+: DH=
ꢀ13.7 kcalmolꢀ1 and DS=ꢀ28.0 calKꢀ1 molꢀ1.
Analogous procedures were repeated to investigate the
formation equilibrium of (Ph-MPZn)22+. Figure S7a,b in the
Figure 6. a) The change in absorbance at l=960 nm at various concentra-
3+
tions of TPA-TPZn3 ([C]) at 298 K. Inset: Plot of (A960/eD)1/2 versus
([C]ꢀ2A960/eD). b) vanꢂt Hoff plot for the formation constant of (TPA-
6+
TPZn3)2
.
Supporting Information shows the Vis/NIR spectral changes
of Ph-MPZn·+ at various temperatures and the vanꢂt Hoff
plot for the formation constant of (Ph-MPZn)22+, respec-
tively. The formation constants and thermodynamic parame-
2+
6+
ters of (Ph-MPZn)2 and of (TPA-TPZn3)2 are summar-
6+
ized in Table 1. The formation constant of (TPA-TPZn3)2
is much larger than that of (Ph-MPZn)2 at any tempera-
ture. The formation enthalpy of (TPA-TPZn3)2 is signifi-
2+
6+
2+
cantly more negative than that of (Ph-MPZn)2
(DH=
ꢀ13.7 and ꢀ5.9 kcalmolꢀ1, respectively). These results indi-
6+
cate that (TPA-TPZn3)2
is stabilized by forming three
p bonds at each porphyrin moiety. A nonlinear increase in
the absorbance at l=960 nm upon the electron-transfer oxi-
dation of TPA-TPZn3 (Figure 2) indicates that once the first
p bond is formed the second and third p-bond formations
are significantly enhanced. The structural restriction by mul-
tiple p bonding between porphyrin radical cations in (TPA-
3+
Figure 5. Vis/NIR spectral changes of a solution of TPA-TPZn3
in
CH2Cl2 (4ꢃ10ꢀ5 m) at various temperatures. Inset: The absorbance
changes at l=655 and 960 nm.
6+
TPZn3)2 leads to a more negative formation entropy than
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Three-Electron Oxidation of a Porphyrin Trimer
FULL PAPER
6+
Table 1. Formation constants (K), thermodynamic parameters (DH, DS, and DG), and absorption maximum
(lmax) of charge-resonance bands for (TPA-TPZn3)2 and (Ph-MPZn)2 in CH2Cl2 at various temperatures.
constant K of (TPA-TPZn3)2
6+
2+
estimated from EPR spectro-
scopic measurements is larger
(3ꢃ104 mꢀ1 at 293 K) than that
determined from Vis/NIR spec-
troscopy because the apparent
T
[K]
K
A
DG
DH
DS
lmax
[mꢀ1
]
[kcalmolꢀ1
]
[kcalmolꢀ1
]
[calKꢀ1 molꢀ1
]
[nm]
243
263
283
298
2.0ꢃ106
1.2ꢃ105
2.6ꢃ104
1.0ꢃ104
ꢀ6.9
ꢀ6.3
ꢀ5.8
ꢀ5.4
6+
(TPA-TPZn3)3
ꢀ13.7
ꢀ28.0
960
3+
EPR intensity of TPA-TPZn3
is slightly less (Figure S8 in the
Supporting Information), which
leads to an overestimation of
the K value.[32] However, it
should be noted that the
change in EPR intensity of
(TPA-TPZn3)n+ (0ꢃnꢃ 3) cor-
responds well to the increase in
DA value at l=960 nm due to
233
253
263
273
298
2.6ꢃ103
7.7ꢃ102
5.1ꢃ102
4.3ꢃ102
ꢀ3.6
ꢀ3.4
ꢀ3.3
ꢀ3.2
ꢀ3.0
2+
(Ph-MPZn)2
ꢀ5.9
ꢀ9.9
1020
A
[a] Little increase in the absorbance at l=1020 nm was observed to preclude the precise determination of the
K value from Vis/NIR spectroscopy. The value in parenthesis is the K value estimated from the DG value at
298 K.
that of (Ph-MPZn)22+. Nevertheless, the gain in enthalpy
that results from strong p-bond formation fully compensates
for the loss in entropy to afford a larger stabilization free
dimeric
species
(inset,
energy for (TPA-TPZn3)2 than for (Ph-MPZn)22+. The dif-
6+
6+
ference in the free energy between (TPA-TPZn3)2
and
(Ph-MPZn)2 is DDG=2.4 kcalmolꢀ1 (ꢂ0.10 eV) at 298 K,
2+
in accordance with the value estimated from the difference
in the absorption maxima of charge resonance bands lmax
=
60 nm (ꢂ0.08 eV). To the best of our knowledge, this exam-
ple is the first that exhibits the synergistic effect of the
number of p-conjugated macrocycles on the formation of di-
meric radical cations.
The DFT(B3LYP/3-21G basis set) calculation also sup-
ports the multiple p-bond formation between porphyrin rad-
6+
ical cations in (TPA-TPZn3)2 (Figure 7a). The mean plane
distance between the porphyrin plane and its adjacent coun-
terpart is approximately 3.5 ꢄ,[29] which is close to reported
values.[11a,b,12] The electrostatic potential map for (TPA-
6+
TPZn3)2 together with the observation of NIR absorption
bands indicates that positive charges are delocalized over
the whole p system (Figure 7b; colored in red), contrary to
the case of electron hopping in p-conjugated macro-
ACHTUNGTRENNUNG
cycles.[6a,10,14,30]
6+
The spin states in (TPA-TPZn3)2 were investigated by
EPR spectroscopic analysis. Figure 8 shows the temperature
3+
dependence of the EPR spectra of TPA-TPZn3 in CH2Cl2.
The EPR intensities of the samples were quantitatively eval-
uated using 1,1-diphenyl-2-picrylhydrazyl (DPPH) as an ex-
ternal standard (see the Experimental Section for details).
3+
The EPR intensities assigned to TPA-TPZn3 changes re-
versibly and decreases to nearly zero on lowering the tem-
perature. It is estimated from EPR intensity that over 90%
3+
6+
of TPA-TPZn3 exists as (TPA-TPZn3)2 at 233 K, which
is fairly consistent with the data of the temperature-modu-
lated Vis/NIR spectroscopic measurements (Figure 5).
Therefore, the remarkable decrease in the EPR signal inten-
sity can be attributed to the intermolecular spin coupling in
the dimer species (TPA-TPZn3)26+. Because of this antifer-
romagnetic spin coupling, no triplet signal for (TPA-
Figure 7. a) Optimized structure and b) electrostatic potential map of
(TPA-TPZn3)2 calculated by DFT at the B3LYP/3-21G level. b-Sub-
stituents of TPA-TPZn3 are replaced by methyl groups for simplicity.
6+
6+
TPZn3)2 was observed at 77 K, contrary to the case of di-
rectly linked porphyrin dimer dications.[31] The formation
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S. Fukuzumi, J.-M. Barbe et al.
Experimental Section
Chemicals and reagents: Silica gel (Merck; 70–120 mm) was used for
column chromatography. Analytical TLC was performed on Merck 60
F254 silica gel (precoated sheets, thickness: 0.2 mm). The reactions were
monitored by TLC, UV/Vis spectroscopic, and MALDI TOF mass-spec-
trometric analysis. Triphenylamine (Tokyo Chemical Industry Co., Ltd.)
and 1,1-diphenyl-2-picrylhydrazyl (Wako Pure Chemical Industries, Ltd.)
were obtained commercially and used without further purification. The
preparation of Ph-MPZn was described elsewhere.[19a] Tris(2,2’-
bipyridyl)rutheniumCAHTUNGTREN(NNGU III) hexafluorophosphate ([RuAHCTUNRTGEGN(NNU bpy)3]ACHTNUGTRNE(NUGN PF6)3) was
prepared from tris(2,2’-bipyridyl)ruthenium(II) chloride hexahydrate by
oxidation with PbO2.[33] Absolute dichloromethane (Carlo Erba) for syn-
thesis and spectroscopic-grade CH2Cl2 and acetonitrile (Nacalai Tesque,
Inc.) for analysis were obtained commercially and purified prior to use.
Physicochemical and photophysical measurements: 1H NMR spectra
were recorded on a Bruker Avance II 300 (300 MHz) or on a Bruker
Avance DRX 500 (500 MHz) spectrometer; chemical shifts are expressed
in ppm relative to benzene (d=7.15 ppm) or pyridine (d=7.22, 7.58, and
8.74 ppm). Mass spectra and accurate mass measurements (HRMS) were
obtained on a Bruker Daltonics Ultraflex II spectrometer in the MALDI
TOF reflectron mode with dithranol as a matrix or on a Bruker Micro-
QTof instrument in ESI mode. Both measurements were made at the Pla-
teforme dꢂAnalyse Chimique et de Synthꢅse Molꢀculaire de lꢂUniversitꢀ
de Bourgogne (PACSMUB). UV/Vis/NIR spectra were recorded on a
Shimadzu UV-3100PC spectrometer or a Hewlett Packard 8453 diode
array spectrophotometer at various temperatures.
Figure 8. a) EPR spectral changes of a deaerated solution of TPA-TPZn3
Electrochemical measurements: CV and DPV were carried out with a
BAS 100W electrochemical analyzer in a deaerated solvent containing
0.10m tetrabutylammonium hexafluorophosphate (TBAPF6) as a support-
ing electrolyte at 298 K. A conventional three-electrode cell was used
with a platinum working electrode and a platinum wire as a counterelec-
trode. The redox potentials were measured with respect to the Ag/
AgNO3 (1.0ꢃ10ꢀ2 m) reference electrode. The potential values (versus
Ag/AgNO3) are converted into those versus SCE by adding 0.29 V.[34]
in CH2Cl2 (1.6ꢃ10ꢀ4 m) in the presence of three equivalents of [Ru-
ACHTUNGTRENNUNG
(bpy)3]3+ ions at different temperatures. b) Dependence of the ratio of
the EPR intensity (IP/ID, where IP and ID are the EPR intensity of TPA-
3+
TPZn3 and DPPH, respectively) on the temperature. The EPR intensi-
ty was determined by double integration of the corresponding first-deriv-
ative EPR spectrum. Inset: Dependence of the EPR intensity (filled
circle, IP; half-open square, ID) on the temperature. The concentrations
3+
of TPA-TPZn3 and DPPH are 1.6 and 4.8ꢃ10ꢀ4 m in CH2Cl2, respec-
EPR measurements: The EPR spectra were measured at various temper-
atures with a JEOL X-band spectrometer (JES-RE1XE). The EPR spec-
tra were recorded under nonsaturating microwave-power conditions. The
magnitude of the modulation was chosen to optimize the resolution and
the signal-to-noise (S/N) ratio of the observed spectra. The g values were
calibrated with a Mn2+ marker. Solutions of the porphyrins in CH2Cl2
were deaerated by purging with argon for 10 min prior to use. The con-
centrations of the radical species were determined by double integration
of the first-derivative EPR signal in reference to that of a known amount
of a stable radical, 1,1-diphenyl-2-picrylhydrazyl (DPPH) under the same
experimental conditions. Because the radical concentration of DPPH is
the same over the entire temperature range, variations in the EPR inten-
sity only depend on the Curie law and instrumental factors such as cavity
tively.
Figure 2). Dependence of EPR intensity on the concentra-
tion of TPA-TPZn3 (Figure S8c in the Supporting Infor-
mation) is also in accordance with the UV/Vis titration of
TPA-TPZn3 at different concentrations (Figure 3a).
3+
3+
Conclusion
2+
Q factor. Thus, by comparing the ratio of the EPR intensity of TPZn3
The present study has shown that the formation constant of
the diamagnetic dimer (TPA-TPZn3)2 is remarkably larger
than those of radical cation dimers of TPA-MPZn and Ph-
MPZn (e.g., 2000-fold at 233 K). Multiple electronic inter-
2+
(IP) with that of DPPH (ID), the radical concentrations of TPZn3 can
6+
be determined at various temperatures.
+
C
Theoretical calculations: DFT calculations were performed on a 32-pro-
cessor QuantumCube. Geometry optimizations were carried out by using
the Becke3LYP functional and 3–21G basis set with the Gaussian 09 pro-
gram (revision A.02).[35] The graphics were drawn using the Gauss View
software program (version 5.0) developed by Semichem Inc.
+
C
3+
actions between porphyrin radical cations in TPA-TPZn3
facilitate the formation of a stable diamagnetic dimer
cation. Such a synergistic effect can be generalized for a va-
riety of organic p-conjugated macrocycles, which may pro-
vide new strategies for the design of organic supramolecular
assemblies related to photofunctional and electroconductive
materials.
2,3,7,8,12,18-Hexamethyl-5-triphenylamine-13,17-diethylporphyrinatozinc
(TPA-MPZn): A mixture of 4-(diphenylamino)benzaldehyde (250 mg,
0.915 mmol) and a,c-biladiene dihydrobromide (500 mg, 0.829 mmol) was
dissolved in hot absolute ethanol (100 mL). PTSA (1.5 g, 8.70 mmol) in
ethanol (20 mL) was slowly added over 12 h, and the reaction mixture
was stirred and heated to reflux for 48 h. The mixture was cooled to
room temperature, and the solution was evaporated under vacuum. The
residue was dissolved in dichloromethane (200 mL), neutralized with a
saturated solution of NaHCO3, and washed thoroughly with water (3ꢃ
500 mL). The organic phase was dried over MgSO4 and filtered, and the
solvent was removed in vacuo. The residue was dissolved in chloroform
3426
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Chem. Eur. J. 2011, 17, 3420 – 3428

Three-Electron Oxidation of a Porphyrin Trimer
FULL PAPER
(35 mL) and methanol (15 mL) and heated to reflux for 90 min in the
presence of sodium acetate (89 mg, 1.00 mmol). The metalation reaction
was monitored by UV/Vis spectroscopic and MALDI TOF mass-spectro-
metric analysis. At the end of the reaction, the solvent was removed
under reduced pressure and the crude product was dissolved in dichloro-
methane (50 mL), washed with water (3ꢃ250 mL), and dried over
MgSO4. The solvent was removed in vacuo and the residue was purified
by column chromatography on silica gel (CH2Cl2 as the eluent). After
evaporation, TPA-MPZn was obtained as a red microcrystalline solid
(355 mg, 0.47 mmol, 57%). 1H NMR (C6D6, 298 K): d=1.86 (t, J=
7.4 Hz, 6H; CH3), 2.81 (s, 6H; b-CH3), 3.46 (s, 6H; b-CH3), 3.51 (s, 6H;
b-CH3), 4.37 (q, J=7.4 Hz, 4H; CH3-CH2), 7.11 (dd, 2H; Ph), 7.41 (dd,
4H; Ph), 7.52 (dd, 2H; Ph), 7.64 (dd, 4H; Ph), 7.82 (d, 2H; Ph), 9.80 (s,
1H; Hmeso), 10.04 ppm (s, 2H; Hmeso); UV/Vis (CH2Cl2): lmax (e
ꢃ10ꢀ3 mꢀ1 cmꢀ1)=405.0 (318), 534.0 (17.6), 570.0 nm (15.5); HRMS
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C
Figure S10 in the Supporting Information).
Trisporphyrin triszinc (TPA-TPZn3): The trisporphyrin tripod (33%,
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tris-
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(aminophenylaldehyde) (250 mg, 0.759 mmol) and a,c-biladiene dihydro-
bromide (1.371 g, 2.27 mmol). 1H NMR ([D5]pyridine, 298 K): d=1.89 (t,
J=8.3 Hz, 18H; CH3), 3.61 (s, 18H; b-CH3), 3.65 (s, 18H; b-CH3), 4.15
(q, J=8.3 Hz, 12H; CH2), 5.01 (s, 18H; b-CH3), 6.89 (d, J=8.0 Hz, 6H;
Ph), 7.43 (d, J=8.0 Hz, 6H; Ph), 10.46 (s, 3H; Hmeso), 10.53 ppm (s, 6H;
Hmeso); UV/Vis (CH2Cl2): lmax (e ꢃ10ꢀ3 mꢀ1 cmꢀ1)=399.9 (523), 530.0 (54),
570 nm (62); HRMS (MALDI TOF): m/z: 1775.6508 [M] +, 1775.6485
C
calcd for C108H105N13Zn3 (see Figure S11 in the Supporting Information).
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This research was supported by Grants-in-Aid (No. 20108010) from the
Ministry of Education, Culture, Sports, Science and Technology, Japan
and KOSEF/MEST through WCU project (R31-2008-000-10010-0) from
Korea. A.T. appreciates
a support from the Global COE program
“Global Education and Research Center for Bio-Environmental Chemis-
try” of Osaka University (S.F.) and JSPS fellowship for young scientists.
The Centre National de Recherche Scientifique (CNRS, UMR 5260) and
“Rꢀgion Bourgogne” are also acknowledged for funding.
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that of ferrocene (Fc/Fc+ at 0.49 V versus SCE) under the same
conditions, thus confirming that both the first and second oxidation
processes of TPA-TPZn3 are three-electron processes (see Figure S6
in the Supporting Information for CV and DPV analysis).
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6+
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even in the presence of the supporting electrolyte TBAPF6; hence,
the first oxidation potential of TPA-TPZn3 is strongly associated
with the dimerization equilibrium under the present conditions.
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Received: October 1, 2010
Published online: February 15, 2011
3428
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 3420 – 3428