Porphyrin dimers bridged by an electrochemically switchable unit

Article abstract of DOI:10.1002/chem.200800517

A simple synthetic approach that allows to incorporate a redox-active bridge into a porphyrin dimers Ni₂2, Ni₂3, and Ni ₂4 was investigated. The oxidation reaction of dimer Ni₂3 with PbO₂ gave porphyrin Ni₂4. The molecular structures of these porphyrins were confirmed by various spectroscopic methods such as NMR, UV/Vis, IR spectroscopy, and mass spectrometry. The electronic absorptions of porphyrin dimers Ni₂2 and Ni₂3 exhibit significant peak-broadening and red shifts, in comparison with those of dimer Ni ₂4. The redox potentials of the first and second oxidations for Ni₂2, corresponding to the formation of Ni₂2⁺ and Ni₂2²⁺ species, are separated by 90 mV. This type of porphyrin dimers holds promise to be used in the application of controllable molecular switches.

Full text of DOI:10.1002/chem.200800517

DOI: 10.1002/chem.200800517
Porphyrin Dimers Bridged by an Electrochemically Switchable Unit
Chi-Lun Mai,[a] Yi-Lin Huang,[a] Gene-Hsiang Lee,[b] Shie-Ming Peng,[b] and
Chen-Yu Yeh*[a]
Dedicated to Professor Chi-Kwong Chang on the occasion of his 60th birthday
In the past decade much effort has been devoted to the
synthesis and studies of molecular systems comprising por-
phyrin units bridged by well-defined p-conjugated spacers at
the meso- or b-positions.[1–4] These conjugated oligoporphyr-
in systems are expected to have potential applications in
molecular wires and electronic devices due to their unique
optical, physical, and chemical properties. Photophysical
studies on electron transfer of a series of ferrocene–oligo-
porphyrin–fullerene triads show that a meso–meso buta-
diyne-bridged oligoporphyrin acts as an efficient molecular
wire for long-range charge transfer over 65 Š.[5] More re-
cently, the conductivity measurements on butadiyne-bridged
porphyrin polymers indicate that the formation of double-
strand 4,4’-bipyridyl ladder complexby addition of bipyri-
dine to the single-strand porphyrin polymer leads to an in-
crease in conductivity by an order of magnitude.[6] EPR
studies performed by Therien and co-workers on meso–
meso ethyne-linked porphyrin oligomers show that this type
of conjugated molecules is able to mediate charge migration
over a distance of about 75 Š.[7] The synthesis of meso–
meso, b–b, b–b triply-linked porphyrin tapes reported by
Osuka represents a milestone in the area of porphyrin mo-
lecular wires.[3] The fully conjugated porphyrin tapes exhibit
remarkable properties including very low optical HOMO–
LUMO gap, low oxidation potential and rigid structure, and
they may find use in a variety of molecular electronics.
Despite extensive studies on the conjugated porphyrin
arrays, oligoporphyrin molecular wires that show switchable
conductance states by chemical or electrochemical means
are rare. One example is the porphyrin arrays in which the
individual macrocyles are laterally-linked by a quinonoid
unit at the b,b’-positions of the porphyrin rings.[8] In this
system, the electronic communication between porphyrin
units can be modulated by quinonoid/benzenoid conversion
using chemical or electrochemical means. Another example
of switchable porphyrin wires is the systems where proqui-
noidal units bridge two porphyrins.[9] It is well known that
the reversible conversion between quinone and hydroqui-
none can be achieved by chemical or electrochemical meth-
ods.[10] Incorporation of a quinone unit into the central part
of a porphyrin dimer via acetylene linkers would allow the
interporphyrin interaction to be switched off and on in a
controllable fashion and this type of molecules may have
the potential for the application in switchable molecular
wires. Therefore, we set out to design and synthesize a series
of porphyrin dimers Ni22, Ni23, and Ni24 to explore their po-
tential as redox-controllable switching molecules.
The synthetic routes to porphyrin dimers Ni22, Ni23, and
Ni24 were outlined in Scheme 1.[11] The coupling reaction of
porphyrin Ni1 with the diiodide gave porphyrin dimer
Ni22.[12] Demethylation of Ni22 to give Ni23 employing
excess BBr3 was unsuccessful. However, the demethylation
can be achieved by the reaction of excess BBr3 with the free
base of the zinc analogue Zn22, obtained by a procedure
similar to that for Ni22.[13] Subsequent nickel insertion af-
forded porphyrin dimer Ni23. The oxidation reaction of
dimer Ni23 with PbO2 gave porphyrin Ni24.[14] For compari-
son purposes, porphyrin Ni5 was synthesized by employing a
procedure similar to that for Ni22. The molecular structures
of these porphyrins were confirmed by various spectroscopic
methods such as NMR, UV/Vis, IR spectroscopy, and mass
spectrometry.
[a] C.-L. Mai, Y.-L. Huang, Prof. C.-Y. Yeh
Department of Chemistry
National Chung Hsing University
Taichung 402 (Taiwan)
Fax: ( +886) 4-2286-2547
E-mail: cyyeh@dragon.nchu.edu.tw
[b] Dr. G.-H. Lee, Prof. S.-M. Peng
Department of Chemistry
National Taiwan University
Taipei 106 (Taiwan)
Figure 1 shows the crystal structure of compound Ni5.[15]
The bond lengths and angles fall in the normal range. The
porphyrin ring adopts an essentially planar conformation
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
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COMMUNICATION
Scheme 1. Synthesis of porphyrin dimers. a) AsPh3, [Pd
(Ni22, 82%; Zn22, 89%). b) i) Zn22, 20% HCl, CH2Cl2; ii) BBr3, CH2Cl2;
iii) Ni(OAc)2·4H2O, CHCl3/AcOH (Ni23, 43% over 3 steps); c) PbO2,
2ACHTER(UNG dba)3], NEt3
ACHTREUNG
CH2Cl2 (90%).
and is nearly coplanar with the dimethoxyphenyl ring. The
coplanar arrangement can be ascribed to the crystal packing
forces.[11,16] It is possible that the two porphyrin rings and
the bridge in dimers Ni22 and Ni23 are coplanar in the solid-
state. The rigid and conjugated spacers in Ni22 and Ni23
would facilitate the electronic coupling between the porphy-
rin units although the spacers can undergo free rotation in
solution.
The absorption spectra of porphyrin dimers Ni22, Ni23,
and Ni24 and reference compound Ni5 in THF are summar-
ized in Figure 2 and Table 1. The spectrum of dimer Ni22
displays a quite similar feature to that of Ni23, suggesting
that the dimethoxybenzene and hydroquinone units in Ni22
and Ni23, respectively, mediate the interporphyrin interac-
tion in a similar way. As compared to that of Ni24, the Soret
bands of Ni22 and Ni23 show peak-broadening and red-shift.
The full width at half maximum (FWHM) is calculated to
be 4386 cmÀ1 for Ni23, which is much larger than that for
Ni24 (FWHM=2098 cmÀ1). It should be noted that Ni24
shows a broad band at 15965 cmÀ1 in THF, which shifts to
15210 cmÀ1 in a mixture of CHCl3/MeOH (2:1), while the
Figure 1. X-ray crystal structure of Ni5. a) Top view. b) Side view (meso-
substituted aryl groups omitted for clarity). The thermal ellipsoids were
drawn at the 50% probability level.
and dimethoxylphenyl units. A significant increase in the in-
tensity of Q bands for Ni22 and Ni23 relative to that for Ni5
was observed. These spectral features for Ni22 and Ni23 can
be explained by a decrease of the energy gap between
HOMO and LUMO as a result of the extended p-conjuga-
tion and indicate that there is strong interporphyrin interac-
tion mediated by the benzenoid unit.[17] In the case of Ni24,
the quinone bridge containing isolated double and single
bonds effectively disrupt the electronic coupling between
the two porphyrin ends.
Soret and Q
in the Supporting Information). Most likely this broad band
consists of both the Q(0,0) band and charge transfer band
ACHTREUNG(1,0) bands are only slightly shifted (Figure S1
AHCTREUNG
from the porphyrin to the quinone unit. Further studies on
the nature of this broad band will be continued. Comparison
of the Soret band for porphyrin Ni22 with that for reference
compound Ni5 confirms that peak-broadening of Ni22 is not
induced by the electronic interaction between the porphyrin
Figure 2. Absorption spectra of Ni22 (g), Ni23 (c), Ni24 (c), and
Ni5 (a) in THF.
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C.-Y. Yeh et al.
Table 1. Absorption data for porphyrins Ni22, Ni23, Ni24, and Ni5.
via the quinone bridge. The second two-electron oxidation
corresponding to the formation of Ni244+ was observed at
lB [cmÀ1
]
lQ [cmÀ1
(loge)[a]
Neutral form
]
FWHM[b] Absorption Absorption
[c]
[c]
H
G
l [cmÀ1
Mono-
cation
]
l [cmÀ1
Di-
]
E
1/2 =+1.27 V. In the reduction region, the first two one-
B-band
[cmÀ1
electron processes at E1/2 =À0.26 and Epc =À0.72 V can be
assigned to the reductions of the quinone unit and the two-
electron process at a potential of À1.37 V corresponds to
the first electron addition to both the porphyrin rings.
]
cation
Ni22 22371 (5.35), 18215 (4.60), 4262
23529,
16666,
12345,
3385
24390,
20408,
13888,
7576
21008 (5.36) 16287 (4.99)
Ni23 22371 (5.35), 18248 (4.60), 4386
21008 (5.30) 16313 (4.96)
Ni24 23365 (5.48) 19305 (4.52), 2098
16026 (4.71)
Ni5 22831 (5.52) 18315 (4.41), 1380
17212 (4.41)
[a] The absorption spectra was taken in THF. [b] FWHM is the full width
at half-maximum height. [c] The oxidized species of Ni24 and Ni5 are rel-
atively unstable.
A simple way to evaluate intramolecular interaction be-
tween the porphyrin units in porphyrin arrays is to study
their electrochemical properties. It is anticipated that two
one-electron oxidations for the first electron abstraction
from the porphyrin units of Ni23 should be observed if the
individual macrocycles are electrochemically coupled. How-
ever, the electrochemical reactions of dimer Ni22 instead of
Ni23 were studied since the oxidation of the hydroquinone
bridge in Ni23 occurs prior to that of the porphyrin rings
and the 1,4-diethynyl-2,5-hydroxyphenyl and 1,4-diethynyl-
2,5-dimethoxyphenyl bridges in Ni22 and Ni23, respectively,
should modulate the interporphyrin interaction in a similar
way as evinced by the absorption spectra. The electrochem-
istry of Ni22 and Ni24 were investigated by cyclic voltamme-
try and differential pulse voltammetry. As shown in
Figure 3, porphyrin dimer Ni22 exhibits two overlapping
one-electron reversible oxidations at potentials of E1/2-
Figure 3. a) Cyclic voltammograms (c) and differential pulse voltam-
mograms (g) for Ni22 and b) cyclic voltammograms for Ni24 in CH2Cl2
containing 0.1m TBAPF6 at a scan rate of 0.1 VsÀ1
.
Calculations of molecular orbitals of Ni22 were performed
by the PM3 method using Spartan ’06 (Figure S2 in the Sup-
porting Information). The building blocks used for the con-
struction of Ni22 were taken from the crystal structure of
porphyrin Ni5, the symmetry of the molecule was uncon-
strained to C1. For Ni22, the electron density is significantly
distributed to the p-system of both porphyrin rings and the
bridge at the HOMO and LUMO, suggesting significant in-
teraction between the two porphyrin rings, that is, the por-
phyrin units would behave as electrochemically dependent
groups. In the case of the quinone-bridge dimer Ni24, the
electron density of the HOMO is essentially localized on
the two porphyrin rings and the LUMO is mainly contribut-
ed from the diethynylquionone unit. Furthermore, slight
splitting (0.1 eV) of HOMO and HOMO-1 for Ni22, which
was absent in Ni24, was observed. These results are in a
good agreement with the absorption spectra and electro-
chemical measurements.
A
1/2ACHTREU(NG ox2)=+0.99 V, which are resolved
CHTREUNG
oxidation processes can be assigned to successive formation
of the monocation and dication, indicative of strong inter-
porphyrin interaction in Ni22. The separation of the redox
potentials E1/2ACHTREUNG(ox1) and E1/2ACHRTE(UGN ox2) is 90 mV and the compro-
portionation constant (Kc) was estimated to be 22.[18] As
compared to the 1,4-ethynylphenyl-bridged nickel porphyrin
dimer,[19] in which the first two one-electron oxidation po-
tentials in CH2Cl2 at 298 K are separated by 50 mV, our por-
phyrin Ni22 shows a stronger electronic coupling between
the porphyrin units via the bridge. This may be ascribed to
the better energy proximity of the frontier orbitals of the
porphyrin units and those of the 1,4-diethynyl-2,5-dimethox-
yphenyl linker because the electron-donating nature of the
methoxyl groups would elevate the energy of both HOMO
and LUMO of the linker. The cyclic voltammogram of Ni24
(Figure 3) shows that the first two-electron oxidation occurs
at E1/2 =+1.08 V corresponding to the first electron abstrac-
tion from both the porphyrin rings to form Ni242+, indicating
that the two porphyrin units do not show electronic coupling
To gain insight into the interporphyrin interaction in
dimer Ni22, the oxidations were further investigated by elec-
tronic absorption. Figure 4 shows the UV/Vis-near IR spec-
tra of Ni22, Ni22+, and Ni222+. The monocation Ni22+ was
generated in situ by reacting Ni22 with one equivalent of
[(p-BrC6H4)3N]
ion Ni222+ can be generated by further addition of another
equivalent of [(p-BrC6H4)3N][SbCl6]. Both monocation
ACHTREU[NG SbCl6] in CH2Cl2. The corresponding dicat-
AHCTREUNG
Ni22+ and dication Ni222+ are relatively stable since they
can be converted back to the neutral form of compound
Ni22 by reacting with a slight excess of ferrocene. In the
near-IR region, the monocation Ni22+ displays a broad in-
tervalence charge transfer (IVCT) band near the detection
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Porphyrin Dimers
COMMUNICATION
limit of 3385 cmÀ1, which disappears upon further oxidation
to the corresponding dication Ni222+. In this porphyrin
dimer system, the existence of the IVCT band upon one-
electron oxidation again demonstrates strong electronic cou-
pling between the two porphyrin units. For comparison pur-
poses, Ni24 and Ni5 were oxidized under similar conditions
and a broad IVCT band was not observed for the oxidized
species (Figures S3 and S4 in the Supporting Information).
Acknowledgements
The authors acknowledge the National Science Council of Taiwan for fi-
nancial support. This work is supported in part by the Ministry of Educa-
tion of Taiwan, under the ATU plan.
Keywords: conjugation · cross-coupling · electrochemistry ·
molecular electronics · porphyrinoids
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We proposed a simple synthetic approach that allows us
to incorporate a redox-active bridge into a porphyrin dimer.
In comparison with those of dimer Ni24, the electronic ab-
sorptions of porphyrin dimers Ni22 and Ni23 exhibit signifi-
cant peak-broadening and red shifts. The redoxpotentials of
the first and second oxidations for Ni22, corresponding to
the formation of Ni22+ and Ni222+ species, are separated by
90 mV. The mixed-valence species Ni22+ shows a broad
IVCT band and this broad IVCT band disappears for Ni222+
species. These spectroscopic and electrochemical data con-
firm that the two porphyrin units in Ni22 and Ni23 exhibit
strong electronic coupling while those in Ni24 are essentially
independent. Thus, we propose that in dimer Ni24 the qui-
none bridge can be reversibly switched off and on by elec-
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nonoid to through-conjugated benzenoid states (Scheme 2).
This type of porphyrin dimers holds promise to be used in
the application of controllable molecular switches. Further
studies on the switching properties of these porphyrin arrays
and related analogues are underway in our laboratory.
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40.4192(14), b=8.9943(3), c=41.0014(14) Š, b=118.8853(9)8, V=
13051.3(8) Š3, Z=8, 1calcd =1.233 gcmÀ3, T=150(2) K, 27950 mea-
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(all data), GOF=1.028. CCDC 668541 contains the supplementary
crystallographic data for this paper. These data can be obtained free
Scheme 2. Redox-controlled molecular switches.
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Received: March 20, 2008
Published online: April 17, 2008
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