Transition metal-directed self-assembly of porphyrins bearing redox-active phenylenediamine pendant

Article abstract of DOI:10.1016/j.tetlet.2010.02.120

Synthesis and Zn(II)-directed self-assembly of the porphyrin-aniline trimer bearing the pyridyl group were demonstrated. Its Au nanoparticle hybrid was also synthesized.

Full text of DOI:10.1016/j.tetlet.2010.02.120

Tetrahedron Letters 51 (2010) 2416–2419
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Transition metal-directed self-assembly of porphyrins bearing
redox-active phenylenediamine pendant
*
Toru Amaya, Yasutomo Shimizu, Yasuhide Yakushi, Yumiko Nishina, Toshikazu Hirao
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and Zn(II)-directed self-assembly of the porphyrin-aniline trimer bearing the pyridyl group
were demonstrated. Its Au nanoparticle hybrid was also synthesized.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 21 January 2010
Revised 16 February 2010
Accepted 22 February 2010
Available online 24 February 2010
Keywords:
Porphyrin
Oligoaniline
p-Conjugated compound
Self-assembly
Au nanoparticles
Controlled assembly of p-conjugated molecules can provide an
organized redox-active nano-space.1 Transition metal-directed
assembly is regarded as a useful approach to architecturally con-
trolled formation of conjugated polymeric complexes with
jugated ligands.2 We have focused on the design, synthesis, and
catalytic and material application of dimensionally controlled
conjugated systems.3 In the previous letters, aniline oligomers as
redox-active -conjugated ones were arranged on the porphyrin
p-con-
p-
p
scaffold,4 in which intramolecular photo-induced electron transfer
is performed from the phenylenediamine moiety to the porphyrin.
We also demonstrated the sandwich-type assembly of the zinc
complexes of such molecules by coordination of bidentate ligands.5
Introduction of a metal coordination site into such molecules is
considered to allow the metal ion-directed self-assembly. Due to
the existence of a vacant axial coordination site in a five-coordi-
nated metal ion/porphyrin complex, such as a Zn(II) one, poly-
meric, and/or oligomeric nanowire structures are considered to
be formed through intermolecular self-assembly (Fig. 1). The
assembly on the metal nanoparticle surface is also expected.
According to this concept, oligoaniline–porphyrin hybrids 1 and
2 were designed, where a pyridine moiety as a metal coordination
site was incorporated into the terminal of the oligoaniline chain.
Here, we report the synthesis and Zn(II)-directed self-assembly of
porphyrin-aniline trimer hybrids 1 and 2 (Scheme 1). The assembly
of 1 on Au nanoparticles is also described.
Figure 1. Schematic representation of transition metal-directed self-assembly of
porphyrins bearing an oligoaniline moiety with a metal coordination site.
Scheme 1 shows the synthesis of 1, 2, and their Zn(II) complexes
1
1
0
Zn-1 and Zn-2. Carboxylic acid 3 and N ,N -(1,4-phenylene)diben-
zene-1,4-diamine were prepared according to the reported proce-
dures.6,7 In the presence of an excess amount of the diamine,
condensation reaction of 3 with the diamine using PyBOP gave
the coupling product 4. The obtained porphyrin 4 was coupled
with isonicotinic acid or 2-(pyridin-4-ylthio)acetic acid to afford
the amide 1 or 2 (two-step yield, 61% and 62%, respectively). The
model aniline chains 5 and 6 without the porphyrin moiety were
also prepared (see Supplementary data). 4-Pyridylthio group has
a higher basicity than the pyridyl group of the isonicotinic acid
moiety. Therefore, the former is expected to show a greater affinity
* Corresponding author. Tel.: +81 6 6879 7413; fax: +81 6 6879 7415.
E-mail address: hirao@chem.eng.osaka-u.ac.jp (T. Hirao).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.120

T. Amaya et al. / Tetrahedron Letters 51 (2010) 2416–2419
2417
Scheme 1. Synthesis of 1, 2, and their Zn(II) complexes Zn-1 and Zn-2.
Table 1
Chemical shifts of 1, 2, Zn-1, Zn-2, and 7 (DMSO-d6, 600 MHz)
–OCH2CO–
–NH1–
a (D
d)
b (
D
d)
–NH2–
c
d
–NH3–
e
f
–NH4–
1
2
Zn-1
Zn-2
7
4.55
4.55
4.56
4.56
4.63
7.30
7.24
6.89
6.87
9.84
5.55 (1.89)
5.51 (1.93)
5.36 (2.08)
5.34 (2.10)
7.44
5.87 (1.06)
5.83 (1.10)
5.85 (1.08)
5.82 (1.11)
6.93
7.46
7.40
7.43
7.41
7.81
6.67
6.63
6.65
6.62
6.99
6.95
6.90
6.93
6.89
6.99
7.89
7.79
7.87
7.80
7.86
6.98
6.92
6.97
6.92
6.96
7.60
7.40
7.60
7.40
7.56
10.33
10.10
10.32
10.12
10.29
toward Zn(II)-porphyrins than the latter. Treatment of 1 and 2 with
Zn(OAc)2ꢀ2H2O gave the self-assembled products Zn-1 and Zn-2 in
96% and 91% yields, respectively. Conformation of 1, 2, Zn-1, and
Zn-2 in a DMSO-d6 solution was investigated by 1H NMR. DMSO-
d6 inhibits the coordination of the pyridyl group to Zn-1 and Zn-
2, which are probably present in a monomeric state. Table 1 shows
the selected chemical shifts for the aniline chains. The protons
close to the porphyrin ring exhibited the remarkable higher field
shift in all compounds. The
Dd, which is the difference in the chem-
ical shift from the model aniline chain 7, reached approximately 2
and 1 ppm at the protons a and b, respectively (Table 1). These
shifts are explained by the ring-current effect of the porphyrin p-
system. This suggests that the aniline chain leans toward the por-
phyrin ring.
To estimate the binding constant of the pyridyl group to the
Zn(II)–porphyrin, the titration experiment was conducted using
the model compounds 5 and 6 (Scheme 2). The CH2Cl2 solution
of 5 or 6 was added to the CH2Cl2 solution of Zn(II)-tetraphenyl
porphyrin (Zn-TPP), and the complexation behavior was tracked
by UV–vis absorption spectroscopy (Figs. S1 and S2). The peaks
of the Soret and Q bands red-shifted as the coordination of the pyr-
idyl group progressed. The binding constants K for 5 and 6 were
calculated to be 2.7 ꢁ 103 and 6.8 ꢁ 103 Mꢂ1, respectively. As ex-
pected, the latter exhibited the larger value.
Scheme 2. Equilibrium for the coordination of oligoaniline 5 or 6 toward Zn-TPP to
form the complex 5-Zn-TPP or 6-Zn-TPP.
Electronic environment of 1, 2, 4, TPP, and their Zn(II) com-
plexes Zn-1, Zn-2, Zn-4, and Zn-TPP was investigated by UV–vis

2418
T. Amaya et al. / Tetrahedron Letters 51 (2010) 2416–2419
Figure 3. Synthesis of NPs-1 and its TEM image.
the solution of TPP, the decreasing of emission intensity was ob-
served with the increasing of the concentration of the aniline
chain. Stern–Volmer plot gave a linear relationship, and the
Stern–Volmer constant was 1.6 ꢁ 103 Mꢂ1 (Fig. S5). Concerning
about the Zn(II)-complexes Zn-1, Zn-2, and Zn-4 bearing the ani-
line chain, their emission intensity was also smaller than that of
Zn-TPP. Zn-1 and Zn-2 showed the larger emission than Zn-4,
where the coordination from the pyridyl moiety to the Zn(II) center
is likely to contribute.
Figure 2. (a) UV–vis absorption spectra of Soret and Q band region for Zn-1, Zn-2,
Zn-4, and Zn-TPP. (b) Emission spectra of 1, 2, TPP, Zn-1, Zn-2, Zn-4, and Zn-TPP
with excitation of the Qy(1,0) or Q(1,0) band for the free base porphyrins or Zn-
porphyrins, respectively. 5.0 ꢁ 10ꢂ6 M solution of CH2Cl2/THF = 995:5 under argon
atmosphere.
absorption spectroscopy. Measurement was carried out in the
5.0 ꢁ 10ꢂ6 M solution of CH2Cl2/THF = 995:5 under argon atmo-
sphere.8 Porphyrins bearing an aniline chain showed a broad
absorption around 280–350 nm derived from the aniline chain as
well as the characteristic Soret and Q bands (Fig. S3). Red-shift of
Soret and Q bands was observed with the Zn complex, Zn-1 and
Zn-2, as compared with Zn-TPP, however, the smaller shift was
observed with the Zn-4 complex which does not have a pyridyl
moiety (Figs. 2a and S4). The coordination of the pyridyl group to
the Zn-porphyrin is likely to contribute to the red-shift. Thus, the
self-assembly of the porphyrin bearing an aniline chain was
suggested.
Fluorescence emission spectroscopy with excitation of the
Qy(1,0) or Q(1,0) band was studied with 1, 2, TPP, and their Zn(II)
complexes, Zn-1, Zn-2, Zn-4, and Zn-TPP (Fig. 2b).9 Measurement
was also carried out in the 5.0 ꢁ 10ꢂ6 M solution of CH2Cl2/
THF = 995:5 under argon atmosphere. Significant quenching was
observed with 1 and 2 bearing the aniline chain, as compared with
that of TPP. It is likely due to the intramolecular photo-induced
electron transfer. This phenomenon was also consistent with our
previous reports.4 When the model aniline chain was added to
First oxidation and reduction potentials were obtained from the
differential pulse voltammograms of 1, 2, 7, TPP, and their Zn com-
plexes Zn-1, Zn-2, and Zn-TPP (Fig. S6). Measurement was carried
out in their 0.25 mM THF solution with Bu4NClO4 as an electrolyte
under argon atmosphere. Table 2 shows the potentials versus Fc/Fþ
c
and the driving force of photo-induced charge separation
DGCS
estimated by the equation:
D
GCS = E(DÅ+/D) ꢂ E(A/AÅꢂ) ꢂ E00, where
E00 is the energy of photoexcitation. First oxidation potentials are
approximately ꢂ0.14 V for 1, 2, Zn-1, Zn-2, and 7, assigned to
the one-electron oxidation of the aniline chain. On the other hand,
first reduction potentials are approximately ꢂ1.7 V for free base
porphyrins 1, 2, and TPP, and ꢂ1.9 V for Zn(II)-complexes Zn-1,
Zn-2, and Zn-TPP. Negative
DGCS was observed with 1, 2, Zn-1,
and Zn-2, which supports the results of decreasing of the emission
and suggests the intramolecular electron transfer.
Pyridyl group is known to act as a protecting group for Au nano-
particles.10 So, Au nanoparticles NPs-1 were synthesized by the
reduction of Na(AuCl4) using NaBH4 in the presence of the porphy-
rin 1. Its transmission electron microscopy (TEM) image is shown
in Figure 3. The average diameter is 3.0 nm and the particles are
independent. 1H NMR showed the presence of the porphyrins
without decomposition. There is no doubt that the porphyrins
act as a protecting group for the Au nanoparticles although direct
evidence was not obtained for the coordination of the pyridyl
group to the Au nanoparticles.
In summary, synthesis and Zn(II)-induced self-assembly of 1
and 2 were demonstrated. Au nanoparticles protected with 1 were
also synthesized. They are of potential use in a variety of applica-
tions such as redox-active receptors and photo-active catalysts or
materials. Further investigation is now in progress.
Table 2
The redox potential (V vs Fc/Fþc ) and driving force of the photo-induced charge
separation
DGCS (V) for 1, 2, TPP, Zn-1, Zn-2, Zn-TPP, and 7 (0.25 mM in THF
containing 0.1 M Bu4NClO4)
a
Redox potential (V vs Fc/Fcþ
)
D
GCS (V)
E(Por/PorÅꢂ
)
E(DÅ+/D)
1
2
ꢂ1.68
ꢂ0.14
ꢂ0.14
+0.60
ꢂ0.14
ꢂ0.15
+0.40
ꢂ0.14
ꢂ0.38
ꢂ0.38
—
ꢂ0.26
ꢂ0.29
—
ꢂ1.68
ꢂ1.70
ꢂ1.93
ꢂ1.90
ꢂ1.93
—
TPP
Zn-1
Zn-2
Zn-TPP
7
Acknowledgments
The authors thank Ms. Toshiko Muneishi at Osaka University for
the measurement of the NMR spectra. This work was partially sup-
ported by a Grant-in-Aid for Scientific Research on Priority Areas
—
a
E(DÅ+/D) ꢂ E(A/AÅꢂ) ꢂ E00, where E00 is the energy of photoexcitation.

T. Amaya et al. / Tetrahedron Letters 51 (2010) 2416–2419
2419
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