Coordination-induced branched self-assembly of porphyrins bearing two redox-active phenylenediamine chains

Article abstract of DOI:10.1016/j.tetlet.2010.04.088

The porphyrins bearing two three-dimensionally regulated oligoaniline chains with terminal pyridyl groups were synthesized. The self-assembled branched polymer complex by introducing Zn(II) to the porphyrin was achieved in solution, which underwent dropca

Full text of DOI:10.1016/j.tetlet.2010.04.088

Tetrahedron Letters 51 (2010) 3376–3379
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Coordination-induced branched self-assembly of porphyrins bearing
two redox-active phenylenediamine chains
*
Toru Amaya, Taiki Ueda, 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:
The porphyrins bearing two three-dimensionally regulated oligoaniline chains with terminal pyridyl
groups were synthesized. The self-assembled branched polymer complex by introducing Zn(II) to the
porphyrin was achieved in solution, which underwent dropcasting on the surface of mica to result in
dome-like nanostructures.
Received 24 March 2010
Revised 16 April 2010
Accepted 22 April 2010
Available online 28 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Porphyrin
Oligoaniline
p-Conjugated compound
Self-assembly
Controlled assembly of
p
-conjugated molecules can provide an
regulated oligoaniline chains with terminal pyridyl groups
(Scheme 1) and their Zn(II)-induced assembly in solution, which
underwent dropcasting on the surface of mica to result in dome-
like nanostructures.
organized redox-active nano-space.1 Especially, construction of
such structures on a surface as well as in solution is promising to
make a nanotechnology press forward.2 Transition metal-directed
assembly is regarded as a useful approach to architecturally con-
trolled formation of various complexes.3 We have focused on the
design, synthesis, and catalytic and materials application of dimen-
Scheme 1 shows the synthesis of 1, 2, and their Zn(II) complexes
Zn-1 and Zn-2. cis-Diester 3, where the cis-configuration was
determined by the X-ray crystallographic analysis after the com-
sionally controlled
line oligomers as redox-active
p
-conjugated systems.4 In previous Letters, ani-
-conjugated ones were arranged
plexation with Zn(II) (Fig. S1),8 was hydrolyzed to result in the
0
p
carboxylic acid, followed by the amidation with N1,N1 -(1,4-pheny-
on the porphyrins scaffold,5 in which intramolecular photoinduced
electron transfer is performed from the phenylenediamine moiety
to the porphyrin scaffold. We also demonstrated the sandwich-
type assembly of the zinc complexes of such molecules by
coordination of bidentate ligands.6 Furthermore, Zn(II)-directed
intermolecular one-dimensional self-assembly was achieved by
introduction of a pyridyl group at the end of an oligoaniline chain.7
The assembly on the Au nanoparticles surface was also carried
out.7 In this context, we designed the porphyrin bearing two
oligoaniline chains, which have the pyridyl groups at the chain ter-
minal. Such molecule is considered to lead to a branched self-
assembly to form a kind of a dendric supramolecular structure as
shown in Figure 1. There are a lot of examples of the self-assembly
using axial coordination to the Zn–porphyrins. On the other hand,
such branched assembly using coordination to Zn–porphyrins with
two coordination moieties as a AB2 monomer has not been re-
ported, to the best of our knowledge. Herein, we report the synthe-
sis of the porphyrins 1 and 2 bearing two three-dimensionally
lene)dibenzene-1,4-diamine9 using EDCI in the presence of HOBt
Figure 1. Schematic representation of a branched self-assembly to form a kind of a
dendric supramolecular structure of the porphyrins bearing oligoaniline moieties
with terminal pyridyl groups.
* 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.2010.04.088

T. Amaya et al. / Tetrahedron Letters 51 (2010) 3376–3379
3377
Scheme 1. Synthesis of 1, 2, and their Zn(II) complexes Zn-1 and Zn-2.
and NEt3 to give the coupling product 4. The thus-obtained por-
phyrin 4 was coupled with isonicotinic acid or 2-(pyridin-4-
ylthio)acetic acid to afford the amide 1 or 2 in 51% or 88% yields,
respectively. 4-Pyridylthio group has a higher basicity than the
pyridyl group of the isonicotinic amide moiety. Therefore, the for-
mer is expected to show a greater affinity toward Zn(II)–porphy-
rins than the latter. Treatment of 1 and 2 with Zn(OAc)2ꢀ2H2O
gave the self-assembled product Zn-1 and Zn-2 in 97% and 64%
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 a pyridyl group to Zn-1 and Zn-2, which are
probably present in a monomeric state. Table 1 shows the se-
lected chemical shifts for the aniline chains. The protons close
to the porphyrin ring exhibited the remarkable higher field shift
in all compounds. The
D
d, which is the deference of the chemical
shift from the model aniline chain 5, reached 2.1–2.4 and 1.2–
1.3 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 chains lean toward the
porphyrin ring.10 Furthermore, the porphyrins with two cis-oli-
goaniline chains exhibit the higher field shift of the protons at
the oligoaniline chains in 1H NMR, as compared with 6 or Zn-67
with a single chain (Table 1). It suggests that the conformation
of the porphyrins with two cis-chains might be stabilized by the
intramolecular hydrogen bonds and/or
p–p stacking between
two cis-oligoaniline chains.11 In the present study, details of the
synthesis and characterization are described in Supplementary
data.
Table 1
Chemical shifts of 1, 2, 6, Zn-1, Zn-2, Zn-6, and 5 (DMSO-d6, 600 MHz)
–OCH2CO–
–NH1–
a (
D
d)
b (
D
d)
–NH2–
c
d
–NH3–
1
2
4.49
4.48
7.13
7.12
5.36 (2.1)
5.35 (2.11)
5.72 (1.21)
5.71 (1.22)
7.31
7.31
6.61
6.60
6.92
6.89
7.79
7.73
6
4.55
4.49
4.42
4.56
4.77
7.30
6.73
6.69
6.89
9.91
5.55 (1.89)
5.07 (2.39)
5.03 (2.43)
5.36 (2.08)
7.46
5.87 (1.06)
5.63 (1.3)
5.60 (1.33)
5.85 (1.08)
6.93
7.46
7.29
7.26
7.43
7.82
6.67
6.61
6.58
6.65
7.00
6.95
6.94
6.88
6.93
7.00
7.89
7.81
7.72
7.87
7.86
Zn-1
Zn-2
Zn-6
5

3378
T. Amaya et al. / Tetrahedron Letters 51 (2010) 3376–3379
To estimate the binding constant of the pyridyl group to the
Zn(II)–porphyrin, the titration experiment was conducted using
the model oligoaniline compounds S1 and S2 with a pyridyl and
a thiopyridyl group, respectively (Fig. S4). The CH2Cl2 solution of
S1 or S2 was added to the CH2Cl2 solution of Zn-3, and the com-
plexation behavior was tracked by UV–vis absorption spectroscopy
(Fig. S4). The peaks of the Soret and Q bands red-shifted as the
coordination of the pyridyl group progressed. The binding con-
stants K for S1 and S2 were calculated to be 5.6 ꢁ 103 and
7.6 ꢁ 103 Mꢂ1, respectively. As expected, the latter exhibited the
larger value.
First oxidation and reduction potentials were obtained from the
differential pulse voltammograms of 1, 2, 3, 5, and their Zn com-
plexes Zn-1, Zn-2, and Zn-3 (Fig. S5). Measurement was carried
out in their 2.5 ꢁ 10ꢂ4 M THF solution with Bu4NClO4 as an electro-
lyte under argon atmosphere. Table 2 shows the potentials versus
Fc/Fc+ and the driving force of photoinduced charge separation
D
GCS estimated by the equation:
D
GCS = E(DÅ+/D) ꢂ E(A/AÅꢂ) ꢂ E00
,
where E00 is the energy of photoexcitation. First oxidation poten-
tials are approximately ꢂ0.16 to ꢂ0.14 V for 1, 2, Zn-1, and Zn-2
assigned to the one-electron oxidation of the two aniline chains.
On the other hand, the first reduction potentials are ꢂ1.70 to
ꢂ1.66 V for free base porphyrins 1, 2, and 3, and ꢂ1.97 to
ꢂ1.90 V for Zn(II) complexes Zn-1, Zn-2, and Zn-3. Negative
DGcs
was observed with 1, 2, Zn-1, and Zn-2.
Electronic environment of 1, 2, 4, and their Zn(II) complexes Zn-
1, Zn-2, and Zn-4 was investigated by UV–vis absorption spectros-
copy. Measurement was carried out in a 5.0 ꢁ 10ꢂ6 M solution of
CH2Cl2/THF = 995/5 under argon atmosphere.12 Porphyrins bearing
the aniline chains showed a broad absorption around 280–350 nm
derived from the aniline chain as well as the characteristic Soret
and Q bands (Fig. 2a and Fig. S6). In the Zn complexes, the red-shift
of Soret and Q bands was observed with Zn-1 and Zn-2, as com-
pared with Zn-4 which does not have a pyridyl moiety (Fig. 2a).
The coordination of the pyridyl group to the Zn–porphyrin is likely
to contribute to the red-shift. Thus, the branched self-assembly of
the porphyrin Zn-1 and Zn-2 bearing two oligoaniline chains was
demonstrated in solution.13
Figure 2. (a) UV–vis absorption spectra of Zn-1, Zn-2, and Zn-4. (b) Emission
spectra of Zn-1, Zn-2, Zn-3, Zn-4, and Zn-6 with excitation at 549, 549, 542, 544,
and 561 nm, respectively. 5.0 ꢁ 10ꢂ6 M solution of CH2Cl2/THF = 995/5 under argon
atmosphere.
Fluorescence emission spectroscopy was studied with 1, 2, 3, 4,
and their Zn(II) complexes, Zn-1, Zn-2, Zn-3, Zn-4, and Zn-6
( Fig. 2b and Fig. S7). Measurement was also carried out in a
5.0 ꢁ 10ꢂ6 M solution of CH2Cl2/THF = 995/5 under argon atmo-
sphere. Significant quenching was observed with 1, 2, 4, Zn-1,
Zn-2, and Zn-4 bearing two aniline chains, as compared with that
of 3 or Zn-3. It is likely due to the intramolecular photoinduced
electron transfer as supported by the negative
DGcs (Table 2). This
phenomenon was also consistent with our previous reports.5 On
the other hand, Zn-6 with a single aniline chain did not show such
significant quenching despite the negative
fore, the thus-observed significant quenching in Zn-1 and Zn-2
D
Gcs (ꢂ0.26 V).7 There-
Table 2
The redox potential (V vs Fc/Fc+) and driving force of the photoinduced charge
separation
containing 0.1 M Bu4NClO4)
D
GCS (V) for 1, 2, 3, Zn-1, Zn-2, Zn-3, and
5
(2.5 ꢁ 10ꢂ4
M in THF
Redox potential (V vs Fc/Fc+)
D
GCS (V)
a
E(Por/PorÅꢂ
)
E(DÅ+/D)
1
2
3
Zn-1
Zn-2
Zn-3
5
ꢂ1.68
ꢂ0.14
ꢂ0.16
+0.61
ꢂ0.14
ꢂ0.16
+0.35
ꢂ0.11
ꢂ0.51
ꢂ1.66
ꢂ1.70
ꢂ1.90
ꢂ1.91
ꢂ1.97
—
ꢂ0.47
—
ꢂ0.38
ꢂ0.39
—
—
a
E(DÅ+/D) ꢂ E(A/AÅꢂ) ꢂ E00, where E00 is the energy of photoexcitation.
Figure 3. AFM images of (a) Zn-1 and 1, (b) Zn-2 and 2.

T. Amaya et al. / Tetrahedron Letters 51 (2010) 3376–3379
3379
clearly depends on the effect of two oligoaniline chains, which
Supplementary data
might be accounted for by the close conformation of the aniline
chains to the porphyrin scaffold induced by the hydrogen bonds
and/or p–p stacking as described above.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.04.088.
The assembly on the surface of mica was investigated with 1, 2,
Zn-1, and Zn-2 by atomic force microscopy (AFM) (Fig. 3a for Zn-1
References and notes
and 1, Fig. 3b for Zn-2 and 2). The dilute solution of THF (1 lM)
1. Redox Systems under Nano-Space Control; Hirao, T., Ed.; Springer: Berlin,
Heidelberg, New York, 2006.
was dropcasted on mica. After drying, AFM observation was carried
out (observed point is illustrated in Fig. S8). As shown in Figure 3a,
the uniformly sized dome-like structures (diameter: 50–70 nm,
height: ca. 3 nm, see the sectional view from a to b), which are
characteristic with a dendric structure,14 were observed with Zn-
1, but not with 1. Such images with the similar tendency were also
observed with Zn-2 and 2. Thus, the branched self-assembly of the
porphyrin bearing two aniline chains was also clearly demon-
strated on the surface of mica.
In summary, the porphyrins 1 and 2 bearing two three-dimen-
sionally regulated oligoaniline chains with terminal pyridyl groups
were synthesized. The branched complexation by introducing
Zn(II) to the porphyrins was achieved in solution, which under-
went dropcasting on the surface of mica to result in dome-like
nanostructures. The present method provides a new approach of
branched assembly using axial coordination to Zn–porphyrins. This
system is of potential use in a variety of applications such as redox-
active receptors and photo-active catalysts or materials. Further
investigation is now in progress.
2. Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671.
3. (a) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K.
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2001, 40, 2022; (c) Kobuke, Y.; Ogawa, K. Bull. Chem. Soc. Jpn. 2003, 76, 689; (d)
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8. Crystal data for Zn-3: triclinic, space group P1(#2), a = 9.9402(5) Å,
b = 12.2531(7) Å,
c
c = 17.5960(7) Å,
a
= 95.0608(16)°,
b = 98.3489(14)°,
= 93.3964(19)°, V = 2106.54(18) Å3, Z = 2; R1 = 0.0497; wR2 = 0.0773. The
data have been deposited with the Cambridge Crystallographic Data Centre:
CCDC-767568.
_
9. Rózalska, I.; Kulyk, P.; Kulszewicz-Bajer, I. New J. Chem. 2004, 28, 1235.
10. As Supplementary data for the conformation, the X-ray crystal structure of the
related porphyrin with a single chain is shown in Figure S2. Hirao, T.; Naka, S.
Unpublished result.
Acknowledgments
11. MM2 optimization of the related porphyrin with a double chain supports such
a conformation (Fig. S3).
12. THF (0.5%) co-solvent was used due to the solubility of the compounds, where
the complexation occurs through ligand exchange from THF.
13. The degree of coordinative self-assembly depends on the concentration due to
the equilibrium.
14. (a) Franz, A.; Bauer, W.; Hirsch, A. Angew. Chem., Int. Ed. 2005, 44, 1564; (b)
Fernández, G.; Pérez, E. M.; Sánchez, L.; Martín, N. J. Am. Chem. Soc. 2008, 130,
2410.
The authors thank Professor Shu Seki and Dr. Toshiyuki Moriu-
chi at Osaka University for the measurement of AFM and X-ray
crystallography, respectively. This work was partially supported
by a Grant-in-Aid for Scientific Research on Priority Areas ‘Chemis-
try of Concerto Catalysis’ from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.