Design and characterization of ferrocene-peptide-oligoaniline conjugates

Article abstract of DOI:10.1016/j.tetlet.2010.06.101

Ferrocene-peptide-oligoaniline conjugates were designed by the introduction of two ferrocene-peptide conjugates into a π-conjugated phenylenediamine spacer to demonstrate the luminescent switching by changing the redox states of the π-conjugated phenylenediamine moiety, wherein the self-aggregation of the π-conjugated moiety was achieved in a higher concentration to induce the chirality organization with a red shift of the maximum emission wavelength.

Full text of DOI:10.1016/j.tetlet.2010.06.101

Tetrahedron Letters 51 (2010) 4530–4533
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Tetrahedron Letters
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Design and characterization of ferrocene–peptide–oligoaniline conjugates
*
*
Toshiyuki Moriuchi , Nami Kikushima-Honda, Satoshi D. Ohmura, 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:
Ferrocene–peptide–oligoaniline conjugates were designed by the introduction of two ferrocene–peptide
conjugates into a -conjugated phenylenediamine spacer to demonstrate the luminescent switching by
changing the redox states of the -conjugated phenylenediamine moiety, wherein the self-aggregation of
the -conjugated moiety was achieved in a higher concentration to induce the chirality organization with
a red shift of the maximum emission wavelength.
Received 28 April 2010
Revised 14 June 2010
Accepted 21 June 2010
Available online 25 June 2010
p
p
p
Ó 2010 Elsevier Ltd. All rights reserved.
Architectural control of molecular arrangement of
p
-conjugated
phenylenediamine spacer as shown in Figure 1. The advantage in the
use of the -alanyl-proline sequence as a dipeptide chain depends on
a hydrogen bonding alanyl moiety and a sterically constrained
proline as a well-known turn inducer in proteins. The ferrocene–
peptide–phenylenediamine conjugate 1red was synthesized from
(chlorocarbonyl)ferrocene and the corresponding dipeptidyl
oligoaniline 2red in 66% yield. The redox properties of 1red were
investigated by cyclic voltammetry. The ferrocene–peptide–phenyl-
enediamine conjugate 1red in dichloromethane exhibited two oxi-
dation waves at 0.30 and 0.67 V versus Fc/Fc+ as shown Figure 2.
The peak with threefold height at 0.30 V might be assignable to
the one-electron oxidation processes of two ferrocene moieties
and one-electron oxidation of the phenylenediamine moiety. The
wave at 0.67 V is assumed to be attributable to the one-electron oxi-
dation of the partially oxidized phenylenediamine radical cation to
the oxidized quinonediimine. The ferrocene–peptide–phenylenedi-
amine conjugate 1red was easily oxidized into the ferrocene–pep-
tide–quinonediimine conjugate 1ox by treatment with lead(IV)
acetate as an oxidant in 80% yield. The oxidized form 1ox could be
again reduced to 1red with hydrazine monohydrate (Fig. 3). These
ferrocene–peptide–oligoaniline conjugates were fully characterized
by spectral data and elemental analyses.
systems has gained increasing interest due to their potential appli-
cations as advanced molecular devices based on defined functional
properties.1 The utilization of molecular self-organization is one of
the most important strategies for the development of functional
materials.2 Control of hydrogen bonding3 has attracted much atten-
tion in the design of various molecular assemblies by virtue of the
directionality and specificity.4 The utilization of self-assembling
properties of amino acids, which possess chiral centers and hydro-
gen bonding sites, is considered to be a convenient approach to
highly ordered molecular assemblies. Recently, we have embarked
upon the introduction of peptides or amino acids into molecular
scaffold to induce chirality organization through the chirality of
amino acids and the intramolecular interchain hydrogen bond-
ing.5–7 In previous Letters, the introduction of one dipeptide chain
(-Ala-Pro-) into the ferrocene scaffold was demonstrated to permit
well-defined chirality organization through multiple hydrogen-
bonded network.5b,f,j,l,n,o,8 On the other hand, the redox-active
phenylenediamine, which exists in three redox forms: the reduced
phenylenediamine, the partially oxidized phenylenediamine radical
cation, and the oxidized quinonediimine, is a molecular unit of poly-
anilines as p-conjugated polymers. The redox switching of the emis-
sion properties of Ru(II) dipyridyl complexes bearing the
phenylenediamine moieties by changing the redox states of the
phenylenediamine moiety has already been demonstrated in
previous Letters.9 The utilization of self-assembling properties of
ferrocene–peptide conjugates is envisioned to regulate the molecu-
In the 1H NMR spectra of 1red in CDCl3 (5.0 Â 10À4 M), the NH
resonances of the central phenylenediamine moiety were hardly
perturbed by the addition of aliquots of DMSO-d6 to CDCl3 (CDCl3:
8.74 ppm, CDCl3/DMSO-d6 (9:1): 8.71 ppm). On the contrary, a
down-field shift was observed with the Ala and C-terminal amide
NH resonances (CDCl3: 6.48 and 9.09 ppm, CDCl3/DMSO-d6 (9:1):
7.10 and 9.56 ppm, respectively). These results indicate the forma-
tion of intramolecular hydrogen bonds between the NH of the cen-
tral phenylenediamine moiety and the CO of the ethyl ester moiety
although the Ala and C-terminal amide NHs do not participate in
intramolecular hydrogen bonding in solution. In the case of 1ox,
the Ala and C-terminal amide NH resonances were perturbed by
the addition of aliquots of DMSO-d6 to CDCl3 (CDCl3: 6.45 and
9.33 ppm, CDCl3/DMSO-d6 (9:1): 6.89 and 9.78 ppm, respectively),
lar arrangement of
p-conjugated systems. From these points of
view, we herein report the design and characterization of ferro-
cene–peptide–oligoaniline conjugates.
Our design is based on the symmetrical introduction of two fer-
rocene–peptide conjugates (Fc-CO-L-Ala-L-Pro-) into a p-conjugated
* Corresponding authors. Tel.: +81 6 6879 7413; fax: +81 6 6879 7415.
E-mail addresses: moriuchi@chem.eng.osaka-u.ac.jp (T. Moriuchi), 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.2010.06.101

T. Moriuchi et al. / Tetrahedron Letters 51 (2010) 4530–4533
EtO
4531
O
H
H
Boc
N
N
N
O
O
N
H
H
N
O
O
N
N
H
N
Boc
H
O
OEt
2red
HCl / Et2O
EtOH, rt, 15 h, Ar
COCl
Fe
, DMAP, NEt3
º
CH2Cl2, 0 C, 1 h →rt, 18 h, Ar
EtO
O
O
H
N
H
N
N
Fe
N
H
O
O
H
N
O
O
N
N
H
N
Fe
H
O
O
OEt
1red
Figure 1. Synthesis of ferrocene–peptide–phenylenediamine conjugate 1red.
indicating the absence of intramolecular hydrogen bonds in a solu-
tion state.
The UV–vis spectra of the reduced form 1red and the oxidized
form 1ox in dichloromethane showed a broad absorption at around
400–550 and 400–650 nm, respectively, which are probably due to
a low-energy charge-transfer transition of the p-conjugation moi-
10 μA
ety (Fig. 4a). The reduced form 1red exhibited luminescence at
593 nm (Fig. 4b). It is noteworthy to mention that weak lumines-
cence was observed with the oxidized form 1ox. These findings
indicate that the redox switching of the luminescent properties is
achieved by changing the redox states of the p-conjugated moiety.
0.2 0.4 0.6 0.8
V vs Fc/Fc+
1.0
1.2
-0.4
-0.2 0.0
The ferrocene–peptide–phenylenediamine conjugate 1red is
expected to self-assemble through contribution of all available
hydrogen bonding donors of the Ala and C-terminal amide moieties
in a high concentration to change the luminescent properties.
Circular dichroism (CD) spectrometry is a useful tool to determine
Figure 2. Cyclic voltammogram of 1red in dichloromethane (5.0 Â 10À4 M)
containing 0.1 M Bu4NClO4 at a glassy carbon working electrode with scan rate
100 mVsÀ1 under an atmosphere of argon.
EtO
O
O
H
H
N
N
N
Fe
N
H
O
O
H
N
O
O
N
H
N
H
N
Fe
O
O
OEt
1red
Pb(OAc)4
CH2Cl2, rt, 3 h, Ar
H2NNH2 H2O
CH2Cl2, rt, 1 h, Ar
EtO
O
O
H
N
N
N
Fe
N
H
OO
H
N
O
O
N
N
H
N
Fe
O
O
OEt
1ox
Figure 3. Redox interconversion between 1red and 1ox.

4532
T. Moriuchi et al. / Tetrahedron Letters 51 (2010) 4530–4533
a
b
1red
1ox
1red
1ox
2.0
1.0
0.0
550
600
650
700
750
800
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Figure 4. (a) Electronic spectra of 1red and 1ox in dichloromethane (5.0 Â 10À5 M). (b) Emission spectra of 1red (kexc = 475 nm, 5.0 Â 10À5 M, 298 K) and 1ox (kexc = 517 nm,
5.0 Â 10À5 M, 298 K) in dichloromethane.
5
a
b
5.0 x 10−5 M
5.0 x 10−3 M
5.0 x 10−5 M
5.0 x 10−3 M
0
−5
θ
−10
550
600
650
700
750
800
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Figure 5. (a) CD spectra of 1red in 5.0 Â 10À5 M and 5.0 Â 10À3 M dichloromethane solution. (b) Emission spectra of 1red in 5.0 Â 10À5 M and 5.0 Â 10À3 M dichloromethane
solution (kexc = 475 nm, 298 K).
an ordered structure in solution. Interestingly, an induced circular
dichroism (ICD) at the absorbance region of the -conjugated moi-
Acknowledgments
p
ety was observed in the CD spectrum of 1red in a high concentration
dichloromethane solution (5.0 Â 10À3 M) although such ICD was not
detected in a dilute dichloromethane solution (5.0 Â 10À5 M) as
shown in Figure 5a. The ferrocene–peptide conjugates bearing one
This work was supported by Grant-in-Aid for Science Research
on Priority Areas from Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan. Thanks are also due to the Analytical
Center, Graduate School of Engineering, Osaka University, for the
use of the NMR, MS, and CD instruments.
dipeptide chain (-L-Ala-L-Pro-) have been demonstrated to induce
well-defined chirality organization through multiple hydrogen-
bonded network in the crystal packing.5b,f,o Chirality organization
Supplementary data
of the
p-conjugated moiety of 1red might be achieved by self-
assembly through intermolecular hydrogen bondings. Although
luminescence was observed at 593 nm in a dilute dichloromethane
solution of 1red (5.0 Â 10À5 M), 1red exhibited luminescence at
609 nm in a high concentration dichloromethane solution (5.0 Â
Supplementary data (general information and experimental de-
tails for syntheses and characterization of 1red and 1ox) associated
with this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.06.101.
10À3 M) (Fig. 5b). A red shift of the luminescence suggests
p–pinter-
action based on the intermolecular aggregation with increasing
concentration. This self-aggregation is consistent with the result of
the CD spectrum in a high concentration.
In conclusion, ferrocene–peptide–oligoaniline conjugates were
designed to demonstrate the luminescent switching by changing
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The introduction of the ferrocene–peptide conjugates (Fc-CO-
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found to induce the self-aggregation of the -conjugated moiety,
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