Supramolecular assemblies formed by new L-lysine derivatives of viologens?

Article abstract of DOI:10.1039/b106513k

L-Lysine derivatives of viologens form supramolecular assemblies of fibers and ribbons in some aromatic solvents, and the charge separation reaction in these self-assembling systems proceeds with a similar efficiency to the MV²⁺ system.

Full text of DOI:10.1039/b106513k

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Supramolecular assemblies formed by new
viologens†
L-lysine derivatives of
a
b
c
a
c
Masahiro Suzuki,* Chad C. Waraksa, Hiroko Nakayama, Kenji Hanabusa, Mutsumi Kimura
and Hirofusa Shiraic
a
Graduate School of Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan.
E-mail: msuzuki@giptc.shinshu-u.ac.jp; Fax: +81-268-24-7248
b
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
c
Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu
University, Ueda, Nagano 386-8567, Japan
Received (in Cambridge, UK) 1st August 2001, Accepted 4th September 2001
First published as an Advance Article on the web 19th September 2001
L
-Lysine derivatives of viologens form supramolecular
constants of the elemental reactions were measured by laser
flash photolysis.8
assemblies of fibers and ribbons in some aromatic solvents,
and the charge separation reaction in these self-assembling
systems proceeds with a similar efficiency to the MV2+
system.
Both 1 and 2 were completely dissolved with heating in all
organic solvents in which the gelation ability was examined.
After dissolution of 1 and 2 by heating, they formed gels in
aromatic solvents (benzene, toluene, nitrobenzene, chloro-
benzene) and reprecipitated in alcohols. In DMF, DMSO, and
chloroform, both 1 and 2 have a high solubility and give an
isotropic solution. TEM images of samples prepared from these
solvents demonstrated that the viologens formed fibrous
assemblies in aromatic solvents and alcohols, but not in
chloroform; in alcohols, small fibers of 1 and 2 were observed
for samples prepared from the dilute solutions (see ESI†). These
facts imply that 1 and 2 hardly interact with each other in
chloroform and strongly interact in alcohols, which leads to the
isotropic solution in chloroform and reprecipitation in the
alcohols. As shown in Fig.1, 1 (A) and 2 (B) prepared from
toluene create nanometer scale network structures with ca.
250–700 nm width for 1 and ca. 300–400 nm width for 2 formed
by entangling of fine fibers (ca. 30–100 nm width) or ribbons
One of the keywords in supramolecular chemistry, which
utilizes relatively weak, noncovalent interactions such as
hydrogen bonding, p–p stacking, electrostatic, and van der
1
Waals interactions, is self-assembly. Self-assembly, which is
the spontaneous organization of molecules driven by non-
covalent interactions or objects into stable aggregates, has been
well recognized in biological systems: lipid bilayers, the DNA
2
duplex, and tertiary and quaternary structure of proteins. In
chemical synthesis, self-assembly offers considerable advan-
tages over stepwise bond formation in the construction of large
supramolecular assemblies. Many reports have appeared on
supramolecular assemblies formed from organic compounds,
dendrimers, macromolecules, and coordination compounds.3
Ordered, self-assembling aggregates of low molecular weight
compounds formed through noncovalent interactions such as
hydrogen bonding, van der Waals interactions and p–p stacking
9
(ca. 70–200 nm width). It is noteworthy that the viologens
form different nanostructures; a fibrous structure for 1 and a
ribbon structure for 2.
4
have been of great interest. These supramolecular assemblies
are to some extent similar to macromolecules, as repeating units
linked to each other by either noncovalent or covalent bonds
To consider the factor of self-assembling behavior, we
measured the FTIR spectrum of 1 and 2. The FTIR spectra in
chloroform, in which no self assembly occurs, show the
(
can be both) give a network structure in various solvents,
2
1
resulting in gelation or a high viscosity. In contrast to the gels
obtained from supramolecules, which can be disintegrated by
addition of a cosolvent or by heating, the network structures
obtained from macromolecules are decomposed only by
breaking chemical bonds. Several low molecular mass com-
absorption bands at 3440 and 1665 cm , characteristic of non-
hydrogen bonded N–H and CNO stretching vibrations. In
contrast, the FTIR spectra in toluene show almost the same
spectra in the solid state and are characterized by a band at 3320
2
1
cm
which is assigned to N–H stretching vibrations of
10
pounds which form network structures in organic fluids have
hydrogen bonded amide groups, which indicates that the
driving force for the self-assembly is mainly hydrogen bonding.
The fine nanofibers and nanoribbons are entangled in a complex
way probably through van der Waals interaction between the
terminal alkyl groups and the three-dimensional network
been reported.5 Recently,
,6
L
-lysine derivatives have been found
to form supramolecular fibrous network structures on the
nanometer scale.7 Here we describe the formation of self-
assembling fibers from new
L
-lysine derivatives of viologens
and their photosensitized charge separation using tris(2,2A-
2+
bipyridine)ruthenium(II) [Ru(bpy)
3
] as a photosensitizer.
The viologen derivatives (1, 2) were prepared in high yield
according to the synthetically simple approach as shown in
Scheme 1. The gelation ability of 1 and 2 was evaluated after
allowing solutions to stand at rt for 6 h followed by dissociation
into organic solvents with heating. Photosensitized charge
separation reactions were carried out in methanol–toluene
2
5
2+
23
containing 1.0 3 10 M Ru(bpy)
3
, 1.0 3 10 M viologen,
and 0.1 M triethanolamine (TEOA) under an argon atmosphere.
Visible light irradiation was performed using a 300 W Xe lamp
equipped with a UV cutoff filter (l > 440 nm). The rate
†
Electronic supplementary information (ESI) available: experimental
details and Figs. S1 and S2. See http://www.rsc.org/suppdata/cc/b1/
b106513k/
Scheme 1
2012
Chem. Commun., 2001, 2012–2013
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106513k

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Table 1 Kinetic parameters obtained from laser flash photolysis
(M2
1
s
21 a
)
k
b
(M21
s
21 b
)
(M21
21 c
k s )
s
k
q
7.0 3 108
1.7 3 10
2.3 3 10
5.1 3 10
10
10
10
2.7 3 107
1
8
7
2
6.2 3 10
1.9 3 10
2+
6.9 3 108
1.5 3 107
MV
a
Stern–Volmer plot (t
0
/t vs. [viologen]) using laser flash photolysis. b Rate
constants for back electron transfer. c The rate constants of re-reduction
reaction of Ru(iii) with TEOA corresponding to eqn. (5).
In summary, we have found that new -lysine derivatives of
L
viologens form gels containing nanometre scale organized
assemblies in some organic solvents. In particular, the viologens
create a network structure by the entangling of nanofibers for
viologen 1 and nanoribbons for viologen 2 in some aromatic
Fig. 1 TEM images of viologens 1 (A) and 2 (B) prepared from toluene
solution.
2+
solvents. In the viologen assembly–Ru(bpy)
3
systems con-
taining a sacrificial electron donor, the charge separation
2+
2+
reaction proceeds with a similar efficiency to MV –Ru(bpy)
3
because of the restriction of the back electron transfer and
acceleration of re-reduction of the Ru(III). Furthermore, we find
that the nanostructure is controllable by changing the length of
the alkylene spacer.
This work was supported by the Division of Chemical
Sciences, Office of Basic Energy Sciences, Department of
Energy, under contract DE-FG02-93-ER14374 and Grant-in-
Aid for COE research (10CE2003) from the Ministry of
Education, Culture, Sports, Science, and Technology of
Japan.
Notes and references
Fig. 2 Viologen radical formation upon visible light irradiation for V1 (2),
1
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1
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2
3
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segments.
L
-lysine
Next, the photosensitized charge separation reaction was
11
carried out in a mixture of methanol and toluene (1+1 v/v).
Fig. 2 shows the formation of a viologen radical upon visible
light irradiation. Very interestingly, the viologen radical
formation in the self-assemblies is the almost the same as in
2
+
4 D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed. Engl., 1996, 35, 4;
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separation proceeds according to eqns. (1)–(5):
Ru(II) + hv ? *Ru(II
)
(1)
(2)
5 Y.-C. Lin, B. Kacher and R. G. Weiss, J. Am. Chem. Soc., 1989, 111,
5
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1
2
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q
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(
3)
6
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+
k
b
2+
Ru(iii) + V ææÆRu(ii) + V
(4)
(5)
7
k
s
Ru(iii) + TEOA ææÆRu(ii) + TEOAox
0 q b s
where k , k , k , and k represent the rate constants for the
corresponding reactions. These values are obtained by laser
flash photolysis and are listed in Table 1. The rate constants of
8
9
Similar TEM images were observed for samples prepared from other
solvents that formed gel.
q
the quenching reaction (forward reaction), k , are almost the
same values regardless of the viologens. In contrast, the self-
assembling viologens restrict the back electron transfer and
accelerate the re-reduction of the Ru(III) to Ru(II) with TEOA.
This is probably due to an electrostatic repulsion between the
10 In toluene, a hydrogen bonded CNO stretching vibration could not be
detected in the FTIR spectra because the toluene’s own spectrum
2
1
overlaps with viologens around 1650 cm
.
2+
1
1 To dissolve Ru(bpy)
3
and triethanolamine (TEOA), the reaction was
carried out in methanol–toluene. The viologens did not gel in methanol–
toluene under the reaction conditions, but we obtained the TEM images
showing that the viologens formed fibrous nanostructures (but not
networks).
positively charged viologen assembly and the ruthenium(III
)
complex and a concentration of TEOA in the vicinity of the
assemblies.
Chem. Commun., 2001, 2012–2013
2013