The self-assembly of amphiphilic oligothiophenes:Hydrogen bonding and poly(glutamate) complexation

Article abstract of DOI:10.1246/bcsj.80.1703

The self-organization behavior of an amphiphilic sexithiophene bearing amide functionalities is studied and compared to that of a derivative bearing an ester group at the same position. The introduction of hydrogen-bond interactions in assemblies of these Π-conjugated oligomers is found to affect the molecular organization both in protic media and in thin deposits on mica. The amphiphilic 2,2′;5′,2″;5″,2?;5?, 2″″;5″″,2′″″-sexithiophene-5, 5′″″-dicarboxylic acid bis[(4,7,10,13,16-pentaoxaheptadecyl) amide] forms assemblies in n-butanol and water and partially aggregates in toluene. Spectroscopy reveals that the presence of a hydrogen-bonding moiety increases the thermal stability of the assemblies in n-butanol and even more in water solution. On mica surfaces, the formation of rod-like one-dimensional nano-structures is observed after deposition from toluene solutions. In addition, transmission electron microscopy in combination with selected area electron diffraction shows that in water plate-like structures are formed built from parallel oriented stacks, with a Π-Π distance of 3.5 A. Comparison of these data to molecular modeling and quantum chemistry calculations is used to better understand the influence of the amide group on the stacking of these compounds. The introduction of these H-bonding interactions leads to denser and more stable stacks. Furthermore, we show that a derivative of the amide compound, bearing terminal ammonium groups, forms a complex with chiral polyanions in aqueous media such that the sexithiophene segments are stacked in a meta-stable helical fashion with preferred handedness. We observed that poly(glutamate) and DNA generate a chiral sexithiophene assembly. In time the induced chirality disappears, which is explained by the meta-stability of the kinetically formed adduct. This constitutes one step forward towards the controlled formation of functional multi-component systems in aqueous solution.

Full text of DOI:10.1246/bcsj.80.1703

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ꢀ 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 9, 1703–1715 (2007) 1703
BCSJ Award Article
The Self-Assembly of Amphiphilic Oligothiophenes:
Hydrogen Bonding and Poly(glutamate) Complexation
Fiorella Brustolin,1 Mathieu Surin,2 Vincent Lemaur,2 Giuseppe Romanazzi,3
1
2
2
1
´ ˆ
Qianyao Sun, Jerome Cornil, Roberto Lazzaroni, Nico A. J. M. Sommerdijk,
ꢀ1;2
ꢀ1
`
Philippe Leclere,
and E. W. Meijer
1Laboratory for Macromolecular and Organic Chemistry, Eindhoven University of Technology,
P. O. Box 513, 5600 MB Eindhoven, The Netherlands
2Laboratory for Chemistry of Novel Materials, University of Mons-Hainaut,
B-7000, Mons, Belgium
3Department of Water Engineering and Chemistry (DIAC), Polytechnic of Bari,
via Orabona 4, I-70125 Bari, Italy
Received March 12, 2007; E-mail: e.w.meijer@tue.nl
The self-organization behavior of an amphiphilic sexithiophene bearing amide functionalities is studied and com-
pared to that of a derivative bearing an ester group at the same position. The introduction of hydrogen-bond interactions
in assemblies of these ꢀ-conjugated oligomers is found to affect the molecular organization both in protic media
and in thin deposits on mica. The amphiphilic 2,20;50,200;500,2000;5000,20000;50000,200000-sexithiophene-5,500000-dicarboxylic acid
bis[(4,7,10,13,16-pentaoxaheptadecyl)amide] forms assemblies in n-butanol and water and partially aggregates in tolu-
ene. Spectroscopy reveals that the presence of a hydrogen-bonding moiety increases the thermal stability of the assem-
blies in n-butanol and even more in water solution. On mica surfaces, the formation of rod-like one-dimensional nano-
structures is observed after deposition from toluene solutions. In addition, transmission electron microscopy in combi-
nation with selected area electron diffraction shows that in water plate-like structures are formed built from parallel ori-
˚
ented stacks, with a ꢀ–ꢀ distance of 3.5 A. Comparison of these data to molecular modeling and quantum chemistry
calculations is used to better understand the influence of the amide group on the stacking of these compounds. The in-
troduction of these H-bonding interactions leads to denser and more stable stacks. Furthermore, we show that a deriva-
tive of the amide compound, bearing terminal ammonium groups, forms a complex with chiral polyanions in aqueous
media such that the sexithiophene segments are stacked in a meta-stable helical fashion with preferred handedness. We
observed that poly(glutamate) and DNA generate a chiral sexithiophene assembly. In time the induced chirality disap-
pears, which is explained by the meta-stability of the kinetically formed adduct. This constitutes one step forward
towards the controlled formation of functional multi-component systems in aqueous solution.
The possibility to generate materials with improved elec-
tronic properties by controlling the molecular organization of
conjugated polymers and oligomers has been driving the de-
sign of self-assembling ꢀ-conjugated materials for several
years now.1 In particular, the pre-organization of well-defined
oligomeric systems such as ꢁ,!-disubstituted sexithiophene
derivatives has been demonstrated as a promising approach
to conjugated materials with a high degree of spatial orienta-
tion and packing of the constituent building blocks.2 In this
context, the possibility of organizing these materials via as-
sembly in aqueous media has recently attracted attention, lead-
ing to the design of amphiphilic ꢀ-conjugated systems.3 This
is exemplified by the formation of aggregates from sexithio-
phenes bearing oligo(ethylene oxide)-substituted end groups4
(2, Chart 1) or cationic end groups5 and oligo(p-phenylene-
vinylene) (OPV) and others with oligo(ethylene oxide)6 or
sulfonic acid7 side groups, in which hydrophobic interactions
have been utilized as the driving force for the formation of
the assemblies.
The induction of helical order in conjugated polymers has
been achieved, e.g., through the interplay of supramolecular
and ionic interactions of enantiopure low molecular weight
compounds with various polymer side chains,8 and through
the synthesis of poly(acetylene)s within a chiral nematic
liquid-crystalline phase.9 In contrast, the construction of heli-
cal assemblies of ꢀ-conjugated oligomers has been mainly
accomplished through the use of intrinsically chiral building
blocks10 or via ‘‘Sergeant-and-Soldiers’’ effects11 and only
Published on the web September 13, 2007; doi:10.1246/bcsj.80.1703

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1704 Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007) BCSJ AWARD ARTICLE
O
O
S
O
O
O
O
O
O
O
O
O
N
H
N
H
O
O
O
6
1
O
O
S
O
O
O
O
O
O
O
O
O
O
O
O
6
2
O
O
S
O
O
O
O
-
NH + 2 CF COO
O
O
N
H
N
H
O
O
O
3
3
+H N
3
6
3
Chart 1.
CN
CN
O
O
O
O
O
O
O
O
OH
4
1% KOH
O
S
S
Br
Cl
1) BH3.THF, reflux
2) CH3OH, HCl
6
O
O
NH
2
O
O
O
Et2O, RT
5
S
S
O
Sn
Sn
S
S
O
O
N
O
O
O
Br
8
H
1
7
Pd(PPh3)4, DMA, 120°C
Scheme 1.
few examples exist in which helicity is generated through
induction by solvents or other chiral molecules.12,13 Since
it has been long demonstrated that the use of hydrogen-bond-
ing moieties can improve the positional order of component
molecules, e.g., in surfactant aggregates,14 low molecular
weight organogels15–17 and liquid-crystalline discotics,18,19
we designed two amphiphilic oligomers 1 and 3 (Chart 1)
consisting of a sexithiophene unit end-capped by two oligo-
(ethylene oxide) chains that are linked to the heteroaromatic
core by amide bonds. These are expected to lead to the forma-
tion of a hydrogen-bond motif that directs and stabilizes the
ordered assembly and the resulting ꢀ–ꢀ stacking imposed
by the hydrophobic interactions of 1 and 3 in polar protic
media.
The possibilities of organizing ꢀ-conjugated oligomers
were further extended by the introduction of a terminal ammo-
nium group in the oligo(ethylene oxide) chains (3). This allows
the possibility of complexation with chiral anionic macromole-
cules and thereby the introduction of helical order in the
assemblies of such oligomers. Here, we show how the com-
plexation with poly(glutamate) gives rise to a templating inter-
action that assembles the molecules of 3 such that the chirality
of the polymer is reflected in the organization of the stacks.
These results may bear implications for the application of
ꢀ-conjugated materials in the molecular recognition of bio-
macromolecules.9b–e,20,21
Results and Discussion
Sexithiophene 1 was synthesized from 50-
bromo-2,20-bithiophene-5-carbonyl chloride (6),4 which was
reacted with amine 5 to give the asymmetrically substituted
bithiophene 7 (Scheme 1). Amine 5 was obtained through a
Michael addition of tetra(ethylene glycol) mono methyl ether
on acrylonitrile in the presence of KOH yielding 4, the latter
Synthesis.
was reduced to the corresponding amine with BH3 THF. Sub-
ꢁ
sequently, two equivalents of 7 were coupled with 5,50-bis(tri-
methylstannyl)-2,20-bithiophene (8) in the presence of a cata-
lytic amount of tetrakis(triphenylphosphine)palladium(0) in
dimethylacetamide (DMA). The synthesis of sexithiophene 2
was published before.4b Sexithiophene 3 was synthesized fol-
lowing the procedure as described in Scheme 2. 50-Bromo-
2,20-bithiophene-5-carbonyl chloride (6)4b was reacted with
the mono-protected amine 11 to give the asymmetrically sub-
stituted bithiophene 12. The synthesis of mono Boc-protected
amine 11 proceeded from a Michael addition of tetraethylene
glycol on acrylonitrile in the presence of KOH to give the sym-
metrical product 9, which was reduced to the corresponding di-
amine 10 with BH3 THF. Reaction with di-tert-butyl dicarbon-
ꢁ
ate yielded the mono-protected product 11 in 68% yield; the
difference in solubility of the mono-, non-, and double-protect-
ed product made separation easy. Subsequently, 5,50-bis(tri-
methylstannyl)-2,20-bithiophene (8)42 was reacted with two

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F. Brustolin et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007) 1705
CN
NC
1) BH .THF, reflux
CN
3
O
O
O
O
O
O
O
O
HO
O
OH
2) CH OH, HCl
3
9
1% KOH
O
O
O
O
O
O
O
NH
H N
2
O
O
O
O
O
2
O
10
S
S
Br
Cl
6
O
N
H
BOC
H N
2
O
Et O, RT
2
11
S
S
Sn
Sn
O
8
S
S
O
O
N
BOC
Br
N
H
O
O
O
Pd(PPh ) , DMA, 120°C
H
3 4
12
O
O
6
TFA
CHCl
S
O
O
O
O
3
N
BOC
O
O
O
N
N
O
O
O
BOC N
3
H
H
H
H
13
Scheme 2.
equivalents of 12 in the presence of a catalytic amount
of tetrakis(triphenylphosphine)palladium(0) in DMA giving
13 in 50% yield. Deprotection of 13 with trifluoroacetic
acid (TFA) yielded the desired bisammonium salt 3 in 100%
yield.
Self-Organization in Solution and at Surfaces. In dilute
chloroform solutions (ꢂ10ꢃ5 M), 1 displays a UV–vis absorp-
tion band with a maximum at ꢂmax ¼ 435 nm, typical for
molecularly dissolved sexithiophene chromophores (Fig. 1a).
In support of this, the corresponding fluorescence spectrum
reveals a structured broad band at ꢂmax ¼ 520 with a shoulder
at ꢂmax ¼ 550 nm, also pointing to the presence of isolated
oligomer molecules in solution (Fig. 1b).
gation. The specific packing and exciton coupling with favored
probability of the highest energy transition explains this blue
shift. The stability of these aggregates is illustrated by the fact
that upon heating these films to 150 ꢅC no spectral changes are
observed. AFM reveals that these films are discontinuous and
consist of interconnected islands with a uniform thickness of
ꢂ6 nm (Fig. 3a). This value corresponds to the length of the
molecule which indicates that the observed solid-state assem-
bly is not guided by molecule-surface interaction with the sub-
strate but more interaction between molecules themselves (see
below). Therefore, we tentatively suggest that aggregates are
formed at an early stage during solvent evaporation with the
molecules standing up within the aggregates.
1H NMR shows that upon going to higher concentrations
(>10ꢃ3 M) a downfield shift from 7.16 to 7.24 ppm is observed
for the signal associated with the amide proton, along with
an upfield shift of the sexithiophene proton resonances of
ꢂ0:1 ppm (Fig. 1b). The shifts are attributed to the formation
of assemblies in which hydrogen bonds between the amide
groups, in tandem with ꢀ–ꢀ interactions, lock the aromatic
cores on top of one another. Since no concomitant line broad-
ening is observed in the sexithiophene resonances, it is
deduced that a high local mobility is still present and that
the observed shifts are probably the result of the onset of
aggregation.
When 1 is deposited on mica from toluene solutions, AFM
revealed besides the presence of large aggregates with no dis-
tinct morphologies also the formation of rod-like structures
with lengths from 200–500 nm and widths of ꢂ6 nm (after cor-
rection, see Experimental Section), which are oriented accord-
ing to the three-fold symmetry of the mica substrate (Figs. 3b
and 3c). Inspection of a molecular model indicates that the
width of the observed structures is consistent with the forma-
tion of one-molecule-wide stacks based on the overlap of the
heteroaromatic cores. Correspondingly, the broad UV–vis
absorption spectrum indicates that in toluene 1 is present as
a mixed population of aggregated and molecularly dissolved
When films are prepared by drop-casting a dilute chloro-
form solution onto mica substrates, the UV–vis spectra of
these films show a remarkable blue-shift of the absorption
band to 390 nm and the appearance of a vibronic progression,
whereas fluorescence spectra display an emission band at
states (Fig. 4). After a few days, the absorption band at ꢂmax
¼
395 nm becomes even more predominant, indicating that in
time molecules of 1 form more or larger aggregates in this
solution, this is in marked contrast to the solution behavior
of 2 which does not aggregate in toluene.4a A similar differ-
ence is observed in THF solution, which is also a good solvent
for 2. It is likely that the presence of the amide groups in 1 is
responsible for this enhanced tendency of these molecules to
aggregate.
ꢄ
ꢂmax ¼ 607 nm with a shoulder at ꢂmax 630 nm (Fig. 2),
¼
both pointing to the formation of aggregates22,23 of 1 in the
solid state. Where 3-substituted oligo- and polythiophenes
typically show a red shift on aggregation, non-substituted or
!–!0 substituted oligothiophenes show a blue shift on aggre-
We propose that the observed rod-like aggregates are

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1706 Bull. Chem. Soc. Jpn. Vol. 80, No. 9 (2007) BCSJ AWARD ARTICLE
formed from the molecularly dissolved material; the relatively
(a)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
slow evaporation rate of toluene may be a key factor in con-
trolling their shape and orientation, as this allows a higher
degree of interaction with the mica surface during the growth
of the assemblies. The importance of the amide groups in the
formation of these nanostructures is demonstrated again by
comparison with the corresponding sexithiophene-based ester
derivative 2, that only shows the formation of islands when
deposited onto mica from the same solvent.24,25 This difference
is tentatively attributed to the possibility of 1 to form arrays of
hydrogen bonds that rigidify and structure the aggregates.
When dissolved in n-butanol at room temperature, 1 showed
an absorption maximum at ꢂmax ¼ 390 nm, indicating the
presence of aggregates as the predominant species, even in
dilute solutions (Fig. 5a).26 The fluorescence in this state con-
comitantly shows a considerable quenching (compared to
chloroform solutions) of the emission bands at ꢂmax ¼ 520
and 550 nm and now revealed a contribution of the aggregated
state at ꢂmax ꢂ 600 nm.4
500
400
300
200
100
0
300 350 400 450 500 550 600 650 700
Wavelength (nm)
(b)
Variable-temperature UV–vis spectroscopy reveals that
upon heating from ꢃ10 to 90 ꢅC a transition occurs at 72 ꢅC
20
B
A
0.30
0.25
15
0.20
10
0.15
5
0.10
7.5
7.4
7.3
7.2
7.1
7.0
6.9
δ (ppm)
0.05
0
350 400 450 500 550 600 650 700 750
Fig. 1. (a) UV–vis and fluorescence spectra of 1 in chloro-
form. Concentration: 2:5 10ꢃ5 M (UV), 2:5 10ꢃ6 M (PL).
Wavelength (nm)
ꢁ
ꢁ
(b) 1H NMR spectra of 1 in CDCl3 for a dilute solution (A)
and for concentrations higher than 10ꢃ3 M (B).
Fig. 2. UV–vis and fluorescence spectra of a film of 1
drop-cast from chloroform.
A
B
C
0 nm
10 nm
0 nm
5 nm
0 °
10 °
Fig. 3. (A) 2 ꢆ 2 mm2 AFM tapping-mode topographic image of 1 deposited from chloroform on a mica substrate. (B) 2 ꢆ 2 mm2
AFM tapping-mode topographic (C) and phase images of 1 deposited from toluene on a mica substrate.