Highly ordered assembly of π-stacked distyrylbenzenes by oligoadenines

Article abstract of DOI:10.1021/ol101571h

A well-defined nanofibrous structure with lengths of several hundred nanometers and cross-sectional width of a single size (～6±0.5 nm) was self-assembled by oligoadenines, dA₂₀, and thymine-appended distyrylbenzene through binary complementary A-T hydrogen bond formation and the strong π-π stacking interactions. This demonstrated a useful supramolecular self-assembling approach to control the packing order of π-conjugated molecules and provided a practical means to enhance the optical properties of a material.

Full text of DOI:10.1021/ol101571h

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4018-4021
Highly Ordered Assembly of π-Stacked
Distyrylbenzenes by Oligoadenines
Wanggui Yang,† Ping Fang Xia,† and Man Shing Wong*,†,‡
Institute of Molecular Functional Materials and Department of Chemistry & Centre for
AdVanced Luminescence Materials, Hong Kong Baptist UniVersity, Kowloon Tong,
Hong Kong, SAR China
mswong@hkbu.edu.hk
Received July 17, 2010
ABSTRACT
A well-defined nanofibrous structure with lengths of several hundred nanometers and cross-sectional width of a single size (∼6 ( 0.5 nm)
was self-assembled by oligoadenines, dA20, and thymine-appended distyrylbenzene through binary complementary A-T hydrogen bond formation
and the strong π-π stacking interactions. This demonstrated a useful supramolecular self-assembling approach to control the packing order
of π-conjugated molecules and provided a practical means to enhance the optical properties of a material.
There has been tremendous progress in designing π-conju-
gated molecules/polymers for various technologically useful
functional properties such as light-emission,1 optical non-
linearity,2 photovoltaic response,3 and charge mobility;4
however, the device performance of these materials is greatly
dependent on the supramolecular organization of molecules/
polymer chains at the molecular level or the morphology of
these materials. Thus, the ability to control the molecular
arrangement and packing order provides a useful means to
tune and optimize the material performance in a device, apart
from the chemical structure modification.5 It has been shown
that covalently organized π-conjugated aggregates and su-
pramolecularly assembled dye aggregates can exhibit en-
hanced optical and electronic properties6 due to intra- or
interchromophoric interaction that may potentially be useful
in the optoelectronic and photonic devices. There has been
increasing attention to explore strategies and tools to control
the molecular order and arrangement of an aggregate as well
as shape, size, and length of nanostructure in the past years.7
Supramolecular self-assembly is a widely used and most
promising approach to construct well-defined molecular
† Department of Chemistry & Centre for Advanced Luminescence
Materials.
‡ Institute of Molecular Functional Materials.
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10.1021/ol101571h  2010 American Chemical Society
Published on Web 08/17/2010

assemblies, nanostructures, and materials.8 Among various
supramolecular building blocks, oligonucleotides9 with pre-
defined structural architectures are particularly useful tem-
plates to arrange and organize functional molecules, construct
nanostructures, and control the size of the supramolecular
structures. Several groups have used the ssDNA as a template
or building block for the assembly of functional π-conjugated
molecules to form well-defined aggregates or nanostruc-
tures.10
Scheme 1. Synthesis of DSB-EOT
We have demonstrated a useful supramolecular self-as-
sembling approach that can be used to properly control the
packing order of π-conjugated molecules, which in turn provides
a practical means to enhance the optical properties. We report
herein an investigation of the supramolecular assembly of
oligoadenines, dA20, and thymine-appended distyrylbenzene
through binary complementary adenine-thymine (A-T) hy-
drogen-bond formation and the π-π stacking interactions
leading to the highly ordered assemblies, where the distyryl-
benzene cores are closely π-π stacked in a cofacial fashion,
hence resulting in greatly enhanced fluorescence properties. This
exemplifies a practical supramolecular assembly principle that
can be used to precisely position π-conjugated molecules into
highly ordered aggregates capable of exhibiting enhanced
functional properties.
A highly planar π-conjugated system, distyrylbenzene,
laterally tethered with two thymine moieties by the short
ethoxy chains and equipped with water-solublizing, multiple
poly(ethylene glycol)s at both ends, namely DSB-EOT, was
synthesized as outlined in Scheme 1. By adapting the
convergent approach established previously,11 double Wad-
sworth-Emmons reaction of 3,4,5-tris[2-(2-methoxyeth-
oxy)ethoxy]benzaldehyde and p-xylylene(bis(phosphonate))
derivative was used as the key step to synthesize the water-
soluble trans-distyrylbenzene skeleton. Alkylation of 3,4,5-
trihydroxybenzoate with 1-chloro-2-(2-methoxyethoxy)ethane
in the presence of potassium carbonate in DMSO afforded
3,4,5-tris[2-(2-methoxyethoxy)ethoxy]benzoate, 1, in 76%
yield. Reduction of benzoate 1 with LiAlH4 in dry THF at
rt afforded benzyl alcohol 2 in an excellent yield. Oxidation
of alcohol 2 using PCC in DCM afforded 3,4,5-tris[2-(2-
methoxyethoxy)ethoxy]benzaldehyde, 3, in 86% yield. On
the other hand, alkylation of p-hydroquinone with 2-chlo-
roethanol in the presence of sodium hydroxide in water
solution afforded dialcohol 4 in 74% yield. Bromomethyla-
tion of 4 with formaldehyde and concd HBr in acetic acid
at rt afforded dibenzyl bromide 5 in 54% yield. Michaelis-
Arbuzov reaction of 5 with triethyl phosphite gave bis-
phosphonate 6 in 97% yield. Double Wadsworth-Emmons
reaction of benzaldehyde 3 and bis-phosphonate 6 in THF
in the presence of NaH afforded trans-distyrylbenzene which
was subsequently hydrolyzed using lithium hydroxide in THF
and water solution affording compound 7 in 58% yield in
two steps. Mesylation of 7 with MsCl and pyridine in DCM
followed by nucleophilic substitution with KI in THF
afforded diiodo product 8 in 63% yield in two steps. Double
substitution of 8 with thymine in the presence of potassium
carbonate at 60 °C afforded the desired product DSB-EOT
in 45% isolated yield, which was fully characterized with
spectroscopic techniques and found to be in good agreement
with its structure (see the Supporting Information).
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The absorption spectrum of DSB-EOT exhibits a sig-
nificant decrease in absorption, spectral broadening, and blue
shift of absorption maximum (λabsmax) in aqueous solution
(λabs ) 350 nm) relative to that in CHCl3 (λabs ) 396
max
max
nm). The fluorescence spectrum also shows notable spectral
broadening with less vibronic structures in aqueous solution.
(Figure S1, Supporting Information). Its fluorescence quan-
tum efficiency (ΦPL) is greatly reduced from 58% in CHCl3
to 31% in phosphate buffer solution. In addition, after the
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Humphrey, M. G. J. Mater. Chem. 1998, 8, 2005–2009. (b) Wong, M. S.;
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Org. Lett., Vol. 12, No. 18, 2010
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aqueous solution of DSB-EOT was heated to 353 K and
then slowly cooled to 283 K over 6 h, both of its absorption
and emission spectra showed an apparent decrease in
absorptivity and fluorescence intensity, respectively. All these
findings consistently suggest that DSB-EOT is solvated in
CHCl3 but self-assembles into π-stacked aggregates in
aqueous solution where the hydrophobic π-conjugated units,
i.e., distyrylbenzene cores and thymine moieties, are rela-
tively tightly packed, with π-π interactions, and the hy-
drophilic poly(ethylene glycol) chains are pointed into the
aqueous medium.12 The temperature dependence of aggregate
formation of DSB-EOT in water was also examined.
Interestingly, there was an increase in absorption of
DSB-EOT at 275 nm and a slightly blue shift and a decrease
in absorption at 350 nm upon heating from 10 to 60 °C. No
such temperature dependence spectral changes were observed
for DSB-EOT in CHCl3 (Figure S2, Supporting Informa-
tion). These observations further affirm that the DSB-EOT
aggregates can be thermally dissociated in water resulting
in a reduction of the π-stacking interactions among the
π-conjugated moieties.
Upon an addition of adenine (4 × 10-7 M) into the
DSB-EOT (2 × 10-7 M) solution, initially there is no
significant change in the spectral characteristics of DSB-EOT
indicating that aggregation prevents the appended thymines
from interacting with adenines. However, upon heating and
prolonged cooling treatment, in addition to the spectral shift,
the absorption and fluorescence (ΦPL ) 8%) of DSB-EOT
show a substantial decrease in intensity, which hints that the
deaggregated DSB-EOT may bind with adenines through
the complementary hydrogen bonding interactions (Figure
S3, Supporting Information). The complementary binding
of DSB-EOT and adenine is further affirmed by the high-
resolution ESI-MS measurements in which the spectrum of
the mixture of DSB-EOT and adenine shows peaks at m/z
742.8412 and 799.3753 with expected isotopic distribution
that correspond to [DSB-EOT + adenine + Na + H]2+ and
[DSB-EOT + 2adenine +2H]2+ ions, respectively (Figure
S4, Supporting Information).
To make use of such strong binary complementary A-T
base pair interactions and the strong π-π stacking interac-
tions of the planar π-conjugated system, a highly organized,
plane-to-plane π-stacking aggregate can be constructed by
the self-assembly of DSB-EOTs with oligoadenines. Upon
mixing of oligoadenines dA20 (0.4 nmol) with DSB-EOT
(0.2 nmol) in 1 mL of the buffered solution, the initial
spectral change is not so prominent. Nevertheless, after
heating and prolonged cooling treatment, there are remark-
able changes in various spectroscopic characteristics. The
absorption at 258 nm, corresponding to the absorption of
oligoadenine, shows a dramatic decrease in intensity, and
the absorption around 354 nm, corresponding to the absorp-
tion of the distyrylbenzene core, shows a strong increase in
absorbance concomitant with spectral shift. In addition to
the red shift of emission spectrum, there is a 2-fold and 3-fold
enhancement in fluorescence of the distyrylbenzene moiety
as compared to that of DSB-EOT solution and the assembly
of DSB-EOT-adenine, respectively (Figure 1b). Further-
Figure 1
.
(a) Absorption spectra of DSB-EOT (2 × 10-7 M), dA20
(4 × 10-7 M), and DSB-EOT + 2dA20 (2 × 10-7 M) measured
in phosphate buffer solution after heating and cooling treatment.
(b)ComparisonofemissionspectraofDSB-EOT,DSB-EOT-adenine,
and DSB-EOT-dA20 after heating and cooling treatment. (c) CD
spectra of dA20 and DSB-EOT + dA20 with and without heating
and cooling treatment. (d) Plot of temperature dependent absorbance
of DSB-EOT-dA20 assembly measured at 260 nm versus tem-
perature.
more, the circular dichroism spectrum of dA20 exhibits almost
completely diminished Cotton effect indicating that dA20
unfolds itself in the presence of DSB-EOT. Native poly-
acrylamide gel electrophoresis (PAGE) analysis of dA20 and
a mixture of 1:2 of DSB-EOT and dA20 revealed that upon
mixing and heat-cool treatment, the identity of dA20
disappeared but a new band with very low mobility was
observed (Figure S5, Supporting Information), suggesting
the association of the dA20 to the DSB-EOT molecules to
form a polymeric assembly. The temperature-dependent
UV-vis profile of DSB-EOT-dA20 complex shows an
abrupt increase in 260 nm absorbance upon heating with a
“thermal denaturation” temperature (Tm) of 35 °C (Figure
1d) reminiscent of the dissociation of double helix DNA
dsAT20 duplex (Tm ) 42 °C) (Figure S6, Supporting
Information). This result further affirms the presence of a
cooperative self-assembly of A-T hydrogen-bonded com-
plexes. All these findings consistently point to the fact that
oligoadenines unfold themselves and bind with the deaggre-
gated DSB-EOTs through the complementary hydrogen-
bonding interactions, and then the bound DSB-EOTs stack
up in a parallel fashion leading to a highly ordered assembly.
In contrast, when the noncomplementary ssDNA oligoth-
ymine, dT20, is mixed with DSB-EOTs under the same
experimental conditions, there are no changes in absorption
and fluorescence spectra indicating DSB-EOT binds specif-
ically to dA20 (Figure S7, Supporting Information). The cause
of the spectral changes of DSB-EOT such as the strong
fluorescence enhancement upon self-assembling into highly
(12) Hoeben, F. J. M.; Shklyarevskiy, I. O.; Pouderoijen, M. J.;
Engelkamp, H.; Schenning, A. P. H. J.; Christianen, P. C. M.; Mann, J. C.;
Meijer, E. W. Angew. Chem., Int. Ed. 2006, 45, 1232–1236.
4020
Org. Lett., Vol. 12, No. 18, 2010

ordered aggregates is attributed to the excitonic coupling of
proximate π-π-stacked chromophores,13 which gives rise
to the increase in the absorption of DSB-EOT (Figure 1a)
and, hence, the fluorescence enhancement, although the
fluorescence quantum efficiency (ΦPL) of DSB-EOT-dA20
assembly is only 19%. Even when the compound is kept at
low temperature (∼10 °C) for more than 6 months, the
spectral characteristics of these DSB-EOT-dA20 assemblies
are prevalent, indicating the highly stability of these nanofi-
brous structures.
the nanofiber using the TEM images is found to be 6 ( 0.5
nm, which is consistent with the proposed molecular model-
ing nanostructures of closely π-π stacked DSB-EOT
molecules bound to two oligoadenine chains in a side by
side fashion. The proposed molecular model of hydrogen-
bonded DSB-EOT·dA20 assembly calculated by Hyperchem
7.0 is shown in Figure 3. In contrast, AFM and TEM analyses
To further investigate the morphology of the resulting
hydrogen-bonded DSB-EOT-dA20 assembly, we carried
out an atomic force microscopy (AFM) study in air which
distinctly shows the formation of long, stable, and well-
defined nanofibers with lengths extending over several
hundred nanometers. Lateral cross-sectional analysis of the
nanofibers shows them to be of a similar height, indicating
a well-defined nanofibrous assembly of a single size (Figure
2a). However, sample preparation of the DSB-EOT-dA20
Figure
3. Proposed molecular model of hydrogen-bonded
DSB-EOT·dA20 assembly (DSB-EOT in green and dA20 in red)
calculated by Hyperchem 7.0.
of samples containing only DSB-EOT molecules find no
such long supramolecular nanofibrous structures. Further-
more, another DSB derivative (DSB-POT) bearing the
longer flexible ethoxyethoxy thymine-linkers show no nanofi-
bers formation upon mixing with oligoadenine under the
same experimental conditions.
In summary, we have demonstrated that using a predefined
template (e.g., oligoadenines) to direct the supramolecular
organization of π-conjugated molecules into highly ordered
nanostructures which give rise to a strong fluorescence
enhancement provide a useful means to tune and modify the
material performance. This also highlights the importance
of the ability to control the supramolecular packing order of
π-conjugated molecules to enhance the optical properties of
a material.
Figure 2. (a) Tapping mode AFM images of DSB-EOT-dA20
assembly on a mica plate (bar ) 1 µm). Bottom: lateral cross-
sectional profiles of DSB-EOT-dA20 assemblies along the line.
(b) TEM images of DSB-EOT-dA20 assembly stained with the
Nanovan at pH 8 (bar ) 200 nm).
Acknowledgment. This work was supported by the
General Research Fund (GRF) of the Hong Kong Research
Grant Council (HKBU 202408). The Institute of Molecular
Functional Materials is funded by an Areas of Excellence
Scheme, University Grants Committee (Hong Kong).
assembly for AFM imaging resulted in nanofibers that are
somewhat embedded on the mica surface. Thus, it was
difficult to obtain accurate values for the width of the
nanofibers using AFM height images.
In addition, DSB-EOT-dA20 assemblies were examined
under dry conditions using transmission electron microscopy
(TEM) after being stained with the Nanovan at pH 8. Again,
the self-assembled nanofibers were evident (Figure 2b). The
experimentally obtained cross-sectional width analyzed on
Supporting Information Available: Synthesis and ex-
perimental details, characterization data, and spectra. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(13) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721–722.
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