Low-dimensional aggregates from stilbazolium-like dyes

Article abstract of DOI:10.1002/anie.200454131

Dye hard: Strong intermolecular interactions of stilbazolium-like dyes with different structures and conformations seem to act as the main driving force for the formation of nanostructures with distinct morphological and spectral properties. Nanorods and

Full text of DOI:10.1002/anie.200454131

Communications
Organic Nanostructures
Low-Dimensional Aggregates from Stilbazolium-
Like Dyes**
Zhiyuan Tian, Yu Chen, Wensheng Yang, Jiannian Yao,*
Lingyun Zhu, and Zhigang Shuai
Organic low-dimensional nanostructures used as nanoscale
building blocks have attracted considerable research interest
in the development of novel nanodevices.[1] This interest is
driven by the fact that organic nanostructures may exhibit a
wide range of electrical and optical properties that depend
sensitively on both their shapes and sizes,and thus is likely to
provide a new method for modifying the optical and
electronic properties of organic functional materials.[2] From
this standpoint,an important challenge in the discovery of
novel mesoscopic properties and the development of nano-
technology is to fabricate organic nanomaterials with the
desired shape and size. Although extensive studies on the
size-dependent optical and electronic properties of organic
nanostructures such as particles,[2a–d] wires,[2e] and tubes[2f]
have been performed recently,the ability to understand and
predict the final structures of nanomaterials,which is critical
to guide the rational fabrication of nanoscale materials with a
Scheme 1. Structures of model compounds 1–3.
The aggregation of molecule 1 was induced by injecting a
certain amount of stock solution of compound 1 in ethanol
into a 1:1 mixed solvent of hexane and methylcyclohexane
(MCH) with vigorous stirring. Ethanol is a good solvent but
the mixed solvent is a poor one for compound 1,so
aggregation of molecule 1 occurred as a consequence of the
change in the solvent quality. The size of nanostructures of 1
was controlled by varying the concentration of the stock
solution. For example,nanorod sizes of 110 and 80 nm were
obtained when 60 mL of the 1.0 ” 10À3 m stock solution and
120 mL of the 5.0 ” 10À4 m stock solution were injected,
respectively. Water was used as the poor solvent for the
aggregation of compound 2 to obtain the colloids. The
average size of nanostructures of 2 was controlled through a
ripening process by variation of the aging time,that is,by
changing the time intervals after injection of the stock
solution. For example,aging times of 15 and 30 minutes
gave nanospheres with diameters of 60 and 100 nm,respec-
tively. The nanorods obtained from compound 1 and the
nanospheres from compound 2 were very uniform in size,as
indicated by the scanning (SEM) and tunneling electron
microscopy (TEM) images (Figure 1). It can be seen that the
sample derived from 1 consists of rather straight nanorods
[2a,3]
desired shape,size,and therefore function,is still limited.
Herein,we report the facile fabrication of organic nano-
structures with well-defined shapes and uniform sizes from
stilbazolium-like dyes (Scheme 1). Compounds 1–3 were
synthesized with the original intention of exploring the
possible influence of the substituents on the shape,and
therefore the optical properties,of the resulting nanostruc-
tures. Low-dimensional nanorods and nanospheres with high
monodispersities in both shape and size were prepared by the
self-aggregation of compounds 1 and 2,respectively. The
concomitant property changes upon formation of nanorods
with intermolecular charge-transfer (inter-CT) characteris-
tics,as well as highly monodisperse nanospheres with size-
dependent spectral features,may be exploited in many
fields.[1b]
[*] Dr. Z. Tian, Y. Chen, W. Yang, Prof. J. Yao
Key Laboratory of Photochemistry
Center for Molecular Science
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100080 (P. R. China)
Fax: (+86)10-8261-6517
E-mail: jnyao@mail.iccas.ac.cn
Dr. L. Zhu, Prof. Z. Shuai
Key Laboratory of Organic Solids
Center for Molecular Science
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100080 (P. R. China)
[**] This work was supported by the National Natural Science
Foundation of China, the Chinese Academy of Sciences, and the
National Research Fund for Fundamental Key Projects No.973
(G19990330).
Figure 1. a) Medium-magnification SEM images of the nanorods
formed from 1. b) Low-magnification SEM images showing a large
quantity of the nanospheres formed from 2. c) TEM image of the nano-
rods. d) TEM image of the nanospheres.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200454131
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with lengths up to the micrometer range that have a uniform
size along the entire length. In addition,the surfaces of the
nanorods are clean and smooth. The product from 2 consists
of a large quantity of perfect nanospheres with very high
monodispersity in terms of size,and some nanospheres are
assembled into regular hexagonal arrays in a small domain
(see Supporting Information). We note that there are scarcely
any previous examples of nanostructures with such high
monodispersity from small organic molecules without the use
of templates. Furthermore,the distinct morphological fea-
tures as well as the high monodispersity in size of the
nanostructures of 1 and 2 lead us to assume that there would,
most probably,be specific driving forces involved in the
formation of these nanostructures.
shift with increasing nanorod size and simultaneously an
additional band centered at approximately 500 nm (2.48 eV)
emerges and gradually becomes predominant at the expense
of the intra-CT band. This new band is attributed to an inter-
CT transition between neighboring molecules in the nano-
rods.[1a] The spectrum of dilute 2 in ethanol solution also
exhibits an intensive p–p* band at about 409 nm (3.03 eV),
while the spectra of the nanospheres shift gradually to shorter
wavelengths on increasing the particle size. In addition,both
the excitation and emission fluorescence spectra of the
nanospheres show similar hypsochromic shifts as the absorp-
tion spectra (see Supporting Information). However,only
very weak fluorescence emission was detected for the nano-
rods of 1,which indicates that strong quenching occurred in
the aggregates of model molecule 1.
Molecule 1 consists of a strong electron-withdrawing
moiety N-methylpyridinium (Nmpd) and a strong electron-
releasing moiety N-methylpyrrole (Nmpr) connected by a
conjugated system.[5] For molecule 2,two weak electron-
releasing 4-hexadecyloxyphenyl (4Hop) moieties,relative to
Nmpr,are symmetrically connected to the Nmpd moiety
through a conjugated ethylene group. One noteworthy
conformational difference between molecules 1 and 2 is the
planarity: the dihedral angles q between the p systems of the
donor and acceptor groups are nearly 08 for molecule 1 but
338 for molecule 2,as indicated by the molecular mechanics
optimizations of single molecules of 1 and 2 (Figure 3a,b). It
can also be seen from Figure 3 that molecule 1 has a linear D–
p–A structure,while this is not the case for 2. These molecular
structural and conformational differences are expected to be
responsible for the different spectral features,such as the
0.26 eV difference in the p–p* transition energy between
monomers 1 and 2 that reflects the different extent of p-
electron delocalization between the donor and acceptor
moieties in the molecules.
It is known that the spectra of the nanostructures can give
information about the formation process.[2a–e] Figure 2 dis-
plays the absorption spectra of the colloidal dispersions of the
as-prepared nanorods and nanospheres. The monomer of 1 in
ethanol exhibits an intensive intramolecular charge-transfer
(intra-CT) absorption band at about 446 nm (2.77 eV),which
is assigned to the transition from the HOMO of the donor to
the LUMO of the acceptor (that is, pD–pA).[4] In the spectra of
*
nanorods of 1,the intra-CT band shows a slight gradual red
SEM and TEM observations,along with the analysis of
the molecular structural features and spectroscopic data,
provide a clue for the understanding of the possible formation
mechanism of the structurally distinct shapes of the nano-
structures formed from 1 and 2. It is well-documented that the
spatial configuration of organic molecules plays an important
role in determining their stacking mode and therefore the
properties of the resulting aggregates.[2e,6] For example,an
approximately face-to-face arrangement of the donor–accep-
tor (D–A) pair,which gives the largest orbital overlap,is
necessary for the formation of inter-CT complexes. For
compounds having the typical molecular structural character-
istics of a strong D–A pair,a linear D– p–A structure,and a
[7]
more planar conformation,strong D–A interactions
are
known to act as the main driving force for the aggregation of
molecules and the formation of one-dimensional (1D) nano-
structures.[7c] The molecular structural characteristics of
compound 1 discussed above enabled a similar mechanism
to be proposed for the aggregation of molecule 1,that is,by a
donor-to-acceptor growth process (Figure 3c) with eventual
formation of rodlike nanostructures. Inter-CT processes[7c]
and the extended delocalization of p electrons are expected
as concomitant spectral features of the strong D–A inter-
actions between adjacent molecules in the nanorods. In fact,
Figure 2. a) Absorption spectra of dispersions of nanorods with differ-
ent diameters: 1) 40 nm, 2) 80 nm, 3) 110 nm, 4) 150 nm. b) Absorp-
tion spectra of dispersions of nanospheres with different sizes:
1) 30 nm, 2) 60 nm, 3) 100 nm, 4) 130 nm. For comparison, the spec-
tra of the corresponding monomers in ethanol are also shown as (m)
in (a) and (b) (dashed lines). A=absorbance (arbitrary units).
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Figure 3. Energy-optimized single molecules of 1 (a) and 2 (b); computer-generated 1D aggregates of 1 (c) and 0D aggregates of 2 (d). In Fig-
ure 3d, the length of the hydrocarbon chains was shortened to eight carbon atoms for simplification.
the gradual appearance of a CT band and the slight bath-
ochromic shift of the p–p* band in the spectra are consistent
with the proposed mechanism for the formation of the
nanorods. In addition,the quenching of the nanorod fluores-
cence emission can also be attributed to the presence of low-
lying CT states in the nanorods.[8] XRD measurements show
that the nanorods grow preferentially along the crystallo-
graphic [011] direction (see Supporting Information). This
kind of preferential growth along a specific crystallographic
direction is prone to forming 1D structures.
aggregation of molecule 2 in a manner different from that of
molecule 1. Somewhat surprisingly,the nanospheres obtained
after the ripening process are very monodisperse both in
terms of shape and size (Figure 1b and Supporting Informa-
tion). Similar results were also obtained in our previous work
concerning organic nanostructures derived from pyrazoline
derivatives.[2c,d] As discussed above,the growth of the nano-
spheres appears to be involved in the process consisting of the
attraction,fusion,and rearrangement of relatively small
particles,which suggests that the Ostwald ripening mecha-
nism most probably does not apply to the present case. The
relatively small particles are expected to aggregate preferen-
tially through solvophobic interactions and then fuse as a
result of their relatively large surface areas,eventually giving
the focused size distribution of the nanospheres produced.[11]
The underlying basis of the narrow size distribution of the as-
prepared nanostructures is still under investigation.
Molecular modeling studies also provide a coherent
picture of the process by which the resulting nanostructures
are formed. The spectral features generated from the
proposed aggregation models,namely the slight red shift of
the intra-CT band and the inter-CT process between adjacent
molecules in nanorods of 1,as well as the blue-shifted intra-
CT transition in the nanospheres of 2,agree well with our
experimental results. In particular,the calculated results
clearly show the charge-transfer process from the donor to
acceptor units between neighboring molecules in the case of
nanorods (see Supporting Information).
The morphological and spectral features of the nano-
structures formed by reference model compound 3 were also
investigated. Molecule 3 is found to aggregate into rodlike
nanostructures,as indicated by SEM studies (see Supporting
Information). Although molecule 3 has a linear structure,it
has a weak D–A interaction between the Nmpr and 4Hop
moieties compared to molecule 1,and thus may aggregate in a
head-to-tail fashion through intermolecular D–A interac-
tions,which should be much weaker than those occurring
within the rods. However,molecule 3 bears only one long
flexible alkyl chain while two are present in molecule 2. Thus,
the solvophobic interaction is unlikely to result in the
formation of H-type aggregates with a nearly parallel
In the case of 2,however,the possibility of D–A
interaction acting as the dominating driving force for the
aggregation of molecules was excluded because of the weak
D–A pair,nonlinear molecular structure,and nonplanar
configuration,as discussed above. Molecule 2 has two long
flexible alkyl side chains substituted on the ionized pyridine
ring,and therefore is a typical amphiphilic molecule bearing a
hydrophilic “head” (ionized pyridine ring) and two long and
flexible hydrophobic “arms” (alkyl chains),as shown in
Figure 3b. The gradually blue-shifted spectral characteristics
of the nanospheres compared with the monomer suggest the
formation of H-type aggregates[9] in which the pyridine units
are arranged in an almost parallel manner (see Figure 3d).
Apparently,this type of aggregation is energetically favorable
because it maximizes favorable stacking interactions between
pyridine units and minimizes the unfavorable interactions
between the molecule and the solvent. Subsequently,these H-
type aggregates are presumed to attract each other—this is
the so-called collective behavior generally observed in
biology and chemistry that originates from interactions
between units with the same shape,[10] and which is expected
to be followed by a process involving fusion and then
rearrangement. Nearly the same spectral features (UV/Vis
absorption,fluorescence emission and excitation) as those
discussed above,as well as spherical nanostructures,were
observed when the mixed solvent of 1:1 hexane/MCH was
used as the poor solvent during the aggregation of 2. This
finding indicates that it is mainly the difference between the
compatibility of the two moieties (4Hop and Nmpd) of
molecule 2 with the poor solvent,as well as the specific
molecular structural features,that eventually lead to the
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J. G. Cao,F. M. Raymo,J. F. Stoddart,A. J. P. White,D. J.
arrangement of the pyridine ring moieties of molecule 3. As a
result,neither the apparent inter-CT feature similar to that
observed in the spectra of the nanorods nor the gradually
blue-shifted spectral feature on increasing the particle size,
seen in the spectra of the nanospheres,is observed for the
rodlike nanostructures in our experiments (see Supporting
Information). The morphology of aggregates from 3 is
relatively undefined compared to that of the rods and
particles as a result of competition between the two relatively
weaker D–A and solvophobic interactions.
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In conclusion,we have successfully prepared stilbazolium-
like dye nanorods and nanospheres by changing the solvent
quality during the synthesis of model compounds. Sufficiently
strong interactions between the constituent molecules allow
the formation of nanostructures with well-defined shape and
highly monodisperse size. Furthermore,different dominant
intermolecular interactions,based on molecular components
with different structural and conformational characteristics,
are likely to be responsible for the formation of organic
nanostructures with distinct shapes. This work provides
specific examples and,therefore,the possibility of construct-
ing nanomaterials that are expected to be useful in the
development of photoelectronic nanodevices.
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Received: February 27,2004 [Z54131]
Keywords: aggregation · charge transfer · donor–acceptor
.
systems · nanostructures · solvophobic interaction
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