Controlling the structures and photonic properties of organic nanomaterials by molecular design

Article abstract of DOI:10.1002/anie.201302894

Exploring the stacks: Two different but related naphthalene compounds were shown to form different nanostructures (see picture) depending on the π-π stacking and hydrogen bonding of the molecules. These nanostructures had unique photonic confinement and l

Full text of DOI:10.1002/anie.201302894

Angewandte
Chemie
Communications
Organic Nanophotonics
Exploring the stacks: Two different but
related naphthalene compounds were
shown to form different nanostructures
(see picture) depending on the p–p
stacking and hydrogen bonding of the
molecules. These nanostructures had
unique photonic confinement and light-
propagation characteristics, which show
potential for nanophotonic circuits.
W. Yao, Y. Yan, L. Xue, C. Zhang, G. Li,
Q. D. Zheng, Y. S. Zhao,* H. Jiang,*
J. N. Yao*
&&&& — &&&&
Controlling the Structures and Photonic
Properties of Organic Nanomaterials by
Molecular Design
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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DOI: 10.1002/anie.201302894
Organic Nanophotonics
Controlling the Structures and Photonic Properties of Organic
Nanomaterials by Molecular Design**
Wei Yao, Yongli Yan, Lin Xue, Chuang Zhang, Guoping Li, Qingdong Zheng,
Yong Sheng Zhao,* Hua Jiang,* and Jiannian Yao*
Controlling the flow of photons at the nano/microscale is
crucial to the development of integrated optical circuits,
which offer the potential to transmit and route information
stacking, causing different preferential growth directions
and nanostructures. Thus, the rational design of molecular
components with different intermolecular recognitions would
be promising for the construction of nanostructures with
desired morphologies, which can confine and guide photons in
specific dimensions.
Herein, we constructed 1D and 2D nanoarchitectures by
altering the structures of the components of the self-
assembled nanostructures, which is an important guide to
the molecular design of tailor-made nanostructures and
photonic properties. The molecules used in this study are
naphthalene derivatives (Scheme 1) consisting of functional p
[1]
more efficiently than electronic circuits. Because of the
strong impact of the shape and dimensions of the photonic
building blocks on the confinement and guiding of optical
[
2]
waves, an essential strategy for controlling photonic flow is
the fabrication of diverse structures. One-dimensional (1D)
structures at the nano- and microscales, with field confine-
ment in two dimensions, have been demonstrated as effective
[
3]
photonic elements, such as Fabry-Pꢀrot (F-P) resonators,
[4]
[5]
[6]
waveguides, lasers, and transducers. In comparison, two-
dimensional (2D) sheet-like structures that confine photons
in one dimension, can be integrated into a chip scale planar
photonics system, including waveguides for interconnection,
[
7]
couplers, and modulators.
Organic small molecules are attractive photonic materials
because they exhibit high photoluminescence quantum effi-
ciencies, color tunability, and size-dependent optical proper-
[
8]
ties. Moreover, flexibility in their synthesis and chemical
modification makes organic molecules useful for the con-
Scheme 1. Molecular formulas of the model compounds.
[
9]
struction of various nanostructures. Altering the organic
molecular structure can tune the intermolecular interactions
p–p interaction, hydrogen bonding), which influence their
systems. Introduction of different amino groups on the
naphthalene core gave distinct intermolecular interactions
that were favorable for 1D or 2D stacking arrangements. This
geometrical discrepancy at the molecular level yields distinct
nanostructures (1D nanowires and 2D rhombic nanosheets)
with disparate photonic properties. The nanowires showed
low transmission loss during the light propagation process,
while the nanosheets displayed interesting asymmetric wave-
guiding behaviors, which we believe was determined by the
molecular packing modes in the nanostructures. This method
of controlling different structures and photonic properties by
structural modification could provide insight and guidance for
the development of organic-material-based photonic devices.
(
packing mode during self-assembly and determine their final
[
10]
aggregated structures. Self-assembly of aromatic organic
molecules through strong p–p stacking is a common approach
for forming 1D nanostructures with uniform morphological
dimensions. The introduction of a hydrogen-bonding group
in specific positions would disturb the balance of p–p
[11]
[
*] W. Yao, Dr. Y. Yan, Dr. L. Xue, C. Zhang, G. Li, Prof. Y. S. Zhao,
Prof. H. Jiang, Prof. J. N. Yao
Beijing National Laboratory for Molecular Sciences (BNLMS), CAS
Key Laboratory of Photochemistry, Institute of Chemistry, Chinese
Academy of Sciences
2
-Acetyl-6-dimethylaminonaphthalene (ADN, Scheme 1)
and 2-acetyl-6-methylaminonaphthalene (AMN) were syn-
thesized (see Supporting Information) as model com-
Beijing 100190 (China)
E-mail: yszhao@iccas.ac.cn
[
12]
hjiang@iccas.ac.cn
jnyao@iccas.ac.cn
pounds,
to explore the influence of the substituents on
their self-assembled nanostructures and photonic properties.
Both molecules are semi-rigid and almost fully conjugated
with the naphthalene core. Notably, ADN has a tertiary
amine on the naphthalene core, while AMN has a secondary
amine at the same position. This difference is expected to
influence their intermolecular interactions and self-assembly
behavior. AMN may have less steric repulsion with neighbor-
ing molecules compared with ADN when assembled into
nanostructures. More importantly, the presence of an amide
hydrogen-bonding site in AMN prevents packing along
Prof. Q. D. Zheng
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences
(
China)
[
**] This work was supported by the National Natural Science
Foundation of China (Nos. 21125315, 51203165), the Chinese
Academy of Sciences, and the Ministry of Science and Technology of
China (2012YQ120060 and National Basic Research 973 Program).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302894.
2
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a single direction, thus leading to nanostructures with differ-
ent morphological features.
a separation of 2.96 ꢁ. Hydrogen bonding advances the
growth along the b axis, which results in a different packing
behavior from that of the ADN molecules. Therefore, the
intermolecular interactions of p–p stacking and hydrogen
bonding cooperatively direct the self-assembly pathway
toward the formation of 2D sheet-like structures by AMN.
The designed molecules were allowed to aggregate into
their preferred morphologies by inducing self-assembly in the
liquid phase. Depending on the intermolecular interactions,
the molecules were expected to assemble into different nano-
or sub-micrometer structures. The ADN molecules aggre-
gated into 1D nanowires with a smooth surface and uniform
diameter of about 800 nm (Figure 2a). The TEM image and
To determine the structural characteristics and intermo-
lecular interactions of these compounds, we simulated the
equilibrium shape of ADN and AMN crystals by minimizing
[13]
the total surface energy using the Materials Studio package.
The crystal data and ORTEP drawing of the two single
crystals used in the calculation are shown in the Supporting
Information (Table S1 and Figure S1). From the simulation
results, we found that the predicted growth pattern and
thermodynamically stable morphology for compound ADN is
a 1D wire-like structure (Figure 1a). The calculated surface
energies g(hkl) of the various crystal faces (hkl) follow the
Figure 2. a) SEM and b) TEM images of the 1D nanowires aggregated
from ADN molecules. Inset of (b): SAED pattern of a single wire. c) PL
microscopy image of nanowires excited with the UV band of a mercury
lamp. d) SEM and e) TEM images of the 2D rhombic nanosheets
aggregated from AMN molecules. Inset of (e): SAED pattern of
a single sheet. f) PL microscopy image of nanosheets excited with the
UV band.
Figure 1. a) The predicted growth morphology of ADN molecules
based on the attachment energies. b) The arrangement of ADN
molecules under p–p stacking. c) The predicted growth morphology of
AMN molecules based on the attachment energies. d) The packing
arrangement of AMN molecules when viewed perpendicular to the
(
001) plane.
selected area electron diffraction (SAED) pattern in Fig-
ure 2b reveal that the wires have single crystal structures
growing along the [010] crystal direction. This direction can
be attributed to the p–p interaction and is consistent with our
calculated results. The nanowires exhibited intense blue
emission under excitation of unfocused UV irradiation
(330–380 nm) with the typical characteristics of an active
optical waveguide, such as bright photoluminescent (PL)
spots at the wire ends and weaker PL from the bodies
(Figure 2c). This indicates that PL energy can propagate
efficiently along the axial direction, leading to a low-loss
optical waveguide.
In contrast with ADN, AMN formed well-defined 2D
rhombic nanosheets. These 2D nanosheets have an edge
length of tens of micrometers (Figure 2d, SEM image) and
a thickness around 400 nm (Figure S2, AFM image). A typical
TEM image of an individual nanosheet is given in Figure 2e
and its corresponding SAED patterns are shown in the inset.
The SAED pattern can be indexed according to the crystal
lattice constants of AMN (see Table S1). The SAED results
indicate that the growth directions parallel to the substrate of
the nanosheet are [100] and [010], which is determined by the
p–p stacking and hydrogen-bonding interactions. As shown in
the PL image in Figure 2 f, the bright green edges of the 2D
À
Á
ꢀ
order: g(010) > g 102 > g(100) > g(002) (Table S2). The
(
growth face because the kinetic barrier of growth for (hkl) is
inversely proportional to the surface energy of g(hkl). The
ADN molecules are arranged in slip-stacks along the short
molecular axis with a p–p stacking distance of 3.57 ꢁ
010) face, with the highest surface energy, is the preferred
[14]
(
Figure 1b). Without H-bonding and other strong interactions
between two adjacent molecules, the p–p interaction is the
predominant driving force during self-assembly, which facil-
itates the ADN crystal growth along the [010] direction
forming a 1D structure.
In contrast, simulation of the compound AMN using
a growth morphology algorithm (Figure 1c) gives a thermo-
dynamically stable sheet-like structure. The larger attachment
energies obtained from the (100) and (010) faces (Table S3)
suggest that both the [100] and [010] directions have high
growth rates. Figure 1d shows the packing diagrams of AMN
along the [001] crystal direction, where the four lavender lines
À
Á À Á À
Á
ꢀ
ꢀ
ꢀꢀ
indicate the (110) 110 110 110 planes. Besides p–p
stacking along the a axis, there is also hydrogen bonding
between the imide hydrogen and carbonyl oxygen with
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nanosheets indicate an obvious optical waveguiding charac-
teristic and a strong 1D field confinement.
loss caused by scattering. Second, the aggregation-induced
red shift and narrowing of the fluorescent spectra reduced the
overlap with its absorption spectra (Figure 3d), which can
effectively diminish the reabsorption of light during prop-
agation along the wire.
In contrast, the photons in the 2D nanosheets can migrate
within the 2D plane parallel to the substrate with confinement
in one dimension. One typical rhombic nanosheet with an
edge length of about 30 mm (Figure 4a) was used to study the
The optical waveguide properties of the as-prepared 1D
nanowires and 2D nanosheets were studied to investigate the
structure–property relationship of the nanostructures. Spa-
tially resolved PL imaging and spectroscopy measurements
were performed by locally exciting a single wire or sheet with
a 351 nm focused laser beam. Figure 3a shows the micro-area
Figure 3. a) Bright-field and PL images obtained from a single ADN
1
D nanowire by exciting the wire at different positions. Scale
Figure 4. a) Bright-field and b) PL images obtained from a single AMN
nanosheet by exciting the center with a UV laser (l=351 nm). Scale
bar=10 mm. Solid arrows show the predominant guiding directions;
dashed arrows show the blocked directions. c) The corresponding
spatially resolved PL spectra of the excited spot and the four edges
(marked with 1–4) of the nanosheet shown in (b). d) The molecular
packing arrangement in the sheet. Black arrow shows the direction of
the electric field of incident light. Solid yellow arrows show the
components of the transition dipole moments for the two front surface
molecules (1,2) in the (001) plane, while the dashed yellow arrows
indicated those of the two molecules on the back side (3,4).
bar=5 mm; b) Spatially resolved PL spectra from the tip of the
nanowire for different separation distances between the excitation spot
and tip of the wire shown in (a). c) The ratio of the intensity I /I
against the distance D. Curves were fitted by an exponential decay
tip body
function I /I =Aexp(ÀRD). d) Normalized absorption and emis-
tip body
sion spectra of the ADN monomer and nanowires.
PL images obtained from a nanowire (length ꢀ 40 mm) by
accurately shifting the excitation laser spots. In this 1D
structure, the photons are confined in two dimensions and
propagate along the axis of the nanowires in two predominant
transmission directions. The propagation loss of the materials
was evaluated by looking at the spatially resolved spectra of
the emitted light with respect to the distance travelled.
Figure 3b illustrates the corresponding PL signals detected
from the wire terminus by changing the position of the
excitation laser beam.
The distance dependent intensity of the nanowire, shown
in Figure 3c, indicates that the intensity of the out-coupled
light decays almost exponentially with the increase in
propagation distance, which is a typical characteristic of
active waveguides. The intensity at the excited site along the
light propagation behaviors in the 2D structure. As displayed
in the photographs (Figure 4b), when the laser beam was
focused at the center of the plane, the neighboring two out-
coupled edges (marked with 1 and 2) have large, bright out-
coupled light beams, which shows significant contrast with the
small, obscure out-couplings at the opposite edges (3 and 4).
This observation clearly indicates that the propagation
efficiencies along the two opposite directions are entirely
different. The corresponding PL spectra of the four edges
(Figure 4c) quantitatively demonstrate that the emission
intensities of edges 1 and 2 are hundreds of times stronger
than that of their opposite edges, showing that the propaga-
tion efficiencies of these nanosheets are highly sensitive to the
propagation direction. To estimate the asymmetry of the light
propagation, we calculated the optical loss coefficient R in the
different propagation directions according to the aforemen-
tioned formula. The loss coefficient R of the guide path
body of the nanowire (Ibody) and at the emitting tip (I ) were
recorded and the optical-loss coefficient (R) was calculated by
tip
single-exponential fitting I /I = Aexp(ÀRD), where D is
tip body
[15]
the distance between the excited site and the emitting tip.
À1
Accordingly, R = 0.069 dBmm at 450 nm, which is much
[16]
À1
lower than the value for other organic materials. There are
two important factors that may contribute to the excellent
optical waveguide behavior of the ADN nanowires. First, the
smooth surface and high crystallinity minimized the optical
towards edges 1 and 2 (0.076 dBmm ) is much lower than that
for
propagation
along
the
opposite
directions
À1
(0.457 dBmm ). It is this sharp difference in the loss behavior
that results in the stark contrast in the output signals, which
4
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[
19]
confirms that these nanosheets could be applied for asym-
metric light propagation in photonic circuits.
backward output intensity, respectively. According to this
formula, the calculated C value of the examined nanosheet is
0.89, which is high enough for applications in nanoscale
photonic devices. By changing the size of the nanosheets
(Figure S4), we can further control the propagation length to
modulate the intensity contrast and diode efficiency. When
the laser beam was focused on two opposite obtuse corners
(Figure 5d,e), the nanosheet showed two asymmetric light
propagation channels, providing a way to manipulate the
photonic signals in multiple directions. According to the
corresponding spatially resolved PL spectra (Figure 5 f), the
calculated C values of the two propagation channels are 0.93
and 0.88. We believe this kind of asymmetric propagation will
play a fundamental role in creating a new generation of
miniaturized all-optical logic devices.
This asymmetric light propagation in the nanosheets is
pertinent to the molecular-packing modes in the crystal,
because the molecular stacking as well as the direction of its
optical transition dipole determine the final optical proper-
[
17]
ties. If the electric field of the incident light is parallel to the
transition dipole moment of oriented molecules, the PL is
maximized with a high absorption cross section and prop-
agation efficiency. In our case, the transition dipole of the
AMN molecule is parallel to the naphthalene long axis in the
p-conjugated plane, pointing in the direction of the amido
groups. The well-defined rhombic nanosheet crystal has four
AMN molecules in one unit cell, in which the four molecules
display a centrosymmetric arrangement (Figure 4d; Fig-
ure S3). When a unit cell was excited, the emitted light was
guided mainly along the directions of the components of the
transition dipole for the two front molecules (solid arrows).
As a result of this ensemble behavior, characteristic of
repeated unit cells in a single-crystal nanosheet, the two out-
coupled edges (Figure 4b, solid arrows) are much brighter
than their opposite edges.
In summary, we have controlled the formation of distinct
self-assembled nanostructures (nanowires and nanosheets)
and the photonic properties of organic materials by rational
molecular design. Different dominant intermolecular inter-
actions (H bonding, p–p stacking), based on molecular
moieties with different structural and conformational char-
acteristics, were responsible for the formation of organic
nanostructures with distinct morphologies. The nanowires
had low dissipation of light during propagation, while the
nanosheets displayed interesting asymmetric light propaga-
tion, which originated from the molecular-packing modes in
the aggregated nanostructure. The ability to asymmetrically
propagate light makes these nanosheets promising candidates
for optical planar diodes, which is important to control the
flow of photons. These results should be useful for the
investigation of active photonic devices and could have
significant potential for future integrated photonic circuits.
This asymmetric propagation makes the organic rhombic
nanosheets suitable for optical planar diodes, which are
optical analogues of the well-known and widely used elec-
[18]
tronic diodes.
By changing the input position on the
nanosheet, we can achieve different functions for this optical
device. When the excitation laser is focused on two opposite
edges with the same excitation power, emitted light is allowed
to travel from one side to the other in the forward direction,
while transmission in the opposite direction is inhibited
(
Figure 5a,b). The corresponding PL spectra (Figure 5c)
quantitatively demonstrated the excellent performance for
unidirectional signal transfer with different loss coefficients R
Received: April 8, 2013
Published online: && &&, &&&&
À1
À1
(
0.106 dBmm forward, and 0.573 dBmm backward). The
intensity contrast (C) between the two output edges is defined
as C = (I ÀI )/(I +I ), where I and I are the forward and
Keywords: materials science · nanophotonics ·
organic nanomaterials · self-assembly · waveguides
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