Controlling growth of molecular crystal aggregates for efficient optical waveguides

Article abstract of DOI:10.1039/c2cc32501b

Controllable crystal aggregate structures which show highly uniform crystal tubule, rod and cubic like architectures were achieved and the well-defined microrods exhibit outstanding optical waveguide properties.

Full text of DOI:10.1039/c2cc32501b

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Chemical Communications
www.rsc.org/chemcomm
Volume 48 | Number 72 | 18 September 2012 | Pages 8981–9096
ISSN 1359-7345
COMMUNICATION
Yuliang Li et al.
Controlling growth of molecular crystal aggregates for efficient optical
waveguides
1359-7345(2012)48:72;1-Y

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Cite this: Chem. Commun., 2012, 48, 9011–9013
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COMMUNICATION
Controlling growth of molecular crystal aggregates for efficient optical
waveguidesw
Songhua Chen,a Nan Chen,a Yong Li Yan,b Taifeng Liu,a Yanwen Yu,a Yongjun Li,a
Huibiao Liu,a Yong Sheng Zhaob and Yuliang Li*a
Received 7th April 2012, Accepted 16th May 2012
DOI: 10.1039/c2cc32501b
Controllable crystal aggregate structures which show highly
uniform crystal tubule, rod and cubic like architectures were
achieved and the well-defined microrods exhibit outstanding
optical waveguide properties.
temperature, while BTN-7 crystals were grown by slow
evaporation of CHCl3 (Fig. 1).z Analysis of the crystal struc-
ture demonstrates that BTN-6 has a nonplanar conformation
and a twisted structure in the crystals. The angle between the
carbazole unit and neighboring benzene ring is about 53.11.
While, two benzene rings linked by the acetylenic and ethyl-
enic linkage bond are nearly coplanar with the benzothiadi-
azole unit, suggesting that an extended p-conjugated system
exists in this part of the molecule. BTN-6 has four molecules per
unit cell with the molecules arranged into two non-equivalent
stacks and with their dipoles pointing in opposite directions.7
One remarkable feature of the crystal structure is the existence
of short S1ꢀ ꢀ ꢀN2 interheteroatom contacts (3.09 A) between the
two 1,2,5-thiadiazole rings and S1ꢀ ꢀ ꢀH–C interactions (2.97 A)
from the benzene ring, which also played an important role in
the parallel creation of this packing structure (Fig. S1w).
Furthermore, the presence of the terminal NO2 group on
benzothiadiazole in BTN-6 gives rise to intermolecular aromatic
hydrogen bonding: C–Hꢀ ꢀ ꢀO (2.59 A) hydrogen bond, and the
p–p interactions between the ethylenic bond and benzothiadi-
azole ring, which are also critical for the head-to-tail crystal
packing. These hydrogen bonds extend over the whole crystal
structure and the multiple intermolecular interactions help lock
the molecular conformations.8 The stabilized conformations are
resistant to intramolecular motion induced by thermal agitation.
This kind of molecular configuration can effectively promote the
p–p interaction in the solid state and may have caused the intense
fluorescence in the aggregation state.9 When viewed down the
crystallographic b-axis, the molecules are p-stacked (interplanar
distance of 3.20 A) in two translational stacks.
During the past decade, well-defined nano- and microstructured
materials have been paid more and more attention for their
potential applications in the electronics and photonics fields.1–3
They are also attractive as building blocks as their individual
structures can function as both device elements and inter-
connects.4 Typically, during this phase of development of a
new science and technology area, researchers focus mainly on
understanding the nature of structure–function relationships.
Intramolecular charge-transfer (ICT) molecules that exhibit
multifunctionality such as multiple electroactivity or photo-
activity offer wide applications in organic electronics, due
to their semiconductor character and high charge carrier
mobilities.5 ICT molecules in particular have potential in
applications of photoelectric devices.6 The results are able to
help reach the goal of physical measurement. In this communi-
cation, we have designed and synthesized donor–acceptor
(D–A) molecules based on benzothiadiazole, where the donor
part consists of carbazole units, while benzothiadiazole works
as the acceptor. Supramolecular self-assembly processes offer
considerable synthetic advantages for fine tuning the size, shape
and distribution and exploiting the supramolecular interaction
for controlling the growth of these ICT compounds in the
form of microtubes, microrods and cubic structures in large
quantities and with high morphological and chemical purity.
Single crystals of BTN-6 were obtained by slow diffusion
of n-hexane into tetrahydrofuran (THF) solution at room
BTN-7 shares some structural similarities with compound
BTN-6, such as the perfect planarity of the benzothiadiazole
acceptor and p-conjugated linkage. Hence, the p-orbital overlap
between the rings of a single molecule is close to its maximum
value.7 Whereas, the dihedral angle between the carbazole unit
and neighboring benzene ring is 67.81, slightly larger than the
dihedral angle found in BTN-6. The closest approaches of a
molecule in one stack to a molecule in an adjacent stack are as
a CAS Key Laboratory of Organic Solids, Beijing National
Laboratory for Molecular Science (BNLMS), Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190,
P. R. China. E-mail: ylli@iccas.ac.cn
b CAS Key Laboratory of Photochemistry, Beijing National
Laboratory for Molecular Science (BNLMS), Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190,
P. R. China
w Electronic supplementary information (ESI) available: Synthetic
details, UV-Vis and fluorescence spectra, AFM images. CCDC-
859087 for BTN-6 and CCDC-859088 for BTN-7. For ESI and
crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc32501b
Fig. 1 Crystal structures of BTN-6 (a) and BTN-7 (b).
Chem. Commun., 2012, 48, 9011–9013 9011
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follows: N1ꢀ ꢀ ꢀH–C (2.68 A); C–Hꢀ ꢀ ꢀp (2.69 A) between
adjacent carbazole rings. One remarkable feature of the crystal
structure is the existence of S1ꢀ ꢀ ꢀp (3.15 A) contacts between
the heteroatom and carbazole.
These compounds have a narrow peak at B340 nm
(Fig. S2w) attributable to the p - p* transition of the
benzothiadiazole unit,10 and a feature absorbance band
indicating the intramolecular charge transfer from the carba-
zole group to the benzothiadiazole acceptor core.11 The results
showed the ICT band of 448 nm for BTN-6 was longer than
384 nm for BTN-7 due to the stronger electron accepting of the
NO2 group.12 In this way, the position of the ICT band in the
visible region could be modulated by using the different
electron acceptor on the benzothiadiazole. BTN-7 showed
intense fluorescence in solution. While, no emission was
detected for BTN-6 in solution (CH2Cl2, CHCl3, THF, etc.).
However, the nanostructure suspension or solid state of
BTN-6 was a good emitter. The fluorescence enhancement
process was studied with different ratios of hexane/THF
mixture. Hexane was continually added to the BTN-6 solution
in THF while keeping a total concentration of 2.0 ꢁ 10ꢂ5 M.
The PL intensity remained almost unchanged when the
content of hexane was lower than 40% and increased greatly
when the hexane content was higher than 50%. Meanwhile,
the emission peak showed a blue-shift on increasing the hexane
(Fig. S3w). However, if the content of hexane was higher than
85%, the fluorescence intensity of the suspension gradually
weakened instead of intensifying, because the mixed solvent
becomes so poor that the molecular aggregates begin to
precipitate in the solvents. The enhanced fluorescence emission
of BTN-6 is a typical aggregation-induced emission system.13
The crystal data showed that the BTN-6 molecule is a sticky
structure. Effective p–p interactions, hydrogen bonding and a
large dipole–dipole moment were observed in the solid state.
The solubility of BTN-6 was poor in conventional organic
solvents such as methanol, acetonitrile, hexane, and toluene,
etc. Slow crystallization was performed at the interface
between a ‘‘good’’ and a ‘‘poor’’ solvent. Well-defined micro-
tube structures of BTN-6 were successfully fabricated by a
phase transfer methodology.14 Hexane was used as a ‘‘poor’’
solvent, while THF was used as a ‘‘good’’ solvent in the
system. After the addition of 90% volume fractions of hexane
into the THF solution, 1-D microtubule structures were observed
as precipitates in the mixture solution and stable even for
4 months (Fig. 2a). Atomic force microscopy images (Fig. S4w)
showed the tubes have a smooth surface with a typical height and
width of 500 nm and 3.0 mm, respectively.
Fig. 2 Large-area SEM and TEM images of BTN-6: microtubes
(a–b), microrods (c–d), microcrystals (e–f). Microrod structure of
BTN-7 (g–h).
solution into poor, same volume solvent hexane with sufficient
stirring (Fig. 2g). BTN-7, with the predominant p–p inter-
action, were rapidly dispersed from a ‘‘good’’ solvent into a
‘‘poor’’ solvent, where the molecules have limited solubility
and thus self-assembly of these molecules was expected to
occur instantaneously.15 This approach provides in situ pre-
paration of microrods on a surface which is suitable for optical
or microscopy investigation. By these simple solution methods,
these ICT compounds were constructed as well-ordered and
stable microsized suprastructures without any addition of
surfactant, catalyst, or template.
X-Ray diffraction experiments (Fig. S5 and S6w) confirmed
that these supramolecular self-assemblies were crystalline,
according to the sharp diffraction peaks. In the case of
BTN-6, there are distinct peaks corresponding to the (0 0 2),
(1 0 ꢂ3) and (1 2 ꢂ4) planes in the microtube structures and
(0 0 2), (0 2 2) and (1 2 ꢂ4) planes in the microrod structures,
revealing that the molecules may tend to stack along the c-axis
to form the 1-D structures. As described, the operation para-
meters such as temperature, concentration, and growth
time remarkably affected the morphologies of the molecular
crystals in different self-assembled systems, due to the inter-
actions of hydrogen bonds and p–p stacking in different
chemical environments. The morphologies of the final formed
architectures were also affected by the shape of their precursor
crystal seeds. In the forming process (Fig. 3) of BTN-6
microtubes, the crystallization is performed on the interface
of the THF and hexane, which provides a limited factor as a
soft template. Due to the poor solubility of BTN-6 in hexane,
the rate of crystallization at the interface of two phases is
higher than that of diffusion of BTN-6 from THF into hexane.
The aggregates of BTN-6 tend to stack along the c-axis due to
The shape and size of the microstructures of BTN-6 can be
tuned by changing the ‘‘poor’’ solvent. As described above,
large-scale 1-D microrods were observed as precipitates in
ethanol/THF mixture solutions. The microrods were about
2–3 mm in width and 10–15 mm in length (Fig. 2c). However,
when the saturated refluxing solution of BTN-6 in THF was
cooled to room temperature, the precipitates were observed to
be ‘‘cubic’’ superstructure assemblies (Fig. 2e). Beyond the
solution processing methods,
a solvent-vapor annealing
technique has been successfully used for self-assembling of
BTN-7 molecules on substrates. Well-defined 1-D microrods
can be simply obtained by injection of a concentrated CH2Cl2
Fig. 3 Schematic representation of the self-assembly processes of BTN-6.
c
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The electrical conductivity value of BTN-6 was estimated to be
1.64 ꢁ 10ꢂ5 S mꢂ1, showing the semiconducting character of
this material.
In summary, two kinds of intramolecular charge-transfer
molecules have been synthesized for studying the tuning of
molecular aggregate structures by self-assembly technology.
The resulting controllable aggregate structures of these intra-
molecular charge transfer compounds show highly uniform
tubule, rod and cubic like architectures in large quantities and
with high morphological and chemical purity. The fabricated
BTN-7 microrods exhibit outstanding optical waveguide prop-
erties with a waveguide efficiency a of 0.018 dB mmꢂ1 and no
obvious red-shift was observed. As a result, they could possess
highly interesting potential applications in optical devices.
We thank the National Nature Science Foundation of China
(201031006 and 90922017) and the National Basic Research 973
Program of China (2011CB932302 and 2012CD932900).
Fig. 4 PL images of microstructures of BTN-6 (a) and BTN-7 (b).
Scale bars are 50 mm. (c) Microarea PL images obtained by exciting an
identical microrod at different positions, down arrow (excited site) and
up arrow (emitted tip). Scale bar is 20 mm. (d) Corresponding PL
spectra in c.
the strong p–p interactions and hydrogen bonds. The tubular
structure of the molecular crystal was easily formed along the
growth direction of the template wall. The growth rates along
the wall exceed greatly the growth in the middle of the
template. Simultaneously, owing to the surface energy among
different facets of the 1-D aggregation, the redissolving starts
at the center of the high energy facet, and then continues
toward the inside along the c-axis.16,17 Therefore, a perfect
crystal tubule-like structure can be obtained. However, in the
mixture of ethanol/THF, the growth rate and direction of the
molecular crystal aggregates is the same in the soft template,
due to the influence of the hydrogen bond interactions being
weak; the 1-D rod-like structures were easily formed. The rate
of crystallization at the interface of two phases is near to that
of the diffusion of BTN-6 THF solution into ethanol due
to the good miscibility of THF and ethanol, which results in
rod-like aggregation.16–18
Notes and references
z Crystal data for BTN-6: C34H20N4O2S, Mr = 548.60, monoclinic,
space group: P21/n, a = 10.064(2), b = 8.9812(18), c = 28.804(6) A,
a = 90, b = 97.80(3), g = 901, V = 2579.5(9) A3, Z = 4, T = 173(2) K,
Dcalcd = 1.413 mg mꢂ3, R1 = 0.0991 (I 4 2s(I)), Rw = 0.1908
(all data), GOF = 1.143; BTN-7: C34H21N3S, Mr = 503.60, mono-
clinic, space group: P21/c, a = 9.0702(18), b = 39.586(8), c =
7.4369(15) A, a = 90, b = 106.217(3), g = 901, V = 2564.0(9) A3,
Z = 4, T = 173(2) K, Dcalcd = 1.305 mg mꢂ3, R1 = 0.0746
(I 4 2s(I)), Rw = 0.1501 (all data), GOF = 1.199. CCDC-859087
for BTN-6 and CCDC-859088 for BTN-7.
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Fluorescence microscopic images (Fig. 4) confirmed that the
microstructures of ICT compounds were intense emitters. The
results indicated that the prepared microrods of BTN-6
exhibited strong red emission, while BTN-7 exhibited green
emission. Furthermore, the distance-dependent photoluminescence
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the corresponding spatially resolved PL spectra were detected
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observed with the increase of propagation distance. Here, the
negligible reabsorption, smooth surface and distinctly flat end
facets contributed to this excellent optical waveguide behavior.
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conditions, and the collected current–voltage (I–V) curves
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Chem. Commun., 2012, 48, 9011–9013 9013