Supramolecular interaction-induced self-assembly of organic molecules into ultra-long tubular crystals with wave guiding and amplified spontaneous emission

Article abstract of DOI:10.1039/c1jm14815j

Ultra-long tubular crystals (length up to 10 mm and diameter of 70 μm) of a small organic functional molecule, 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB), are successfully prepared through a physical vapor transport (PVT) method. On the basis of crys

Full text of DOI:10.1039/c1jm14815j

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PAPER
Supramolecular interaction-induced self-assembly of organic molecules into
ultra-long tubular crystals with wave guiding and amplified spontaneous
emission†
Yuanxiang Xu, Houyu Zhang, Feng Li, Fangzhong Shen, Huan Wang, Xianjie Li, Yang Yu and Yuguang Ma*
Received 27th September 2011, Accepted 28th October 2011
DOI: 10.1039/c1jm14815j
Ultra-long tubular crystals (length up to 10 mm and diameter of 70 mm) of a small organic functional
molecule, 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB), are successfully prepared through
a physical vapor transport (PVT) method. On the basis of crystal structural analysis and density
functional theory calculations, we show that the formation of such tubular crystals is drawn by the
branched hydrogen bonds between the BCPEB molecules. High crystalline quality, highly ordered
molecular orientation, and hollow-like topological structure enable the crystal to exhibit optical wave
ꢀ1
guided emission behaviors with low optical loss (3 dB mm ) and highly polarized emission (the
polarized ratio is about 20). Amplified spontaneous emission (ASE) characteristics of the tubular
crystals were also studied; the net gain coefficients at the peak wavelength and the threshold are 91.7
ꢀ1
ꢀ2
cm and 36 kW cm , respectively.
8
crystals. Recently organic crystals with topological structures, in
Introduction
the shape of e.g. hollow tubes, have attracted much attention due
9
to potential applications in fields such as optics and electronics.
Organic crystals constructed of p-conjugated molecules have
attracted great attention in the field of organic optoelectronic
Artificial preparation of high quality, long range order and
regular shaped crystal tubes according to the requirements of
function and device is of great importance. This, however,
remains a challenge because of the difficulty in controlling the
growth and directionality of organic molecules on a large scale
under the driving forces of weak intermolecular interactions. On
the basis of investigation of crystal growth, characterization of
crystal structure and optical property measurements, we describe
here a example of ultra-long organic crystal tubules from the
p-conjugated molecule 1,4-bis(2-cyano-2-phenylethenyl)benzene
1
materials. The advantages of organic crystal materials lie in their
definite structures, which provide a model for the deep under-
standing of basic interactions between the molecules (supramo-
lecular interactions), and structure–property relationships
between the molecular packing arrangement and optoelectronic
2
properties (luminescence and carrier mobility). The academic
motivation in hunting for organic crystals with desired high
thermal stability, highly ordered structures and high carrier
mobility is due to their potential application in optoelectronic
3
4
devices such as optically pumped lasers, field-effect transistors,
5
6
electroluminescence, and photovoltaic cells. Different from
crystalline inorganic semiconductors, which are constructed of
atoms joined by strong chemical bonds (bond energy 100–400 kJ
ꢀ1
mol ), organic crystals are built from molecules connected by
weak intermolecular interactions, such as van der Waals inter-
actions, p–p stacking interactions, and hydrogen bonding
ꢀ1 7
(
interaction energy 10–65 kJ mol ). This gives rise to the
diversities of organic crystal structures (polymorphs and topol-
ogies) and complexities for the controlled growth of organic
State Key Laboratory of Supramolecular Structure and Materials, Jilin
University, 2699 Qianjin Avenue, Changchun, 130012, P. R. China.
E-mail: ygma@jlu.edu.cn; Fax: +86 431-85168480; Tel: +86 431-
Fig. 1 (a) The chemical structure of a single BCPEB molecule. (b) Image
of a single tube under UV light (365 nm). (c) SEM image of the BCPEB
tube. (d) Confocal fluorescence microscopy image of the BCPEB tube’s
cross-section.
8
5168480
Electronic supplementary information (ESI) available. CCDC
†
reference number 815708. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1jm14815j
1
592 | J. Mater. Chem., 2012, 22, 1592–1597
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(
(
BCPEB, see Fig. 1a) through the physical vapor transport
hanging inside the wall of the growth tube (see Fig. S2, ESI†).
Fig. 1b shows the photograph of the club-shaped crystals under
ultraviolet (UV) light (365 nm), which depicts a single club of
uniform size. The clubs have a length of up to 10 mm and
diameter of 70 mm. Very strong blue fluorescence is emitted from
the ends of the club. The scanning electron microscopy (SEM)
and confocal fluorescence microscopy images (Fig. 1c and
Fig. 1d) demonstrate that the club-shaped crystals have a hollow
pentagonal cross section (see Fig. S3, ESI†).
10
PVT) method (see Fig. S1, ESI†). In the PVT environment,
high energy BCPEB molecules in vapor are cooled to a relatively
low temperature, thus they aggregate slowly through the self-
assembly process driven by supramolecular interactions. In
principle, the growth of a one dimensional (1D) structure
requires a directional interaction, which has dominance in
interaction energy over other intermolecular interactions. In
BCPEB molecules, the directional interaction is branched
hydrogen bonds between N (from the cyano-segment) and two H
atoms (from the aromatic segment and double bond).
X-ray crystallography indicates the ultra-long tube is a high
quality crystal.‡ The crystal structure is triclinic with space group
ꢁ
P1, the molecule is on the center of symmetry (inversion center)
and half of the molecule is crystallographically independent. The
ORTEP drawing of BCPEB is shown in the ESI.† The perspec-
tive view of molecular alignment in the BCPEB crystal from the
top of the bc plane (along the a axis) is shown in Fig. 2a. It
indicates that all BCPEB molecules are in a parallel alignment
with identical conformation in the crystalline tube and exhibit
very regular uniaxially oriented packing. As shown in Fig. 2b,
along the a axis in the crystal molecules are bound by two types
of N/H–C hydrogen bond, I and II (the distance of H to N and
angle of the hydrogen bonds are summarized in Table 1). Driven
Experimental section
Materials and synthesis
All chemicals were purchased commercially, and used without
further purification. The synthetic procedures and characteriza-
tion data are described in the ESI.†
Measurements
NMR spectra were recorded on a Bruker AVANCE 500 MHz
spectrometer with chloroform-d as solvent and tetramethylsilane
(
TMS) as internal standard. The morphology and size of the
samples were measured by field emission scanning electron
microscopy (SEM) on a JSM-6700F electron microscope (JEOL,
Japan). Ultraviolet-visible absorption and fluorescence spectra
were recorded on UV-3100 and RF-5301 PC spectrophotome-
ters, respectively. The X-ray diffraction experiment was per-
formed on a Bruker D8 with GADDS X-ray diffractometer with
ꢀ
Cu-Ka radiation (l ¼ 1.5418 A) at 40 KV and 20 mA. The
diffractometer is equipped with a graphite monochromator. The
wall of the tubular BCPEB crystal was selected randomly as
a sample for X-ray crystallography test and crystallographic data
ꢂ
were collected at a temperature of 20 ꢁ 2 C on a Bruker ApexII
CCD diffractometer with graphite monochromated Mo-Ka
ꢀ
radiation (l ¼ 0.71073 A). Data processing was accomplished
1
1
with the SAINT processing program. The structure was solved
Fig. 2 (a) Uniaxially oriented alignment of BCPEB molecules in the
crystal. (b) C–H/N hydrogen bonds I and II viewed along the a axis. (c)
CH/p interactions III and IV between adjacent molecules.
by direct refinement with a full-matrix least-squares technique
12
using the SHELXTL-97. Experimental details for the structural
determination are described in Table S2, ESI.† The confocal
fluorescence microscopy images were taken on an OLYMPUS
FLUOVIEW FV1000. For details of amplified spontaneous
emission (ASE), optical waveguide and polarization tests see the
ESI.†
Table 1 Hydrogen bonds in the BCPEB crystal
a
a
Hydrogen bond
I: C(8)–H(8)/N(1)
II: C(10)–H(10)/N(1)
ꢀ
D/A distances (A)
3.42(3)
2.49(7)
155(4)
3.59(7)
2.73(3)
147(1)
ꢀ
H/A distances (A)
ꢂ
D–H/A angle ( )
Results and discussion
a
Symmetry transformations used to generate equivalent atoms: ꢀ1+x, y,
Fabrication and structural characterization of tubular BCPEB
crystals
z.
The PVT growth of BCPEB crystals was carried out in
a temperature-controlled quartz tube. The BCPEB powder
‡
Crystal data for BCPEB: C24
ꢀ
H
16
ꢀ
N
2
, M ¼ 332.39, triclinic, a ¼ 6.737(2)
(
about 10 mg) was placed into the source zone at a predetermined
ꢀ
ꢂ
ꢂ
A, b ¼ 8.541(3) A, c ¼ 8.965(3) A, a ¼ 79.742(7) , b ¼ 68.025(7) , g ¼
ꢂ
ꢀ
3
ꢁ
temperature. Ultrapure argon was used as carrier gas and
protection gas to prevent the BCPEB from being oxidized at high
67.714(7) , V ¼ 442.2(3) A , T ¼ 293(2)K, space group P1, Z ¼ 1,
2845 reflections measured, 1824 independent reflections (Rint ¼ 0.0394).
2
values were 0.0563 (I > 2s(I)). The final wR(F ) values
10,13
The final R
1
temperature during the PVT process.
The crystal grows in
1
were 0.1176 (I > 2s(I)). The final R values were 0.1464 (all data). The
2
final wR(F ) values were 0.1574 (all data). The goodness of fit on F
was 0.946.
a growth zone at relatively low temperature. After several hours,
club-shaped crystals as well as slice-like crystals were found
2
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Table 2 Aromatic CH/p interactions in the BCPEB crystal
Hydrogen bond III: C11–H(11)/ring1[C1/C6]
IV: C4–H(4)/ring2[C9/C11a]
ꢀ
H/ring distances (A)
3.02(1)
3.72(2)
129(5)
2.94(3)
3.75(2)
136(2)
ꢀ
C/ring distances (A)
ꢂ
C–H/ring angle ( )
by these branched hydrogen bonds, as described above,
a molecular layer formed. In the bc plane, molecules are stacked
in a parallel displaced manner along the c axis in length of
a phenylene vinylene moiety. The interconnections between two
stacked molecules are aromatic CH/p interactions III and IV (the
distance from H to the central point of the phenyl ring and angle
of the CH/p ring are summarized in Table 2), as shown in Fig. 2c.
The detailed formation of tubular crystals with pentagonal
topological structure is so far not very clear. In principle, the
tubular crystal growth is related to the intermolecular interac-
In order to elucidate this self-waveguided mode of light
emission in the tubular crystals, the orientation of the BCPEB
molecules in the crystal was determined. We have tested five faces
of each tubular crystal and found them to have the same XRD
pattern. Fig. 3a is an X-ray diffraction pattern of the crystal tube
(bottom) and X-ray diffraction pattern predicted for BCPEB
based on single crystal data (top). By comparing the two curves,
the diffraction spacing is determined and it is assigned to the
(0 2 ꢀ1) plane of the BCPEB crystal. In the (0 2 ꢀ1) plane, the
14
tions and the growth conditions. As described above, the
BCPEB molecules were mainly affected by N/H–C hydrogen
bonds and CH/p interactions in the tubular crystal. To deter-
mine the dominant direction of crystal growth with the larger
energy among these intermolecular interactions, a computational
study of the interaction energy has been carried out by density
functional theory. By a supermolecular approach, the calculated
ꢀ1
interaction energy is 27.7 kJ mol for a hydrogen bonded dimer
ꢀ
1
along the a axis, while the interaction energies are 18.5 kJ mol
ꢀ
1
and 9.5 kJ mol for CH/p interactions in the two configurations,
respectively (see ESI†). This indicates that the branched
hydrogen bonds are the directional driving forces, which make
the BCPEB molecules arrange along the same direction leading
to the formation of 1D straight crystals.
Numerous fabrication experiments indicated the formation of
tubular crystals is controllable and repeatable. The length and
diameter of the BCPEB tubes can be readily controlled by
adjusting the experimental conditions, such as growth time and
temperature of the growth zone. Longer tubes can be obtained by
prolonging the growth period. For example, at a fixed growth
ꢂ
temperature of 248 C, the final lengths of the tubes were 3 mm, 5
mm, and 10 mm at growth periods of 8, 16 and 24 h, respectively.
Tubes of different average thickness have also been obtained at
ꢂ
various temperatures. For example, when 238, 243 and 248 C
were adopted at a fixed growth time of 16 h, we obtained tubular
walls with an average thickness of 8.1, 15.8 and 17.4 mm
respectively (see Fig. S6, ESI†). When the temperature of the
ꢂ
growth zone was below 238 or above 248 C, the tubular crystals
were not obtained.
Optical waveguide and polarized emission properties
Under UV light, the tubes exhibit blue emission with very bright
luminescence spots observed from the ends of each tube and
a weaker emission observed from the bulk surface of tube, as
shown in Fig. 1b. If excited with a focused laser (l ¼ 355 nm) at
different positions along the tube, stronger emission from both
tips than from the body was also found (see Fig. S7, ESI†). It
indicates that the light emitted at the excited region was confined
inside the crystal and propagated along the tube axis, then
Fig. 3 (a) X-ray diffraction pattern of BCPEB tubular crystals obtained
by prediction based on single crystal data (top) and experimental test
(
bottom). Inset: BCPEB molecules lay in the (0 2 ꢀ1) plane. (b) Fluo-
rescence spectra taken with polarizations of E and E . Inset: depen-
t
k
dence of the output intensity at l ¼ 482 nm on polarization of angle,
where q denote the angles of the polarization plane of the emitted light
with respect to the excited surface.
3,15,16
radiated from the tips.
1
594 | J. Mater. Chem., 2012, 22, 1592–1597
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BCPEB molecules interact with each other through the branched
hydrogen bonds in a uniaxial orientation (see the inset in Fig. 3a).
This means that the transition dipoles of BCPEB molecules in the
crystal are uniaxially aligned (the calculation results show that
the orientation of the S0–S1 transition dipole moment of BCPEB
is along the molecular long axis, see Fig. S10, ESI†), so the
3,17
emission light from the tips may be polarized,
which can be
proved by polarization experiments. Polarization-dependent
fluorescence spectra are shown in Fig. 3b, and a schematic
illustration of spectra obtained parallel (E
) to the excited surface of the tube is shown in Fig. 4. The
maximum and minimum intensity of spectra correspond to the
and E , respectively, with a ratio of about 20. This result
indicates that the self-waveguided emitted light is polarized
k
) and perpendicular
(E
t
E
k
t
3,17,18
parallel to the excited surface of the tube.
Thus the self-
waveguided mode of light emission in the tubular crystals is clear,
schematically shown in Fig. 4.
ꢀ1
Further, we measured the value of its optical loss as 3 dB mm
at 482 nm (for a detailed description of the optical waveguide and
optical loss test see the ESI†). Compared with the many organic
nanowires/nanofibes and films that have been reported (>100 dB
ꢀ
1 19
Fig. 4 Schematic representation of molecular arrangement in the
tubular BCPEB crystal and configuration of the polarized emission from
the crystal.
mm ), the propagation loss of the BCPEB tube is much lower;
approximately that of subwavelength-diameter silica wires
ꢀ1 20
(
about 1.7 dB mm ). This can be attributed to the following
reasons. Firstly, its low self-absorption due to well separated
19,21
absorption and emission bands of the BCPEB tube
(the
absorption and emission spectra of BCPEB are in Fig. S9, ESI†).
Secondly, the smooth surface and uniform diameter (as shown in
Fig. 1b) are beneficial for decreasing the optical loss due to the
15a,18
reduction of light scattering.
Thirdly, photons are spatially
confined in two dimensions in the thin tubular wall due to the air
medium inside the tubes. Such confinement of the light decreases
the interaction of the light reflected from the internal wave-
guiding surface with the substrate, which gives rise to lower
16,19
propagation losses.
Amplified spontaneous emission (ASE) properties
Generally, waveguided propagation of emission is thought to be
one prerequisite for lasing or amplified spontaneous emission
2
2
(
ASE), and the uniaxially oriented molecules should be a suit-
able configuration to amplify emitted light and achieve the
Fig. 5 (a) PL spectra of BCPEB crystal tube as a function of the pump
laser energy. (b) Dependence of the peak intensity and FWHM of
emission spectra on the pump laser energy.
Fig. 6 Peak intensity of PL spectra as a function of the pump stripe
length.
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Table 3 Comparison of the ASE performance of different organic crystals
a
Threshold value
b
Net gain coefficient
ꢀ1
ꢀ
2
ꢀ2
Crystal
(mJ cm
)
(kW cm
)
(cm
)
Reference
ꢀ
2
BCPEB
DPDSB
CNDPASDB
CNDPDSB
BP1T
BP2T
BC4
AC5
360 (10 ns)
7000 (10 ns)
305 (10 ns)
395 (10 ns)
400 (30 ns)
355 (500 ps)
50 (500 ps)
560 (16 ns)
36
700
30.5
39.5
13.3
710
100
35
91.7 (180 kW cm
–
)
This work
27
18
24
28
23
23
29
ꢀ
2
55 (1900 kW cm
ꢀ
)
2
33 (375 kW cm
57.1 (620 kW cm
)
ꢀ
2
)
–
–
ꢀ
2
75 (61.3 kW cm
)
a
ꢀ2
ꢀ2
b
For changing mJ cm into kW cm , the pulse durations were given in brackets. Net gain coefficient: these parameters were obtained at the
corresponding excitation energies in the brackets.
organic crystal laser with low energy threshold. This is because
the light emission of organic materials is from excitons localized
on a single molecule, and dipole moments with the same
configuration can easily interact into a certain phase with respect
to the optical field, which should induce the coherent emission
highest values. The comparison indicates that the high quality
tubular BCPEB crystal with regular uniaxially oriented mole-
cules is suitable to amplify emitted light and achieve the laser
with low threshold.
18
and the light amplification. To study the ASE property, an
Conclusions
isolated single BCPEB tube was excited with a pulsed laser (l ¼
By means of supramolecular assembly in the gas phase, tubular
crystals with a hollow pentagonal cross section were prepared via
PVT. It is found that BCPEB molecules in the tubes are uni-
axially orientated, which is verified by characterization of crystal
structure by X-ray diffraction, and polarized emission measure-
ments. The unique morphology and molecular arrangement give
tubular BCPEB crystals possible applications as optical wave-
guides with low optical loss and organic lasers with low
threshold. Directional non-covalent interactions such as p–p
interactions and hydrogen bonds could be utilized to build well
controlled structures.
3
55 nm) and PL emissions were subsequently collected from the
23,24
tip area.
As the pump energy increased, a gain-narrowed peak
rises out of the broad emission spectrum (Fig. 5a). Fig. 5b
represents dependence of the peak intensity and full widths at
half-maximum (FWHM) of emission spectra on the pump laser
energy. The relationship between the peak intensity and the
pump energy is nonlinear. This is characterized as the ASE
25
caused by stimulated emission. The slope of the peak intensity
ꢀ2
versus pump energy curve changes at 36 kW cm , which should
be the threshold value of ASE. Fig. 6 shows the peak intensity
(
482 nm) of PL spectra as a function of pump stripe length at
ꢀ2
pump energy of 150 kW cm (for the experimental process see
the ESI†). The peak intensity increases exponentially with
increasing pump stripe length. The solid curves are fits to the data
using the following equation:
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 50820145304, 21073077, 50733002),
and the Ministry of Science and Technology of China
(2009CB623605).
AðlÞP
gðlÞ
0
À
Á
gðlÞl
IðlÞ ¼
e
ꢀ 1
(1)
where A is a parameter proportional to the PL quantum yield, P0
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26
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J. Mater. Chem., 2012, 22, 1592–1597 | 1597