C-H???π interaction induced formation of microtubes with enhanced emission

Article abstract of DOI:10.1021/cg201211e

High luminescent bis(salicylaldehyde)o-phenylenediimine(salophen) microtubes with rectangular cross sections were successfully synthesized by a self-assembly method. Accompanied by the formation of microtubes, a remarkable enhanced emission was observed.

Full text of DOI:10.1021/cg201211e

Article
pubs.acs.org/crystal
C−H···π Interaction Induced Formation of Microtubes with Enhanced
Emission
Cuiping Zhao,†,# Zhongliang Wang,‡,# Yinlong Yang,† Chao Feng,‡ Wei Li,† Yanan Li,‡ Yuping Zhang,†
Feng Bao,† Yuliang Xing,† Xiujuan Zhang,*,† and Xiaohong Zhang*,‡
†Functional Nano & Soft Materials Laboratory (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials &
Devices, Soochow University, Suzhou, Jiangsu 215123, China
‡Nano-organic Photoelectronic Laboratory and Laboratory of Organic Optoelectronic Functional Materials and Molecular
Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101
S
* Supporting Information
ABSTRACT: High luminescent bis(salicylaldehyde)o-phenylenediimine-
(salophen) microtubes with rectangular cross sections were successfully
synthesized by a self-assembly method. Accompanied by the formation of
microtubes, a remarkable enhanced emission was observed. Crystal structure
analysis and theoretical studies were both investigated in detail. It was found
that a conformation change induced by multiple C−H···π interactions
between adjacent molecules was responsible for the formation of microtubes.
The edge-to-face C−H···π interactions also resulted in molecular structural
rigidification, which made salophen a stronger emitter in microtubes.
INTRODUCTION
EXPERIMENTAL SECTION
■
■
Materials. 1,2-Phenylenediamine was obtained from Alfa Aesar
(China) and used without further treatment. Ethanol (A. R.) and 1,4-
dioxane (A. R.) were purchased from Beijing Chemical Reagent Corp.,
China. High purity water was generated with a Milli-Q apparatus
(Millipore) and was filtered by an inorganic membrane with a pore
size of 0.02 μm (Whatman International, Ltd.) just before use.
Bis(salicylaldehyde)o-phenylenediimine (Salophen) was synthesized
using standard procedures involving the reaction of salicylaldehyde
with 1,2-phenylenediamine (2:1 molar ratio) in ethanol.14 The yellow
needle products were purified by recrystallization from ethanol (yield:
80−90%). The product was then confirmed by MS and NMR
(1HNMR: δ 6.6−7.4 (m, 12 H, phenyl), 8.6 (s, 2 H, NCH), 13.2 (s,
2H, OH)).
Characterization. The morphologies and sizes of salophen
nanoaggregates and microtubes were measured by a field-emission
scanning electron microscope (FESEM, Hitachi S-4300) operated at
an accelerating voltage of 5 kV. The transmission electron microscopy
(TEM) study was performed with a Philips CM200FEG microscope
operated at an accelerating voltage of 200 kV. Powder X-ray diffraction
(PXRD) patterns were investigated using an X-ray diffractometer
(Mac Science M18AHF, Japan) equipped with Cu K-alpha radiation
(λ = 1.54050 Å), employing a scanning rate of 0.1 deg/s in the 2θ
range from 5° to 60°. The UV−visible absorption spectra were
recorded using a Hitachi U-3010 spectrophotometer. Photolumines-
cenc (PL) spectra were measured at an excitation wavelength of 332
nm by a Hitachi F-4500 spectrometer.
In recent years, π-conjugated materials based organic
nanostructures have aroused a surge of interest, due to their
outstanding optical and electrical properties. Applications in
light emitting diodes,1 optical waveguides and lasers,2,3 and
field-effect transistors4−6 have been successfully demonstrated.
Different from their inorganic counterparts, noncovalent
intermolecular face-to-face π−π interactions (or π−π stacking)
usually play the key role in crystal engineering and
corresponding properties tuning.7 Meanwhile, it is also
evidenced that face-to-face π−π interactions would result in a
cofacial configurations, which would increase the amount of
π−π orbital overlap and thus be detrimental to luminescence in
the nanostructures.8,9 So far, there has been little evidence that
edge-to-face π−π interaction termed as C−H···π interaction
can lead to the formation of organic nanostructures. Although a
single C−H···π interaction is kinetically labile and the enthalpy
is only about 1.6−2.5 kcal/mol,10−13 could multiple C−H···π
interactions be applied to engineer nanoscale molecular
assembly as well as affect their luminescence behavior? Herein,
in this paper we describe the formation of bis(salicylaldehyde)
o-phenylenediimine (salophen) microtubes mediated by C−
H···π interactions. Accompanied by the formation of micro-
tubes, a remarkable enhanced emission is observed. Crystal
structure analysis and theoretical studies are systematically
investigated.
Received: September 15, 2011
Revised: December 25, 2011
Published: January 18, 2012
© 2012 American Chemical Society
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Computational Method. The geometric structures of salophen
molecule were optimized by employing the Hartree−Fock theory and
an economic basis set,15 and the energies of the minimum energy
configurations were then calculated by using density functional theory
(DFT) and standard 6-31G* basis set. All DFT calculations were
performed using the B3LYP exchange-correlation functional which
combined the Becke three-parameter exchange functional with the
gradient-corrected correlation functional of Lee, Yang, and Par.16,17 All
the calculations were conducted by the Gaussian03 package. The
precise energy calculation for those structures was performed in the
framework of DFT with the generalized gradient approximation
(GGA) by the LYP functional for exchange-correlation energy, as
implemented in the SIESTA code.18 It should be noted that the
double-ζ polarized (DZP) numerical atomic orbital basis set is
adopted to provide sufficient accuracy, and the core electrons are
represented by norm-conserving pseudopotentials. Periodic boundary
condition was employed for both crystal and molecule models, using
the same lattice parameters as discussed above. The cutoff of the grid
integration of charge density was set to 150 Ry, and the Brillouin zone
integration was performed on a Monkhorst−Pack (MP) 8 × 8 × 8 k-
point mesh.
Figure 2. XRD patterns of (a) salophen microtubes and (b) the
starting powders.
RESULTS AND DISCUSSION
■
Salophen microtubes was prepared by a simple solution
method. In a typical preparation procedure, initial salophen
powder was dissolved in dioxane to a concentration of 3 × 10−3
M and 200 μL of solution was injected into 5 mL of
continuously stirred high purity water drop by drop at room
temperature (25 °C). Then the solution was left undisturbed
for about 4 h for stabilization. Figure 1 shows the SEM images
Figure 3. SEM images of salophen sub-microtubes at different aging
time. (a) 5 min, (b) 10 min, (c) 30 min, (d) 60 min, (e) 120 min, and
(f) 4 h.
Figure 1. (a) Typical SEM image of salophen microtubes; inset is the
enlarged SEM image of a single tube; (b) TEM image of a single
salophen microtube; inset is the corresponding selected-area electron
diffraction pattern.
fused together (Figure 3c). Increasing the aging time to 1 h,
half-tubular structures came to emerge among these particles.
With ripening proceeding, the number of particles greatly
decreased (Figure 3d), and these half-tubular structures slowly
sewed together into single rectangular tubes with smooth
surfaces and open ends (Figure 3e), indicating that growth
proceeded by oriented attachment of particles. If the aging time
was extended to 4 h, particles finally disappeared, leaving
behind all rectangular microtubes with very high morphological
purity (Figure 3f). The schematic illustration of the formation
process was proposed in Figure 3g. Strong directional
molecular interactions are supposed to be the main driving
forces for the particles to self-organize and join together along a
certain direction to form microtubes, in a tendency to reduce
the high surface energy of the whole system. This process is
very similar to the “oriented attachment” mechanism proposed
by Banfield et al. in the formation of anatase (TiO2)
nanorods.20
of the as-prepared samples, which are mainly composed of a
large number of microtubes with smooth surface, uniform
diameter, and distinctive rectangular cross section. The average
length and outside diameter of these tubes are ca. 10−20 μm
and 500 nm, respectively. Inset of Figure 1a shows a single
microtube, with the wall thickness of about 100 nm. The
microstructures of the salophen microtubes were also studied
by TEM; the sharp spots in the selected area electron
diffraction (SAED) pattern clearly demonstrate that the
microtube is single-crystalline. X-ray diffraction (XRD) patterns
were recorded to further investigate the crystal structure of the
microtubes and the starting materials as shown in Figure 2.
Both XRD patterns can be readily indexed by a monoclinic
space group P21 with the unit cell dimensions a = 6.064 Å, b =
16.306 Å, c = 16.541 Å, α = β = 90°, and γ = 91.5° (CCDC
Refcode: OPHSAL10).19
To investigate the formation mechanism of the rectangular
microtubes, samples with different ripening durations were
studied. As shown in Figure 3a, disordered rod-like nano-
particles with diameters of 80−120 nm and lengths of 400−600
nm were first observed 5 min right after stirring. The rod-like
particles began to aggregate (Figure 3b) and then gradually
We also investigated the time-dependent photoluminescence
(PL) properties as shown in Figure 4a. It was found that there
was a dramatic change of fluorescence intensity during the
formation of microtubes. The dioxane solution and the initial
nanoparticle suspensions both had two very weak emission
peaks centered at 545 and 435 nm. With increasing the aging
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Figure 4. Time-dependent (a) fluorescence and (b) UV−visible spectra of salophen aggregates in water.
time, emission at 545 nm decreased obviously, while the
emission at 435 nm experienced a red shift to 451 nm,
accompanied with the intensity gradually enhanced and finally
reaching the maximum of about 100 times higher than original.
To gain the insight into the cause of the unique PL behavior,
the UV absorption spectra were measured as well. Figure 4b
shows the UV−vis spectra acquired every 5 min in the
formation process. It can be seen that a new peak at 335 nm
was emerging and gradually become predominant, while the
absorption band at 410 nm, associated with n → π* transition
of keto tautomer decreased correspondingly. In addition, the
absorption band associated with π → π* transitions centered at
257 nm also increased with the ripening proceeding. The iso-
absorptive point appeared at 266 nm, indicating a conversion
from the keto to the enol form existed during the formation
process of salophen microtubes. This is also consistent with the
PL results, where the emission of the enol form increased
gradually with the crystallization proceeding, while the emission
of initial nanoparticles, which mainly consisted of the keto
Figure 5. (a) Chemical structure of salophen molecule (red, blue,
green, and white spheres represent oxygen, nitrogen, carbon, and
hydrogen atoms, respectively); (b) calculated energy of salophen
molecule at different distortion angles and the structures of two
optimized configurations with lowest energies. (c) Projection of
salophen structures viewed along the [100] direction; (d) perspective
view of the aromatic C−H···π interaction in the crystal, which are
denoted by dotted lines.
forms, dropped off accordingly.
When the salophen molecules meet with protic solvent
(water), hydrogen bondings between the CO group in
salophen and hydrogen atom in water would be easily formed,
which is in favor of formation of keto tautomers. Thus, as
proposed by Herzfeld,21 the amorphous particles formed at the
early stage should be mostly composed of keto tautomers.
However, according to the absorption spectra, in the following
crystal growth process, most of keto tautomers gradually
transformed into enol tautomers. Accompanied with the
changes, obvious emission enhancement was observed as
well. What is the driving force for this transformation? Theory
calculation and crystal structure analysis were further
investigated to explore the conformation and energy difference
of salophen molecules in the isolated state and in the
microtubes.
We define the dihedral angle between C1−C2−N1−C3 as
the distortion angle and calculated the energy of different
configurations of isolated salophen molecules, with the C2−N1
δ bond rotating from 0° to 180°. Two lowest energy
configurations appeared with the distortion angle of about
35° and 140°, as shown in Figure 5. The two configurations
were then further optimized by using the B3LYP/6-31G*
method. It was found that they were almost isoenergetic, with
an energy difference of 2.01 kJ/mol, and the dihedral angle
C1−C2−N1−C3 being 38.1° and 142.5°, respectively. We also
studied the molecular configuration in the crystals. Figure 5c
shows the crystal structure viewed along the a axis; it is found
that R2 is almost coplanar to the middle phenyl ring R1 with
only 2.5° titled in the molecule (the enol tautomer). The
coplanarization of the groups R2 and R1 will enlarge the π-
conjugated system and thus activate the radiation process,
making the salophen molecules more luminescent,22−24 while
the coplanarization is also in favor of forming a slipped cofacial
π−π stacking between adjacent molecules, which may induce
certain luminescence quenching. In addition, R3 is significantly
distorted from R1, with a twisted angle up to 53°, which is
obviously different than that optimized in the isolated state. But
at this point, most of the phenyl rings take on the edge-to-face
mode, which favors the formation of C−H···π interactions
between molecules (Figure 5d). C−H···π interaction is a weak
attractive force occurring between a soft acid (C−H group from
the edge of π system) and a soft base (the face of π system).
Multiple C−H···π interactions would not only lock the twisted
conformation and rigidify the structure, making salophen
stronger emitter, but also would act as the driving force to
induce the directional crystal growth to form one-directional
microtubes. Thus the combination effect of planarization
between R1 and R2 and twisted conformations between R1
and R3 should be responsible for the enhanced emission in the
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microcrystals. The multiple C−H···π interactions is the major
driving force for the conformation change and the formation of
microtubes.
The intensity of intermolecular interactions between
salophen molecules was also investigated by calculating the
Crystal structure analysis indicates that salophen molecules take
on more twisted conformations in the crystalline state
compared to those in the isolated state. The multiple aromatic
C−H···π interactions induce and stabilize the twisted
conformation, which endow microtubes with enhanced
emission. Theory calculations indicate that conformation
change of salophen molecules also causes large negative ΔG,
which encourages self-assembly of salophen molecules into
microtubes.
binding energy Eb, which is defined by Eb = Et,cell/4 − Et,mol
where Et,cell is the total energy of the unit cell of salophen
crystal, Et, is the total energy of an isolated salophen
,
mol
molecule, and 4 denotes the unit cell consist of four salophen
molecules. The atomic coordinates of the cell and the lattice
parameters were adopted from CCDC, while the optimal
geometric structure of the isolated salophen molecule was
obtained from the above optimization calculations. The total
optimized binding energy was calculated to be 262.3 kJ/mol,
which was the driving force for the assembly of microtubes.
This means the large negative Gibbs energy ΔG (binding
energy) mainly comes from the multiple C−H···π interactions
along with some π−π interactions.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figure S1: MS of the as-synthesized salophen. Figure S2 IR
spectra of the samples collected at the ripening time of 5 min
and 8 h. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
On the basis of experimental observations, crystal structure
analysis and calculation results, we proposed the growth
process of salophen microtubes, as illustrated in Figure 6. When
■
Corresponding Author
*(Xiujuan Zhang) Fax: +86-512-65882846; tel:+86-512-
65880955; e-mail:xjzhang@suda.edu.cn. (Xiaohong Zhang)
Tel: +86-10-82543510; e-mail: xhzhang@mail.ipc.ac.cn.
Author Contributions
#These authors contributed equally to this work.
ACKNOWLEDGMENTS
■
This work was supported by Major Research Plan of the
National Natural Science Foundation of China (Nos.
91027021, 91027041), National Natural Science Foundation
of China (Nos. 50903059, 51173124, 51172151, 50825304),
National Basic Research Program of China (973 Program,
Grant Nos. 2010CB934500, 2011CB808400), and the
Specialized Research Fund for the Doctoral Program of Higher
Education of China (Grant No. 20093201120020). We also
thank Natural Science Foundation of Jiangsu Province (No.
BK2010003) and a Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institu-
tions.
Figure 6. Schematic energy and conformation diagram of salophen
molecules during the self-assembly process.
salophen dioxane solution were dropped into water,
aggregation of salophen molecules would occur due to the
low solubility in the mixed solvent, during which competitive
equilibrium existed between the keto tautomer and the enol
tautomer. Particles composed of keto tautomers would be
much more easily formed at the beginning, because the keto
tautomer has a lower energy barrier Ek. However, in the mixed
solution, water would offer protons and then facilitate the
isomerization transform from keto form to enol form. More
importantly, during the next Ostwald ripening process, strong
intermolecular interactions (multiple C−H···π interactions)
will strengthen the molecular conformation change and lock
the molecules in enol forms in crystals, as the spectra indicated.
Accordingly, particles composed of keto tautomers will
gradually evolve into particles constituted with enol tautomers
and then eventually grow into microtubes under directional
intermolecular interactions as the driving force.
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CONCLUSIONS
■
In summary, single-crystal salophen microtubes with rectan-
gular cross sections have been prepared. During the formation
of microtubes, a remarkable fluorescence increase is observed.
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