A new V-shaped organic fluorescent compound integrated with crystallization-induced emission enhancement and intramolecular charge transfer

Article abstract of DOI:10.1002/asia.201300331

The emission behavior of a new V-shaped organic fluorescent compound (p,p′-bis(2-aryl-1,3,4-oxadiazol-5-yl)diphenyl sulfone (OZA-SO), consisting of diethylamino (donor) and sulfone (acceptor) units, has been studied in various polar solvents and with diff

Full text of DOI:10.1002/asia.201300331

FULL PAPER
DOI: 10.1002/asia.201300331
A New V-Shaped Organic Fluorescent Compound Integrated with
Crystallization-Induced Emission Enhancement and Intramolecular Charge
Transfer
Pei-Yang Gu,[a] You-Hao Zhang,[a] Gao-Yan Liu,[a] Jian-Feng Ge,[a] Qing-Feng Xu,*[a]
Qichun Zhang,*[b] and Jian-Mei Lu*[a]
Abstract: The emission behavior of
a new V-shaped organic fluorescent
compound (p,p’-bis(2-aryl-1,3,4-oxadia-
zol-5-yl)diphenyl sulfone (OZA-SO)),
consisting of diethylamino (donor) and
sulfone (acceptor) units, has been stud-
ied in various polar solvents and with
different morphologies. As expected,
there is the gradual transition from the
locally excited state to the intramolecu-
lar charge-transfer (ICT) state with the
increasing solvent polarity. The photo-
luminescence intensity of OZA-SO ini-
tially decreases with a low water frac-
tion (fw), owing to ICT effect, and then
increases with a high fw, owing to crys-
tallization-induced emission enhance-
ment. At the same time, the fluores-
cence lifetime of OZA-SO increases
from 0.062 ns in dimethylformamide
(DMF) to 5.80 ns in a solution contain-
ing 90% water, and then to 7.49 ns in
a solution containing 60% water. Fur-
thermore, the solid-state emission of
OZA-SO can be tuned reversibly from
green to yellow by fuming/grinding or
fuming/heating owing to morphological
changes. This color-switchable feature
of OZA-SO may have potential appli-
cations in optical-recording and tem-
perature-sensing materials.
Keywords: charge transfer · donor–
acceptor systems · fluorescence ·
oxadiazoles · sulfones
Introduction
can emit strong fluorescence not only in the solid state or
aggregation forms, but also in dilute solutions.[4] For exam-
ple, Zhu and co-workers reported a new donor–acceptor
system, tetraphenylethene–naphthalimide (TPE-NI), which
displayed dual photoluminescence (PL) activity from both
ICT and AIE.[5c] In addition, strong solvatochromism was
observed with the emission covering most of the visible
spectrum. The fluorescence quantum yields (QYs) of TPE-
NI were 20.4% in the aggregates (tetrahydrofuran (THF)/
water 1:9, v/v) and 22.2% in chloroform. Zhu et al. also
noted that if the polarity of the solvent remained the same,
for hexane and paraffin oil, the quantum efficiencies of the
latter were enhanced 10-fold. However, the integration of
AIE and ICT into one molecule is still rare and it is worth
studying the relationship between the structure and optical
properties. In particular, it is rarely reported whether the in-
termolecular distance and molecular shape affects the fluo-
rescence. This gap in knowledge strongly encouraged us to
develop a new V-shaped ICT-active dye molecule, which
could show tunable light emission both in solution and in
the solid state.
Fluorescent molecules have been widely used as active ele-
ments in optoelectronic devices, chemosensors, and biop-
robes.[1,2] However, most of these compounds only have rea-
sonable emission in dilute solution and their emission in ag-
gregated states becomes much weaker or disappears, owing
to the notorious effect of aggregation-caused quenching
(ACQ).[3] Recently, Tangꢀs group and others separately re-
ported an intriguing phenomenon that aggregated molecules
could display stronger fluorescence than their dilute solu-
tions; this now is well known as the effect of aggregation-in-
duced emission (AIE).[4a] More recently, the integration of
AIE and intramolecular charge transfer (ICT) into one com-
pound has led to a new phenomenon:[5] this type of material
[a] P.-Y. Gu, Y.-H. Zhang, G.-Y. Liu, Prof. J.-F. Ge, Prof. Q.-F. Xu,
Prof. J.-M. Lu
Department of Polymer Science, College of Chemistry
Chemical Engineering and Materials Science
Soochow University, Suzhou, 215123 (P. R. China)
Fax : (+86)0512-65880367
Herein, we have tried to integrate 1,3,4-oxadiazole and
the V-shaped structural characteristic of a sulfone into one
new compound, p,p’-bis(2-aryl-1,3,4-oxadiazol-5-yl)diphenyl
sulfone (OZA-SO), because 1) 1,3,4-oxadiazoles are classic
heterocyclic compounds that have attracted significant inter-
est for use in organic light-emitting diodes (OLEDs) as elec-
tron-accepting materials,[6] and 2) it has been demonstrated
that sulfones possess a special V-shaped structure, which is
helpful for weakening p–p stacking.[7] We anticipated that
E-mail: xuqingfeng@suda.edu.cn
lujm@suda.edu.cn
[b] Prof. Q. Zhang
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Avenue, 639798 (Singapore)
Fax : (+65)67904705
E-mail: qczhang@ntu.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300331.
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Qing-Feng Xu, Qichun Zhang, Jian-Mei Lu et al.
OZA-SO could offer us the
synergistic effect of ICT and
AIE. Herein, we report the syn-
thesis, structure, and physical
properties of a new V-shaped
molecule, OZA-SO. Our results
show that the new V-shaped
molecule displays an interesting
AIE, which emits light weakly
in the amorphous phase, but
strongly in the crystalline
phase. Furthermore, the solid-
state emission of OZA-SO can
be tuned reversibly from green
to yellow by fuming/grinding or
Figure 1. Absorption (a) and emission (b) spectra of OZA-SO in different solutions at the same concentration
(10ꢀ5 mollꢀ1, lex =380 nm). DMSO=dimethylsulfoxide.
Solvent Effect
fuming/heating, owing to morphological changes.
Initially, the optical properties of OZA-SO were investigat-
ed in different solvents with the same concentration and the
results are shown in Figure 1. In DMF, the absorption spec-
trum of OZA-SO exhibits two prominent bands at l=316
and 379 nm, which can be ascribed to a localized aromatic
p–p* transition and ICT character from the diethylamino
moiety to the sulfone, respectively. The absorption peak
(lab) is slightly redshifted with increasing solvent polarity;
however, the shift is only of a few nanometers. In contrast,
the PL behavior of OZA-SO varies dramatically with
changes in solvent polarity. The PL intensity is gradually re-
duced with increasing solvent polarity and the fluorescent
QYs decrease from 0.80 to 0.004 when the solvent changes
from toluene to DMF. The emission peaks (lem) are general-
ly redshifted from l=464 nm in toluene to l=614 nm in
DMF. The bathochromic shift, combined with the decreased
QY, can be ascribed to stabilization of the excited state.
This behavior originates from the strong ICT effect.[5b]
To understand whether there is any correlation between
solvent polarity and the optical properties of absorption and
PL, the Lippert–Mataga model is used in a series of solvents
with a broad range of polarity. The solvent polarity parame-
ters (Df) are expressed by Equation (1):
Results and Discussion
Synthesis of the Target Compound
The synthetic procedure for the preparation of OZA-SO is
depicted in Scheme 1. The target compound was synthesized
in three-step steps: First, p,p’-dicarboxydiphenyl sulfone was
esterified with absolute ethanol. Then the as-prepared prod-
e ꢀ 1
n2 ꢀ 1
Df ¼ fðeÞ ꢀ fðn2Þ ¼
ꢀ
ð1Þ
2e þ 1 2n2 þ 1
in which e and n are the permittivity (or dielectric constant)
and refractivity (or refractive index) of the solvent, respec-
tively.
Scheme 1. Synthesis of OZA-SO: a) CH3CH2OH, H2SO4, reflux;
b) excess hydrazine hydrate, reflux; c) aromatic acids, phosphorous oxy-
chloride, reflux.
As shown in Figure 2, and Figure S4 and Table S1 in the
Supporting Information, the solvatochromism in absorption
is very weak and the shifted wavelength is only a few nano-
meters. However, the solvatochromism in PL is very strong
and the maximum redshifted wavelength is about 160 nm.
This result indicates that the ICT excited state has a larger
dipolar moment than the ground state, owing to substantial
charge redistribution, which probably receives a contribution
from the relaxation of the initially formed Franck–Condon
excited state. It is well known that ICT states are effectively
stabilized in polar media, such as DMF and water, owing to
uct was condensed with hydrazine hydrate to yield p,p’-dihy-
drazinocarbonyl diphenylsulfone (HCPS, 72% yield). Final-
ly, through a condensation reaction with an aromatic acid in
the presence of phosphorous oxychloride, the hydrazide
(HCPS) from the second step was converted into the target
compound OZA-SO in 58% yield.
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environment of the aggregates becoming less polar than the
outside.[5]
The trends of PL intensity and lem, depending on fw in the
DMF/water mixtures, are shown in Figure 3a and c. OZA-
SO was nonemissive when it was dissolved in DMF, but did
show strong emission in the aggregate state. The PL spectra
of OZA-SO in solvent mixtures with a low fw show two
peaks (locally excited (LE) emission peak and ICT emission
peak) and redshifted as well as decreasing PL intensity,
owing to the increasing polarity of the solvent mixtures. The
emission of OZA-SO was revitalized when
a “large”
Figure 2. Dependence of the Stokes shift (Du) as a function of the solvent
polarity parameter (Df) for OZA-SO.
amount of water (fw ꢁ50%) was added to DMF, owing to
aggregation. The PL spectra of OZA-SO in the mixture
with high fw show only one peak and the peak of ICT emis-
sion disappears. When OZA-SO was in the aggregates state,
the solvent polarity effect disappeared and the fluorescence
was quenched. Surprisingly, the PL intensity decreased with
a further increase of the fw. In the mixture with a low water
content of 50%, OZA-SO might steadily aggregate to form
ordered, crystalline particles. On the other hand, in solvent
mixtures with a high fw (about 90%), the dye molecules
might abruptly agglomerate to form random, amorphous ag-
gregates.[9] At the same time, the fluorescence lifetime of
OZA-SO increased from 0.062 ns in DMF to 5.80 ns in 90%
fw, then to 7.49 ns in 60% fw. To confirm these results, we in-
vestigated the PL spectra of OZA-SO in different DMF/
water mixtures (see Figure S10 in the Supporting Informa-
tion). The PL emission intensity reached a maximum at
60% fw (Figure 3b). It is worth noting that the change in the
relative PL intensity (I/I0) in 60% fw increases from 30-fold
in a 10 mmollꢀ1 solution to 45-
dipole–dipole interactions of the polarized excited state with
the solvents.[8] This result further confirms that there is
strong ICT behavior in OZA-SO.
Crystallization-Induced Emission Enhancement
The optical properties of OZA-SO in DMF/water mixtures
were studied to understand how the aggregation affected
the absorption and emission of the ICT molecules. The lab
of OZA-SO was slightly redshifted when a “small” amount
of water was added to DMF (Figure 3a, left) owing to the
increased solvent polarity. However, when a “large” amount
of water was added to DMF, the lab of OZA-SO was slightly
blueshifted and the peak intensity of ICT decreased; this
was attributed to the formation of aggregates and the inner
fold in a 20 mmollꢀ1 solution to
165-fold in a 50 mmollꢀ1 solu-
tion. This could serve as further
proof for the crystallization-in-
duced emission enhancement
(CIEE) nature of OZA-SO be-
cause the emission of an ACQ
fluorophore is normally weak-
ened, rather than strengthened,
with increasing concentration.
To further confirm that the
crystalline aggregates of OZA-
SO were more efficient emitters
than amorphous counterparts,
crystals of OZA-SO were ob-
tained from an ethanol/hexane
mixture and its amorphous
powders were obtained by
rapid precipitating from chloro-
form. The crystalline and amor-
phous structures of these sam-
ples were verified by XRD
studies. As shown in Figure 4,
the former exhibits many sharp
reflection peaks, whereas the
latter gives only a weak, broad
Figure 3. Absorption (a) and emission (c) spectra of OZA-SO in DMF/water mixtures (10ꢀ5 mollꢀ1, lex
380 nm); b) change in the relative PL intensity (I/I0) of OZA-SO in different water fractions with different
concentrations. I0 =PL intensity in DMF.
=
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Qing-Feng Xu, Qichun Zhang, Jian-Mei Lu et al.
Figure 4. XRD patterns of the OZA-SO samples a) obtained from etha-
nol/hexane; b) pristine; c) pristine sample quickly fumed with chloro-
form; d) pristine sample slowly fumed with chloroform; e) ground pris-
tine sample; f) ground sample quickly fumed from chloroform; and
g) ground sample slowly fumed from chloroform.
peak. The lem of crystals is 20 nm redshifted relative to the
amorphous powders (Figure S9 in the Supporting Informa-
tion). Furthermore, the QY of crystals (10.2%) is much
higher than amorphous pow-
Scheme 2. A schematic representation of the treatment procedures. The
pictures were taken at room temperature under l=365 nm UV light.
ders (<1%).
Mechanofluorochromism and
Thermochromism
OZA-SO showed significant
changes in emission when they
were ground (Scheme 2). PL
emission spectra, wide-angle
XRD patterns, and differential
scanning calorimetry (DSC)
curves of the OZA-SO samples
are shown in Figures 4 and 5a,
as well as in Figures S11 and
S12 in the Supporting Informa-
Figure 5. PL emission spectra of the OZA-SO samples obtained a) under different morphologies and b) under
different temperature.
tion. The crystals of OZA-SO show a strong green emission
at l=510 nm. Interestingly, after grinding with a mortar and
pestle, the obtained powders emitted strong yellow light at
l=570 nm with QY 6.5%. When the solutions of OZA-SO
in chloroform were evaporated to dryness at the different
speeds, two different aggregates were obtained: an amor-
phous aggregate with rapid evaporation and a crystalline ag-
gregate with slow evaporation. This result was confirmed by
SEM (Figure S16 in the Supporting Information). Interest-
ingly, when the pristine powders were heated to different
temperatures (125, 195, or 2458C) and then cooled slowly,
the color changed from green (l=510 nm) to yellow (l=
580 nm; Figure 5b).
To gain insight into the mechanism for such phenomena,
OZA-SO in different aggregated states was studied through
powder XRD (Figure 4). The XRD diffractograms of crys-
tals exhibited many sharp diffraction peaks, whereas pristine
samples gave few diffraction peaks. In contrast, the amor-
phous sample, obtained through fast evaporation, exhibited
only a big peak in its XRD pattern. The PL emission of the
ground sample is redshifted relative to that of crystals and
amorphous powders, which suggests that there is a stronger
interaction in the ground sample than those in crystals and
amorphous samples. In particular, dimers might be formed
in the ground sample; this was further confirmed by drop-
ping OZA-SO into poly(methyl methacrylate) (PMMA; Fig-
ure S15 in the Supporting Information). In polymer films,
the conformation of the chains is frozen. An excited OZA-
SO can only interact with the nearest molecule to form
a
dimer state. Alternatively, the OZA-SO-containing
PMMA film might be considered a solid solution, in which
PMMA acts as a solvent with high polarity. The interaction
between dye and PMMA increased with increasing dye con-
centration. Hence, the PL emission was redshifted and QYs
increased from 1.2 to 2.3 to 4.7 and finally to 5.4% with in-
creasing OZA-SO concentration. As shown in Figure S12 in
the Supporting Information, the diffraction curve of the an-
nealed sample at 1258C displays many sharp and intense re-
flection peaks that coincide with the crystalline orders. The
diffraction of the sample annealed at 1958C also exhibits
some reflections, but does not show too many sharp peaks,
which indicates a partially crystalline phase. However, the
diffraction of the annealed sample at 2458C (which is higher
than its melting point) does not display any fine peaks. All
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Qing-Feng Xu, Qichun Zhang, Jian-Mei Lu et al.
of these results indicate that the morphology of OZA-SO
Conclusion
could be tuned through changing the temperature or grind-
ing/evaporation speed.
The emission behavior of OZA-SO, which consisted of di-
ethylamino (donor) and sulfone (acceptor) units, was stud-
ied in various polar solvents and in the aggregated states.
There was a gradual transition from the LE state to the ICT
state with increasing solvent polarity. In weakly polar sol-
vents, such as toluene, the PL spectra of the luminogens
were associated with emissions from LE states with a high
QY, whereas in polar solvents, such as DMF, the redder and
weaker emissions from the ICT states dominated the main
contribution, which indicated that there was a transition
from the LE state to the ICT state. The PL intensity of
OZA-SO decreased with low fw, owing to the ICT effect,
and then increased with high fw, owing to CIEE. At the
same time, the fluorescence lifetime of OZA-SO increased
from 0.062 ns in DMF to 5.80 ns in a solution containing
90% water, then to 7.49 ns in a solution containing 60%.
Furthermore, the solid-state emission of OZA-SO could be
tuned reversibly from green to yellow by fuming/grinding or
fuming/heating, owing to morphological changes. This color-
switchable feature of OZA-SO may have potential applica-
tions in optical-recording and temperature-sensing materials.
Theoretical Study of OZA-SO
To understand more about the originality of the AIE pro-
cesses of OZA-SO, semiempirical molecular orbital calcula-
tions were carried out by using the PM3 formalism with
a simplified model structure. The optimal intermolecular
separation (3.69 ꢂ, which agrees with the single-crystal
structure) was calculated to be a little larger than the
normal p–p interaction distance (ca. 3.3 ꢂ); this might be
helpful to reduce the distance-dependent intermolecular
quenching effects and produce the intense fluorescence in
the aggregated state (Figure 6). Furthermore, there is only
Experimental Section
General
Figure 6. The dimer model for cluster calculation.
All starting materials (analytic pure) were purchased from TCI or Sino-
pharm Chemical Reagent and were used as received without further pu-
rification.
partial overlap between adjacent molecules, which implies
that there is a weak p–p interaction. The unique structure
may play important roles in suppressing excimer formation
and closing nonradiative pathways in the aggregated state.
The energies of the HOMO and LUMO of OZA-SO are
shown in Figure 7. The electron density distributions of
By using CDCl3 or [D6]DMSO as the solvent and tetramethylsilane
1
(TMS) as the internal standard, H and 13C NMR spectra were measured
on INOVA 300 or 400 MHz NMR spectrometers at ambient temperature.
UV/Vis absorption spectra were determined on a Shimadzu RF540 spec-
trophotometer. The emission and excitation spectra were recorded at
room temperature no a Edinburgh-920 fluorescence spectrophotometer.
Thermogravimetric analysis (TGA) was performed on a Universal V3.7A
TA instrument under a flow of pure N2 gas (10 mLminꢀ1) at a heating
rate of 108Cminꢀ1 from room temperature to 7008C. Wide-angle X-ray
diffraction (XRD) patterns were recorded on an XꢀPert-Pro MPD X-ray
diffractometer. The ground-state geometry of the OZA-SO was opti-
mized by the hybrid density functional theory (B3LYP) with the 6-31G*
basis set using the Gaussian 03 program package. Forcite and CASTEP
were used to optimize the packed structures of OZA-SO. The fluorescent
QY in solution and in the aggregate states were determined by using qui-
nine bisulfate (FF =0.546 in 0.1n H2SO4), whereas that of the solid film
and powder were measured by using a calibrated integrating sphere.
Figure 7. a) HOMO and b) LUMO levels of OZA-SO.
Preparation of the Target Compounds
p,p’-Dicarbethoxydiphenyl sulfone and HCPS were synthesized according
to reported procedures.[10]
OZA-SO change significantly between the HOMO and
LUMO. The electron density distributions of the HOMO
are mostly allocated to aromatic amine groups; however,
the LUMOs are mainly concentrated on the sulfone core.
Generally, this type of dye molecule with such an electron
distribution indicates the ICT nature, which is consistent
with the experimental results.
p,p’-Dicarbethoxydiphenyl Sulfone (CPS)
A mixture of p,p’-dicarboxydiphenyl sulfone (30.6 g, 0.1 mol), absolute
ethanol (100 mL), and sulfuric acid (2.7 mL) was heated at reflux for 6 h
and then excess ethanol was removed by distillation. The resulting resi-
due was poured onto crushed ice. The white solid (20.7 g, 62%) was ob-
tained after recrystallization from absolute ethanol and dried under
vacuum. 1H NMR (400 MHz, [D6]DMSO): d=8.14 (q, J=8.7 Hz, 8H),
4.33 (q, J=7.1 Hz, 4H), 1.31 ppm (t, J=7.1 Hz, 6H).
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p,p’-Dihydrazinocarbonyl diphenylsulfone (HCPS)
Xiao, Z. Wang, E. R. T. Tiekink, H. M. Doung, H. Zhang, F. Boey,
X. W. Sun, F. Wudl, Chem. Eur. J. 2010, 16, 7422.
Hydrazine hydrate (5 mL, 85%) and p,p’-dicarbethoxydiphenyl sulfone
(3.35 g, 0.01 mol) was mixed in ethanol (30 mL) and the mixture was
heated to reflux for 6 h. After cooling to room temperature, the reaction
mixture was poured into ice water. The resulting white solid was collect-
ed and recrystallized in absolute ethanol and dried under vacuum (2.42 g,
72%). 1H NMR (400 MHz, [D6]DMSO): d=10.02 (s, 2H), 8.07 (d, J=
8.6 Hz, 4H), 8.00 (d, J=8.6 Hz, 4H), 4.60 ppm (s, 4H).
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p,p’-Bis[2-(4-Diethylaminophenyl)-1,3,4-oxadiazol-5-yl]diphenylsulfone
(OZA-SO)
A
mixture of 4-diethylaminobenzoic acid (3.86 g, 0.02 mol), HCPS
(3.34 g, 0.01 mol), and phosphorous oxychloride (5 mL) was heated at
reflux for 6 h, and then poured onto crushed ice after cooling to room
temperature. The dark-green solid (3.76 g, 58 mmol, 58%) was obtained
after recrystallization from acetic ether/hexane and dried under vacuum.
M.p. (232ꢂ1)8C; 1H NMR (300 MHz, CDCl3): d=8.27 (d, J=6.3 Hz,
4H), 8.11 (d, J=7.5 Hz, 4H), 7.95 (d, J=7.2 Hz, 4H), 6.78 (s, 4H), 3.45
(s, 8H), 1.22 ppm (s, 12H); 13C NMR (300 MHz, CDCl3): d=166.3, 161.9,
150.4, 143.0, 129.1, 128.9, 128.6, 127.5, 111.2, 109.2, 44.6, 12.6 ppm;
HRMS: m/z calcd for: C36H36N6O4S: 671.2411 [M+Na]+; found:
671.2406.
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Acknowledgements
We thank the Chinese Natural Science Foundation (21071105 and
21176164), the Project of Science and Technology of Suzhou
(SYG201114), and the China Scholarship Council.
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Received: March 13, 2013
Revised: April 12, 2013
Published online: July 2, 2013
Chem. Asian J. 2013, 8, 2161 – 2166
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