Synthesis, characterization, aggregation-induced emission, solvatochromism and mechanochromism of fluorinated benzothiadiazole bonded to tetraphenylethenes

Article abstract of DOI:10.1039/c8ra01448e

Compounds consisting of unsubstituted, monofluoro and difluoro substituted benzothiadiazole bonded to two tetraphenylethenes were successfully prepared by palladium catalyzed Suzuki-Miyaura cross-coupling reaction of their corresponding co-monomers. All compounds exhibited aggregation-induced emission characteristics when the water fraction was higher than 60% in the THF/water mixtures. The emission maximum for the three compounds was blue-shifted when the water content reached 90% compared to that in THF solution. The intensity of emission maximum of difluorinated benzothiadiazole linked with two tetraphenylethenes was 2.5 times higher in 90% water compared to those in THF solution. Surprisingly, two liquid crystal phases with two distinct emission colors were observed only for the compound containing difluorinated benzothiadiazole bonded to two tetraphenylethene. All compounds showed remarkable solvatochromic properties in selected solvents with different polarities. The powder XRD results and mechanochromism of the compounds suggested that the solid state structures can change from one form to another by grinding, fuming or annealing processes.

Full text of DOI:10.1039/c8ra01448e

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Synthesis, characterization, aggregation-induced
emission, solvatochromism and mechanochromism
of fluorinated benzothiadiazole bonded to
tetraphenylethenes†
Cite this: RSC Adv., 2018, 8, 12619
*
Chin-Yang Yu,
Chia-Chieh Hsu and Hsi-Chen Weng
Compounds consisting of unsubstituted, monofluoro and difluoro substituted benzothiadiazole bonded to
two tetraphenylethenes were successfully prepared by palladium catalyzed Suzuki–Miyaura cross-coupling
reaction of their corresponding co-monomers. All compounds exhibited aggregation-induced emission
characteristics when the water fraction was higher than 60% in the THF/water mixtures. The emission
maximum for the three compounds was blue-shifted when the water content reached 90% compared
to that in THF solution. The intensity of emission maximum of difluorinated benzothiadiazole linked with
two tetraphenylethenes was 2.5 times higher in 90% water compared to those in THF solution.
Surprisingly, two liquid crystal phases with two distinct emission colors were observed only for the
compound containing difluorinated benzothiadiazole bonded to two tetraphenylethene. All compounds
showed remarkable solvatochromic properties in selected solvents with different polarities. The powder
XRD results and mechanochromism of the compounds suggested that the solid state structures can
change from one form to another by grinding, fuming or annealing processes.
Received 15th February 2018
Accepted 28th March 2018
DOI: 10.1039/c8ra01448e
rsc.li/rsc-advances
aggregation induced emission characteristics. Very recently, the
TPE based molecules have been synthesized and investigated by
Misra et al.16–18 The rational molecular design of the TPE based
molecules makes it potential uses in non-doped OLEDs with
a good color contrast. In addition, the obvious color change
ranging from blue to red with reversible mechanochromism
based on molecules containing the TPE linked to a variety of
electron acceptors has also been demonstrated.
Typically, the emission colors of conjugated carbocyclic
compounds with AIE characteristics are between blue and
green.19 In fact, the emission colors of compounds can be
altered through molecular design such as when the structures
contain electron donor–acceptor (D–A) units and heteroatoms.
In addition, the rational design and synthesis of D–A molecules
could realize full visible colors by manipulating the strength of
intramolecular charge transfer (ICT) between the constituent
units.20 Benzothiadiazole (BT) is one of the most important
classes of electron acceptors due to its relatively high electron
affinity and p-extended planar structure.21 Recently, uorina-
tion of BT has been utilized and shown to be an efficient means
to modify the optical, electronic and electrochemical properties
of BT.22,23 Tang and coworkers have synthesized the electron
donor–acceptor–donor (D–A–D) chromophores containing BT
derivatives and TPE.24 These molecules exhibit high emission
with different emission colors in the solid state. The other
asymmetrical D–A or D–A–D types of chromophores such as
cyano-substituted diphenylethene derivatives with AIE
Introduction
Fluorescent organic molecules have attracted much attention
on account of their outstanding sensitivity and distinct selec-
tivity1,2 and have been widely used for the detection of a variety
of analytes in the solution state.3–5 Considering their potential
use in optoelectronic devices such as organic light-emitting
diodes (OLEDs), organic light-emitting eld-effect transistors
(OLEFETs) and organic solid lasers, the molecules must exhibit
high emission intensity in the solid state. However, conven-
tional uorescent molecules exhibit relatively low uorescence
in the solid state due to the strong aggregation caused
quenching effect.6–8 Recently, the aggregation induced emission
(AIE) characteristics of small molecules and polymers has been
proposed.9–12 Tetraphenylethene (TPE)13–15 with its propeller-
shaped structure is one of the most studied examples for
Department of Materials Science and Engineering, National Taiwan University of
Science and Technology, 43, Section 4, Keelung Road, Taipei, 10607, Taiwan.
E-mail: cyyu@mail.ntust.edu.tw
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of compounds 1, 2, 3, 4, BT-2TPE, FBT-2TPE and 2FBT-2TPE and HR-EI mass
spectra of compounds BT-2TPE, FBT-2TPE and 2FBT-2TPE and normalized
UV-Vis absorption spectra and PL emission spectra of BT-2TPE, FBT-2TPE and
2FBT-2TPE in different solvents. The details of the absorption and emission
maximum, quantum yield, Stokes shi and optical bandgap for BT-2TPE,
FBT-2TPE and 2FBT-2TPE. Metrical parameters for the structure of 2FBT-2TPE.
CCDC 1815763 for 2FBT-2TPE. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c8ra01448e
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characteristics, vapochromic and mechanochromic properties
have also been reported.25–27 The compounds revealed a revers-
ible conversion between the crystalline and amorphous forms
by different processes.
Results and discussion
Synthesis and characterization of BT-2TPE, FBT-2TPE and
2FBT-2TPE
Solvatochromism is the ability of compounds to change
color when dissolved in different solvents.28,29 Recently, a variety
of solvatochromic molecules with D–A structures including
symmetrical or asymmetrical benzothiadiazide based cyano-
substituted diphenylethene derivatives26,30,31 have been synthe-
sized and studied due to their applicability as probes for the
determination of volatile solvent vapor as well as their potential
application in optical recording and security papers. These
compounds exhibit remarkable solvatochromic and vapochro-
mic characteristics by conjugation effects, molecular packing
and conversion of crystalline to amorphous states. Mechano-
uorochromic materials are a sort of smart material which
display a changing color of emission in response to external
stimuli and these materials can be potentially used in memory
chips, security ink and sensors.32–34 Araki and coworkers have
reported that organic mechanouorochromic compounds
exhibited piezochromic luminescence by controlling the mode
of molecular packing.35 Mechanouorochromism of donor–
acceptor p-conjugated (D–p–A) uorescent dyes was attributed
to a reversible switching between crystalline and amorphous
states with a change in the dipole–dipole and intermolecular p–
p interactions.36,37 Secondary bonding interactions and D–A
pairs play important roles in the design of new mechanochro-
mic materials.38–40 Recently, a number of D–A compounds have
been prepared and found to possess mechanouorochromic
nature.41–43
The synthetic routes towards the D–A–D types of molecules BT-
2TPE, FBT-2TPE and 2FBT-2TPE are shown in Scheme 1. Fluo-
rinated phenylenediamine derivatives were reacted with thionyl
chloride (SOCl2) in the presence of triethylamine in anhydrous
chloroform to afford the uorinated 2,1,3-benzothiadiazole
derivatives 1 and 2. A high selectivity of bromination at the 4
and 7 position of 1 and 2 was carried out by the dropwise
addition of bromine in hydrobromic acid containing 1 or 2 to
give 3 and 4. The 4-(1,2,2-triphenylvinyl)phenylboronic acid
pinacol ester 5 was prepared by a modication of a procedure
reported previously.45 The D–A–D types of molecules BT-2TPE,
FBT-2TPE and 2FBT-2TPE were synthesized by Suzuki–Miyaura
cross-coupling reaction using equimolar amounts of the corre-
sponding comonomers, 10 mol% Pd(PPh3)4 and aqueous K2CO3
to give BT-2TPE, FBT-2TPE and 2FBT-2TPE in a yield of 85%,
64% and 38%, respectively.
The compounds BT-2TPE, FBT-2TPE and 2FBT-2TPE were
fully characterized by 1H NMR, 13C NMR, 19F NMR, 1H–1H COSY
spectroscopy and high resolution mass spectrometry (see ESI†).
Clearly, a low isolated yield of compounds FBT-2TPE and 2FBT-
2TPE was obtained compared to that of BT-2TPE possibly due to
the low reactivity of the uorinated BT moieties as the intro-
duction of the uorine atom to the BT leads to a deactivation of
the aromatic ring.
Compounds BT-2TPE, FBT-2TPE and 2FBT-2TPE were char-
1
acterized by H NMR spectroscopy in CD2Cl2 (Fig. 1). A singlet
In general, D–A type molecular systems show low uores-
cence quantum efficiency in solution as well as in the solid
state. Although the AIE active molecules containing BT bonded
to two TPE with remarkable mechanochromism have been re-
ported and investigated by Misra,44 no examples of uorinated
substituted BT bonded to two TPE are available. The modica-
tion of the uorine atoms onto the BT unit seems to be an
efficient way that the molecules can be easily prepared and the
optical, electronic and electrochemical properties as well as
aggregation-induced emission characteristics, solvatochrom-
ism and mechanochromism can be ne-tuned which can
compare to that of the unsubstituted BT bonded to two TPE.
The aim of this research work is mainly to investigate the
properties and the effect of uorine atoms attached to the
central BT unit of D–A–D type molecular systems as well as their
aggregation-induced emission characteristics, solvatochrom-
ism and mechanochromism. In this work, we report the
synthesis of unsubstituted, monouoro and diuoro
substituted BT bonded to two TPE by palladium catalyzed
Suzuki–Miyaura cross-coupling reaction of their corresponding
comonomers. The structures of the D–A–D types of three chro-
mophores have been fully characterized. Aggregation induced
emission characteristics of small molecules in the THF/water
mixtures and in the solid state have been studied as well as
their solvatochromic and mechanochromic effects related to
their structures.
peak at 7.74 ppm integrating to 2 hydrogens corresponds to the
hydrogens of BT unit for BT-2TPE (Fig. 1a). Two AA0BB0 systems
at around 7.78 and 7.20 ppm integrating to 4 hydrogens each
were assigned to the hydrogens of TPE relative to ortho and meta
position of carbons linked to the BT moieties, respectively. The
multiple signals between 7.05 and 7.18 ppm integrating to 30
hydrogens were attributed to the remaining hydrogens of the
TPE. The doublet at around 7.63 ppm (JH–F ¼ 11.8 Hz) inte-
grating to 1 hydrogen was assigned to the hydrogen of BT unit
Scheme 1 Synthetic routes to BT-2TPE, FBT-2TPE and 2FBT-2TPE.
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Optical properties of BT-2TPE, FBT-2TPE and 2FBT-2TPE
The UV-vis absorption spectra of BT-2TPE, FBT-2TPE and 2FBT-
2TPE were recorded in dilute dichloromethane (5 ꢀ 10ꢁ6 M)
(Fig. 2a). The band between 280 and 350 nm can be attributed to
the p–p* transition and another band between 360 and 470 nm
corresponds to the charge transfer (CT) transition between the
TPE and the BT units.46 The absorption maximum of BT-2TPE,
FBT-2TPE and 2FBT-2TPE is at 414, 405 and 391 nm, respec-
tively. The absorption maximum was blue-shied aer the
introduction of the uorine atom onto the BT possibly due to
a collapsing of the planarity of the molecules. The emission
maximum wavelength of BT-2TPE, FBT-2TPE and 2FBT-2TPE in
dilute dichloromethane (5 ꢀ 10ꢁ6 M) (Fig. 2b) is very similar at
around 553 nm. Thin lms of BT-2TPE, FBT-2TPE and 2FBT-
2TPE were prepared by spin-coating 150 mL of the solution (3 mg
mLꢁ1 in toluene) onto glass slides (2.0 cm ꢀ 2.0 cm) at a rota-
tion speed of 1500 rpm for 20 seconds and then 3000 rpm for 20
seconds. The solid state absorption maximum of BT-2TPE, FBT-
2TPE and 2FBT-2TPE was red-shied by 30, 24 and 26 nm,
respectively compared to that of the solution. This bath-
ochromic shi can be attributed to intermolecular interactions
in the solid state as we expected. However, the solid state
emission maximum wavelength of BT-2TPE, FBT-2TPE and
2FBT-2TPE is blue-shied by 18, 26 and 38 nm compared to that
of the solution. The optical bandgap calculated by the onset of
the absorption edge of BT-2TPE, FBT-2TPE and 2FBT-2TPE is
2.35, 2.40 and 2.46 eV, respectively. The introduction of
Fig. 1 1H NMR spectra of BT-2TPE, FBT-2TPE and 2FBT-2TPE in
CD2Cl2.
relative to the ortho position of carbon linked to uorine atom
in FBT-2TPE (Fig. 1b). The two AA0BB0 systems at 7.79 and
7.60 ppm integrating to 2 hydrogens each were attributed to the
hydrogens of phenyl ring of TPE moieties relative to the ortho
position of carbon linked to BT units. The multiple signals
between 7.04 and 7.23 ppm integrating to 34 hydrogens were
attributed to the remaining hydrogens of the TPE. The multiple
signals appearing at 7.61 and 7.22 ppm (two AA0BB0 systems)
integrating to 4 hydrogens each correspond to the hydrogens of
the phenyl ring relative to the ortho and the meta position of
carbons attached to the diuoro-substituted BT, respectively
(Fig. 1c). Again, the multiple signals between 7.05 and 7.20 ppm
integrating to 30 hydrogens are attributed to the remaining
hydrogens of the TPE.
Fig. 2 (a) UV-Vis spectra and (b) emission spectra of BT-2TPE, FBT-
2TPE and 2FBT-2TPE in dichloromethane solution (solid line) and in
solid state (dashed line). Pictures of films under UV light irradiation at
a wavelength of 365 nm are shown in the insets.
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a uorine atom onto the BT units leads to a collapse of the was 2.5 times higher than that of in THF solution. The pictures
planarity of the molecules, therefore the optical bandgap of BT-2TPE, FBT-2TPE and 2FBT-2TPE in different THF-water
increases as we expected. The details of the absorption and mixtures under UV light irradiation at a wavelength of 365 nm
emission maximum, quantum yield, Stokes shi and optical is shown in Fig. 3f. All compounds BT-2TPE, FBT-2TPE and
bandgap are shown in the ESI (Table S1†).
2FBT-2TPE showed emission color changes upon increases in
The emission spectra of compounds BT-2TPE, FBT-2TPE and the fraction of water.
2FBT-2TPE with different water fractions in THF/water mixture
are shown in Fig. 3a–c. Upon increasing the water fraction up to
60%, the maximum emission wavelength of BT-2TPE, FBT-2TPE
Single crystal structure of 2FBT-2TPE
and 2FBT-2TPE was red-shied by 15, 18 and 23 nm, respec-
tively and the maximum emission intensity gradually decreases.
This could be attributed to the stabilization of the intra-
molecular charge-transfer state upon increasing the polarity of
the solvents.46,47 However, upon a further increase in the water
fraction from 60% to 90%, the maximum emission wavelength
of BT-2TPE, FBT-2TPE and 2FBT-2TPE was blue-shied and the
maximum emission intensity generally increases due to
formation of nanoaggregates that can enhance aggregation-
induced emission. All compounds showed distinct responses
in different THF and water percentages. At lower water fractions
(<60%), the emission is mainly attributed to intramolecular
charge transfer between the electron donor and the electron
acceptor which is controlled by solvent polarity, however, at
higher water contents (>60%), AIE dominates over solvent
polarity. The maximum photoluminescence quantum yield of
compounds BT-2TPE, FBT-2TPE and 2FBT-2TPE (Fig. 3d) is
0.24, 0.29 and 0.31, respectively compared to quinine sulfate
(0.546) in 0.1 M H2SO4. It should be noted that the emission
maximum intensity of 2FBT-2TPE (Fig. 3e) in 90% water content
In order to obtain a single crystal of 2FBT-2TPE, a saturated
dichloromethane solution was allowed to slowly evaporate at
room temperature. The solid state structure of 2FBT-2TPE
(Fig. 4a) showed symmetrical disorder in the BT core (space
group: P1, Z ¼ 1) and solvent packing in the crystal structure.
The crystal structure of 2FBT-2TPE conrmed that the dihedral
angles between BT and phenyl rings of TPE were 46.32ꢂ and
46.48ꢂ, respectively which are relatively large due to the highly
twisted structure of 2FBT-2TPE. The packing diagram of 2FBT-
2TPE (Fig. 4b) showed the intermolecular distance of C–H/S
˚
was 2.945 A from the disordered sulfur atom, the intermolec-
˚
ular distance of C–H/F was 3.038 and 3.134 A from the
disordered uorine atom and intramolecular distance of
˚
C–H/F was 2.553 and 2.663 A. The packing structure of 2FBT-
2TPE (Fig. 4c) exhibited a ladder-like packing structure with
˚
a distance of 4.811 and 4.349 A. An attempt to acquire the single
crystal of BT-2TPE and FBT-2TPE failed possibly due to the
Fig. 3 Emission spectra of (a) BT-2TPE, (b) FBT-2TPE and (c) 2FBT-
2TPE in THF-water mixtures (concentration:5 mM; excitation wave-
length: lmax) and plots of (d) fluorescence quantum yields and (e) I/I0
vs. water fraction of BT-2TPE, FBT-2TPE and 2FBT-2TPE and (f) Fig.
4
(a) Solid-state structures, (b) inter- and intramolecular
pictures of BT-2TPE, FBT-2TPE and 2FBT-2TPE in different THF-water distances and (c) packing diagram of 2FBT-2TPE. Thermal ellipsoids
mixtures under UV light irradiation at a wavelength of 365 nm.
are set at 50% probability.
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highly random nature of the unsubstituted and mono- by polarized optical microscopy with a heating rate of 2
uorinated substituted BT.
ꢂCminꢁ1. Upon cooling, the recrystallization temperature of
FBT-2TPE and 2FBT-2TPE was observed at 202 and 250 ꢂC,
respectively. No recrystallization temperature for BT-2TPE could
be observed possibly due to the random nature of BT-2TPE.
Thermal properties of BT-2TPE, FBT-2TPE and 2FBT-2TPE
The decomposition temperatures of BT-2TPE, FBT-2TPE and
2FBT-2TPE were determined by thermal gravimetric analysis Solvatochromic behaviors
ꢁ1
ꢂ
(TGA) at a heating rate of 10 C min under a nitrogen ow.
Clearly, one step weight loss was observed for all compounds
(Fig. 5a). The 5% weight loss of BT-2TPE, FBT-2TPE and 2FBT-
2TPE was at 408 ꢂC, 416 ꢂC and 402 ꢂC, respectively with a small
amount of solid residue remaining when heating up to 800 ꢂC.
All compounds exhibited an excellent thermal stability. The
DSC traces of BT-2TPE, FBT-2TPE and 2FBT-2TPE at the second
heating cycle (Fig. 5b) showed an endothermic peak at 266, 260,
In general, the spectral properties of D–A molecules are sensi-
tive towards the solvent polarity. In order to study the sol-
vatochromic effect of BT-2TPE, FBT-2TPE and 2FBT-2TPE,
various solvents with different polarities such as hexane,
toluene, chloroform, dichloromethane, ethyl acetate, 1,4-
dioxane, acetone, acetonitrile, dimethylsuoxide and dime-
thylformaldehyde were selected. Compounds BT-2TPE, FBT-
2TPE and 2FBT-2TPE were soluble in all of the selected organic
solvents and exhibited a pronounced solvatochromic perfor-
mance. Generally, the absorption maximum of BT-2TPE, FBT-
2TPE and 2FBT-2TPE were all slightly blue-shied when dis-
solved in a solvent of increasing polarity while the uorescence
maximum of BT-2TPE, FBT-2TPE and 2FBT-2TPE revealed
a high degree of red-shiing with increasing solvent polarity.
The emission maximum intensity gradually decreased, indi-
cating that the larger charge separation and dipole moment
existed in the excited state than in the ground state. The
absorption spectra and emission spectra of BT-2TPE, FBT-2TPE
and 2FBT-2TPE in different solvents and the solvatochromic
ꢁ1
ꢂ
310 C with an enthalpy value of 68.0, 46.6 and 17.5 kJ mol
,
respectively which can be attributed to the melting point of the
compounds. In addition,ꢂtwo broad exothermic transitions for
BT-2TPE at 172 and 224 C were observed upon heating corre-
sponding to the formation of a self-organized structure. The
polarized optical micrographs associated with different transi-
tions for 2FBT-2TPE were shown in the insets of Fig. 5b. There
are an additional two endothermic peaks at 293 and 303 ꢂC for
2FBT-2TPE which can be attributed to the two transitions
related to the two distinct liquid crystal mesophases observed
Fig. 5 (a) TGA curves of BT-2TPE, FBT-2TPE and 2FBT-2TPE at
a heating rate of 10 ꢂC minꢁ1 under a nitrogen atmosphere and (b) DSC
diagrams of the second heating and cooling curves of BT-2TPE, FBT-
2TPE and 2FBT-2TPE at a heating and cooling rate of 10 ꢂC minꢁ1 Fig. 6 (a) Plot of Stokes shift versus solvent polarity parameter of BT-
under a nitrogen atmosphere. Polarized optical micrographs associ- 2TPE, FBT-2TPE and 2FBT-2TPE in various solvents and (b) pictures of
ated with the transition temperature of 2FBT-2TPE are shown in the BT-2TPE, FBT-2TPE, 2FBT-2TPE in different solvents under UV light at
insets.
a wavelength of 365 nm.
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behavior data have been summarized in the ESI (Fig. S32 and
Table S3†). The plots of Stokes shi versus solvent polarity
parameter based on the Lipper-Mataga equation of BT-2TPE,
FBT-2TPE and 2FBT-2TPE were shown in Fig. 6a. As the solvent
polarity parameter increases, the Stokes shi also increases
with a linear correlation of 0.799 for BT-2TPE, 0.841 for FBT-
2TPE and 0.863 for 2FBT-2TPE. In addition, the calculated slope
of tting line for BT-2TPE, FBT-2TPE and 2FBT-2TPE was 4954,
7169 and 10 334, respectively. These results indicate that the
uorescent properties of three compounds are strongly related
to the solvent polarity. Compound 2FBT-2TPE exhibited a rela-
tively steep slope indicating the formation of large charge
separation and high dipole moment in the excited state.
Pictures of BT-2TPE, FBT-2TPE and 2FBT-2TPE in different
solvents under UV light at a wavelength of 365 nm were shown
in Fig. 6b. A notable color change of three compounds in
different polarities of solvents was observed indicating
a remarkable solvatochromism.
Mechanochromic behaviors
The ground samples were obtained from pristine powders via
the grinding process using spatula, mortar and pestle. The
annealed samples were obtained upon heating pristine powders
at the specied temperature for 20 min and then allowing to
slowly cool to room temperature. In this case, the annealing
temperature for BT-2TPE, FBT-2TPE and 2FBT-2TPE was 200 ꢂC,
200 ꢂC and 270 ꢂC, respectively. Surprisingly, the pristine
powders BT-TPE obtained by evaporating dichloromethane and
hexane exhibited a yellow and yellow-green color, respectively.
The pristine powders FBT-2TPE and 2FBT-2TPE obtained by
evaporating dichloromethane and hexane exhibited no color
change upon visual inspection.
Fig. 7 (a) Normalized PL spectra (b) pictures and (c) PXRD diagrams of
BT-2TPE in pristine, ground, annealed and fumed powders.
The mechanochromism of three compounds was investi-
gated by emission spectroscopy. The emission spectra of
compound BT-2TPE prepared by different processes were
shown in Fig. 7a. Pristine powders obtained by evaporating
dichloromethane exhibited an emission maximum at 545 nm.
Aer fuming with dichloromethane vapor for 5 min, the emis-
sion maximum was at 515 nm. In addition, aer annealing the
pristine powders for 20 min, the emission maximum was at
513 nm. Pristine powders obtained by evaporating dichloro-
methane via fuming or annealing process both exhibited
a notable hypsochromic shi of around 30 nm with color
changes from yellow to green (Fig. 7b) due to a decreased D–A
interaction. Pristine powders obtained by evaporating hexane
had an emission maximum at 514 nm. Aer the grinding
process, the ground compound revealed an emission maximum
of 539 nm which was red-shied by 25 nm with a color change
from green to yellow (Fig. 7b) indicating an enhanced D–A
interaction. Further annealing of the ground compound resul-
ted in a shiing of the emission maximum to 516 nm with
a color change from yellow to green. This suggested that
conjugation or D–A interaction of compounds can be generated
by simple grinding process which leads to a notable bath-
ochromic shi. Powder X-ray diffraction (PXRD) measurements
were employed to elucidate the micro-structures of the pristine
powders and powders through grinding and annealing process.
PXRD results (Fig. 7c) revealed that fumed and annealed
powders exhibited relatively sharp diffraction peaks compared
to that of the pristine powders obtained by evaporating DCM
indicating the generation of a crystalline form. Upon grinding
pristine powders obtained by evaporating hexane, the sharp
peaks disappeared suggesting a transition from the crystalline
to the amorphous state. The ground powders were then
annealed and the sharp diffraction peaks appeared again indi-
cating the regeneration of the crystalline form.
The emission spectra of compounds FBT-2TPE and 2FBT-
2TPE as pristine, ground and annealed powders were shown in
Fig. 8a and b. The pristine, ground and annealed powders of
FBT-2TPE exhibited an emission maximum at 512, 519 and
530 nm, respectively. In addition, the pristine, ground and
annealed powders of 2FBT-2TPE exhibited an emission
maximum at 498, 509 and 484 nm, respectively. It should be
noted that a pronounced color change from green to blue aer
annealing of the ground compound 2FBT-2TPE was observed
due to a decreased D–A interaction. PXRD results (Fig. 8c) for
FBT-2TPE exhibited less intense diffraction peaks aer the
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(V-670) UV-Vis-NIR spectrophotometer and on a Jasco (FP-8500)
uorescence spectrophotometer, respectively. The photo-
luminescence quantum yields (F) of compounds were per-
formed in THF and water solutions, respectively, relative to
a quinine sulfate (F ¼ 0.54 in 0.1 N H2SO4) standard. Ther-
mogravimetric analysis was carried out by using a TA ꢁin1 stru-
ments TGA Q-500 under a heating rate of 10 ꢂC min and
a nitrogen ow rate of 60 mL minꢁ1. Differential scanning
calorimetry (DSC) was carried out using a PerkinElmer DSC
4000 under a heating and a cooling rate of 10 ꢂC minꢁ1 and
a nitrogen ow rate of 20 mL minꢁ1. Powder X-ray diffraction
(PXRD) data was recorded on a D2 PHASER X-ray Diffractometer
˚
with Cu Ka radiation (l ¼ 1.5418 A). Polarized optical micros-
copy (POM) images were obtained from Olympus BX51 polar-
ized light microscopy.
Synthesis of 5-uorobenzo[c][1,2,5]thiadiazole 1. 4-Fluoro-o-
phenylenediamine (2.20 g, 17.4 mmol) was dissolved in
a mixture of 200 mL of anhydrous CHCl3 and 12 mL of anhy-
drous Et3N under an argon atmosphere and then thionyl chlo-
ride (2.8 mL, 36.4 mmol) was added dropwise by syringe. The
mixture was reuxed overnight. The reaction mixture was then
cooled down to room temperature and most of the CHCl3 was
evaporated. Deionized water (250 mL) was added to the reaction
mixture and then concentrated HCl was added slowly until the
pH value was 1.0. Through direct steam distillation was used to
collect all condensed liquid, extracted three times then dried
over anhydrous MgSO4. The solvent was removed by rotary
evaporator to give a white solid (0.88 g, 5.7 mmol) in a yield of
Fig. 8 Normalized PL spectra of (a) FBT-2TPE and (b) 2FBT-2TPE and
PXRD results and photo images of (c) FBT-2TPE and (d) 2FBT-2TPE in
pristine, ground and annealed powders. Photo images of pristine,
ground and annealed powders FBT-2TPE and 2FBT-2TPE are shown in
the insets.
grinding process and new sharp peaks generated by the
annealed process indicating the transition from one crystalline
form to amorphous state to another crystalline form. In addi-
tion, the ground powders of 2FBT-2TPE (Fig. 8d) showed rela-
tively sharp diffraction peaks compared to pristine powders
indicating the high degree of crystallinity. The ground samples
were then annealed and a selection of low intensity and new
peaks were observed possibly due to the conversion of one
crystalline form to another. This suggests that compounds FBT-
2TPE and 2FBT-2TPE could form other types of well-organized
structures by annealing and grinding processes, respectively
compared to that of the pristine powders.35
1
33%. H NMR (600 MHz, CDCl3, ppm) d: 7.92 (dd, 1H, J ¼ 9.6,
5.4 Hz), 7.56 (dd, 1H, J ¼ 8.6, 2.5 Hz), 7.37 (ddd, 1H, J ¼ 9.6, 8.6,
2.5 Hz). 13C NMR (150 MHz, CDCl3, ppm) d: 163.48 (d, 1JCF ¼ 253
3
3
Hz), 154.87 (d, JCF ¼ 14 Hz), 151.95, 122.49 (d, JCF ¼ 11 Hz),
121.27 (d, 2JCF ¼ 30 Hz), 104.76 (d, 2JCF ¼ 24 Hz). 19F NMR (564
MHz, CDCl3, ppm) d: ꢁ110.38. EI-MS: calculated for
[C6H3FN2S]+: m/z 154, found m/z 154.
Experimental
Materials and general considerations
Synthesis of 5-uorobenzo[c][1,2,5]thiadiazole 2. A similar
procedure was used as that described for the synthesis of
All chemicals and reagents were purchased from commercial compound 1 using 1,2-diamino-4,5-diuoro-benzene (2.52 g,
sources (Alfa Aesar, Acros and Sigma-Aldrich) without further 17.4 mmol) as the starting material. Compound 2 was obtained
1
purication. Solvents used for spectroscopic measurements to give a white solid (0.92 g) in a yield of 31%. H NMR (600
were spectrograde. All air sensitive reactions were carried out MHz, CDCl3, ppm) d: 7.76 (dd, 2H, J ¼ 9.0, 4.5 Hz). 13C NMR (150
1
2
under an argon or a nitrogen atmosphere. Anhydrous organic MHz, CDCl3, ppm) d: 153.80 (dd, 2C, JCF ¼ 261 Hz, JCF ¼ 20
solvents were distilled under a nitrogen atmosphere over Hz), 148.85 (pseudo triplet, 2C, 3JCF ¼ 5 Hz, 3JCF ¼ 2 Hz), 106.12
calcium hydride or sodium/benzophenone. Well-degassed (dd, 2C, 2JCF ¼ 16 Hz, 3JCF ¼ 5 Hz). 19F NMR (564 MHz, CDCl3,
solutions were carried out through bubbling argon for at least ppm) d: ꢁ128.96. EI-LRMS: calculated for [C6H2F2N2S]+: m/z
30 min. Column chromatography was performed using Silica 172, found m/z 172.
1
1
1
Flash G60 70–230 mesh silica. H, H–1H COSY, H–13C HSQC
Synthesis of 4,7-dibromo-5-uorobenzo[c][1,2,5]thiadiazole
and 13C NMR spectra were recorded on Bruker AVIII HD-600 3. Compound 1 (0.88 g, 5.7 mmol) and 12 mL of 48% HBr(aq) were
MHz spectrometer. The chemical shis were reported in ppm. charged into a two-neck ask and then bromine was added
All coupling constants were reported in hertz (Hz). The dropwise with a dropping funnel. The suspension was heated to
following abbreviations were used to indicate multiplicity: s ¼ 120 ꢂC for 2 days. The suspension was cooled to room tempera-
singlet, d ¼ doublet, dd ¼ doublet of doublets, ddd ¼ doublet of ture and was quenched by saturated aqueous NaHSO3. The
doublet of doublets t ¼ triplet and m ¼ multiplet. Electron mixture was extracted with DCM three times and the combined
impact mass spectra (EI-MS) were recorded on a JEOL JMS-700 organic layers were dried over anhydrous MgSO4. The solvent was
spectrometer. Electrospray ionization mass spectra (ESI-MS) removed by rotary evaporator and the residue was puried by
were recorded on a Thermo/Finnigan LCQ spectrometer. UV- column chromatography (petroleum ether : DCM ¼ 8 : 2) to give
vis absorption and emission spectra were recorded on a Jasco a white solid (0.82 g) in a yield of 46%. 1H NMR (600 MHz, CDCl3,
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ppm) d: 7.79 (d, 1H, JH–F ¼ 8.3 Hz). 13C NMR (150 MHz, CDCl3,
ppm) d: 160.40 (d, 1JCF ¼ 256 Hz), 154.87 (d, 3JCF ¼ 14 Hz), 150.33,
Synthesis of 5-uoro-4,7-bis(4-(1,2,2-triphenylvinyl)phenyl)
benzo[c][1,2,5]thiadiazole FBT-2TPE. A similar procedure was
used as that described for the synthesis of BT-2TPE using
compound 3 (0.25 g, 0.8 mmol) as the starting material. The
compound FBT-2TPE was obtained to give a yellow solid (0.42 g) in
a yield of 64%. 1H NMR (600 MHz, DCM-d2, ppm) d: 7.79
(d(AA0BB0), 2H, J ¼ 8.4 Hz), 7.62 (d, 1H, JH–F ¼ 11.8 Hz), 7.60
(d(AA0BB0), 2H, J ¼ 8.4 Hz), 7.23–7.04 (m, 34H). 13C NMR (150 MHz,
CDCl3, ppm) d: 133.97, 133.34, 133.27, 131.71, 131.48, 131.43,
131.36, 131.34, 131.31, 131.20, 129.83, 129.81, 129.22, 128.38,
127.84, 127.79, 127.76, 127.72, 127.65, 127.62, 126.67, 126.61,
126.57, 126.54, 126.50, 126.46, 119.51, 119.30, 117.11, 117.01. 19F
NMR (564 MHz, CDCl3, ppm) d: ꢁ115.14. HR-MS (EI, [M]+):
calculated for [C58H39FN2S]+: m/z 814.2818, found m/z 814.2817.
Synthesis of 5,6-diuoro-4,7-bis(4-(1,2,2-triphenylvinyl)
phenyl)benzo[c][1,2,5]thiadiazole 2FBT-2TPE. A similar proce-
dure was used as that described for the synthesis of BT-2TPE
using compound 4 (0.25 g, 0.75 mmol) as the starting material.
The compound 2FBT-2TPE was obtained to give a yellow solid
123.88 (d, 2JCF ¼ 32 Hz), 114.00 (d, 3JCF ¼ 11 Hz), 98.21 (d, 2JCF
¼
24 Hz). 19F NMR (564 MHz, CDCl3, ppm) d: ꢁ103.25. EI-MS:
calculated for [C6H1Br2FN2S]+: m/z 312, found m/z 312.
Synthesis of 4,7-dibromo-5,6-diuorobenzo[c][1,2,5]thiadia-
zole 4. A similar procedure was used as that described for the
synthesis of compound 3 using 5-uorobenzo[c][1,2,5]thiadia-
zole (0.92 g, 5.3 mmol) as the starting material. Compound 4
was obtained to give a white solid (0.82 g) in a yield of 47%. 1H
NMR (600 MHz, CDCl3, ppm) d: no peaks. 13C NMR (150 MHz,
1
2
CDCl3, ppm) d: 151.84 (dd, 2C, JCF ¼ 262 Hz, JCF ¼ 22 Hz),
148.85 (s, 2C), 99.35 (dd, 2C, 2JCF ¼ 16 Hz, 3JCF ¼ 6 Hz). 19F NMR
(564 MHz, CDCl3, ppm) d: ꢁ119.53. EI-MS: calculated for
[C6Br2F2N2S]+: m/z 330, found m/z 330.
Synthesis of 4,4,5,5-tetramethyl-2-(4-(1,2,2-triphenylvinyl)
phenyl)-1,3,2-dioxaborolane
5.
1-(4-Bromophenyl)-1,2,2-
triphenylethylene (3.00 g, 7.3 mmol), bis(pinacolato)diboron
(2.24 g, 8.8 mmol), potassium acetate (2.87 g, 29.2 mmol) and
Pd(dppf)Cl2 (266 mg, 0.36 mmol) were charged into a two-neck
ask and then 30 mL of anhydrous 1,4-dioxane was added by
syringe. The reaction mixture was reuxed under an argon
atmosphere for 12 hours. Aer cooling to room temperature,
the catalyst was removed by ltration. The ltrate was extracted
with DCM three times and the combined organic layers were
dried over anhydrous MgSO4. Aer removal of the solvent under
reduced pressure, the residue was puried by column chro-
matography (petroleum ether : ether ¼ 9 : 1) to give a white
solid (2.54 g) in a yield of 76%. 1H NMR (600 MHz, CDCl3, ppm)
d: 7.54 (d (AA0BB0), 2H, J ¼ 8.1 Hz), 7.12–7.06 (m, 9H), 7.06–6.98
(m, 8H), 1.32 (s, 12H). 13C NMR (150 MHz, CDCl3, ppm) d:
1
(0.25 g) in a yield of 38%. H NMR (600 MHz, CDCl3, ppm) d:
7.61 (d(AA0BB0), 4H, J ¼ 8.4 Hz), 7.22 (d(AA0BB0), 4H, J ¼ 8.4 Hz),
7.20–7.05 (m, 30H). 13C NMR (150 MHz, CDCl3, ppm) d: 151.27,
151.13, 150.39, 150.37, 150.35, 149.55, 149.42, 144.48, 143.60,
143.46, 143.45, 141.89, 140.33, 131.45, 131.35, 131.34, 131.32,
129.79, 128.25, 127.81, 127.76, 127.64, 126.68, 126.56, 126.53,
118.33, 118.31, 118.26, 118.24. 19F NMR (564 MHz, CDCl3, ppm)
d: ꢁ113.79. HR-MS (EI, [M]+): calculated for [C58H38F2N2S]+: m/z
832.2724, found m/z 832.2725.
Conclusions
146.80, 143.73, 143.63, 143.55, 141.42, 140.88, 134.13, 131.38, Fluorinated BT linked with TPE were successfully synthesized by
131.35, 131.34, 130.73, 127.76, 127.69, 126.59, 126.49, 126.48, a Suzuki–Miyaura cross-coupling reaction using Pd(PPh3)4 as the
83.71, 24.94. ESI-MS: calculated for [C32H31BO2]+: m/z 459, catalyst and K2CO3 as a base to give the compounds in a moderate
found m/z 459.
yield. All compounds exhibited aggregation-induced emission
Synthesis of 4,7-bis(4-(1,2,2-triphenylvinyl)phenyl) benzo[c] characteristics upon increasing the water fraction over 60% in the
[1,2,5]thiadiazole BT-2TPE. 4,7-Dibromobenzo[c][1,2,5]thiadia- THF/water mixtures. In addition, the emission maximum was
zole (0.29 g, 1 mmol), compound 5 (0.82 g, 1.79 mmol), blue-shied when the water content reached 90% compared to the
Pd(PPh3)4 (0.11 g, 0.1 mmol) and potassium carbonate (1.10 g, THF solution. The effect of uorine atoms attached to the central
8.0 mmol) were charged into a two-neck ask under vacuum for BT unit makes these D–A–D molecules force to be planar which led
30 minutes. Aer purging with argon, 80 mL of toluene, 10 mL to a more signicant blue-shied in absorption and emission
of ethanol and 10 mL of water was added by syringe. The maximum in the aggregate state. Interestingly, the emission
ꢂ
reaction mixture was reuxed at 120 C for 12 hours under an maximum intensity of the compound containing diuorinated BT
argon atmosphere. Aer cooling to room temperature, organic bonded to two TPE in 90% water content was found to be 2.5 times
solvent was removed by rotary evaporator. The residue was higher than that in THF solution which reveal a signicant
extracted with DCM and water three times and the combined increase in uorescence compared to that of the unsubstituted
organic layers were dried over anhydrous MgSO4. Aer removal and monouorinated substituted BT. Surprisingly, single crystal
of the solvent under reduced pressure, the residue was puried with ladder-like packing structures as well as liquid crystal meso-
by column chromatography (DCM : petroleum ether ¼ 6 : 4) to phases were observed only for compound comprising of diuori-
give a yellow solid (0.68 g) in a yield of 85%. 1H NMR (600 MHz, nated BT and TPE. This molecule with the unique feature can be
CDCl3, ppm) d: 7.78 (d(AA0BB0), 4H, J ¼ 8.4 Hz), 7.74 (s, 2H), 7.20 used as a key component in the liquid crystal devices, organic
(d(AA0BB0), 4H, J ¼ 8.4 Hz), 7.18–7.05 (m, 30H). 13C NMR (150 light-emitting diodes and organic eld-effect transistors. All
MHz, CDCl3, ppm) d: 154.01, 143.78, 143.74, 143.65, 143.64, compounds showed remarkable solvatochromic effects in various
141.48, 140.51, 136.26, 132.59, 131.59, 131.47, 131.37, 131.35, selected solvents with different polarities and revealed notably
128.35, 127.88, 127.81, 127.72, 127.63, 126.58, 126.50, 126.46. emission color changes. The powder XRD results and the mecha-
HR-MS (EI, [M]+): calculated for [C58H40N2S]+: m/z 796.2972, nochromism of the compounds suggested that the solid state
found m/z 796.2970.
structures change from the one form to another. Theses
12626 | RSC Adv., 2018, 8, 12619–12627
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There are no conicts to declare.
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