Benzothiazolium-functionalized tetraphenylethene: An AIE luminogen with tunable solid-state emission

Article abstract of DOI:10.1039/c2cc33780k

Melding a benzothiazolium unit with tetraphenylethene generates a new hemicyanine luminogen with aggregation-induced emission characteristics; the luminogen exhibits crystochromism and its solid-state emission can be repeatedly tuned from yellow or orange to red by grinding-fuming or grinding-heating processes due to the transformation from the crystalline to the amorphous state and vice versa.

Full text of DOI:10.1039/c2cc33780k

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COMMUNICATION
Benzothiazolium-functionalized tetraphenylethene: an AIE luminogen
with tunable solid-state emissionw
a
a
a
a
a
a
Na Zhao, Zhiyong Yang, Jacky W. Y. Lam, Herman H. Y. Sung, Ni Xie, Sijie Chen,
b
a
a
b
ac
Huimin Su, Meng Gao, Ian D. Williams, Kam Sing Wong and Ben Zhong Tang*
Received 26th May 2012, Accepted 10th July 2012
DOI: 10.1039/c2cc33780k
Melding a benzothiazolium unit with tetraphenylethene generates
a new hemicyanine luminogen with aggregation-induced emission
characteristics; the luminogen exhibits crystochromism and its
solid-state emission can be repeatedly tuned from yellow or
orange to red by grinding–fuming or grinding–heating processes
due to the transformation from the crystalline to the amorphous
Scheme 1 Synthetic route to luminogen 1.
state and vice versa.
Cyanine dyes have been widely used for fluorescent sensory
9
applications for hundreds of years. Recently, they have
Creation of efficient luminogenic materials is a hot research
topic. Of particular interest are those luminogens with tunable
and reversible emission in the solid state due to their potential
1
been successfully applied as nonlinear optical materials and
0
1
1
chemosensors as their absorption and emission colour can easily
reach the red and near infrared region. However, they suffer some
1
applications in biotechnology and memory systems. A problem
12
associated with the emissions of most luminogenic materials in
the solid state is aggregation-caused quenching (ACQ): during
film formation, the dye molecules are located in close vicinity
and are inclined to form aggregates, which favour the formation
drawbacks such as small Stokes shift and ACQ effect, thus
making realization of their full potential a daunting task.
To enlarge the family of AIE-active red emitters, in this
communication, we generated a hemicyanine dye by attaching a
benzothiazolium unit, a building block for cyanine dye, to tetra-
2
of detrimental species such as excimers. In 2001, we found that
7b
some propeller-shaped molecules exhibit the phenomenon of
aggregation-induced emission (AIE) which is exactly opposite
3
to the ACQ effect. Instead of quenching, aggregate formation
phenylethene (TPE), a typical AIE luminogen, through vinyl
functionality (1; Scheme 1). Luminogen 1 inherits the AIE feature
of TPE and emits at longer wavelength. Whereas many dye
molecules show tunable light emission in the solution state,
luminogen 1 exhibits crystochromism and its solid-state emission
can be repeatedly tuned by grinding–fuming and grinding–heating
has induced them to emit intensely. Since then, a variety of AIE
4
–6
luminogens have been prepared
and utilized in various
7
applications. Most of the AIE luminogens prepared so far,
however, emit blue or green light. Few emitters with longer
emission wavelengths have been prepared, possibly due to
synthetic difficulty, though they suffer little interferences from
13
processes, which are rarely reported in the AIE system.
Luminogen 1 was synthesized as a yellow solid in a yield of
62% according to the synthetic route shown in Scheme 1.
Detailed procedures for its synthesis and characterization can
be found in the ESI.w
Crystals of 1 were obtained by slow evaporation of its
DCM–ethanol, THF–hexane and DCM–ethyl acetate (EtOAc)
mixtures and analyzed by single crystal X-ray diffraction. Their
ORTEP drawings are shown in Fig. S1 (ESIw) and the crystal
data are given in Table S1 (ESIw). Interestingly, the crystals grown
from different solvent mixtures emit at different wavelengths with
different efficiencies (Fig. S2, ESIw and Table 1). To gain insight
into the distinct emission behavior of the crystals, we checked
their geometric structures and packing arrangements. Due to the
propeller-shaped TPE unit, all the crystals adopt a highly twisted
8
optical absorption and light scattering.
a
Department of Chemistry, Institute for Advanced Study,
State Key Laboratory of Molecular Neuroscience, Institute of
Molecular Functional Materials and Division of Biomedical
Engineering, The Hong Kong University of Science & Technology
(HKUST), Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk
Department of Physics, HKUST, Clear Water Bay, Kowloon,
Hong Kong, China
Department of Polymer Science & Engineering, Zhejiang University,
Hangzhou, 310027, China
b
c
w Electronic supplementary information available: Synthesis and
characterization of 2 and 3; crystal data for 1, 1Á2/3 THF and
1
ÁEtOAc and their HOMO and LUMO energy levels; views of inter-
actions between molecules in crystals of 1; TEM and SEM images and
ED patterns of amorphous and crystalline aggregates of 1; change in
the PL spectrum of 1 by repeated grinding–heating cycles; and XRD
1
conformation. The torsion angles (y ) between the bridged phenyl
ring and the vinyl core of TPE in crystals of 1, 1Á2/3 THF and
1
ÁEtOAc are 70.421, 70.371 and 67.941, respectively, suggesting
and DSC profiles of 1 at different aggregated states. CCDC
83789–883791. For ESI and crystallographic data in CIF or other
that the molecular conjugation is in the order 1 o 1Á2/3 THF o
1ÁEtOAc. This agrees well with their observed emission maximum,
8
electronic format see DOI: 10.1039/c2cc33780k
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8637–8639 8637

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Table 1 Photophysical properties, torsion angles and energy gaps of
crystals of 1
a
Crystal
l
em (nm)
F
F
(%)
y
1
(1)
DE (eV)
1
1
1
565
578
591
18.0
28.2
43.6
70.42
70.37
67.94
1.89
1.83
1.79
Á2/3 THF
ÁEtOAc
a
F
Abbreviations: lem = emission maximum, F = fluorescence quantum
yield determined using a calibrated integrating sphere, DE = energy band
gap determined using the B3LYP/6-31G(d) basis set.
in which crystals of 1ÁEtOAc are the redder emitters. As shown
in Fig. S3–S5 (ESIw), except weak p–p stacking interaction
between the benzothiazolium units, multiple C–HÁ Á Áp and
C–HÁ Á ÁF hydrogen bonds and SÁ Á ÁF interaction are observed
in all crystals. Additional C–HÁ Á Áp and C–HÁ Á ÁO hydrogen
bonds that arise from interactions with the solvent molecules
are also found in crystals of 1Á2/3 THF and 1ÁEtOAc. These
multiple bonds and interactions help further rigidify the
molecular conformation, which reduces the energy loss
through the nonradiative rotational relaxation channel and
thus enhances the emission efficiency of 1Á2/3 THF and
Fig. 1 (A) PL spectra of 1 in THF and THF–water mixtures with
different water fractions (f ). (B) Plots of fluorescence quantum yields
w
1ÁEtOAc. The HOMO and LUMO energy levels of the crystals
versus the composition of the aqueous mixtures of 1. (C) Change in the
PL spectrum of 1 in 90% aqueous mixtures with time from 0 to 30 min.
were calculated using the 3LYP/6-31G* basis set and the results
are given in Fig. S6 (ESIw). The HOMO of all crystals is
dominated by the orbitals of the TPE unit, while the orbitals of
the benzothiazolium component contribute mainly to the LUMO
energy levels. The energy band gaps of 1, 1Á2/3 THF and 1ÁEtOAc
are calculated to be 1.89, 1.83 and 1.79 eV, respectively, nicely
correlating with their different emission colors. Clearly, the
crystal emission of 1 can be tuned readily by solvent molecules,
which is extraordinary, if not unprecedented in the AIE system.
Luminogen 1 absorbs at 440 nm in diluted THF solution
(
D) Plot of I/I
0 and 99% water contents. I
solution. Solution concentration: 20 mM; excitation wavelength:
25 nm. Inset: photographs of 1 in (B) THF–water mixtures with f
0
value versus time in THF–water mixtures of 1 with
9
0
= emission intensity in pure THF
4
w
values of 0 and 90 vol% and (C) 90% aqueous mixture at different
time intervals (0 and 30 min) taken under 365 nm UV illumination.
accompanied with a remarkable enhancement in the emission
intensity. In contrast, such a phenomenon was not observed in
the 99% aqueous mixture (Fig. 1D). Since the ultimate emission
maximum is close to that of crystals, it is implied that the
aggregates crystallize over time. This is supported by the TEM
and SEM images and ED patterns shown in Fig. S7 (ESIw). The
aggregates formed in the 90% aqueous mixture may possess a
more loose structure than those in the THF–water mixture
with 99% water fraction, which provides more free volume for
the molecules to reorient and pack in a more ordered fashion.
The unusual blue-shift observed in the crystalline phase, on the
other hand, may be due to the conformation twisting of the
aromatic rings of 1 in order to fit into the crystalline lattice.
Without such constraint, the molecules in the amorphous phase
may assume a more planar conformation and thus show a redder
emission.
(
Fig. S2, ESIw) due to the intramolecular charge transfer (ICT)
from the electron-donating TPE unit to the electron-accepting
1
benzothiazolium unit. Similar to TPE, luminogen 1 emits
4
faint photoluminescence (PL) at 663 nm with a fluorescence
quantum yield (F ) of 3.66% when its diluted solution is
F
photoexcited (Fig. 1A). When a small amount of water is
added to the THF solution, the emission intensity as well as
the F
The higher the water content, the lower the light emission
and the F value because the solution polarity becomes higher
F
value becomes lower, presumably due to the ICT effect.
F
progressively. Interestingly, at water fraction >90%, the mixture
emits even more intensely and efficiently than pure THF solution.
F
At 99% water content, the F value is 13.12%, which is more
than 3-fold higher than that in pure THF solution. Clearly, 1
is AIE-active. We have hypothesized that restriction of
intramolecular rotation (RIR) is the main cause for the AIE
phenomenon, which blocks the nonradiative relaxation channel
Mechanochromic luminescent materials have received consider-
able interest in recent years in view of their potential applications
13b
in camouflage and optical information storage systems. The
crystals of 1 show strong yellow emission at 565 nm. Intriguingly,
after gentle grinding using a pestle and a mortar, red powders are
formed, which show red PL at 650 nm (Fig. 2 and 3A). After
fuming with acetone vapor for 10 min, the initial (yellow)
appearance is reinstalled (Fig. 3A). The conversion between yellow
and red emission colors can be repeated many times without
fatigue as these stimuli are nondestructive in nature (Fig. 3B). On
the other hand, heating the ground sample at 150 1C for 10 min
changes its colour from red to orange (Fig. 2 and Fig. S8A, ESIw).
15
and populates radiative excitons. Since 1 is not soluble in water,
its molecules must have been aggregated in aqueous mixtures
with high water fractions. However, at water fraction r90%, the
ICT effect still dominates. Afterwards, the RIR process prevails,
which turns 1 into a strong emitter.
Amusingly, the emission intensity and color of a freshly
prepared 90% aqueous mixture change when standing at room
temperature with time. As depicted in Fig. 1C, the PL spectrum
initially peaks at 644 nm, which shifts progressively to 566 nm
8
638 Chem. Commun., 2012, 48, 8637–8639
This journal is c The Royal Society of Chemistry 2012

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Its solid-state emission can be tuned reversibly from yellow or
orange to red by grinding–fuming and grinding–heating processes
due to the morphological change from the thermodynamically
stable crystalline phase to the metastable amorphous state.
We are currently utilizing 1 as a fluorescent visualizer for
tumor cell targeting and imaging. The details will be published
in due course.
Fig. 2 Switching the solid-state emission of 1 by repeated grinding–
fuming and grinding–heating processes. The photographs were taken
under 365 nm UV irradiation.
The work reported in this communication was partially
supported by the National Science Foundation of China
(
20974028), the RPC and SRFI Grants of HKUST
RPC10SC13, RPC11SC09 and SRFI11SC03PG), the Research
Grants Council of Hong Kong (604711, 603509, HKUST2/CRF/
0 and N_HKUST620/11), the Innovation and Technology
(
1
Commission (ITCPD/17-9) and the University Grants Committee
of Hong Kong (AoE/P-03/08 and T23-713/11-1). B.Z.T. thanks
the support from the Cao Guangbiao Foundation of Zhejiang.
Notes and references
Fig. 3 (A) Change in the PL spectrum of 1 by grinding–fuming
process. (B) Repeated switching of the solid-state fluorescence of 1
by repeated grinding and fuming cycles.
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sample exhibits many sharp diffraction peaks, indicative of its
crystalline nature (Fig. S9A, ESIw). In contrast, the ground
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state and vice versa. It is noteworthy that the fumed sample shows
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amorphous powders cannot be completely recovered as yellow-
emissive crystals by the latter method.
7
Analysis by differential scanning calorimetry (DSC) also
substantiates the above claim. The DSC curve of crystals of 1
recorded during the heating scan is basically a straight line
parallel to the abscissa (Fig. S9B, ESIw). In contrast, an
endothermic peak at 138 1C is detected in the ground sample.
Thermogravimetric analysis shows that 1 exhibits a 5% weight
loss at 280 1C. The peak at 138 1C thus should not stem from
the decomposition of the molecule or its glass-transition
temperature as such thermal transition involves only a small
enthalpy change. Instead, it is more likely to be associated with
the crystallization of the luminogen. No signals are detected in
thermally-treated and fumed samples as they are crystalline.
In summary, an AIE hemicyanine dye was synthesized by
incorporation of a benzothiazolium unit into TPE through
vinyl functionality. Luminogen 1 exhibits crystochromism and its
crystals emit stronger and bluer PL than its amorphous aggregates.
9
1
¨
¨
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8637–8639 8639