pH-Responsive dye with dual-state emission in both visible and near infrared regions

Article abstract of DOI:10.1007/s11426-017-9221-6

A new dual-state emission (DSE) dye comprised of tetraphenylethene (TPE), triphenylamine (TPA), and indoline groups has been synthesized, which showed efficient fluorescence in both solution and solid. The dye is comprised of three parts and these parts s

Full text of DOI:10.1007/s11426-017-9221-6

SCIENCE CHINA
Chemistry
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pH-Responsive dye with dual-state emission in both visible and
near infrared regions
*
Titao Jing & Lifeng Yan
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, CAS Key Laboratory of Soft Matter
Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials (iCHEM), University of Science and Technology of China,
Hefei 230026, China
Received November 30, 2017; accepted February 10, 2018; published online April 24, 2018
A new dual-state emission (DSE) dye comprised of tetraphenylethene (TPE), triphenylamine (TPA), and indoline groups has
been synthesized, which showed efficient fluorescence in both solution and solid. The dye is comprised of three parts and these
parts show different fluorescence properties which can be very useful in some applications since the dye can produce in-
formation-rich responses. For example, the dye is pH-sensitive in both solution and solid states, and it emits yellow fluorescence
in normal pH and red/NIR fluorescence in acidic condition. Cytotoxicity of the dye is low at concentration of 3 μM which was
confirmed by a methyl thiazolyl tetrazolium (MTT) experiment, and in vitro experiments revealed that the pH responsive
performance can be used in bioimaging. It provides a novel pH-sensitive DSE dye ever reported, which has potential application
in many fields.
dual-state emission, fluorescence, near infrared, pH-responsive, dye
Citation:
Jing T, Yan L. pH-Responsive dye with dual-state emission in both visible and near infrared regions. Sci China Chem, 2018, 61, https://doi.org/
0.1007/s11426-017-9221-6
1
1
Introduction
However, AIE/ACQ can only work for the state of ag-
gregation or solution. AIE materials were active in only
aggregation state, strongly decided by the intermolecular
interaction. When it has strong interaction with other mole-
cules, AIE materials might be disaggregated and exhibit no
or weak fluorescence. The quenching of ACQ materials in
solid or high concentration also limited the application of the
traditional dyes. Thus, incorporating the merits of ACQ and
AIE dyes to create efficient dyes in both solution and solid
states are attractive [7].
Organic fluorescent dyes have attracted much attention for
their applications in a wide range of bio-imaging, chem/bio-
sensors, and optoelectronic materials, etc. [1], and the me-
chanism of luminescence is complex and mystic. Traditional
organic luminophores are mainly composed of planar aro-
matic rings and emit efficiently in dilute solutions [2,3]. But
the emission is weak or quenched in concentrated solution or
in the aggregated state, as the aggregation-caused quenching
(
ACQ) [4]. Tang et al. [2] developed aggregation-induced
Recently, a new concept called dual-state emission (DSE)
appeared, and the dye exhibit highly efficient luminescence
in both solution and the solid state [8–13], and some inter-
esting works have been reported by filling the gap between
ACQ and AIE [3,5,9,10,14–16], Xu et al. [5] reported a
deep-blue luminescent compound that emits efficiently both
in solution and solid state. An arch-bridge-type fluorophore
emission (AIE or AIEE) fluorescent dyes which show
quenching or weakened emission in solution, but exhibit
strong fluorescence upon aggregation [5]. Recently, thou-
sands of AIE dyes were prepared and well-studied [6].
*
Corresponding author (email: lfyan@ustc.edu.cn)
©
Science China Press and Springer-Verlag GmbH Germany 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

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also exhibits dual photoluminescence [10]. However, as the
contradiction of ACQ and AIE, dyes featuring DSE is hard to
design and synthesis. In addition, DSE with intelligent per-
formance, such as stimulation-responsive emission, have not
been reported by now. Stimulation-responsive emission dyes
are thus of great importance essentially and arrest much at-
tention [17,18].
Tetraphenylethene (TPE) is the most widely used unit in
AIE as it is convenient for the preparation and high fluor-
escence quantum yield in solid or aggregation state [19]. In
solution, the propeller shape of TPE consume the energy of
luminophores, result in non-emission (AIE) or very low
quantum yield (AIEE) [20,21]. For ACQ dyes, substantial
rigidity with limited intramolecular motions is necessary in
solution [9]. Indoline group is the basic unit of cyanine dyes,
featuring rigidity and large π plane. Triphenylamine (TPA)
also enjoys rigidity and large π plane. TPE-TPA has been
reported by Tang et al. [22], which is AIE. Here, while in-
doline was conjugated to TPE-TPA, a new dye (TTIN) was
synthesized (Scheme 1 and Figure S1, Supporting Informa-
tion online) and it exhibits dual photoluminescence in both
solution and solid. Moreover, the dye was pH sensitive, it
emits yellow light in the normal pH range while it shows
emission in far red and NIR region in acidic conditions, both
in solution and solid states. So, a pH-responsive dye which
emitting in both solution and solid states was synthesized and
its photoluminescence performance was studied (Figure 1).
Figure 1 DSE of TTIN dye with pH sensitivity. (a) Solution state which
dissolved in tetrahydrofuran (THF):water=2:1. (left: photo image, right:
fluorescence image). (b) Solid state (up: photo image, down: fluorescence
image) (color online).
2
.3 Synthesis of TPE-B(OH)2
As shown in Scheme 1 [23], in a flame dried Schlenk flask,
.2 g TPE-Br was dissolved in 30 mL anhydrous THF fol-
1
lowed by evacuating under vacuum and flush with nitrogen
for five times. The solution was cooled down to −78 °C with
liquid nitrogen-ethanol bath, and then 2.5 mL n-butyllithium
2
Experimental
2
.1 Materials
hexanes solution (2.5 M) was injected under N . The reaction
2
maintained for 5 h, and the color of the system turned to
brownish dark and 1.8 mL trimethyl borate was injected
THF, n-hexaneand other organic solvent were purchased
from Sinoreagent Corporation (China). Diphenylmethane, 4-
bromobenzophenone, 2,3,3-trimethylindolenine, and 4-(N,
N-Diphenylamino) benzaldehyde were obtained from Alad-
din Corporation (China) and used without further purifica-
tion.
under the protection by N gas. The solution was heated to
2
room temperature and stirred for 4 h. HCl (2 M, 10 mL)
aqueous solution was added and the mixture was stirred
overnight. Finally, the mixture was extracted with 50 mL
dichloromethane (DCM) three times and then the organic
solvent dried over anhydrous sodium sulfate. After DCM
was evaporated, and the crude product was purified on a
2
.2 Characterization
1
column chromatography with petroleum ether/ethylacetate
Bruker AC 300 spectrometer was applied to obtain H-NMR
spectra of compounds (Germany). Size and size distribution
of the aggregates were determined by dynamic light scat-
tering (DLS) carried out on a Malvern Zetasizer Nano ZS90
with a He-Ne laser (633 nm). F97pro fluorescence spectro-
photometer (Shanghai Lengguang industrial Co. Ltd., China)
used to carry out fluorescence measurements. All excitation
and emission slit width was 10 nm, but photomultiplier tube
1
(
EA)=5:1 eluted. 0.7 g, 63%. H-NMR (300 MHz, DMSO-
d6) δppm: 7.96 (s, 1H), 7.57–7.50 (m, 2H), 7.15–7.05(m,
9
H), 7.01–6.90 (m, 8H).
2
.4 Synthesis of TPA-CHO-Br
In a 50 mL round-bottom flask which equipped with con-
denser, 4-(N,N-Diphenylamino)benzaldehyde (0.54 g) and
N-bromosuccinimide (NBS) (0.35 g) were refluxed in 10 mL
(PMT) sensitivity was adjusted as requirement. UV-Vis
spectra were obtained on an UV1700pc (Shanghai AuCy
Scientific Instrument Co. Ltd., China) Ultraviolet spectro-
photometer. TEM: JEOL-2010 (Japan).
CCl for 6 h, and the reaction was then cooled to room
4
temperature and 10 mL water was added. The crude product

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Scheme 1 Synthesis of TTIN.
was extracted with 20 mL DCM three times and the dried.
then washed by deionization (DI) water five times followed
by 1 mL ethanol twice. The mixture was then purified by
3
mL ethanol was added and the product was purified by
1
recrystallization. 0.5 g. 71%. H-NMR (300 MHz, CDCl )
preparation thin liquid chromatography (PTLC) and 70 mg
3
1
δppm: 9.84 (s, 1H), 7.73 (d, J=9 Hz, 2H), 7.47 (d, J=9 Hz,
TTIN was obtained, yield 71%. H-NMR (300 MHz, CDCl )
3
2
4
1
H), 7.39–7.34 (m, 2H), 7.21–7.15 (m, 3H), 7.7–7.03(m,
δppm: 7.70 (d, J=9 Hz, 1H), 7.52–7.49 (m, 4H), 7.37–7.33
(m, 6H), 7.20–7.00 (m, 27H), 1.57 (s, 6H). C-NMR
13
13
H). C-NMR (75 MHz, CDCl ) δppm: 189, 131.78,
3
+
30.35, 128.87, 128.73, 126.42, 125.29, 118.91. MS (ESI )
(75 MHz, CDCl ) δppm: 183.54, 148.98, 146.91, 146.34,
3
+
m/z: 352.0326 (M+H ), C H ONBr.
146.23, 143.75, 143.71, 142.56, 141.08, 140.53, 138.04,
19
15
1
1
1
35.76, 131.82, 131.41, 131.25, 129.54, 129.48, 128.72,
27.87, 127.63, 116.47, 125.02, 123.92, 122.54, 121.09,
2
.5 Synthesis of TPE-TPA-CHO
+
20.26, 60.39, 52.55, 24.04. MS (ESI ) m/z: 745.35706 (M
+
TPA-CHO-Br (0.4 g) and TPE-B(OH) (0.35 g) were dis-
+H ), C H N .
2
56 45
2
solved in 20 mL THF. 2 mL Na CO aqueous solution (2 M)
2
3
and Pd(PPH ) (10 mg) were then added. The mixture was
3
4
2
.7 Cell imaging and MTT
evacuated under vacuum and backfilled with N three times.
2
The reaction was refluxed at 80 °C overnight, which bubbled
HepG2 (liver hepatocellular cells) cells were seeded at a
density of 3×10 in a plate incubated for 16 h. Afterwards,
5
with N gas [23]. After cooling down to room temperature,
2
1
0 mL brine was poured to quench the reaction. The product
the original medium was replaced with 3 μΜ of TTIN in
culture medium and incubated for another 4 h. Then the
medium was removed and washed twice with various pH
values PBS buffer from 4.0 to 7.4. Next, the cell was
maintained in phosphate buffer saline (PBS) buffer with
various pH for 10 min before took photos [25]. In a 96-well
plate, HepG2 cells were seed at a density of 5000 cells per
well and incubated for 24 h. Culture medium was replaced
with 100 μL of medium containing TTIN at various con-
centrations or complete DMSO (control). After 24 h of in-
cubation, the cells were washed with PBS, the viability was
then evaluated by MTT assay.
was extracted with 20 mL DCM three times and then was
evaporated to remove DCM. TPE-TPA-CHO was purified on
a column chromatography with petroleum ether/EA=4:1
1
eluted. Obtain 320 mg product in yield of 60%. H-NMR
(300 MHz, CDCl ) δppm: 9.83 (s, 1H), 7.72 (d, J=9 Hz, 2H),
3
7
.54 (d, J=9 Hz, 2H), 7.36 (d, J=6 Hz, 2H), 7.21 (d, J=9 Hz,
13
2
H), 7.14–7.01 (m, 24H); C-NMR (75 MHz, DMSO-d )
6
δppm: 190.62, 152.50, 145.54, 145.90, 142.23, 140.76,
1
1
40.07, 137.03, 135.92, 132.78, 130.61, 130.01, 128.86,
27.77, 126.39, 125.64, 125.52, 125.45, 119.00, 118.77,
+
+
118.11. MS (ESI ) m/z: 604.26398 (M+H ), C H ON.
45
34
2
.6 Synthesis of TPE-TPA-indolenine (TTIN)
3
Results and discussion
TPE-TPA-CHO (82 mg), 2,3,3-trimethylindolenine (62 mg),
and benzyltriethylammonium (BTEA) (14 mg) were added
in a tube. 2 mL 50% NaOH aqueous solution was added by
ten portions in 20 min. Under vigorous stirring, the reaction
mixture was performed at 50 °C overnight [24]. NaOH
aqueous solution was removed by suction and the solid was
At first, DSE behaviors of the as-prepared TTIN dye were
studied in THF-hexane mixture as shown in Figure 2(a).
TTIN can be well dissolved in THF but not in hexane. In
THF, the dye shows emission at 508 nm, yellow, with about
19% quantum yield. When fraction of hexane was increased

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Figure 2 (a) Fluorescence spectra of TTIN in various THF-hexane mixture with changing content of THF, (b) in different solvents, (c) in DMF-glycerol
mixture with changing content of glycerol, (d) fluorescence intensity and emission peak in different DMF-glycerol mixture. All the concentration is 25 μM
(color online).
up to 10%, the quantum yield swiftly decreased to 13% with
emission at 508 nm. No aggregation formed with low con-
centration of hexane. The change of quantum yield might be
due to the solvent effect (Figure S2). When the concentration
of THF was progressively decreased to 1%, the quantum
yield maintained at around 11%. Different to the stable
quantum yield, the emission peak shifted to the blue region
gradually. In 99% hexane solvent, two emission peaks were
observed at both 467 and 446 nm, respectively. As shown in
Figure 1 and Scheme 1, TTIN molecule could be divided into
three parts: TPE, TPA and indoline. TPE is an AIE unit,
while indoline is ACQ, and the contradictive parts occupied
both the ends of TTIN, conjugated by a dual photo-
luminescence unit TPA [9]. Some reports showed that TPA is
adaptive to modify group, or TPE-TPA is AIE [22] while
TPA-indoline have strong fluorescence in solution [26]. The
two totally different effects of the two TTIN ends were
harmonized by the TPA unit, and none of them was in
dominant. In solution, substantial rigid TPA-indoline part is
photoluminescence, and the intramolecular rotations of TPE
do not consume the whole energy. While in aggregation or
solid, considering the remote distance between TPE and in-
doline, the indoline part of TTIN might be quenched, as the
propeller shape of TPE might have little influence on stop-
ping the aggregation of indoline. However, the restriction of
intramolecular rotations in TPE-TPA unit induces the pho-
toluminescence. The harmony of AIE an ACQ thus might
features DSE of TTIN. The fluorescence spectra in different
solvents also were tested, as shown in Figure 2(b). In THF
and toluene, middle polarity solvent, the fluorescence in-
tensity is strong and the emission peak is less than 510 nm.
Upon increasing polarity of solvent (TCM, DMF, methanol),
emission peak shifts to long wavelength companied with
attenuating of fluorescence intensity, which is consistent
with the TICT mechanism [9,17,27,28]. While in water, a
poor solvent for the dye, TTIN was in aggregation state. The
fluorescence intensity was recovered and the emission peak
blue-shifted to 533 nm compared to that in methanol.
To better understand the TICT and AIE behavior of the
TTIN dye and exclude the effect of hydrogen bond, the
fluorescence spectra of TTIN in different alcohols with
changing of the lengths of alkyl groups has also been in-
vestigated, as shown in Figure 3. While the molecular length
of alcohols was extended, the polarity of solvent decreased,
resulting in weak TICT effect. The fluorescence is gradually
intensified along with blueshift of the emission peak. Except
the TICT, the solubility of TTIN dye in long chain alcohol is
poor, and the aggregation might also contribute to the
fluorescence enhancement and blueshift. The counteractive
effect of TICT and AIE has also been tested in DMF-glycerol
mixed solution, as glycerol is more polar than that of DMF,
along with high viscosity. At first, the decrease in fluores-
cence intensity and the redshift of the emission peak was
observed by the increasing the content of glycerol, indicating

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the polarity of the mixture increased which induced TICT.
But the viscosity also increased to suppress the in-
tramolecular rotations [20], which is counterproductive to
TICT, and the fluorescence intensity and emission peak be-
came stable. When the glycerol fraction is more than 60%,
aggregation take control, combined assistance of viscosity to
restrict the intramolecular rotations, and the fluorescence
intensity was recovered with a blueshift of the emission
peak.
Interestingly, the TTIN dye is pH sensitive, and the pH
sensitivity is not only in solution but also in solid or nano-
aggregative states, which make it outstanding from the pH
sensitive dyes for only AIE or ACQ. The indoline group of
TTIN could be protonation, which turned into an electron-
withdrawing group. The strong electron-withdrawing thus
induce TICT, which make it a potential pH probe. TTIN dye
can be dissolved in THF and the mixture of THF and water,
and the pH sensitivity of the solution state was studied in its
Figure 3 Fluorescence spectra of the TTIN dye in different alcohols with
changing molecular lengths (E =400 nm). The insert picture is the photo-
graph which exposed to 365 nm UV lamp. The first in the left is THF
which was a reference, followed by water, methanol, ethanol, butylalcohol,
x
1
-hexanol, and octanol (color online).
THF-H O (2:1) mixture, as shown in Figure 4(a–d). Inter-
2
TICT dominates. When the content of glycerol is over 30%,
estingly, only one emission peak 566 nm was found in
Figure 4 Absorption and fluorescence spectra of TTIN in different solvents (25 μM) or solid state at various pHs. (a–d) in THF/H
2
O (2:1): (a) absorption,
(b) excited by 400 nm, (c) excited by 520 nm, and (d) ratiometric fluorescence responses to pHs; (e) absorption and (f) fluorescence spectra of solid TTIN,
inset shows the photo and fluorescence images of the solid at normal and acidic condition; (g–i) in pure water: (g) absorption, (h) excited by 400 nm, (i)
excited by 520 nm (color online).

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fluorescence spectra. With gradually increasing acidity, the
emission is continually weakened, in consistence with the
colour changes of the photo images (Figure S3). At solution
sorption spectra also showed pH sensitivity (Figure 4(g, i)),
which is colorimetric. The color of the TTIN aqueous solu-
tion is yellow at high pH solution. When pH decreased to 0, it
turned into red. The reversibility has also been tested, as
shown in Figure 5(d). After 3 times cycles, TTIN still be
reversible to acidic environment. The decrease of basic en-
vironment may due to the aggregation state of the TTIN
state in THF-H O solution, the new absorption peak 527 nm
2
emerged at pH 3, and the colour change could be observed by
naked eyes at pH 2.
Solid of the TTIN dye also shows pH sensitive behavior, as
shown in Figure 4(e, f). The solid was prepared by volati-
lizing from TTIN ethyl acetate solution, which emits in the
yellow region normally. However, when the film was im-
mersed in pH 0 solution for 30 min, colour changes to red
and the emission redshifted to red and NIR region, indicating
that TTIN is pH sensitive in solid. pH sensitivity of TTIN in
nano-aggregative states in water (or 10% FBS solution,
Figure S4) was also investigated, as shown in Figure 4(d, g–
i). At pH 9, 8 and 7, the fluorescence intensity is stable and
emission peak is 532 nm, located in the yellow region.
However, when the pH decreased gradually, the intensity of
−
molecules, which make OH hard to get inside of nanoag-
gregations.
Another interesting phenomenon is the morphology of
nano-aggregates which was observed by TEM (Figure 5). In
DI water, the TTIN aggregate is sphere, but it displays reg-
ular triangle morphology when it was protonated by HCl
aqueous solution (pH 0). The protonated TTIN is positive
charge, and the intermolecular interaction might different
from normal TTIN molecules, results in the interesting
morphology. In addition, for aggregative state in water, when
pH below 1, the new absorption peak emerged at 538 nm,
and only at pH 0 red colour could be observed by naked eyes.
The difference might due to the steric hindrance in ag-
gregation or solid state. Proton must overcome the hindrance
to touch the inner TTIN molecules and make it hard proto-
nation. When in solution state, proton in solution contact
easily with TTIN molecules. Furthermore, a newly TICT
state might form in the aggregation or the solid state as the
crowded molecules were forced by twisting. That induces
dual emission peaks of TTIN when excited by 400 nm.
5
32 nm becomes weak as the TICT effect. Meanwhile, a new
emission peak in the red region emerges from fluorescence
spectra when pH below 5.0. The new emission peak exhibits
redshift and attenuation of intensity with the enhancement of
acidity. When the pH below 1.0, the new emission peak
moved to the near infrared (NIR) region. The dual emission
peaks of TTIN enable it a ratiometric fluorescent probe as
shown in Figure 4(d), and a near 1000 fold enhancement of
fluorescence ratio of I /I can be found [29,30]. The ab-
695 532
Figure 5 DLS (radius) and TEM of TTIN in different solvent. (a) DI water; (b) pH 0 HCl aqueous solution; (c) n-hexane. All three scale bar were 600 nm.
d) Reversible switching of the emission TTIN by repeated adjustment of its aqueous solution to acidic or neutral environments (color online).
(

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While in solution, the molecules did not twist by the inter-
action, inducing a longer wavelength emission peak in far
red and NIR region by the TICT effect.
Comparison test was also carried out and the results were
shown in Figure 6 to demonstrate the advantage of TTIN.
AIE dye TPE-Br, ACQ dye rhodamine B (RB) and TTIN
were papered in solution and aggregate states. As shown in
Figure 6, TPE was emitted blue fluorescence in aggregation
state but non-emission in solution state. The orange fluor-
escence of RB is strong in solution state but quenched in
aggregation states. That is a non-emission state would exist,
no matter ACQ or AIE dyes. The non-emission state limited
its application. However, the fluorescence of TTIN could be
observed in both states, endows it a wide potential applica-
tion.
The pH responsive performance can be used in bioima-
ging. As the pH sensitivity and multi-emission fluorescence
of TTIN, particularly in the far red and NIR regions, it is a
good probe for bioimaging [31–33]. As shown in Figure 7
HepG2 cells were incubated with TTIN for 4 h in plate and
the intracellular pH of the cells was adjusted using PBS
buffer from 7.4 to 5.0. When excited by light in blue channel,
cells could be observed and the visible fluorescence is
dominating, which have little difference for acidic environ-
ment. While excited by light in the green channel, far red and
NIR fluorescence take as control, and the pH sensitivity
becomes obvious. In normal pH 7.4, the fluorescence is weak
and cells were hard to be recognized. While in acidic en-
vironment pH 5.0, the fluorescence is much stronger than
that of normal pH. For visible fluorescence, it is hard to
penetrate deep into tissues but can be visible without
Figure 8 MTT assay of the TTIN molecules over HepG2 cells (color
online).
equipment. For NIR emission, it can penetrate deep into
tissues and provide useful diagnostic information [34], but
not visible for naked eyes. TTIN featuring multi-emission,
including in visible and NIR region, is definitely much
promising in the application of bio-imaging. The pH sensi-
tivity of TTIN also makes it a potential pH probe in vitro and
in vivo such as lysosome and tumour. The MTT assay also
shows the noninvasive of TTIN in the concentration 3 μM
(Figure 8).
4
Conclusions
In summary, a DSE dye TTIN has been synthesized, which
emits yellow or red/NIR fluorescence in both solution and
solid states. The photoluminescence is efficient without in-
tramolecular rotation consuming and it filled the gap of AIE
and ACQ. Moreover, the dye is pH sensitive, both in solid
and solution states. When the solution or solid is in acidic
condition, both of them emit red/NIR fluorescence, makes it
a potential material in bioimaging, chem/bio-sensors, and
optoelectronic materials.
Figure 6 Photo images of TPE-Br, RB, and TTIN. The left side was in
solution state and right side was in aggregative state. For the solution, all
three dyes were dissolved in THF. For aggregation, TPE-Br was dispersed
in water while RB and TTIN were dispersed in hexane (color online).
Acknowledgements This work was supported by the National Natural
Science Foundation of China (51673180, 51373162).
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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Figure 7 The HepG2 tumor cells fluorescence images after treatment by
TTIN (concentration: 3 μM, incubation time: 4 h) (color online).

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