Photoactivatable aggregation-induced emission fluorophores with multiple-color fluorescence and wavelength-selective activation

Article abstract of DOI:10.1002/chem.201406026

Photoactivatable (caged) fluorophores are widely used in chemistry, materials, and biology. However, the development of such molecules exhibiting photoactivable solid-state fluorescence is still challenging due to the aggregation-caused quenching (ACQ) effect of most fluorophores in their aggregate or solid states. In this work, we developed caged salicylaldehyde hydrazone derivatives, which are of aggregation-induced emission (AIE) characteristics upon light irradiation, as efficient photoactivatable solid-state fluorophores. These compounds displayed multiple-color emissions and ratiometric (photochromic) fluorescence switches upon wavelength-selective photoactivation, and were successfully applied for photopatterning and photoactivatable cell imaging in a multiple-color and stepwise manner.

Full text of DOI:10.1002/chem.201406026

DOI: 10.1002/chem.201406026
Full Paper
&
Fluorescence
Photoactivatable Aggregation-Induced Emission Fluorophores
with Multiple-Color Fluorescence and Wavelength-Selective
Activation
Lu Peng, Yue Zheng, Xiaoyan Wang, Aijun Tong, and Yu Xiang*[a]
Abstract: Photoactivatable (caged) fluorophores are widely
used in chemistry, materials, and biology. However, the
development of such molecules exhibiting photoactivable
solid-state fluorescence is still challenging due to the aggre-
gation-caused quenching (ACQ) effect of most fluorophores
in their aggregate or solid states. In this work, we developed
caged salicylaldehyde hydrazone derivatives, which are of
aggregation-induced emission (AIE) characteristics upon
light irradiation, as efficient photoactivatable solid-state
fluorophores. These compounds displayed multiple-color
emissions and ratiometric (photochromic) fluorescence
switches upon wavelength-selective photoactivation, and
were successfully applied for photopatterning and photo-
activatable cell imaging in a multiple-color and stepwise
manner.
materials for applications,[10] such as luminescent probes,[9c,11]
imaging reagents,[9c,12] optoelectronic devices,[13] and stimuli-re-
sponsive materials.[9b,14] These AIE fluorophores usually exhibit
a large Stokes’ shift with almost no overlap between their exci-
tation and emission spectra, so that little ACQ effect is present
even when the molecules are closely packed in their aggregate
or solid states. In addition, they are normally non-fluorescent
in solutions because of the intramolecular motions that cause
non-radiative decay of the excited states, while this process
is restricted in aggregates or solids to maintain strong
fluorescence.[9d,15]
Introduction
Photoactivatable (or caged) fluorophores[1] are molecules
showing fluorescence switches upon light irradiation, which
are widely used as functional groups and probes in chemis-
try,[2] materials,[3] and biology.[4] Unfortunately, most fluoro-
phores, such as conventional fluorescein or rhodamine dyes,
undergo aggregation-caused quenching (ACQ),[5] which makes
the activated fluorescence of their photoactivatable derivatives
usually much weaker in aggregate or solid states compared to
that in solutions. Organic compounds with strong solid-state
fluorescence are attractive materials suitable for the fabrication
of luminescent materials and many other real world applica-
tions where molecules tightly packed as aggregates or even
solids are required.[6] However, fluorophores exhibiting both
photoactivatable characteristics and strong solid-state
fluorescence without ACQ upon light irradiation are still much
less investigated, despite of their potential applications as
functional solid materials for photopatterning,[7] photo-
activatable imaging,[8] etc.
Therefore, AIE fluorophores can be an ideal platform for the
development of photoactivatable solid-state fluorescent mate-
rials. By attaching a 2-nitrobenzyl group as a quencher to an
AIE-active tetraphenylethene molecule, Tang and co-workers
have constructed a caged fluorophore that displays a strong
cyan emission with high fluorescence quantum yields in its
aggregate or solid states upon UV irradiation, and applied the
compound as a solid material for photopatterning and anti-
counterfeiting.[7a] Nevertheless, it is still very challenging to
develop caged solid-state fluorophores with photoactivatable
fluorescence in multiple colors (including ratiometric fluores-
cence switches) by wavelength-selective photoactivation, as
well as to apply them in biological systems.
Unlike many fluorescent dyes showing little or very weak
solid-state fluorescence due to the ACQ effect, fluorophores
with aggregation-induced emission (AIE) properties are an
emerging class of molecules displaying strong fluorescence in
their aggregate or solid states.[9] So far, many AIE fluorophore
systems[9d] made of hydrocarbons, heteroatoms or organome-
tallics have been characterized and developed as fluorescent
In this work, we developed a serial of salicylaldehyde hydra-
zone derivatives,[16] which are caged by 2-nitrobenzyl, phenac-
yl, or 7-methoxylcoumarin-4-yl, to serve as photoactivatable
solid-state fluorophores with multiple fluorescence colors upon
wavelength-selective activation (Figure 1). The emission colors
of these compounds upon UV irradiation were tunable from
green to orange by changing the substitutions on the
salicylaldehyde hydrazone,[16] while the photoactivation of the
fluorophores was selectively carried out by UV irradiation at
365 or 300 nm through varying the caging groups.[17] Interest-
[a] L. Peng, Y. Zheng, X. Wang, Prof. A. Tong, Prof. Y. Xiang
Department of Chemistry, Beijing Key Laboratory for Analytical Methods
and Instrumentation, Key Laboratory of Bioorganic Phosphorus Chemistry
and Chemical Biology, Tsinghua University, Beijing 100084 (PR China)
E-mail: xiang-yu@tsinghua.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406026.
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of salicylaldehyde hydrazones, which are AIE-active by ESIPT,
can be quenched by caging the hydroxyl groups and subse-
quently activated by uncaging through UV irradiation. As
shown in Figure 1a, no ESIPT is possible in the presence of
caging groups on the salicylaldehyde hydrazones 1–9 due to
the absence of hydroxyl protons, thus these caged compounds
should be non-fluorescent. Upon UV irradiation to remove the
caging groups and yield 10–12, the hydroxyl and ESIPT can be
recovered to restore the AIE and strong solid-state fluores-
cence. In addition to the capability of realizing multiple-color
emissions upon UV irradiation by changing the substitutions
on the salicylaldehyde hydrazone,[9d,16a,19] another advantage of
this design principle over previous reports is that the caging
groups may not necessarily be quenchers,[7a,8,20] so that the
choice of caging groups is very flexible to enable wavelength-
selective photoactivation[3b] as well as ratiometric (photochro-
mic) fluorescence switches upon UV irradiation.
Caged fluorophores with multiple-color photoactivatable
fluorescence
Figure 1. a) Chemical structures of compounds 1–12 and the scheme of
photouncaging 1–9 to yield 10–12, which are fluorescent at different wave-
lengths (colors) by UV irradiation at different wavelengths. ESIPT, excited-
state intramolecular proton transfer; NB, 2-nitrobenzyl; PH, phenacyl; CM,
7-methoxylcoumarin-4-yl. b) The multiple-color fluorescence enhancement
or change upon irradiation at 365 or 300 nm for 1 (green), 2 (yellow), 3
(orange), 6 (light orange), and 8 (from blue to white purple) in their solid
and aggregate (colloid solution) states.
The photoactivatable fluorescence of 2-nitrobenzyl-caged[17b,21]
salicylaldehyde hydrazones 1–3 were studied first to testify our
design principle (Figure 2a). All these caged compounds dis-
played little fluorescence in their aggregate (colloid solution,
Figure S1 in the Supporting Information) or solid states, which
suggests the successful caging effect of 2-nitrobenzyl; while
ingly, a ratiometric (photochro-
mic) fluorescence switch upon
UV irradiation was also achieved
when using a fluorescent cou-
marin derivative as the caging
group. Furthermore, these pho-
toactivatable solid-state fluoro-
phores were successfully applied
for photopatterning and photo-
activatable cell imaging, in a mul-
tiple-color and stepwise manner.
Results and Discussion
Design of the photoactivatable
AIE fluorophores
One class of AIE-active fluoro-
phores is based on the excited-
state
intramolecular
proton
transfer (ESIPT) mechanism,[18] in
which the hydroxyl groups of
these fluorophores are responsi-
ble for ESIPT and essential for
their AIE characteristics as well
Figure 2. a) The transformation of 2-nitrobenzyl-caged salicylaldehyde hydrazones 1–3 by UV irradiation at
365 nm. b) Fluorescence spectra of 1–3 in aggregate states before and after UV irradiation at 365 nm. c) Partial
1H NMR spectra (1 mm solutions in [D6]DMSO) of 2 as prepared, 2 after irradiation at 365 nm, 2 after standing in
as
solid-state
fluorescen-
ce.[9d,16a,19] Therefore, we expect
that the solid-state fluorescence the dark, and 11, which suggests the complete transformation of 2 to 11 by the UV irradiation.
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upon continuous irradiation under a 12 W 365 nm handheld
UV lamp, 1–3 underwent a gradual fluorescence enhancement
over time until strong emissions in green (521 nm), yellow
(541 nm), and orange (566 nm) were observed for 1, 2, and 3,
respectively, in both of their aggregate and solid states
(Figure 1b). The time-dependent fluorescence spectra of 1–3 in
the colloid solutions were recorded in the absence and
presence of UV irradiation at 365 nm (Figure S2–S4 in the Sup-
porting Information), showing that the photoactivation only
occurred under UV light and could be fully accomplished.
Figure 2b illustrates the fluorescence spectra of 1–3 in their ag-
gregated states before and after the irradiation, in which the
fluorescence of 1–3 after the ir-
the result strongly supported the formation of 11 from 2
(Figure 2a), including the removal of the 2-nitrobenzyl and the
recovery of the hydroxyl group, by UV irradiation.
Wavelength-selective photoactivation and ratiometric
(photochromic) fluorescence switch
Due to the caging groups in our photoactivatable system are
not necessarily quenchers, it is then possible for us to use
other photosensitive groups other than 2-nitrobenzyl to con-
struct caged solid-state fluorophores exhibiting wavelength-
selective photoactivation. As depicted in Figure 3a, phenacyl-
radiation was almost identical to
that of 10–12 in the same sol-
vent (Figure S5, Supporting In-
formation), which supports the
hypothesis in our design. The
photoactivatable fluorescence of
1–3 was also studied in their
solid states (on a solid support
for solid-state fluorescence mea-
surement). Only background
fluorescence of the solid support
was observed for 1–3 without
UV irradiation, regardless of the
time for standing in the dark.
However, upon irradiation at
365 nm, the solids of 1–3 dis-
played similar fluorescence emis-
sions as those in the colloid sol-
utions and the fluorescence
gradually enhanced over time
(Figure S6–S9, Supporting Infor-
mation). These results demon-
strated the usefulness of 1–3 as
photoactivatable fluorescent materials in both colloid solutions
and solids with multiple-color fluorescence emissions upon
light irradiation. The photophysical properties of 1–3 are listed
in Table S1 (Supporting Information).
Figure 3. The transformation of a) 6 to 12 and b) 8 to 11 by UV irradiation selectively at 300 nm, and the
fluorescence spectra of 6 and 8 before and after UV irradiation at 300 or 365 nm.
caged[17b,22] salicylaldehyde hydrazones 6 showed little fluores-
cence in the dark or after prolonged irradiation at l=365 nm
in its aggregate or solid states, whereas the light-orange fluo-
rescence was gradually lighted up when the irradiation was at
l=300 nm (Figure 1b), which is close to the optimal wave-
length capable of removing the phenacyl group[17b,22] from
non-fluorescent 6 to yield fluorescent 12. The kinetics of the
fluorescence enhancement upon 300 nm irradiation was moni-
tored (Figure S10, Supporting Information), which was slower
To investigate the fate of the caged fluorophores after pho-
toactivation, we further studied the 1H NMR spectra of 2 in
[D6]DMSO before and after the UV irradiation. As shown in Fig-
ure 2c, 2 exhibited two sharp peaks around d=5.6 and
8.9 ppm, corresponding to the signals of the CH2 (connecting
salicylaldehyde hydrazone and nitrobenzyl groups) and the N= compared to those of 1–3 (Figure S2–S4, Supporting Informa-
CH (salicylaldehyde hydrazone). There was no change in the
1H NMR spectrum of 2 after standing in the dark for 1 h, which
indicated the compound was stable in DMSO in the absence
of UV light. Nevertheless, when 2 was irradiated at 365 nm for
1 h, the chemical shifts of the CH2 and N=CH underwent disap-
pearance and 0.1 ppm downfield shift, respectively, while
a new peak at around 11.1 ppm (OH) emerged. This spectrum
of 2 after the irradiation was very similar to that of 11 (five
characteristic peaks around d=6.9, 7.4, 7.7, 9.0, and 11.1 ppm),
with some minor peaks most likely ascribed to some by-
products from the released 2-nitrobenzyl moiety. Therefore,
tion) due to the lower quantum efficiency of phenacyl versus
that of 2-nitrobenzyl.[17b,22] The time-dependent solid-state fluo-
rescence spectra of 4–6 after standing in dark or irradiated by
l=365 or 300 nm UV light were also studied. They were all
found to display photoactivated fluorescence enhancement
over the background only in the presence of 300 nm
irradiation, while almost no change occurred in the dark or
under irradiation at 365 nm (Figure S11–S13, Supporting
Information), thereby enabling multiple wavelength-selective
photoactivation.
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More interestingly, when we utilized 7-methoxylcoumarin-4-
yl[17b,23] as a caging group that was fluorescent itself, the fluo-
rophores 8 exhibited a ratiometric (photochromic) fluorescence
switch instead of solely photoactivation after UV irradiation. As
shown in Figure 3b, 8 displayed blue fluorescence in its aggre-
gate or solid states because of the weakly fluorescent coumar-
in as a caging group (significantly quenched compared to its
fluorescence in solution, as displayed in Figure S14 Supporting
Information). Upon light irradiation at 300 nm, the coumarin
should be removed from 8 and the resulting compounds ex-
hibited combined fluorescence characteristics of both 7-me-
thoxylcoumarin (blue) and salicylaldehyde hydrazones 11
(yellow), which appeared as fluorescence in white purple (Fig-
ure 1b). This photochromic fluorescence was only observed
after 300 nm irradiation, not in dark or under 365 nm light (Fig-
ure S15, Supporting Information), because the caging group’s
photosensitive wavelength is close to 300 nm.[17b,23] The solids
of 7–9 were also measured on the solid support to investigate
their photoactivatable fluorescence properties, and they were
all found to undergo similar fluorescence switches solely upon
irradiation at 300 nm (Figure S16–S19, Supporting Information).
The photophysical properties of 4–9 are also listed in Table S1
(Supporting Information).
at 365 nm through a photomask, the words “CHEMISTRY”
fluorescent in green, yellow and orange were successfully pat-
terned on filter papers preloaded loaded with solids of 1, 2
and 3, respectively, whereas the use of 8 enabled the pattering
of a fluorescent “CHEMISTRY” in white purple on a blue back-
ground after UV irradiation at 300 nm. By drawing an image of
flowers and leaves using 1–3 as the “dyes” on the same paper
(Figure 4b), the initially invisible image containing three colors
showed up when the paper was irradiated under l=365 nm
UV light, which indicated the success of multiple-color photo-
patterning in one image. More interesting, if some of the flow-
ers in the image were drawn using 8 instead of 2, the photo-
patterning could be carried out in not only a multiple-color
fashion but also a stepwise manner by sequential light irradia-
tions at l=365 and 300 nm (Figure 4c). At first, the image was
only visible as two blue flowers. Subsequently, orange flowers
and green leaves emerged upon light irradiation at 365 nm.
Further, the blue flowers turned white purple as the light
irradiation was altered to 300 nm.
Photoactivatable cell imaging
In addition to the application of the caged salicylaldehyde
hydrazones in their solid states for photopatterning, we also
investigated the use of these molecules in their colloid solu-
tions (aggregate states) for photoactivatable cell imaging.[8]
The formation of colloids of 1–3 and 8 in aqueous solutions
was supported by the results of dynamic light scattering (DLS)
analysis (Figure S1, Supporting Information). The diameters of
the particles in the colloid solutions were in the range of 130–
190 nm, which is suitable for cellular uptake through endocy-
tosis. After incubating MCF-7
Multiple-color and stepwise photopatterning
Encouraged by the above results showing the efficient
photoactivatable fluorescence of our caged salicylaldehyde hy-
drazones in solid states, we further applied these molecules as
materials for photopatterning[7] in a multiple-color and step-
wise manner. As displayed in Figure 4a, upon light irradiation
cells with each of the colloid sol-
utions of the caged fluorophores
1–3, the cells were washed thor-
oughly and irradiated at l=
365 nm or in dark for 40 min
before imaged under a fluores-
cence microscope. As shown in
Figure 5a–c, cells containing 1–3
displayed almost no detectable
fluorescence under the micro-
scope before UV irradiation or
after the incubation in dark.
However, upon irradiation at
365 nm, green, yellow and
orange fluorescence gradually
arose inside the cells with 1, 2
and 3, respectively (Figure 5a–c,
and Figure S20–S22 in the Sup-
porting Information). The fluo-
rescent dots were found mostly
Figure 4. a) Pattering words of “CHEMISTRY” in green (1), yellow (2), orange (3), and white purple on a blue back-
ground (8) using a photomask on papers loaded with one of the photoactivatable fluorophores as solids.
b) Photoactivating a multiple-color fluorescent image of flowers (orange and yellow) and leaves (green) made of
1–3 as solids by UV irradiation at 365 nm. c) Stepwise photoactivating a multiple-color fluorescent image of
flowers (blue, orange and white purple) and leaves (green) made of 1, 3 and 8 as solids by sequential UV
irradiations at 365 and 300 nm.
localized in the cytoplasm of the
cells rather than in the nucleus,
mainly because some of the unc-
aged fluorophore molecules
(containing phenol groups) on
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Figure 5. Fluorescent live cell images of MCF-7 cells containing a) 1, b) 2, or c) 3 after incubation without or with UV irradiation at 365 nm. d) Ratiometric
fluorescent live cell images of MCF-7 cells containing 8 after incubation without or with UV irradiation at 300 nm. “Dark”, in dark; “UV”, with UV irradiation;
“B”, “G”, “Y”, “R”, and “L”: blue, green, yellow, red, and bright channels, respectively; “Y/B”, intensity ratio of yellow channel divided by blue channel.
the surface of the particles might undergo ionization to gener-
ate negative charges. On the other hand, cells incubated with
8 were found to exhibit ratiometric fluorescence switches after
UV irradiation at 300 nm (Figure 5d, and Figures S23–S24 in
the Supporting Information), as illustrated in the images using
color scales of intensity ratio (yellow channel divided by blue
channel). Ratiometric fluorescence switches are usually much
less susceptible to the interference from photobleaching or
concentration distributions of fluorophores inside cells,
compared with fluorescence enhancement alone.[24] These
results demonstrated that our caged fluorophores were
successfully delivered into live cells and fully preserved their
photoactivatable fluorescence in the cellular environment.
activatable solid-state fluorescence of multiple emission colors
and wavelength-selective activation were achieved, respective-
ly. The use of a fluorescent caging group also enabled a ratio-
metric (photochromic) fluorescence switch upon UV irradiation.
The caged salicylaldehyde hydrazones were further successfully
applied for photopatterning and photoactivatable cell imaging
in a multiple-color and stepwise manner, which suggests their
promising potential as photoactivatable solid-state fluoro-
phores in chemistry, materials, and biology.
Experimental Section
Materials and instrumentation
All chemicals were purchased either from Alfa Aesar (Tianjin,
China) or Sigma–Aldrich (St. Louis, USA) and used as received with-
out further purification. The 10 mm HEPES buffer at pH 7.0 was
prepared by adding a proper amount of NaOH to HEPES in Milli-
pore water under adjustment by a pH meter. All absorption and
fluorescence spectra were measured on a JASCO V-550 spectro-
photometer and a JASCO FP-6500 spectrophotometer (Tokyo,
Japan). All NMR spectra were recorded using a JOEL JNM-ECA300
spectrometer (Tokyo, Japan) operated at 300 MHz. Cell imaging
was performed on an Olympus FV 1000 confocal microscope
(Tokyo, Japan).
Conclusions
In summary, we have developed caged salicylaldehyde
hydrazone derivatives showing photoactivatable solid-state
fluorescence in this study. Upon UV irradiation, the caging
groups blocking the hydroxyl moieties of these fluorophores
were removed, thereby recovering ESIPT and strong fluores-
cence in their aggregate (colloid solution) and solid states. By
changing the substitutions on the salicylaldehyde hydrazone
and varying the caging groups on the hydroxyl group, photo-
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tion and brine, dried over anhydrous sodium sulfate, filtered and
evaporated under reduced pressure. After drying, 2-nitrobenzyl-
caged salicylaldehydes as precursors for the synthesis of 1–3 were
obtained. One of the 2-nitrobenzyl-caged salicylaldehydes (2 equiv)
was dissolved in CH2Cl2 or DMF, followed by the addition of hydra-
zine hydrate (1 equiv). After being stirred overnight at room tem-
perature, the crude product was precipitated by adding ethanol,
filtered and washed by ethanol twice to afford 1–3 after drying.
Photoactivation of the fluorophores in aggregate states
(colloid solutions)
Stock solutions (2.0 mm) of compounds 1–9 were prepared in
DMSO. In a typical experiment, the colloid solution was prepared
by placing 25 mL of the stock solution into 475 mL 10 mm HEPES at
pH 7.0 with rigorous mixing. The solutions were irradiated by
a 12 W hand-held UV lamp (5 cm above the solutions) at 365 or
300 nm for a desired time, then absorption and fluorescence
spectra were recorded at 258C. Excitation was at 370 nm.
Compounds 7–9 were prepared by a similar procedure as 1–3
using 7-methoxylcoumarin-4-yl bromide instead of 2-nitrobenzyl
bromide. Compounds 10–12 were synthesized following the re-
ported procedure.[16b] Compounds 4–6 were obtained by a similar
procedure as 1–3 using 10–12 and phenacyl bromide instead of
salicylaldehydes and 2-nitrobenzyl bromide, respectively.
Photoactivation of the fluorophores in solid states
Stock solutions (2.0 mm) of compounds 1–9 were prepared in THF.
In a typical experiment, 3 mL of each stock solution was dropped
on a filter paper strip. After complete evaporation and dried, the
paper strips was irradiated by a 12 W hand-held UV lamp (5 cm
above the spots) at 365 or 300 nm for a desired time, then
fluorescence spectra were recorded at 258C. Excitation was at
370 nm.
For additional figures and more details of the synthesis, see the
Supporting Information.
Acknowledgements
We thank for the financial support from the National Natural
Science Foundation of China (nos. 2142200199, 21390410 and
21375074), the National Key Scientific Instrument and Equip-
ment Development Project of China (no. 2012YQ030111), the
Tsinghua University Initiative Scientific Research Program (no.
20131089220), and the Recruitment Program of Global Youth
Experts of China.
Multiple-color and stepwise photopattering
Papers were fully soaked by stock solutions (2.0 mm) of 1, 2, 3 and
8 in THF, and dried under ambient condition. The photopattering
of the word “CHEMISTRY” was carried out by irradiating the papers
at 365 or 300 nm for 30 min under a plastic photomask (with the
“CHEMISTRY” pattern) printed by a standard printer. For images of
flowers and leaves, the stock solutions were used as the “dye” for
the drawings on a paper by brush pens. After having been dried
under ambient conditions, the paper was irradiated at l=365 or
300 nm for 30 min. The visualization of the fluorescent images was
through the excitation by a l=365 nm UV lamp for a short time
(within 5 s) to minimize unwanted activation.
Keywords: aggregation-induced emission · cell imaging ·
fluorescence · hydrazones · photoactivatable fluorophores
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MCF-7 cells were obtained from NIH (Bethesda, USA) and the re-
agents for culture were purchased from HyClone (Waltham, USA).
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bottomed dishes and cultured in Dulbecco’s modified Eagle
medium (DMEM) containing 10% fetal bovine serum, 100 I.U.mLÀ1
penicillin and 100 mgmLÀ1 streptomycin at 378C in a 5% CO2
incubator. After 24 h of incubation in DMEM, the cells were
washed twice with DMEM.
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prepared by placing 25 mL of their DMSO stock solutions (2.0 mm)
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at 378C. The cells were then washed with 1 mL DMEM for three
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dark or irradiated at l=365 or 300 nm for a desired time, the
fluorescence imaging of the cells was implemented. Excitation
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Published online on && &&, 0000
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FULL PAPER
Get photoactive: Photoactivatable
aggregation-induced emission (AIE)
fluorophores based on caged salicyl-
aldehyde hydrazone derivatives display
strong solid-state fluorescence upon UV
irradiation, and are successfully applied
for photopatterning and photoactivata-
ble cell imaging in a multiple-color and
stepwise manner (see figure).
&
Fluorescence
L. Peng, Y. Zheng, X. Wang, A. Tong,
Y. Xiang*
&& – &&
Photoactivatable Aggregation-Induced
Emission Fluorophores with Multiple-
Color Fluorescence and Wavelength-
Selective Activation
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Chem. Eur. J. 2015, 21, 1 – 8
www.chemeurj.org
8
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!