Selective Dual-Channel Imaging on Cyanostyryl-Modified Azulene Systems with Unimolecularly Tunable Visible–Near Infrared Luminescence

Article abstract of DOI:10.1002/chem.201700947

Although organic light-emitting molecules have received a growing attention and applicability in modern bioimaging science, the design and control of complex photoluminescent properties in unimolecularly selective imaging remains a challenging topic. Cons

Full text of DOI:10.1002/chem.201700947

A Journal of
Accepted Article
Title: Selective Dual-Channel Imaging on Cyanostyryl-Modified
Azulene Systems with Unimolecularly Tunable Visible-
Nearinfrared Luminescence
Authors: Yunyun Zhou and Liangliang Zhu
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To be cited as: Chem. Eur. J. 10.1002/chem.201700947
Link to VoR: http://dx.doi.org/10.1002/chem.201700947
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Selective Dual-Channel Imaging on Cyanostyryl-Modified Azulene
Systems with Unimolecularly Tunable Visible-Nearinfrared
Luminescence
Yunyun Zhou,[a] Yaping Zhuang,[a] Xin Li,[b] Hans Ågren,[b] Lin Yu,[a] Jiandong Ding,[a] and Liangliang
Zhu*[a]
Abstract: While organic light-emitting molecules have received a
growing attention and applicability in modern bioimaging science, the
design and control of complex photoluminescent properties in
unimolecularly selective imaging remains a challenging topic. Since
tunable multipathway imaging can be advantagedly connected with
treatment processes in therapy, we here report the integration of an
Azulene, a typical dipolar molecule consisting of an electron-
deficient 7-membered ring and an electron-rich 5-membered ring,
is well known for its particular organization of excited states and
optical tranisitions.[5] Azulene is prone to emit from the S2 or
higher states thus disobeying Kasha's rule.[6] Such a high-energy
radiative decay largely constrains the corresponding emission
wavelength within a UV-Vis spectral region. Although the
regiochemistry with different substitutions on azulene,
interestingly, can help to regulate the S1-S2 energy gap so as to
vary the absorption behaviour,[7] the characteristics of the NIR
emissive pathways for azulene and its derivatives have been
only rarely reported..
azulene and
a cyanostyryl moiety into one skeleton for the
generation of in situ stimuli-responsive luminescent materials, with
the aim to achieve tunable and effective emissions in distinct
channels via smart molecular design on a single-molecular platform.
This strategy takes advantage of 1) the Z/E isomerization of the
cyanostyryl unit that can vary the push-pull effect of the substitution
on azulene, accompanied by altering absorption and emission of
individual excited states, and 2) an optimized excited-state
regulation for opening a nearinfrared emissive channel and making
up for a controllable dual-pathway luminescent system together with
the utilization of visible emission. As exemplified by a demonstration
of manipulating the luminescence at the cell level, the materials
exhibit a superior application potential for unimolecularly selective
imaging, labeling and probing events.
(a)
CN
CN
A1
A2
O
O
O
(Endgroup: PEG2000-yl )
(Endgroup:n-butyl )
n H
H
(b)
CN
CN
H+
+
Microscale and nanoscale photoluminescent techniques have
enabled a grand scientific advancement in bioimaging and
biosensing.[1] Among such techniques, those involving materials
with nearinfrared (NIR) luminescence have attracted
considerable attention as alternative to visible (Vis) light that is
readily hampered by the biological self-luminescent signals and
the limited ability to penetrate in vivo.[2] Although the past years
have witnessed extensive progress in the NIR luminescent
systems for different biomedical applications,[3] the material
design and choice still face with challenges at the molecular
level. For examples, the generation of NIR emission has largely
relied on big and planar π conjugated structures.[4] To seek
simple but rational structural strategy for generating and tuning
of NIR emissions on a unimolecular scaffold, so as to build a
straightforwardly controllable dual-pathway luminescent system
together with the utilization of Vis emission, remains a difficult
but desirable proposition.
Z-Ax
Z-AxH
O
R
O
R
h

R
O
R
O
H
- H+
x = 1, R = n-butyl
+
x = 2, R = PEG2000-yl
CN
CN
E-AxH
Z-form
E-Ax
(OFF)
(c)
330-405 nm
633-710 nm
330-405 nm
633-710 nm
(ON)
Visible Emission
NIR Emission
E-form
[a]
[b]
Dr. Y. Zhou, Ms. Y. Zhuang, Dr. L. Yu, Prof. Dr. J. Ding, Prof. Dr. L.
Zhu
Figure 1. (a) Chemical structures of the cyanostyryl-modified azulenyl
compounds A1 and A2 and the (b) protonation-assistant photoisomerization
process. (c) Optimized conformational (up) Z- and (down) E-forms of the
cyanostyryl-modified azulenyl moiety at the B3LYP/6-31G* level of theory and
the illustration of the tunable dual-pathway emission behaviors: the Vis and
NIR emissions are both quenched in Z-form whereas both strengthened in E-
form.
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science
Fudan University, Shanghai 200433 china
E-mail: zhuliangliang@fudan.edu.cn
Dr. X. Li, Prof. Dr. H. Ågren
Division of Theoretical Chemistry and Biology
School of Biotechnology
KTH Royal Institute of Technology
SE-10691 Stockholm, Sweden
We herein put forward an excited-state regulation approach,
as well as the role of structural rigidity for the azulene system, to
address the possibility of a dual-pathway (Vis and NIR) emission.
Supporting information for this article is given via a link at the end of
the document.
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Cyanostilbene, a polar light-active moiety, provides a tunable
push-pull effect through molecular derivation and Z/E-
isomerization, accompanied by the adjustment of its emission
wavelength and intensity.[8] As the first publication of this series
of work, we demonstrate a unimolecular strategy by combining
azulene and cyanostyryl units into one skeleton for a realization
of the above-mentioned hypothesis from perspective of precisely
photoisomerization was accelerated and the compound reached
a photostationary state shortly within 2 h under at a relatively low
power (8 W).
7
1
NC
4
6
9
2
8
O
3
5
regulating π-electron. We expect that
a
dual-pathway
(a) Z-A1
luminescent system can be built in this way, on accounts of the
excited-state regulation and conformational control via stimuli-
responsive push-pull effect to enable versatile electron-transition
processes.
The demonstrated π-functional skeleton was synthesized by
knoevenagel condensation within only three steps, leaving a
phenolic hydroxyl endgroup for additional derivatizations. In this
(b) Z-A1H
(c) E-A1H
work,
photochemical and photophysical studies in organic solution
(compound A1), whereas PEG2000-yl chain, modified
a
n-butyl group is introduced for fundamental
a
a
(d) E-A1
poly(ethylene glycol) (PEG), is linked to facilitate the water
solubility and biocompatibility for cell imaging (compound A2).
Figure 1 outlines the chemical structures as well as the
protonation-assistant photoisomerization process of the two
compounds. Their preparation route and details are referenced
in Experimental Section and Supporting Information (SI).
(e)
(f)
At the initial state, the cyanostyryl-modified azulene structure
adopts a Z-form evidenced by well-assigned 1H NMR signals
(Figure 2a and S2). Unfortunately, direct photoisomerization was
not observed upon irradiation on Z-A1 either by UV or visible
light (see the unchanged NMR or optical spectra in Figure S2
and S3), probably because the azulene moiety quenched the
intermediate triplet formation making the photoisomerization
insensitive.[9] However, azulene can exhibit stimuli-responsive
Figure 2. Characterization of the protonation-assistant photoisomerization: 1H
NMR spectra (400 MHz, CDCl3) of compound A1 (a) at initial state (Z-A1), (b)
after protonation by TFA (0.3 mol/L, Z-A1H), then (c) after photoirradiation (E-
A1H), and followed by (d) heating (E-A1). 1H-1H COSY spectra of (e) Z-A1 and
(f) E-A1.
behavior in acidic environments[5-7,10]
. Hence we turn to
investigate the synergy effect of pH and light on our compounds.
Upon addition of trifluoroacetic acid (TFA) into Z-A1 solution,
the resonances of proton 2, 3, 4, 5, 8 on the cyanostyryl and 7-
membered rings of azulene underwent a downfield shift in the 1H
NMR spectra, whereas that of proton 1 on 5-membered ring
experienced an upfield shift and proton 7 turned broad (see
Figure 2b and Figure S4). These results confirmed that the
protonation is dominant on the 5-membered ring, yielding a
stable tropylium cation, in accordance with related findings in
literature.[5-7,10] The protonation made the absorption and
emission spectra change slightly (Figure S5 and S6). In contrast,
these optical properties altered greatly when the sample was
photoirradiated.
[11]
Similar to other photoisomerizable species,
the Z-to-E
photoisomerization of compound A1 diminishes the main
absorption band (~420 nm) of the molecule (see Figure 3a and
S8). However, a remarkable absorption band around 680 nm
appeared simultaneously in our case (Figure 3a), featuring that
the S0-to-S1 transition was strengthened by irradiation whereas
the S0-to-S2 one was suppressed. The remarkable alternation of
the different ground-to-excited state transitions can be well
distinguished by naked eye (see the solution color change from
greenish yellow to dark-green in Figure 3a). Our
photoluminescent spectral studies thus revealed that the
compound is able to exhibit an enhancement feature. As seen
from Figure 3b, the E-isomer produces a strong emission at
~480 nm as compared to the relatively quenched emission in its
Z-isomer (see also the photographs under a UV light in Figure
3b). Here the optical properties along with the protonation-
assistant Z/E-photoisomerization process are clearly featured as
the control study without irradiation on the tropylium species that
did not show any apparent absorption and emission changes
(Figure S10 and S11). The S0-to-S1 transition and the
significantly intensified emission are maintained after the
The Z-to-E photoisomerization occurs when the tropylium
species Z-A1H was irradiated by visible light or UV light at 365
1
nm. With the employment of the H-1H COSY spectra to assist
the characterizations, the proton resonances can be well
assigned in both of the Z- and E-forms (Figure 2e and 2f). The
resultant E-isomer was confirmed by the 1H NMR spectrum
where a group of upfield shifted resonances of the cyanostyryl-
modified azulene skeleton were observed except for proton 1
and 7 (Figure 2c and Figure S7). The peaks of proton 1 and 7
shifted oppositely to the others due to their facing of the
electron-withdrawing group -CN in the E-form. The maximum
photoisomerization efficiency is over 40% according to the NMR
integral studies. Upon the irradiation at 365 nm, the Z-to-E
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deprotonation. These results indicate that the E-isomer is also a
stable state. It could be converted back to its Z-form by heating
the sample at high temperature over 65 °C (see Figure S12).
The extinction coefficient of the main peaks for compounds Z-A1,
E-A1, Z-A2, and E-A2 are 5.12×104, 1.52×104, 1.87×104 and
7.54×103 M-1cm-1, respectively.
In addition to figuring out the tunable visible emission (Figure
3b, 4c and S15), we also explored the NIR emissive pathway by
excitation of low energy with appropriate longpass filters applied.
Since a chemical channel for rapid relaxation exists in the S1
state, only emission from higher excited states can be seen for
azulene and its derivatives [6] For the present cases, however, a
substantial NIR emission around 800 nm was observed in the E-
form (Figure 4d and S16), the quantum yield of which (~2%) can
be evenly matched with that of the visible emission band (3.7%).
Such a strong NIR emission can be attributed to 1) the
enhancement of the S0-to-S1 transition that increases the
probability of the radiative decay from the S1 state (referenced
from the UV-Vis spectral change), 2) the conformational
restriction of the E-form that can be helpful for inhibiting the
nonradiative relaxation processes. The time-resolved emission
study indicates a fluorescence mechanism with a short lifetime
of 5 ns (Figure S17), ensuring the emission can be insensitive to
oxidizers or sensitizers (Figure S15 and S16) so as to be fit for
better applications in vivo. In contrast, the NIR luminescence
cannot be observed when the S0-to-S1 transition is unmatched
like for the precursors (Figure S18). Thus, dual-pathway (Vis
and NIR) luminescence for the unimolecular systems can be
realized by structural design and Z/E-isomerization (see the
proposed illustration in Figure 4f). A kit of dual-pathway
luminescent functions is valuable as an optical micro-apparatus
for applications, exemplified by the unimolecular selective
imaging studies discussed below.
The change in luminescent intensity is related to
conformational restrictions of the Z/E-photoisomerization
process. Computational studies at the B3LYP/6-31G (d,p) level
indicate that a twisted intramolecular charge transfer (TICT) [12]
within the cyanostyryl moiety was generated upon photo-
excitation of the initial Z-isomer, followed by intramolecular
rotation of the azulenyl group under no steric hindrance from the
opposite phenoxy group with a dihedral angle of 177.6° (Figure
1b). This conformation allows for an efficient nonradiative decay
path that leads to quenching of the luminescence. Upon
photoisomerization to the E-isomer, the TICT pathway was
inhibited since the molecular rotation was sterically restricted as
evidenced from a relatively rigid conformation with a dihedral
angle of 9.6° (Figure 1b),[13] which in turn presents a strong
emission. Such photophysics essentially originate from the
newly built π-functional skeleton, and compound A2 displays the
identical optical properties as A1 during the Z/E-
photoisomerization process (Figure S13 and S14). In addition to
the major emission band at around 480 nm, the compounds also
showed a tunable long-wavelength emission located at the NIR
spectral region (~810 nm, see insert of Figure 3b). This signal
will be more efficient while also excitated by a NIR light power
source.
Figure 3. Optical properties along with the protonation-assistant
photoisomerization: (a) UV-Vis spectra and (b) emission spectra (λex=330 nm)
of A1 at different states in CHCl3 (5×10-5 mol/L) at rt. The insets shows the
photographs of the solutions with Z- and E-forms, respectively, under daylight
and a UV light (λex = 365 nm). The emission signals in the NIR spectral region
of (b) are also highlighted.
Having characterized the tunable optical transition processes
along with Z/E-isomerization, we now turn to explore the
different luminescent behaviors of these molecules. For a
potential bioimaging application in vivo, luminescence in water
as well as biocompatibility are required. Both the Z- and E-forms
of compound A2 can form vesicular nanostructures in water,
whereby the hydrophobic π-functional skeleton self-organized
internally whereas hydrophilic PEG2000 chain exposed to the
aqueous environment, as shown by the TEM images (Figure 4a
and 4b). Such morphologies are favorable for cell endocytosis
without affecting the luminescent signal expressions (vide infra).
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biological studies,[15] our system compromises the merits of dual-
Figure 4. Tuning for dual-pathway luminescence: TEM images of (a) Z-A2 and
(b) E-A2 prepared from water solution (scale bar: 100 nm). Emission spectra (c:
λex=330 nm, scanning range = 350 - 900 nm, d: λex=710 nm, scanning range =
730 - 900 nm) of A2 at Z- and E-form in water (5×10-5 mol/L) at rt. (e) Proposed
mechanism for the tuning of the dual-pathway luminescence in Z- and E-form.
pathway luminescence and
long wavelength (>700 nm)
excitation. As in situ photoisomerization of A2 undergoes a
relatively slow process in acidic internal environments, the
incubations of normal MC3T3-E1 cells with compound A2 in Z-
or E-form (1 × 10-3 mol/L), in a MEM-α medium at 37 °C for 4 h,
were monitored by confocal microscopy (Figure 5) to showcase
the selective imaging effect. The results indicate that both of the
Z- and E-forms of A2 can be endocytosed by cells for bioimaging
(see the images of bright field and each luminescent channels
for comparison) with low-cytotoxicity (Figure S19). The cell
spreading morphology is quite normal after incubation (see the
Although tunable imaging techniques have been emerging at
the microscale,[14] reports on unimolecular selective imaging
under the control of detection channels are still scarce. In our
work, experiments in living cells were also carried out to further
demonstrate the potential application of the tunable dual-
pathway luminescence provided from the unimolecular platform.
As compared with those tranditional fluorescent probes
(coumarin, diamidinophenylindole, fluorescein, etc.) for
bright field in Figure 5), further indicating that such
a
concentration and exposition time did not cause cell damage
and apoptosis.
Figure 5. Selective dual-channel imaging: representative results of the unimolecular imagings for MC3T3-E1 cells incubated with compound A2 (1 × 10-3 mol/L)
from (left) Vis channel and (right) NIR channel, in a MEM-α medium at 37 °C for 4 h. The brightness intensity changes along the lines within the highlighted areas
are also displayed. Scale bar, 100 μm.
The effect of selective dual-channel imaging can be detected
by confocal microscopes. For the Vis channel, the cell imaging
with compound A2 in the E-form can be clearly observed from
the 405 nm excitation, while the imaging of the same channel is
relatively weak with A2 in the Z-form (Figure 5). The tiny
brightness from the imaging with A2 in Z-form may be distributed
to the cellular self-luminescent signals. The selective imaging
effect from the NIR channel is much remarkable. The imaging
with A2 in the Z-form is completely dark under the 640 nm
excitation, whereas it is greatly strengthened with A2 in the E-
form (Figure 5). The luminescent intensity differences along the
cellular profile can also be highlighted (Figure 5). As we
demonstrated above, the Z/E isomerization allows the emission
of the materials to undergo a significant intensity change both in
the Vis and NIR spectral regions. This exploration suggests that
the conformationally depended photophysical alternation
process also can occur at the cell level, in agreement with the
corresponding photoluminescent behavior in solution. Such
spectrally tunable unimolecular entities might thus present a
promising potential for biomaterial applications at the microscale.
In summary, a cyanostyryl-modified azulene system with in
situ stimuli response and tunable photophysical behavior has
been designed and developed. NIR emissive fashion was
opened in this system and consequently a dual-pathway (Vis
and NIR) luminescent strategy with intensity adjustable was
developed, whereby two factors: the S0-to-S1 transition
regulation and control of molecular conformation, play key roles.
The tunable Vis-NIR emission takes place on a unimolecular
entity. Selective dual-channel imaging using the smart
compound has been demonstrated through intracellular studies.
The present interdisciplinary results can be valuable for inspiring
new design or development of organic intelligent materials with
potential applications for multi-mode light-emission.
Acknowledgements
This work was supported by the NSFC/China (21644005),
National Program for Thousand Young Talents of China, and
State Key Project of Research and Development
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Nakayama, Y. Ohba, H. Katagiri, J. Am. Chem. Soc. 2013, 135, 19095-
19098.
(2016YFC1100300). Y. Z. acknowledges NSFC/China
(21502229) and China Postdoctoral Science Foundation funded
project (2016M601491) for additional research supports. X. L.
and H.Å. thank the Swedish National Infrastructure for
Computing (SNIC) for providing computational resources for
project SNIC 2014-11/31. Y. Z. and L.Z thanks Prof. M. Ji and
Prof. H. Zhu for helpful discussions.
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Keywords: dual-pathway emission • azulene • nearinfrared
luminescence • Z/E-isomerization • selective imaging
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10.1002/chem.201700947
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Yunyun Zhou, Yaping Zhuang, Xin Li,
Hans Ågren, Lin Yu, Jiandong Ding,
Liangliang Zhu*
Selective Dual-Channel Imaging on
Cyanostyryl -Modified Azulene
Systems with Unimolecularly Tunable
Visible-Nearinfrared Luminescence
By integrating an azulene and a cyanostyryl moiety into one skeleton, we
employed the excited-state regulation and conformational control via stimuli-
responsive push-pull effect to enable the generation of NIR emissive channel of the
azulene system, so as to address the dual-pathway (Vis and NIR) emission on
unimolecular scaffold and a well application in selective dual-channel cell imaging.
This article is protected by copyright. All rights reserved.