Pillar[5]arene-BODIPY host-guest interaction induced fluorescence enhancement and lysosomes targetable bioimaging in dilute solution

Article abstract of DOI:10.1016/j.tet.2020.131698

Supramolecular complex by pillar[5]arenes and BODIPY dyad through host-guest interaction was developed and showed aggregation induced emission (AIE) property. ¹H NMR titration and 2D NOESY NMR measurements were performed to evidence the host-gu

Full text of DOI:10.1016/j.tet.2020.131698

Tetrahedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Pillar[5]arene-BODIPY host-guest interaction induced fluorescence
enhancement and lysosomes targetable bioimaging in dilute solution
,
, c, *
Jia-Yi Chen a, Xing-Yu Li a, Jie Wu b, Yongquan Wu b **, Gui-Chao Kuang a
a State Key Laboratory of Power Metallurgy, Central South University, Changsha, Hunan, 410083, PR China
b Key Laboratory of Organo-pharmaceutical Chemistry, School of Chemistry and Chemical Engineering, Gannan Normal University, Ganzhou, Jiangxi,
341000, PR China
c State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai, 200438, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Supramolecular complex by pillar[5]arenes and BODIPY dyad through host-guest interaction was
developed and showed aggregation induced emission (AIE) property. 1H NMR titration and 2D NOESY
NMR measurements were performed to evidence the host-guest interaction and the complex was
formed by 1:1 M ratio. The fluorescence intensities of the BODIPY dyad were enhanced by increasing
solution viscosity or and host-guest supramolecular interaction with pillar[5]arenes. Preliminary living
cell imaging results demonstrated the highly emissive complex showed lysosome targetable property.
© 2020 Elsevier Ltd. All rights reserved.
Received 3 August 2020
Received in revised form
12 October 2020
Accepted 16 October 2020
Available online xxx
Keywords:
BODIPY
pillar[5]arenes
Host-guest
Aggregation induced emission
Supramolecular chemistry
Bioimaging
1. Introduction
temperature, solvents viscosity and polarity, etc. The third strategy
to influence BODIPY dyads emissions by restricting the intra-
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyads
have gained great attention in the last three decades due to their
excellent photophysical properties [1] and various applications [2].
Through structural modification, these BODIPY dyads showed
tunable photophysical properties and three well-reported strate-
gies were developed to regulate these dyads’ fluorescent emission
intensities and wavelength [3]. For the BODIPY dyads containing
donor or acceptor structural moieties, their fluorescence emissions
were quenched by photoinduced electron transfer (PET) [4e6].
After binding or reacting with a chemical, the electron transfer
pathway was blocked and thus fluorescent emission was restored.
Therefore, these BODIPY dyads could be used as fluorescent probes
[7]. Charge transfer (CT) is the second strategy to tune their pho-
tophysical properties [8]. These dyads’ fluorescence properties
were highly correlated with their environment such as solution
molecular rotation at the meso position has just been recognized
about ten years [9e11]. To make these aggregation induced emis-
sion (AIE) dyads to be highly emissive was usually by adding poor
solvents, increasing the solvents viscosity or decreasing tempera-
ture [12e14]. Although some progress has been made in this area,
there are few reports referring to using host-guest interaction to
get highly emissive BODIPY complex. Due to the BODIPY dyads
large size, it is difficult to select a suitable host to capture BODIPY
guest [15,16]. Therefore, to develop supramolecular host-guest
system to make BODIPY highly emissive in solution remains
interesting and challenging.
On the other hand, five kinds of macrocycles behaved as su-
pramolecular hosts have been demonstrated [17,18]. In the past
decade or so, the pillararene-based host-guest chemistry has been
intensively and extensively studied [19]. These macrocycles have
shown a wide range of applications in supramolecular hosts due to
their easy tunable cavity size. The repeating unit could be changed
from 5 to 15 [20]. In addition, appropriate outer structural modi-
fication led to these macrocycles to be water soluble, which would
be very important for these host-guest complexes biological
application [21]. For example, the host-guest complexes between P
* Corresponding author. State Key Laboratory of Power Metallurgy, Central South
University, Changsha, Hunan, 410083, PR China.
** Corresponding author.
E-mail addresses: wyq@gnnu.edu.cn (Y. Wu), gckuang@csu.edu.cn (G.-C. Kuang).
https://doi.org/10.1016/j.tet.2020.131698
0040-4020/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: J.-Y. Chen, X.-Y. Li, J. Wu et al., Pillar[5]arene-BODIPY host-guest interaction induced fluorescence enhancement and
lysosomes targetable bioimaging in dilute solution, Tetrahedron, https://doi.org/10.1016/j.tet.2020.131698

J.-Y. Chen, X.-Y. Li, J. Wu et al.
Tetrahedron xxx (xxxx) xxx
Scheme 1. Molecular structures and cartoon representation of BDP-1 and P[5].
Scheme 2. Synthetic route for BDP-1.
[5] hydrophobic anticancer drug tamoxifen could be efficiently
delivered to cancer cells and showed enhanced pesticide effect [22].
Inspired by the well-known AIE fluorogen tetraphenylethene de-
rivatives exhibited strong fluorescent emission with P[5] [23,24],
herein, we designed a BODIPY dyad with pyridinium unit at meso
position. Considering the similar structural moiety baring at the
different dye, we expected that this BODIPY dyad could form host-
guest supramolecular complex with P[5].
Our group has been interested in water soluble BODIPY de-
rivatives since 2014 [25e32]. We found the dendritic oligo(ethylene
glycol) modified BODIPY showed enhanced fluorescent intensity
during the thermoresponsive process [25]. Detailed photophysical
studies demonstrated that, due to dehydration of oligo(ethylene
glycol), the microviscosity around the BODIPY dyad increased,
which restricted the intramolecular rotation and hampered
photoelectron transfer pathway. In this work, a novel BODIPY de-
rivative BDP-1 was prepared and its host-guest complex with water
soluble P[5] was investigated (Scheme 1). BDP-1 showed enhanced
fluorescence emission in viscous solutions. The supramolecular
complex formed by BDP-1 and P[5] through host-guest interaction
was examined by Proton nuclear magnetic resonance (1H NMR)
titration and 2D nuclear Overhauser effect spectroscopy (NOESY)
NMR. The fluorescent intensities of the complex exhibited gradu-
ally enhancement during the P[5] addition. Preliminary living cell
imaging results demonstrated the highly emissive complex could
stain lysosomes specifically.
Scheme 3. Synthetic route for P[5].
be sensitive to solvents polarity. The fluorescence spectra of BDP-1
were collected and showed apparent solvents dependent (Fig. S1).
Weak fluorescent emission could be detected in polar solvents such
as methanol, N,N-dimethylformide (DMF), acetonitrile (MeCN) and
dimethylsulfoxide (DMSO), but strong fluorescent emission was
observed in non-polar dioxane and carbon tetrachloride.
2. Results and discussions
2.1. Synthesis and characterization
The fluorescence properties of BDP-1 were correlated to the
solvents viscosity as well. Due to rotation of the single bond be-
tween the phenyl linkage and BODIPY in free state, BDP-1 showed
weak emission in good solvents because the energy was lost
through non-radioactive pathway. In viscous solution or aggre-
gated state, the intramolecular rotation was restricted and the
fluorescence intensity would be restored. Therefore, the photo-
physical properties in mixture solvents were investigated. When
the BDP-1 was dissolved in water/THF mixture solvents, the fluo-
rescence emission intensities increased gradually with the fraction
of water increase, indicating the formation of BDP-1 aggregate.
However, the intensity was still weak even when the fraction of
water reached 99% (Fig. 1a). These results demonstrated that BDP-1
showed good dispersity in polar aqueous solution because of the
hydrophilic pyridinium units. Therefore, we further conducted
another fluorescent measurement by dissolving BDP-1 in glycerol/
water with varying fraction of viscous glycerol. Naked eye obser-
vation revealed dramatic intensities changes (Fig. S2). Further
quantitative measurements were performed by fluorescent spectra,
which showed about 105 times intensity enhancement (Fig. 1b).
Two preliminary conclusions could be drawn from these results.
BDP-1 was synthesized through three steps reaction (Scheme
2). Starting from 4-bormobenzaldehyde (1) and 4-pyridylboronic
acid (2), compound 3 could be obtained after Suzuki coupling re-
action with catalytic palladium triphenylphosphine. Thereafter, one
pot classic acid-catalysis condensation, oxidation by p-chloranil
and complexation with boron trifluoride etherate (BF3$Et2O) re-
actions gave rise to BODIPY derivative 4. Followed by reaction with
iodomethane, our target guest molecule BDP-1 was obtained.
While the host molecule P[5] was synthesized according to re-
ported procedure (Scheme 3) [33]. Detailed synthesis and charac-
terization for both host and guest molecules have been
demonstrated in Experimental section.
2.2. Photophysical properties
The photophysical properties of BDP-1 in various polarity sol-
vents and viscous solution were investigated. Charge deficient
methyl pyridium unit behaves as acceptor while BODIPY fluorogen
becomes as donor. Therefore, strong CT effect would lead to BDP-1
2

J.-Y. Chen, X.-Y. Li, J. Wu et al.
Tetrahedron xxx (xxxx) xxx
Fig. 1. Fluorescent spectra of BDP-1 in (a) water/THF; (b) glycerol/water mixture sol-
vents. Condition: concentration is 1
nm/10 nm.
mM, excitation wavelength is 504 nm, slits are 10
First, BDP-1 would show very strictly confined environment that
the intramolecular rotation has been restricted. Second, the intra-
molecular charge transfer was not the dominant factor to influence
the fluorescent emission because in the polar viscous solution, the
fluorescent emission was very strong.
2.3. Host-guest interaction
Fig. 3. 2D NOESY NMR spectrum of supramolecular complex BDP-1/P[5]. Measure-
ment condition: the concentration of BDP-1 is 5 mM, solvent is D2O/CD3OD ¼ 4/3. The
molar ratio of BDP-1/P[5] is 1:1.
With above results in hands, the host-guest supramolecular
complex between BDP-1 and P[5] was investigated. 1H NMR
titration tests were performed to determine the molar ratio of BDP-
1/P[5] complex (Fig. 2). We chose deuterated water and methanol
mixture solvents for 1H NMR measurement due to bad solubility of
BDP-1 in water. BDP-1 exhibited well-resolved peaks in both aro-
matic and aliphatic regions. However, after 0.25 M ratio of P[5]
addition, the peaks in low field region showed dramatic changes,
that is, these peaks became broad, shifted to high field and merged.
The terminal methyl group at 5.85 ppm also shifted up-field a little
bit. When the molar ratio of BDP-1/P[5] reached to 1:0.5, these
aromatic peaks showed small shift but their intensity decreased
further, which indicated the strong host-guest interaction. After
molar ratio reached 1:1, no peak shift was observed. This result
demonstrated that the supramolecular host-guest complex was
formed with 1:1 M ratio. Job plot analysis further confirmed this
result (Fig. S3). The binding constant was determined to be
first one was attributed to correlation from the Ha and H8, which
should be strong evidence of the supramolecular complexation
because large space existed between the two protons. Another spot
was ascribed to Hb and H1, which was merged with Hc and Hd.
Considering the space between Hc,d and Hb, this spot should be
generated from new supramolecular complex.
2.4. Cell imaging
The size and morphology of this supramolecular complex in
aqueous solution were investigated by dynamic light scattering
(DLS) and scanning electron microscopy (SEM). The average size of
the complex aggregate in aqueous is around 190 nm (Fig. 4a), which
was further evidenced by the tube morphology from SEM image
study (Fig. 4b).
The fluorescence titration measurement was performed by
addition P[5] to BDP-1 dilute aqueous solution (Fig. 5). The com-
plex showed gradually enhanced fluorescent intensities with P[5]
addition. The original emission might from partially aggregate of
BDP-1 due to its bad solubility in water. As P[5] addition, the su-
pramolecular host-guest interaction might work and BDP-1 would
be encapsulated to the hydrophobic cavity. Thus, the intra-
molecular rotation would be restricted, which leads to the
enhanced fluorescent emission.
3600 Mꢀ1
.
To further investigate the host-guest complex, 2D NOESY NMR
was performed (Fig. 3). Beside the correlation spots between Hc,d
and H3, 6, 7, two characteristic spots could be clearly observed. The
Motivated by the above photophysical studies, cell image tests of
Fig. 2. 1H NMR titration spectra of complex with varying molar ratio of BDP-1/P[5], (a)
1:0; (b) 1:0.25; (c) 1:0.5; (d) 1:1 and (e) 1:2. Measurement condition: the concen-
tration of BDP-1 is 5 mM, solvent is D2O/CD3OD ¼ 4/3.
Fig. 4. (a) DLS result and (b) SEM image of supramolecular complex BDP-1/P[5] in
aqueous solution. Measurement condition: Concentration is 5 mM, molar ratio is 1:1.
3

J.-Y. Chen, X.-Y. Li, J. Wu et al.
Tetrahedron xxx (xxxx) xxx
images revealed that the green emissive spots were well dispersed,
which implied us that the supramolecular complex might stain
specific cell organ. Based on our previous experiences and the
positive pyridinium structural moieties, we proposed that these
aggregates might stain lysosomes. Therefore, co-staining experi-
ment with the commercial Lyso-Tracker Red (LTR) was performed.
The merged yellow image (Region 1, 2, 3 and 4) revealed that our
supramolecular complex was lysosome targetable (Fig. 6i), which is
consistent with our previous observation for supramolecular
polymer [32].
3. Conclusions
In summary, a novel BODIPY based supramolecular complex
BDP-1/P[5] was prepared. The host-guest interaction between
BDP-1 and P[5] was evidenced by 1H NMR titration and 2D NOESY
NMR measurements. The fluorescence intensities of the BODIPY
dyad were enhanced by increasing solution viscosity or host-guest
supramolecular interaction with pillar[5]arenes. Bioimaging
studies indicated the supra-molecular complex BDP-1/P[5] could
stain lysosomes specifically. Further studies along this line to pre-
pare other functional BODIPY dyads are undergoing in our
laboratory.
Fig. 5. Fluorescent spectra of supramolecular complex BDP-1/P[5] in dilute aqueous
solution with increasing molar ratio of P[5]. Condition: BDP-1 concentration is 10
excitation wavelength is 504 nm, slits are 10 nm/10 nm.
mM,
the supramolecular complex were conducted. Michigan Cancer
Foundation-7 (MCF-7) cells were selected for bioimaging. After
staining these cells for certain time, BDP-1 and BDP-1/P[5] com-
plex exhibited apparent intensities difference (Fig. 6). Laser scan-
ning fluorescence microscopy (LSFM) results presented that, after
1 h incubation, BDP-1 only displayed weak fluorescent spots
(Fig. 6b), while the supramolecular complex BDP-1/P[5] showed
strong green emission (Fig. 6e). This result indicated that the strong
fluorescent emission came from the supramolecular host-guest
interaction. Further detailed observation of the complex cell
4. Experimental
4.1. Materials
All chemical reagents were commercially available and used as
received unless otherwise stated. Dichloromethane (DCM) was
dried by standard methods using CaCl2 before distillation. All re-
actions were performed under nitrogen atmosphere. Analytical
thin-layer chromatography (TLC) was performed using TLC plates
pre-coated with silica gel (TLC: 10e40
mm, 0.20 0.03 mm). Flash
column chromatography was performed using 40e63
mm
(230e400 mesh) silica gels as the stationary phase. MCF-7 cells
were provided by the Institute of Biochemistry and Cell Biology
(Chinese Academy of Sciences) and were grown in RPMI 1640
(Roswell Park Memorial Institutes’ Medium) at 37 ꢁC under 5% CO2.
Commercial Lyso-Tracker Red (LTR) was purchased from Beyotime
(China). Confocal fluorescence imaging was performed on an
inverted microscope (IX81, Olympus) with a confocal scanning unit
(FV1000, Olympus).
4.2. Equipments
The 1H and 13C NMR spectra were obtained from a Bruker
Advance spectrometer at 500 and 125 MHz, respectively. All
chemical shifts were reported in
d units relative to tetramethylsi-
lane (TMS). A Hitachi U-5100 and Hitachi F-2700 were used to
measure UVevis absorption and fluorescence emission spectra,
respectively. High resolution mass spectra were obtained by using a
Tsq quantum access max from Thermo. The diameter of the com-
plex was determined at 298 K on ZEN3600 MALVERN via dynamic
light scattering (DLS). Scanning electron microscopy (SEM) mea-
surements were conducted in a FEI SIRION200 microscopy with an
accelerating voltage of 10 kV.
4.3. Synthesis
Fig. 6. (a, d) Bright-field images, (b, e) confocal luminescence of MCF-7 cells and (c, f)
overlay images of the bright-field and the confocal images incubated with BDP-1 and
4.3.1. Synthesis of compound 3
BDP-1/P[5] complex (10
MCF-7 cells stained with BDP-1/P[5] complex (10
m
M) for 1 h, respectively. Confocal fluorescence images of (g)
M) and (h) Lyso-Tracker Red (LTR,
Compound 3 was prepared according to the reported literature.
Compound 1 (184 mg, 1 mmol) and solution of K2CO3 (80 mg,
m
[33]
200 nM). (i) Overlay image of (g) and (h). Excitation wavelength is 515 nm, emission
wavelength is from 525 to 545 nm, LTR: Excitation wavelength is 543 nm, emission
wavelength is from 580 to 600 nm.
0.57 mmol, H2O) were added into 1,4-dioxane (60 mL) in flask
under N2 atmosphere. The mixture was frozen by liquid nitrogen
4

J.-Y. Chen, X.-Y. Li, J. Wu et al.
Tetrahedron xxx (xxxx) xxx
and degassed for 5 min. After that, compound 2 (145 mg, 1.2 mmol)
was added into flask. After dissolving, the system was frozen,
pumped for three times. After this procedures, palladium triphe-
nylphosphine (12 mg, 0.01 mmol) was added into this system. Then
the mixture was heated to 110 ꢁC and kept stirring for 20 h. When
the system cooled to room temperature, solid was obtained by
evaporating the solvents. The crude product was purified by silica
gel chromatography with PE: DCM ¼ 1:1 to get brown powder 3
https://doi.org/10.1016/j.tet.2020.131698.
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The authors declare that they have no known competing
financial interests or personal relationships that could have
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This work was supported by the State Key Laboratory of Powder
Metallurgy, Central South University, the National Natural Science
Foundation of China (21501031), the Natural Science Foundation of
Hunan (2018JJ2487, 2020JJ4679), the Natural Science Foundation of
Jiangxi, China (20151BAB213001), State Key Laboratory of Molec-
ular Engineering of Polymers (Fudan University) and Beijing Na-
tional Laboratory for Molecular Sciences (BNLMS201829).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
5