Stable Radical Cation-Containing Covalent Organic Frameworks Exhibiting Remarkable Structure-Enhanced Photothermal Conversion

Article abstract of DOI:10.1021/jacs.9b07695

The production of a radical cation-containing covalent organic framework (COF) has been accomplished by sequential in situ reactions, quaternization, and one-electron reduction of the 2,2′-bipyridine-based COFs. The acid-catalyzed COF formation enables the cis configuration of 2,2′-bipyridyl moieties in the structure, of which the stability arises from the eclipsed stacking of the two-dimensional layered structure. The postfunctionalization generates cyclic alkylated diquats as the sole products from the controlled quaternization. The reduction of diquat cations on the COF skeletons results in a large number of radical cations, which delocalize and uniaxially stack on top of one another by virtue of interlayered ?-electronic couplings. The absorption of the near-infrared (NIR) region exhibited by the cationic radical COF is remarkably high owing to the intercharge transfer across the ?-coupling interlayers. Also, the long-range array of extended and planar frameworks in such a COF leads to the extra stability of the radical cations against external stresses. The structure-enhanced performance of the COF material is witnessed with photothermal conversion efficiencies of as high as 63.8 and 55.2% when exposed to 808 and 1064 nm lasers, respectively. Further PEG modification on such a COF allows photoacoustic imaging and photothermal therapy in vivo under NIR light illumination to be manifested.

Full text of DOI:10.1021/jacs.9b07695

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Article
Stable Radical Cations-Containing Covalent Organic Frameworks
Exhibiting Remarkable Structure-Enhanced Photothermal Conversion
Zhen Mi, Peng Yang, Rong Wang, Junjuda Unruangsri, Wuli Yang, Changchun Wang, and Jia Guo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07695 • Publication Date (Web): 19 Aug 2019
Downloaded from pubs.acs.org on August 20, 2019
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Stable Radical Cations-Containing Covalent Organic
Frameworks Exhibiting Remarkable Structure-Enhanced
Photothermal Conversion
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Zhen Mi†, Peng Yang†, Rong Wang†, Junjuda Unruangsri‡, Wuli Yang*†, Changchun Wang†, and Jia
Guo*†
†State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University,
Shanghai 200433, China.
‡Research group on Materials for Clean Energy Production STAR, Department of Chemistry, Chulalongkorn University,
Bangkok 10330, Thailand.
KEYWORDS: Covalent organic frameworks; Radical cations; Near infrared; Photothermal effect; Redox
ABSTRACT: The production of radical cation-containing covalent organic framework (COF) has been accomplished by sequential
in-situ reactions, quaternization and one-electron reduction, of the 2,2’-bipyridine-based COFs. The acid-catalyzed COF formation
enables the cis-configuration of 2,2’-bipyridyl moieties in the structure, of which the stability arises from the eclipsed stacking of
two-dimensional layered structure. The post-functionalization generates cyclic alkylated diquats as sole products from the controlled
quaternization. The reduction of diquat cations on the COF skeletons results in a large number of radical cations, which delocalize
and uniaxially stack on top of one another by virtue of interlayered π-electronic couplings. The absorption of the near-infrared (NIR)
region exhibited by the cationic radical COF is remarkably high owing to the inter-charge transfer across the π-coupling interlayers.
Also, the long-range array of extended and planar frameworks in such the COF renders the extra stability of the radical cations against
external stresses. The structure-enhanced performance of the COF material is witnessed with the photothermal conversion efficiencies
as high as 63.8% and 55.2%, when exposed to 808 nm and 1064 nm lasers, respectively. The further PEG modification on such the
COF allows photoacoustic imaging and photothermal therapy in vivo under the NIR light illumination to be manifested.
INTRODUCTION
Jiang et al., adopted the post-functionalization to attach radical
containing compounds, i.e. TEMPO, on the side of framework
skeletons providing neutral radicals in the pore cavities.10 The
in-situ redox reaction of COFs is another often-used strategy to
generate charged radicals. Such the methodology requires the
pre-construction of redox COFs with neutral and conjugated
planar molecules such as acenes11,12 and tetrathiafulvalene,13,14
which were converted into charged radical frameworks with
preserved crystallinity. The uniqueness of the charged radicals
locally produced on COFs is that the eclipsed stacking of 2D
organic frameworks could induce the linear alignment of
radicals for the interlayered charge transfer and long-range
migration toward interfaces. However, if the redox modules are
positively charged such as viologens, the overlapping
arrangement of COF layers is severely impaired and thus the
ordering alignment of charged radicals is altered, resulting in
weakened radical properties. Even though the viologen-based
COFs with positively charged skeletons have been constructed
previously, either their weak crystallinity15 or staggered layer-
by-layer arrangement16,17 is far from being satisfactory to allow
further exploration of distinct redox properties induced by the
eclipsed stacking of cationic radical frameworks.
Conjugated radical cations are stable in redox chemistry and
associated to the diverse redox-driven changes. They have laid
the important foundation for
applications such as redox-catalysis, energy storage, and
electrochromic display.1 The stability of the radical cation
species can be elevated through supramolecular assembly into
foldamers,2 dimers,3 and supramolecular organic frameworks.4,5
Nevertheless, highly ordering arrangement of conjugated
radical cations in long-range domains has remained challenging
due to the presence of electrostatic repulsion.
a plethora of advanced
Two-dimensional covalent organic frameworks (2D COFs)
are emerging as a family of novel crystalline materials, which
can be molecularly pre-designed to a highly periodic atomic
framework, and in synthesis, can give the uniaxial stacking of
2D macromolecular frameworks at atomic level.6-9 The
incorporation of organic radicals into COF frameworks has
imparted distinct redox activity to such materials with
prominent optical, electronic and magnetic functionalities.
There are two different designing strategies developed so far.
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Figure 1. (a) The two reversible redox states of diquat. (b) Transformation of Py-BPy-COF into cationic Py-BPy2+-COF and cationic radical
Py-BPy+•-COF by two-step post-modification. During the process, the trans-form of 2,2’-BPy-DCA is converted to the mono-cationic cis
conformer in an acidic condition, and hence enables the formation of cyclic ethylated diquats, which could be reduced further with Na2S2O4.
Herein, we propose a novel strategy to convert a 2,2’-
bipyridine-based COF from neutral to positively charged and
ultimately to cationic radical framework, which enables the
superimposition of redox centers with each other in framework.
The inter-charge transfer, occurring through the π-coupling
multilayers, contributes to the outstanding photophysical
properties, namely near-infrared (NIR) absorption and
photothermal conversion by nonradiative relaxation process.
Furthermore, the structure-to-activity relationship on
photothermal effect has been established to acquire the
exceptionally high efficiency of heat generation from NIR
photoexcitation. Building upon this intriguing phenomenon, the
cationic radical framework has also been explored for
photoacoustic imaging and photothermal therapy in vivo.
RESULTS AND DISCUSSION
We first commenced our studies from the reversible redox
compounds, viologens, in order to bestow radical formation in
COFs. Viologens, (C5H4NR)2n+, with tunable pyridyl groups are
well known as redox active compounds with desirable electron-
accepting ability. Their di-cationic derivatives are air-stable
salts, which can undergo two-step reversible reductions forming
a radical cation and a neutral species, respectively (Figure 1a).
Viologen radical cations are considered among the most stable
organic radicals.18 In light of this, the 2,2’-bipyridine-
containing COF was solvothermally synthesized through the
Schiff-base condensation of 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)
tetraaniline (Py-TA) and [2,2'-bipyridine]-5,5'-dicarbaldehyde
(2,2’-BPy-DCA) (Figure 1b). The reaction proceeded in a
mixed solvent of mesitylene and dioxane (v/v of 1:1) at 120 oC
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for 7 days, catalyzed by 6M HOAc aqueous solution. The
yellow product was obtained in 76.8% yield.
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The use of HOAc solution not only catalyzes the aldimine
condensation, but also facilitates the inter-conversion of the
trans-form 2,2’-BPy-DCA to the more stable mono-cationic
cis-conformer. The pKa value of 2,2’-BPy-DCA was estimated
by potentiometric titration at 3.83 (Figure S1a), which is larger
than the pH (2.60) of the reaction mixture
[mesitylene/dioxane/6M HOAc], indicating that 2,2’-BPy-
DCA should be singly protonated during the reaction. As early
reported, 2,2’-bipyridine exists in a trans-conformation in the
solid state as well as in organic solvents and in basic solutions.19
However, when 2,2’-bipyridine is protonated in acidic solution,
the trans-conformer of mono-cationic species could be
interconverted into the much more stable cis-form. This
transformation arises from the formation of intramolecular [N-
H+•••N] hydrogen-bonding interaction, along with a small
contribution from the increasing extend of π-conjugation.19 To
prove this, the UV-Vis absorption spectra were recorded to
elucidate the favored configuration of 2,2’-BPy-DCA in the
reaction solution. When 6M HOAc was added to the aqueous
solution of 2,2’-BPy-DCA, the two distinct bands appear at 253
nm and 312 nm (Figure S1b), which are both red-shifted when
compared to the solution without HOAc. These two bands were
attributed to the characteristic absorption of the mono-cation of
2,2’-bipyridine. Moreover the spectrum is in line with that
obtained from metal chelated bipyridine compounds such as
[Ni(bipy)3]Cl2,19 implying the cis-configuration of the mono-
cationic 2,2’-BPy-DCA in an acidic medium. The trans di-
cation species were not observed in the 6M HOAc solution
because they only exist in extremely strong acids. As a result,
we can be certain that the large population of cis-form 2,2’-
bipyridyl mono-cations was incorporated into the COF
framework during the reaction. Lastly the cis configuration of
the 2,2’-bipyridine derivatives in Py-BPy-COF is rigidly locked
in place by the extended π-interaction in the stacked layers. This
is an essential prerequisite for the formation of cyclic alkylated
diquats in the post-quaternization of Py-BPy-COF.
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Figure 2. (a) Solid-state 13C CP/MAS NMR spectrum of the Py-
BPy2+-COF. (b) XPS spectra of N 1s and Br 3d (inset) for the Py-
BPy2+-COF.
To optimize the conditions for a quaternization reaction, the
model reaction between 2,2’-bipyridine and 1,2-dibromoethane
was systematically studied. The optimal yield of cyclic diquats
was found in nitrobenzene at elevated temperature and
prolonged reaction time with 1.4 equiv. 1,2-dibromoethane
(Table S1 and Figure S2). Py-BPy-COF was hence quaternized
according to the optimized conditions. During the reaction, a
distinctive change from yellow to brown solution was observed.
The more precise quantitative measurement of quaternization
conversion was performed by potentiometric titration analysis
of the counter anions of the diquats, Br-. The quaternization
conversion can also be reflected from the relative molar ratio of
cyclic diquats in Py-BPy2+-COF to the total of bipyridine
moiety. The conversions depend largely on the reaction
conditions, of which the maximum conversion of 72.7% was
obtained (Table S2 and Figure S3). Although higher
quaternization conversion was achieved when prolonging
reaction time, lower yield was obtained indicating the
possibility of COF destruction. It is noteworthy that only certain
degrees of quaternization can attain layered structure of cationic
frameworks through the multicenter π-π interaction of pyrene
moieties, otherwise the exfoliation of the multilayers is likely.
The quaternized Py-BPy2+-COF was further validated
employing FT-IR spectroscopy. The FT-IR analysis showed the
stretches of methylene (-CH2-) appeared at 2923 cm-1 and 2850
cm-1, indicative of the ethylene groups attached on the Py-
BPy2+-COF framework. The signals were absent in the
corresponding spectrum of Py-BPy-COF (Figure S4). Solid-
state CP/MAS 13C NMR spectrum again verified the presence
of alkyl C(1) at 50 ppm, assigned to the ethylene group, as well
as the other multiple signals corresponding to the pyridine C(2-
6), phenyl C(8-11), pyrene C(12-16) and imine C(7),
respectively (Figure 2a). X-ray photoelectron spectroscopy
(XPS) spectrum provided an insight into the different nitrogen
forms in the quaternized Py-BPy2+-COF. As the broad peak of
N 1s signal was deconvoluted into the three individual forms
(Figure 2b), the imine and diquat forms of nitrogen appearing
at 399.5 eV and 402.0 eV were predominant, while the signal
corresponding to the pyridine nitrogen at 398.3 eV was mostly
negligible. This is in stark contrast to the parent Py-BPy-COF,
where the diquat form of N atom is absent (Figure S5). Also,
the cis-form 2,2’-bipyridyl mono-cations are not found in the
Py-BPy-COF, implying its complete deprotonation after the
wash with plenty amount of deionized water during the work-
up step. As shown in Figure 2b-inset, without deconvolution,
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the Br 3d signal of Py-BPy2+-COF was well fitted into a single Both poly(Py-BPy2+)-A and -B gave no diffraction peaks in the
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peak appearing at 67.6 eV, assigning to the bromide anion. This
strong evidence points toward the formation of cyclic alkylated
diquats as sole species present in Py-BPy2+-COF. The formation
of the linear alkylated diquats species is negligible as no XPS
signal corresponding to bromoalkane was witnessed.
Additionally, the thermal stability of the main skeleton in Py-
BPy-COF was corroborated by the thermogravimetric analysis,
showing that the decomposition was not observed until 400 oC
PXRD patterns (Figure S7), confirming the amorphous nature.
This is largely due to the immense electron repulsion impeding
the formation of layer-stacked structure during the bottom-up
growth and to the milder reaction condition in the post-
polymerization synthesis.
H2N
NH2
o
(Figure S6). The initial weight loss during 250-400 C of Py-
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BPy2+-COF could be attributed to the loss of diquat moieties.
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H2N
NH2
(A)
(B)
+
N
OHC
CHO
OHC
CHO
N
N
N
Br
Br
Dioxane
6M HOAc, r.t. 24h,
Mesitylene/Dioxane (1/1)
6M HOAc, 120oC, 7days,
1. BrCH2CH2Br
2. Na2S2O4
Na2S2O4
Poly(Py-BPy+•)-A
Poly(Py-BPy+•)-B
Figure 4. The syntheses of cationic radical containing polymeric
materials, via the bottom-up (A) and the post-modification (B)
methods, respectively. The quaternization in (B) and reduction
reaction in (A) and (B) followed the typical routes for the post-
modification of Py-BPy-COF.
Figure 3. PXRD patterns of Py-BPy-COF, Py-BPy2+-COF and Py-
The detailed investigation using XPS revealed the poly(Py-
BPy2+)-A gives one single and symmetric peak around 67.6 eV
for the Br 3d signal, which is characteristic of Br- anion (Figure
S8). The result is expected for poly(Py-BPy2+)-A as the
dialdehyde-containing cyclic alkylated diquat was used as a
starting material. Nevertheless, the Br 3d signal in the XPS
spectrum from the poly(Py-BPy2+)-B could be deconvoluted
into the two peaks, one at 67.6 eV corresponding to Br- anion
and the other at 70.5 eV corresponding to bromoalkane. This
reflects that some trans 2,2’-bipyridyl moiety is present in the
poly(Py-BPy2+)-B, resulting in the formation of linear alkylated
quats with remaining terminal bromoalkane moieties. A further
spectroscopic test was carried out to examine whether the linear
alkylated diquats could be reduced to generate radicals by
Na2S2O4 in the same manner as did the cyclized diquats. As
displayed in Figure S9, there is no any change in the whole
spectrum from UV to NIR windows before and after reduction
reaction of linear alkylated diquats, [2,2’-BPy(CH2CH2Br)2]2+.
Furthermore, the colors of the reaction solution remained
unchanged. This observation is in a stark contrast to the feasible
reduction of the cyclic ethylated diquat, [2,2’-BPy(CH2)2]2+, as
BPy+•-COF.
Next, the redox reaction on Py-BPy2+-COF was carried out
o
using aqueous sodium dithionite (Na2S2O4) at 50 C for 24 h.
The color change from brown to dark green solution implied the
successful reduction from cyclic diquats [BPy]2+ to cationic
radicals [BPy]+• moieties, hence giving the cationic radical
containing COF. As evidenced from the powder X-ray
diffraction (PXRD) measurement (Figure 3), the reduced Py-
BPy+•-COF is as crystalline as the quaternized Py-BPy2+-COF.
By referencing to the PXRD pattern of the parent Py-BPy-COF,
the dominant peak at 3.16o corresponds to the (100) reflection
plane, and the others at 4.62o, 6.38o, 9.74o, 12.98o and 23.86o are
assigned to the (020), (220), (330), (440) and (001) lattice
facets.20 Aside from the crystalline structure unchanged, the as-
synthesized Py-BPy+•-COF showed the same FT-IR spectrum
and thermal decomposition tendency with the Py-BPy2+-COF
(Figures S4 and S6), indicating that the framework composition
remained intact through the post-modification.
To evaluate the physical properties arising from the
uniqueness of the COF materials, comparative experiments
were performed to synthesize the cationic radical containing
amorphous polymers via two possible methods. The reaction
between aldehyde-substituted diquats dibromide and Py-TA
was conducted by the bottom-up approach, giving poly(Py-
BPy2+)-A. While the post-modification approach, which was
reminiscent of that employed in the synthesis of the COF
counterpart, provided poly(Py-BPy2+)-B. Both cationic
polymers were in-situ reduced under the identical condition,
affording poly(Py-BPy+•)-A and -B, respectively (Figure 4).
a
comparative model. Therefore, the poly(Py-BPy2+)-B
comprises some ethylated trans-2,2’-bipyridyl bromides which
cannot be further reduced, and some of diquats which after
reduction give rise to the cationic radicals. It should be noted
that the quaternization conversion of the poly(Py-BPy2+)-B was
calculated to be 34.7% (Table S2).
N2 sorption measurement was carried out at 77 K to analyze
the pore characters. As shown in Figure 5a, Py-BPy-COF
features a type IV adsorption isotherm, indicative of a typical
mesopore character. Brunauer–Emmett–Teller (BET) surface
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areas and pore volumes were calculated to be as high as 1903
singly reduced monomer [2,2’-BPy-DCA]+• was much weaker
than those of polymer radicals (Figure S11). This underlines
that the unpaired electrons with the assembled dimers are
readily recombined or quenched in air.
m2/g and 1.8322 cm3/g. In comparison, the other two derivative
COFs give type-I-like adsorption profiles, and smaller BET
surface areas and pore volumes of 461 m2/g and 0.2745 cm3/g
for Py-BPy2+-COF, and 398 m2/g and 0.1984 cm3/g for Py-
BPy+•-COF, respectively. Also, the pore-size distributions of
the derivatized COFs are much broader than that of the parent
Py-BPy-COF and shift to the micropore range of 1-2 nm (Figure
5b). The change in porosity reflects the effect of the
quaternization on the pore channels. The amorphous poly(Py-
BPy2+)-A and -B both had small BET surface areas as well as
flawed pore-size distributions (Figure S10).
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Figure 6. EPR spectra of amorphous poly(Py-BPy+•)-B and Py-
BPy+•-COF.
To investigate the photophysical properties in aqueous
solution, the quaternized Py-BPy2+-COF solids were dispersed
by ultrasound and modified with NH2-ended polyethylene
glycol (PEG) through the Schiff-base reaction between amines
and residual aldehydes on the Py-BPy2+-COFs. FT-IR spectrum
proved that the disappearance of C=O stretching frequency
peak at 1697 cm-1 in Py-BPy2+-COF/PEG signified the
successful chemical functionalization between the aldehyde
group residue of Py-BPy2+-COFs and PEG-amine (Figure S12).
The grafting content of PEG was determined to be 9.9 wt.%
from the TGA curve (Figure S6). Such the external
modification improves hydrophilicity and dispersibility of
COFs in physiological media, while there is only marginal
change in their crystallinity and porosity (Figure S13). The
PEG-modified Py-BPy+•-COF was prepared by the direct
reduction of Py-BPy2+-COF/PEG, by which the amount of
modifying PEG chains has retained (Figure S6). This could be
illustrated as both Py-BPy2+-COF/PEG and Py-BPy+•-
COF/PEG exhibited the similar grain sizes of ca. 90 nm in SEM
images (Figure S14) as well as the similar hydrodynamic
diameters of ca. 164 nm with the equal polydispersity index of
0.12 in aqueous solution (Figure S15). The presence of PEG
altered a positively charged surface to an electrically neutral one
(Figure S16), facilitating the cell membrane crossing as well as
lengthening the circulating time in body.
Figure 5. (a) N2 sorption isotherms of Py-BPy-COF, Py-BPy2+-
COF, and Py-BPy+•-COF. (b) Pore-size distributions of Py-BPy-
COF, Py-BPy2+-COF and Py-BPy+•-COF.
Electronic paramagnetic resonance (EPR) spectroscopy was
employed to provide the radical-forming evidence. With the
same treatment and measurement conditions, Py-BPy+•-COF
and poly(Py-BPy+•)-B both gave the typical localized EPR
signals, while much stronger peak was observed for the Py-
BPy+•-COF, indicating an increment of radical concentration
(Figure 6). This proves again that the formation of cis-2,2’-
bipyridyl moiety is dominant in the bottom-up growth of COF,
thus being more favorable than the amorphous counterpart on
radical production. The g-factor of Py-BPy+•-COF (2.0037) was
almost equal to that of a free electron (2.0023), indicative of an
organic radical character. As expected, the radical signal on the
UV-Vis-NIR spectra provided the evidence for the
absorption in the long-wavelength window of each COFs
(Figure 7). The dispersion of Py-BPy+•-COF/PEG exhibited a
broad featureless band in the range of 600-1300 nm, which the
absorbance was remarkably higher than those of Py-BPy-
COF/PEG and Py-BPy2+-COF/PEG at the same concentration.
The light scattering effect was eliminated by the diffuse
reflectance attachment in measurement. The calculated
extinction coefficients exhibit the similar increasing trends at
the wavelengths of 808 nm and 1064 nm, respectively. The
maximum extinction coefficient of the Py-BPy+•-COF/PEG is
as high as 16.6 (L/g cm) at 808 nm and 15.9 (L/g cm) at 1064
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nm (Table S3). Therefore, such the remarkable long- Herein, the functionalized COF is featured in structure with the
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wavelength absorption can be attributed to the columnar
alignment of diquat radical cations in a long-range order, which
may be constituted by the eclipsed stacks of radical cations
[BPy+•] or mixed-valences of [BPy2+] and [BPy+•] entities
alternating along the stacking axis. Furthermore, the radical
cations-containing COF not only possesses the single occupied
molecular orbital (SOMO), which is higher in energy than the
highest occupied molecular orbital (HOMO), but also generates
the fine electronic bands within the narrow bandgap of
SOMO/HOMO.[21] This results in a broad absorption in the NIR
region and no hyperfine lines being observed in Figure 7.
Additionally, a slight decrease of 5% in absorbance was
observed for the Py-BPy+•-COF after 6 days in an oxygen
deficient condition (Figure S17a), marking the advantage of
stable radical cations embedded in the COF skeletons. While,
the absorptions of amorphous poly(Py-BPy+•)-A and -B at 808
nm and at 1064 nm were distinctively lower than those of the
COF analogues and decreased to the levels of the corresponding
cationic polymers over 6 days. The absorption of cationic
radical monomer, cyclic ethylated [2,2’-BPy(CH2)2]+•, at an 808
nm-wavelength completely disappeared within an hour,
emphasizing the more active and less stable nature of cationic
radical in the monomer compared to the polymer and COF,
respectively (Figure S17b). In addition, the fluorescence
emission was largely reduced for the Py-BPy2+-COF/PEG and
nearly quenched for the Py-BPy+•-COF/PEG when compared
with the Py-BPy-COF/PEG. This demonstrates that the
predominant relaxation of excitons is non-radiative favoring the
heat generation in Py-BPy+•-COF/PEG (Figure S18).
singly reduced radicals that could be delocalized across π-
coupling interlayers. Such the structure-enhanced inter-charge
transfer offers the remarkable photoenergy conversion as we
expected. Therefore, the photothermal performance of cationic
radical COFs has been successively explored in detail. Besides
an 808-nm NIR laser, a 1064-nm NIR laser which is in the range
of second NIR window (1000-1350 nm, NIR-II), has also been
applied. This longer wavelength NIR laser can provide the
maximum penetration through tissues as well as lower
background autofluorescence emission.25
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Figure 8. Elevated temperature change vs. time for the dispersions
of different PEG-modified COFs (100 μg/mL) and PBS as a control
set upon exposure to 808-nm laser (a) and 1064-nm laser (b) for 5
min at a power of 1 W/cm2.
Figure 7. UV-Vis-NIR spectra of the dispersions of Py-BPy-
COF/PEG, Py-BPy2+-COF/PEG and Py-BPy+-COF/PEG in
phosphate buffer saline, respectively, at a concentration of 50
μg/mL. The inset is the photographs of the aqueous dispersions of
the different PEG-modified COFs.
Upon exposure to the two NIR lasers for 5 min, the phosphate
buffer saline (PBS) solution as a blank control experiment
generated insignificant amount heat, signifying negligible
amount of heat being produced by the NIR lasers. Meanwhile,
among all COFs, Py-BPy+•-COF/PEG (100 µg/mL) generated
highest amount of heat resulting in rapid increases in
temperatures up to 75 oC and 65 oC when irradiated with 808-
nm and 1064-nm lasers at 1 W/cm2 power, respectively (Figures
8a and 8b). The heating process could be steadily recycled for
all COFs with the lasers being switched on/off for the five
Photothermal effect generally arises as a result of non-
radiative relaxation upon photoexcitation. Both strong
absorption and low fluorescence quantum yield are significant
factors to maximize the conversion of absorbed photons into
heat. For the first time, we have demonstrated the photothermal
effect promoted by the enhanced π-π interaction between layers
in the keto-enamine-linked COFs,22 albeit with moderate
efficiency compared to other known photothermal agents.23,24
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consecutive runs (Figure S19). Similar temperatures were
treated with Py-BPy+•-COF/PEG (100 µg/mL) were most
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reached in each run. The photothermal conversion efficiency
(PCE) was calculated by the time constant method26 to be
19.3%, 47.2% and 63.8% at 808 nm, and 10.4%, 40.1% and
55.2% at 1064 nm, for Py-BPy-COF/PEG, Py-BPy2+-COF/PEG
and Py-BPy+•-COF/PEG, respectively (Figures S20 and S21).
On the other hand, the discrete radical diquats, [2,2’-
BPy(CH2)2]+•, just gave only 3.7% of PCE at 808 nm under the
identical conditions (Figure S22). The finding strongly supports
the structure-enhanced photothermal effect in the COF
materials. Amid the well-known photothermal agents, the
cationic radical COFs presented here is ranked among the top
materials generating heat when irradiated with NIR lasers
(Figure S23).
noticeably damaged (Figure 9b and 9c). The cumulative
viability was as low as 10% and 12% after illuminated with an
808-nm and a 1064-nm laser, respectively. The confocal laser
scanning microscope (CLSM) again verified the effectiveness
of photothermal ablation of the COF-incubated cells under a
1064-nm NIR laser. It was found that the cell death only
occurred when the laser was on (Figure 9d). The similar effect
was also found using an 808-nm laser (Figure S37).
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As the post modification strategy gives functional groups and
sites in a control manner, the structural effect on the
photothermal efficiency has been investigated in detail. By
varying the quaternization conversion of the COF from 3.3% to
72.7%, it was found that the PCE values of Py-BPy2+-COF/PEG
were largely promoted from 23.1% to 47.2% at 808 nm, and
16.9% to 40.1% at 1064 nm, respectively (Table S4 and Figures
S27-S32). Also, the subsequent redox reaction for the radical
Py-BPy+•-COF/PEG could be optimized to make the PCE
escalated from 54.4% to 63.8% at 808 nm and 46.1% to 55.2%
at 1064 nm (Table S5 and Figures S33 and S34).
As shown in Table 1, the 2-3 times higher PCE values were
obtained from the optimal PEG-modified COFs, in comparison
to those obtained from the PEG-modified amorphous materials.
Irrespective of the preparation methods used, both of the
amorphous poly(Py-BPy+•) afforded the moderate PCE values
of 19-30% (Figures S35 and S36). The underlying mechanism
elucidates that the high heat generation is dependent on the
uniaxial stacking of the cationic radical diquats with COFs,
which facilitates the inter-charge transfer in the long-range
domains. Additionally, as the poly(Py-BPy2+)-B partly
comprises ethylated trans-2,2’-bipyridyl bromide, its post
reduction cannot attain much improvement on photothermal
efficiency.
Figure 9. (a) Cytotoxicity to A549 cells in the presence of PEG-
modified COFs at the concentrations of 25 to 400 μg/mL. Viability
assay of A549 cancer cells incubated with the different COFs and
PBS as a control set, respectively, and followed by illumination of
808-nm (b) and 1064-nm laser (c) at 1 W/cm2 power. (d) CLSM
images of A549 cells incubated in the presence of Py-BPy+-
COF/PEG with or without 1064-nm laser irradiation. The cells
were co-stained by calcein-AM and propidium iodide (PI) for
imaging of live cells (green) and dead cells (red), respectively.
In light of the prominent photothermal conversion of the
COFs both in the NIR-I (750-900 nm) and NIR-II (1000-1350
nm) windows, the photoacoustic (PA) imaging performance
was estimated due to the photothermal-induced thermoelastic
expansion when the pulse NIR laser irradiated on the
photothermal agents (Figure S38a). The PA intensity increased
noticeably at the excitation wavelengths around 700-800 nm
both for the Py-BPy+•-COF/PEG and Py-BPy2+-COF/PEG. That
is presumably owing to the broad and featureless absorption in
the long-wavelength window. The maximum of PA signal was
obtained for the Py-BPy+•-COF/PEG around 700 nm, which
subsides the interference of hemoglobin (760 nm) and
oxygenated hemoglobin (850 nm). In stark contrast, the signal
intensity of Py-BPy-COF/PEG is almost similar with that of
PBS, indicating that the pristine non-modified COF provides no
PA images. Meanwhile, it was found that the PA intensity
linearly increased with concentrations of Py-BPy+•-COF/PEG
and Py-BPy2+-COF/PEG, respectively (Figure S38b and S38c),
demonstrating again that they serve as stable contrast agents in
PA imaging. To testify the in vivo PA imaging, the colloidal
stability of PEG-modified COFs was evaluated in PBS and
serum, respectively, by dynamic light scattering analysis, which
showed no evident difference in the hydrodynamic diameters
(ca. 173 nm in PBS and ca. 184 nm in serum) of the PEG-
Table 1. Comparison of photothermal conversion efficiencies
for the different PEG-modified polymers and COFs synthesized
by the bottom-up (A) and post-modification (B) routes.
Quaternizati
PCE (%)
PCE (%)
on
conversion
(%)
Materials
at 808 nm
at 1064 nm
Poly(Py-BPy2+)-A/PEG
Poly(Py-BPy+•)-A/PEG
Poly(Py-BPy2+)-B/PEG
Poly(Py-BPy+•)-B/PEG
Py-BPy2+-COF/PEG
Py-BPy+•-COF/PEG
100
N.A.
34.7
N.A.
72.7
N.A.
24.6
29.6
21.2
23.7
47.2
63.8
19.5
23.3
15.4
19.1
40.1
55.2
The photothermal ablation of cancer cells with the PEG-
modified COF nanomaterials were examined in vitro in the
MTT assay. Prior to the laser exposure, there was almost no
cytotoxicity against A549 cells in the presence of COFs with
varied concentration, 25-400 µg/mL (Figure 9a). Upon
irradiated with the 1 W/cm2 NIR lasers for 5 min, the A549 cells
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modified COFs (Figure S39). Then A549 tumor-bearing nude 6.62 ± 1.53% ID/g to 3.52 ± 1.44% ID/g, it was shown that the
mice were used as animal models and the Py-BPy+•-COF/PEG
dispersion (6 mg/mL, 100 μL) was administered to the mice via
a tail vein injection. The PA signals in the tumor region were
collected at specific intervals (0, 2, 4, 6, 8, and 24 h). As shown
in Figure 10a, the red spots-distributed region is responsible for
the tumor site, wherein the PA intensity reaches a maximum at
4 h and gradually declines until it nearly vanishes at 24 h. This
implies that the highest accumulation of COF nanomaterials in
tumor takes around 4 h through the enhanced permeability and
retention effect during the circulation in body (Figure S40).
significant amount of the COF uptake reduced in liver, spleen
and lungs, while increased in kidneys. The finding reflects that
the COF nanoparticles could be metabolized after 24 h post-
injection, indicating a good biosafety of the PEG-modified
COFs in vivo. Furthermore, the polyradicals in the COFs
showed a certain degree of stability during the circulation in
body (Figure S41). Since Fe3+ ions could act as optical sensors
to probe the presence of radicals, the Py-BPy+•-COF/PEG could
be found in blood and its concentration was much higher in the
early stage (< 6 h) than at 24 h, which is well in line with the
profile of pharmacokinetics.
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Figure 10. (a) PA imaging in the tumor area after intravenously
injected Py-BPy+•-COF/PEG were recorded at different time points.
(b) Thermal images of A549 tumor-bearing mice exposed to laser
irradiation at 1 W/cm2 power for 5 min after 4 h post-tail
intravenous injection of PBS, and Py-BPy+•-COF/PEG.
Further, in vivo thermal imaging with Py-BPy+•-COF/PEG
was carried out in A549 tumor-bearing mice. The tumor sites in
nude mice were irradiated by 808-nm or 1064-nm lasers,
respectively, at 1 W/cm2 power for 5 min after 4 h post-tail
intravenous injection of Py-BPy+•-COF/PEG and PBS as a
control set. The temperature of the tumor tissue containing Py-
BPy+•-COF/PEG rapidly increased from 31°C to 67.1°C using
Figure 11. The pharmacokinetic profiles (a) and biodistribution (b)
of COF modified with PEG in A549 tumor-bearing mice at 4 h and
24 h post-injection. Error bars indicate the SD (n=4).
o
an 808-nm laser and 30.9 C to 63.2°C using a 1064-nm laser
In the investigation of the anticancer efficiency after
intravenous injection, A549 tumor-bearing BALB/c nude mice
were randomly divided into four groups (n=6) and then the PBS
and Py-BPy+•-COF/PEG dispersion were injected through the
tail veins, respectively. As shown in Figure 12a, no apparent
weight loss could be seen in any groups of mice, indicating that
Py-BPy+•-COF/PEG nanomaterials were reasonably safe in
vivo. Upon exposure to the lasers for 5 min (808 nm or 1064
nm, 1 W/cm2), the tumor with the accumulated Py-BPy+•-
COF/PEG were completely eliminated (Figure 12b). The
physical measurement of sizes and weights of the tumors was
carried out to assure the tumor-eradication in the Py-BPy+•-
COF/PEG administered groups (Figure 12c). The observation
was in contradiction to the control groups administered with
PBS. At the 14th day post-injection, all mice were sacrificed
because of the extensive tumor burden of the control groups.
After that, the major viscera (heart, liver, spleen, lungs, and
(Figure 10b), while only marginal increase in temperature was
observed in the control group. This suggests that the Py-BPy+•-
COF/PEG nanomaterials demonstrate equally effective in vivo
NIR-I and -II photothermal conversions.
To access the circulation capability in blood and the
distribution in organs for Py-BPy+•-COF/PEG, BALB/c mice
were intravenously injected with the Py-BPy+•-COF/PEG
dispersion (6 mg/mL, 100 μL). Blood was collected at given
intervals and the concentration of COFs was measured using
high resolution preclinical PA imaging system as reported.27
There was only 4.18 ± 2.27% ID/g of COFs remaining in the
blood at 24-h post-injection (Figure 11a). Meanwhile, the in
vivo biodistribution of COFs was investigated at 4 h and 24 h-
post injection, respectively. Main organs and tumors were
dissected to quantify the Py-BPy+•-COF/PEG residues (Figure
11b). Besides the tumor with the decreased accumulation from
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Figure 12. (a) Time-dependent body weight curves of nude mice after different treatments. Error bars indicate the SD (n=6). (b) Time-
dependent tumor growth curves after different treatments. Error bars indicate the SD (n=6) and the treatments were performed only once. (c)
Photographs (left) and weights (right) of typical tumors at the end of the experiment (14th day). (d) H&E staining images of major organs
(heart, liver, spleen, lungs, and kidneys). Tissues dissected from each group on the 14th day after different treatments. The scale bar
corresponds to 50 μm. *P < 0.05, **P < 0.01, ***P < 0.001.
Supporting Information
kidneys) were excised and co-stained by hematoxylin and eosin
The Supporting Information is available free of charge on the ACS
Publications website.
(H&E). No obvious tissue damages or noticeable pathological
changes were found in the Py-BPy+•-COF/PEG administered
groups as compared to the control groups (Figure 12d).
Therefore, the in vivo antitumor outcomes agree well with the
in vitro cytotoxicity results, indicating that the Py-BPy+•-
COF/PEG could serve as a promising imaging-treatment
integrated platform.
Materials and methods, UV-Vis-NIR spectra, potentiometric
1
titration curves, H NMR spectra, elemental analysis data, FT-IR
spectra, EPR spectra, fluorescence emission spectra, TGA curves,
PXRD patterns, N2 sorption isotherms, pore-size distributions, FE
SEM images, hydrodynamic diameter distributions, zeta potentials,
XPS spectra, recycling-heating profiles, photothermal efficiencies
calculation and their comparison among the different materials,
CLSM images, in vitro photoacoustic imaging, and time-dependent
photoacoustic signal intensity in tumor sites (PDF)
CONCLUSIONS
In conclusion, we report the conversion of 2,2’-bipyridine-
based COFs to the cationic radical containing framework with
high crystallinity. The long-distance uniaxial stacking of COF
layers could suppress the rotation of 2,2’-bipyridyl moieties
along the central carbon-carbon bond and contribute to the extra
stabilization of the cis-conformer, even in the conditions where
trans/cis conversion is possible. Thus, controlled quaternization
of cis-form 2,2’-bipyridine moieties in the COF renders the
cyclic diquats containing frameworks as well as retains the
structural periodicity. The one-electron reduction on the
quaternized COF generates cationic π-radicals overlapped by an
eclipsed fashion toward a long-range ordering domain. Such
radicals embedded within the COF skeletons are extended,
stable, and responsible for the broad and intense NIR
absorption. More impressively, the cationic radical COF
demonstrates the outstanding structure-enhanced photothermal
conversion, which is accounted for by the extended inter-charge
transfer across the π-coupling interlayers within COF. Hence
the photothermal therapy and photoacoustic imaging in vivo is
applicable upon exposure to 808-nm or 1064-nm wavelength
lasers. As the vast majority of organic mixed valence systems
are charged species, our strategy is a feasible and general
solution for the versatile design of COF-based mixed valence
systems for multidisciplinary applications.
AUTHOR INFORMATION
Corresponding Author
*wlyang@fudan.edu.cn
*guojia@fudan.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work is supported by the NSFC (21774023), STCSM
(18520744800) and State Key Project of Research and
Development (2016YFC1100300).
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