A Photoresponsive Smart Covalent Organic Framework

Article abstract of DOI:10.1002/anie.201503902

Ordered π-columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external-stimuli-responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π-building blocks. The COF is switchable upon photoirradiation to yield a concavo-convex polygon skeleton through the interlayer [4π+4π] cycloaddition of anthracene units stacked in the π-columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external-stimuli-induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π-electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.

Full text of DOI:10.1002/anie.201503902

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201503902
German Edition: DOI: 10.1002/ange.201503902
Smart Materials Very Important Paper
A Photoresponsive Smart Covalent Organic Framework**
Ning Huang, Xuesong Ding, Jangbae Kim, Hyotcherl Ihee, and Donglin Jiang*
Abstract: Ordered p-columnar structures found in covalent
organic frameworks (COFs) render them attractive as smart
materials. However, external-stimuli-responsive COFs have
not been explored. Here we report the design and synthesis of
a photoresponsive COF with anthracene units as the photo-
responsive p-building blocks. The COF is switchable upon
photoirradiation to yield a concavo-convex polygon skeleton
through the interlayer [4p+4p] cycloaddition of anthracene
units stacked in the p-columns. This cycloaddition reaction is
thermally reversible; heating resets the anthracene layers and
regenerates the COF. These external-stimuli-induced structural
transformations are accompanied by profound changes in
properties, including gas adsorption, p-electronic function, and
luminescence. The results suggest that COFs are useful for
designing smart porous materials with properties that are
controllable by external stimuli.
channels.[1–7] The p-units in 2D COFs not only constitute p-
columns that control the electronic functions but also shape
the channel walls that form the interface for the adsorption of
gases.[2–4] The integration of stimuli-responsive p-units into
COFs is likely to yield structurally dynamic frameworks in
which the structure can be transformed upon external
stimulation. However, a “smart” COF is unprecedented and
the possibility of structural transformation is to be exempli-
fied.
Herein, for the first time, we report a photo-responsive,
structurally dynamic COF. The photo-responsive 2D COF
(Figure 1a, Ph-An-COF) was synthesized by condensation of
2,3,6,7-tetrahydroxyanthracene and 1,3,5-benzenetriboronic
acid under solvothermal conditions in 92% isolated yield
(Supporting Information, Tables S1–S4; Figures S1–S3). Ph-
An-COF consists of ordered 1D channels that are 2.9 nm in
diameter and periodic p-columns with benzene vertices and
anthracene edges, in which the anthracene units function as
photo-responsive building blocks (Figure 1b–d). Thermogra-
vimetric analysis revealed that Ph-An-COF did not show
decomposition up to 4008C under nitrogen (Figure S2). Ph-
An-COF exhibits an X-ray diffraction (XRD) profile with
prominent peaks at 3.04, 6.02, 6.72, 8.81 and 26.38, which are
assigned to the 100, 110, 200, 210 and 001 facets, respectively
(Figure 1e, blue curve). The presence of a 001 facet at 26.38
corresponds to a p–p stacking distance of 3.4 Š, indicating
that molecular ordering occurs along the direction perpen-
dicular to the 2D planar sheets. Pawley refinement generated
an XRD profile (black curve) that reproduced the exper-
imentally observed curve, as evidenced by their negligible
difference (dotted black curve). Structural simulations with
an eclipsed AA-stacking structure generated an XRD curve
(green curve) that is consistent with the experimental profile.
By contrast, the staggered AB mode (orange curve) could not
reproduce the experimental result. These XRD results
indicate that Ph-An-COF is a crystalline material with 1D
mesopores and ordered anthracene p-columns (Figure 1b).
The anthracene columns in Ph-An-COF are configured in
a face-on-face p-stacking mode with a vertical interlayer
separation of 3.4 Š between two anthracene units. Such
a stacking mode, together with a suitable distance (< 4.2 Š),[8]
is favorable for photoinduced [4p+4p] cycloaddition. As
shown in Figure 1c, photoinduced [4p+4p] anthracene cyclo-
addition is a symmetrically allowed reaction that occurs at the
9- and 10-positions of anthracene (An) and forms concavo-
convex-shaped cycloaddition dimers (AnCD). The cycloaddi-
tion results in a decrement of aromaticity because the
extended p-system is broken in the dimer (Figure 1c, inset),
M
aterials with structures that are transformable in
response to external stimuli, such as light, heat and pressure,
are attracting increasing attention because of their broad
applications in various fields. In particular, when the struc-
tural transformations are accompanied by changes in phys-
icochemical properties, these materials are considered
“smart” and “dynamic” and can function as stimuli-respon-
sive materials. Two-dimensional covalent organic polymers
(2D COFs) and their layered covalent organic frameworks
(COFs) are a class of crystalline porous polymers that allow
atomically precise integration of organic units into periodic
columnar p-arrays and ordered one-dimensional (1D) open
[*] N. Huang, Dr. X. Ding,[+] Prof. Dr. D. Jiang
Department of Materials Molecular Science, Institute for Molecular
Science, National Institutes of Natural Sciences
5-1 Higashiyama, Myodaiji, Okazaki 444-8787 (Japan)
E-mail: jiang@ims.ac.jp
Dr. J. Kim, Prof. Dr. H. Ihee
Center for Nanomaterials and Chemical Reactions
Institute for Basic Science, KAIST
Daejeon 305-701 (Republic of Korea)
[+] Present address: National Center of Nanoscience and Technology,
No.11 ZhongGuanCun, BeiYiTiao, Beijing, 100190 (China)
[**] D.J. acknowledges the support of a Grant-in-Aid for Scientific
Research (A) (24245030) from the Ministry of Education, Culture,
Sports, Science and Technology (Japan) (MEXT). This work was
supported by IBS-R004-G2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503902.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial License, which permits
use, distribution and reproduction in any medium, provided the
original work is properly cited and is not used for commercial
purposes.
À
whereas a direct single C C bond of 1.54 Š connecting the
two units gives rise to a drastic conformational change,
resulting in a shortened distance between the central portions
of the dimeric molecules and in expanded spatial separation
8704
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Figure 1. a) Synthesis of anthracene COF (Ph-An-COF). b) Top and side views of Ph-An-COF (blue: anthracene unit). c) [4p+4p] anthracene (An)
cycloaddition upon irradiation to form the cyclodimer (ANCD) and its thermally allowed reverse reaction (inset: singlet crystal structure of AnCD).
d) Side and top views of the structural changes of one hexagonal macrocycle of the COFs upon photoirradiation and thermal stimulation (blue:
anthracene Unit, purple: cycloaddition dimer of anthracene). e) XRD patterns of Ph-An-COF: experimentally observed (blue curve), Pawley refined
(black curve), their difference (dotted curve), the patterns simulated using the eclipsed AA (green curve) and staggered AB (orange curve)
stacking modes.
at both ends (C-to-C distance of 4.7 Š). This cycloaddition
reaction is thermally reversible, regenerating two planar
anthracene units (Figure 1c).
calculated to be 47% dimers, which is similar to the values of
other anthracene systems reported to date.[8]
The XRD patterns of the photoirradiated COF samples
(Figure 2c, red curve) were similar to that of Ph-An-COF
(blue curve), with similar peak positions of the 100, 110, 200,
210 and 001 facets. Photoirradiation stimulated a cycloaddi-
tion-induced structural transformation, whereas the crystal-
linity of the polygon skeletons was retained (Figure 2c). The
observed decrease in the intensity of the XRD peaks is likely
to be related to the fact that the concavo-convex polygon
skeletons have a relatively flexible conformation, which leads
to weak X-ray diffraction.
With these results in mind, we prepared films of Ph-An-
COF by in situ polymerization to deposit a Ph-An-COF film
(thickness 242 nm) onto a quartz substrate (Supporting
Information). Ph-An-COF exhibited two strong electronic
absorption bands at 278 and 373 nm; these bands were
assigned to the n–p* and p–p* transition bands of the
anthracene units, respectively (Figure 2a, blue curve). Irradi-
ation of the film under Ar with 360 nm light using a xenon
lamp through a band-path filter (360 Æ 10 nm) caused the
intensity of the absorption bands at 278 and 373 nm to
decrease because the cycloaddition reaction disrupted the p-
conjugation of the anthracene units.[8] By contrast, the
intensity of the band at 305 nm, which was assigned to the
n–p* transition band of the cycloaddition dimer, increased.
These spectral changes occurred in conjunction with the
appearance of clear isosbestic points at 289, 325 and 410 nm,
which indicate that the cycloaddition reaction is free of side
reactions and proceeds cleanly to form the cycloaddition
dimers without any byproducts. We monitored the cyclo-
addition reaction using time-dependent electronic absorption
spectroscopy. A plot of absorbance versus time yielded
a linear curve (Figure 2a, inset); the reaction was first
order, with a rate constant of 4.1 ” 10À3 minÀ1. All of these
spectral changes indicate that Ph-An-COF is responsive to
irradiation, which triggers the cycloaddition of the anthracene
units in the p-columns, thus transforming the planar 2D sheets
The alignment of building units along the c-axis in Ph-An-
COF constitutes 1D mesoporous channels, in which the
channel walls may serve as an interface for gas adsorption.
The channel walls of the concavo-convex polygon Ph-AnCD
-
COF differ from those of the Ph-An-COF. As revealed by
nitrogen sorption isotherm measurements, the photoinduced
structural transformation is accompanied by a change in gas
storage capability. For example, Ph-An-COF exhibits typical
type-IV curves, which are characteristic of mesoporous
materials (Figure 2d, blue curve). The Brunauer–Emmett–
Teller (BET) and Langmuir surface areas were evaluated to
be 1864 and 2782 m2 gÀ1, respectively, whereas the pore size
and pore volume were calculated to be 2.9 nm and
1.24 cm3 gÀ1, respectively (Figure 2e and S5), using the non-
local density function theory (NLDFT) method. The photo-
generated Ph-AnCD-COF samples exhibited type-IV sorption
profiles (Figure 2d, red curve) along with BET and Langmuir
surface areas of 1456 and 2142 m2 gÀ1, respectively. In relation
to this observation, the pore volume decreased to 1.08 cm3 gÀ1
(Figure S4). Notably, the pore size distribution profile
revealed that the photo-generated Ph-AnCD-COF consists of
only one type of mesopore, with a pore size of 2.9 nm
(Figure 2 f and S5). This result is reasonable because the
into concavo-convex polygon skeletons (Figure 1d, Ph-AnCD
-
COF). We hydrolyzed the photogenerated Ph-AnCD-COF
1
samples and conducted H NMR spectroscopy, which con-
firmed the production of only cyclic anthracene dimers
(Figure S4). On the basis of the proton integrations, the
photostationary state of the [4p+4p*] cycloaddition was
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The results show that the changes of properties, including
the porosity, p-electronic absorption and luminescence, are
dramatic and correlate with the structural transformation
induced by irradiation. Reversibility of such properties is
highly desired for wide applications of this type of materials
and thus we examined its reversibility. To test the possibility
of thermal reversibility, we heated the photo-generated Ph-
AnCD-COF samples at 1008C in the dark and observed that
the dimers returned back to the anthracene units. Electronic
absorption spectroscopy confirmed that the cycloaddition
dimers were thermally transformed into anthracene units,
showing enhanced absorbance at 278 and 373 nm with clear
isosbestic points (Figure 2b). This spectral change also reveals
that the thermally induced recovery of anthracene units was
nearly quantitative. From the time-dependent profile (Fig-
ure 2b, inset), the reaction was determined to be in first order,
with a rate constant of 1.3 ” 10À3 minÀ1. Time-dependent
fluorescence spectroscopy also suggests that the fluorescence
of the anthracene excimer was recovered upon heating
(Figure S6). Nitrogen sorption isotherm measurements
revealed an enhanced BET surface area of 1684 m2 gÀ1
(Figure 2d, black curve). The pore size and pore volume
were evaluated to be 2.9 nm and 1.14 cm3 gÀ1, respectively
(Figure 2 g and S5). XRD profiles demonstrated that the
COF samples recovered the intensity of the peaks while
retaining the original diffraction patterns (Figure 2c, black
curve). These observations indicate that thermal stimuli
enabled the regeneration of Ph-An-COF. Notably, cycle
performance tests revealed that the external-stimuli-respon-
sive transformation is repetitive (Figure 2h and Figures S7,
S8).
The anthracene units stacked in the p-columns of Ph-An-
COF are responsive to irradiation, which induces interlayer
[4p+4p] cycloaddition reactions, causes conformational
changes of the p-columns, and triggers a structural trans-
formation of the layers. These photoinduced hierarchical
transformations are reversible by virtue of the thermally
allowed reversibility of the cycloaddition reaction. Notably,
the structural transformations are accompanied by profound
changes in properties and functions, including gas adsorption,
p-electronic adsorption and luminescence. Our results dem-
onstrate the first example of photoresponsive structurally
dynamic COFs and suggest that COFs could be designed as
“smart” materials whose gas adsorption, molecular storage,
sensing, and semiconducting properties are controllable by
external stimuli.
Figure 2. a) Electronic absorption spectral changes of Ph-An-COF upon
irradiation at 360 nm (inset: time-dependent profile). b) Electronic
absorption spectral changes of Ph-AnCD-COF upon heating at 1008C in
the dark (inset: time-dependent profile). c) XRD patterns of Ph-An-
COF (blue curve), Ph-AnCD-COF (red curve), and Ph-An-COF (black
curve) generated by heating Ph-AnCD-COF at 1008C under Ar in the
dark. d) Nitrogen sorption isotherm curves (blue: Ph-An-COF; red: Ph-
AnCD-COF; black: Ph-An-COF generated by heating Ph-AnCD-COF at
1008C under Ar in the dark; filled circles: adsorption; open circles:
desorption). Pores size distribution profiles of e) Ph-An-COF, f) Ph-
AnCD-COF, and g) Ph-An-COF generated by heating Ph-AnCD-COF.
h) Cycle performance as revealed by normalized absorbance change.
irradiation mainly causes the structural transformation along
the c-axis, whereas the pore diameter is less affected.
The photoinduced structural transformation significantly
affects the p-electronic properties of the samples. Ph-An-
COF is highly blue-luminescent, exhibiting an emission band
centered at 429 nm, as a result of excimer emission of the p-
stacked anthracene units (Figure S6). The absolute fluores-
cence quantum yield of the Ph-An-COF solid samples was
evaluated to be 5.4%. Irradiation of Ph-An-COF caused
a monotonic decrease of the fluorescence intensity because
the resulting cycloaddition dimers were not luminescent.
Time-dependent spectral changes revealed that 42% of the
anthracene units in Ph-An-COF was transformed into dimers
in the photostationary state, which is consistent with the value
calculated on the basis of the electronic spectral changes.
Keywords: anthracene · covalent organic frameworks ·
photoresponsive materials · porous polymer · smart materials
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Received: April 28, 2015
Revised: May 21, 2015
Published online: June 10, 2015
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