Azobenzene-Equipped Covalent Organic Framework: Light-Operated Reservoir

Article abstract of DOI:10.1021/jacs.9b09643

Light-operated materials have gained significant attention for their potential technological importance. To achieve molecular motion within extended networks, stimuli-responsive units require free space. The majority of the so far reported 2D-extended org

Full text of DOI:10.1021/jacs.9b09643

Subscriber access provided by Université de Strasbourg - Service Commun de la Documentation
Article
Azobenzene-Equipped Covalent Organic
Framework: Light-Operated Reservoir
Gobinda Das, Thirumurugan Prakasam, Matthew A. Addicoat, Sudhir Kumar Sharma, Florent
RAVAUX, Renny Mathew, Maria Baias, Ramesh Jagannathan, Mark A. Olson, and Ali Trabolsi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09643 • Publication Date (Web): 27 Oct 2019
Downloaded from pubs.acs.org on October 27, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
Azobenzene-Equipped Covalent Organic Framework: Light-
Operated Reservoir
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Gobinda Das,a‡ Thirumurugan Prakasam, Matthew A. Addicoat, Sudhir Kumar Sharma, Florent Ravaux,
a‡
b
c
d
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
a
c
e
a
Renny Mathew, Maria Baias, Ramesh Jagannathan, Mark A. Olson, Ali Trabolsi*
Chemistry Program, New York University Abu Dhabi (NYUAD), Saadiyat Island, United Arab Emirates
a
b
c
School of Science and Technology, Nottingham Trent University Clifton Lane, NG11 8NS Nottingham (UK)
Engineering Division, New York University Abu Dhabi (NYUAD), United Arab Emirates
d
e
Electrical and Computer Engineering Department, Khalifa University, 127788 Abu Dhabi, United Arab Emirates
School of Pharmaceutical Science and Technology, Tianjin University, Tianjin, P. R. China
ABSTRACT: Light-operated materials have gained significant attention for their potential technological importance. To
achieve molecular motion within extended networks, stimuli-responsive units require free space. The majority of the so
far reported 2D-extended organic networks with responsive moieties restrict their freedom of motion on account of their
connectivity providing constrained free volume for efficient molecular motion. We report here a light-responsive
azobenzene-functionalized covalent organic framework (TTA-AzoDFP) designed in a way that the pendent azo groups
are pointing towards the pore channels with sufficient free volume necessary for the unencumbered dynamic motion to
occur inside the pores of the COF and undergo a reversible trans-cis photoisomerization upon light irradiation. The
resulting hydrophobic COF was used for the storage of rhodamine B and its controlled release in solution by the
mechanical motion of the azobenzene units triggered by ultraviolet (UV) light irradiation. The TTA-AzoDFP displayed
unprecedented photo-regulated fluorescence emission behavior upon UV light irradiation. Size, emission, and degree
of hydrophobicity with respect to trans-cis-trans photo-isomerization could be reversibly controlled by alternating UV
and visible light exposure. The results reported inhere demonstrate once again the importance of the careful design of
the linkers to allow not only the incorporation of molecular switches within the chemical structure of COFs but also to
provide the required free space for not hindering their motion. The results demonstrate that; responsive COF could be
suitable platforms for delivery systems that can be controlled by external stimuli.
2
0, 23-24, 34-36
INTRODUCTION
Photo-responsive materials,1-8 that can change their
organic frameworks,
structures.
and other nano-
37-38
structures and properties in response to external stimuli
Covalent organic frameworks (COFs)39-42 are a class of
robust two- and three-dimensional extended network
materials that are known for their artistic structures and
5, 9-10
11-12
11
13
such as light,
pH,
chemicals, and heat, have
variety of
attracted tremendous attention for
applications in drug delivery, smart windows, and
molecular machines.
stimuli, light is one of the most appealing and
environmentally friendly stimuli that has been frequently
used for a variety of purposes. In particular, the merits
of photo-responsive materials capable of capturing and
releasing guest molecules in response to light irradiation
a
14
15
43
potential for a wide range of applications, such as gas
7,
16-17
44
45
chemical sensing,46-50 and
Compared to other external
storage,
separation,
The strategic incorporation of functional
groups on the framework building blocks is a key way to
introduce “triggers” into the COF structure, thus allowing
them to change their properties in response to external
stimuli. A key parameter of this strategy is to create
what has been identified as the free volume needed for
an uninhibited molecular motion to occur if applicable.
The integration of azobenzene derivatives into COFs
was reported by a number of research groups,
far, none of them have shown complete reversible photo-
switching behavior. Such is the case because there
exists restrictions to the free motion of the azobenzene
units, which were always used as the linkers that form
5
1-53
catalysis.
17
18
54
8,
11,
16-18
have garnered considerable regard.
19
55-
Azobenzene
is one of the most common light-
57
responsive molecular switches that can be reversibly
switched between its trans and cis isomers by applying
light of a particular wavelength. Azobenzene-based
compounds form the cis-isomer upon exposure to UV
58-61
but so
20-21
light,
while the reverse trans-isomerization reaction
2
2-24
can be achieved upon visible light irradiation.
This
59
photo-isomerization process induces a large geometrical
change with the non-planar cis conformer having a
shorter distance between the aryl termini (d4–4′ ≈ 6 Å)
the COF wall and are not oriented towards the pores
with enough free space that permits the isomerization to
20
occur. Zhang et al. reported the trans-cis isomerization
24-
compared to the planar trans conformer (d4–4′ ≈ 9 Å).
of azobenze in imine-linked porous covalent organic
polymers featuring pendent azo functional groups.
However, due to the low crystallinity of their polymers,
25
On account of these significant structural and
geometrical changes, photo-active azobenzene
molecules were widely incorporated into a variety of
20
conclusive structural elucidation was not successful
26-32
33
supramolecular polymeric materials,
gels, metal
ACS Paragon Plus Environment

Journal of the American Chemical Society
and reversible photo-switching behavior was not Synthesis and characterization
Page 2 of 13
2
0, 62
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
explored in those polymers.
The photo-switchable COF (TTA-AzoDFP) was
synthesized by the condensation of TTA (42.3 mg, 0.30
mmol) with Azo-DFP (106.2 mg, 0.45 mmol) in 3 mL of a
To overcome these shortcomings, we envisioned a
strategy to introduce light-responsive azobenzene units
as dangling groups within the COF pore structure (Figure
1) with enough free space that permits their photo-
isomerization. The COF was obtained from the imine
2
1,4-dioxane:H O (1:1, v:v) mixed solvent in the presence
of a catalytic amount of acetic acid in a 25 mL high-
pressure flask under solvothermal conditions at 120 °C
for 1 hour, which afforded a yellow colored powder. The
product was purified by washing with 1,4-dioxane and
ethanol and subsequently dried at 110 °C for 12 hours.
The product was completely insoluble in water and
common organic solvents (details in SI).
condensation
triyl)trianiline
of
4,
(TTA)
4',
4"-(1,3,5-triazine-2,4,6-
and (E)-4-(4-
(
(
phenyldiazenyl)phenyl)pyridine-2,6-dicarbaldehyde
Azo-DFP, Scheme S1, SI). The synthesized material
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
displays some degree of crystallinity and the installation
of dangling trans azobenzene groups inside the COF
pores permitted achieving a reversible light-responsive
trans-cis-trans photo-isomerization within the COF cavity
without any significant degradation of the framework.
The formation of imine (C=N) bonds was confirmed by
13
FT-IR spectroscopy (Figure S1) and solid-state C NMR
spectroscopy (Figure S2). The TTA-AzoDFP displayed
1
an IR-stretching band at 1619 cm confirming the
formation of imine linkages. Furthermore, disappearance
of the stretching signals of the primary amine (∼ 3200
1
1
cm ) and aldehyde (1693 cm ) groups indicated
13
complete consumption of the starting precursors.
cross-polarization magic-angle spinning (CP/MAS) NMR
(Figure S2) spectrum of the product showed
C
a
characteristic signal at ∼ δ = 160.5 ppm, which could be
attributed to the imine groups (–C=N). The absence of
any aldehyde signal at δ = 190 ppm indicates the
consumption of the Azo-DFP precursor. A typical peak at
around δ = 152.7 ppm was attributed to the azo-linked
20
aromatic carbons (–CAr–N=N–CAr–). The peak at δ =
69.3 ppm could be ascribed to the triazine ring present
1
63
in TTA-AzoDFP. Thermogravimetric analyses (TGA)
revealed that the TTA-AzoDFP is thermally stable up to
390 °C (Figure S3) without obvious weight loss.
Figure 1: Synthesis of TTA-AzoDFP obtained under
solvothermal conditions (ST) at 120 C in a 1:1 (v:v) 1,4-
2
dioxane:H O mixed solvent.
The azobenzene-functionalized COF was hydrophobic
and could easily trap hydrophilic cargo such as
Rhodamine B (RhB) in water. Upon UV-light irradiation,
the mechanical motion of the trans to cis- isomerization
facilitated the release of the encapsulated cargo. This
example of photo-responsive COF can be used as a
platform for light-triggered cargo release through the
isomerization of the azobenzene units within the COF
pores. Thus, this design strategy is capable of providing
COFs as light-controlled reservoirs.
Figure 2: Microscopic characterization of the as-
synthesized trans-TTA-AzoDFP. SEM (a), TEM (b, c),
and AFM (d, inset showing surface roughness).
The size and surface morphology of the as-synthesized
trans-TTA-AzoDFP were characterized by scanning and
transmission electron microscopy (SEM and TEM), as
well as atomic force microscopy (AFM). The SEM image
RESULTS AND DISCUSSION
ACS Paragon Plus Environment

Page 3 of 13
Journal of the American Chemical Society
3
shown in Figure 2a displays uniform aggregated
an accessible volume of 2965 Å . The spacing between
the π−π stacking planes was found to be ~3.6 Å and
~3.9 Å for the trans and the cis form, respectively. In
both isomers, twisting of the azobenzene moieties
causes distortion of the π–π stacking interactions
between the COF layers, which would lead to PXRD
peak broadening in wide angle region compared to our
earlier reported azobenzene free TTA-DFP COF (Figure
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
spherical particles with a mean diameter of ca. 115 nm.
The spherical morphology and the size of the COF were
confirmed using HR-TEM. Closer inspection of the HR-
TEM images revealed that the spherical particles have
rough surfaces and the diameter of the particles was
found to be 117 ± 1.0 nm (Figures 2b, 2c and S4).
Spherical shape and roughness of the surface were also
observed in AFM images (Figures 2d and S5) with an
average diameter of 120 nm, which is in good agreement
with the dimensions observed by SEM and TEM.
64
S8).
Photo-isomerisation of TTA-AzoDFP
The trans/cis photo-isomerization process was first
studied for the azobenzene-functionalized diformyl
pyridine monomer (Azo-DFP) and was monitored in
solution by H-NMR. The solution H NMR spectra of the
Azo-DFP ligand were recorded before and after UV light
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Structural analysis
The as-synthesized trans-TTA-AzoDFP COF exhibits a
powder X-ray diffraction (PXRD) pattern (Figure 3a) with
prominent peaks at 5.8° and a relatively broad peak at
1
1
1
26.3°, which are assigned to the 110, and 003 facets,
(360 nm) irradiation. The stacked H NMR plot (Figure
respectively. The structural model was constructed
based on distorted hcb layered topology in trigonal P1
space group. Models of trans-TTA-AzoDFP COF in
eclipsed (AA) and slipped-AA configurations were made
and compared to the experimental PXRD pattern. Slip-
AA stacking was found to match the experimental PXRD
and is energetically favorable (Figures 3b and S6). The
COF adopts a layer stacked structure with unit cell
parameters of a = 32.9 Å, b = 32.8 Å and c = 6.9 Å. The
3
S9) shows several new peaks when a CDCl solution of
Azo-DFP at room temperature was exposed to UV light
for 15 minutes. After the trans-to-cis isomerization of
Azo-DFP, the proton signals of the aldehyde (H
10.27 and H - δ 8.49 ppm, Figure S9 for proton labeling)
show slight up-field shifts of 0.05 and 0.13 ppm,
f
- δ
e
67
respectively. In a similar fashion, the signals for the
aromatic protons of the azobenzene unit (H δ = 7.54
ppm, H δ = 7.98 ppm, H δ = 7.93 ppm, and H δ = 8.10
a
b
c
d
2,6-aldehyde groups in the pyridine linker connect non-
ppm) exhibit significant up-field shifts of 0.26, 1.08, 0.26
and 1.10 ppm respectively. Those up-field shifts are
caused by mutual shielding of the pyridine and phenyl
rings connected to the azo bond. After continuous
irradiation of the Azo-DFP solution with UV light for more
than 180 minutes, 76 % of trans Azo-DFP was converted
into the cis isomer and extended UV light exposure did
not increase the rate of conversion.
linearly with the triamine moiety in an overall triangular
fashion, analogous to the structure we recently reported
in
our
previous
COFs
obtained
from
2,6-
64
pyridinedicarboxaldehyde. It can be seen from the
simulated structure that the dangling azobenzenes are
directed towards the pore channel, to form a second
64-65
pseudo-hcb net.
. The total volume of the unit cell is
3
3
6483 Å , of which 1795 Å was calculated to be
The azobenzene-functionalized TTA-AzoDFP COF
exhibited the expected reversible trans/cis isomerization
of its azobenzene units when alternatingly irradiated with
UV and visible lights. The photo-isomerization was
monitored by UV/Vis spectroscopy (Figure 4). To
achieve the isomerization, TTA-AzoDFP was first
dispersed in ethanol and was irradiated with 365 nm UV
light for varying periods of time. Upon irradiation, the
intensity of the π−π* absorption band originally present
at 339 nm decreased continuously and a new weak band
appeared at 431 nm corresponding to n−π* transition
(Figure 4a and Figure S10). With increasing irradiation
time, the π−π* absorption band showed a significant
bathochromic shift to 353 nm. These spectral changes
indicated that the azo groups of the TTA-AzoDFP clearly
accessible to a nitrogen probe. Simulation of the same
structure with removing the azobenzene moieties yields
3
an accessible volume of 3311 Å . The stacking observed
in the dangling azobenzene groups is not as “perfect” as
observed in the COF backbone as perfect alignment
would result in the three azobenzene groups overlapping
in the center of the pore. The azobenzene moieties twist
slightly away from the center of the pore (Figure 3b) to
form a propeller arrangement and interact both by π-
66
stacking and CH-π interactions (T-shape stacking ),
where alternate azobenzene moieties twist away from
o
the COF plane (C-C-Nazo-Nazo  = 37 ). This combination
of geometric effects ensures adequate free volume for
trans-cis isomerization to occur. We also simulated the
structure in cis form (Figures 3c and S7). The cis form
22,
underwent the transformation from trans- to cis-isomer.
3
3
has a total unit cell volume of 6148Å , of which 1421 Å
is accessible. Removing the azobenzene groups yields
24, 34
The yield of the trans to cis isomerization
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 13
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3: Structural characterization of trans- and cis-TTA-AzoDFP. (a) Comparison of the experimental powder X-ray
diffraction (PXRD) patterns of the trans-TTA-AzoDFP (red), cis- TTA-AzoDFP (green), and the simulated PXRD pattern
(blue) of the slip-AA stacking model (inset: side view of a ball-and-stick representation of four stacked layers of both
simulated structures); (b, c) Slip-AA-ball-and-stick model of the simulated structures of trans-TTA-AzoDFP and cis-TTA-
AzoDFP. The zoom out shows the orientation of the azobenzene units inside the pore.
was found to be 62 % (Figure S11).68 Two isosbestic
points centered at 304 and 379 nm indicate that the
compared to the AzoDFP monomer which showed
negligible fluorescence in solution (Figure S15). This
observation reflects an electronic coupling between the
68
70
photoreaction
occurs
uniformly.
Once
the
photostationary state was reached after 30 minutes of
UV light irradiation, the dispersed solution of the resulted
cis-TTA-AzoDFP form was subsequently irradiated with
azobenzene units in the COF. During the COF formation,
the azobenzene units are brought to within close
proximity of one another, possibly resulting in an
4
50 nm visible light for 30 minutes. The absorption band
aggregation-induced
stemming from restricted motion of the azobenzene.
fluorescence
enhancement
7
1-73
of the π−π* interaction centered at 339 nm increased in
intensity instantly (within 60 seconds of irradiation) and
remained unaltered after 30 minutes of irradiation. This
irradiation restores 95 % of the π − π * absorption band
When the solution containing the trans-TTA-AzoDFP
was exposed to 365 nm light for 5 minutes, the emission
signal underwent a large blue shift from 456 nm to 430
nm with a 40-fold increase in the emission intensity. The
calculated HOMO–LUMO gap of the cis-TTA-AzoDFP
(ΔE = 2.07 eV) is larger than that of trans-TTA-AzoDFP
(ΔE = 1.76 eV), which clearly justifies the blue shift in the
emission maximum observed upon irradiation.
Furthermore, the planar structure of trans-TTA-AzoDFP
compared to the distorted structure of cis-TTA-AzoDFP
facilitates efficient electron transfer between the HOMO
(Figures 4a and S12). The trans−cis−trans photo-
isomerization was switched for fifty consecutive cycles
(Figures 4b, and S13). The unchanged absorption band
of TTA-AzoDFP at 339 nm revealed no signs of photo-
degradation. The photostability and reversibility of trans-
TTA-AzoDFP were also confirmed by PXRD, SEM, and
TEM (Figures S13c and S14). Photo-isomerization was
also observed by photo-luminescence (Figure 4c). An
interesting feature of the azo molecular switch is that
while it is weakly emissive in the trans form, the cis
49-50
and LUMO energy levels.
Therefore cis-TTA-
AzoDFP exhibited stronger luminescence with respect to
the trans-TTA-AzoDFP. The original weak emission of
trans-TTA-AzoDFP is regenerated by irradiating the
solution with 450 nm visible light. The reversible on-off
photoluminescence study was performed for three
consecutive cycles (Figure 4d) during which the
luminescence intensities obtained after each irradiation
remained intact for the entire process confirming the
photo-stability of the network.
69
isomer was characterized by an intense blue emission.
The emission spectrum of the as-synthesized trans-TTA-
AzoDFP uniformly dispersed in an ethanoic solution at
room temperature was measured using an excitation
wavelength of 339 nm, corresponding to trans-TTA-
AzoDFP, and showed a weak emission peak centered at
456 nm. Although relatively weak, the emission of the
as-synthesized trans-TTA-AzoDFP was enhanced
ACS Paragon Plus Environment

Page 5 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4: a) UV−Vis spectral changes of the isomerization of trans-TTA-AzoDFP (1) to cis- TTA-AzoDFP (2) dispersed
in ethanol upon irradiation with a 365 nm UV-light and the recovered UV−Vis spectra of trans-TTA-AzoDFP (3) upon
irradiation at 450 nm; c) Fluorescence spectra of trans-TTA-AzoDFP, before (1) and after (2) trans-to-cis isomerization.
Fluorescence intensity decreases upon irradiation with 450 nm light (3). b) and d): Three photo-switching cycles of
TTA-AzoDFP dispersed in ethanol at room temperature upon alternating the irradiation wavelength between 365 and
4
50
The permanent porosity of the as-synthesized trans-
TTA-AzoDFP was evaluated using adsorption
isotherms (Figure S16) recorded at 77 K. The material
exhibited type-II adsorption isotherm, which is
nm
light.

1
g . The increase in the surface area of cis-TTA-AzoDFP
confirms that when the azobenzene units are in the cis-
form, the pores become more accessible allowing for the
diffusion of N2 molecules into the pores. The pore
diameters were found to be 10 Å and 11 Å for trans-TTA-
AzoDFP and cis-TTA-Azo respectively (Figure S16b).
Analogously, changes in pore diameter before and after
light irradiation were also noted for the azobenzene-
functionalized covalent organic polymers reported by
Zhang et al. The PXRD of TTA-AzoDFP COF was
measured before and after UV-light irradiation at
different time intervals. As shown in Figure 3 (green
line), the crystallinity of the as-synthesized trans-TTA-
AzoDFP decreases upon irradiation with 365 nm UV
light, which can be attributed to the lack of planarity in
the cis-isomer of azobenzene as well as the concurrent
increase in π−π stacking distance. A similar observation
was previously reported for an azo-functionalized
N
2
a
characteristic of micoporous materials. The Brunauer–
Emmett–Teller (BET) surface area was calculated to be
45 m g . The low surface area compared to the
2
1
2
isostructural azobenzene-free TTA-DFP COF (900 m

1
64
g , Figure S17) could be attributed to the presence of
the pendent azobenzene groups which are directed
towards the pores and consequently occupying them.
The observed pore volume for the non-functionalized
20
3
1
TTA-DFP COF was found to be 0.76 cm g , whereas,
the pore volume for trans-TTA-AzoDFP was calculated
3
1
to be 0.08 cm g . This result clearly indicates that the
presence of the azobenzene unit at the para position of
DFP significantly influences the porosity. The poor
crystallinity of our material could also possibly account
for the low surface area measured for trans-TTA-
AzoDFP COF. To evaluate the influence of trans-to-cis
photo-isomerization on the porosity of our material, we
preformed the BET analysis on TTA-AzoDFP powder
which was irradiated in the solid state with UV-light for 5
5
9
covalent organic framework. The theoretical surface
areas of trans-TTA-AzoDFP and cis-TTA-AzoDFP were
2
1
2
1
calculated to be 2020 m
g
and 2089 m g ,
respectively. These values are very high compared to
the experimental surface areas, which is not surprising,
considering that the simulated surface areas were
calculated for highly ordered materials.
2
hours. The measured surface area increased to 78 m

1
3
g , and the pore volume slightly increased to 0.09 cm
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 13
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5: Light-induced morphological changes monitored by microscopy (a-c) and size distribution plots obtained from
TEM (d). SEM (a), AFM (b) and TEM (c) images and changes in particle size distribution (d) of trans-TTA-AzoDFP,
before and after UV-light (365 nm) irradiation followed by visible light (450 nm) irradiation.
To gain more insight into the solid-state structural
with visible light (450 nm) for 10 minutes. This photo-
induced morphological change can be explained from
the simulated structure, whereby the azobenzene
moieties within the pore channels of the network twist
out-of-plane, resulting in an increase of interlayer
spacing from 3.6 Å (trans-TTA-AzoDFP) to 3.9 Å (cis-
TTA-AzoDFP). This process lead to an approximately 10
% expansion of the network along the 100 plane (Figure
S19). Dynamic light scattering (DLS) experiments
provided further evidence for the reversible expansion
and contraction of the network which was concordant
with theoretical results (Figure S19). DLS measurements
carried out on the dispersed material in ethanol at room
temperature revealed an average hydrodynamic size of
13
changes induced by photo-irradiation, we performed
C
CP-mass analysis on the as-synthesized trans-TTA-
AzoDFP COF, as well as on samples which were
irradiated for 1 hour with 365 nm UV light (Figure S18). A
minimal up-field shift of the –C=N (C
.8 ppm) was observed with a concomitant reduction of
intensity. In addition, a noticeable up-field shift of the
signal of the aromatic carbon of the pyridine ring (C , 
3 ppm) in trans-TTA-AzoDFP. This up-field chemical
e
) bond signal ( =
0
f
=
67
shift confirmed the trans-to-cis isomerization.
In
addition, a notable increase in the intensity of the peak
corresponding to the azo-linked aromatic carbons (–CAr
N=N–CAr–, C and C,  = 0.5 ppm) at δ = 152.7 ppm
was observed upon UV light irradiation.
–
i
j
530  20 nm for the as-synthesized trans-TTA-AzoDFP.
Exposing the sample to UV-light (365 nm) for 10 minutes
induced an expansion of the size to 615  20 nm. A
contraction in size to 549  30 nm was observed when
the sample was irradiated with visible light (450 nm) for
Light-controlled morphology and surface wettability
The spherical as-synthesized trans-TTA-AzoDFP
undergoes important morphological changes upon UV
irradiation with a 365 nm UV light. The morphological
changes were monitored by SEM (Figure 5a), AFM
(Figure b), and HRTEM (Figure 5c) analyses. It can be
clearly seen from the microscopic images and size
distribution plots (Figure 5d) that the original diameter
size of 115 nm for the nano-assemblies increased
significantly to ∼272 nm when exposed to UV light for 30
minutes. The initial size was restored upon irradiation
10 minutes. This expansion and contraction in size
observed by DLS correlates well with the observed 10 %
expansion of the network along the 100 plane. The
larger size observed by DLS as compared to the size
distributions obtained by AFM, SEM, and HRTEM is a
result of the presence of solvent during the DLS
measurements.
ACS Paragon Plus Environment

Page 7 of 13
Journal of the American Chemical Society
Further insight into the light-induced morphological
changes were obtained by measuring the surface
topographies of trans/cis-TTA-AzoDFP-coated
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a
substrate before and after isomerization by AFM. 3D
AFM -topographic images (Figures S19d-f) revealed that
under UV-light irradiation, the planar-ordered trans-TTA-
AzoDFP isomerizes into disordered cis-TTA-AzoDFP,
which produced a rough surface that may cause a
contraction along the a-axis. Interestingly, the AFM
measurements also demonstrated that the particles’ size
increased upon the isomerization from trans-TTA-
AzoDFP to cis-TTA-AzoDFP. The average roughness
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6: Images and changes in contact angles (a-c) of
a water droplet on a trans-TTA-AzoDFP-coated on glass
surface before and after UV-light (365 nm) irradiation
followed by visible light (450 nm) irradiation; (d) three
cycles showing the reversible wettability switching of the
TTA-AzoDFP COF upon irradiation with UV-light (365
nm) for 30 minutes followed by visible light (400 nm)
irradiation again for 30 minutes.
a
values (R ) for trans-TTA-AzoDFP and cis-TTA-AzoDFP
were measured to be 6.5 ± 0.5 nm and 14.3 ± 1 nm,
respectively. The expansion of the particles’ size in
response to light is further strong evidence for the
74
occurrence of photoisomerization.
Smart photo-switchable materials whose surface
wettability can be reversibly controlled by light have
attracted much interest because of their numerous
7
5
76
Capture and release of cargo: light-operated reservoir.
Because of the ability to modulate the hydrophobicity of
the TTA-AzoDFP and generate an internal mechanical
motion by the reversible photoisomerization of the
azobenzene units, we considered the possibility of using
trans-TTA-AzoDFP as a light-controlled reservoir for the
hydrophilic cargo Rhodamine B (RhB). RhB was chosen
because its size (1 nm  0.3 nm  0.7 nm) is comparable
to the 1.1 nm pore aperture of trans-TTA-AzoDFP,
obtained from pore size distribution, which facilitated its
diffusion into the pores (Figure 7a). Loading the RhB into
trans-TTA-AzoDFP was achieved by immersing 1 mg of
applications
such as biosensors,
biomedical
7
7
78
79
applications, in microfluidic devices, data storage,
80
and molecular machines. It is well established that azo-
functionalized materials show different surface wettability
that could be triggered by trans-cis isomerization through
UV light irradiation. To evaluate the ability of our COF to
change its surface wettability, we performed contact
angle measurements on a glass surface coated initially
with the as-synthesized trans-TTA-AzoDFP. The surface
coated with trans-TTA-AzoDFP COF was found to be
hydrophobic in nature, as was seen from the 134 ± 1.5
contact angle (Figure 6a) measured for a water droplet
placed on it. When the coated surface was irradiated
with a 365 nm UV light for 30 minutes, the contact angle
decreased to 91 ± 1.5 (Figure 6b). The significant
reduction in the contact angle in response to light is the
result of the trans-to-cis photo-isomerization of the
azobenzene groups. The azobenzene trans-to-cis
isomerization is accompanied by a large change in its
–
5
the as-synthesized trans-TTA-AzoDFP COF in a 2  10
M (2 mL) solution of RhB in pure water at room
temperature while stirring. The absorption of the RhB
solution was monitored by UV-Vis spectroscopy before
and after the addition of the as-synthesized trans-TTA-
AzoDFP (Figure S20). The concentration of RhB in the
aqueous solution decreased with time and the removal
uptake was found to be 0.097 mg of RhB adsorbed per 1
mg of trans-TTA-AzoDFP (calculated from the time-
dependent UV-Vis spectra of RhB in solution, Figure
S20). The calculated dimensions of a single molecule of
70
dipole moment. The dipole moment of the cis isomer is
significantly higher than that of the trans isomer causing
a decrease of the surface free energy and thus produce
a
more hydrophilic surface. Similar behavior was
78-80
RhB is 8.3 Å  14.2 Å with a volume of 1168 Å
3
observed for other azo-functionalized polymers.
To
–
check the reversibility of the surface wettability, we re-
irradiated the surface with 450 nm visible light for 30
minutes. The observed contact angle increases to 131 ±
(including a Cl counterion). The average accessible
pore volume of a bilayer unit cell of trans-TTA-AzoDFP is
3
found to be 1795 Å . Therefore, at first glance,
approximately 1.5 RhB molecules could fit in the bilayer
unit cell. This accessible volume of trans-TTA-AzoDFP is
clearly divided into three approximately equal hexagonal
1.5 (Figure 6c), which is comparable to the original
value measured for the as-synthesized material. The
control over the contact angle in response to UV and
visible light was repeated for three consecutive cycles
(Figure 6d).
3
channels, each with a volume of ~600 Å . Each channel
has a pore limiting diameter of ~11.2 Å, which is greater
than the “width” of a RhB molecule. Therefore one
molecule of RhB can bind in each channel. Taking into
account the unit cell height of 7Å and the “height” of
each RhB molecule, 14.2 Å, each RhB molecule would
take up approximately four layers of trans-TTA-AzoDFP
COF (i.e. two bilayer unit cells). If all three channels
were filled, this equates to 3 molecules per 2 unit cells
which is equivalent to 1.5 RhB molecules per unit cell,
and is consistent with the volume analysis (Figure S21).
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 13
release of RhB from the pores is purely resulting from
the mechanical motion of the azobenzenes
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
isomerization, we investigated the effect of heat on the
RhB release. Heating an aqueous solution containing
suspension of RhB-loaded trans-TTA-AzoDFP to 80 °C
did not induce any RhB release as proven by the lack of
changes in the absorption spectrum (Figure S24)
confirming that the release of RhB is purely resulting
from the mechanical movement of the azobenzene units
installed in the pores.
To investigate the framework integrity during this
process, we performed PXRD and BET analyses on the
recovered material. The corresponding PXRD peak
positions as well as their intensity are unchanged (Figure
S25a) compared to the diffraction pattern observed for
the as-synthesized material. This observation highlighted
the stability of the material and indicated that the
framework architecture was retained. However, the
surface area of the regenerated COF was found to be 41
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
–1
m g (Figure S25b), which indicates that some of the
RhB dye remained within the pores.
CONCLUSION
Figure 7: a) Schematic representation of the release of
RhB from the pores of TTA-AzoDFP after UV light
irradiation (max = 365 nm); b) UV-Vis generated release
profile of RhB-loaded TTA-AzoDFP upon UV irradiation
in water at 20 °C, performed in a 1 cm quartz cuvette.
Prior to exposure to UV light, no release of RhB could be
detected. Upon UV-light irradiation the temperature of
the solution was maintained at 20 °C.
In summary, we have designed and synthesized a light-
responsive azobenzene-containing covalent organic
framework, TTA-AzoDFP. The dangling photo-active
azobenzene units within the pores have enough free
space for the trans-cis photo-isomerization to happen
and permit an efficient UV and visible lights-induced
reversible trans–cis photo-isomerization that could be
monitored by UV-Vis absorption and emission
spectroscopy. The isomerization produces large
structural changes within the network which significantly
affect morphology, surface wettability, and guests’
adsorption behavior. The particles’ size was observed to
expand upon isomerization and the surface wettability of
the framework was controlled by alternately illuminating
the material with UV and visible light. The light-
mechanized structure was used to control the release of
RhB dye molecules loaded within the structure of the
covalent organic framework. Arranging mobile,
switchable and functional molecular components in a
highly dense and predictable display is a fundamental
and crucial step towards the generation of solid-state
devices with numerous functions and properties. As it
has been recently identified by others, our material
points out once again the importance of the building
blocks design for the creation of functional solid-state
switches and machines.
The BET surface area of RhB-loaded trans-TTA-AzoDFP
2
–
(
trans-TTA-AzoDFP@RhB) was measured to be 12 m g
(
1
Figure S22) which was lower than the BET surface
2
–
area of the as-synthesized trans-TTA-AzoDFP (45 m g
1
)
, confirming that RhB was successfully loaded into the
pores. In order to evaluate the importance of the
azobenzene units in the adsorption process, we
investigated the adsorption of RhB under the same
conditions with the previously reported TTA-DFP COF
64
(
Figure S23a), with un-functionalized DFP units and
have contact angle of 85 °C (Figure S22 inset). Despite
64
the high porosity of TTA-DFP, no noticeable adsorption
of RhB was observed (Figure S22b). This clearly
indicates that the presence of azobenzene units within
the COF pores not only improves the material’s
hydrophobicity but also enhances its capture of
hydrophilic guests. The isolated RhB-loaded trans-TTA-
AzoDFP was washed with water for several times to
remove any surface adsorbed dye molecules and then
dried in oven at 110 °C for 12 hours. The dry RhB-
loaded trans-TTA-AzoDFP was suspended in 2 mL of
water at room temperature for 30 minutes and no dye
leakage from the COF channels was observed as
witnessed by the lack of absorption signal (Figure 7b)
from the solution. Upon irradiation with 365 nm UV-light,
RhB molecules were released from the COF channels as
confirmed by an increase in the absorption intensity
corresponding to RhB. The absorption intensity
increased overtime and the amount of RhB released was
calculated to be 92 % within 180 minutes at 20 °C
(Figure 7b and Figure S23). In order to confirm that the
ASSOCIATED CONTENT
Supporting Information
Synthetic procedures and TGA data of the synthesized
materials; SEM, and TEM images of TTA-AzoDFP; porosity
measurements, PXRD analysis, stability study and UV data.
AUTHOR INFORMATION
Corresponding Author
*
ali.trabolsi@nyu.edu
ACS Paragon Plus Environment

Page 9 of 13
Journal of the American Chemical Society
Present Addresses
If an author’s address is different than the one given in the
affiliation line, this information may be included here.
10. Hansen, M. J.; Hille, J. I. C.; Szymanski, W.; Driessen,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
†
A. J. M.; Feringa, B. L., Easily Accessible, Highly
Potent, Photocontrolled Modulators of Bacterial
Communication. Chem 2019, 5 (5), 1293-1301.
Author Contributions
1
1. Kundu, P. K.; Olsen, G. L.; Kiss, V.; Klajn, R.,
Nanoporous frameworks exhibiting multiple stimuli
responsiveness. Nat. Commun. 2014, 5, 3588.
‡
These authors contributed equally.
Funding Sources
Any funds used to support the research of the manuscript
should be placed here (per journal style).
Notes
12. Park, C.; Oh, K.; Lee, S. C.; Kim, C., Controlled Release
of Guest Molecules from Mesoporous Silica Particles
Based on a pH-Responsive Polypseudorotaxane Motif.
Angew. Chem., Int. Ed. 2007, 46 (9), 1455-1457.
The authors declare no competing financial interest.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3. Chu, Z.; Han, Y.; Bian, T.; De, S.; Král, P.; Klajn, R.,
Supramolecular Control of Azobenzene Switching on
Nanoparticles. J. Am. Chem. Soc. 2019, 141 (5), 1949-
1960.
ACKNOWLEDGMENT
The research described here was sponsored by New York
University Abu Dhabi (NYUAD), UAE. G.D., T.P., S.S., R.J.
and A.T. thank NYUAD for its generous support of the
research program at NYUAD. The research was carried out
by using the Core Technology Platform resources at
NYUAD. MAA thanks the UK Materials Chemistry
Consortium for HPC time on Thomas (EP/P020194).
14. Li, Z.; Barnes, J. C.; Bosoy, A.; Stoddart, J. F.; Zink, J. I.,
Mesoporous silica nanoparticles in biomedical
applications. Chem. Soc. Rev. 2012, 41 (7), 2590-2605.
1
1
1
5. McCarthy, W.; Powers, R. M., Thermochromic filters
and stopband filters for use with same. Google
Patents: 2014.
6. Wang, L.; Li, Q., Photochromism into nanosystems:
towards lighting up the future nanoworld. Chem. Soc.
Rev. 2018, 47 (3), 1044-1097.
7. Danowski, W.; van Leeuwen, T.; Abdolahzadeh, S.;
Roke, D.; Browne, W. R.; Wezenberg, S. J.; Feringa, B.
L., Unidirectional rotary motion in a metal–organic
framework. Nat. Nanotechnol. 2019, 14 (5), 488-494.
8. Bigdeli, F.; Lollar, C. T.; Morsali, A.; Zhou, H.-C., A
Study on Switching in Metal-Organic Frameworks.
Angew.
REFERENCES
1
.
Yagai, S.; Kitamura, A., Recent advances in
photoresponsive supramolecular self-assemblies.
Chem. Soc. Rev. 2008, 37 (8), 1520-1529.
2
.
Jiang, Y.; Park, J.; Tan, P.; Feng, L.; Liu, X.-Q.; Sun, L.-
B.; Zhou, H.-C., Maximizing Photoresponsive
Efficiency by Isolating Metal–Organic Polyhedra into
Confined Nanoscaled Spaces. J. Am. Chem. Soc. 2019,
1
1
1
41 (20), 8221-8227.
3
4
.
.
Petermayer, C.; Dube, H., Indigoid Photoswitches:
Visible Light Responsive Molecular Tools. Acc. Chem.
Res. 2018, 51 (5), 1153-1163.
Chem., Int. Ed. 2019 in press;
DOI:10.1002/anie.201900666.
Mondal, B.; Ghosh, A. K.; Mukherjee, P. S., Reversible
9. Müller, K.; Knebel, A.; Zhao, F.; Bléger, D.; Caro, J.;
Heinke, L., Switching Thin Films of Azobenzene-
Containing Metal–Organic Frameworks with Visible
Light. Chem. - Eur. J. 2017, 23 (23), 5434-5438.
0. Zhu, Y.; Zhang, W., Reversible tuning of pore size and
CO2 adsorption in azobenzene functionalized porous
organic polymers. Chem. Sci. 2014, 5 (12), 4957-4961.
1. Borah, P.; Sreejith, S.; Anees, P.; Menon, N. V.; Kang,
Y.; Ajayaghosh, A.; Zhao, Y., Near-IR squaraine dye–
loaded gated periodic mesoporous organosilica for
Multistimuli
Switching
of
a
Spiropyran-
Functionalized Organic Cage in Solid and Solution. J.
Org. Chem. 2017, 82 (15), 7783-7790.
Grzelczak, M.; Liz-Marzán, L. M.; Klajn, R., Stimuli-
responsive self-assembly of nanoparticles. Chem. Soc.
Rev. 2019, 48 (5), 1342-1361.
5
.
2
2
6
7
.
.
Klajn, R., Spiropyran-based dynamic materials. Chem.
Soc. Rev. 2014, 43 (1), 148-184.
Hoorens, M. W. H.; Medved’, M.; Laurent, A. D.; Di
Donato, M.; Fanetti, S.; Slappendel, L.; Hilbers, M.;
Feringa, B. L.; Jan Buma, W.; Szymanski, W.,
Iminothioindoxyl as a molecular photoswitch with
photo-oxidation of phenol in
a continuous-flow
device. Sci. Adv. 2015, 1 (8), e1500390.
2
2
2. Park, J.; Sun, L.-B.; Chen, Y.-P.; Perry, Z.; Zhou, H.-C.,
Azobenzene-Functionalized Metal–Organic Polyhedra
for the Optically Responsive Capture and Release of
Guest Molecules. Angew. Chem., Int. Ed. 2014, 53 (23),
1
00ꢀnm band separation in the visible range. Nat.
Commun. 2019, 10 (1), 2390.
8
9
.
.
Roke, D.; Sen, M.; Danowski, W.; Wezenberg, S. J.;
Feringa, B. L., Visible-Light-Driven Tunable Molecular
Motors Based on Oxindole. J. Am. Chem. Soc. 2019, 141
5
842-5846.
3. Yanai, N.; Uemura, T.; Inoue, M.; Matsuda, R.;
Fukushima, T.; Tsujimoto, M.; Isoda, S.; Kitagawa, S.,
Guest-to-Host Transmission of Structural Changes for
Stimuli-Responsive Adsorption Property. J. Am. Chem.
Soc. 2012, 134 (10), 4501-4504.
(
18), 7622-7627.
Yu, Z.; Hecht, S., Control over Unfolding Pathways by
Localizing Photoisomerization Events within
Heterosequence Oligoazobenzene Foldamers. Angew.
Chem., Int. Ed. 2013, 52 (51), 13740-13744.
24. Park, J.; Yuan, D.; Pham, K. T.; Li, J.-R.; Yakovenko,
A.; Zhou, H.-C., Reversible Alteration of CO2
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 13
Adsorption upon Photochemical or Thermal
Treatment in a Metal–Organic Framework. J. Am.
Chem. Soc. 2012, 134 (1), 99-102.
Frameworks: Optically Triggered Release from a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Molecular Container. ACS Nano 2014, 8 (2), 1463-1467.
36. Lyndon, R.; Konstas, K.; Ladewig, B. P.; Southon, P.
D.; Kepert, P. C. J.; Hill, M. R., Dynamic Photo-
Switching in Metal–Organic Frameworks as a Route to
Low-Energy Carbon Dioxide Capture and Release.
Angew. Chem., Int. Ed. 2013, 52 (13), 3695-3698.
37. Samanta, D.; Gemen, J.; Chu, Z.; Diskin-Posner, Y.;
Shimon, L. J. W.; Klajn, R., Reversible photoswitching
of encapsulated azobenzenes in water. Proc. Natl.
Acad. Sci. 2018, 115 (38), 9379-9384.
38. Jones, C. L.; Tansell, A. J.; Easun, T. L., The lighter side
of MOFs: structurally photoresponsive metal–organic
frameworks. J. Mater. Chem. A 2016, 4 (18), 6714-6723.
39. Lohse, M. S.; Bein, T., Covalent Organic Frameworks:
Structures, Synthesis, and Applications. Adv. Funct.
Mater. 2018, 28 (33), 1705553.
40. Huang, N.; Wang, P.; Jiang, D., Covalent organic
frameworks: a materials platform for structural and
functional designs. Nat. Rev. Mater. 2016, 1, 16068.
41. Liu, Y.; Diercks, C. S.; Ma, Y.; Lyu, H.; Zhu, C.;
Alshmimri, S. A.; Alshihri, S.; Yaghi, O. M., 3D
Covalent Organic Frameworks of Interlocking 1D
Square Ribbons. J. Am. Chem. Soc. 2019, 141 (1), 677-
683.
42. Vitaku, E.; Dichtel, W. R., Synthesis of 2D Imine-
Linked Covalent Organic Frameworks through
Formal Transimination Reactions. J. Am. Chem. Soc.
2017, 139 (37), 12911-12914.
43. Waller, P. J.; Gándara, F.; Yaghi, O. M., Chemistry of
Covalent Organic Frameworks. Acc. Chem. Res. 2015,
48 (12), 3053-3063.
44. Lee, G.-Y.; Lee, J.; Vo, H. T.; Kim, S.; Lee, H.; Park, T.,
Amine-Functionalized Covalent Organic Framework
for Efficient SO2 Capture with High Reversibility. Sci.
Rep. 2017, 7 (1), 557.
45. Yuan, S.; Li, X.; Zhu, J.; Zhang, G.; Van Puyvelde, P.;
Van der Bruggen, B., Covalent organic frameworks for
membrane separation. Chem. Soc. Rev. 2019, 48 (10),
2665-2681.
46. Li, Z.; Huang, N.; Lee, K. H.; Feng, Y.; Tao, S.; Jiang,
Q.; Nagao, Y.; Irle, S.; Jiang, D., Light-Emitting
2
5. Brown, J. W.; Henderson, B. L.; Kiesz, M. D.; Whalley,
A. C.; Morris, W.; Grunder, S.; Deng, H.; Furukawa,
H.; Zink, J. I.; Stoddart, J. F.; Yaghi, O. M.,
Photophysical pore control in an azobenzene-
containing metal–organic framework. Chem. Sci. 2013,
4
(7), 2858-2864.
2
2
2
6. Gao, J.; He, Y.; Xu, H.; Song, B.; Zhang, X.; Wang, Z.;
Wang, X., Azobenzene-Containing Supramolecular
Polymer Films for Laser-Induced Surface Relief
Gratings. Chem. Mater. 2007, 19 (1), 14-17.
7. Schuh, C.; Lomadze, N.; Rühe, J.; Kopyshev, A.;
Santer, S., Photomechanical Degrafting of Azo-
Functionalized Poly(methacrylic acid) (PMAA)
Brushes. J. Phys. Chem. B 2011, 115 (35), 10431-10438.
8. Ji, X.; Ahmed, M.; Long, L.; Khashab, N. M.; Huang,
F.; Sessler, J. L., Adhesive supramolecular polymeric
materials constructed from macrocycle-based host–
guest interactions. Chem. Soc. Rev. 2019, 48 (10), 2682-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
697.
2
3
9. Wang, L.; Yin, L.; Zhang, W.; Zhu, X.; Fujiki, M.,
Circularly Polarized Light with Sense and
Wavelengths
To
Regulate
Azobenzene
Supramolecular Chirality in Optofluidic Medium. J.
Am. Chem. Soc. 2017, 139 (37), 13218-13226.
0. Wang, K.; Yin, L.; Miu, T.; Liu, M.; Zhao, Y.; Chen, Y.;
Zhou, N.; Zhang, W.; Zhu, X., Design and synthesis of
a novel azobenzene-containing polymer both in the
main- and side-chain toward unique photocontrolled
isomerization properties. Mater. Chem. Front. 2018, 2
(6), 1112-1118.
3
3
3
1. Zhang, W.; Yoshida, K.; Fujiki, M.; Zhu, X.,
Unpolarized-Light-Driven Amplified Chiroptical
Modulation Between Chiral Aggregation and Achiral
Disaggregation of an Azobenzene-alt-Fluorene
Copolymer in Limonene. Macromolecules 2011, 44 (13),
5
105-5111.
2. Yin, L.; Zhao, Y.; Liu, M.; Zhou, N.; Zhang, W.; Zhu,
X., Induction of supramolecular chirality by chiral
solvation in achiral Azo polymers with different
spacer lengths and push–pull electronic substituents:
where will chiral induction appear? Polym. Chem.
Covalent
Organic
Frameworks:
Fluorescence
Improving via Pinpoint Surgery and Selective Switch-
On Sensing of Anions. J. Am. Chem. Soc. 2018, 140 (39),
12374-12377.
2
017, 8 (12), 1906-1913.
3. Choi, Y.-J.; Kim, J.-T.; Yoon, W.-J.; Kang, D.-G.; Park,
M.; Kim, D.-Y.; Lee, M.-H.; Ahn, S.-k.; Jeong, K.-U.,
Azobenzene Molecular Machine: Light-Induced
47. Ascherl, L.; Evans, E. W.; Hennemann, M.; Di Nuzzo,
D.; Hufnagel, A. G.; Beetz, M.; Friend, R. H.; Clark, T.;
Bein, T.; Auras, F., Solvatochromic covalent organic
frameworks. Nat. Commun. 2018, 9 (1), 3802.
Wringing
Gel
Fabricated
from
Asymmetric
Macrogelator. ACS Macro Lett. 2018, 7 (5), 576-581.
4. Meng, X.; Gui, B.; Yuan, D.; Zeller, M.; Wang, C.,
Mechanized azobenzene-functionalized zirconium
metal-organic framework for on-command cargo
release. Sci. Adv. 2016, 2 (8), e1600480.
3
3
48. Gao, Q.; Li, X.; Ning, G.-H.; Leng, K.; Tian, B.; Liu, C.;
Tang, W.; Xu, H.-S.; Loh, K. P., Highly
photoluminescent
two-dimensional
imine-based
covalent organic frameworks for chemical sensing.
Chem. Commun. 2018, 54 (19), 2349-2352.
5. Heinke, L.; Cakici, M.; Dommaschk, M.; Grosjean, S.;
Herges, R.; Bräse, S.; Wöll, C., Photoswitching in Two-
49. Dong, J.; Zhang, K.; Li, X.; Qian, Y.; Zhu, H.; Yuan, D.;
Component
Surface-Mounted
Metal–Organic
Xu, Q.-H.; Jiang, J.; Zhao, D., Ultrathin two-
ACS Paragon Plus Environment

Page 11 of 13
Journal of the American Chemical Society
dimensional porous organic nanosheets with
molecular rotors for chemical sensing. Nat. Commun.
017, 8 (1), 1142.
characterization and application. J. Mater. Chem. A
2019, 7 (10), 5650-5655.
64. Das, G.; Benyettou, F.; Sharama, S. K.; Prakasam, T.;
Gándara, F.; de la Peña-O’Shea, V. A.; Saleh, N. i.;
Pasricha, R.; Jagannathan, R.; Olson, M. A.; Trabolsi,
A., Covalent organic nanosheets for bioimaging.
Chem. Sci. 2018, 9 (44), 8382-8387.
65. Das, G.; Nagaraja, S.; Sridurai, V.; Shinde, D. B.;
Addicoat, M.; Prakasam, T.; Gándara, F.; Ravaux, F.;
Sharma, S. K.; Nair, G. G.; Lai, Z.; Jagannathan, R.;
Olson, M. A.; Trabolsi, A., Redox-Triggered Buoyancy
and Size Modulation of a Dynamic Covalent Gel.
Chem. Mater. 2019, 31 (11), 4148-4155.
66. Sinnokrot, M. O.; Valeev, E. F.; Sherrill, C. D.,
Estimates of the Ab Initio Limit for π−π Interactions:ꢀ
The Benzene Dimer. J. Am. Chem. Soc. 2002, 124 (36),
10887-10893.
1
2
3
2
5
5
0. Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh, V.;
Kaur, G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee,
R., Chemical sensing in two dimensional porous
covalent organic nanosheets. Chem. Sci. 2015, 6 (7),
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
931-3939.
1. Han, X.; Xia, Q.; Huang, J.; Liu, Y.; Tan, C.; Cui, Y.,
Chiral Covalent Organic Frameworks with High
Chemical Stability for Heterogeneous Asymmetric
Catalysis. J. Am. Chem. Soc. 2017, 139 (25), 8693-8697.
2. Sun, Q.; Aguila, B.; Ma, S., A bifunctional covalent
organic framework as an efficient platform for cascade
catalysis. Mater. Chem. Front. 2017, 1 (7), 1310-1316.
3. Ding, S.-Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.-G.;
Su, C.-Y.; Wang, W., Construction of Covalent
Organic Framework for Catalysis: Pd/COF-LZU1 in
Suzuki–Miyaura Coupling Reaction. J. Am. Chem. Soc.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
5
67. Procházková, E.; Čechová, L.; Kind, J.; Janeba, Z.;
Thiele, C. M.; Dračínský, M., Photoswitchable
2
011, 133 (49), 19816-19822.
Intramolecular
Hydrogen
Bonds
in
5-
5
4. Huang, N.; Ding, X.; Kim, J.; Ihee, H.; Jiang, D., A
Photoresponsive Smart Covalent Organic Framework.
Angew. Chem., Int. Ed. 2015, 54 (30), 8704-8707.
5. Deng, H.; Olson, M. A.; Stoddart, J. F.; Yaghi, O. M.,
Robust dynamics. Nat. Chem. 2010, 2, 439.
Phenylazopyrimidines Revealed By In Situ Irradiation
NMRꢀSpectroscopy. Chem. - Eur. J. 2018, 24 (2), 492-
498.
5
5
5
68. Das, D.; Agarkar, H., Unexpected Nonresponsive
Behavior of a Flexible Metal-Organic Framework
under Conformational Changes of a Photoresponsive
Guest Molecule. ACS Omega 2018, 3 (7), 7630-7638.
69. Tsai, B.-K.; Chen, C.-H.; Hung, C.-H.; Hsiao, V. K. S.;
Chu, C.-C., Photoswitchable fluorescence on/off
behavior between cis- and trans-rich azobenzenes. J.
Mater. Chem. 2012, 22 (39), 20874-20877.
70. Korich, A. L.; Iovine, P. M., Boroxine chemistry and
applications: A perspective. Dalton Trans. 2010, 39 (6),
1423-1431.
71. Yuan, X.; Wang, Z.; Li, L.; Yu, J.; Wang, Y.; Li, H.;
Zhang, J.; Zhang, Z.; Zhou, N.; Zhu, X., Novel
fluorescent amphiphilic copolymer probes containing
azo-tetraphenylethylene bridges for azoreductase-
triggered release. Mater. Chem. Front. 2019, 3 (6), 1097-
1104.
72. Bo, Q.; Zhao, Y., Fluorescence from an Azobenzene-
Containing Diblock Copolymer Micelle in Solution.
Langmuir 2007, 23 (10), 5746-5751.
73. Han, M.; Hara, M., Intense Fluorescence from Light-
Driven Self-Assembled Aggregates of Nonionic
Azobenzene Derivative. J. Am. Chem. Soc. 2005, 127
(31), 10951-10955.
74. Hao, Y.; Huang, S.; Guo, Y.; Zhou, L.; Hao, H.; Barrett,
C. J.; Yu, H., Photoinduced multi-directional
deformation of azobenzene molecular crystals. J.
Mater. Chem. C 2019, 7 (3), 503-508.
6. Olson, M. A., Shuttling in the solid state. Nat. Chem.
2
015, 7, 470.
7. Vogelsberg, C. S.; Garcia-Garibay, M. A., Crystalline
molecular machines: function, phase order,
dimensionality, and composition. Chem. Soc. Rev. 2012,
1 (5), 1892-1910.
4
5
8. Chandra, S.; Kundu, T.; Kandambeth, S.; BabaRao, R.;
Marathe, Y.; Kunjir, S. M.; Banerjee, R., Phosphoric
Acid Loaded Azo (−N═N−) Based Covalent Organic
Framework for Proton Conduction. J. Am. Chem. Soc.
2
014, 136 (18), 6570-6573.
5
6
9. Zhang, J.; Wang, L.; Li, N.; Liu, J.; Zhang, W.; Zhang,
Z.; Zhou, N.; Zhu, X., A novel azobenzene covalent
organic framework. CrystEngComm 2014, 16 (29), 6547-
6
551.
0. Ge, R.; Hao, D.; Shi, Q.; Dong, B.; Leng, W.; Wang, C.;
Gao, Y., Target Synthesis of an Azo (N═N) Based
Covalent Organic Framework with High CO2-over-
N2 Selectivity and Benign Gas Storage Capability. J.
Chem. Eng. Data 2016, 61 (5), 1904-1909.
1. Dey, K.; Pal, M.; Rout, K. C.; Kunjattu H, S.; Das, A.;
Mukherjee, R.; Kharul, U. K.; Banerjee, R., Selective
Molecular Separation by Interfacially Crystallized
Covalent Organic Framework Thin Films. J. Am. Chem.
Soc. 2017, 139 (37), 13083-13091.
6
6
2. Liu, C.; Zhang, W.; Zeng, Q.; Lei, S.,
A
Photoresponsive
Framework:
Surface
Surface-Confined
Covalent
Organic
Synthesis,
75. Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K.,
Photoreversibly
Switchable
Superhydrophobic
Isomerization, and Controlled Guest Capture and
Release. Chem. - Eur. J. 2016, 22 (20), 6768-6773.
Surface with Erasable and Rewritable Pattern. J. Am.
Chem. Soc. 2006, 128 (45), 14458-14459.
63. Wu, X.; Wang, B.; Yang, Z.; Chen, L., Novel imine-
76. Chen, M.; Besenbacher, F., Light-Driven Wettability
linked covalent organic frameworks: preparation,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 13
Changes on a Photoresponsive Electrospun Mat. ACS
Nano 2011, 5 (2), 1549-1555.
7. Sun, T.; Feng, L.; Gao, X.; Jiang, L., Bioinspired
Surfaces with Special Wettability. Acc. Chem. Res. 2005,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7
7
3
8 (8), 644-652.
8. Lin, I. H.; Cheng, C.-C.; Li, K.-F.; Chen, J.-K.; Chiu, C.-
W.; Chang, F.-C., Nucleobase-grafted supramolecular
polymers for tuning the surface properties. Polym.
Chem. 2014, 5 (3), 702-705.
9. Han, M.; Ishikawa, D.; Honda, T.; Ito, E.; Hara, M.,
Light-driven molecular switches in azobenzene self-
assembled monolayers: effect of molecular structure
on reversible photoisomerization and stable cis state.
Chem. Commun. 2010, 46 (20), 3598-3600.
0. Jiang, W.; Wang, G.; He, Y.; Wang, X.; An, Y.; Song, Y.;
Jiang, L., Photo-switched wettability on an
electrostatic self-assembly azobenzene monolayer.
Chem. Commun. 2005, (28), 3550-3552.
7
8
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment

Page 13 of 13
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
TOC
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment