Switching on and off Interlayer Correlations and Porosity in 2D Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.9b02800

Two-dimensional covalent organic frameworks (2D COFs) attract great interest owing to their well-defined pore structure, thermal stability, high surface area, and permanent porosity. In combination with a tunable chemical pore environment, COFs are intriguing candidates for molecular sieving based on selective host-guest interactions. Herein, we report on 2D COF structures capable of reversibly switching between a highly correlated crystalline, porous and a poorly correlated, nonporous state by exposure to external stimuli. To identify COF structures with such dynamic response, we systematically studied the structural properties of a family of two-dimensional imine COFs comprising tris(4-aminophenyl)benzene (TAPB) and a variety of dialdehyde linear building blocks including terephthalaldehyde (TA) and dialdehydes of thienothiophene (TT), benzodithiophene (BDT), dimethoxybenzodithiophene (BDT-OMe), diethoxybenzodithiophene (BDT-OEt), dipropoxybenzodithiophene (BDT-OPr), and pyrene (Pyrene-2,7). TAPB-COFs consisting of linear building blocks with enlarged π-systems or alkoxy functionalities showed significant stability toward exposure to external stimuli such as solvents or solvent vapors. In contrast, TAPB-COFs containing unsubstituted linear building blocks instantly responded to exposure to these external stimuli by a drastic reduction in COF layer correlation, long-range order, and porosity. To reverse the process we developed an activation procedure in supercritical carbon dioxide (scCO₂) as a highly efficient means to revert fragile nonporous and amorphous COF polymers into highly crystalline and open porous frameworks. Strikingly, the framework structure of TAPB-COFs responds dynamically to such chemical stimuli, demonstrating that their porosity and crystallinity can be reversibly controlled by alternating steps of solvent stimuli and scCO₂ activation.

Full text of DOI:10.1021/jacs.9b02800

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Article
Switching On and Off Interlayer Correlations and
Porosity in 2D Covalent Organic Frameworks
Torben Sick, Julian M. Rotter, Stephan Reuter, Sharath Kandambeth, Nicolai N. Bach, Markus
Döblinger, Julia Merz, Timothy Clark, Todd B. Marder, Thomas Bein, and Dana D Medina
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02800 • Publication Date (Web): 28 Jun 2019
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Switching On and Off Interlayer Correlations and Porosity in 2D Cova-
lent Organic Frameworks
Torben Sick,1 Julian M. Rotter,1 Stephan Reuter,1 Sharath Kandambeth,1 Nicolai N. Bach,1 Markus Dö-
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blinger,1 Julia Merz,2 Timothy Clark,3 Todd B. Marder,2 Thomas Bein1* and Dana D. Medina1*
1 Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstraße 5-13, 81377
Munich, Germany
2 Institute für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-
Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
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Computer-Chemistry-Center, Department of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-
Nuremberg, Naegelsbachstr. 25, 91052 Erlangen, Germany.
Covalent Organic Frameworks, COFs, porous polymers, dynamic structure, porosity, crystallinity, supercritical carbon
dioxide, scCO2, benzodithiophene
ABSTRACT: Two-dimensional covalent organic frameworks (2D COFs) attract great interest owing to their well-defined pore
structure, thermal stability, high surface area and permanent porosity. In combination with a tunable chemical pore environment,
COFs are intriguing candidates for molecular sieving based on selective host-guest interactions. Herein, we report on 2D COF
structures capable of reversibly switching between a highly correlated crystalline, porous and a poorly correlated, non-porous state
by exposure to external stimuli. To identify COF structures with such dynamic response, we systematically studied the structural
properties of a family of two-dimensional imine COFs comprising tris(4-aminophenyl)benzene (TAPB) and a variety of dialdehyde
linear building blocks including terephthalaldehyde (TA) and dialdehydes of thienothiophene (TT), benzodithiophene (BDT), di-
methoxybenzodithiophene (BDT-OMe), diethoxybenzodithiophene (BDT-OEt), dipropoxybenzodithiophene (BDT-OPr) and py-
rene (Pyrene-2,7). TAPB-COFs consisting of linear building blocks with enlarged π-systems or alkoxy functionalities showed sig-
nificant stability towards exposure to external stimuli such as solvents or solvent vapors. In contrast, TAPB-COFs containing un-
substituted linear building blocks instantly responded to exposure to these external stimuli by a drastic reduction in COF layer
correlation, long-range order and porosity. To reverse the process we developed an activation procedure in supercritical carbon
dioxide (scCO2) as a highly efficient means to revert fragile non-porous and amorphous COF polymers into highly crystalline and
open porous frameworks. Strikingly, the framework structure of TAPB-COFs respond dynamically to such chemical stimuli,
demonstrating that their porosity and crystallinity can be reversibly controlled by alternating steps of solvent stimuli and scCO2
activation.
In recent years 2D COFs have been examined in the context
INTRODUCTION
of applications related to functional interfaces capable of spe-
cific molecular uptake. Due to their well-defined pore channel
structure, sufficiently large pore apertures and tailored pore
chemical environment the selective adsorption of molecular
agents such as pollutants,27-28 chiral compounds29 and chemi-
cals valuable for green chemistry reactions5, 30-33 was realized
for 2D COF powders and active membranes. Control over the
pore chemical environment in principle enables the permeabil-
ity of these porous frameworks to be tuned to the desired task.
This is achieved mainly by installing anchor functional groups
providing favorable interactions with the guest molecules.
Upon the adsorption of the desired guest molecules, the crys-
tallinity and the integrity of the pore system are expected to be
maintained, which can be beneficial for the reverse process of
desorption, thereby allowing for cycling of the process.
Covalent organic frameworks (COFs) are a class of organic
crystalline porous materials obtained by connecting organic
building blocks through covalent bonds.1 The dynamic nature
of the bonds formed, in classes of compounds comprising
mainly boroxines,2-3 boronic esters,4-9 imines,10-13 azines,14-15
imides,16-17 hydrazones18-20 and triazines,21-24 and the defined
geometries of the COF building blocks are two essential con-
ditions for attaining ordered framework structures.25-26 In two-
dimensional COF materials (2D COFs), extended molecular
sheets interact through weak attractive forces to form defined
molecular stacks and open porous channels. Therefore, the π-
stacking interactions between adjacent COF layers play a
pivotal role in achieving accessible surface area, and structural
stability.2
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Another way for modulating the framework’s permeability
multidentate building block with a variety of linear dialde-
hydes. Using unsubstituted linear dialdehydes, namely tereph-
thalaldehyde (TA), thieno[3,2-b]thiophene-2,5-
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is to control the pore accessibility, namely triggering a switch
between an open and a closed pore state by exposure to exter-
nal stimuli. For example, such a responsive 2D COF platform
can be realized by switching between a crystalline, porous and
an amorphous, non-porous COF phase in the solid state. As
described above, a crystalline 2D COF phase forms via the
self-assembly of individual COF layers into well-defined
molecular stacks yielding uniform pore shapes, whereas an
amorphous phase can be envisioned as COF layers that are
randomly shifted in-plane with respect to the adjacent layers,
yielding irregular pores and limited or no pore accessibility.
Switching between these two states implies that the stacking
interactions can be modulated in a 2D COF, namely dynami-
cally tightening and weakening them under specific condi-
tions.
dicarboxaldehyde (TT) and benzo[1,2-b:4,5-b']dithiophene-
2,6-dicarboxaldehyde (BDT) resulted in imine COFs with
moderate crystallinity, limited pore accessibility and moderate
structural stability towards a variety of external stimuli. These
COFs were therefore classified as ‘fragile’. COFs featuring a
higher degree of structural robustness were obtained by em-
ploying BDT cores decorated with alkoxy side groups as
building blocks, such as 4,8-dimethoxybenzo[1,2-b:4,5-
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b']dithiophene-2,6-dicarbaldehyde
(BDT-OMe),
4,8-
diethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-dicarbaldehyde
(BDT-OEt) and 4,8-dipropoxybenzo[1,2-b:4,5-b']dithiophene-
2,6-dicarbaldehyde (BDT-OPr). Alternatively, structural ro-
bustness was also achieved by a significant enlargement of the
dialdehyde’s π-system using pyrene-2,7-dicarboxaldehyde
(Pyrene-2,7). In contrast to the fragile COFs, these frame-
works display long-range structural order and high surface
area even after long exposure to organic solvents and solvent
vapors.
Crystalline 2D COFs with weak π-stacking interactions are
prone to exfoliation into individual layers upon exposure to
different solvents and solvent mixtures during sonication19, 34-36
or by mechanical exfoliation.37-38 The reason for the exfolia-
tion in solution is still to be revealed, nevertheless it is be-
lieved that an intercalation of solvent molecules might be the
trigger.19 COFs featuring enhanced π-stacking interactions
were reported to be highly stable towards treatments such as
post-synthetic solvent immersion. Robust 2D COFs are usual-
ly constructed by reinforcing the π-stacking interactions be-
tween the COF layers, by methods such as intra-molecular
hydrogen bonding, alkoxy group incorporation,39-40 defined
docking of propeller moieties41 and armchair building
blocks,42 or by controlling dipolar interactions between COF
layers.43 Jiang and coworkers introduced a strategy to use
arene/perfluoroarene-subunits to influence adjacent layers’
multipole moments, yielding highly crystalline, alternatingly
stacked frameworks.44 These methods are aimed at producing
structures that maintain their key properties, namely crystallin-
ity and pore integrity, under harsh conditions such as at ex-
treme pH. Therefore, they are suitable for applications requir-
ing defined backbones and stability in a range of chemical
conditions.45-48 The realization of a reversible activation and
deactivation protocol, switching between two structurally
different COF modes in the solid state, is an important funda-
mental aspect contributing to the understanding of COF
framework assembly and allows for control over structural
features. To date, reversible switching of crystallinity or po-
rosity in the solid state has been shown only for a very limited
number of framework materials. In molecular superstructures49
or cages50-51 it was possible to switch repetitively at least one
of those key materials properties back and forth, mainly due to
conformational flexibility in the soft organic material and
uptake of guest molecules. In a recent pioneering study, the
concept of reversible crystallinity was demonstrated by includ-
ing guest molecules into a non-crystalline floppy 2D polymer
constructed via urea-linkages.52 However, to the best of our
knowledge, a dynamic switch of both crystallinity and porosi-
ty in ‘rigid’ 2D materials by the exclusion of guest molecules
in the solid state is yet to be reported.
Furthermore, we developed a fast, simple and efficient acti-
vation protocol using supercritical carbon dioxide (scCO2) to
transform poorly crystalline and moderately porous TAPB-
COFs into highly crystalline and open porous frameworks.
The scCO2-activated crystalline and porous fragile COFs were
still found to be highly sensitive towards a large variety of
organic solvents and vapors. In contrast, the scCO2-activated
robust COFs showed stability towards external stimuli except
for 1,4-dioxane exposure. Depending on the fragility of the
COF, a significant reduction in long-range order and porosity
was recorded upon brief immersion of the COFs in solvents
(e.g. rinsing with 1,4-dioxane) or exposure to vapor atmos-
pheres. Remarkably, alternating scCO2 activation and solvent
vapor exposure cycles allowed us to turn on and off the COF’s
crystallinity and porosity reversibly.
RESULTS AND DISCUSSION
To identify COFs featuring a dynamic response towards ex-
ternal stimuli we focused our study on COFs comprising
TAPB building blocks. Those TAPB-COFs were reported to
exhibit excellent structural properties such as long-range or-
der, tunable structural properties and high surface areas.16, 40, 47,
53-62
To explore the stability and crystallinity of TAPB-based
frameworks, we synthesized a family of TAPB-COFs with
linear counterparts featuring π-systems of different extensions
such as TA, TT, BDT and Pyrene-2,7. In addition, we synthe-
sized TAPB-COFs of substituted BDT cores exhibiting differ-
ent alkoxy side chain lengths, namely BDT-OMe, BDT-OEt,
and BDT-OPr. The series of these imine-linked TAPB-COFs
was synthesized under solvothermal conditions. Briefly, 1
equiv. of TAPB (0.02 to 0.06 mmol) and 1.5 equiv. of linear
dialdehyde (0.03 to 0.09 mmol) were suspended in a mixture
of 1,4-dioxane and mesitylene (1.0 mL, 8:1 v:v) in a 6 mL
glass tube under argon. The reaction mixture was sonicated for
2 min to obtain a finely dispersed suspension of precursors.
After addition of acetic acid as catalyst (100 µL, 6 M) the
reaction tube was sealed and placed in a preheated oven at 100
°C for 72 h. The as-synthesized precipitates were isolated by
filtration under reduced pressure. Interestingly, powder X-ray
diffraction (PXRD) analysis of the as-synthesized, filtered
powders revealed moderate to poor crystallinity for all of the
examined precipitates (Figures S3–S5). To improve the struc-
Herein, we report the synthesis of a series of hexagonal-pore
2D COFs featuring different levels of structural robustness. In
the context of responsive interfaces, their dynamic behavior
towards chemical guests such as solvent molecules in the
liquid and vapor phases are presented. The COFs were synthe-
sized via an acetic acid-catalyzed Schiff-base condensation
reaction of 1,3,5-tris(4-aminophenyl)benzene (TAPB) as a
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tural features, with regard to crystallinity and porosity, of the desired improvement in crystallinity. Surprisingly, employing
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obtained COFs, namely their long-range order, different isola-
tion and purification procedures were employed. Attempts at
removing residual monomers, catalyst agents and solvents
solely by vacuum drying and heat activation did not lead to the
solvent rinsing of the COF powders with the main reaction
solvent, namely 1,4-dioxane, resulted in amorphous powders
(Figure S2).
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Figure 1. a) Schematic representation of 2D imine COF synthesis from TAPB and a variety of linear building blocks. b)-h) Comparison of
the experimental (red) and refined simulated PXRD patterns (black) in conjunction with the difference plot (blue) for the different TAPB-
COFs.
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Further solvent stability screenings were carried out to iden-
(see experimental section for further details). After 2 h, the
pressure was slowly reduced and the scCO2-activated powders
were recovered.
tify the optimal conditions for a suitable recovery of crystal-
line products by a post-synthetic solvent treatment. We chose
common commercially available solvents with a polarity gra-
dient ranging from water to toluene and submersed the COF
powders for 24 h (5 mg/mL) in the respective solvent to obtain
a mild solvent exchange process. We observed a relationship
between the extension of the π-system of the linear building
block and the structural stability of the COFs. The TA TAPB-
COF, comprising a linear building block with comparably
small π-system, showed instability towards all the employed
solvents, whereas the TT and BDT TAPB-COFs showed a
certain degree of stability towards toluene and ethanol, respec-
tively (Figure S4). Nevertheless, the general stability observed
towards solvent exchange for all three COFs was moderate
(Figures S3a and S4). Investigation of the accessible porosity
of TT and BDT TAPB-COFs revealed non-porous or moder-
ately porous frameworks, respectively (Figure S44). There-
fore, these unsubstituted COF structures were classified as
“fragile” (Figure 1). In contrast, the alkoxy substituted COFs
BDT-OMe, BDT-OEt and BDT-OPr TAPB-COFs as well as
the Pyrene-2,7 TAPB-COF were found to be highly stable
towards the solvent exchange treatment with acetone, ethanol,
and toluene. Furthermore, these COFs could even be Soxhlet
extracted with toluene, and they exhibited substantially higher
crystallinity compared to the as-synthesized powders, indicat-
ing a sufficient elution of residual agents along with retention
of the structures (Figures S3b and S5). These findings are in
accordance with the substantial open porous character of these
TAPB-COFs (Figures 2 and S42), which thereby were classi-
fied as “robust” (Figure 1).
Structural Characterization. The crystallinity of scCO2-
activated fragile and robust COF samples was investigated by
PXRD (Figure 1). Strikingly, for all the COF structures stud-
ied, a very intense and sharp reflection at low 2θ values corre-
sponding to the 100 crystal plane was observed, indicating
highly crystalline materials. For the TA and TT TAPB-COFs,
this reflection appears at 2θ = 2.53°, d = 34.9 Å and 2θ =
2.73°, d = 32.3 Å, respectively. The unsubstituted and substi-
tuted BDT TAPB-COFs (BDT TAPB-COF, BDT-OMe
TAPB-COF, BDT-OEt TAPB-COF, BDT-OPr TAPB-COF)
exhibit the 100 reflection at 2θ = 2.30°, corresponding to a d-
spacing of 38.4 Å. For the Pyrene-2,7 TAPB-COF, the 100
reflection appears at 2θ = 2.28°, corresponding to the largest
unit cell of the TAPB-COF series with a d-spacing of 38.7 Å.
The intense 100 reflection is followed by weaker, higher order
reflections that are attributed to the 110, 200, 210, 220, 230,
310, and 001 planes. We modeled the COF structures in the
P6 space group, corresponding to a fully overlapped AA-
eclipsed stacking model. The unit cell parameters of the COFs
were determined by performing Pawley refinements with
respect to the experimental diffraction patterns (Figure 1). The
comparison between the experimental and the simulated
PXRD patterns shows close matching for all reflection posi-
tions (Figure 1b-g). The d-spacings corresponding to the broad
001 reflections positioned at 2θ = 24-27° with maxima at ca.
2θ = 25.1° were calculated to be ca. 3.55 Å and are attributed
to the COF’s interlayer distances. As an exception, Pyrene-2,7
TAPB-COF exhibits a 001 reflection at 2θ = 25.7° correspond-
ing to a comparably closer stacking distance of adjacent layers
of 3.46 Å. In a previous report, the TAPB was shown to be a
key factor for obtaining highly crystalline frameworks at-
tributed to its inherent propeller structure induced by steric
constraints.41 Thereby, the TAPB allows for spatial control
and a precise arrangement of adjacent layers by influencing
the interlayer distance. Herein, we reveal that the interlayer
distances can be controlled by the nature of the linear counter-
parts. Using a linear pyrene linker between the TAPB building
blocks results in a closer interlayer distance of the adjacent
COF-layers, demonstrating that the TAPB propeller has a
certain degree of flexibility.
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In the search for a general isolation method that is suitable
for both fragile and robust TAPB-COFs, we employed a su-
percritical fluid activation method using supercritical carbon
dioxide. In principle, the supercritical fluid activation is based
on the effect of a liquid-like substance above its critical point
on a target material. Above the critical point, the liquid-like
matter exhibits very low surface tension and thereby capillary
forces acting on the walls of the extracted material are signifi-
cantly reduced. This activation procedure is well established
for obtaining highly porous materials63-67 and was already
reported to be useful for the recovery of COFs.52, 57, 62, 68-70 In a
typical scCO2 activation process employed in our work, fresh-
ly filtered COF powders exhibiting low to moderate crystallin-
ity were transferred to a metal holder and placed in an auto-
clave, which was then tightly closed and cooled to 5 °C. Sub-
sequently, CO2 was flushed through the system and finally the
closed autoclave was heated to 40 °C, while adjusting the
pressure to 85 bar to establish supercritical CO2 conditions
The long-range order established by employing the scCO2
activation process was monitored over the course of several
weeks (Figure S15). The scCO2-activated fragile and robust
TAPB-COFs maintained their highly crystalline character
during this time, rendering these COFs stable under ambient
conditions, thus exhibiting a significant shelf life.
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Figure 2. Space filling model showing the hexagonal pore channels of (a) the fragile BDT TAPB-, and (b) the robust BDT-OMe
TAPB-COF. (c) Schematic representation of the impact of solvent treatment and subsequent vacuum drying on BDT TAPB-COF
(left) and BDT-OMe TAPB-COF (right). (d) PXRD of the BDT TAPB-COF upon solvent treatment and subsequent vacuum dry-
ing. (e) PXRD of the BDT-OMe TAPB-COF upon solvent treatment and subsequent vacuum drying.
The porosity of the scCO2-activated fragile and robust COF
samples was investigated by nitrogen physisorption. The type
IVb isotherms are characteristic for mesoporous materials with
pores smaller than 5 nm (Figures S39 – 42).71 For all scCO2-
activated COFs, high surface areas of up to 2000 m2g-1 were
determined, which illustrates the power of the scCO2 activa-
tion as a general method providing highly porous and crystal-
line COF materials (for details on BET surface areas, pore
volumes and pore size distributions see Table S2 and Figures
S39-S43 in the Supporting Information).
sole/ethanol as the solvent mixture.41 Under these synthesis
conditions, the monodisperse TT TAPB-COF particles ob-
tained are 2 µm in diameter and, in some cases, self-
organization into larger spherical hollow superstructures was
observed (Figure S36).
Remarkably, structural analysis of the unsubstituted COF
powders via transmission electron microscopy (TEM) reveals
crystallites with domain sizes of up to 500 nm (Figure S37).
Specifically, the Pyrene-2,7 TAPB-COF exhibits well-faceted
hexagonally shaped crystals and the BDT TAPB-COF exhibits
large domain sizes of up to 500 nm. To illustrate further the
impact of scCO2 activation on structural features, we charac-
terized an as-synthesized and vacuum dried fraction, a 1,4-
dioxane treated fraction (24 h exposure to 1,4-dioxane vapor)
and a scCO2-activated fraction of the same sample of BDT
TAPB-COF by TEM (Figure S38). The scCO2-activated BDT
TAPB-COF sample shows enlarged crystalline domains of up
Scanning electron micrographs (SEM) of the highly crystal-
line COF powders revealed either spherical or rod-like COF
particle assemblies (Figures S34-S36). Among the different
COFs, the TT TAPB-COF forms unique monodispersed spher-
ical particles of ca. 5 µm (Figure S35c). Furthermore, we
investigated the morphological properties of TT TAPB-COF
following a previously reported reaction scheme using ani-
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to 100 nm (Figure S38c). In contrast, the filtered and vacuum mbar) for 30 min to remove residual guest molecules located
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dried and the 1,4-dioxane-rinsed BDT TAPB-COF fractions
only exhibit short-range ordered domains of up to 10 nm (Fig-
ure S38a and b). Interestingly, the scCO2-activated robust
BDT-COFs show significantly smaller domain sizes of 20-80
nm (Figure S37). This can be attributed to the sterically de-
manding side chains and electrostatic repulsion caused by the
alkoxy chains, which can impact the packing of the COF lay-
ers.
in the pores prior to PXRD analysis (Figures S9–S11). Sur-
prisingly, the fragile COFs were still affected by the interac-
tion with solvent vapors and suffered a reduction in long-range
order demonstrated by the 100 line broadening and relative
intensity (Figures S9a and S10). Similar to what was observed
upon immersion in solvents, the robust TAPB-COFs main-
tained their high crystallinity upon the exposure to vapors of
acetone, ethanol and toluene. Upon exposure to water and 1,4-
dioxane vapors, the robust COFs showed high vulnerability
(Figures S9b and S11). To investigate the strong impact of
solvent vapors on the TAPB-COF structures, time-dependent
PXRD measurements were conducted for a fragile and a ro-
bust TAPB-COF. As representative model systems, BDT
TAPB-COF (fragile) and BDT-OMe TAPB-COF (robust)
were selected for these experiments and were exposed to dif-
ferent solvent vapors. We chose 1,4-dioxane and ethanol as
solvent vapors, because both frameworks were highly affected
by 1,4-dioxane immersion, while ethanol immersion did not
lead to complete framework degradation in both cases. For
these experiments, highly crystalline scCO2-activated TAPB-
COF powders spread on a silicon wafer were placed in a plas-
tic bag and immobilized on the PXRD measuring stage. An
open vessel filled with the respective solvent was included
within the plastic bag, which then was tightly closed allowing
for a solvent vapor saturated atmosphere inside the bag. Sub-
sequently, PXRD patterns of the COF powders were collected
in a continuous cyclic mode with 2.5 min measurement time.
In these experiments, we found that exposing both BDT and
BDT-OMe TAPB-COF powders to 1,4 dioxane vapor resulted
in a poorly crystalline material. This is an interesting observa-
tion as 1,4-dioxane is the main solvent in the TAPB-COF bulk
syntheses, aiding the progressive construction of the frame-
works. The loss of structural order was observed by a steady
decrease in the diffraction intensity of the 100 reflections in
the course of increasing vapor exposure time. Notably, we did
not observe changes in the diffraction reflection positions of
the 100 and higher order reflections in the ab-plane during
these experiments (Figure 3b).
TABP-COF Response Towards Solvent and Vapor
Stimuli. After achieving structural stability and permanent
porosity under ambient conditions through scCO2 activation of
the TAPB-COFs, their stability towards external solvent stim-
uli was investigated. To this end, the crystalline and porous
scCO2-activated COF powders were immersed for 24 h in the
different solvents of the series described earlier (acetone,
ethanol, toluene, water, 1,4-dioxane, 5 mg/mL). Prior to
PXRD analysis, the powders were filtered and vacuum dried
to remove residual solvent molecules. In the case of the fragile
COFs, a pronounced decrease in long-range order was detect-
ed (Figure 2). In contrast, the robust TAPB-COFs maintained
their crystallinity when exposed to all of the employed sol-
vents excluding water and 1,4-dioxane (Figures S6b and S8).
To rule out the possibility of simple decomposition of the
fragile COF samples during exposure to solvents, we conduct-
ed IR measurements of the scCO2-activated and the 1,4-
dioxane treated fragile BDT TAPB-COF and the robust BDT-
OMe TAPB-COF samples (Figures S32 and S33). All IR
spectra of the solvent-treated COFs exhibit vibrational absorp-
tion bands comparable to those of the highly porous and crys-
talline scCO2-activated samples, hence excluding decomposi-
tion or new bond forming processes.
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Next, we examined the interactions of the scCO2-activated
crystalline frameworks with different solvent vapors. We
anticipated that the vapor treatment would have a smaller
impact on the COF structures compared to exposure to liquid
solvents. For these experiments, the scCO2-activated TAPB
COF samples were placed, for 24 h, in a closed vapor chamber
containing an open reservoir of the respective solvent. The
resulting powders were subjected to reduced pressure (10-3
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Figure 3: (a) Schematic representation of the structural transformation of highly ordered COF layers upon vapor treatment with 1,4-
dioxane to low ordered and non-porous materials and a subsequent supercritical reactivation to highly ordered porous polymers. (b) PXRD
and TEM of BDT TAPB-COF after synthesis and activation with scCO2 (left), after exposure to 1,4-dioxane vapor (middle) and after
reactivation with scCO2 (right). c) N2-physisorption isotherms and calculated pore size distributions of BDT TAPB-COF after synthesis
and activation with scCO2 (left), after exposure to 1,4-dioxane vapor (middle) and after reactivation with scCO2 (right). (d) PXRDs of
BDT TAPB-COF after (re-)activation in scCO2. (e) PXRDs of BDT TAPB-COF after treatment in 1,4-dioxane vapor. (f) Comparison of
the calculated BET surface areas of BDT TAPB-COF after scCO2 (re-)activation (blue) and 1,4-dioxane vapor treatment (red).
The amorphization process can be associated with a random
displacement of the COF layers. In contrast to the exposure to
1,4-dioxane, both model systems (fragile BDT TAPB-COF;
robust BDT-OMe TAPB-COF) showed a certain degree of
tolerance towards exposure to ethanol vapor. Importantly, we
detected a pronounced shift of the 001 reflection towards
lower 2θ values corresponding to an increased stacking dis-
tance of the adjacent COF layers upon vapor exposure for both
the fragile BDT TAPB-COF as well as the robust BDT-OMe
TAPB-COF (Figure S16). In combination with the IR investi-
gations indicating a chemically intact imine polymer layer
(Figures S32 and S33), this observation points to a slow aniso-
tropic displacement of the COF layers promoted by the weak-
ening of the stacking interactions, thereby resulting in the loss
of most structural features.
As the solvent vapors show a slowly progressing effect on
the 001 reflections accompanied by a decrease in all other
reflection intensities, we studied the effect on the 001 reflec-
tions in further detail for both model systems exposed to a
small volume of different solvents. We therefore conducted
PXRD measurements with scCO2-activated TAPB-COF sam-
ples being soaked with a droplet of either ethanol, 1,4-dioxane
or toluene to examine the immediate impact on the frame-
work’s stacking distances. We detected shifts of the 001 re-
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flections towards smaller 2θ values (Figure S18–S19). For the S62). Stable BDT dimers were surrounded by 1,4-dioxane to
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fragile BDT TAPB-COF and a droplet of 1,4-dioxane we
detected a shift from 2θ = 25.1 to 22.7° corresponding to inter-
layer distances of 3.52 Å and 3.91 Å, respectively. In compari-
son, the 001 reflections of the robust BDT-OMe TAPB-COF
shifted from 2θ = 25.1 to 24.2° upon treatment with a droplet
of 1,4-dioxane, corresponding to interlayer distances of 3.52 Å
and 3.68 Å, respectively. The dynamic character of the layer
distance enlargements can also be seen by comparison of the
samples being treated with toluene droplets. For the robust
BDT-OMe TAPB, the interlayer distance shifted from 2θ =
25.1 to 24.4°, corresponding to interlayer distances of 3.52 Å
and 3.64 Å upon exposure to toluene. The stacking distance of
BDT TAPB-COF shifted from 2θ = 25.1 to 24.7° upon the
treatment with a droplet of toluene, corresponding to interlayer
distances of 3.52 Å and 3.60 Å, respectively. These results
underline the significant effect of solvents on the COF’s stack-
ing distance and the amorphization process.
identify possible binding interactions. Two binding modes of
1,4-dioxane to the BDT-core, π-face or edge-on, were identi-
fied that give complexes that are competitive in energy (Fig-
ures S62, S63). Each BDT unit can accommodate two 1,4-
dioxane molecules or other oxygen containing molecules such
as ethanol to give complex structures (Figure S62). Because
edge-on coordination is the most relevant scenario for stacked
oligomers in a COF, its effect on the stacking energy deter-
mines whether the solvent can dissociate the stacks. In the
calculations, the 1,4-dioxane molecules in the BDT complex
do not affect the stacking distance significantly but rather
cause the BDT units to twist relative to each other (Figure
S64). This weakens the stacking, so that the dimerization
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1
−
energy is calculated to be -25.8 and -19.6 kcal mol at MP2
and B3LYP-D3 level, respectively. These values compare
with the dimerization energy with ethanol of -35.6 and -30.9
1
−
kcal mol at MP2 and B3LYP-D3 level, respectively. Thus,
even in the absence of further solvent molecules, four 1,4
dioxane molecules weaken the stacking interaction significant-
ly, particularly when compared to ethanol. Interestingly, we
note that BDT-TAPB COF shows a certain stability towards
EtOH exposure as depicted in Figure 2d. Our experiments also
suggest that non-BDT TAPB COFs such as TA- and Py-2,7-
TAPB COF also show lability towards 1,4-dioxane. Therefore,
model calculations within Cs-symmetry on the coordination of
chair dioxane to typical bonding moieties that occur in the
COFs were carried out. These calculations gave the binding
energies of 1,4-dioxane and are shown in Table S4. Moreover,
to validate the hypothesis of specific binding of 1,4-dioxane
onto the COF backbone, we examined, in a solvation experi-
ment, the effect of a dioxane isomer, namely 1,3-dioxane, on a
stable and crystalline BDT-OMe TAPB COF powder. Strik-
ingly, the exposure of the COF powder to 1,3-dioxane had
only a marginal effect on the crystallinity of the COF (Figure
S21). This underlines that the specific interactions of 1,4-
dioxane with the backbone and this specific dioxane isomer is
required for achieving the switch between the amorphous and
crystalline COF phases (For detailed discussion see SI).
Switching On and Off Crystallinity and Porosity in
TAPB-COFs. Strikingly, by employing a scCO2 reactivation
process for the vapor and solvent treated samples, we were
able to retrieve highly crystalline COFs featuring the close
stacking distances observed prior to vapor and wetting exper-
iments (Figures S17–19). This indicates that the process of
gradual loss of structural order is reversible for all TAPB-
COFs and that a poorly-crystalline TAPB-based polymer can
be converted back into a highly crystalline COF simply by
using the scCO2 activation protocol. We therefore conclude
that the TAPB-COF layers can be mobilized and shifted in all
three dimensions upon exposure to external stimuli such as
solvent vapors. This “breathing” feature of the frameworks
results in a nearly complete amorphization in the case of frag-
ile TAPB-COFs, while the robust TAPB-COFs resist a loss of
order upon exposure to different solvents such as acetone,
toluene and ethanol. Retrieving the structural order for these
robust COFs can simply be achieved by the removal of the
solvents by common means such as employing reduced pres-
sure.
Among the solvents used in this study, 1,4-dioxane stands
out by exhibiting a profound role in two different processes.
This solvent is essential for the formation of the TAPB-
frameworks and is the main component in the solvent mixture
for the synthesis of the bulk materials. However, it has a
strong impact on the long-range order when used in post-
synthetic solvent or vapor treatment for all of the fragile or
robust structures examined. As none of the TAPB-COFs can
withstand exposure to 1,4-dioxane, this is a general method for
the deconstruction of highly crystalline and open porous
TAPB-COFs. Even more remarkable, this process is reversible
for every solvent or vapor used by employing a scCO2 reacti-
vation treatment whereby all TAPB-COFs regain their crystal-
line structure and their porosity (see Table S3 in Supporting
Information).
As the stacking of the TAPB-COFs can be modulated by
employing solvent vapors and subsequent scCO2 activation,
we used 1,4-dioxane as an external stimulus for multiple acti-
vation and amorphization cycles. In a typical cycle, a highly
crystalline and open porous scCO2-activated COF powder was
immersed in 1,4-dioxane for 24 h, filtered and dried under
reduced pressure, resulting in the poorly crystalline, non-
porous state (Figure S21a). Then, the poorly crystalline dried
powder was scCO2-reactivated following the protocol de-
scribed earlier. For all of the examined powders, the desired
COF properties such as crystallinity and porosity were indeed
restored in a comparable quality by the scCO2 activation
(Figure 4). This cycle was repeated several times, resulting in
alternation between the crystalline, open porous and the poorly
crystalline, non-porous COF states. Attempts at replacing the
scCO2 activation protocol with other activation methods, such
as immersing the poorly crystalline robust COFs in a weaker
solvent for the cycling experiments, failed to produce a pow-
der with properties comparable to those resulting from the
scCO2 activation protocol (Figures S13, S14 and S21b).
To study the interactions of 1,4-dioxane with the COF
frameworks, we conducted a theoretical study of the interac-
tions of 1,4-dioxane with functional sites in the COF back-
bone. For the study, we chose molecular BDT-based dimers as
model systems to mimic the BDT stacks in the COF backbone
and imine motifs based on the TAPB scheme (Figures S59-
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Figure 4: a) Schematic model showing the effect of intercalation with solvent molecules on crystalline and ordered TAPB-COF layers. In
the case of 1,4-dioxane acting as intercalated solvent molecules, the resulting framework does not retain order or porosity. Fragile COFs
result in only short-range ordered and non-porous polymers, while robust COFs regain their order and porosity after removal of solvents.
The PXRDs in (b) and (c) show the changes in crystallinity of the fragile BDT TAPB-COF over time after exposure to 1,4-dioxane (b) and
EtOH (c) vapors. The PXRDs in (d) and (e) show the changes in crystallinity of the robust BDT-OMe TAPB-COF over time after expo-
sure
to
1,4-dioxane
(d)
and
EtOH
(e)
vapors.
To exclude the possibility that only the pressure and the
temperature of the scCO2 activation is relevant for successful
(re-)activation, a BDT TAPB-COF and a BDT-OMe TAPB-
COF were treated with argon, simulating the CO2 activation
conditions concerning pressure and temperature. Notably,
under these conditions argon is at a super critical state. These
experiments yielded poorly crystalline materials indicating
that scCO2 has a crucial role in the dynamic process. Further-
more, we utilized scC2H6 as a hydrocarbon source for activa-
tion of BDT-OMe TAPB COF powder treated with 1,4-
dioxane. For the activation, the exact same activation condi-
tions were applied as those for using scCO2 or Ar. Notably,
after the scC2H6 activation treatment the obtained powder did
not feature crystallinity or substantial porosity. Modification
of the CO2 extraction protocol, employing temperature and
pressure parameters that leave the CO2 sub-critical, yielded
poorly crystalline BDT-OMe TAPB COF powders. These
experiments further support our conclusion that supercritical
CO2 plays a special role in enabling the recovery of the crys-
talline state (Figure S21).
Of particular interest is the recovery of TAPB-COFs from
aqueous media, which would allow for efficient removal of
polar guests captured within the porous structure. To study the
behavior of the TAPB-COFs in an aqueous medium, we im-
mersed a scCO2-activated, fragile BDT TAPB-COF sample in
deionized water (15 mg/5 mL, pH 7) for a period of 24 h.
After the solvent treatment, the COF sample was filtered,
dried under vacuum and examined by PXRD. As observed
following the treatment with organic solvents, the PXRD of
the water-treated fragile COF sample showed a complete loss
of long-range order. This powder was then subjected to scCO2
reactivation, resulting in the complete recovery of the COF
structural order (Figure S20). As with organic solvents, the
robust TAPB-COFs showed higher resistance to the water
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solvent treatment. After 24 h in water, the crystallinity of these li. Freshly synthesized fragile TAPB-COFs instantly lose
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robust COFs was still high as indicated by the observed reflec-
tions (Figures S6b and S8).
crystallinity and porosity upon exposure to solvents, heat or
vacuum. We attribute the sensitivity of these COFs mainly to
weaker π-π interactions of the linear dialdehydes between
adjacent layers. We found that the fragile TAPB-COFs can be
stabilized by the incorporation of alkoxy substituents on the
BDT-cores. While the usual extraction workup strategies for
the fragile TAPB-COFs failed, we developed a successful
workup procedure based on scCO2 extraction for these fragile
as well as the robust TAPB-COFs of our test series. The
scCO2 extraction is not only a fast and efficient process, but it
also minimizes the amount of solvents needed in the workup
procedure.
To gain further insights regarding the impact of the scCO2
activation on the TAPB frameworks, we studied the interac-
tion of poorly crystalline and crystalline COF powders with
nitrogen and carbon dioxide by means of physisorption exper-
iments. The porosity of the crystalline COF and of the corre-
sponding poorly crystalline analog of the fragile BDT TAPB-
COF were analyzed by N2 and CO2 adsorption measurements
at 273 K and 1 bar. The crystalline BDT TAPB-COF dis-
played CO2 uptake of 1.41 mmol/g and N2 uptake of 0.57
mmol/g at 273 K (Figure 5). The powder was immersed in
1,4-dioxane (5 mg/mL) for 24 h yielding the short-range or-
dered BDT TAPB-COF analog after vacuum drying. This
BDT TAPB-COF showed a lower N2 uptake of 0.364 mmol/g.
This can be attributed to the reduced surface area of the fragile
COF due to fewer open pores. Interestingly, we observed that
the short-range ordered COF analog displayed a significant
increase in CO2 uptake (1.94 mmol/g at 273 K), which is
higher than that of the crystalline COF (Figure 5). This indi-
cates a high affinity of CO2 to the poorly crystalline frame-
work and can possibly explain the effective activation of the
COFs via scCO2. We postulate that due to the smaller kinetic
diameter of CO2 (3.3 Å), as compared to N2 (3.64 Å), CO2
molecules are still able to access voids between displaced
adjacent layers. In addition, these experiments illustrate the
importance of controlling the framework characteristics, al-
lowing for adjusting the interface affinity in the solid state.
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Furthermore, we have established that successfully scCO2-
activated, highly crystalline and open porous COFs can be
completely converted into poorly crystalline and non-porous
polymers by exposure to solvents in liquid and vapor phases.
Intriguingly, this process is completely reversible by applying
the scCO2 activation approach, and the crystalline/short-range
ordered switching cycle can be repeated many times. The
dramatic transformation of fragile COFs from the crystalline
to the poorly crystalline state also leads to substantial changes
in the gas adsorption behavior. Hence, the 1,4-dioxane treated,
poorly crystalline BDT TAPB-COF shows a higher CO2 to N2
uptake ratio for adsorption in comparison to the crystalline
analog at 1 bar pressure (~6.3 vs ~2.9, Figure 5). Therefore, as
CO2 is still able to access the voids in-between displaced COF
layers, in its supercritical state it allows for a full reconstruc-
tion of the poorly crystalline, non-porous polymer into a crys-
talline and open-porous COF. In view of the above, our results
provide deeper insight into the decisive role of subtle interlay-
er interactions in determining the porosity and crystallinity of
COFs.
ASSOCIATED CONTENT
The Supporting Information is available free of charge on
the ACS Publications website at DOI: XXXXXX
The Supporting Information contains detailed information
on synthetic procedures, work-up approaches, and structural
investigations of the TAPB-COFs.
AUTHOR INFORMATION
Corresponding Author
*dana.medina@cup.uni-muenchen.de
*bein@lmu.de
Notes
The authors declare no competing financial interest.
Figure 5: Switching of N2 (diamond) and CO2 (circle) storage
properties of BDT TAPB-COF upon 1,4-dioxane vapor treatment
at 273 K. Red represents long-range ordered COF sample before
vapor treatment, blue represents short-range ordered COF sample
after the 1,4-dioxane vapor treatment.
ACKNOWLEDGMENTS
The authors are grateful for funding from the German Sci-
ence Foundation (DFG) for SPP 1613, the NIM Excellence
Cluster (DFG), and GRK 2112 (DFG), and from the Free State
of Bavaria (research networks ‘Solar Technologies Go Hy-
brid’ and UMWELTnanoTECH). Research leading to these
results received funding from the European Research Council
under the European Union’s Seventh Framework Programme
(FP7/2007-2013)/ERC Grant Agreement 321339. The authors
CONCLUSION
In summary, we have shown that hexagonal imine COFs
consisting of TAPB and different linear dialdehyde building
blocks exhibit very different stabilities towards external stimu-
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