Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.6b09787

Covalent organic frameworks (COFs), formed by reversible condensation of rigid organic building blocks, are crystalline and porous materials of great potential for catalysis and organic electronics. Particularly with a view of organic electronics, achievi

Full text of DOI:10.1021/jacs.6b09787

Article
pubs.acs.org/JACS
Synchronized Offset Stacking: A Concept for Growing Large-Domain
and Highly Crystalline 2D Covalent Organic Frameworks
Florian Auras,†,‡ Laura Ascherl,† Amir H. Hakimioun,§ Johannes T. Margraf,§ Fabian C. Hanusch,†
Stephan Reuter,† Derya Bessinger,† Markus Doblinger,† Christina Hettstedt,† Konstantin Karaghiosoff,†
̈
Simon Herbert,† Paul Knochel,† Timothy Clark,§ and Thomas Bein*,†
†Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstraße 5-13, 81377
Munich, Germany
‡Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom
§Friedrich-Alexander-University Erlangen-Nurnberg (FAU), Computer-Chemie-Centrum, Nagelsbachstraße 25, 91052 Erlangen,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: Covalent organic frameworks (COFs), formed
by reversible condensation of rigid organic building blocks, are
crystalline and porous materials of great potential for catalysis
and organic electronics. Particularly with a view of organic
electronics, achieving a maximum degree of crystallinity and
large domain sizes while allowing for a tightly π-stacked
topology would be highly desirable. We present a design
concept that uses the 3D geometry of the building blocks to
generate a lattice of uniquely defined docking sites for the
attachment of consecutive layers, thus allowing us to achieve a
greatly improved degree of order within a given average
number of attachment and detachment cycles during COF
growth. Synchronization of the molecular geometry across
several hundred nanometers promotes the growth of highly crystalline frameworks with unprecedented domain sizes.
Spectroscopic data indicate considerable delocalization of excitations along the π-stacked columns and the feasibility of donor−
acceptor excitations across the imine bonds. The frameworks developed in this study can serve as a blueprint for the design of a
broad range of tailor-made 2D COFs with extended π-conjugated building blocks for applications in photocatalysis and
optoelectronics.
molecular stacks can be used to alter the properties or extend
the functionality of the host material through the incorpo-
ration of guest molecules, such as fullerenes,5,12,13 tetracya-
noquinodimethane,14 or iodine.15,16
INTRODUCTION
■
Growing extended and high quality crystals of molecular
framework materials has remained a challenge, particularly in
the case of covalent organic frameworks (COFs), where the
connections between the individual building blocks are
formed by covalent bonds. COFs are porous long-range
ordered materials that have recently attracted considerable
scientific attention as candidates for gas storage and
separation,1,2 catalysis,3,4 and as new materials for organic
electronics and optoelectronics.5−7 In view of the last
potential application, COFs that are covalently linked in
two dimensions while held together by π-stacking in the third
dimension (referred to as 2D-COFs), are of particular
interest.8,9 The self-assembled π-stacked columns that are
formed in these materials enable electronic transport across
the layers.10,11 Additionally, if the linkage between the
individual building blocks that constitute the 2D layers is π-
conjugated, as for example in the case of imine-linked
frameworks, the conductivity might be extended to the other
two dimensions. The aligned open channels that surround the
In view of these potential applications it is essential to
construct highly crystalline (i.e., a maximum degree of
coherent long-range order within a crystal domain) frame-
works with large crystal domains and to be able to select from
a broad range of specialized building blocks. Possible
applications in gas separation or catalysis require fully
accessible pores that are not blocked due to stacking faults
or amorphous regions. The factor of crystallinity becomes
even more important if applications in electronics or
optoelectronics are intended, as stacking faults could disrupt
the conductive π-stacked columns, and defect sites and grain
boundaries could act as traps and recombination sites for
excitons or charges.
Received: September 18, 2016
© XXXX American Chemical Society
A
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Figure 1. Possible molecular conformations and corresponding stacking arrangements of 1,3,6,8-tetrakis(4-aminophenyl)pyrene when
incorporated into a COF (C, gray; H, white; N, blue). In the “propeller” conformation (a) the normal vectors of the phenylenes describe a
circle. These molecules form offset stacks of alternating left- and right-handed propellers, whereby the phenylenes arrange in an edge-on-face
sequence. In the “armchair” configuration (b) the normal vectors of the phenylenes point into the same direction. Owing to the reduced steric
demands in this configuration the molecules can stack more closely and with reduced lateral offset. (c) Forming defect-free COF domains
requires ensuring the same stacking direction of neighboring pyrene stacks. We propose that this can be achieved by synchronizing the
orientation of the phenylenes across flat, cofacially stacked bridges. (d) Chemical structures of the building blocks used in the synthesis of the
new pyrene-based COFs.
COFs are synthesized via reversible formation of covalent
bonds, the most widely applied linkage motifs to date being
boronate esters,17−20 imines,21−24 and hydrazones.25,26 In all
of these cases, this reversibility of the bond formation under
reaction conditions provides the growing COF crystal with a
functional self-healing mechanism, a factor of key importance
for obtaining a long-range ordered network.27 However, as the
formation and cleavage of these covalent bonds involve
several reaction partners and intermediates, these processes
will inherently be more complex and typically slower than for
example the coordinative network formation in metal−organic
frameworks (MOFs), or the crystallization of small organic
molecules, which is driven mainly by Coulomb and dispersion
interactions. As a result, crystalline COF structures can be
achieved only on very small length scales of typically a few
tens to hundred nanometers. If it were possible, however, to
direct the attachment of a building block to the growing COF
domain such that the correct orientation is highly favored
over all other possible attachment geometries, one could
expect to obtain enhanced crystallinity within a given number
of average attachment and detachment cycles per building
block. This could ultimately pave the way for the develop-
ment of COF single crystals.
We have recently developed a concept for growing highly
crystalline 2D COFs that exhibit well-defined hexagonal facets
and are devoid of amorphous regions between the individual
crystallites.28 In this concept, the 3D molecular conformation
generates a uniquely defined docking site for the attachment
of a successive COF layer, thus greatly lowering the
probability of stacking faults and strained regions due to
defects within the individual layers. While we have realized
this deterministic approach to COF growth using propeller-
shaped central building blocks, it would be desirable to
formulate a geometric concept that imposes less strict
boundary conditions on the selection of building blocks.
Additionally, enabling significant π-orbital overlap between
extended opto- and electroactive building blocks such as
acenes, porphyrins, or tetrathiafulvalenes would be a
prerequisite for highly conductive frameworks.
Here, we present a design concept for growing highly
crystalline COFs with domain sizes on the order of half a
micrometer based on the synchronized offset-stacking of the
B
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Figure 2. (a) Experimental PXRD pattern (black dots) of the Py-1P COF. Rietveld refinement (red line) provides a very good fit to the
experimental data with only minimal differences (the green line shows the difference plot between the experimental PXRD pattern and the one
obtained by Rietveld refinement; Rwp = 4.91%, Rp = 10.32%). Bragg positions are indicated by blue ticks. Inset, magnified view of the 2θ > 8°
region. (b) The corresponding unit cell with the viewing direction normal to the pyrene core (left) and onto the side (right). These data are
available as Supporting Information. (c) High resolution TEM image showing the large crystal domains of the Py-1P COF. Inset, magnified view
showing the pseudoquadratic arrangement of the mesopores. The white frame indicates the magnified area. (d) Nitrogen sorption isotherm
recorded at 77 K. Inset, QSDFT calculation using an equilibrium model yields the very narrow pore-size distribution that is anticipated for a fully
crystalline lattice.
building blocks. The core moiety of the multidentate building
block is hereby allowed to π-stack, thus providing the
framework with a very stable interlayer distance and enabling
electronic contact between the layers. The four phenylene
substituents on this core serve a dual purpose. They bear the
chemical functionality used to cross-link the framework, but
also define the magnitude and direction of the offset between
two COF layers. In this way, the molecular geometry of the
central building block gives rise to a uniquely defined docking
site that can guide the attachment of successive layers.
Implementing this concept, we synthesized a series of highly
crystalline pyrene-based COFs with a slip-stacked quasi-
quadratic structure. Time-resolved and steady-state optical
spectroscopy revealed considerable delocalization of excita-
tions along the π-stacked columns and, depending on the
selection of the building blocks, electronic coupling and
charge-transfer transitions across the imine bonds.
is the case for most imine-linked COFs, the amine
functionality required to cross-link the network is not directly
attached to the core, but added in the form of a 4-
aminophenyl substituent. In addition to practical reasons such
as straightforward synthesis via cross-coupling reactions and a
more core-independent reactivity of the aniline groups
compared to a directly attached amine, these anilines provide
the nonplanarity of the building block that is required to
generate a geometric docking site.
Steric repulsion between the phenylene hydrogens and the
six hydrogen atoms of the core causes the four phenylenes to
be rotated against the pyrene by typically 40−60°.31 In
contrast to smaller building blocks such as 1,1,2,2-tetrakis(4-
aminophenyl)ethene,28 the phenylenes on the pyrene are not
sterically coupled and thus the molecule can in theory adopt
several geometric configurations; the most symmetrical ones
are shown in Figures 1a and b.
In order to study possible stacking modes of the
tetraphenylpyrene subunit when incorporated into a COF,
we synthesized a molecular model compound 2 by imine
condensation between 1 and four equivalents of benzaldehyde
(see the Supporting Information for experimental details).
This model compound was found to crystallize in the
RESULTS AND DISCUSSION
■
To implement our concept we chose 1,3,6,8-tetrakis(4-
aminophenyl)pyrene (1) as central building block. Pyrene-
based building blocks used in previous COF studies were
found to produce appreciably well-ordered networks.2,29,30 As
C
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arrangement is approximately 0.41 nm, considerably larger
than typical distances of π-stacked acenes.11
We then simulated a possible COF structure for this
stacking motif, linking 1 with the smallest aromatic
dialdehyde, i.e., terephthalaldehyde (SI, Figure S3a). Balancing
the attractive interactions between the pyrene cores and the
steric demands of the edge-on-face stacked phenylenes leads
to the formation of fairly offset pyrene stacks in the
framework (Figure 1a). We find φ = 50.9°, and d = 0.42
nm for the simulated COF, in good agreement with the
experimental parameters for the molecular crystals of 2.
As this “propeller” structure is not particularly close-packed,
we reasoned that there might be another geometric
arrangement that could allow for stronger interactions
between the COF layers and thus produce a thermodynami-
cally more stable framework. The “armchair” configuration, in
which the normal vectors of the four phenylenes point in the
same direction, would allow for the cofacial arrangement of
both the pyrene cores and the phenylenes (Figure 1b). The
distances in this geometry would be determined by the π−π
interactions. Although this molecular configuration has, to the
best of our knowledge, not yet been observed in molecular
crystals of tetraphenylpyrene derivatives, recent examples of
tetra(2-pyridyl)ethene complexes have shown that external
interactions, for example the complexation of a transition
metal ion, can provide sufficient driving force for the
transition from the “propeller” to the “armchair” config-
uration.32
Simulating the above COF with the pyrenes in the
“armchair” configuration indeed led to a much closer-packed
framework (SI, Figure S3b). Here the pyrenes form slip-
stacked columns with greatly reduced offset and shorter core-
to-core distances of φ = 70.3° and d = 0.39 nm, respectively
(Figure 1b).
Comparing the two molecular configurations regarding their
ability to direct the attachment of successive COF layers
reveals further differences. The “armchair” conformation
generates a uniquely defined docking site in which the
stacking distance and the lateral offset are defined by the π−π
interactions between the pyrene cores and the phenylenes,
respectively (SI, Figure S4b). Here, also the direction of the
offset is uniquely defined through the tilt of the phenylenes
with respect to the core. In the case of the “propeller”
configuration, however, a single building block can only define
the stacking distance and the magnitude of the offset. As in
this configuration the phenylenes of a single molecule do not
possess a preferred direction, the second building block can
attach in two positions (SI, Figure S4a). Directional
information that can be transferred to successive building
blocks via the chains of edge-on-face oriented phenylenes only
exists once a dimer has formed.
At this stage, it remains difficult to predict which of the
above configurations will be present in our COFs, because
this might also be influenced by the choice of the linear
building block. As the two conformers stack at very different
offset angles, however, the experimentally determined unit cell
might allow for conclusions regarding the molecular geometry.
In a defect-free crystal, all pyrene stacks would be offset in
the same direction. As the incorporation of a pyrene stack
with the “wrong” offset direction would cause immense strain
and a large number of defects to compensate it, the
synchronization of all pyrene stacks throughout a crystal
domain is of key importance. In the “armchair” case, we
Figure 3. Experimental PXRD patterns (black dots), Pawley-refined
patterns (red lines), difference plots (green lines), and Bragg
positions (blue ticks) of (a) the Py-2PE, (b) the Py-3PE, and (c)
the Py-3PEBTD COFs, respectively. Insets, the corresponding refined
unit cells. Compared to the Py-1P COF, the reflection intensities of
these COFs appear weaker due to the inclusion of trapped oligomers
in the pores that cannot be fully removed without compromising the
crystallinity of the framework.
“propeller” configuration also observed for the parent
tetraphenylpyrene,31 with slipped stacks of alternating right-
and left-handed propellers (SI, Figure S2). Because of the
sterically demanding edge-on-face configuration of the four
phenylenes, the angle φ at which the pyrene layers are offset
is 48.9°. The average spacing of the pyrenes d in this
D
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anticipate that this can be achieved if the linear building block
is able to transport information about the orientation of the
phenylene moieties from one pyrene to its neighbors,
ensuring coplanar orientation (Figure 1c). This synchroniza-
tion mechanism is expected to work well with flat and rigid
linear building blocks, such as terephthalaldehyde, whereas
twisted molecules such as biphenyl-4,4′-dicarbaldehyde fail to
ensure the coplanar orientation of the phenylenes and hence
are not expected to form a stable framework. For geometric
reasons the “propeller” conformation, on the other hand,
cannot incorporate aligned and closely packed linear building
blocks. We thus expected it to be less sensitive to the flatness
of the linear units, but also less effective for achieving long-
range synchronization.
We used the 1,3,6,8-tetrakis(4-aminophenyl)pyrene building
block (1) in the solvothermal synthesis of imine-linked COFs
in combination with a series of linear dialdehydes (Figure 1d,
see the SI for experimental details). Regarding the choice of
the building blocks, in view of the above considerations we
reasoned that the differences between the twisted biphenyl
and terphenyl dialdehydes and their flat ethynylene-bridged
counterparts should provide us with clear conclusions about
the preferred stacking mode in our COFs. The flat building
blocks, and in particular the 2,1,3-benzothiadiazole-substituted
8, would moreover allow us to study the electronic coupling
between the pyrene cores and their π-conjugated electron-rich
(7) or -deficient (8) counterparts.
The powder X-ray diffraction pattern of the Py-1P COF
exhibits a large number of well-defined reflections and only
very weak background, confirming the formation of a highly
crystalline framework (Figure 2a). In the following we use the
term “crystallinity” to refer to the degree of coherent long-
range order within a COF domain (as opposed to the domain
size). Rietveld refinement using the “armchair” configuration
and assuming C2/m symmetry reproduced the experimental
pattern very well and yielded the lattice parameters a = 3.33
0.01 nm, b = 3.43 0.01 nm, c = 0.388 0.005 nm, β =
76 1° (Figure 2b).
Since the unit cell of the Py-1P COF contains a large
number of light atoms and the reflections are broadened even
for this well-crystallized framework, it was not possible to
refine the coordinates of individual atoms and thus directly
observe the conformation of the tetraphenylpyrene. Given the
differences in stacking behavior between the two possible
structures, however, it is possible to draw conclusions on the
molecular geometry from the following considerations:
The comparison between the experimental pattern and the
theoretical patterns calculated from the two simulated COF
structures yields a good match for the “armchair” config-
uration with only slightly shifted reflection positions, whereas
the “propeller” structure does not resemble the experimental
pattern very well (Figures S3c and S3d).
These quantum-mechanical simulations yielded β = 75.2°, in
very good agreement with the experimental unit cell.
In order to obtain independent experimental evidence for
the presence of the “armchair” configuration in the Py-1P
COF, we synthesized this framework using a 1:1 mixture of
terephthalaldehyde and 2,3,5,6-tetrafluoroterephthalaldehyde.
This mixture has been found to add an additional electrostatic
stabilization to COFs via the formation face-on-face oriented
stacks of alternating fluorinated and nonfluorinated phenyl-
enes.33 The PXRD pattern of this Py-1PF COF exhibits the
same sequence of reflections and in particular contains the
double-peak that stems mainly from the 111 and 001
reflections, here slightly shifted to 23.6° (SI, Figure S5).
The unit cell parameters for this double-layer COF after
Rietveld refinement in the space group P2/m are a = 3.38
0.02 nm, b = 3.40 0.02 nm, c = 0.772 0.01 nm, β = 75
2°. The experimental finding that the forced face-to-face
arrangement of the bridges in the Py-1PF COF gives rise to a
framework that is almost identical to the nonfluorinated Py-
1P COF, is another strong indication for the presence of
cofacially stacked bridges and hence “armchair”-type pyrenes
in our COFs.
In conclusion, based on the above findings we offer strong
theoretical and experimental evidence for the presence of the
“armchair” configuration of the tetraphenylpyrenes in the Py-
1P COF. To the best of our knowledge, this is the first time
that the crystallization of a COF framework has been
observed to change the geometry of one of its constituents
into a conformation that does not occur in molecular solids.
Transmission electron microscopy (TEM) images of the
Py-1P COF reveal that this material crystallizes in the form of
platelets with very large domain sizes of 300−500 nm (Figure
2c). Domains oriented with their crystallographic a−b plane
perpendicular to the viewing direction show the highly
ordered pseudoquadratic arrangement of the mesopores.
Moreover, the individual crystallites feature well-defined facets
that are devoid of any visible amorphous regions.
With pore diagonals of 2.00, 2.44, and 2.76 nm (bridge-to-
bridge and the two pyrene-to-pyrene distances, respectively)
in the refined structure, the Py-1P COF is at the border
between micro- and mesoporosity. Its type IV nitrogen
sorption isotherm, which is typical for mesoporous materials,
exhibits a sharp step and an H1 hysteresis loop at low p/p0
values, confirming the expected small pore size (Figure 2d).
The pore diameter calculated by quenched-solid density
functional theory (QSDFT) using an equilibrium model and
assuming a cylindrical pore geometry is 2.20 nm, in very good
agreement with the averaged pore diagonals. The Brunauer−
Emmett−Teller (BET) surface area of the Py-1P COF is
2210
50 m2 g−1 with a total pore volume of 1.09
0.03
cm3 g−1. These values are in excellent agreement with the
Connolly surface and total pore volume of 2160 m2 g−1 and
0.98 cm3 g−1, respectively, confirming that the pores are open
and fully accessible.
The lattice parameters c and β (see above), which can be
refined with high accuracy from the position and splitting of
the double-peak at 2θ = 23.4° and which are very sensitive to
the packing of the pyrenes, provide a second indication in
favor of the less offset and closer-packed “armchair”
configuration.
The packing geometry of the “armchair” structure was
further confirmed by density functional theory (DFT)
calculations using the CASTEP code with the generalized-
gradient-approximation (GGA) PBE functional and a
correction for dispersion interactions (see the SI for details).
The formation of the COF during the solvothermal
synthesis proceeds, similar to other imine-linked frame-
works,34 via an initially formed amorphous network that is
converted into the crystalline Py-1P COF over the course of
several hours (SI, Figure S7). After 1 d the main reflections
hardly change in intensity any more, indicating nearly full
conversion to the crystalline COF. Weaker reflections,
however, continue to become more distinct, suggesting an
ongoing recrystallization of the material accompanied by
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Figure 4. Diffuse reflectance (a) and PL spectra (b) of the four COFs. (c−f) TCSPC traces (black), instrument response functions (IRF, gray),
and the corresponding triexponential deconvolution fits of the COFs (red lines). The fraction of collected photons corresponding to the
respective lifetimes are given in brackets. Photoexcitation was achieved with a picosecond diode laser at 403 nm.
further improvements in crystal quality. In combination with
our lock-and-key design of the building blocks, this
mechanism can reliably produce fully crystallized nano- to
microcrystalline COFs. Even larger crystals can be anticipated
if future improvements in the linkage chemistry could
suppress the fast condensation of the amorphous network.
In order to broaden the scope of the above concept we
used a series of directly linked and ethynylene-bridged
building blocks in the COF syntheses. As expected from the
above geometric considerations, the twisted biphenyl (2P)
and terphenyl (3P) derived building blocks fail to produce
stable crystalline frameworks. The Py-2P COF forms in the
reaction solution, but quickly loses its long-range order upon
evaporation of the solvent (SI, Figure S6).
If an acetylene bridge, however, is introduced between the
phenylenes, the molecule can adopt a planar configuration
that should allow for the synthesis of well-ordered “armchair”
type frameworks.35 Indeed, we were able to obtain crystalline
COF materials using the acetylene-bridged building blocks
(Figures 3a and 3b). Pawley refinements carried out in the
space group C2/m with the angle β fixed at 76° (i.e.,
assuming similar stacking offset as for the Py-1P COF)
yielded the lattice parameters a = 4.59 0.05 nm, b = 4.11
0.05 nm, c = 0.394 0.01 nm, β = 76° (fixed) and a = 5.74
0.05 nm, b = 4.84 0.05 nm, c = 0.388 0.01 nm, β =
76° (fixed) for the Py-2PE and the Py-3PE COFs,
respectively.
in the framework, produced the lattice parameters a = 5.60
0.05 nm, b = 5.00 0.05 nm, c = 0.386 0.01 nm, β = 76°
(fixed) (Figure 3c).
The new pyrene-based COFs are intensely colored solids
ranging from bright orange (Py-1P and Py-2PE COFs) to
deep red (Py-3PEBTD COF). UV−vis diffuse reflectance
spectra corroborate these observations (Figure 4a). Compared
to a solution of compound 1, the framework absorption edge
is red-shifted by about 100−150 nm, indicating considerable
electronic delocalization across the pyrene stacks (SI, Figure
S12). The Py-1P and Py-2PE COFs feature particularly sharp
absorption onsets around 560 nm, whereas the absorption
spectrum of the Py-3PE COF is more curved in the
normalized representation due to an overlap of the pyrene
and the acetylene bridge absorption. In all three cases, the
steepest rise in the absorption spectrum is at almost the same
wavelength, indicating that the optical transitions are
dominated by the central pyrene building block and only
slightly affected by the additional π-conjugation of the
acetylenes. This interpretation is supported by the corre-
sponding photoluminescence (PL) spectra (Figure 4b),
showing similar emission at around 600 nm.
In order to obtain a better understanding of the electronic
processes in these materials, we determined the absolute
external photoluminescence quantum yields (PLQYs) using
an integrating sphere (SI, Table S2),36,37 and studied the PL
decay dynamics via time-correlated single photon counting
(TCSPC). All three COFs exhibit PLQYs of around 0.2% and
triexponential decay curves with almost identical lifetimes.
These results confirm that, irrespective of the length of the π-
conjugated bridge, the electronic processes are dominated by
the tetraphenylpyrene moiety.
This concept allows us also to incorporate additional
electronic functionality into the building blocks and thus
modulate the electronic structure of the resulting frameworks.
We demonstrate this by applying a 2,1,3-benzothiadiazole-
modified (BTD) version of the 3PE building block. Pawley
refinement of the Py-3PEBTD COF in the space group P2/m,
assuming the most symmetric arrangement of the BTD units
Placing a strongly electron-withdrawing BTD group into
the bridge, however, does change the band gap of the
F
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material. The red-shifted absorption and emission spectra are
indicative of a charge-transfer excitation at energies below the
π−π* transition of the individual building blocks. This is
supported by the significantly lower PLQY of the Py-3PEBTD
COF. The recombination of the emissive states, however,
appears at very similar time constants as for the other three
COFs, highlighting the strong influence of the central building
block on the optoelectronic properties.
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CONCLUSION
■
In this study, we used the three-dimensional geometry of a
tetraphenylpyrene-derived building block to generate a
periodic lattice of synchronized docking sites for the
attachment of consecutive COF layers. This way we were
able to grow highly crystalline 2D COFs with domain sizes on
the order of half a micrometer that feature well-defined facets
and are devoid of any observable amorphous regions. In
combination with future improvements in the linkage
chemistry, this geometric guidance could pave the way for
the development of COF single crystals. Our design principle
also allows for the simultaneous incorporation of π-stacked
central building blocks and π-stacked bridges, thus enabling
electronic communication between all subunits of the
framework. Studying the optoelectronic properties of a series
of pyrene-based COFs revealed significant delocalization of
excitations along the pyrene stacks and showed that charge-
transfer excitations are possible across the imine bonds. The
frameworks developed in this study can serve as a blueprint
for designing a broad range of tailor-made 2D COFs with
extended π-conjugated building blocks for applications in
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b09787.
Experimental methods, synthetic procedures, and addi-
tional structural and spectroscopic data (PDF)
Crystal data (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*bein@lmu.de
ORCID
Florian Auras: 0000-0003-1709-4384
Paul Knochel: 0000-0001-8167-272X
Timothy Clark: 0000-0001-7931-4659
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for funding from the German Science
Foundation (DFG; Research Cluster NIM) and the Free State
of Bavaria (Research Network SolTech). The research leading
to these results has received funding from the European
Research Council under the European Union’s Seventh
Framework Programme (FP7/2007-2013)/ERC Grant Agree-
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