Oligothiophene-Bridged Conjugated Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.7b01631

Two-dimensional covalent organic frameworks (2D-COFs) are crystalline, porous materials comprising aligned columns of π-stacked building blocks. With a view toward the application of these materials in organic electronics and optoelectronics, the construction of oligothiophene-based COFs would be highly desirable. The realization of such materials, however, has remained a challenge, in particular with respect to laterally conjugated imine-linked COFs. We have developed a new building block design employing an asymmetric modification on an otherwise symmetric backbone that allows us to construct a series of highly crystalline quaterthiophene-derived COFs with tunable electronic properties. Studying the optical response of these materials, we have observed for the first time the formation of a charge transfer state between the COF subunits across the imine bond. We believe that our new building block design provides a general strategy for the construction of well-ordered COFs from various extended building blocks, thus greatly expanding the range of applicable molecules.

Full text of DOI:10.1021/jacs.7b01631

Article
pubs.acs.org/JACS
Oligothiophene-Bridged Conjugated Covalent Organic Frameworks
Niklas Keller,† Derya Bessinger,† Stephan Reuter,† Mona Calik,† Laura Ascherl,† Fabian C. Hanusch,†
Florian Auras,*,†,‡ 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
S
* Supporting Information
ABSTRACT: Two-dimensional covalent organic frameworks
(2D-COFs) are crystalline, porous materials comprising
aligned columns of π-stacked building blocks. With a view
toward the application of these materials in organic electronics
and optoelectronics, the construction of oligothiophene-based
COFs would be highly desirable. The realization of such
materials, however, has remained a challenge, in particular with
respect to laterally conjugated imine-linked COFs. We have
developed a new building block design employing an
asymmetric modification on an otherwise symmetric backbone
that allows us to construct a series of highly crystalline
quaterthiophene-derived COFs with tunable electronic proper-
ties. Studying the optical response of these materials, we have
observed for the first time the formation of a charge transfer state between the COF subunits across the imine bond. We believe
that our new building block design provides a general strategy for the construction of well-ordered COFs from various extended
building blocks, thus greatly expanding the range of applicable molecules.
optical absorption throughout the visible spectrum, and good
chemical stability.24−26
INTRODUCTION
■
Since the development of the first covalent organic frameworks
(COFs) in 2005,1 these materials have attracted considerable
scientific interest for applications in gas storage and
separation,2,3 in photocatalysis,4,5 as proton conducting
materials,6,7 and in optoelectronics.8,9 Cross-linking multi-
dentate organic building blocks via reversible covalent bond
formation into a crystalline porous framework allows for the
construction of robust materials with tunable functionality
based on the selection of their building blocks.10−16 This way, a
great variety of highly ordered materials can be realized having
specific combinations of properties that can offer advantages
over already established materials, such as zeolites or
conductive polymers. With a view on optoelectronic properties,
sheet-like 2D-COFs are of particular interest.17 In these
materials, the covalently linked layers are tightly π-stacked,
thus forming columns with significant electronic overlap that
enable efficient migration of excitations and charge car-
riers.18−20
The incorporation of this important class of molecular
semiconductors into a COF, however, has remained a
challenge. Only the smallest thiophene-based building blocks,
i.e., thiophene, bithiophene (2T),27 and small fused thiophenes
such as thieno[3,2-b]thiophene (TT)8,11 and benzo[1,2-b:4,5-
b′]dithiophene (BDT),28−30 have been applied as COF
building blocks to date, and mostly in boronate-linked COFs
that lack electronic conjugation within the COF layers.
With a view on COFs for optoelectronics, it would thus be
highly desirable to develop a method to integrate larger
oligothiophenes (nT, n ≥ 4) that allow for significant π-overlap
and spectral coverage into a conjugated COF.
As flat, extended conjugated building blocks typically suffer
from very low solubility in all common solvents, solubilizing
groups such as alkyl chains are required for tuning the solubility
into the optimal regime for COF growth.30,31 However, even
short alkyl chains can severely impede the close face-on
oriented packing that is a prerequisite for the formation of
conductive 2D-COFs.
Oligo- and polythiophene-based materials, such as
2,2′:5′,2″:5″,2‴-quaterthiophene (4T), α-sexithiophene (6T),
and poly(3-hexylthiophene-2,5-diyl) (P3HT), are among the
most prominent molecular semiconductors in organic field
effect transistors (OFETs) and organic photovoltaics
(OPV).21−23 These materials benefit from high intrinsic charge
carrier mobilities of both electrons and holes, typically broad
In this work, we have developed a building block design
concept that allows us to overcome these limitations through
an asymmetric functionalization of an otherwise C2-symmetric
Received: February 15, 2017
Published: June 6, 2017
© 2017 American Chemical Society
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Figure 1. (a) Co-condensation of 1,3,6,8-tetrakis(4-aminophenyl)pyrene (Py) with 2 equiv of the quaterthiophene dialdehyde bearing asymmetric
butyl chains (a4T) leads to the formation of the Py-a4T COF. When employing a quaterthiophene dialdehyde with a symmetric configuration of the
alkyl chains (s4T), however, no crystalline material can be obtained. (b) Cut-out of the DFT-optimized hypothetical Py-s4T COF structure. Slipped
cofacial stacking of the s4T building block would place the alkyl chains of adjacent layers in close vicinity, causing considerable steric repulsion and
impeding the close packing required for 2D-COF formation. (c) This steric repulsion can be avoided by attaching the alkyl chains to the first and
third thiophene unit (a4T) instead, allowing for sufficiently spaced alternating alkyl chains while a close-packed cofacial arrangement of the
thiophene backbone is supported. (d) DFT-optimized structure of the Py-a4T COF with alternating orientation of the alkyl chains viewed along the
pseudoquadratic pore.
4T backbone. This way, we can incorporate solubilizing alkyl
chains and allow them to integrate into the COF lattice at
sufficiently large distances, while maintaining the desirable close
contacts between the thiophene layers. We then extended this
concept to electronic modifications of the backbone via the
incorporation of thiophene-based acceptor units, such as 4H-
thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) and thieno[3,4-
b]thiophene (TT). Combining these modified 4T building
blocks with pyrene-based moieties, we synthesized a first series
of highly crystalline quaterthiophene-linked COFs with tunable
optical absorption and emission properties.
For constructing the imine-linked COFs, we followed a
synchronized offset stacking approach.15 This COF design uses
geometrically interlocked layers for superior crystallinity while
enabling close-packed arrangements of both subunits and is
perfectly compatible with the quasi C2-symmetric flat geometry
of the 4T bridges. The quaterdentate 1,3,6,8-tetrakis(4-
aminophenyl)pyrene (Py) has previously been applied in the
synthesis of several imine-linked frameworks and has proven to
generate stable and exceptionally crystalline 2D-COFs.15,32,33
Pyrene-based frameworks feature a quasi-quadratic geometry
comprising close-packed, slightly slip-stacked pyrene columns
and equally close-packed, slip-stacked linear bridges, thus
providing the ideal geometric environment for our new 4T
building blocks.
On the basis of published pyrene COF structures and density
functional theory (DFT) simulations, we expect the 4T unit to
be incorporated as face-on oriented stacks, whereby each layer
is slipped by about 0.1 nm, mainly perpendicular to the 4T axis
(Figure 1b,c). In the case of s4T, this would bring the alkyl
chains of adjacent layers very close together and most likely
cause considerable steric repulsion, forcing the framework into
a less closely stacked conformation and sacrificing in part the π-
overlap between the thiophenes. On the other hand, a4T can
stack with the butyl chains alternating on both sides of the
RESULTS AND DISCUSSION
■
Starting from commercially available 3-bromothiophene, we
synthesized the quaterthiophene (4T) dialdehyde building
blocks via Ni- and Pd-catalyzed Kumada and Suzuki cross-
coupling reactions [see the Supporting Information (SI) for
details]. In order to study the effect of the configuration of the
alkyl chains, we synthesized a symmetric and an asymmetric
version of the dibutyl-4T, in which the alkyl chains are attached
to the 3-positions of the second and third (s4T) and the first
and third thiophene units (a4T), respectively (Figure 1a).
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Figure 2. (a) Experimental PXRD data (black dots) of Py-a4T COF. Pawley refinement (red line) using the C2/m symmetric structure model
shown in part b provides a very good fit to the experimental pattern with only minimal differences (the green line shows the difference plot between
the experimental and refined patterns; Rwp = 7.5%, Rp = 13.4%). Bragg positions are indicated with blue ticks. The alkyl chains of the a4T building
block were treated in the refinement as a superposition of both possible orientations with 50% occupancy each (both shown in the figure). The
simulated PXRD pattern based on the C2/m symmetric unit cell (gray line) is in very good agreement with the experimental and the refined pattern.
Differences in the peak intensities, in particular the altered intensity ratios between lower- and higher-angle reflections, might stem from small
oligomeric fragments that are trapped in the large COF pores (the attenuation of peak intensities due to this will be strongest at small 2θ angles).
stack, thus allowing for much larger spacing between these alkyl
chains while maintaining a close-packed face-on orientation of
the 4T backbone (Figure 1c,d).
observation confirms our above conclusions drawn from the
structure modeling and highlights the importance of the
building block design.
Acid-catalyzed solvothermal synthesis using a4T and Py in a
2:1 molar ratio yielded the anticipated Py-a4T COF as a dark
red powder (see the SI for details). The successful formation of
a crystalline material was confirmed by powder X-ray diffraction
(PXRD; Figure 2a). The presence of multiple higher-order
reflections reveals the high degree of long-range order present
in this new COF. Pawley refinement using a force-field-
optimized C2/m symmetric structure model (Figure 2b)
provides a very good fit to the experimental data and produced
the lattice parameters a = 4.80 0.05 nm, b = 5.14 0.05 nm,
The asymmetric modification strategy also allows us to
extend one of the thiophene units into a more-electron-
deficient fused heterocycle, thus forming a donor−acceptor
(D−A) building block with modified optical and electronic
properties while retaining the overall geometry of the 4T
backbone.
Incorporation of a single 4H-thieno[3,4-c]pyrrole-4,6(5H)-
dione (TPD) or thieno[3,4-b]thiophene (TT) via sequential
Stille coupling reactions yielded the 4TTPD and 4TTT building
blocks, respectively (Figure 3a; see the SI for experimental
details). For the best possible crystallinity, the 4TTT building
block was synthesized as an isomer-pure material with the
orientation of the TT subunit as shown in Figure 3a. In both
cases, the asymmetric incorporation enables the 4T derivatives
to stack in a close-packed alternating sequence despite the
sterically demanding acceptor moieties (Figure 3b,c).
The acid-catalyzed reaction of Py with 2 equiv of the
modified 4T building blocks under solvothermal conditions
yielded the Py-4TTPD and Py-4TTT COFs, respectively. The
formation of crystalline frameworks was confirmed by PXRD
analysis (Figure 3d,e). Pawley refinements of the Py-4TTPD and
Py-4TTT COFs using the force-field-optimized structure models
displayed as insets in Figure 3d,e reproduced the experimental
patterns very well. Following the same reasoning as for the
simulation and refinement of the Py-a4T COF (see above), the
asymmetric 4TTPD and 4TTT building blocks were approxi-
mated as a superposition of both possible orientations with 50%
occupancy each. The refined unit cell parameters are a = 5.16
0.05 nm, b = 5.30 0.05 nm, c = 0.384 0.02 nm, and β = 72°
5° for the Py-4TTPD COF and a = 4.76 0.05 nm, b = 5.10
0.05 nm, c = 0.384 0.02 nm, and β = 72° 5° for the Py-
4TTT COF. The longer pore diagonals a and b in these COFs
compared to the Py-a4T COF can be attributed to a slight
variation in the optimal stacking offset perpendicular to the 4T
axis and the resulting differences in the tilt of the 4T columns
versus the pyrene core. The length of the crystallographic c axis,
on the other hand, is dominated by the stacking of the pyrene
units and, thus, hardly affected by the tilt of the 4T bridge.
c = 0.388
0.01 nm, and β = 72°
5°. From the above
discussion and the computational modeling we anticipate an
alternating sequence of the alkyl chains as the predominant
stacking motif within a single a4T stack. The four a4T stacks
that constitute the walls of a COF pore, however, are separated
well enough such that they will not form an ordered
superstructure. We thus treated the alkyl chains in the Pawley
refinement and the simulation of the PXRD pattern as a
superposition of both possible orientations with 50%
occupancy each.
The formation of an ordered porous framework was further
confirmed by transmission electron microscopy (TEM) (SI,
Figure S14). The contrast of the pore channels is visible for a
number of crystallites with a projected periodicity of 3.4 nm,
which corresponds very well to the channel repeat distance of
3.45 nm in the refined structure.
In order to probe the influence of the alkyl chain length on
the resulting framework, we synthesized a short-chain analogue
of the a4T building block bearing two methyl groups instead of
the butyl chains. Due to the significantly lower solubility of this
a4TMe building block, the COF synthesis required a 2.5 times
more dilute reaction mixture. The resulting framework is
isostructural to the original a4T COF, with a slightly smaller
unit cell (SI, Figure S15).
Under identical synthetic conditions as those chosen for the
Py-a4T COF and under all other solvent combinations and
concentrations tested, the reaction between Py and 2 equiv of
s4T yielded only orange-red, fully amorphous networks. This
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Figure 3. (a) The asymmetric modification approach can also be used to construct quaterthiophene-based donor−acceptor building blocks, in which
one thiophene is replaced by an electron-deficient 3,4-extended thiophene. (b, c) Despite the increased steric demands of these subunits, a close-
packed cofacial arrangement of the asymmetric building blocks is possible via alternating orientation of the electron-deficient moieties. (d, e)
Experimental PXRD data (black dots) of the Py-4TTPD and Py-4TTT COFs, respectively, and corresponding Pawley-refined patterns (red lines) and
difference plots (green lines; Rwp = 5.2%, Rp = 14.3% and Rwp = 9.3%, Rp = 16.1%, respectively). Bragg positions are indicated with blue ticks. The
acceptor moieties (TPD and TT) were treated in the refinements as a superposition of both possible orientations with 50% occupancy each (both
shown in the representations of the unit cells). The simulated PXRD patterns (gray lines) based on the C2/m symmetric unit cells (insets) agree
very well with the experimental and refined patterns of the respective frameworks. Differences in the peak intensities, in particular the altered
intensity ratios between lower- and higher-angle reflections, might stem from unreacted precursor materials or small fragments adsorbed in the COF
pores (the attenuation of peak intensities due to this is strongest at small 2θ angles).
Figure 4. (a) UV−vis−NIR absorption of the three thiophene-bridged COFs, calculated from the diffuse reflectance spectra of the COF powders
dispersed in BaSO4. (b) PL spectra measured with pulsed 378 nm excitation and (c) the corresponding TCSPC traces recorded at the respective
emission maximum of each COF. The lines represent exponential deconvolution fits taking into account the instrument response function and
scattering (see the SI for details; the rise at 1.35 ns originates from the diode laser). The COFs exhibit fast biexponential (Py-a4T) or triexponential
(Py-4TTPD and Py-4TTT) decays with sub-nanosecond components. The fractional intensities, i.e., the fractions of collected photons corresponding
to the respective lifetimes, are stated in brackets.
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The new 4T-based COFs are intensely colored powders that
are capable of absorbing most of the visible spectrum (Figure
4a). The optical band gaps estimated from Tauc plots are 1.57,
1.17 (indirect), and 1.40 eV for the Py-a4T, Py-4TTPD, and Py-
4TTT COFs, respectively (SI, Figure S8). Electronic integration
of the 4T subunit is apparent from the strong red-shift of the
absorption in comparison to previously reported pyrene COFs
with small acene bridges. Furthermore, the increased
absorption capabilities of the D−A building blocks (SI, Figures
S6 and S7) translate to red-shifted absorption maxima of the
corresponding COFs (Py-a4T COF, 563 nm; Py-4TTPD COF,
580 nm; Py-4TTT COF, 638 nm). All three COFs feature an
additional absorption band below the energy of the main π−π*
transition that is relatively weak for the Py-4TTPD and strongest
for the Py-a4T COF. This might indicate the formation of a
charge transfer state between the electron-rich pyrene and the
slightly electron-deficient 4T subunits (see below).
columns. We demonstrate that this strategy also provides a
facile route for modifying the electronic properties of the 4T
backbone via incorporation of electron-deficient subunits, thus
forming donor−acceptor type molecules. The absorption and
emission spectra confirm that the 4T-based building blocks are
electronically integrated into the framework. Spectral features
below the energy of the π−π* transition and the analysis of the
corresponding emission decay time traces reveal the fast and
efficient formation of a charge transfer state between the imine-
linked pyrene and quaterthiophene subunits. We believe that
our new asymmetric building block design provides a general
strategy for the construction of well-ordered COFs from
various extended building blocks. This will greatly expand the
range of applicable molecules for realizing frameworks with
tailor-made optoelectronic properties.
ASSOCIATED CONTENT
■
The photoluminescence (PL) spectra follow the same trends
as the absorption spectra with emission maxima of the COFs at
654, 718, and 773 nm, respectively (Figure 4b and SI, Figures
S9 and S10).
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b01631.
Experimental methods, synthetic procedures, and addi-
tional structural and spectroscopic data (PDF)
The hypothesis of a photoinduced charge transfer between
the two imine-linked building blocks is supported by the time-
correlated single photon counting (TCSPC) traces of the
COFs (Figure 4c). Dilute solutions of the 4T building blocks
exhibit monoexponential decay curves with lifetimes of about
0.5 ns (a4T, 4TTT) or 1 ns (4TTPD) (SI, Figures S12 and S13).
In the solid state, the PL lifetimes of the pure building blocks
are increased to τ1 ∼ 0.5 ns and τ2 ∼ 1 ns (biexponential
decays). The Py building block shows even longer lifetimes of
above 2 ns in solution and a main component of >1 ns in the
solid state. If integrated into a COF, however, the PL lifetimes
are drastically reduced. The fastest component, which
represents more than 50% of the emitted photons, drops to
below 200 ps (Py-4TTT) or even below 100 ps (Py-a4T, Py-
4TTPD). This indicates that the imine-linked pairing with the
tetraphenylpyrene has opened up a new pathway for fast and
efficient conversion of the COF π* excited state, potentially via
electron transfer to the quaterthiophenes and hole transfer to
the pyrene. This is further supported by comparison with the
PL lifetimes reported for electron-rich acene-bridged pyrene
COFs that do not enable strong intramolecular charge
transfer.15 In these materials, the lifetimes are much longer,
ranging from 0.35 ns to well above 1 ns.
AUTHOR INFORMATION
■
Corresponding Authors
*fa355@cam.ac.uk
*bein@lmu.de
ORCID
Florian Auras: 0000-0003-1709-4384
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-
ment No. 321339. The authors thank Dr. Markus Doblinger for
̈
the transmission electron microscopy.
The existence of an efficient charge transfer pathway that
competes effectively with the radiative recombination of the
COF π* excited state is additionally supported by the
quenching of the 4T PL upon incorporation into the COF
(SI, Figure S11). This effect is most significant for the Py-a4T
COF, which is consistently the material that exhibits the
strongest charge transfer absorption band in the diffuse
reflectance spectrum. To the best of our knowledge, this is
the first observation of the formation of a charge transfer state
between the subunits of an imine-linked COF.
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