Microtubular Self-Assembly of Covalent Organic Frameworks

Article abstract of DOI:10.1002/anie.201708526

Despite significant progress in the synthesis of covalent organic frameworks (COFs), reports on the precise construction of template-free nano- and microstructures of such materials have been rare. In the quest for dye-containing porous materials, a novel

Full text of DOI:10.1002/anie.201708526

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Accepted Article
Title: Microtubular Self-Assembly of Covalent Organic Frameworks
Authors: Bappaditya Gole, Vladimir Stepanenko, Sabrina Rager,
Matthias Grüne, Dana D. Medina, Thomas Bein, Frank
Würthner, and Florian Beuerle
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708526
Angew. Chem. 10.1002/ange.201708526
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Microtubular Self-Assembly of Covalent Organic Frameworks
Bappaditya Gole,[a,b] Vladimir Stepanenko,[a,b] Sabrina Rager,[c] Matthias Grüne,[a] Dana D. Medina,[c]
Thomas Bein,[c] Frank Würthner[a,b] and Florian Beuerle*[a,b]
Dedicated to Sir Fraser Stoddart on the occasion of his 75th birthday
Abstract: Despite significant progress in the synthesis of covalent
organic frameworks (COFs), reports on a precise construction of
template-free nano- and microstructures for such materials have
been rare. In search of dye-containing porous materials, a novel
conjugated framework DPP-TAPP-COF with enhanced absorption
capability up to 800 nm has been synthesized utilizing reversible
imine condensations between 5,10,15,20-tetrakis(4-aminophenyl)-
belts,[11] fibers[12] and spheres[13] have been observed for some
COFs, but detailed mechanistic investigations have so far only
been conducted in two examples for COF-based hollow
spheres.[14] The template-assisted synthesis of COF nanotubes
has also been reported.[15] However, in this case additional effort
was required to initially prepare and finally remove the templates
without causing irreversible modifications of the COF properties.
porphyrin (TAPP) and
a diketopyrrolopyrrole (DPP) dialdehyde
derivative. Surprisingly, the obtained COF exhibited spontaneous
aggregation into hollow microtubular assemblies with outer and inner
tube diameters of around 300 and 90 nm. A detailed mechanistic
investigation revealed the time-dependent transformation of initial
sheet-like agglomerates into the tubular microstructures.
The formation of well-defined nanoscale superstructures has
been a major achievement for supramolecular chemistry in
recent years.[1] For a precise control over function and materials
properties however, molecular organization often needs to be
mastered on even larger spatial regimes, e.g. the µm-scale.[2] In
natural systems, function often emerges from defined
microarchitectures that are assembled via protein-templated
biomineralization.[3] While the defined bottom-up fabrication of
artificial microstructures is still quite challenging, it would
significantly improve the understanding of structure-property
relationships for real applications.
Covalent organic frameworks (COFs), representing a class of
crystalline porous polymers,[4] have recently emerged as
promising materials for potential applications in gas adsorption,[5]
energy storage,[6] heterogeneous catalysis[7] or sensing.[8] In
particular, two-dimensional (2D) COFs comprising extended π-
systems or well-defined donor-acceptor heterojunctions in nm-
sized regimes render such materials promising candidates for
optoelectronic applications.[9] In most cases, however, 2D COFs
are prepared and isolated as microcrystalline powders. Thereby,
the limited long-range crystal growth and morphological
definition is presumably due to internal defects[10] and kinetic
trapping of smaller crystallites as a result of dispersive π-
stacking of individual layers. Defined morphologies such as
Figure 1. a) Synthesis and b) proposed microtubular self-assembly of DPP-
TAPP-COF.
Herein, we report on the synthesis of DPP-TAPP-COF
containing diketopyrrolopyrrole (DPP) and tetraphenylporphyrin
(TPP) moieties. This imine COF adopts a unique hollow
microtubular morphology with uniform diameters, rendering it, to
the best of our knowledge, the first example for bottom-up
microtubular self-assembly based on COF materials.
[a]
Dr. B. Gole, Dr. V. Stepanenko, Dr. M. Grüne, Prof. Dr. F. Würthner,
Dr. F. Beuerle
Universität Würzburg, Institut für Organische Chemie
Am Hubland, 97074 Würzburg (Germany)
E-mail: florian.beuerle@uni-wuerzburg.de
During our quest for dye-containing COFs, we studied the
[b]
[c]
Dr. B. Gole, Dr. V. Stepanenko, Prof. Dr. F. Würthner, Dr. F. Beuerle
Center for Nanosystems Chemistry & Bavarian Polymer Institute
(BPI)
Theodor-Boveri-Weg, 97074 Würzburg (Germany)
S. Rager, Dr. D. D. Medina, Prof. Dr. T. Bein
Ludwig-Maximilians-Universität München, Department of Chemistry
& Center for NanoScience (CeNS)
reaction
of
5,10,15,20-tetrakis(4-aminophenyl)-porphyrin
(TAPP)[16] and the organic semiconductor DPP[17] dialdehyde
derivative DPP-1 bearing solubilizing ethylhexyl side chains.
Microcrystalline precipitates were obtained after an AcOH-
catalyzed solvothermal reaction in n-BuOH/mesitylene (3:1) at
120 °C for five days. Washing with anhydrous THF and acetone
followed by drying under high vacuum yielded DPP-TAPP-COF
(Figure 1a) as a dark purple material in 53% yield. Remarkably,
Butenandtstraße 5-13, 81377 München (Germany)
Supporting information for this article is given via a link at the end of
the document.
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even small deviations from these optimized conditions only
resulted in amorphous products (see Table S1).
microtubular structures (Figure 2) that are extended up to 20 μm.
The majority of the microtubes were aggregated into bundles, in
some cases, however, individual tubes were observed, which
were possibly separated mechanically by sonication and sample
preparation. Energy-dispersive X-ray (EDX) spectroscopy on
various spots of different tubes revealed a uniform atomic
composition, thus indicating the homogeneous formation of a
composite material (Figure S10). SEM and scanning
transmission electron microscopy (STEM) images (Figures 2b,e)
clearly demonstrated the hollow nature and remarkably smooth
surface of the tubes. Statistical analysis yielded mean values for
outer and inner diameters of 303 ± 38 nm and 87 ± 21 nm,
respectively (Figure 2f), which corresponds to a mean wall
thickness of 105 ± 9 nm (Figure S23). High-resolution TEM
(Figure 2d) revealed a periodic rhomboidal framework with
domain sizes in the range of several tens of nm.
Indeed, framework formation was proven by several analytical
techniques. The FT-IR spectrum (Figure S5) shows almost
complete disappearance of the aldehyde band at 1649 cm−1 and
the simultaneous rise of a new band at 1582 cm−1 corresponding
to the C=N stretching mode. In addition, the N-H stretching band
for the amino groups of TAPP at 3316 cm−1 is significantly
weakened after polymerization. Similar spectral trends indicating
identical functionalities and connectivity were observed for
model compound M-1 as a representative “cutout” possessing
two TPP units attached to one DPP-1. 13C Cross-polarization
magic-angle spinning (CP-MAS) NMR spectra (Figure 3d-f) for
both DPP-TAPP-COF and M-1 are in excellent agreement with
13C NMR solution data of M-1 obtained in CDCl3. The absence
of any aldehyde signal, expected to appear beyond 180 ppm,
indicates virtually quantitative consumption of the DPP precursor.
Elemental analysis of DPP-TAPP-COF revealed the efficient
formation of a polymeric material composed of both monomers
(see SI for details). Thermogravimetric analysis (TGA) revealed
a thermal stability up to 350 °C followed by a weight loss of
around 20%, which is tentatively attributed to the loss of the alkyl
side chains,[18] and ultimate decomposition at 450 °C (Figure S6).
Powder X-ray diffraction (PXRD) revealed Bragg reflections
centered at low 2θ angles of 2.68°, 3.51°, 4.26°, 5.49° and 7.17°
corresponding to 110, 020, 120, 220 and 040 planes,
respectively (Figure 3a), thus implying the formation of small
Figure 2. a), b) SEM and c) TEM micrographs of DPP-TAPP-COF nanotubes;
d) high resolution TEM image of a microtube’s outer wall indicating crystalline
domains; e) STEM image of singular microtube indicating the hollow nature of
the tube; f) statistical distribution of inner and outer tube diameters.
Strikingly, scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) revealed that DPP-
TAPP-COF predominantly assembles into well-defined
Figure 3. a) PXRD patterns of DPP-TAPP-COF: experimental (red), Pawley refinement (black), simulated pattern (green), and difference plot (blue). b)
Simulated unit cell for a monoclinic crystal system of space group C2/m. c) Model compound M-1. d) 13C-CP-MAS-NMR spectra of DPP-TAPP-COF and e)
of solid M-1. f) 13C-NMR spectrum (CDCl3, 400 MHz, rt) of M-1.
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COF domains. A simulated diffraction pattern in the monoclinic
C2/m space group (see SI for details) with an eclipsed but
slightly offset (~ 1 Å) AA stacking provides a good description of
DPP-TAPP-COF (Figure 3b). The final unit cell parameters were
obtained by performing Pawley refinement and correspond to a
elevated temperatures under high vacuum for 12 hours. The
obtained sorption isotherm (Figure S9) and calculated BET
surface area of 139 m2 g−1 indicate a fairly low N2 uptake, which
we attributed to the offset stacking and primarily to the sterically
demanding side chains protruding into the pores.
= 45.3 Å, b = 48.1 Å, c = 3.9 Å; α = γ = 90°, β = 74.3° (Rwp
3.82% and Rp = 2.86%).
=
The absorption spectrum of M-1 nearly matches an overlay of
NH2-TPP and DPP-1, except for slightly increased Q-bands at
590 and 650 nm (Figure 4a). Steric interactions of the phenyl
rings with the porphyrin possibly induce a significant twist, thus
resulting in limited π-conjugation. Diffuse reflectance spectra for
DPP-TAPP-COF showed a significant shift of the maximum
absorption to 670 nm (Figure 4b), which can be rationalized by a
planarization of the π-system and pronounced aggregation of the
individual layers within the COF.[16b] In addition, the relative
intensity ratio for the Q- versus the Soret-bands increased from
0.4 and 0.41 for TAPP and M-1, respectively, to 1.47 for the
COF. Due to this enhanced absorption, DPP-TAPP-COF more
efficiently harvests photons in the visible and near IR region.
For time-dependent morphological studies, COF reactions were
distributed to several pyrex tubes and quenched at different time
intervals. SEM Images after one day indicated the formation of
plate-like agglomerates of small individual crystallites (Figure 5a).
After four days, significantly smoother plate surfaces were
observed alongside initial signs for a scrolling of some of the thin
sheets (Figures 5b,c). After five days, hollow microtubes were
isolated as the major product (Figure 5d) in addition to some
remaining plate-like aggregates. Further increase of the reaction
time up to 15 days resulted in roughening and fracturing of the
tube walls as evidenced by SEM (Figures S17e,f) and PXRD
(Figure S16). However, the isolated COF microtubes are stable
for several months and PXRD measurements showed no signs
for structural collapse. Based on these data, there is no
evidence for tube growth via Ostwald ripening, as it was recently
invoked for the formation of spherical COF particles.[14a] Instead,
we propose the following mechanism for microtube formation
(Figure 5). Initially, small crystallites of imine-condensation
products are formed that agglomerate into sheet-like aggregates,
which is presumably induced by van der Waals interactions
between the branched alkyl chains. Over time, initial crystallites
grow further by condensation of unreacted precursors or grain
boundary consumption via reactive aldehydes and amines at the
interfaces (Figures S19 and S20). This transformation is
Figure 4. a) UV/Vis absorption spectra (CHCl3, rt) of M-1, NH2-TPP and DPP-
1. Insets show enlarged region from 500 to 700 nm and the visual colors of the
compounds in CHCl3. b) Kubelka-Munk function for diffuse reflectance spectra
of DPP-TAPP-COF, M-1 and molecular precursors TAPP and DPP-1. Spectra
are normalized to global absorption maximum.
Nitrogen sorption analysis was performed after activation at
Figure 5. Proposed mechanism for microtube formation: a) agglomeration of small DPP-TAPP-COF crystallites into sheet-like aggregates, b) smoothing
and densification of sheets by reversible imine condensations, c) rolling of the sheets due to solvent removal and d) tube formation and recombination by
reversible imine condensations.
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Financial support from the Fonds der Chemischen Industrie
(Liebig fellowship for F.B.) and the Bavarian Research Program
‘Solar Technologies Go Hybrid’ is gratefully acknowledged. T.B.
thanks the DFG for support through the research cluster
Nanosystems Initiative Munich (NIM). The research has also
received funding from the European Research Council under the
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Keywords: covalent organic frameworks • diketopyrrolopyrroles
• porphyrins • imines • microtubes
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
B. Gole, V. Stepanenko, S. Rager, M.
Grüne, D. D. Medina, T. Bein, F.
Würthner and F. Beuerle*
Page No. – Page No.
Microtubular Self-Assembly of
Covalent Organic Frameworks
Rolling up the COFs: Tetraphenylporphyrins and Diketopyrrolopyrroles have been
incorporated as functional dye components in covalent organic frameworks via
reversible imine condensations. Upon formation, these crystalline polymers
spontaneously self-assemble into hollow microtubes.
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