A 2D covalent organic framework with 4.7-nm pores and insight into its interlayer stacking

Article abstract of DOI:10.1021/ja206242v

Two-dimensional layered covalent organic frameworks (2D COFs) organize π-electron systems into ordered structures ideal for exciton and charge transport and exhibit permanent porosity available for subsequent functionalization. A 2D COF with the largest p

Full text of DOI:10.1021/ja206242v

ARTICLE
pubs.acs.org/JACS
A 2D Covalent Organic Framework with 4.7-nm Pores and Insight into
Its Interlayer Stacking
Eric L. Spitler,† Brian T. Koo,‡ Jennifer L. Novotney,† John W. Colson,† Fernando J. Uribe-Romo,†
Gregory D. Gutierrez,† Paulette Clancy,*,‡ and William R. Dichtel*,†
†Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
‡School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States
S Supporting Information
b
ABSTRACT: Two-dimensional layered covalent organic frameworks (2D COFs)
organize π-electron systems into ordered structures ideal for exciton and charge
transport and exhibit permanent porosity available for subsequent functionaliza-
tion. A 2D COF with the largest pores reported to date was synthesized by
condensing 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 4,40-dipheny-
lbutadiynebis(boronic acid) (DPB). The COF was prepared as both a high surface
area microcrystalline powder as well as a vertically oriented thin film on a
transparent single-layer graphene/fused silica substrate. Complementary molec-
ular dynamics and density functional theory calculations provide insight into the
interlayer spacing of the COF and suggest that adjacent layers are horizontally
offset by 1.7ꢀ1.8 Å, in contrast to the eclipsed AA stacking typically proposed for
these materials.
the true AA eclipsed packing structure previously proposed for
these materials. Though these offsets are relatively small relative
to the size of the COF unit cell, similar changes in packing dra-
matically affect the charge mobility of discotic liquid crystals.20,21
Thus, our simulations are critical for optimizing future 2D layered
COFs for efficient vertical charge transport.22 Given the similar-
ity of the HHTP-DPB COF to other 2D layered COF structures,
we believe these offsets are likely to be found across the entire
class of these materials.
1. INTRODUCTION
Covalent organic frameworks (COFs) organize molecular
building blocks into layered two-dimensional (2D)1ꢀ10 or three-
dimensional (3D)11,12 periodic crystalline networks that feature
high surface areas, excellent thermal stability, and extremely low
densities. The layered 2D variants stack functional π-electron
systems in van der Waals contact with maximal π-orbital overlap
ideal for charge or exciton transport5ꢀ7,13ꢀ15 and exhibit open
porous channels that run parallel to the direction of stacking.
These properties, in addition to the predictable nature of COF
design, have attracted great interest as structurally precise optoelec-
tronic materials. COFs are usually isolated as insoluble and un-
processable powders, but we recently reported the first oriented,
crystalline COF films on transparent conductive substrates.16
Oriented thin film morphologies broaden the potential applica-
tions of COFs significantly, as they might template the formation
of other nanomaterials or enable nanometer-scale patterning;17
however, these applications require pores larger than the typical
2ꢀ3 nm size range.8,18 Here we describe a 2D COF (HHTP-
DPB COF) with 4.7-nm wide hexagonal pores, the largest yet
reported, which we synthesized as both an insoluble powder and
as a vertically oriented thin film.
Nearly all 2D layered COFs have been described as fully eclipsed
structures based on their powder X-ray diffraction (PXRD) patterns,
but the relatively broad peaks do not rule out small horizontal
offsets between layers.19 We have performed a detailed molecular
mechanics and density functional theory (DFT) modeling analysis
of the interlayer potential energies of various stacking conforma-
tions of HHTP-DPB COF. These studies suggest that adjacent
layers of HHTP-DPB COF are slightly offset (1.7ꢀ1.8 Å) from
2. RESULTS AND DISCUSSION
2.1. COF Powder Synthesis and Characterization. The sol-
vothermal condensation of 4,40-diphenylbutadiynebis(boronic
acid) (1, DPB) with 2,3,6,7,10,11-hexahydroxytriphenylene
(2, HHTP) in a 1:1 mixture of mesitylene:dioxane provided
the HHTP-DPB COF as a microcrystalline powder (Scheme 1).
1 is an intriguing COF building block because of its linear structure,
extended conjugation, and minimal steric conflicts associated
with achieving a planar conformation. The HHTP-DPB COF
powders were isolated from the reaction mixture by filtration and
purified by washing with toluene and drying under vacuum.
Fourier transform infrared spectroscopy (FTIR) indicated bor-
onate ester formation, as evidenced by a sharp BꢀO stretch located
at 1354 cmꢀ1 not found in either of the reactants. The spectrum also
showed strongly attenuated hydroxyl stretches. Spectra taken
prior to activating the pores show intense ꢀCH3 stretches of
Received: July 12, 2011
Published: October 20, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of HHTP-DPB COF from Bis(boronic acid) Linker 1 and HHTP 2 (left) and Model of the Idealized bnn
Topology (right)
toluene or mesitylene, which disappear upon heating the COF at
90 ꢀC under vacuum for 72 h. In contrast, we have successfully
removed these solvents from other 2D layered COFs containing
PXRD patterns of HHTP-DPB COF showed poor crystallinity
prior to activation (Figure S15, Supporting Information), which
improved dramatically after activating the COF under vacuum at
100 ꢀC for 12 h. The diffraction pattern of the activated material
shows peaks at 2.17ꢀ, 3.79ꢀ, 5.90ꢀ, 7.93ꢀ, and 8.02ꢀ, which
correspond to the (100), (110), (210), (220), and (310) Bragg
peaks of a primitive hexagonal lattice. The crystal structure
was simulated using the Materials Studio suite of programs25
by assembling eclipsed triphenylenes into a bnn net (P6/mmm).26
The experimental PXRD displayed a diffraction pattern in
agreement with one simulated from this model, allowing facile
indexing of the diffraction peaks. Pawley refinement (Figure S18,
Supporting Information) of the pattern gave unit cell param-
eters a = b = 46.9 Å. The broad (001) diffraction peak
at 25.8ꢀ corresponds to a vertical spacing between stacked
sheets of 3.37 Å, indicating that adjacent layers are in van der
Waals contact. The relative positions of the butadiyne groups
in the COF lattice lack the appropriate spacing and angular
offset necessary to undergo topochemical polymerization,27ꢀ29
and we saw no evidence for this process by differential
scanning calorimetry or by heating the COF powders. We
also considered an alternate structure wherein adjacent sheets
are not cofacially stacked but offset by half of the unit cell
distance in the horizontal a and b planes, thus creating a gra
net (P63/mmc; Figure S19, Supporting Information). The simu-
lated PXRD pattern of these structures did not match the
experimental data.
13
HHTP at room temperature under vacuum. 1Hꢀ C CP/MAS
NMR of HHTP-DPB COF confirmed the formation of the
expected structure, and its 11B NMR spectrum consisted of a
single resonance consistent with the formation of a single type of
boronate ester linkage (see Supporting Information).
The gas adsorption properties of HHTP-DPB COF were
evaluated by N2 gas adsorption at 77 K (Figure 1A). The material
exhibits a reversible isotherm typical of mesoporous materials
most resembling Type IV, in which gas adsorption by the pores
occurs in two steps at P/P0 < 0.10 and 0.20 < P/P0 = 0.40
pressures.23 Analysis of the low-pressure region (0.05 < P/P0 < 0.20)
of the isotherm provides a Langmuir surface area of 1290 m2 gꢀ1
and BET surface area of 930 m2 gꢀ1 (see Figure S21, Supporting
Information).24 To compare the measured gas adsorption capa-
city of the prepared COF with its maximum uptakes, we
simulated an isotherm using Monte Carlo simulations using
the Metropolis method in Materials Studio (Figure S22, Sup-
porting Information), from which a maximum BET surface area
of 2640 m2 gꢀ1 was calculated. This value is quite similar to the
calculated Connolly surface area of 2670 m2 gꢀ1 for the eclipsed
HHTP-DPB COF structure and consistent with that observed in
other frameworks. These calculations suggest that the measured
COF has been activated to approximately 40% of its maximum
uptake, and further optimization is ongoing. The hysteresis
observed during desorption is typical of interparticle adsorption
and has been observed in other COFs. These particles were
uniform spheres with 4 μm diameters, as observed by scanning
electron microscopy (SEM, Figure 1B). HHTP-DPB COF retained
92% of its mass up to 350 ꢀC by thermal gravimetric analysis
(Figure S20, Supporting Information).
2.2. Simulation of HHTP-DPB COF Interlayer Packing. It is
important to note that almost all other 2D COFs have been
described as eclipsed layered structures, but the X-ray diffraction
peaks of these materials are too broad to preclude slight interlayer
offsets. We performed complementary simulations to under-
stand the 2D stacking of HHTP-DPB COF beyond the informa-
tion available from the PXRD data. Two levels of theory were
used to model the intermolecular interactions between COF layers:
The crystallinity and unit cell parameters of the HHTP-DPB
COF were determined by PXRD (Cu Kα line, Figure 2). The
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Figure 3. Potential energy surface generated from the semiempirical
MM3 potential. The surface can be divided into three regions with
distinct energetic properties: A, Low energy region containing eclipsed
and near-eclipsed two-layer structures; B, region containing all staggered
structures, shown schematically in Figure 4A; C, high-energy region
containing the structures shown in Figure 4B.
The MM3-derived PES (Figure 3) was generated by varying
the x- and y-translational offsets of two adjacent COF layers and
recalculating the interlayer spacing at each point to minimize the
interaction energy. The PES contains three regions (labeled
AꢀC) of relatively flat ‘terrain’ bounded by steep gradients. Region
A exhibits significantly more stabilizing interaction energies
(ꢀ2.8 eV) than regions B and C (ꢀ0.8 and ꢀ0.4 eV, respectively)
and corresponds to structures that are eclipsed or nearly eclipsed.
The interlayer spacing that minimizes the interaction energy in
this region is 3.45 Å, which is within 0.1 Å of the experimental
value. In the MM3 calculations, the potential energy of the fully
eclipsed structure is only 0.13 eV higher than the minimum
energy calculated in region A, which occurs at offsets that define a
ring of radius 1.6 Å around the origin. This result indicates almost
no preference for nearly eclipsed structures, but repulsive electro-
static forces that occur at the origin are likely to be underestimated at
the MM3 level of theory.
Figure 1. (A) N2 adsorption (blue circles) and desorption (red squares)
isotherms for HHTP-DPB COF. (B) Scanning electron microscope
(SEM) image of HHTP-DPB COF powder.
Regions B and C of the PES correspond to offsets that reduce
the van der Waals contact between layers, each of which is higher
in energy than region A. The flatter regions at the corners of the
PES form region B, which correspond to gra layers (see Figure 4B).
The potential energy in this region varies from a peak of about
ꢀ0.7 eV to a valley of about ꢀ0.9 eV. The average binding energy
of this region is less than a third of the binding energy of near-
eclipsed structures in region A. The energy difference between
regions A and B is about 2.0 eV, which is a very significant
potential energy increase from the bottom of the potential energy
well in region A. There are no significant potential energy barriers
that might prevent reorganization of COF layers from region B to
region A, suggesting that forming these structures is unlikely. As
shown in Figure 4, there is less overlap in this configuration than
in a more eclipsed configuration, which lowers the contribution
of the van der Waals component to the intermolecular energy.
However, monopoles in adjacent layers are staggered by 60ꢀ so
that the repulsive interaction between them is less than in the
eclipsed case. The potential energy gradient near staggered confi-
gurations is less steep than the gradient near eclipsed configura-
tions because near-staggered configurations balance the decrease
in HHTP overlap with an increase in linker overlap. Configurations
Figure 2. Experimental (blue) and Pawley refined (red) vs predicted
(green) PXRD patterns of HHTP-DPB COF and difference plot
(experimental, refined; black). Major observed reflections are labeled.
molecular mechanics, using the MM3 force field, which describes
hydrocarbon and ringed aromatic systems reliably,30,31 and DFT,
a more accurate quantum mechanical treatment that accounts for
electronic interactions.32 Molecular dynamics simulations are
orders of magnitude faster than static DFT calculations and can
explore a far broader set of configurations. Therefore, we used
molecular mechanics to broadly define the potential energy
surface (PES) associated with offsetting two layers of the HHTP-
DPB COF and subsequently refined this model in regions of
interest using DFT.
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Figure 4. (A) Expansion of region A in Figure 3 calculated with density functional theory. (B) Expansion of a subset of region B in Figure 3 calculated
with density functional theory and atomic representation of the two-layer staggered structure.
of layers with the highest potential energy are the triangle-shaped
protrusions near the edges of the PES, labeled as region C in
Figure 3. The energy of arrangements in this region is approx-
imately ꢀ0.4 eV, which is 0.4 eV higher than the energies in
region B, and 2.4 eV higher than arrangements in region A. The
gradient between regions B and C attains a maximum value of
0.1 eV/Å. Because there is a constant negative gradient in region
C, these structures are also unstable and unlikely to form. These
are the most energetically unfavorable configurations because the
structures have no molecular overlap between HHTP units and
only minimal overlap of linkers in adjacent layers. This reduces
the van der Waals component of the intermolecular energy to the
smallest value possible and significantly raises the potential
energy of structures in this region. On the basis of these findings,
we did not perform further calculations on structures in region C
We next generated a new PES (Figure 4) for regions A and B
using more accurate DFT calculations as expressed in Gaussian09
using the M06 exchange correlation functional.32ꢀ34 The overall
potential energy surface is qualitatively similar to that generated
from the MM3 potential but with more finely corrugated sur-
face features. Configurations with the lowest energy, of about
ꢀ2.9 eV, are found 1.7ꢀ1.8 Å from the origin with an interlayer
spacing of 3.42 Å, which is close to the experimental value and
matches that of other 2D COFs. DFT calculations for structures
in region B confirm that staggered structures remain significantly
higher in energy (2.4 eV) than the eclipsed structures of region A.
Thus, the MM3 model can give a reasonably accurate result for
COF structures in a fraction of the time needed for ab initio
calculations.
significantly higher (1.1 eV) than that for the more stable structures
just 1.7ꢀ1.8 Å away. These differences arise from DFT’s more
precise treatment of electrostatic forces. The variation in poten-
tial energy around the ring is 0.1 eV, compared with the insignificant
3 ꢁ 10ꢀ3 eV variations predicted by MM3. The net result is that,
while MM3-generated results suggest that perfectly eclipsed
structures are accessible, DFT calculations predict this is much
less likely. Because this offset has no preferred direction, the
average representation of the 3D structure of the COF cannot be
reduced to a simple monoclinic unit cell. It is unlikely that there
will be a prevalence of patterns, such as staircase, zigzag, or helical
arrangements of layers that would be observable by X-ray diffraction.
Instead, adjacent layers are more likely to stack in random arran-
gements around a preferred offset from the origin, similar to tur-
bostratic disorder commonly observed in layered materials. These
random offsets are difficult to observe in low resolution powder
diffraction data, as they would generally affect only the relative
intensity and width of certain diffraction peaks, while affecting
peak positions only at small d-spacings (neither of which has been
observed experimentally). We also compared the accessible surface
area from the Connolly surfaces for eclipsed HHTP-
DPB COF with that for 1.0 Å and 1.7 Å interlayer offsets. No
significant differences were found, as each of these structures had
accessible surface areas close to 2700 m2 gꢀ1 (Table S2, Supporting
Information). Thus, most methods used for COF characteriza-
tion are unable to distinguish between eclipsed and slightly offset
structures. In COFs designed for optoelectronic applications,
small changes in these offsets will strongly affect charge mobilities
through the stacked aromatic systems,21 and the computational
methods described above will be important to understand their
performance and design optimal new materials.
Despite the qualitative agreement between the MM3 and DFT
calculations, there are important differences between the two
approaches. The DFT PES suggests that fully eclipsed structures
are unlikely, because they show a potential energy maximum ꢀ1.8 eV,
2.3. Synthesis and Characterization of Vertically Oriented
HHTP-DPB COF Films on Single Layer Graphene. Despite
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Figure 6. (A) Transmission absorption (red) and normalized emission
(blue) spectra of HHTP-DPB COF thin film (λexc = 357 nm) and (B)
photographs of fluorescent COF film on SLG/SiO2 (left) and powder
(right) under 365 nm illumination.
Figure 5. (A) Grazing incidence X-ray diffraction (GID) of HHTP-
DPB COF thin film on SLG (growth time: 24 h). (B) Projection of A
near Q^ = 0. (C) Cross-sectional SEM image of the film.
aspects of the COF structure impact the fluorescence of the
diphenylbutadiyne chromophores. First, catechol boronate
ester linkages have shown evidence of extended conjugation
in linear polymers.36 Though the conjugation efficiency of
these groups is not as high as other linkages, this effect should
be maximized in a COF given the coplanar relationship among
the chromophores. Similarly, diphenylbutadiynes capable of
free rotation of each phenyl group generally have very low fluo-
rescence quantum yields, which increase dramatically when the
phenyl rings are fixed into a coplanar arrangement.37,38 Finally,
vertical stacking of the diphenylbutadiynes can also lead to a red-
shifted emission.39ꢀ41
their intriguing structures, it remains difficult to perform ad-
vanced spectroscopy on COF powders or incorporate them into
devices. We overcame this limitation recently by growing COF
thin films on single-layer graphene (SLG) functionalized substrates.16
We prepared crystalline, vertically oriented HHTP-DPB COF
thin films by condensing 1 and 2 under solvothermal conditions
in the presence of SLG on a transparent fused SiO2 substrate
(SLG/SiO2). Grazing incidence X-ray diffraction (GID, Figure 5A)
indicates scattering intensity at 0.156 Åꢀ1, 0.271 Åꢀ1, 0.311 Åꢀ1
,
0.411 Åꢀ1, 0.543 Åꢀ1, and 0.568 Åꢀ1, corresponding to the same
(100), (110), (200), (210), (220), and (310) peaks observed in
the powder samples. The intensity of these diffractions is concen-
trated near Q^ = 0, indicating that the c-axis of the COF is
oriented normal to the substrate surface. The (001) Bragg peak
that appears at Q = 1.83 Åꢀ1 in powder samples is not observed
in the GID experiment (Figure S25, Supporting Information),
again indicating that the c-axis is specifically oriented normal
to the substrate. Instead, the (001) peak is observed at Q^ =
1.85 Åꢀ1 in measurements performed at large out-of-plane
diffraction angles (Figure S26, Supporting Information). Top-
down SEM images indicate that the films are featureless over
large areas and have only occasional bulk crystallites distributed
across the surface (Figure S27, Supporting Information). Cross-
sectional micrographs obtained by milling the sample using a Ga+
focused ion beam indicate that the films are continuous across
the substrate with thickness 132 ( 18 nm (Figure 5C). The large
pore size obtained in the thin film morphology may also serve as a
useful template for nanopatterning, as features in the 2ꢀ5 nm
region are difficult to obtain using either standard lithographic
techniques or block copolymer lithography.35
3. CONCLUSIONS
We have demonstrated that the pore sizes of oriented 2D
layered COF films can be pushed well into the mesoporous regime,
which significantly expands the range of complementary molec-
ular, polymeric, or inorganic guests that might be co-organized
with these unique materials. Our computational studies strongly
suggest that 2D layered COFs do not adopt true eclipsed structures
usually reported for these materials, but that the layers are slightly
offset from one another. These calculations are easily generalized
to other 2D layered COFs and will be critical for understanding
interlayer exciton and charge transport, two processes of funda-
mental importance for COF-based optoelectronic devices. Highly
luminescent COF films are of interest for fluorescent sensors,42,43
for which structural precision, tunable composition, and high surface
areas will prove extremely useful.
’ ASSOCIATED CONTENT
The HHTP-DPB COF powders and films are strongly photo-
luminescent (Figure 6), which we attribute to the cofacially
packed diphenylbutadiyne subunits, as other HHTP-containing
COFs do not fluoresce strongly. UV/vis spectra of the films
obtained through the transparent SLG/SiO2 substrate are con-
sistent with the presence of the triphenylene and diphenylbuta-
diyne chromophores and show only moderate tailing at longer
wavelengths associated with scattering. Photoemission spectra of
the films show a λmax of 457 nm (λexc = 357 nm) that is red-
shifted by 55 nm relative to that of 1 dissolved in DMF. Several
S
Supporting Information. Experimental procedures and
b
characterization data for all new compounds and materials and
more details on the offset calculations and simulated surface area
of HHTP-DPB COF. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
wdichtel@cornell.edu; pc@cbe.cornell.edu
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W.R.D. acknowledges support from the NSF CAREER pro-
gram (CHE-1056657) and the NSF-funded CCI-I Center for
Molecular Interfacing (CHE-0847926). This work is based upon
research conducted at the Cornell High Energy Synchrotron
Source (CHESS), which is supported by the National Science
Foundation and the National Institutes of Health/National
Institute of General Medical Sciences under NSF award DMR-
0936384. We also made use of the Cornell Center for Materials
Research (CCMR) facilities with support from the NSF Materi-
als Research Science and Engineering Centers (MRSEC) pro-
gram (DMR-0520404). E.L.S. acknowledges the award of the
American Competitiveness in Chemistry postdoctoral fellowship
(ACC-F) from the NSF (CHE-0936988). B.T.K. acknowledges
the NSF Integrative Graduate Education and Research Trainee-
ship (IGERT) Program in Materials for a Sustainable Future
(DGE-0903653). J.L.N. acknowledges the NSF IGERT Program
in the Nanoscale Control of Surfaces and Interfaces (DGE-
0654193). J.W.C. acknowledges the award of a Graduate Re-
search Fellowship from the NSF. We thank Dr. Arthur Woll at
CHESS for helpful discussions.
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