Highly emissive covalent organic frameworks

Article abstract of DOI:10.1021/jacs.6b02700

Highly luminescent covalent organic frameworks (COFs) are rarely achieved because of the aggregation-caused quenching (ACQ) of stacked layers. Here, we report a general strategy to design highly emissive COFs by introducing an aggregation-induced emission (AIE) mechanism. The integration of AIE-active units into the polygon vertices yields crystalline porous COFs with periodic stacked columnar AIE arrays. These columnar AIE arrays dominate the luminescence of the COFs, achieve exceptional quantum yield via a synergistic structural locking effect of intralayer covalent bonding and interlayer noncovalent interactions and serve as a highly sensitive sensor to report ammonia down to sub ppm level. Our strategy breaks through the ACQ-based mechanistic limitations of COFs and opens a way to explore highly emissive COF materials.

Full text of DOI:10.1021/jacs.6b02700

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Communication
Highly Emissive Covalent Organic Frameworks
Sasanka Dalapati, Enquan JIN, Matthew Addicoat, Thomas Heine, and Donglin Jiang
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Highly Emissive Covalent Organic Frameworks
Sasanka Dalapati1, Enquan Jin2, Matthew Addicoat3, Thomas Heine3, and Donglin Jiang1*
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1Field of Energy and Environment, School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai,
Nomi 923-1292, Japan
2Department of Structural Molecular Science, School of Physical Science, SOKENDAI, Hayama, Kanagawa 240-0193, Japan
3Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstrasse 2, 04103 Leipzig, Germany
Supporting Information Placeholder
SCHEME 1. Schematic representation for the syn-
thesis of TPE-Ph COF with TPE units at the ver-
tices of the dual-pore Kagome lattice.
ABSTRACT: Highly luminescent covalent organic frame-
works (COFs) are rarely achieved because of the aggrega-
tion-caused quenching (ACQ) of π-π stacked layers. Here,
we report a general strategy to design highly emissive COFs
by introducing an aggregation-induced emission (AIE)
mechanism. The integration of AIE-active units into the
polygon vertices yields crystalline porous COFs with peri-
odic π-stacked columnar AIE arrays. These columnar AIE π-
arrays dominate the luminescence of the COFs, achieve ex-
ceptional quantum yield via a synergistic structural locking
effect of intralayer covalent bonding and interlayer noncova-
lent π-π interactions, and serve as a highly sensitive sensor to
report ammonia down to sub ppm level. Our strategy breaks
through the ACQ-based mechanistic limitations of COFs
and opens a way to explore highly emissive COF materials.
Covalent organic frameworks (COFs) are a class of crys-
talline porous polymers that enable the precise integration of
organic units into periodic columnar π-arrays and inherent
pores, making them useful in various applications.1-6 The π-π
layered structures easily trigger the thermal decay of photo-
excited states and result in less or non-emissive COFs. This
aggregation-caused quenching (ACQ) mechanism is ubiqui-
tous for COFs, and this mechanistic limitation is the reason
why highly luminescent COFs are rarely achieved6a-c even
though various luminescent π-units have been employed for
the synthesis of COFs.5c,6d,e In this study, we developed a
general strategy for designing highly luminescent COFs by
introducing aggregation-induced emission (AIE) mecha-
nism to the π-frameworks in which the intralayer covalent
link and interlayer noncovalent π-interaction work synergis-
tically to impede the rotation-induced energy dissipation.
none of them consists of π-stacked ordering structures. The
synthesis of AIE-based COFs that satisfy the requirements of
π-ordering, porosity, and high luminescence is important to
the advancement of AIE materials. However, AIE-based
COFs and their functions are unprecedented.
We designed an AIE-active tetraphenylethene (TPE) unit
for the synthesis of a highly emissive two-dimensional COF
(Figure 1, TPE-Ph COF; Supporting Information (SI); Fig-
ures S1-S5). Solvothermal condensation of TPE-cored bo-
ronic acids (TPEBA) and 1,2,4,5-tetrahydroxybenzene
(THB) in a mixture of dioxane/mesitylene (1/1 by vol.) at
AIE is a phenomenon that chromophores become emissi-
ve in aggregated states and has shown broad applications in
light-emitting diodes, light-harvesting, photonic devices,
chemo-sensing, and bio-imaging.7 Although porous poly-
mers with AIE building units have been reported,8,9
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Figure 1. FE-SEM images of the TPE-Ph COF samples prepared at different reaction times of (A) 3 days, (B) 10 days, (C) 20 days,
and (D) 30 days. High-resolution SEM images of the TPE-Ph COF samples prepared at different times of (E) 20 days and (F) 30 days.
(G) PXRD patterns of TPE-Ph COF (experimentally observed (red), Pawley refined (green), difference between experimental and
Pawley refined data (black), calculated for the AA-stacking mode of dual-pore Kagome structure (blue), AB-stacking mode of dual-
pore Kagome structure (pink), and AA-stacking mode of single-pore rhombic structure (orange)). Crystal structures of the dual-pore
Kagome TPE-Ph COF in (F) AA and (I) AB stacking modes, and (J) single-pore rhombic AA-stacking mode.
90 °C for three days yielded light yellow crystallites in 71%
isolated yield (SI). Field-emission scanning electron micros-
copy (FE-SEM) revealed that TPE-Ph COF has rectangular
belts and sharp faces, edges, and corners (Figures 1A-F).
These well-defined morphologies encouraged us to monitor
the time-dependent growth of the TEP-Ph COF crystallites.
The belt length was approximately 4 µm on average for the
COF samples after three-day reaction (Figure 1A). When
the reaction time was extended to 10 days, the belts extended
to 6 µm (Figure 1B) while decreasing their width to nearly 1
µm. We further extended the reaction time to 20 and 30 days,
and observed that the belts further increased to 16 and 53
µm, respectively (Figures 1C-F). These changes show that
the COF belts grow while maintaining rectangular shapes
with clear faces, edges, and corners. The TPE-Ph COF is
stable up to 400 °C under nitrogen atmosphere (Figure S6).
located at the vertices and the phenyl linkers occupy at the
edges of the polygons to form periodic columnar TPE π-
arrays and a dual-pore Kagome lattice with trigonal mi-
cropores and hexagonal mesopores.
Nitrogen sorption measurements were conducted at 77 K
to evaluate the porosity. The sorption curves were classified
as a combination of typical type I and IV isotherms, which
are characteristic of micropores and mesopores, respectively
(Figure 2A). The Brunauer–Emmett–Teller (BET) surface
area and pore volume were estimated to be 962 m2 g–1 and
0.5 cm3 g–1, respectively. Theoretical surface area for the AA-
stacked TPE-Ph COF (Figure S8) was calculated to be 1005
m2 g–1, which is close to experimental value. The pore size
distribution was evaluated using the nonlocal density
functional theory and the pore sizes were calculated to be 1.3
and 2.6 nm (Figure 2B), which are attributed to the
micropores and mesopores, respectively. Both crystal
structure and nitrogen sorption analysis suggest a dual-pore
Kagome lattice for the TPE-Ph COF.
The TPE-Ph COF exhibited strong diffraction signals at
2.74°, 4.82°, 5.58°, 7.28°, 8.54°, 10.30°, and 18.52°, which
were assigned to be 100, 110, 200, 210, 300, 400, and 001
facets, respectively (Figure 1G, red curve; for enlarged pro-
file, see Figure S7). Pawley refinements yielded a PXRD pat-
tern (green curve) that well matched with the experimentally
observed one, as reflected by their negligible differences
(black curve). The lattice parameters were determined as a =
Figures 2C-F shows the fluorescence microscope images
of the TPE-Ph COF samples prepared after 3-, 10-, 20-, and
30-day reactions. Surprisingly, all the TPE-Ph COF samples
exhibited brilliant luminescence in the same color, irrespec-
tive of the reaction time. More interestingly, each belt was
highly luminescent and the luminescence was homogeneous-
ly observed across the whole belt (Figure 2F, inset). These
results indicate that the structural growth does not alter the
luminescent function of TPE-Ph COF.
b = 37.466 Å, c = 4.45 Å, α = β = 90°, and γ = 120° (SI).
DFTB calculations10 revealed a dual-pore Kagome lattice
and a single-pore rhombic lattice with AA and AB stacking
modes, respectively (SI). The AA stacking mode (Figure
1H) of the Kagome lattice reproduced the peak position and
intensity of the PXRD pattern (Figure 1G, blue curve),
whereas the AB stacking mode (pink curve) deviated from
the experimentally observed PXRD pattern. The simulated
peak positions of the rhombic lattice (Figure 1J) were com-
pletely different (Figure 1G, orange curve) from the experi-
mentally observed PXRD pattern. Thus, the TPE units are
The TPE model compound exhibited an absorption band
at 385 nm, whereas that of the TPE-Ph COF exhibited a
peak centered at 390 nm (Figure S9). Upon excitation, the
solid samples of the TPE model compound and TPE-Ph
COF emitted brilliant blue luminescence, with peak maxima
at 480 and 500 nm, respectively (Figure 2G). The absolute
fluorescence quantum yield of the TPE-Ph COF solid sam-
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Figure 2. (A) N2 adsorption (l) and desorption (¡) isotherm curves and (B) pore size distribution profiles of the TPE-Ph COF.
Fluorescence microscopy images of TPE-Ph COF samples prepared at different reaction times of (C) 3 days, (D) 10 days, (E) 20 days,
and (F) 30 days (the yellow rectangle contains an inset image showing an enlarged view of the belts).(G) Fluorescence spectra and
(H) lifetime profile of the TPE model compound (black) and the TPE-Ph COF (blue). The green curve in (H) is the instrument re-
sponse function. (I) Fluorescence quantum yield of the TPE-Ph COF compared to the model compound. (J) Fluorescence spectral
change of the TPE-Ph COF upon addition of ammonia. (K) Stern-Volmer plot of the fluorescence quenching by ammonia.
ples was observed to be as high as 32%, whereas the solid
samples of the TPE model compound exhibited an absolute
quantum yield of 15% (Figure 2I). Compared to the model
compound, the TPE-Ph COF exhibited two to three-fold
high luminescence quantum yields (Figure 2I). This en-
hancement in the solid state suggests that the π-π interac-
tions between layers of COFs additionally contribute to the
higher fluorescence quantum yield. Time-resolved fluores-
cence spectroscopy revealed that the lifetimes of the model
compound and TPE-Ph COF were 3.8 and 0.7 ns, respec-
tively (Figure 2H). By combining the lifetime and quantum
yield, we evaluated the radiative rate constant to be 4.6 × 108
require a rotation energy of 9.9 kcal mol–1 (at 70° dihedral
angle; Figure S10), which is higher than that (7.1 kcal mol–1)
of the TPE vertex analog (Figure S11). By contrast, for the
layered TPE-Ph COF, this barrier was as high as 216.7 kcal
mol–1 (Figure S12). Further enhancement of the dihedral
angle to greater than 70° drastically increased the rotation
barrier energy for the layered TPE-Ph COF, whereas the
barriers were hardly changed for the vertex-analog and the
monolayer (Figures S10-S12). At the monolayer level, the
four phenyl groups of the TPE vertices are connected to four
different linkers that substantially decrease the free rotation
of the phenyl groups. At the layered framework level of the
TPE-Ph COF, the π-π stacking further restricts the rotation
of the four phenyl groups.7 Therefore, the structural syner-
gies, i.e., the intralayer covalent bond and interlayer non-
covalent packing, work together to prevent the excited state
distortion9b, and hence the excited energy immediately re-
leases via fluorescence decay. The above structural insights
explain why the quantum yield of the TPE-Ph COF is much
higher than that of the TPE model compound. To the best of
s–1, which was 3.9 × 107 s–1 for the model compound. These
results indicate that the TPE-Ph COF facilitates the radiative
process, leading to a rate constant one order of magnitude
greater than the model compound.
DFTB calculations of the TPE vertex analog, COF mono-
layer, and AA-stacked COF support the existence of the
structural effects. The phenyl rings in the COF monolayer
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Boronate linkages of the TPE-Ph COF form Lewis acid-
base pair when interacted with ammonia because boronate
serves as Lewis acid and ammonia is a Lewis base.1c Based on
this Lewis acid-base interaction, we explored the TPE-Ph
COF as a highly sensitive fluorescence sensor for ammonia.
Indeed, upon addition of ammonia, the TPE-Ph COF (2 mg
COF in 2 mL toluene) exhibited a rapid response to ammo-
nia and decreased its luminescence (Figure 2J). Stern-
Volmer plot revealed an almost linear curve, whereas the
fluorescence quenching rate constant kq (= kSV/τ) was evalu-
ated to be as high as 4.1 × 1014 M–1 s–1 (Figure 2K). We can
further decrease the amount of COF to 0.25 mg (2 mL tolu-
ene) for the detection of ammonia. In this case, 1 ppm am-
monia could lead to 30% down of fluorescence intensity
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(Figure S13), leading to a kq value of 6.3 × 1014 M–1 s–1. This
ammonia sensing was also observed in cyclohexane, whereas
the kq value was 1.4 × 1014 M–1 s–1 (Figure S14). These ex-
tremely high kq values suggest that the TPE-Ph COF is a
highly sensitive ammonia detector at sub ppm level.
In summary, we have developed a novel way to highly
emissive COFs by exploring aggregation-induced emission
mechanisms for structural design. Integrating TPE units into
the vertices of the COFs significantly reduces the rotation-
induced excitation-energy dissipation and reveals a synergis-
tic structural locking effect originating from the covalent
bonding and noncovalent π-π interactions. This discovery
could lead to a new generation of highly luminescent COF
materials that retain high quantum yields in both solid and
solutions, and function as highly sensitive sensor to detect
specific chemicals. Our AIE-based concept breaks through
ACQ-related mechanistic limitations of COFs and offers a
platform for exploring highly ordered yet emissive frame-
work materials. We anticipate that the high luminescence
together with the confined porous structure will be useful in
sensing, imaging, and lasing applications.
ASSOCIATED CONTENT
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Supporting Information
Materials and methods, syntheses and characterizations, Tables
S1, S2, and Figures S1-S14. These materials are available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
djiang@jaist.ac.jp
ACKNOWLEDGMENT
S.D. is an International Research Fellow of the Japan Society for
the Promotion of Science (JSPS).
REFERENCES
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