Perylene-based covalent organic frameworks for acid vapor sensing

Article abstract of DOI:10.1021/jacs.9b08079

Traditionally, the properties and functions of covalent organic frameworks (COFs) are defined by their constituting building blocks, while the chemical bonds that connect the individual subunits have not attracted much attention as functional components of the final material. We have developed a new series of dual-pore perylene-based COFs and demonstrated that their imine bonds can be protonated reversibly, causing significant protonation-induced color shifts toward the near-infrared, while the structure and crystallinity of the frameworks are fully retained. Thin films of these COFs are highly sensitive colorimetric acid vapor sensors with a detection limit as low as 35 μg L^-1 and a response range of at least 4 orders of magnitude. Since the acidochromism in our COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously the concentration and protonation strength of nonaqueous acid solutions, in which pH electrodes are not applicable, and to distinguish between different acids. Including the imine bonds as function-determining constituents of the framework provides an additional handle for constructing multifunctional COFs and extending the range of their possible applications.

Full text of DOI:10.1021/jacs.9b08079

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Perylene-Based Covalent Organic Frameworks for Acid Vapor
Sensing
Laura Ascherl,† Emrys W. Evans,‡ Jeffrey Gorman,‡ Sarah Orsborne,‡ Derya Bessinger,†
Thomas Bein,† Richard H. Friend,‡ and Florian Auras*,‡
†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: Traditionally, the properties and functions of covalent organic
frameworks (COFs) are defined by their constituting building blocks, while the
chemical bonds that connect the individual subunits have not attracted much
attention as functional components of the final material. We have developed a
new series of dual-pore perylene-based COFs and demonstrated that their imine
bonds can be protonated reversibly, causing significant protonation-induced color
shifts toward the near-infrared, while the structure and crystallinity of the
frameworks are fully retained. Thin films of these COFs are highly sensitive
colorimetric acid vapor sensors with a detection limit as low as 35 μg L−1 and a
response range of at least 4 orders of magnitude. Since the acidochromism in our
COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously
the concentration and protonation strength of nonaqueous acid solutions, in which pH electrodes are not applicable, and to
distinguish between different acids. Including the imine bonds as function-determining constituents of the framework provides
an additional handle for constructing multifunctional COFs and extending the range of their possible applications.
popular imines, on the other hand, can enable electronic
communication across the bond but act as very strong
acceptors, causing extremely fast nonradiative quenching of
excited states.18,20,28 Recently, strategies to overcome these
adverse characteristics of imines via postsynthetic trans-
formations into oxazoles, thiazoles, or quinolines have been
developed.29−31 However, little effort has been undertaken
thus far to make use of the imine bonds themselves and
employ them as a functional part of the final framework.
Here we utilize the lone electron pair of the imine linkages
as the function-determining moiety of a new series of two-
dimensional COFs. The COFs are based on a newly developed
perylene tetraaniline building block and adopt a star-shaped
dual-pore topology. Protonation of the imine bonds in these
materials causes significant absorption red-shifts and the
formation of protonation-induced absorption bands in the
near-infrared, while the structure and crystallinity of the
frameworks are fully retained. We find that the acidochromism
depends on both acid strength and concentration, allowing us
to use this effect for sensing and distinguishing between acids
in solution and in the vapor phase. We demonstrate that
perylene COF thin films can serve as highly sensitive
colorimetric acid vapor sensors with a response range of at
least 4 orders of magnitude.
INTRODUCTION
■
Covalent organic frameworks (COFs) are two-dimensional1−3
or three-dimensional4,5 crystalline, fully organic networks, in
which the key physical properties, such as the type and energy
of optical transitions6,7 or the electrical conductivity,8−10 are
highly tunable. These materials can also be seen as a periodic
lattice of extremely well-defined pores.11−14 The environment
that a guest molecule encounters in these pores can vary over a
wide range; e.g., it can be hydrophilic or hydrophobic or can
include different functional groups on the pore walls.15−17
When constructing new COFs and defining their properties,
the focus lies traditionally on the choice of appropriate
building blocks, such as aryls,18,19 (fused) thiophenes,20−22 or
other extended heteroarenes.23−25 The bond that connects
them, however, is often considered a necessary evil. The
linkage chemistry is selected to be reversible enough to provide
the required self-healing capabilities during the COF synthesis
yet yield a sufficiently stable framework in order to avoid fast
degradation during the intended application.
Apart from these structural aspects, the bond is primarily
meant to interfere as little as possible with the function of the
framework. In fact, the linkage chemistry is often regarded as
one of the limiting factors for potential applications of COFs,
especially in terms of their practicability in optoelectronics or
electrochemical applications. Nonconjugated bonds such as
boronate esters have little impact on the adjacent building
blocks but impede in-plane electronic communication and
charge transport.26,27 Conjugated linkages such as the very
Received: July 28, 2019
© XXXX American Chemical Society
A
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Figure 1. Construction of the new perylene COFs. (a) The solvothermal co-condensation of the perylene tetraaniline with linear dicarbaldehydes
yields the star-shaped dual-pore Per-1P, Per-N, and Per-Py COFs. (b−d) Experimental PXRD patterns of the perylene COFs (black symbols).
Pawley refinements (red lines) provide a good fit with only minimal differences (green) between the experimental and the refined patterns. Bragg
positions are indicated by blue ticks. (e−g) Force-field-derived, Pawley refined structure models of the three perylene COFs. The Connolly surfaces
are calculated for a nitrogen-sized probe (r = 0.184 nm).
The new perylene COFs were synthesized via solvothermal
co-condensation of Per(NH2)4 with the phenylene-, naph-
thalene-, and pyrene-based dicarbaldehydes (see the Support-
ing Information, section B for experimental details).
The powder X-ray diffraction (PXRD) patterns of the COFs
show sharp reflections and several well-resolved higher-order
reflections (Figure 1b−d). Due to the very large unit cells of
these COFs, the overlap between some reflections even at low
diffraction angles is unavoidable. The COFs were indexed and
RESULTS AND DISCUSSION
■
While the range of tetradentate aldehyde building blocks for
2D COFs has expanded significantly over the past years, tetra-
amines are still limited to only three building blocks, i.e., the
pyrene, ethene, and porphyrin tetraanilines.12,16,26 In order to
extend this range toward larger aromatic units, we have
developed a perylene-based building block. The 2,5,8,11-
tetrakis(4-aminophenyl)perylene (Per(NH2)4) generates star-
shaped COFs with small trigonal and large hexagonal pores
(Figure 1).
B
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Figure 2. Protonation of the Per-N COF with TFA vapor. (a) The imine-linked bridges of the Per-N COF can be protonated stepwise, causing a
fully reversible color change from yellow to dark brown. (b) Absorption spectra of a Per-N COF film upon protonation in TFA vapor of increasing
concentration (green to red lines). The COF film has a response range of at least 4 orders of magnitude, and the protonation can be fully reversed
by exposing the protonated COF to triethylamine vapor (gray dashed line). (c) Absorption difference between the protonated and fully
deprotonated COF showing a protonation-induced absorption band across the visible and near IR range, accompanied by a bleach band in the blue
and UV. Inset: absorption difference spectra normalized to their respective maximum. The absorption feature formed at low TFA concentrations
peaks at 500 nm with almost no absorption above 600 nm. At higher acid concentrations, however, an additional absorption feature develops in the
600−800 nm region. We attribute these two features to the monoprotonated and the bisprotonated imine-linked bridges, respectively (see
discussion below). Abbreviations: TFA = trifluoroacetic acid.
Pawley-refined in the hexagonal space group P6 with three
symmetry-equivalent perylenes per unit cell (Figure 1e−g).
For comparison purposes, we also synthesized a range of
COFs based on the literature-known tetraphenylpyrene,7,16,18
tetraphenylethene,3,12 and triphenyltriazine32 building blocks.
The syntheses and structures of these COFs are shown in the
SI, sections B and D.
deprotonation (Figure 2a). The protonated perylene, pyrene,
and triazine COFs can be stored in humid air for several hours
without any noticeable degradation. Only the 4PE-TT COF,
where the steric stabilization is reduced due to its twisted
imine-linked bridges and significantly larger layer spacing (4.6
nm vs ca. 3.5 nm for the other COFs), is unstable toward
humidity when protonated. A video showing the repeated
protonation and deprotonation of a pyrene COF film is
supplied in the SI.
The protonation of the imine bonds can be achieved by
exposing the COF powders or thin films to acid solutions or
vapors and can be followed by infrared spectroscopy (SI,
Figure S7). We observe the appearance of the CNH+
stretching vibration around 1650 cm−1, accompanied by an
attenuation of the imine CN stretching mode around 1610
cm−1.36
The lowest-energy optical transitions of imine-linked COFs
have significant charge-transfer character with the ground-state
electron density being distributed across the frameworks, while
the first singlet excited state is more localized on the imines
(SI, Figure S14). Since the protonated imines are stronger
electron acceptors than their free-base counterparts, the
energies of these transitions are lowered, leading to red-shifted
absorption and emission spectra. Most COFs are constructed
with two electronically coupled imines per bridge unit. In this
case, the simultaneous protonation of both imines increases
the charge transfer character even further and enables an
additional stabilization via quinoidal resonance structures.
For the perylene COF series, we observe a color change
from greenish-yellow to dark brown, caused by the appearance
of a protonation-induced absorption band in the 450−800 nm
region that is accompanied by a bleach band below 450 nm
(SI, Figure S5-1). Similar color shifts are observed for the other
COFs (SI, Figures S8 and S14a).
In order to facilitate the spectroscopic analysis and
application as sensors, the COFs were also grown as thin
films on fused silica, sapphire, or indium tin oxide coated glass
substrates (see the SI, section B for experimental details). 2D
grazing-incidence wide-angle X-ray scattering (GIWAXS)
measurements confirm that the films consist of the crystalline
COFs (SI, Figure S4). For the perylene COFs, we observe that
the intensities of the hk0 reflections are distributed almost
evenly across semicircles with only slightly higher intensities
close to the substrate horizon, indicating that these COF films
are polycrystalline with random crystallite orientation (SI,
Figure S4-1). This is in sharp contrast to the other COFs in
this study and other reported COF films,33,34 which are highly
textured with their respective crystallographic a−b planes
parallel to the substrate. We attribute this random orientation
to the significantly higher reactivity of the Per(HN2)4 building
block compared to the pyrene, ethene, or triazine anilines. The
increased reactivity causes a rapid growth of the initially
formed nuclei on the substrate without the typical equilibrium
between imine condensation and dissolution that provides the
selectivity for a−b-oriented domains. An additional factor
could be nuclei that form in the reaction mixture and are
subsequently incorporated into the growing COF film. The
resulting COF films, however, are optically transparent and
homogeneous with only minimal light scattering.
The nitrogen lone pair of imines can be protonated by
sufficiently strong acids,35 but this process typically leads to
their decomposition if small amounts of water are present. We
found, however, that the close-packed structures of 2D COFs
provide sufficient stabilization to render them stable against
hydrolysis and enable the fully reversible protonation and
In their free-base form, the perylene COFs possess a red
(Per-1P and Per-N COFs) or yellow (Per-Py COF) photo-
luminescence (PL) with comparably long PL lifetimes (SI,
Figures S5-2 and S6). If protonated, however, the PL of the
C
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Figure 3. Protonation of pyrene COF films with acid solutions. (a) Chemical structure of the Py-TT COF. The structures of the Py-1P and the Py-
1POH COFs are shown in the SI, section B. (b) Absorption changes of the Py-TT COF film upon protonation with TFA solutions of various
concentrations in 1,4-dioxane. The protonation can be fully reversed by neutralization with Et3N (gray dashed line). (c) Plot of the absorption
difference between the protonated and the nonprotonated Py-TT COF showing the absorption features of the monoprotonated bridge (see
discussion below). (d) Evolution of the main absorption bands of the pyrene COFs, determined at their respective maxima (Py-TT COF: 578 nm,
Py-1P COF: 544 nm, Py-1POH COF: 590 nm). While the absorption change of the Py-TT and Py-1P COFs follows a logarithmic dependence on
the acid concentration, the protonation of the Py-1POH COF is considerably retarded due to the −OH···N hydrogen bonds. (e) Protonation of
the Py-TT COF with different acid solutions. TFA and TCA solutions produce selectively the monoprotonated bridge unit with a characteristic
absorption band between 550 and 750 nm. The considerably stronger MsOH generates a much stronger absorption band which extends further
into the IR, indicating the formation of the bis-protonated species. Insets: photographs of the deprotonated, monoprotonated, and bis-protonated
Py-TT COF. Abbreviations: TCA = trichloroacetic acid, TFA = trifluoroacetic acid, MsOH = methanesulfonic acid.
Per-Py COF is shifted drastically from 570 nm to about 1000
nm and is greatly reduced in intensity. The emission from the
other protonated COFs is below the detection limit of our IR
detectors.
TFA concentrations, whereas at higher TFA concentrations
also the bis-protonated state (600−800 nm absorption) is
formed. The stepwise protonation will be discussed in more
detail below.
Building on these initial observations, we investigated the
potential of a Per-N COF film as an acid vapor sensor. The
COF film was placed in a sealed cuvette, and a defined acid
atmosphere was generated by evaporating 1 μL of a
trifluoroacetic acid (TFA) solution in tetrahydrofuran, yielding
acid vapor concentrations between 35 μg L−1 and 110 mg L−1.
Even the lowest acid concentration causes a measurable change
in the absorption spectrum with protonation-induced
absorption and bleach bands extending from 440 to 800 nm
and below 440 nm, respectively (Figure 2b,c). These bands
become more intense with increasing acid vapor concentration,
without any saturation effects. We observe, however, the
gradual evolution of an additional IR component as the acid
vapor concentration is increased (Figure 2c, inset). We
attribute these spectral changes to the protonation of only
one of the two imines per bridge (500 nm absorption) at low
The protonation can be fully reversed by exposing the COF
to triethylamine vapor, followed by washing in acetonitrile to
remove the triethylammonium salt (Figure 2b,c, dashed gray
line). To conclude, the Per-N COF acts as a very sensitive
colorimetric acid vapor sensor with a low detection limit and a
response range of at least 4 orders of magnitude.
For a more detailed analysis of how the COF protonation
proceeds, we focus on a series of the well-established pyrene
COFs (Figure 3a). The Py-TT COF is a donor−acceptor
variant of the archetypical Py-1P COF that forms particularly
crystalline films.7,18 In the Py-1POH COF, the hydroxy groups
provide an additional stabilization of the structure via
hydrogen bonding to the adjacent imine nitrogens.16,37
Combining the pyrene tetraaniline with the pyrene tetraalde-
hyde yields the Py-Py COF with only one imine bond per
bridge (SI, Figure S12a).7
D
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If the protonation is carried out using TFA solutions in
dioxane rather than acid vapor, we observe a gradual color
change of the Py-TT COF from initially orange to dark brown
due to the appearance of a strong absorption band between
500 and 750 nm (Figure 3b,c). An additional absorption band
is formed at 450 nm, accompanied by a bleach band around
370 nm. The intensity of the absorption and bleach bands
follows a logarithmic dependence on the acid concentration
(Figure 3d, red triangles). The spectral features, however,
remain unaffected by the acid concentration, suggesting that
under these protonation conditions only a single, well-defined
protonated state is formed. The protonation can be fully
reversed by immersing the COF film in a dilute Et3N solution.
During the protonation and subsequent deprotonation, the
crystallinity and porosity of the COFs and COF films are fully
retained (SI, Figure S11).
acid solutions. Indeed, only one protonated state is formed
regardless of the protonation strength of the acid.
In addition to the COF films being very sensitive acid vapor
sensors, the balance between the mono- and bisprotonated
states provides additional information about the respective acid
mixture. COF films could be employed as colorimetric probes
for determining simultaneously the concentration and the
protonation strength of nonaqueous acid solutions, which are
incompatible with standard pH electrodes. The close-packed
crystalline COF lattice provides the stability, molecular-scale
porosity, and easy read-out required for reliable sensing.
CONCLUSION
■
We have developed a series of star-shaped dual-pore perylene-
based COFs and have demonstrated that their imine bonds can
be reversibly protonated, causing significant color shifts.
Protonation of the phenylene−NCH−bridge−HCN−
phenylene subunits proceeds in a two-step process with the
second protonation requiring significantly stronger acids. The
mono- and the bisprotonated species have distinct absorption
features in the red and near-infrared spectral regions, enabling
the COFs to act as colorimetric sensors for simultaneously
determining the acid strength and concentration in non-
aqueous solutions where pH electrodes are not applicable.
Moreover, the COF films are highly sensitive acid vapor
sensors with a very low detection limit and a response range of
at least 4 orders of magnitude. By using the imine bond itself as
a function-determining moiety, we have provided an additional
handle for constructing functional COFs for a broad range of
applications.
The protonation behavior of the Py-1P COF is very similar,
whereas the protonation of the Py-1POH COF is significantly
delayed, highlighting its stabilization via −OH···N hydrogen
bonds that counteract the imine protonation (Figure 3d and
SI, Figure S8).
When comparing the absorption spectra obtained upon
protonating the COFs with dilute solutions of various acids,
however, we notice considerable differences (SI, Figure S10).
The weakest acids (acetic acid and chloroacetic acid) do not
cause any considerable protonation. The stronger halogenated
acetic acids (dichloroacetic acid, trichloroacetic acid, TFA) can
monoprotonate the Py-TT and Py-1P COFs (and the Py-1POH
COF to some degree), producing the spectral features
discussed above. Exposing the COFs to these acids at higher
concentrations yields the same absorption signatures with
higher intensities. The even stronger p-toluenesulfonic acid
and methanesulfonic acid, however, do not only produce more
intense absorption bands than the other acids at the same
concentration but also give rise to significantly different
spectral features that extend much further into the IR. The
same IR-extended absorption bands can also be obtained with
saturated TFA vapor or other strong acids (SI, Figure S13) or
TFA solutions in solvents that are very poor hydrogen bond
acceptors (SI, Figure S9),38 suggesting that their formation
depends on the protonation strength of the respective acid
mixture rather than the choice of the acid anion.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b08079.
■
S
Experimental methods, synthetic procedures, structure
analysis and additional spectroscopic data (PDF)
Video of the reversible protonation (MPG)
Per-1P COF Pawley-refined structure model (CIF)
Per-N COF Pawley-refined structure model (CIF)
Per-Py COF Pawley-refined structure model (CIF)
Tz-TT COF Rietveld-refined structure model (CIF)
The observations above strongly suggest that the proto-
nation proceeds in two distinct steps with characteristic
absorption bands extending to about 750 nm and beyond 1000
nm, respectively, in the case of the Py-TT COF. No
intermediates or gradual absorption shifts are observed. We
attribute these steps to the mono- and the bisprotonation of
the two electronically coupled imines within one phenylene−
NCH−bridge−HCN−phenylene unit (Figure 3e). The
first protonation can be achieved with a variety of acid
solutions. However, this monoprotonation reduces the basicity
of the neighboring imine, and thus considerably more
protonation strength is required for the second protonation.
This cooperative phenomenon involving electronically coupled
imines sets our COFs apart from previous examples of
acidochromic framework materials, which rely on isolated
acidochromic building blocks.39,40
AUTHOR INFORMATION
Corresponding Author
*fa355@cam.ac.uk
ORCID
Emrys W. Evans: 0000-0002-9092-3938
Jeffrey Gorman: 0000-0002-6888-7838
Thomas Bein: 0000-0001-7248-5906
Richard H. Friend: 0000-0001-6565-6308
Florian Auras: 0000-0003-1709-4384
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The assignments of the spectra to the mono- and
bisprotonated imine bridges are further supported by
comparison with the Py-Py COF (SI, Figure S12). Since this
framework contains only one imine bond per bridge, we do not
expect to observe the second protonation step with stronger
This project has received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (grant agreement no.
670405). The research leading to these results has received
funding from the European Research Council under the
E
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̈
Hanusch, F. C.; Reuter, S.; Bessinger, D.; Doblinger, M.; Hettstedt,
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