Humidity Sensing through Reversible Isomerization of a Covalent Organic Framework

Article abstract of DOI:10.1021/jacs.9b08628

Here we report that a covalent organic framework (COF), which contains 2,5-di(imine)-substituted 1,4-dihydroxybenzene (diiminol) moieties, undergoes color changes in the presence of solvents or solvent vapor that are rapid, passive, reversible, and easily detectable by the naked eye. A new visible absorption band appears in the presence of polar solvents, especially water, suggesting reversible conversion to another species. This reversibility is attributed to the ability of the diiminol to rapidly tautomerize to an iminol/cis-ketoenamine and its inability to doubly tautomerize to a diketoenamine. Density functional theory (DFT) calculations suggest similar energies for the two tautomers in the presence of water, but the diiminol is much more stable in its absence. Time-dependent DFT calculations confirm that the iminol/cis-ketoenamine absorbs at longer wavelength than the diiminol and indicate that this absorption has significant charge-transfer character. A colorimetric humidity sensing device constructed from an oriented thin film of the COF responded quickly to water vapor and was stable for months. These results suggest that tautomerization-induced electronic structure changes can be exploited in COF platforms to give rapid, reversible sensing in systems that exhibit long-term stability.

Full text of DOI:10.1021/jacs.9b08628

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Humidity Sensing through Reversible Isomerization of a Covalent
Organic Framework
Samik Jhulki,†,∥ Austin M. Evans,‡,∥ Xue-Li Hao,† Matthew W. Cooper,† Cameron H. Feriante,†
Johannes Leisen,†† Hong Li,† David Lam,§ Mark C. Hersam,‡,§ Stephen Barlow,†
Jean-Luc Bredas, William R. Dichtel,*,‡ and Seth R. Marder*,†
́
†School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, United States
§
‡Department of Chemistry and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois
60208, United States
S
* Supporting Information
ABSTRACT: Here we report that a covalent organic frame-
work (COF), which contains 2,5-di(imine)-substituted 1,4-
dihydroxybenzene (diiminol) moieties, undergoes color
changes in the presence of solvents or solvent vapor that are
rapid, passive, reversible, and easily detectable by the naked
eye. A new visible absorption band appears in the presence of
polar solvents, especially water, suggesting reversible con-
version to another species. This reversibility is attributed to the
ability of the diiminol to rapidly tautomerize to an iminol/cis-
ketoenamine and its inability to doubly tautomerize to a
diketoenamine. Density functional theory (DFT) calculations
suggest similar energies for the two tautomers in the presence
of water, but the diiminol is much more stable in its absence.
Time-dependent DFT calculations confirm that the iminol/cis-
ketoenamine absorbs at longer wavelength than the diiminol and indicate that this absorption has significant charge-transfer
character. A colorimetric humidity sensing device constructed from an oriented thin film of the COF responded quickly to
water vapor and was stable for months. These results suggest that tautomerization-induced electronic structure changes can be
exploited in COF platforms to give rapid, reversible sensing in systems that exhibit long-term stability.
Covalent organic frameworks (COFs) are permanently
porous and structurally precise polymer networks,2−6 the
chemical versatility and porosity of which make them an
attractive platform for the detection of volatile analytes. For
example, Auras and co-workers recently explored solvato-
chromism in COF films as a mechanism for rapid humidity
sensing.7 Although this COF system exhibited impressive
response times (∼200 ms), the optical response was limited to
the polarity-induced stabilization of an excited state with
charge-transfer character, and as such, the change in the
spectrum was relatively small (a 20−30 nm shift in the
absorption edge). In contrast to solvatochromism, which relies
on the stabilization of a single chromophore, analyte-induced
tautomerism forms a new chromophore, potentially offering a
larger spectral change.
1. INTRODUCTION
Tautomers are constitutional isomers that are interconverted
by the migration of an atom or group of atoms, most
commonly a proton, from one site to another (as shown, for
example, Figure 1A). This chemical change is often rapid, and
the relative stability of the tautomers can depend strongly on
the surrounding environment, including the presence or
absence of volatile species. Because tautomers often display
rapid interconversion, sensitivity to different chemical stimuli,
and switchable optical behavior, they have found extensive use
as molecular probes.1 However, solid-state systems in which
tautomers have distinct properties and sensitivity to relevant
atmospheric changes are much less common. Moreover, the
extent of this property change is correlated to the accessibility
of tautomeric sites to volatile analytes, which presents a
challenge that, to date, has limited solid-state sensing based
upon tautomerization. Because of these challenges, porous
polymer systems might be better suited to tautomeric sensing
than other materials into which diffusion of an analyte is more
difficult.
Iminol-to-ketoenamine tautomerism (Figure 1A) is well
known in COF materials. In particular, iminol groups are
formed when COFs are made from 2,4,6-triformylphloro-
Received: August 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 1. (A) Imine condensation between aniline and o-hydroxybenzaldehyde leads to an iminol that can tautomerize via single proton transfer to
generate a cis-ketoenamine. (B) Imine condensation between TAPB and TFP leads to an iminol product that rapidly and irreversibly converts to its
β-ketoenamine tautomer. (C) Imine condensation between TAPB and 2,5-dihydroxyterephthaldehyde leads to an iminol product (TAPB-PDA-
OH COF) that can tautomerize only to a single cis-ketoenamine and leads to a dynamic equilibrium of the diiminol and iminol/cis-ketoenamine
forms. (D) Imine condensation between TAPB and terephthaldehyde or 2,5-dibutoxyterephthaldehyde leads to the formation of TAPB-PDA or
TAPB-PDA-OBu17 COF, respectively, where tautomerization is not possible.
B
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 2. (A) Synthesis and chemical structures of the TAPB-PDA-OH,13 TAPB-PDA, and TAPB-PDA-OBu COFs. The purple surface shows
the high surface area and accessibility of tautomeric sites for the TAPB-PDA-OH COF. (B, D, F) Experimental and predicted (Pawley-refined,
eclipsed-stacking PXRDs) of the three COFs. (C, E, G) N2 adsorption−desorption isotherms of the three COFs.
glucinol (TFP) and amines, but they irreversibly tautomerize
during polymerization to yield β-ketoenamine-linked COFs
(Figure 1B).8−11 The stability of the β-ketoenamine tautomer
precludes using TFP for tautomeric sensing but suggests that
components with fewer hydroxyl groups might be suitable
candidates for solid-state vapor sensing.12 Tautomerization-
induced color changes in a COF on exposure to water have
been reported; however, in that report, the formation of the
stable trans-ketoenamine form13 was irreversible at room
temperature, meaning that sensors based on this recognition
element would require thermal regeneration (Figure S8). We
hypothesized that COFs based on 2,5-dihydroxyterephth-
aldehyde (PDA-OH) might exhibit dynamic tautomerization
because the bis-ketoenamine species, for which a closed-shell
nonzwitterionic resonance structure cannot be drawn, is likely
to be energetically disfavored after the first ketoenamine
formation (Figure 1C). Importantly, to investigate this
approach, the complementary amine monomer used to form
a PDA-OH-containing COF should be weakly absorbing in the
visible region in order not to mask the tautomerically induced
change in color upon exposure of the COF to water vapor.
This consideration led us to choose the UV-absorbing 1,3,5-
tris(4-aminophenyl)benzene (TAPB) as a suitable condensa-
tion partner. Although the corresponding COF, here named
TAPB-PDA-OH, has previously been reported,14 dramatic
color changes in the presence of polar solvents or humid air
were not described.15 Comparison to structural analogues of
this material using 2,5-dibutoxyterephthaldehyde (PDA-OBu)
and terepthaldehyde (PDA),16 which cannot undergo iminol-
to-ketoenamine tautomerism (Figure 1D), along with the-
oretical modeling of the optical behavior of model tautomers,
supports our hypothesis of water-induced diiminol-to-ketoen-
amine tautomerization. A proof-of-principle TAPB-PDA-OH
COF sensor rapidly responds to changes in relative humidity
and showed no attenuation in performance over a period of at
least 2 months.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of COF Materials.
COFs were synthesized as powders by conventional
solvothermal methods (described in more detail in the
Supporting Information).18 Fourier transform infrared (FT-
IR) spectroscopy confirmed the imine condensation by the
disappearance of the aldehyde (1660−1680 cm−1) and amine
(3300−3450 cm−1) stretching modes and a concomitant
increase in the intensity of the CN stretching modes from
1610 to 1620 cm−1 (Figures S1−S3), consistent with the
presence of predominantly iminol linkages. Powder X-ray
diffraction (PXRD) confirmed the structural regularity and
high quality of the COFs, indicating the appearance of several
diffraction features that can be indexed as the (100), (110),
(200), and (210) Bragg reflections of a hexagonal lattice
(Figure 2B,D,F), while a weak peak at higher angle can be
assigned to a (00l) reflection. By comparison to simulated
powder diffraction patterns, we infer that all COFs are
hexagonally tessellated sheets that eclipse one another with
ca. 3 Å interlayer separations (Figure S20 and CIF files).
Furthermore, Le Bail fitting of the PXRD pattern indicated that
all COFs have an approximate in-plane domain size of ca. 100
C
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 3. (A) Diffuse reflectance spectroscopy (DRS) plots for TAPB-PDA-OH COF powder in different solvents; photographs of the dry and wet
COFs are shown to the right. (B) Plot of change in the absorption strength (measured by the Kubelka−Munk function) at 600 nm (ΔK-M600
)
versus the ETN parameter: the nearly linear trend is broken for strongly H-bonding solvents. (C) Plot of change in absorbance at 600 nm versus
pKa for H-bonding solvents. (D) Calculated UV−vis spectra of the tautomers of model compound BPH in the gas phase and in the presence of
water. The hole and electron natural transition orbitals for the low-energy transition of the single cis-ketoenamine form (with two molecules of
water present) are shown to the right.
nm.19 Nitrogen sorption isotherms were collected for these
materials, all of which displayed hysteresis-free type IV
isotherms indicative of well-ordered mesopores that are filled
as monolayers initially but are subject to capillary condensation
as they form multilayers.20 Brunauer-Emmett−Teller (BET)
analysis revealed that the TAPB-PDA, TAPB-PDA-OH, and
TAPB-PDA-OBu COFs had surface areas of 2100, 2250, and
2000 m2 g−1, respectively (Figure 2C,E,G), near their
respective Connolly surface areas of 2300, 2350, and 2200
m2 g−1.21 The pore-size distribution derived from nonlocal
density functional theory (DFT) shows a narrow distribution
of pore sizes for all materials with an average pore size (30 Å)
consistent with the modeled eclipsed structures (Figure S4).22
Using a Monte Carlo algorithm with water as a probe
molecule, we calculated that ca. 10 water molecules per iminol
moiety can fit within the TAPB-PDA-OH COF pores,
suggesting that this material will be amenable to accepting
solvent vapor into its pores. Furthermore, the experimentally
determined pore sizes and high surface areas of these materials
indicate that analytes can rapidly access the tautomerically
active sites.
polarity. This difference is particularly pronounced when
TAPB-PDA-OH COF is exposed to water, which results in a
dark coloration and absorption spanning the entire visible
region (Figure 3A).
In the previous report23 of humidity sensing in a COF,
which had more pronounced donor−acceptor character than
the present COFs, a red shift of the long-wavelength
absorption edge is seen as the solvent polarity is increased;
this was attributed to solvatochromism (i.e., preferential
stabilization of the excited state in more polar solvents). The
behavior of TAPB-PDA-OH (Figure 3A) is qualitatively
different. Figure 3B,C shows the dependence of the absorption
at 600 nm, as measured by the increase in the Kubelka−Munk
function at that wavelength relative to that of the dry COF,
ΔK-M600, on the solvent H-bonding and polarity parameter,
ETN,24 and, for the case of H-bond-donor solvents, the solvent
acidity, respectively. Both plots show fairly similar trends. For
solvents of low to moderate polaritytetrahydrofuran (THF),
dichloromethane (DCM), acetone, acetonitrile, 2-propanol, 1-
butanol, and ethanolΔK-M600 is approximately linearly
dependent on ETN (Figure 3B).24 However, this increase in
absorption is not accompanied by a significant shift of the main
absorption edge (Figure 3A), as one would expect for simple
solvatochromism, suggesting the growth of a new feature
distinct from that responsible for the main absorption edge. In
higher-polarity solvents (ETN > 0.7), there is a marked
deviation from linearity in the plot of ΔK-M600 vs ETN (or
2.2. Solvent-Dependent Optical Behavior. The TAPB-
PDA-OH COF changes color when its dry form is solvated
(Figure 3A). Diffuse reflectance spectroscopy (DRS) showed
that the orange COF has an absorption onset of 572 nm (2.2
eV). A new band with an onset at approximately 690 nm (1.8
eV) emerges when the COF is soaked in solvents of increasing
D
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 4. (A) CP-MAS 13C NMR spectra of dry and wet TAPB-PDA-OH COF show a diminution of the imine peak at ca. 160 ppm and the
appearance of a new feature at ca. 105 ppm. (B) Synchrotron X-ray scattering pattern of the wet TAPB-PDA-OH COF as it dries. (C) Chemical
structures of the different tautomers of small-molecule model compound BPH with DFT relative energies in the presence of water are shown on
the right.
pKa), and the absorption at 600 nm is more clearly attributable
to the formation of increasing quantities of a new
chromophore. The retention of the feature at ca. 540 nm in
the most polar environment examined indicates that only a
fraction of the initial chromophores are converted to the new
chromophore. To examine the plausibility of assigning the new
chromophore as the cis-iminol/ketoenamine tautomer, we
turned to the small-molecule model compound, BPH,
obtained from condensing aniline with 2,5-dihydroxyterephth-
aldehyde. This and several related compounds were previously
shown to exhibit solvent-dependent equilibria between
diiminol and iminol/ketoenamine forms, which absorb at
λmax = ca. 435 and ca. 540 nm, respectively.25 We also
resynthesized BPH and observed similar behavior (Figure S6);
in particular, both dichloromethane and ethanol solutions
exhibit similar short-wavelength absorptions attributed to the
diiminol, with negligible solvatochromism of this band, while
the long-wavelength band, attributed to the iminol/ketoen-
amine, is seen only in ethanol. Furthermore, time-dependent
(TD) DFT calculations on the diiminol form of BPH (see the
SI for details) also suggest that its spectrum is only weakly
dependent on the presence of water (Figure 3D),26 whereas
that of its iminol/cis-ketoenamine tautomer has a much longer
absorption wavelength (Figure 3D). The natural transition
orbitals indicate that this transition is associated with charge
transfer from the enamine and imine arms to the central
aromatic ring (Figure 3D). We also compared the absorbance
of TAPB-PDA and TAPB-PDA-OBu COFs, neither of which
is capable of tautomerization, under wet and dry conditions
(Figure S5). Although both exhibited limited solvatochromism
(i.e., a solvent-dependent shift of the main absorption edge
consistent with their weak donor−acceptor nature (Figure S13
and S14)), neither exhibited the pronounced optical changes
seen in the TAPB-PDA-OH COF.
To investigate the effect of structural regularity on the
observed optical behavior, a largely amorphous cross-linked
polymer was synthesized from TAPB and PDA-OH. (See
Supporting Information Section VII for synthesis and
E
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
characterization.) While FT-IR confirmed imine polymer-
ization, nitrogen sorption measurements showed that this
material had a much lower surface area (ca. 40 m2 g−1). When
exposed to water, the cross-linked polymer exhibited a
substantially diminished optical response (Figure S7), which
we attribute to the restricted accessibility of the tautomeric
sites in an amorphous cross-linked network. This contrasting
behavior between a crystalline 2D COF and an amorphous
polymer of similar chemical composition expands the growing
body of literature that demonstrates the promise of structurally
regular and porous materials for sensing applications.27
To test the generality of this tautomerization phenomenon,
TAPPy-PDA-OH COF (TAPPy = 1,3,6,8-tetrakis(4-amino-
phenyl)pyrene) was also synthesized (as described in the
Supporting Information). This COF exhibits only moderate
crystallinity as judged from its PXRD diffraction pattern
(Figure S20) and BET surface area analysis (320 m2 g−1,
Figure S21). This COF exhibited qualitatively similar behavior
to TAPB-PDA-OH COF, acquiring a dark coloration in wet
media (Figure S22) and a new absorption feature at ca. 600
nm. Compared to TAPB-PDA-OH COF, the new feature seen
for TAPPy-PDA-OH under wet conditions is less prominent,
suggesting that the position of equilibrium lies further toward
the diiminol in the pyrene COF. This may be partially due to
the lower surface area of the pyrene material, leading to fewer
tautomeric sites being accessible to solvent, but may also
reflect subtle effects of the different chemical structures on the
energetics associated with the equilibria.
resonance centered at 153 ppm, assigned to the imine carbon
using DFT simulations for model compound BPH (Figure
S18), diminishes significantly on exposure of the dry COF to
water. A new peak is observed as a shoulder at ca. 105 ppm
and, by comparison to the DFT calculations, can be assigned
to the carbon atom of the phenyl ring ortho to the enamine
nitrogen atom.9 The absence of a resonance for the wet COF
in the typical “carbonyl” 13C region might initially appear
surprising. However, DFT calculations predict the “carbonyl”
resonance for the cis-ketoenamine form of BPH to be shifted
considerably upfield from this range, suggesting that this
resonance, which as a quaternary carbon is likely to be
intrinsically weak, likely overlaps with the tail of the imine
resonance, complicating its observation.28 The upfield shift can
be explained by the contributions of aromatic zwitterionic
resonance structures (Figure S9); consistent with this, the
DFT-optimized structure of BPH exhibits a C−O bond length
of 1.26 Å in the single cis-ketoenamine form (Figure S17),
intermediate between those of typical C−O (1.4 Å) and CO
(1.2 Å) bonds, and the DFT-predicted C−O stretching
frequency is lower than expected for a typical ketone (Figure
S19). In the literature, TAPPy-TT (TT = thieno[3,2-
b]thiophene-2,5-dicarboxaldehyde) COF, in which isomer-
ization is not possible and the solvent dependence of spectra is
presumably entirely due to conventional solvatochromism,
exhibited nearly identical 13C NMR spectra in its dry and wet
forms.23 Thus, the differences between the 13C NMR spectra
of the dry and wet TAPB-PDA-OH COF further support our
hypothesis of a chemical change in general and tautomerization
in particular in this COF.
We measured the diffraction patterns of the TAPB-PDA-
OH COF in the presence of a variety of solvents using
synchrotron X-ray radiation. Across a range of solvents, the
COF crystallinity was unaffected, with the same number and
approximate shape of diffraction features being observed in
each case. However, the position of the in-plane diffraction
features shifted when immersed in water. In particular, the
(100) diffraction feature shifted to a real-space d spacing of 32
Å, which, as the COF dried, relaxed to its original position of
28 Å (Figure 4B). This expanded unit cell may originate from
the combination of bond-length differences and out-of-plane
distortions of the ketoenamine and iminol tautomers and hints
that responsive breathing behavior could be engineered in
framework materials through the incorporation of tautomeri-
cally active states.29−31
Taken together, the various spectroscopic and structural
changes observed are all consistent with substantial solvent-
mediated tautomerism in the TAPB-PDA-OH COF. DFT
calculations on model compound BPH (Figure 4C) indicate
that the diiminol tautomer is strongly preferred in the gas
phase, consistent with the properties of the dry COF.
However, in the presence of two water molecules, the diiminol
and cis-iminol/ketoenamine are nearly isoenergetic, with the
latter being only 0.09 eV (ca. 2 kcal mol−1, ca. 3.6kBT at room
temperature) less stable (Figure 4C), consistent with the
presence of significant quantities of both tautomers in the
presence of water. As expected, the double cis-ketoenamine
tautomer is significantly less stable (by 0.40 eV/9.2 kcal mol−1/
ca. 16kBT) than the single cis-iminol/ketoenamine, consistent
with the cross-conjugation present in the single cis-ketoen-
amine and the absence of a nonzwitterionic closed-shell
resonance structure for the double ketoenamine, suggesting
that the double cis-ketoenamine is unlikely to be present in
2.3. Other Solvent-Induced Changes. Having estab-
lished the optical response of the TAPB-PDA-OH COF to
solvent, particularly strong H-bond donors, we investigated the
nature and reversibility of other associated structural and
spectroscopic changes. FT-IR spectra of the dry and fully wet
samples of TAPB-PDA-OH COF show subtle changes, with
the most prominent being increased absorption at ca. 1650
cm−1 (Figure S10a). However, there is considerable overlap
with the bending mode of excess water (expected at 1640 cm−1
for liqud water) in the FT-IR spectrum of the fully wet COF
(Figure S10a). The water mode is less prominent in the FT-IR
spectrum of a partially wet sample obtained by exposure to
moist air and shows changes within the framework more
clearly (Figure S10b). Numerous changes in the spectrum in
the range 1000−1650 cm−1 indicate that new vibrational
modes are present in the COF structure that is exposed water
vapor, consistent with tautomerization (Figure S10b) and with
expectations based on DFT calculations for model compound
BPH (Figure S19a). In particular, we note that the C−O
stretch associated with the “keto” moiety is predicted by DFT
to be very weak and at lower frequency than either a typical
isolated CO bond (Figure S19a) or several of the other
skeletal modes, explaining the absence of a typical CO-like
feature in the observed spectrum. Second, the increased
strength of the imine CN stretch at ca. 1650 cm−1 is
consistent with predictions for the cis-iminol/ketoenamine
tautomer (Figure S10a).10,12 Raman spectra, which are less
sensitive to signals from free water, showed similar evidence for
reversible tautomerization (Figure S10c) and were in
qualitative agreement with DFT calculations on BPH (Figure
S19b).
Solid-state 13C cross-polarization magic angle spinning (CP-
MAS) nuclear magnetic resonance (NMR) spectra of both the
dry and wet TAPB-PDA-OH COF were acquired (Figure 4A).
Again, the changes are subtle, but the intensity of the
F
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 5. Orange and blue traces correspond to as-synthesized and post-sensing TAPB-PDA-OH COFs, respectively. As-synthesized COF was
soaked in water and then dried by critical point drying. (A) FT-IR showing no damage to chemical linkages during the wetting process. (B)
Sorption isotherms are similar, showing no deleterious effects in the pore structures due to wetting with water. (C) PXRDs showing wetting in
which the COF does not affect the crystalline diffraction.
Figure 6. (A) AFM images of the TAPB-PDA-OH COF film. (B) GIWAXS of the as-synthesized TAPB-PDA-OH COF films. (C) Humidity
sensing using TAPB-PDA-OH COF films using the change in absorbance at 600 nm. TAPB-PDA COF films show no response to humidity. (D)
Humidity sensing using TAPB-PDA-OH films over multiple cycles. The break shows a gap of more than 1 month of usage. The similar response
observed after 1 month attests to its stability.
more than trace amounts (Figure S16). The formation of trans-
iminol/ketoenamine was also found to be significantly less
favorable (by 0.47 eV/10.8 kcal mol−1/19kBT) than the
formation of the cis-iminol/ketoenamine, strongly suggesting
that the observed optical changes are not attributable to defect
sites in the COF with trans geometries (Figure S15).
pores, the COFs were synthesized as highly oriented films with
the pores perpendicular to the substrate. This orientation is
confirmed by grazing-incidence wide-angle X-ray scattering
(GI-WAXS) where the in-plane diffraction ((100) Bragg
feature) is predominantly seen in the qxy plane and the out-of-
plane diffraction ((001) Bragg feature) is located along the qz
plane (Figure 6B). Upon exposure to humidity, the optical
responses of the COF films were found to be similar to those
of the COF powders (Figure S23).
A humidity sensing experiment (experimental details in
Supporting Information Section XIV) was performed in which
dry and water-saturated air were alternately passed over the
film and changes in the absorbance of the COF at 600 nm,
ΔA600, were monitored (Figure 6C). The TAPB-PDA-OH
COF displayed a fast response of 9 s as the environment
changes from dry to humid and less than 1 s when the humid
air was changed to dry air. Slower responses to humidity (23 s)
and lower values of ΔA600 (60% relative to that obtained using
water-saturated air) were obtained in a film using unsaturated,
but moist, air (Figure S26), and the value of ΔK-M600 for
powders (Figure S27) increases with exposure to air of
increasing relative humidity, together indicating that both films
and powders can potentially be used as sensors of relative
humidity. On the other hand, the amorphous film of TAPB-
PDA-OH cross-linked polymer exhibited a much slower
response (ca. 130 s, Figure S25) than the corresponding
COF likely due to restricted accessibility and slow diffusion of
the vapor and emphasizing the importance of the regular and
As the wet TAPB-PDA-OH COF dries, it reverts to a state
that is indistinguishable from the original sample. For example,
the FT-IR spectra of the two dry materials, before and after
exposure to water, are nearly identical (Figure 5A), indicating
the reversibility of the process, as expected for a tautomeriza-
tion. Importantly, the BET surface area and X-ray diffraction
pattern of the dried COF are almost unchanged from those of
the as-synthesized material (Figure 5B,C). This lack of
hysteresis demonstrates that the structural changes are
reversible when water is removed from the framework. These
experiments suggest that tautomerism is an appealing sensing
mechanism for applications that require long-term stability.
2.4. Humidity Sensing. To exploit the dynamic and
reversible nature of the solvent response of the TAPB-PDA-
OH COF, we fabricated a humidity detector in which the
absorbance of a thin film of the COF is monitored by
transmission UV−vis spectroscopy (see Supporting Informa-
tion for details, Section XIII). We synthesized films with
thicknesses of ca. 200 nm, which we anticipated would allow
for substantial diffusion and thus rapid access of the volatile
analytes but also sufficient optical density to allow for sensing
(Figure 6A). To further enhance diffusive access to COF
G
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
open COF structure for rapid sensing. TAPB-PDA COF films
did not exhibit any humidity-sensing changes in optical
behavior, consistent with the observation in the powder,
while the TAPB-PDA-OBu COF film displayed only small
increases in absorbance (attributable to the weak solvato-
chromism seen in powders) and an overall slower mediachro-
mic response (ca. 15 s, Figure S28). We hypothesize that the
slower response is in part due to its more hydrophobic pores.
However, the slow kinetics of the already weakly responsive
butoxy-functionalized COF suggest that side chains can be
used to tune sensor performance in future COF designs.
The sensors were passively regenerated when the humid
environment was replaced with a dry atmosphere. For example,
we exposed the films to human breath and observed the same
rapid and reversible optical switching (Suppporting Informa-
tion video). The TAPB-PDA-OH COF response is highly
reversible over multiple cycles of exposure to dry and wet air.
Even after more than a month of ambient storage, we found
that the performance of the last sensing experiment was
indistinguishable from that of the first sensing, indicating the
high-fidelity nature of this sensor (Figure 6D). The stable,
reversible, passive nature of this sensing behavior suggests that
specific, highly responsive, rapid tautomeric sensing in a more
engineered COF platform is worth further exploration.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.9b08628.
■
S
Details of materials synthesis, COF characterization (IR,
2
13C and H MAS NMR, PXRD, and SA analysis), DFT
calculations, and humidity sensing (PDF)
Response of the TAPB-PDA-OH COF to human breath
(MOV)
COF structures (CIF)
COF structures (CIF)
COF structures (CIF)
COF structures (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: wdichtel@northwestern.edu.
*E-mail: seth.marder@chemistry.gatech.edu.
ORCID
Austin M. Evans: 0000-0002-3597-2454
Matthew W. Cooper: 0000-0002-3589-8688
Hong Li: 0000-0002-4513-3056
David Lam: 0000-0003-4264-0033
Mark C. Hersam: 0000-0003-4120-1426
Stephen Barlow: 0000-0001-9059-9974
■
3. CONCLUSIONS
A COF containing 2,5-di(imine)-substituted 1,4-dihydroxy-
benzene moieties undergoes a dramatic change in color, clearly
visible to the naked eye, on exposure to strong hydrogen-bond-
donor solvents, especially water, and moist air. UV−vis
spectroscopy, NMR spectroscopy, and density functional
theory calculations clearly distinguished between the solvato-
chromism associated with other MOFs and COFs and the
more complex and dramatic solvent-induced color change
found in the TAPB-PDA-OH COF. In particular, a strong new
absorption feature seen in humid environments is, by
comparison of spectroscopic data with experimental and
computational data for model compounds, assigned to the
tautomerization of some of the diiminol moieties to iminol/cis-
ketoenamine chromophores. We attribute this dynamic
tautomerization to similar energies of these two tautomers
while preventing tautomerization to a double cis-ketoenamine
due to cross-conjugation effects. The new absorption feature is
attributed to a charge-transfer-type excited state, resulting from
a stronger acceptor nature of the central ring of the iminol/
ketoenamine tautomer relative to that of the diiminol. COFs
incorporating similar diiminol motifs with the same con-
nectivity have previously been reported but have also
incorporated chromophores, such as porphyrins or phthalo-
cyanines, whose strong visible absorption may have obscured
́
Jean-Luc Bredas: 0000-0001-7278-4471
William R. Dichtel: 0000-0002-3635-6119
Seth R. Marder: 0000-0001-6921-2536
Author Contributions
∥These authors contributed equally to this work.
Notes
The authors declare the following competing financial
interest(s): Georgia Institute of Technology and Northwestern
University have filed a patent application based on the findings
of this manuscript.
ACKNOWLEDGMENTS
■
We thank the United States Army Research Office for a
Multidisciplinary University Research Initiative (MURI) award
under grant number W911NF-15-1-0447. S.J. thanks the
United States-India Educational Foundation (USIEF, India)
and the Institute of International Education (IIE, USA) for a
Fulbright-Nehru Postdoctoral Fellowship (grant no. 2266/
FNPDR/2017). A.M.E. is supported by the National Science
Foundation Graduate Research Fellowship under grant no.
(DGE-1324585). H.L. and J.-L.B. acknowledge funding of this
work by the United States Army Research Office under award
W911NF-17-10339. D.L. and M.C.H. acknowledge the
Department of Energy (grant DE-SC0019356) for support of
the Raman spectroscopy characterization. We thank Jeremy
Swartz for videography assistance. Portions of this work were
performed at the DuPont-Northwestern-Dow Collaborative
Access Team (DND-CAT) located at Sector 5 of the
Advanced Photon Source (APS). DND-CAT is supported by
Northwestern University, E.I. DuPont de Nemours & Co., and
The Dow Chemical Company. Grazing incidence diffraction
patterns were collected at Sector 8 ID-E of the APS. This
research used resources of the Advanced Photon Source, a U.S.
Department of Energy (DOE) Office of Science User Facility
the optical changes associated with tautomerization.32
A
prototype humidity sensor was found to be rapidly and
passively regenerated and was active for at least 2 months. We
anticipate that our demonstration of the viability of practical
tautomeric sensing will inspire engineered devices capable of
complex sensing responses for volatile analytes in these
materials. Furthermore, we hope that these materials will
find substantial interest as components in internet of things
systems where passive and long-term-stable sensors are a
necessity.33
H
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(19) Le Bail, A.; Jouanneaux, A. A qualitative account for anisotropic
broadening in whole-powder-diffraction-pattern fitting by second-rank
tensors. J. Appl. Crystallogr. 1997, 30, 265−271.
operated for the DOE Office of Science by Argonne National
Laboratory under contract no. DE-AC02-06CH11357.
(20) Sing, K. The Use of Nitrogen Adsorption for the Character-
ization of Porous Materials. Colloids Surf., A 2001, 187, 3.
(21) Connolly, M. L. Analytical Molecular-Surface Calculation. J.
Appl. Crystallogr. 1983, 16, 548−558.
REFERENCES
■
(1) Antonov, L.; Deneva, V.; Simeonov, S.; Kurteva, V.;
Nedeltcheva, D.; Wirz, J. Exploiting tautomerism for switching and
signaling. Angew. Chem., Int. Ed. 2009, 48, 7875−7878.
(2) Colson, J. W.; Dichtel, W. R. Rationally synthesized two-
dimensional polymers. Nat. Chem. 2013, 5, 453−465.
(3) Huang, N.; Wang, P.; Jiang, D. Covalent organic frameworks: a
materials platform for structural and functional designs. Nat. Rev.
Mater. 2016, 1, 16068.
(4) Evans, A. M.; Ryder, M. R.; Flanders, N. C.; Vitaku, E.; Chen, L.
X.; Dichtel, W. R. Buckling of Two-Dimensional Covalent Organic
Frameworks under Thermal Stress. Ind. Eng. Chem. Res. 2019, 58,
9883−9887.
(5) Lohse, M. S.; Bein, T. Covalent organic frameworks: Structures,
synthesis, and applications. Adv. Funct. Mater. 2018, 28, 1705553.
(6) Bisbey, R. P.; Dichtel, W. R. Covalent organic frameworks as a
platform for multidimensional polymerization. ACS Cent. Sci. 2017, 3,
533−543.
(7) Ascherl, L.; Evans, E. W.; Hennemann, M.; Di Nuzzo, D.;
Hufnagel, A. G.; Beetz, M.; Friend, R. H.; Clark, T.; Bein, T.; Auras, F.
Solvatochromic covalent organic frameworks. Nat. Commun. 2018, 9,
3802.
(22) Landers, J.; Gor, G. Y.; Neimark, A. V. Density functional
theory methods for characterization of porous materials. Colloids Surf.,
A 2013, 437, 3−32.
(23) Ascherl, L.; Evans, E. W.; Hennemann, M.; Di Nuzzo, D.;
Hufnagel, A. G.; Beetz, M.; Friend, R. H.; Clark, T.; Bein, T.; Auras, F.
Solvatochromic covalent organic frameworks. Nat. Commun. 2018, 9,
3802.
(24) Nekoeinia, M.; Yousefinejad, S.; Abdollahi-Dezaki, A.
Prediction of ETN Polarity Scale of Ionic Liquids Using a QSPR
Approach. Ind. Eng. Chem. Res. 2015, 54, 12682−12689.
́
(25) Filipczak, K.; Karolczak, J.; Ziołek, M. Temperature influence
on deactivation paths and tautomeric equilibrium of some photo-
chromic Schiff bases studied by time-resolved and stationary
spectroscopy. Photochem. Photobiol. Sci. 2009, 8, 1603−1610.
(26) Reichardt, C. Solvatochromic Dyes as Solvent Polarity
Indicators. Chem. Rev. 1994, 94, 2319−2358.
(27) Lustig, W. P.; Mukherjee, S.; Rudd, N. D.; Desai, A. V.; Li, J.;
Ghosh, S. K. Metal−organic frameworks: functional luminescent and
photonic materials for sensing applications. Chem. Soc. Rev. 2017, 46,
3242−3285.
(8) Chong, J. H.; Sauer, M.; Patrick, B. O.; MacLachlan, M. J. Highly
stable keto-enamine salicylideneanilines. Org. Lett. 2003, 5, 3823−
3826.
(28) The DFT simulations (Figure S18) afford chemical shifts for
the diiminol form of BPH that are generally ca. 5 ppm lower (i.e.,
shifted more upfield) than those observed for the COF (Figure 4A).
In the DFT simulations, the “keto” resonance in the imine/
ketonenamine form of BPH is at ca. 158 ppm (and thus likely at
ca. 163 ppm) and would therefore likely overlap with the observed tail
of the imine resonance. As noted in the text, this resonance is likely
intrinsically weak. (CP-MAS does not efficiently transfer magnet-
ization to quaternary 13C nuclei.) Moreover, incomplete trans-
formation to the imine/ketonenamine tautomer and broadening due
to structural inhomogeneity and perhaps to dynamic processes may
further complicate the observation of this feature.
(29) Wei, Y. S.; Chen, K. J.; Liao, P. Q.; Zhu, B. Y.; Lin, R. B.; Zhou,
H. L.; Wang, B. Y.; Xue, W.; Zhang, J. P.; Chen, X. M. Turning on the
flexibility of isoreticular porous coordination frameworks for
drastically tunable framework breathing and thermal expansion.
Chem. Sci. 2013, 4, 1539−1546.
(9) DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruna, H. D.;
̃
Dichtel, W. R. β-Ketoenamine-linked covalent organic frameworks
capable of pseudocapacitive energy storage. J. Am. Chem. Soc. 2013,
135, 16821−16824.
(10) Kandambeth, S.; Mallick, A.; Lukose, B.; Mane, M. V.; Heine,
T.; Banerjee, R. Construction of crystalline 2D covalent organic
frameworks with remarkable chemical (acid/base) stability via a
combined reversible and irreversible route. J. Am. Chem. Soc. 2012,
134, 19524−19527.
(11) Daugherty, M. C.; Vitaku, E.; Li, R. L.; Evans, A. M.; Chavez, A.
D.; Dichtel, W. R. Improved synthesis of β-ketoenamine-linked
covalent organic frameworks via monomer exchange reactions. Chem.
Commun. 2019, 55, 2680−2683.
(12) Ning, G. H.; Chen, Z. X.; Gao, Q.; Tang, W.; Chen, Z. X.; Liu,
C. B.; Tian, B. B.; Li, X.; Loh, K. P. Salicylideneanilines-Based
Covalent Organic Frameworks as Chemoselective Molecular Sieves. J.
Am. Chem. Soc. 2017, 139, 8897−8904.
(13) Li, X.; Gao, Q.; Aneesh, J.; Xu, H.-S.; Chen, Z.; Tang, W.; Liu,
C.; Shi, X.; Adarsh, K.; Lu, Y. Molecular Engineering of Bandgaps in
Covalent Organic Frameworks. Chem. Mater. 2018, 30, 5743−5749.
(14) Kandambeth, S.; Venkatesh, V.; Shinde, D. B.; Kumari, S.;
Halder, A.; Verma, S.; Banerjee, R. Self-templated chemically stable
hollow spherical covalent organic framework. Nat. Commun. 2015, 6,
6786.
(15) Reference 14 reports that IR spectra and X-ray diffraction
patterns for TAPB-PDA-OH COF are unchanged after exposure to
water, consistent with our observations of this material’s stability.
However, color or UV−vis spectral changes were not discussed.
(16) Li, R. L.; Flanders, N. C.; Evans, A. M.; Ji, W.; Castano, I.;
Chen, L. X.; Gianneschi, N. C.; Dichtel, W. R. Controlled growth of
imine-linked two-dimensional covalent organic framework nano-
particles. Chem. Sci. 2019, 10, 3796−3801.
(30) Finsy, V.; Kirschhock, C. E.; Vedts, G.; Maes, M.; Alaerts, L.;
De Vos, D. E.; Baron, G. V.; Denayer, J. F. Framework breathing in
the vapour-phase adsorption and separation of xylene isomers with
the metal−organic framework MIL-53. Chem. - Eur. J. 2009, 15,
7724−7731.
(31) Serre, C.; Bourrelly, S.; Vimont, A.; Ramsahye, N. A.; Maurin,
G.; Llewellyn, P. L.; Daturi, M.; Filinchuk, Y.; Leynaud, O.; Barnes, P.
An explanation for the very large breathing effect of a metal−organic
framework during CO2 adsorption. Adv. Mater. 2007, 19, 2246−2251.
(32) Kandambeth, S.; Shinde, D. B.; Panda, M. K.; Lukose, B.;
Heine, T.; Banerjee, R. Enhancement of chemical stability and
crystallinity in porphyrin-containing covalent organic frameworks by
intramolecular hydrogen bonds. Angew. Chem., Int. Ed. 2013, 52,
13052−13056.
(33) Miorandi, D.; Sicari, S.; De Pellegrini, F.; Chlamtac, I. Internet
of things: Vision, applications and research challenges. Ad hoc Netw.
2012, 10, 1497−1516.
(17) For the synthesis of thins films of TAPB-PDA-OBu COF, see
Shao, P.; Li, J.; Chen, F.; Ma, L.; Li, Q.; Zhang, M.; Zhou, J.; Yin, A.;
Feng, X.; Wang, B. Flexible Films of Covalent Organic Frameworks
with Ultralow Dielectric Constants under High Humidity. Angew.
Chem. 2018, 130, 16739−16743.
(18) Smith, B. J.; Overholts, A. C.; Hwang, N.; Dichtel, W. R. Insight
into the crystallization of amorphous imine-linked polymer networks
to 2D covalent organic frameworks. Chem. Commun. 2016, 52, 3690−
3693.
I
DOI: 10.1021/jacs.9b08628
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX