Two-Dimensional Chemiresistive Covalent Organic Framework with High Intrinsic Conductivity

Article abstract of DOI:10.1021/jacs.9b03441

This paper describes the synthesis of a novel intrinsically conductive two-dimensional (2D) covalent organic framework (COF) through the aromatic annulation of 2,3,9,10,16,17,23,24-octa-aminophthalocyanine nickel(II) and pyrene-4,5,9,10-tetraone. The intr

Full text of DOI:10.1021/jacs.9b03441

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Article
Two-Dimensional Chemiresistive Covalent Organic
Framework with High Intrinsic Conductivity
Zheng Meng, Robert M. Stolz, and Katherine A Mirica
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03441 • Publication Date (Web): 26 Jun 2019
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Two-Dimensional Chemiresistive Covalent Organic Framework with
High Intrinsic Conductivity
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Zheng Meng,1 Robert M. Stolz,1 Katherine A. Mirica*1
1Department of Chemistry, Burke Laboratory, Dartmouth College, Hanover, New Hampshire, 03755
Supporting Information Placeholder
ABSTRACT: This paper describes the synthesis of a novel intrinsically conductive two-dimensional (2D) covalent organic
framework (COF) through the aromatic annulation of 2,3,9,10,16,17,23,24-octa-aminophthalocyanine nickel(II) and pyrene-
4,5,9,10-tetraone. The intrinsic bulk conductivity of the COF material (termed COF-DC-8) reached 2.5110-3 S/m, and increased
by three orders of magnitude with I2 doping. Electronic calculations revealed an anisotropic band structure, with the possibility
for significant contribution from out-of-plane charge-transport to the intrinsic bulk conductivity. Upon integration into
chemiresistive devices, this conductive COF showed excellent responses to various reducing and oxidizing gases, including NH3,
H2S, NO, and NO2, with parts-per-billion (ppb) level of limits of detection (LOD for NH3=70 ppb, for H2S=204 ppb, for NO=5
ppb, and for NO2=16 ppb based on 1.5 min exposure). Electron paramagnetic resonance spectroscopy and X-ray photoelectron
spectroscopy studies suggested that the chemiresistive response of the COF-DC-8 involves charge transfer interactions
between the analyte and nickelphthalocyanine component of the framework.
INTRODUCTION
The development of chemically robust, porous, and
molecular solids,11 and conductive coordination polymers),5b,
5c, 12
the strategy for maximizing the intrinsic bulk
conductivity of COFs can leverage through-bond and
through-space charge transport characteristics. To achieve
through-bond charge transport, the covalent linkages
formed during COF synthesis must promote efficient charge
delocalization. Previous studies on 1D and 2D conjugated
polymers with single bonds in their backbones rarely showed
high charge carrier mobility values,13 suggesting that borate
electrically conductive nanomaterials drives progress in
electronic devices,1 energy storage,2 catalysis,3 and chemical
sensing.4 Recent advances in the synthesis of electrically
conductive metal−organic frameworks (MOFs) have enabled
a range of applications in electrocatalysis, energy storage,
and chemical sensing that were previously inaccessible using
traditionally insulating MOFs.5 Both through-bond and
through-space charge transport mechanisms have proven to
be effective in promoting conductivity in MOFs.5 In
particular, the molecular design strategy focusing on planar
two-dimensional MOFs, in which the formation of π–d
conjugated sheets can promote the delocalization of charge,
has yielded metallic conductivities.6 Despite progress in the
development of conductive MOFs, the design and synthesis
of electrically conductive π–conjugated covalent organic
frameworks (COFs) connected by chemically robust bonds—
which arguably possess superior chemical stability to
frameworks derived from reversible coordination chemistry
and borate or Schiff-base chemistry —has remained a
tremendous challenge.7 Although doping of COFs with
oxidants and guest molecules has led to conductivities of
~10-2 to 10-1 S/m,8 access to intrinsically conductive COFs
with high bulk conductivity remains limited.9
Capitalizing on the general principles of molecular
engineering for other classes of conductive materials (e.g.,
conductive organic polymers,10 conductive organic
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building blocks: octaamino-derived nickelphthalocyanine
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(NiOAPc) and pyrenetetraone (TOPyr). The annulation of
tetraketone and octaamine precursors to form pyrazine rings
generates a fully aromatic conjugated framework structure
with square apertures (Scheme 1). The utilization of the
nickelphthalocyanine core was inspired by its application in
the construction of conductive frameworks materials
demonstrated by our group18 and others,3d, 19 as well as its
affinity to small gaseous analytes.20 The resulting fully
conjugated 2D lattice with Lieb topology can maximize
through-bond charge delocalization, while the stacking of
the embedded metallophthalocyanine units confined within
the rigid framework material can efficiently promote out-of-
plane charge transfer. The bulk conductivity—which
characterizes the weighted average of contributions from
through-bond and through-space charge delocalization in
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polycrystalline
COF-DC-8—reached
2.5110-3
S/m,
representing the highest bulk conductivity achieved within
an intrinsically conductive COF.
Chemiresistive devices made from this conductive COF
showed excellent responses and ultra-low limits of detection
for gaseous analytes (1.5 min exposure-based LODs: 70 ppb
for NH3, 204 ppb for H2S, 5 ppb for NO, and 16 ppb for NO2).
Increases in resistance towards reducing gases and
decreases in resistance towards oxidizing gases were
consistent with p-type semiconductive character of the COF.
Spectroscopic characterization using electron paramagnetic
resonance (EPR) and X-ray photoelectron spectroscopy (XPS)
suggested that the chemiresistivite response of COF-DC-8
originates from the combination of binding and charge
transfer interactions between the analyte and the Ni-
containing phthalocyanine component of the framework.
The promising chemiresistive performance highlights the
potential application of this modular class of materials in the
fabrication of electronic devices and chemical sensors.
Scheme 1. The synthetic route for 2D conductive COF-DC-8
with nickelphthalocyanine and pyrene subunits connected
by pyrazine rings.
and imine linkages in typical COFs may be inefficient at
facilitating through-bond charge transport. While full
annulation of building blocks through aromatic linkages has
been established as a promising strategy for conjugated 2D
COF formation,7e, 13c, 14 it has not yet yielded materials with
high intrinsic conductivities. Interestingly, recent studies
highlighted the unique role of topology in enhancing
through-bond charge delocalization in conjugated 2D COFs,
with Lieb lattice15 being more favorable over Kagome
lattice.16 A complementary strategy for achieving through-
space transport requires maximizing orbital interactions
within the resulting layered framework structure through
strategic choice of building blocks. Although several reports
have taken advantage of π-stacking for designing COF-
based materials with reasonable charge carrier mobilities (8.1
cm2 V1 s1),7c, 17 their bulk conductivities remained limited.
We reasoned that by simultaneously capitalizing on both
molecular design strategies of through-bond and through-
space transport in a highly planar, two-dimensional, fully
conjugated material based on a Lieb lattice may lead to the
realization of a high intrinsic bulk electrical conductivity
within a COF-based material.
RESULTS AND DISCUSSION
Synthesis and Characterization. As drawn in Scheme
1, the condensation reaction performed in a mixed solvent
of dimethylacetamide (DMAC) and o-diclorobenzene (DCB)
in the presence of sulfuric acid for 10 days gave the desired
COF-DC-8 as a dark green powder (see Section 2.2 in
Supporting Information for details). Microwave heating
significantly reduced the reaction time to 10 hours, albeit at
the expense of slightly diminished crystallinity (entry 10 in
Table S1 and Figure S6). Efforts to optimize reaction
conditions yielded several alternative approaches for
accessing COF-DC-8, including the use of the aqueous
solution of acetic acid in a mixed solvent system of DMAC
and DCB (entry 14 in Table S1 and Figure S6), and the use
of NMP as the solvent in the presence of sulfuric acid or
aqueous solution of acetic acid (entry 17, 18 in Table S1 and
Figure S6).
Fourier-transform infrared spectroscopy of COF-DC-8
(Figure S7) showed the appearance of characteristic
absorption bands of the phenazine system at 1518, 1431,
and 1351 cm–1, while absorption bands of C=O and –NH2
This paper describes the development of a novel
intrinsically conductive COF (COF-DC-8) through the
condensation reaction between the highly planar conjugated
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groups from NiOAPc and TOPyr were absent, indicating the
peaks were observed at 2θ = 4.06°, 8.19°, 11.48°, 12.26°, and
26.65°, that were assigned to the [100], [200], [220], [300],
and [001] facets, respectively (Figure 1b). Although PXRD
displayed peak broadening which was possibly due to small
crystallite size, the peak intensity and position were sufficient
for confirming the framework structure and key cell
parameters of DC-COF-8. The strong diffractions from the
[100] and [200] facets indicated long-range order within the
ab plane of the structure. The Ni-to-Ni distance on the side
of the square, based on those two diffractions, was
calculated to be 2.17 nm. The presence of the [001] facet at
26.65° suggested the structural ordering with a 3.34 Å
separation of layers along the c axis perpendicular to the 2D
sheets. These results were consistent with the structure
optimized by DFT calculations with generalized gradient
approximation functional (See Section S6 in Supporting
Information). The experimental PXRD was in good
agreement with the simulated fully eclipsed AA-stacking
model with a space group of P4/MMM characterized by
cofacial stacking of metallophthalocyanine units in adjacent
layers; in contrast, the staggered model with an offset by
distances of a/2 and b/2 did not match the experimental
PXRD pattern (Figure 2b, Figure S11-S13).
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formation of phenazine linkages. XPS showed characteristic
bands for the K-edge of carbon (285.9 eV) and nitrogen
(398.6 eV) in C=N bond (Figure S9), indicating the presence
of sp2-hybridized nitrogen atoms. Elemental analysis showed
that C:N:Ni ratios were consistent with theoretical values
(Table S2), although the absolute percentages of C, N, and
Ni were slightly lower than theoretical values. The
discrepancy between the theoretical and observed values
could be from the existence of unreacted C=O and NH2
groups at sheet edges7d and the entrapment of small-
molecule volatiles (acetone, H2O, or DMAC) included in the
pores, as suggested by XPS (Figure S9 in section 4) and
thermal gravimetric analysis (see Figure S20 in section 10)
analysis.
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Scanning electron microscopy (SEM) revealed the
presence of rod-shaped crystallites with lengths ranging
from hundreds of nanometers to several micrometers and
widths of hundreds of nanometers (Figure 1c and Figure
S17). Transmission electron microscopy (HR-TEM) provided
visualization of a layered morphology (Figure 1d and Figure
S18). The presence of regular lines with a spacing of ~2.2 nm
in TEM (Figure 1d) was consistent with the interatomic
distances of the [100] plane inferred from PXRD and
computational models. COF-DC-8 exhibited reversible
nitrogen sorption isotherm curves with a Brunauer-Emmett-
Teller surface area of 360 m2 g–1 (Figure S19). The pore-size
distribution profile gave an accessible pore size of 1.7 nm.
Thermogravimetric analysis showed excellent thermal
stability of COF-DC-8 with only a total 10% of mass loss
upon heating to 510 °C (Figure S20). COF-DC-8 maintained
its crystallinity when treated with 12 M HCl and 14 M KOH
for 24 h (Figure S15), confirming its chemically robust
skeleton.
Figure 1. The structure and characterization of COF-DC-8.
(a) Top and side view of the structure of COF-DC-8 with 2 ×
2 square grids in eclipsed stacking mode with a Lieb lattice.
(b) Comparison of the simulated and experimental PXRD
patterns of COF-DC-8. (c) SEM and (d) TEM of COF-DC-8.
Figure 2. (a) Calculated electronic band structure of COF-
DC-8 and (b) the corresponding first Brillouin zone.
Powder X-ray diffractometry (PXRD) further supported
the formation of the desired framework formed under
optimized reaction conditions (Figure 1a-b). Prominent
Electronic Properties. The electrical conductivity of
COF-DC-8 was measured using a four-point probe method
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on a pressed pellet with a thickness of 1.0 mm. The bulk
was not suitable for confirming these calculations
experimentally.
Chemiresistivity of COF-DC-8. 2D COFs are highly
appealing for the fabrication of electronic devices by virtue
of their well-defined lattice with π functionality, large
surface-to-volume ratio, and chemically robust skeleton.
However, the poor to modest conductivity of existing 2D
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conductivity of COF-DC-8 was determined to be 2.5110-3
S/m at 298 K, which is several orders of magnitude higher
than small-molecule nickel phthalocyanine21 and a large
number of other organic solids,22 and is also significantly
higher than many undoped conjugated polymers (10-4–10-8
S/m)23 and other pristine semi-conductive COFs.7a, 8a, 8b, 8d, 9a
The Arrhenius fit to variable-temperature current
measurements revealed an activation energy of 324 meV for
the transport of charge carriers in COF-DC-8 (Figure S22).
Nearly a 103-fold improvement in conductivity was achieved
by doping the COF with a gaseous stream of I2 (~400 ppm in
N2) within 5 minutes of exposure at room temperature
(Figure S23).
COFs has limited their use in electronic devices and other
8a, 8c, 8d, 9a, 24
applications.7a,
conductivity
Given the good electrical
ability of the built-in
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nickelphthalocyanine units to bind a wide range of analytes
through coordination with the metal center,18, 25 we tested
the chemiresistive response of COF-DC-8 towards a series of
gases with different reducing and oxidizing abilities,
including NH3, H2S, NO, and NO2. These gases constitute
well-known toxic pollutants and are also biological signaling
DFT calculations (see Section S7 in Supporting
information) were performed to gain insight into the
electronic structure of the COF. The Dirac bands crossed the
Fermi level in both A−M, Γ−Z, and R-X directions and
exhibited wide band dispersions of more than 0.5 eV,
suggesting the intrinsically conductive nature of COF-DC-8.
The high symmetry points in the first Brillouin zone of COF-
DC-8 showed that the bands crossed the Fermi level through
the out-of-plane directions (Figure 2a). In contrast, the in-
plane direction, including Γ−M, Z−R, Γ−M and Z-A of the
material, exhibited moderate band gaps ranging from
0.5−1.0 eV. These results suggested that the charge
transport in COF-DC-8 may be anisotropic and that a
significant mechanism contributing to the bulk conductivity
may be through pathways along the c-axis.6d The computed
through space charge-transport property is consistent with
those found in other phthalocyanine-17b, 17c and porphyrin-
based COFs,7c, 17d that exhibited high carrier mobility along
the direction of the stacking due to the formation of periodic
π-columns. We hypothesize that the eclipsed stacking mode
of COF-DC-8 has the potential to amplify the alignment of
the π-conjugated units and enhance the through-space
molecules.26 To make chemiresistive devices,
a COF
suspension was drop casted onto interdigitated electrodes,
which gave devices with resistances in the range of kΩ
(Figure S24). The devices were tested under a constant
applied voltage of 1.0 V, and the current was monitored
using a potentiostat.
Upon exposure to 40 ppm of NH3 (balance gas N2) for
30 min, COF-DC-8 showed a normalized response (−ΔG/G0)
of 391% (Figure 3a, Figure S34a). This positive change in
resistance of COF-DC-8 in response to NH3, which typically
serves as an electron donor, suggested that COF-DC-8
behaved as a p-type semiconductor.27 A response of 62%
was observed to 40 ppm of H2S (Figure 3a, Figure S34a).
The higher response to H2S, compared to NH3, may be due
to the stronger interaction between the material and H2S.
This response value was comparable with chemiresistive
devices employing hexahydroxytriphenylene-based MOF28
and metallophthalocyanine-based bimetallic MOF.29 In
contrast to the positive responses to NH3 and H2S, COF-DC-
8 exhibited negative response to both NO and NO2 (Figure
3b, Figure S34b), which further reinforced the p-type
character of COF-DC-8. The exposure of the COF to 40 ppm
NO generated a high response of 3939 317 % (Figure 3c).
This remarkable response to
metallophthalocyanine-to-metallophthalocyanine
charge
transport along the 2D stacks. Since the experimental
validation of these calculations would require access to
highly oriented nanosheets and nanowires of the COF, the
polycrystalline material—which was the focus of this study—
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Figure 3. Chemiresistive responses of devices with integrated COF-DC-8 to (a) NH3, (b) H2S, (c) NO, and (d) NO2 under an
applied voltage of 1.0 V and an atmosphere of dry nitrogen. Responses (−ΔG/G0) of COF-DC-8 after 1.5 min (squares) and 30
min (circles) exposure versus concentration of (e) NH3, (f) H2S, (g) NO, and (h) NO2.
NO was more than 10 times higher than that produced by
triphenylene-based conductive MOFs.28, 30 An even higher
response of –6338 479% was generated after the 30 min
exposure of 40 ppm of NO2 (Figure 3d, Figure S34b). This
modulation of electrical conductivity of this COF by chemical
doping with low-concentrations of small reactive gases has
promising implications for developing advanced electronics
where rectification of conductance is required.
dependences were found for the LOD values of NO and NO2
(Table S8, S10). Longer exposure times led to sub-ppb level
of calculated LODs.
Recovery of Device Performance. Sensors
characterized by high reversibility in response under
repeated exposures to analyte are advantageous for their
reusability. While dosimetric devices are highly irreversible
when cycled through analyte exposures, they are
advantageous to allow a historical record of total exposure
events.31 The response of COF-DC-8 to NH3 was partially
reversible in the concentration range from 5 to 80 ppm
(Figure 3a, Table S3). Full reversibility was observed when
the concentration was below 2 ppm. Across the entire tested
concentration range (2–80 ppm) for H2S, only slight
reversibility (5%-15%) was observed (Figure 3b, Table S5).
The response to both NO and NO2 showed partial
reversibility (7%-33% for NO and 25%-45% for NO2, Figure
3c, 3d, Table S7, S9). These observations demonstrated the
dosimetric or quasi-dosimetric characteristics of COF-DC-8
to H2S, NO, and NO2, in which the response was dependent
on the dosing history.
Determination of LODs. Concentration-dependent
studies (Figure 3a-d) enabled quantitative correlations
between changes in response as a function of analyte
concentration for COF-DC-8. Although the response values
were distinguishable at [NH3] =2–80 ppm, only a relatively
narrow linear range of 2–10 ppm could be identified (Figure
3e, Figure S27). The LOD value for NH3 determined from the
response-concentration relationship in the linear range was
70 ppb based on 1.5 min exposure. Compared with NH3,
COF-DC-8 showed a wider liner range of 5–80 ppm for H2S
that persisted across different exposure times ranging from
1.5 min to 30 min (Figure 3f, Figure S29). The LOD derived
for H2S detection was 204 ppb based on response values
after 1.5 min exposure to the analyte. The calculated LOD
values for both NH3 and H2S showed minimal dependence
on exposure time with only slight changes observed for
exposure time differences in the range of 1.5 min to 30 min
(70-57 ppb for NH3, 204-121 ppb for H2S, see also Table S4,
S6).
In order to investigate the reusability of the devices, the
analyte-exposed COF devices were reactivated by both
thermal recovery and solvent recovery (Section 12.2.6 in
Supporting Information). After solvent recovery by
immersing in deionized water for 1 hour, the responses of
devices were largely restored (108% for H2S, 71% for NO, and
100% for NO2). However, the devices were only partially
restored (60% for H2S, 14% for NO, and 17% for NO2) their
performance after thermal recovery by heating at 60 ºC for
18 h under ~20 mTorr. These results were consistent with the
previous report showing that, in metal–organic frameworks,
washing was more efficient than heating for recovery of
device performance.28 The recovery tests suggested that
analyte molecules bound by the material may be released
Excellent linear response-concentration relationships
(R2=0.98-0.99) were observed for NO and NO2 at relatively
wide concentration ranges of 0.02–40 ppm for NO and 2–40
ppm for NO2(Figure 3g, 3h, Figure S31, Figure S33). The
striking sensitivity of COF-DC-8 led to ultra-low LODs of 5
and 16 ppb for NO and NO2, respectively, based on
responses after 1.5 min exposure. Compared with those
found for NH3 and H2S, significant exposure-time
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from the surface under aqueous conditions, thus restoring
the sensing ability of the COF-DC-8 device.
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Analysis of Sensing Kinetics. The rate and magnitude
of the chemiresistive response is usually governed by several
factors, including the characteristics of the sensing
apparatus,32 the morphology and preferential orientation of
the material,33 and the electronic nature of the analyte.4b
Analyzing the rate of response at the initial stage upon
analyte exposure is a convenient technique that can allow
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convenient assessment of sensing kinetics when factors of
the sensing apparatus and material characteristics are kept
constant.34
Through the analysis of the initial rate of response, we
found that COF-DC-8 could differentiate certain analytes
and their concentrations quantitatively within only 1 minute
of initial analyte exposure (Figure 4, Figure S36-S39).
Plotting the slope of initial response over the first 1 min of
exposure versus the concentration of the four gases
provided a linear relationship, which, in most cases, extended
into much wider concentration ranges compared with those
found in direct response-concentration analysis. This
method provided a simple and fast analysis of the
concentration of the four analytes.
Figure 5. EPR spectrum of COF-DC-8 before (dashed lines)
and after dosing (solid lines) with (a) NH3 and H2S, and (b)
NO and NO2. For comparison, the intensity of the peaks has
been normalized based on the peak intensity before analyte
dosing. (c) XPS of S 2p range of COF-DC-8 after H2S
exposure. (d) XPS of N 1s range of COF-DC-8 after NO2
exposure.
Sensing Mechanism. To investigate the sensing
mechanism, spectroscopic analysis using EPR and XPS was
performed. EPR analysis was carried out in the X-band at 77
K under N2 atmosphere on COF samples before and after
they were exposed to analytes (1% NH3, H2S, NO, and NO2)
for 1 hour. XPS analysis was carried out on analyte-exposed
samples prepared by a similar procedure with those for EPR
analysis, which were subsequently mounted on copper tape
and analyzed under reduced pressure of ~10-7 Torr (see
Section 12.3 in Supporting information for details).
Figure 4. The initial rate of response as a function of
concentration of (a) reducing gases NH3 and H2S and (b)
oxidizing gases of NO and NO2.
The EPR spectrum of pristine COF-DC-8 showed a
ligand centered radical at g = 1.99, which was likely due to
the chemisorption of O2 on the Ni center that induces
unpaired radicals because of the charge transfer from the
phthalocyanine ligand to O2 molecule.35 After NH3 exposure,
only a 15% intensity increase in EPR signal was observed
(Figure 5a). No obvious change was detected on the XPS
spectra of C 1s, N 1s, and Ni 2p (Figure S41) after NH3
exposure. These results suggested that the interaction
between NH3 and COF-DC-8 may be dominated by
reversible and weak charge transfer interactions under
tested conditions.36
Each of the four gases showed distinct dynamics of
response at wide concentration range. Under the
concentration of 40 ppm, the rate of the initial response for
NH3 was 48±5% min-1, while the initial response for H2S
showed a slower rate of 18±3% min-1 (Figure S36, S37).
Compared with reducing gases NH3 and H2S, COF-DC-8
showed much quicker response for NO and NO2 at the initial
state with rates at 513±53% min-1 and 171±8% min-1,
respectively (Figure S38, S39). This observation indicated
that the initial rate of response was strongly dependent on
type of the analyte-material interaction.4
After dosing COF-DC-8 with H2S, we observed a
significant decrease in peak intensity of 70% in the EPR signal
(Figure 5a). This observation may be consistent with the
replacement of surface-bound O2 by H2S at the Ni centers.
By XPS, clear emission lines at binding energies of 163.8 eV
and 165.1 eV (Figure 5c, Figure S42) were detected, that
were similar to S 2p1/2 and S 2p3/2 peaks found in metal
sulfides,37 which indicated the formation of Ni-S bond.
Moreover, a prominent peak at a higher binding energy of
167.5 eV, consistent with the formation of sulphite species,
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Journal of the American Chemical Society
was also observed. This observation suggested that the Ni-
bound H2S may be further transformed into species with
higher oxidation states, likely with the participation of
sensing, electronics, and electrical energy conversion and
storage. Future studies focused on the exploration of
templated epitaxial growth of the COF, the exfoliation of its
thin layer forms, as well as examining the influence of the
central element in the phthalocyanine component on
structure-property relationships, will be required for
enhanced understanding of this COF system and the
fabrication of highly modular advanced electronic devices
and sensing arrays.
1
2
3
4
5
6
7
8
surface adsorbed O2 through
a
catalytic oxidation
reaction.37b This type of irreversible transformation is
consistent with the negligible recovery in response after H2S
exposure shown in Figure 3b.
After the exposure of the COF to oxidizing gases,
prominent increases of 3.2- and 4.2-fold in the EPR signals
were found after NO and NO2 exposure, respectively (Figure
5b). No significant changes were observed for the XPS
spectra of Ni 2p, indicating that the oxidation state of the Ni
centers was not altered (Figure S43, S44). After NO dosing,
a new component corresponding to N 1s appeared at the
binding energy of 402.0 eV, which was absent in the pristine
material. Based on the previous studies of NO adsorption on
metal surfaces, this component was assigned as a Ni-bound
NO species.38 After NO2 dosing, a weak, yet clear, peak was
observed at the binding energy of 406.1 eV (Figure 5d).
Through the comparison with spectra of NO2 adsorbed on
metallophthalocyanines39 and metal oxides40, this peak
suggested the existence of NO2 molecules which were likely
coordinated to the Ni center. Taken together, the EPR and
XPS results suggest that surface adsorption and charge
transfer interactions between the analytes and
nickelphthalocyanine component of the COF were critical to
the observed chemiresistive properties of the COF.
9
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Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Synthetic details, characterization, and conductivity
measurements, and sensing experiments (PDF).
AUTHOR INFORMATION
Corresponding Author
Katherine.A.Mirica@dartmouth.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
K.A.M. acknowledges support from startup funds provided
by Dartmouth College, from the Walter and Constance Burke
Research Initiation Award, Army Research Office Young
Investigator Program Grant No. W911NF-17-1-0398,
National Science Foundation EPSCoR award (#1757371),
Sloan Research Fellowship (FG-2018-10561),3M Non-
Tenured Faculty Award, and Arthur L. Irving Institute for
Energy and Society at Dartmouth. The authors thank the
University Instrumentation Center at the University of New
Hampshire (Durham, NH) for the access to XPS, Mr. Maciej
M. Kmiec for the assistance with EPR, and Mr. Chang Liu for
the assistance with TGA.
CONCLUSION
In conclusion, we have designed and synthesized a
novel intrinsically conductive COF-DC-8 through the
aromatic annulation between of 2,3,9,10,16,17,23,24-octa-
aminophthalocyanine nickel(II) and pyrene-4,5,9,10-tetraone.
The reaction between the tetraketone and octaamine allows
the formation of pyrazine rings to generate a fully aromatic
conjugated structure, and renders the construction of square
apertures with a side length of 2.2 nm and excellent chemical
and thermal stability. The bulk room temperature
conductivity of COF-DC-8 reached 2.51×10-3 S/m, which is
the highest bulk conductivity achieved within an intrinsically
conductive COF. Doping with I2 can further increase the bulk
conductivity by a factor of 103.
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