A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor–Acceptor Fulleretic Covalent-Organic Materials

Article abstract of DOI:10.1002/anie.201914233

The effect of donor (D)–acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for “static” changes in materials properties, an external stimulus was applied for “dynamic” control of the electronic profiles. Overall, the presented D–A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.

Full text of DOI:10.1002/anie.201914233

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: A Dual Threat: Redox-Activity and Electronic Structures of Well-
Defined Donor-Acceptor Fulleretic Covalent-Organic Materials
Authors: Natalia B. Shustova, Gabrielle A. Leith, Allison M. Rice,
Brandon J. Yarbrough, Anna A. Berseneva, Richard T. Ly,
Charles N. Buck, Amy J. Brandt, Donna A. Chen, Benjamin
W. Lamm, Morgan Stefik, Kenneth S. Stephenson, Mark
D. Smith, Aaron K. Vannucci, Perry J. Pellechia, Sophya
Garashchuk, and Denis Chusov
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914233
Angew. Chem. 10.1002/ange.201914233
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10.1002/anie.201914233
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A Dual Threat: Redox-Activity and Electronic Structures of Well-
Defined Donor-Acceptor Fulleretic Covalent-Organic Materials
Gabrielle A. Leith,[a]§ Allison M. Rice,[a]§ Brandon J. Yarbrough,[a]§ Anna A. Berseneva, [a] Richard T.
Ly,[a] Charles N. Buck III,[a] Denis Chusov,[b] Amy J. Brandt,[a] Donna A. Chen,[a] Benjamin W. Lamm, [a]
Morgan Stefik,[a] Kenneth S. Stephenson,[c] Mark D. Smith,[a] Aaron K. Vannucci,[a] Perry J. Pellechia,[a]
Sophya Garashchuk,[a] and Natalia B. Shustova*[a]
Abstract: The effect of donor (D)-acceptor (A) alignment on the
materials electronic structure was probed for the first time using novel
purely organic porous crystalline materials with covalently integrated
two- and three-dimensional acceptors. The first studies towards
estimation of charge transfer rates as a function of acceptor stacking
are in line with the experimentally observed, drastic (ca eight-fold)
conductivity enhancement. Redox behavior of prepared materials,
which are the first studies of this kind reported for any buckyball- or
tetracyanoquinodimethane-integrated crystalline porous scaffolds to
date, was evaluated. Moreover, in parallel with tailoring the D-A
alignment responsible for “static” changes in materials properties, an
external stimulus was applied for the first time to this class of materials
for “dynamic” control of the materials’ electronic profiles. Overall, the
Scheme1. (left) Donor-acceptor alignment inside of a porous well-defined
organic framework resulting in conductivity enhancement. (right) The cyclic
discussed D-A strategic design, in combination with stimuli-controlled
electronic behavior, redox activity, and modularity could be used as a
blueprint for the development of electroactive and conductive
multidimensional and multifunctional crystalline porous materials.
voltammetry curve of crystalline porous material with embedded electron-
accepting moieties (TCNQ[1≡(34%)]; DMF solution containing 0.1
tetrabutylammonium hexafluorophosphate, TCNQ[1≡(34%)] (40 wt.%), carbon
black (60 wt.%), and 0.1 mL of Nafion; saturated calomel reference, platinum
wire counter, and gold working electrodes).
M
Merging the properties of three-dimensional buckyballs (or
buckybowls) and covalent-organic frameworks (COFs) provides
function of acceptor stacking. In addition, we estimated redox
behavior using cyclic voltammetry of pure organic crystalline
porous scaffolds containing covalently decorated pores with
strong electron accepting bound tetracyanoquinodimethane
(TCNQ) and fullerene (C60) moieties,[31–33], which are the first
studies of this kind reported for any buckyball- or TCNQ-
integrated COFs to date. We demonstrate that the concept of
fluorophore tag integration could be an effective avenue to
monitor reaction progress inside of a COF. To achieve that, the
COF interior for the first time was harnessed for performance of
Sonogashira cross-coupling and [2+2] cycloaddition reactions,
followed by ring opening of a strained cyclobutene intermediate in
the case of the latter. In addition to donor (COF)-acceptor
(fullerene or TCNQ) alignment responsible for “static” changes in
the materials’ electronic structure, we also test the hypothesis
that an excitation wavelength could be used for dynamic control
of electronic properties in COFs infiltrated with photoresponsive
units as a function of an external stimulus for the first time.[34–36]
For integration of three-dimensional (3D) acceptors such
access to
a novel class of materials combining ultrafast
energy/electron transport characteristic of three-dimensional (3D)
fulleretic linkers with high modularity, crystallinity, and surface
area, which are intrinsic properties of COFs.[1-13] The precise
donor-acceptor alignment achieved in the materials is imposed by
the rigid COF scaffold and is crucial for efficient energy or charge
transfer as it can influence the distance of exciton diffusion, π-π
stacking, or Förster radius, and as a result, can enhance device
performance.[14–20]
Herein, we demonstrate that the donor-acceptor alignment
in well-defined frameworks could drastically affect the materials’
electronic properties resulting in ca 100,000,000-fold conductivity
enhancement, one of the largest increases reported for COFs.[21–
30] We probed charge transfer rates within the Marcus theory as a
[a]
§
G. A. Leith,§ A. M. Rice, § B. J. Yarbrough, § A. A. Berseneva, R. T.
Ly, C. N. Buck III, A. J. Brandt, Prof. Dr. D. A. Chen, B. W. Lamm,
Prof. Dr. M. Stefik, Dr. M. D. Smith, Prof. Dr. A. K. Vannucci, Dr. P.
J. Pellechia, Prof. Dr. S. Garashchuk, and Prof. Dr. N. B. Shustova
equal contribution
as buckyballs, choice of the host matrix (COF) should satisfy the
following criteria: (i) sufficient pore aperture for accommodation
of bulky electron acceptors (e.g., C60 diameter ~7 Å;), (ii)
presence of reactive functional groups promoting cycloaddition
or coupling reactions, and (iii) maintenance of structural integrity
after derivatization. The first two strategies (1,3- and [2+2]
Department of Chemistry and Biochemistry, University of South
Carolina (USC)
631 Sumter Street, Columbia, SC 29208 (USA)
E-mail: shustova@sc.edu
[b]
[c]
Dr. D. Chusov
A. N. Nesmeyanov Institute of Organoelement Compounds of the
Russian Academy of Sciences
Vavilova St. 28, Moscow 119991, Russian Federation
K. S. Stephenson
cycloaddition followed by ring opening of
a
strained
intermediate) were utilized to integrate 3D and planar electron
acceptors while the third strategy (Sonogashira cross-coupling)
was employed to embed fluorophore tags (vide infra). The COF,
1≡(x%) (where x = 34 = [BPTA]/([BPTA]+[DMTA]) × 100%;
Department of Physics and Astronomy, USC
712 Main Street, Columbia, SC 29208 (USA)
Supporting information for this article is given via a link at the end of
the document.
Figure
1),
prepared
from
2,5-bis(2-propynyloxy)
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1

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terephthalaldehyde (BPTA), 2,5-dimethoxyterephthalaldehyde
(DMTA), and tri-(4-aminophenyl)benzene (TAPB), satisfies the
mentioned criteria. This COF (i) possesses 30 Å-channels, (ii)
contains alkyne groups available for postsynthetic derivatization
(see the Supporting Information for synthetic details), and (iii)
maintains structural integrity over a wide pH range.[37]
reaction conditions suitable for 1,3-cycloaddition starting with less
bulky and more affordable azide-based derivatives, e.g., 2-
azidoethanol and 2-azidoethyl ethyl malonate.Only after such
control experiments, where the reaction conditions which were
developed for cycloaddition of the small azide-based derivatives,
applied for integration of 2-azidoethyl ethyl malonate (NEM)
inside the of the COF (1≡(34%), see the Supporting Information).
As a result, the methodology for development of donor-acceptor
materials includes two-steps: (i) several-day pre-soaking of, for
instance, C60 derivatives inside the large COF channels
promoting acceptor diffusion and (ii) addition of the reagents
required for a 1,3-cycloaddition reaction (Figure 1) followed by
heating. Synthesis of the C60 derivative, 2-azidoethyl-(ethyl)-3’H-
cyclopropa-[1,2](C60)[5,6]fullerene-3’,3’-dicarboxylate (NEM-C60,
Figure 1), suitable for the cycloaddition reaction and accessible
on a gram-scale without the use of labor-demanding high-
pressure liquid chromatography (HPLC) was performed using a
3-step procedure.[38]
Spectroscopic techniques including Fourier-transform
infrared (FTIR) and solid-state 13C cross-polarization magic-angle
spinning (CP-MAS) nuclear magnetic resonance (NMR)
spectroscopies, as well as fluorophore tag integration discussed
below, were used to monitor reaction progress in the solid state.
Control experiments, in which the COF was subjected to the same
reaction conditions but without the addition of the azide-
containing precursor, were also carried out. In the case of FTIR
spectroscopic studies, we monitored the disappearance of 2120
cm-1 (C≡C) and 3300 cm-1 (C≡C–H) bands (Figures 2 and 3) in
the spectra of products in contrast to the control experiment where
both bands were preserved (Figure S11). For instance, in the
case of NEM-C60[1≡(34%)], FTIR spectroscopic studies
confirmed disappearance of the C≡C resonance as well as
appearance of the carbonyl stretch (1716 cm-1) of the diethyl
malonate moiety (Figure 2). The 13C CP-MAS spectroscopic
studies of NEM-C60[1≡(34%)] demonstrated disappearance of the
C≡C resonances in contrast to the control experiment (without
presence of the azide precursor under the same reaction
conditions, Figure 2). Moreover, the substantial decrease of
Brunauer-Emmett-Teller (BET) surface area from 1253 m2g-1
(1≡(34%), Figure S15) to 229 m2g-1 (NEM-C60[1≡(34%)], Figure
S16) also supports incorporation of bulky C60 derivatives inside of
the porous scaffold. Although the surface area for a covalently
linked C60-metalloporphyrin COF was slightly higher (393
m2/g),[39] we report the highest surface area determined for a
purely organic COF with covalently bound C60 acceptors to date.
The steady-state and time-resolved spectroscopic studies are
also in line with integration of electro accepting units inside of the
electron donating host (vide infra). Crystallinity of the synthesized
material was preserved even after multiple cycles of the 1,3-
cycloaddition reaction necessary for maximization of C60 content
as evident through wide-angle X-ray scattering (WAXS, Figure
S9) studies; resulting in the first example of a crystalline, porous,
and fully organic covalently-linked C60-based COF. [39–42]
Figure 1. (top) Development of reaction conditions utilizing molecular TCNQ
moieties for the [2+2] cycloaddition reaction, followed by ring opening of a
strained cyclobutene intermediate. Single-crystal X-ray structure of the product
is shown. Thermal displacement ellipsoids are drawn at the 50% probability
level. (middle) The 1,3-cycloaddition and [2+2] cycloaddition reactions, followed
by ring opening of a strained cyclobutene intermediate in the case of the latter,
for integration of fullerene and TCNQ moieties, respectively, are shown.
(bottom) Integration of a fluorescent tag using Sonogashira cross-coupling
reaction.
While C60 possesses a unique spherical structure allowing
for stabilization of up to six electrons, the LUMO of C60 (-4.50 eV)
is comparable with a planar acceptor, TCNQ (-4.23 eV).[43,44]
TCNQ derivatives usually do not require the labor-intensive HPLC
procedure for their isolation, in contrast to C60 compounds with a
large number of active centers and possible isomers,[14] making it
possible to compare the properties of prepared NEM-
C60[1≡(34%)] with TCNQ-covalently-linked analogs. To develop
suitable reaction conditions to perform a [2+2] cycloaddition
Prior to investigations with bulky and relatively expensive
fullerene-containing derivatives, we systematically studied the
2
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reaction followed by ring opening of an intermediate cyclobutene
necessary to attach TCNQ through a covalent bond to the COF
interior and to monitor reaction progress using “conventional”
anthracene[1≡(50%)]
and
4-methyl-2-oxo-2H-chromene-3-
carbonitrile[1≡(50%)] (Figures S22 and S23). We applied the
developed strategy towards detection of residual alkyne moieties
in TCNQ[1≡(50%)]. In contrast to the reaction with 1≡(50%), there
was no emission detected for TCNQ[1≡(50%)] after the treatment
with both fluorescent tags (Figures S24–25). These findings are
in line with the spectroscopic evidence and confirm the lack of
unreacted alkyne sites concluding reaction completion.
To study the electronic and photophysical properties of the
prepared donor-acceptor materials, comprehensive analysis
including diffuse reflectance (DR), steady-state and time-resolved
photoluminescence (PL), and X-ray photoelectron (XPS)
spectroscopies, cyclic voltammetry (CV), and conductivity
measurements were employed.
Integration of electron acceptors (TCNQ or C60-derivative)
inside the COF was accompanied by a drastic color change from
pale yellow to dark red or brown, respectively (Figure 3). DR
spectroscopic studies confirm that acceptor integration resulted in
the appearance of an additional absorption band leading to a
bathochromic shift of the absorption profile in comparison with the
parent COF, C60, and TCNQ, which is typically associated with
charge transfer (CT).[31,45] To further study the observed behavior
of both C60 and TCNQ-modified COFs, we utilized time-resolved
PL spectroscopy. With the assumption that the PL decay rate
consists of radiative, nonradiative, and CT components, analysis
should reveal shortening of PL lifetimes of the host owing to
integration of strong acceptor moieties (i.e., fullerene-based
derivative or TCNQ).[46] The amplitude-averaged lifetime
estimated by fitting the time-resolved PL decay curves were 474
ps for 1≡(34%) while NEM-C60[1≡(34%)] and TCNQ[1≡(50%)]
exhibited much shorter lifetimes of 32 ps and 103 ps, respectively.
This is indicative of the potential for CT in the system (Figure 2).
Similar behavior was previously reported in literature for C60
inclusion as guest molecules.[41] In addition, the emission
response of the COF itself was quenched when either TCNQ or
NEM-C60 was covalently bound to the COF walls (Figure S27).
As a fast and nondestructive pre-screening technique, XPS
was used to probe the electronic structure of the C60-based COF
by monitoring the density of states (DOS) near the Fermi level (EF,
binding energy = 0 eV). For 1≡(34%) itself, the valence band
spectrum exhibits behavior associated with insulating materials,
given that there is near zero intensity within 3 eV of the Fermi level
(EF = 0 eV binding energy). The spectrum of C60 also shows zero
intensity at 3 eV. In contrast, the XPS valence band spectrum for
NEM-C60[1≡(34%)] exhibits higher intensity at 3 eV from EF. This
indicates a greater DOS near EF, which would be consistent with
a more conductive material.
To further elaborate on the observed changes in the XPS
spectra, we performed conductivity measurements demonstrating
that, for instance, the conductivity of NEM-C60[1≡(34%)] was
found to be 1.97 × 10-6 S/cm in comparison with the parent COF
(2.32 × 10-12 S/cm).[31] Our observations that conductivity
increases with promotion of donor-acceptor alignments are in line
with literature reports.[32] On the example of TCNQ-integrated
materials, we also studied changes in electronic properties as a
function of the degree of acceptor integration. Conductivity
measurements demonstrate an order-of-magnitude difference for
TCNQ[1≡(34%)] (1.67 × 10-7 S/cm) versus TCNQ[1≡(100%)]
(2.12 × 10-6 S/cm) resulting in a six-orders-of-magnitude increase
in comparison to the COF itself (2.32 × 10-12 S/cm). The optical
band gap values derived from the Tauc plot analysis for 1≡(34%),
NEM-C60[1≡(34%)], and TCNQ[1≡(50%)] were found to be 2.32
Figure 2. (a) FTIR spectra of 1≡(34%) (black) and NEM-C60[1≡(34%)] (blue).
Grey areas show H–C≡C present in 1≡(34%) and absent in NEM-C60[1≡ (34%)]
and the appearance of the C=O stretch in NEM-C60[1≡ (34%)]. (b) 13C CP-MAS
NMR spectra of 1≡(34%) (black), a control experiment with 1≡(34%) (orange),
and NEM-C60[1≡(34%)] (blue). (c) Fluorescent decays of 1≡(34%) (black), NEM-
C60[1≡(34%)] (blue), and TCNQ[1≡ (50%)] (red). (d) XPS data for the valence
band region for 1≡(34%) (black), C60 (pink), and NEM-C60[1≡ (34%)] (blue).
solution NMR spectroscopy, we started with the molecular
building blocks, TCNQ and 4-ethynyl-N,N-dimethylaniline (Figure
1). In addition to spectroscopic studies, formation of the desired
product,
2-(4-(4,4-dicyano-2-(4-
(dimethylamino)phenyl)butylidene)cyclohexa-2,5-dien-1-
ylidene)malononitrile, was confirmed by single-crystal X-ray
crystallography (Figures 1 and S12). As a next step, the
developed conditions were applied toward the reaction of TCNQ
with the COF (1≡(x%)). For comprehensive characterization of
the prepared TCNQ[1≡(x%)], we used the same set of techniques
as in the case of NEM-C60[1≡(34%)]. For instance, we observed
disappearance of 2120 cm-1 (C≡C) and 3300 cm-1 (C≡C–H) bands
in the FTIR spectrum, while appearance of the nitrile stretch at
2100 cm-1 (C≡N) persisted even after a 24-hour Soxhlet washing
procedure, highlighting the presence of TCNQ covalently bound
to the COF (Figure 3). Due to the smaller size of TCNQ versus
the relatively bulky NEM-C60, the measured BET surface area
even after TCNQ integration was found to be 812 m2g-1 (Figure
S17). The determined surface area is the highest for a TCNQ-
covalently-linked COF to date in comparison with previous reports
that mainly focused on infiltration of TCNQ molecules as
guests.[23]
In addition to spectroscopic methods and gas sorption
analysis, fluorescent labeling was utilized to address the question
about residual unreacted alkyne sites. Two chromophores, 6-
bromo-3-cyano-4-methylcoumarin and 2-bromoanthracene, were
selected to perform a Sonogashira cross-coupling reaction within
the functionalized COF (Figure 1). As a control experiment, we
integrated coumarin and anthracene moieties inside 1≡(50%). We
observed
a
strong
fluorescent
signal
for
both
3
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eV, 1.81 eV, and 2.04 eV, respectively, and they are in line with
the conductivity measurements (Figures S34-35).[47,48] Further
tuning of electronic properties was achieved through iodine
doping
electron transfer rates compared to the parent COF itself (see the
Supporting Information for more details). In addition, the
electronic structure analysis corroborates donor-acceptor
alignment resulting in a bathochromic shift in optical transitions
after TCNQ-integration (Figure 3) which is in good agreement with
the experimental results (Figure 3).
In addition to “static” changes through acceptor
incorporation inside the electron donating matrix, the electronic
properties of COF materials could also be dynamically controlled
through integration of photoresponsive molecules. To investigate
this possibility, a spiropyran derivative, 1′,3′-dihydro-1′,3′,3′-
trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole]
(SP,
Figure S33), was encapsulated in 1≡(0%). In general, SP
derivatives isomerize under UV irradiation to the charge-
separated merocyanine form which could result in π-electron
delocalization and possibly promote conductivity enhancement.
Indeed, upon UV irradiation, a drastic color change from light
yellow to dark brown was observed for SP@[1≡(0%)], indicating
the transformation from the closed spiropyran form to the open
merocyanine form (Figure S33).[50] Based on 1H NMR
spectroscopic analysis of the digested SP@1≡(0%), one SP
molecule was found per six –OMe units (Figure S32).
Furthermore, after 30-min-UV irradiation, a 39%-increase in
conductivity was observed in comparison with SP@1≡(0%) (5.38
× 10-11 S/cm). As a result, these studies could be considered as a
blueprint for the development of stimuli-responsive crystalline
doped COFs with dynamically controlled electronic behavior.
Due to the presence of redox-active building units, fullerene
and TCNQ moieties, we studied the redox behavior of the
prepared NEM-C60[1≡(34%)] and TCNQ[1≡(34%)]. Significant
modifications of the commonly used electrochemical setup were
employed[51] since cyclic voltammetry studies of COFs are still
relatively rare. The redox-active COFs (40 wt.%) were combined
with carbon black (60 wt.%) and Nafion, and the obtained slurry
was pipetted onto the tip of the electrode and dried under high
vacuum. Cyclic voltammetry of TCNQ itself (1.0 × 10−4 M) was
Figure 3. (a) FTIR spectra of 1≡(34%) (black) and TCNQ[1≡(50%)] (red). Grey
areas show the H–C≡C stretch present in 1≡(34%) and absent in
TCNQ[1≡(50%)] as well as appearance of the C≡N stretch in TCNQ[1≡(50%)].
(b) 13C CP-MAS NMR spectra of 1≡(34%) (black) and TCNQ[1≡(50%)] (red). (c)
Normalized diffuse reflectance spectra of 1≡(34%) (black), C60 (pink), NEM-
C60[1≡(34%)] (blue), TCNQ (grey), and TCNQ[1≡(50%)] (red). (d) Photographs
of 1≡(x%), NEM-C60[1≡ (34%)], and TCNQ[1≡(50%)]. The table shows the
amplitude-weighted average lifetimes for 1≡(34%), NEM-C60[1≡ (34%)], and
TCNQ[1≡(50%)]. (e) The optical transition strength calculated for the COF-
fragment (black) and TCNQ[COF]-fragment (red) and the corresponding
electron transfer rates. The theory level is RPA TDDFT based on B3LYP-D3/6-
31+G**. (f) Schematic representation of the frontier orbitals for the truncated
molecular model comprised of the TCNQ[COF]-fragment.
performed in
a
0.1
M
solution of tetrabutylammonium
hexafluorophosphate in acetonitrile to afford one quasireversible
reduction at Ep = –0.5 V and one oxidation at 0.1 V vs. saturated
calomel electrode (SCE) (Figure S19). Comparison of the
acquired CV data for TCNQ[1≡(34%)] to TCNQ itself, shows that
both potentials of TCNQ[1≡(34%)] are less negative at Ep = –0.3
V and 0.2 V vs. SCE (Scheme 1), which could be evidence of the
stronger electron accepting properties of the prepared material.
Thus, inclusion of TCNQ in a COF matrix affects the redox
potential, which enables this compound to be electrochemically
sensed.[52] In comparison with C60 itself which exhibits four
quasireversible reductions at Ep = –0.3 V, –0.7 V, –1.4 V, and –
1.9 V in DMF vs. SCE, the NEM-C60 shows similar redox features
at Ep = –0.9 V and –1.8 V (Figures S18 and S20). The reduction
waves are more negative than that of pristine C60, which is a
known consequence of the saturation of a double bond on the C60
sphere.[53] The redox potentials of NEM-C60 (Ep = –0.9 V and –1.8
V) and NEM-C60[1≡(34%)] (Ep = –1.2 V and –2.1 V vs. SCE) are
also similar, with a slight shift in potential (Figures S20 and S21)
supporting integration of NEM-C60 and reinforcement of the redox-
active nature within the COF. Such electrochemical behavior (i.e.,
presence of two waves in the cyclic voltammogram) is
comparable with C60-based dimers in the literature,[53] with a
reduced symmetry comparable to NEM-C60. While CVs with
integrated acceptors have been measured for other systems such
as molecular rectangles,[52] these are the first studies, to the best
resulting in further increases of conductivity in the range of two-
to four-orders-of-magnitude with values of 5.06 × 10-8 S/cm, 1.08
× 10-5 S/cm, and 1.41 × 10-4 for COF, TCNQ[1≡(50%)], and
TCNQ[1≡(100%)], respectively. The conductivity of NEM-
C60[1≡(34%)] increased to 3.63 × 10-5 S/cm when the material was
doped with iodine. The detailed procedure of iodine doping can
be found in the Supporting Information. The values obtained
herein are among some of the highest values reported in COFs to
date.[21–27] In addition, these conductivity studies are the first
reports for COFs with covalently bound TCNQ moieties.
To shed light on how acceptor modulation and stacking
(e.g., TCNQ moieties) could potentially affect COF properties,
electronic structure calculations and electron transfer rates were
estimated according to the Marcus theory[49] (Eq. 1).
ꢀ = 2ꢁ/ℏ · |ꢂ |ꢄ/√(4ꢁꢅꢀꢆΤ) ∙ exp (−ꢅ/(4ꢀꢆΤ)) (1)
ꢃ
where k = charge transfer rate, Vc = electron coupling, and l =
reorganization energy of the system; see the Supporting
Information for more details. According to this model, TCNQ
stacking inside of the COF could result in a ca 32-fold increase in
4
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of our knowledge, that demonstrate redox-active behavior of
TCNQ- and C60-integrated COFs.
performed on an HPC cluster funded by the National Science
Foundation under Grant No. CHE-1048629.
The aforementioned results demonstrate preparation of the
first family members of purely organic, crystalline, porous
scaffolds with covalently bound TCNQ and C60 moieties;
integration of the latter ones not only resulted in increases in
conductivity by eight-orders-of-magnitude, but affect the redox
behavior of the material. Moreover, the interior of a COF was
harnessed for the first time to perform a Sonogashira cross-
coupling and [2+2] cycloaddition reactions, followed by ring
opening of a strained intermediate in the case of the latter. The
former reaction was probed for fluorophore tag integration that
opens an avenue to control reaction progress inside of the COF.
Notably, the reported purely organic covalently-integrated-
acceptor scaffolds have the highest surface area and they are the
first buckyball- and TCNQ-integrated COFs that exhibit redox
behavior to date. Our theoretical analysis probing charge transfer
rates within the Marcus theory as a function of TCNQ stacking
within COFs showed a 32-fold increase in electron transfer rates
compared to the parent COF itself. Despite the theory limitations,
it is the first successful attempt to explain charge transfer in COFs
using the Marcus theory that paves the way for further simulations
of electronic structures and CT studies of hierarchical materials.
Shifting from “static” tuning of the electronic structure, dynamic
control of electronic properties in COFs as a function of external
stimulus was achieved through spiropyran guest infiltration in the
porous scaffold. Overall, this work demonstrates the potential for
donor-acceptor alignment on the example of buckyball- and
TCNQ-electron acceptors and COF-based donors for the
development of porous and crystalline materials with tunable
electronic structures that could open a new avenue for the rational
design of electroactive and conductive multidimensional and
multifunctional crystalline porous materials.
Keywords: covalent-organic framework • donor acceptor
interactions • electronic structure • fulleretic materials • redox-
active
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6
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COMMUNICATION
Sometimes a journey into the deep
confines of the forest can lead to new
Gabrielle A. Leith,§ Allison M. Rice,§
Brandon J. Yarbrough,§ Anna A.
Berseneva, Richard T. Ly, Charles N.
Buck III, Denis Chusov, Amy J. Brandt,
Donna A. Chen, Benjamin W. Lamm,
Morgan Stefik, Kenneth S. Stephenson,
Aaron K. Vannucci, Perry J. Pellechia,
Sophya Garashchuk, and Natalia B.
Shustova* Page No. – Page No.
A Dual Threat: Redox-Activity and
Electronic Structures of Well-
Defined Donor-Acceptor Fulleretic
Covalent-Organic Materials
discoveries.
Electron
acceptors
prepared themselves for travel into
the unknown of the COF electron-
donating “forest”. Instead of getting
lost in the deep, dark forest, the
acceptor-based (purely organic)
covalently bound, crystalline and
porous scaffolds exhibited not only
increases in conductivity, but also
retention of the redox-active nature.
This article is protected by copyright. All rights reserved.
7