Redox tunable viologen-based porous organic polymers

Article abstract of DOI:10.1039/c6tc00132g

The use of an organic donor-acceptor polymer containing a viologen electron acceptor and triarylamine electron donor as a platform in the development of multifunctional materials is presented. The highly robust porous organic polymer (POP) system allows f

Full text of DOI:10.1039/c6tc00132g

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Redox Tunable Viologen-based Porous Organic Polymers
a
a
b
c
c
b
Carol Hua, Bun Chan, Aditya Rawal, Floriana Tuna, David Collison, James M. Hook, and
a
Received 00th January 20xx,
Accepted 00th January 20xx
Deanna M. D’Alessandro
The use of an organic donor-acceptor polymer containing a viologen electron acceptor and triarylamine electron donor as
a platform in the development of multifunctional materials is presented. The highly robust porous organic polymer (POP)
system allows for exploration of the interplay between electronic and host-guest interactions in the synthesized polymers,
POP-V1, which contains a redox-active triarylamine core and POP-V2, which contains a redox-inactive benzene core, where
DOI: 10.1039/x0xx00000x
www.rsc.org/
2 2
each of the redox states present can be reversibly accessed. The degree of charge transfer in addition to the H and CO
gas adsorption properties of the polymer are able to be tuned as a function of the electronic state which has important
implications for the potential applications of these polymers in optical, electrochromic and solar cell devices.
[14-15]
function of these materials;
the ability to modify these
Introduction
properties as a function of the redox state opens potentially
new pathways for the application of these materials.
Multifunctional materials, wherein multiple properties can be
accessed within the same material in response to external
stimuli such as light or a change in redox state, offer significant
potential for the development of electroactive, energy storage
and magnetoelectric materials. These systems exhibit
switchable properties that are able to be turned “on” and “off”
in response to external perturbations. Exploiting the
properties of the different accessible states offers the
potential for lower energy and cost alternatives to existing
technologies.
In this work, the combination of electron donor triarylamine
and electron acceptor viologen moieties within porous organic
polymer networks was explored as a strategy to produce
highly versatile donor-acceptor switchable materials which are
responsive to electrochemical and chemical stimuli. The
incorporation of two highly stable and reversible processes –
[16-
the one electron oxidation to the triarylamine radical cation
1
and the one electron reduction to the viologen radical
7]
[18-19]
cation
– was exploited to yield materials with the capacity
to reversibly switch between multiple redox states. As shown
in Figure 1, the POP-V1 polymer, which contains the redox-
active triarylamine core and the POP-V2 polymer, which
contains the redox-inactive benzene core, were compared to
understand the nature of the charge-transfer properties in this
class of donor-acceptor materials. The electroactive properties
of the polymers were interrogated using in situ solid state
Vis/NIR and EPR spectroelectrochemical experiments which
Polymeric donor-acceptor systems are particularly versatile
platforms for the development of multifunctional materials
and have found applications in numerous areas ranging from
[1-9]
optical and electrochromic devices,
to solar cells and
[10-12]
organic semiconductors.
In these systems, oxidation of
the donor substituent and reduction of the acceptor unit
offers at a minimum, three distinct accessible redox states
within the same material. Despite the potential to access the
novel host-guest and physical properties distinctly offered by
[13, 20-25]
have yielded insights into their function.
The ability to
[13]
tune the host-guest properties of POP-V1 and POP-V2 in their
different redox states provides a basis for regulating the
each state, this aspect has, to date, received very limited
attention. Importantly, many of the interesting chemical and
physical properties of donor-acceptor systems can be
attributed to charge-transfer mechanisms which underpin the
2 2 2
uptake and selectivity of CO , N and H which has important
implications for the use of these materials in applications such
as electrical swing adsorption.
a.
School of Chemistry, The University of Sydney, New South Wales 2006, Australia.
*
deanna.dalessandro@sydney.edu.au
NMR Facility, Mark Wainwright Analytical Centre, The University of New South
b
Wales, New South Wales 2052, Australia
School of Chemistry and Photon Science Institute, The University of Manchester,
Manchester M13 9PL, United Kingdom.
c
Electronic Supplementary Information (ESI) available: Additional synthetic
procedures, solid state NMR data, thermal gravimetric analysis, electrochemistry,
EPR and gas sorption data. See DOI: 10.1039/x0xx00000x
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Scheme 1. a) One electron oxidation of triarylamine to the radical cation, b) reduction processes of viologen and c) synthesis scheme for POP-V1 and POP-V2.
doubly reduced neutral quinoid form. The voltammogram of
POP-V2 was characterized by a reversible cathodic process at
+
Results and Discussion
~
1.25 V vs. Fc/Fc due to reduction of the viologen (Figure
Synthesis and Characterisation
POP-V1 and POP-V2 were synthesized via a Sonogashira- model system, mPOP-V2(Cl)
Hagihara palladium catalyzed cross coupling reaction between The second cathodic process was not clearly defined and may
tris(p-ethynylphenyl)amine or 1,3,5-tris(4-
indicate a comproportionation reaction between the neutral
ethynylphenyl)benzene and 1,1’-bis(4-bromophenyl)-4,4’- viologen and the doubly charged species to yield two
S3b), which is consistent with the computed LUMO of the
, centered around this moiety.
2
[
18]
bipyridinium chloride to yield POP-V1 and POP-V2 as dark monoradical cation species.
yellow/brown and yellow solids, respectively (Scheme 1). The observed for POP-V2, which was expected given the absence
No anodic processes were
1
3
polymers were characterized using solid state NMR C CPMAS of the triarylamine core in this polymer.
13
and C CP non-quaternary suppression (NQS) spectra (Figure
S1) where several resonances in the alkynyl region (~90 ppm) The UV/Vis/NIR spectra of POP-V1 and POP-V2 were
were indicative of successful cross coupling of the monomer characterized by intense absorption bands above 20000 cm
-1
units. The broadness of the peaks in the solid state NMR which were assigned to overlapping π to π* transitions of the
spectra was attributed to the amorphous nature of these aromatic moieties present (Figure 1). Our computed spectra
materials. The thermal gravimetric analysis (TGA) indicates (Figure 1a) for the neutral model systems, as well as those for
that both of the polymers displayed high thermal stability up the corresponding oxidized and reduced states, are
to 300 °C (Figure S2).
qualitatively consistent with the experimental spectra. The
dark brown POP-V1, when compared to the light brown POP-
-1
Redox-state Dependent Spectral Properties
V2, displayed an intense absorption band >16000 cm (Figure
a) which was attributed to the donor-acceptor interactions
2
Solid state cyclic voltammetry experiments in [(n-
within the polymer. This assignment is supported by TDA-DFT
calculations, which associate the corresponding absorption in
4 9 4 6 3
C H ) N]PF /CH CN electrolyte for POP-V1 revealed one broad
+
anodic process at ~1.0 V vs. Fc/Fc which was assigned to
oxidation of the triarylamine core and a quasi-reversible
+
cathodic process at -1.0 V vs. Fc/Fc which was assigned to the
mPOP-V1(Cl)
2
with predominantly the HOMO–LUMO
transition. As mentioned earlier, the HOMO in our DFT model
is mainly associated with the amines and the LUMO with the
viologen, the transition is therefore related to a charge
transfer from the triarylamine to the viologen. This resulted in
a smaller optical band gap for POP-V1 (2.17 eV) than POP-V2
reduction of the viologen moiety to its radical cation (Figure
S3a). These assignments are supported by our analysis of the
2
DFT orbitals of the model system [mPOP-V1(Cl) ], which shows
that the HOMO is largely centered around the amines, while
the LUMO is primarily viologen-based (Figure 2b). We attribute
+
the second cathodic process at -2.0 V vs. Fc/Fc to the
(
2.24 eV). In comparison, the calculated HOMO-LUMO gaps for
our model systems, mPOP-V1(Cl) and mPOP-V2(Cl) , are 3.9
2
2
and 4.5 eV, respectively. The weak EPR signal observed for
irreversible reduction of the viologen radical cation to its
2
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-1
[26-
POP-V1 in the neutral state, which was absent in POP-V2, can the reduced viologen between 22000-25000 cm (Figure 2).
2
8]
be attributed to the donor-acceptor properties of the polymer,
The optical band gap for the reduced species was lower
as the electron was spontaneously transferred from the than that of the neutral polymer system (POP-V1: 2.17 eV, red-
triarylamine electron donor to the viologen acceptor. Indeed, POP-V1: 1.81 eV, POP-V2: 2.24 eV, red-POP-V2: 1.94 eV) which
our DFT calculations show that within POP-V1(Cl)
2
, which is was in accordance with computational results from the
overall neutral, there is a net charge of +0.23 associated with literature where the HOMO-LUMO separation decreases upon
the two terminal triarylamine units.
reduction of the dication to the monocation, then to the
[29]
neutral species, as well as the results of our DFT calculations
for the model systems mPOP-V1(Cl) (HOMO-LUMO gap = 3.9
2
•
eV) and mPOP-V1(Cl) (3.4 eV), and mPOP-V2(Cl)
•
mPOP-V2(Cl) (3.3 eV).
2
(4.5 eV) and
2
Figure 1. a) DFT HOMO and LUMO of mPOP-V1(Cl) and computed [TDA-BMK/6-
3
1G(d)] UV/Vis spectra for the model systems b) mPOP-V1 and c) mPOP-V2.
Figure 2. Solid state UV/Vis/NIR spectra of a) POP-V1, ox-POP-V1 and red-POP-
V1, b) POP-V2 and red-POP-V2 over the range 5000-40000 cm .
-
1
A darkening in color of POP-V1 from dark brown to black was
observed upon chemical oxidation with nitrosonium
tetrafluoroborate and was consistent with the appearance of a
-1
In the EPR spectra of the polymers, ox-POP-V1 exhibited a
smaller g-factor (2.001) than POP-V1 (2.003) (Figure S6a)
whilst red-POP-V1 had a larger g-factor (2.004). The shift in the
band at 12000 cm (Figure 2a). This is assigned to the localized
[
17]
D
0
to D
1
transition of the triarylamine radical cation, with
•
our DFT calculations for mPOP-V1(Cl)
3
(where an additional Cl
value of g from g (= 2.0023) indicates that the SOMO of the
e
has been added for charge balance) suggesting a transition
from a low-lying triarylamine-based occupied orbital to the
oxidized radical undergoes coupling with an empty orbital, and
conversely, that the SOMO of the reduced radical couples with
a filled orbital of the polymer. Red-POP-V2 exhibited a smaller
g-factor (1.999) than POP-V2 (2.002) (Figure S6b).
Interestingly, the trend for POP-V2 and its reduced analogue
was opposite to that observed with POP-V1, indicating that
reduction of POP-V2 results in coupling of the SOMO of the
radical with an empty orbital. The difference in the trends for
-1
SOMO. In addition, a red shift of the band at 20000 cm was
also observed.
The chemical reduction of POP-V1 and POP-V2 with lithium
naphthalenide resulted in a darkening of the solids to dark
brown. The UV/Vis/NIR spectra for the reduced forms of POP-
V1 and POP-V2 were characterized by absorption bands due to
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Figure 3. In situ solid state Vis/NIR spectroelectrochemical experiment in [(n-
the g-factors between POP-V1 and POP-V2 indicates that the C H ) N]PF /CH CN for POP-V1 where a) oxidation (0 to 2.0 V), b) reduction (0 to
4
9
4
6
3
2.0 V) and c) photos of the oxidation/reduction.
-
energies of each of the orbitals differ due to the presence of
either an N-centered (triarylamine) in POP-V1 or phenyl-
centered core in POP-V2.
In the neutral state, a small EPR signal was observed for POP-
V1 which may be a consequence of the donor-acceptor
properties of the polymer, where the electron was
spontaneously transferred from the triarylamine electron
donor to the viologen electron acceptor. Upon application of a
Probing the Interplay between Electronic and Optical Properties
Application of a potential of 1.75 V to POP-V1 during the solid
state Vis/NIR spectroelectrochemical experiment resulted in a
color change from dark brown to black and the formation of
two bands in the NIR/Vis spectrum (Figure 3b). As with our
assignment for the chemically oxidized species, the band at
potential of -0.9
V
in the solid state EPR
spectroelectrochemical experiment for POP-V1, the EPR signal
was significantly enhanced from the neutral state indicating
formation of the viologen radical cation (Figure 4). The
intensity of this signal decreased upon application of a
potential of 0 V to reform the neutral state; a potential of 1.7 V
then resulted in an enhancement of the signal intensity due to
formation of the triarylamine radical cation, which was able to
be reduced upon application of a potential of 0 V.
-
1
~
12000 cm was assigned to the formation of the triarylamine
radical cation and corresponds to the localized π to π*
-1
transition while the band at ~14000 cm corresponded to a
charge transfer process from the triarylamine to the viologen
acceptor. As the bands were spectrally reversible upon
application of
a negative potential, the charge transfer
interactions present in the polymer could be tuned in response
to an electrical stimulus. The reduced state of the polymer was
obtained upon application of a potential of -2.0 V (Figure 3a),
The viologen radical cation exhibited a different g-factor
(
(
2.004) when compared to the triarylamine radical cation
2.002), which corresponds to the trend observed for the
-1
where the band at ~21000 cm increased in intensity whilst
chemically oxidized and reduced species for POP-V1 (Figure
S11). The EPR signal due to the viologen radical cation
displayed more anisotropy than that for the triarylamine
radical cation, presumably due to the reduced symmetry of the
viologen moiety. Upon application of a potential of -1.8 V
-1
the band at ~23000 cm decreased in intensity due to
reduction of the viologen linker in the POP-V1 polymer.
(Figure 4), the signal intensity was significantly increased when
compared to the first reduction (Figure S9). The first reduction
and oxidation processes within the framework likely resulted
in structural changes within the polymeric system, which
allowed the counter-ions to diffuse with greater ease into the
pores of the network. Consistent with this argument, our DFT-
•
•
optimized structures for mPOP-V1(Cl) and mPOP-V1(Cl)
3
–
have Cl ions that are more loosely bound than those in mPOP-
–
V1(Cl)
.26 Å to the closest carbon atom, whereas in the oxidized and
reduced analogs the corresponding distances are 2.50 Å and
.84 Å, respectively. This increased the degree of reduction
2 2
. Thus, the Cl ions in mPOP-V1(Cl) have a distance of
2
2
obtainable, which made the system more amenable to
changes in redox state.
For POP-V2, application of a negative potential of -2.0 V
resulted in a color change from yellow to brown with an
-1
increase in the intensity of the band at ~18000 cm and
-1
decrease in the band at 22500 cm (Figure S7). These
processes can be attributed to formation of the viologen
[30-32]
radical cation
and were spectrally reversible upon
application of a positive potential.
The signal intensity increase in the solid state EPR
spectroelectrochemical experiment for POP-V2 upon
application of a potential of -1.7 V was due to formation of the
viologen radical cation (Figure S10). As the potential was
increased to 1.5 V, the signal intensity decreased to a point
where it remained constant, as expected given the absence of
the redox-active triarylamine core. During the second
reduction process, the signal shape changed from one with
4
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axial anisotropy to one higher in symmetry, suggesting that the reduction process, presumably due to structural change within
radical environment had been altered during the first the polymer system in an analogous fashion to POP-V1.
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Figure 4. Solid state EPR spectroelectrochemical data for POP-V1 in [(n-C
4 9 4 6 3
H ) N]PF /CH CN electrolyte. The arrows indicate the direction of spectral
progression throughout the course of the experiment.
Exploiting Tunable Redox States to Modulate H
The nitrogen sorption isotherms at 77 K for POP-V1, ox-POP-
V1, red-POP-V1, POP-V2 and red-POP-V2 displayed
2 2
and CO Uptake
Hydrogen isotherms at 77 K for the neutral polymers (POP-V1:
.04 mmol/g, POP-V2: 5.59 mmol/g at 1150 mbar) displayed a
5
a
significantly higher uptake than the reduced polymers (red-
POP-V1: 4.39 mmol/g, red-POP-V2: 3.51 mmol/g at 1150
significant hysteresis upon desorption which points towards
flexibility within the different interlinked polymer networks
mbar) (Figure 5b). Likewise, POP-V1 also displayed
a
2
(Figure 5a). The step in the N desorption isotherm at ca. 542
significantly higher uptake of hydrogen (5.04 mmol/g at 1150
mbar) when compared to ox-POP-V1 (3.38 mmol/g at 1150
mbar) (Figure 5b). These observations are consistent with the
lower surface area of the polymer upon oxidation or
reduction.
mbar for POP-V1, red-POP-V1, POP-V2 and red-POP-V2
indicated the presence of mesopores within the polymer
structures, whilst the absence of this step for ox-POP-V1 was
likely to arise from the tetrafluoroborate counterion from the
4
chemical oxidant (NOBF ). The pore size analysis indicated
several different sized pores at 6, 10, 12.5 and 22 Å for the
polymer systems (Figure S13). Ox-POP-V1 lacked the 22 Å size
Table 1: BET surface areas of the POP-V materials.
pore which correlated well with the absence of a step in the N
desorption isotherm.
2
2
BET Surface Area [m /g]
POP-V1
Ox-POP-V1
Red-POP-V1
POP-V2
812
390
606
960
591
All polymer systems displayed moderate BET surface areas of
2
90-960 m /g (Table 1). The significant drop in surface area for
3
ox-POP-V1 was presumably due to additional occupancy of the
-
pores by the BF counter-ions from the chemical oxidant. A
4
Red-POP-V2
decrease in the BET surface area upon reduction was contrary
to the expectation that the reduced polymer systems should
exhibit higher surface areas as the doubly reduced quinone
state of the viologen moiety is neutral, negating the need for
counterions in the pores for charge balance. A plausible
rationale for this observation is that removal of the chloride
counterions results in a partial collapse of the networks due to
the loss of stability which is provided by the counterion in the
neutral polymers. In the latter case, electrostatic repulsion
within the positively charged networks would be expected to
support increased network porosity.
The CO
2
uptake was also dependent on the the redox state. A
at 298 K was observed for the reduced
lower uptake of CO
2
networks (red-POP-V1: 0.92 mmol/g, red-POP-V2: 1.09
mmol/g at 1100 mbar) when compared to their neutral
analogues (POP-V1: 0.67 mmol/g , POP-V2: 1.27 mmol/g at
1
100 mbar). However, ox-POP-V1 displayed a higher CO
2
uptake (1.18 mmol/g at 1100 mbar) when compared to POP-
V1, despite having a significantly lower surface area (Figure
5c). To understand the origins of these observations, we have
6
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Figure 5. Gas sorption isotherms of POP-V1 (
), ox-POP-V1(
), red-POP-V1 (
where adsorption = filled symbols and desorption = open symbols. The lines
joining each point are provided as a guide to the reader.
) and POP-V2 (
), red-POP-V2 (
2
) for a) N at 77
2 2 2
K, b) H at 77 K, c) CO at 298 K and d) the isosteric heat of adsorption for CO
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Figure 6. DFT HOMO (-0.20977) and LUMO ( -0.06623) of mPOP-V1(Cl) with BMK/6-31G(d) orbital energies (hartree) in parentheses of the monomer POP-
-
V1 polymer fragments, mPOP-V1 and mPOP-V1(Cl)3 with CO
2
where C = grey, N = blue, H = white, O = red, Cl = green. The interaction of CO
mPOP-V1(Cl) is observed.
2
with Cl in
3
computed the mPOP-V1(Cl)
n 2
–CO complexes (n = 0–3) and the The combination of the electron acceptor viologen with the
corresponding binding energies. The calculated structures electron donor triarylamine has yielded donor-acceptor
–
suggest that the interaction of CO
2
with a Cl ion is important polymers, POP-V1 and POP-V2, with differential electronic and
for its binding within the materials (Figure 6). It is presumed physical properties in their distinct redox states. The reversible
[33]
nature of the redox switching provides access to different
+
that reduction of the polymers with LiNp (-3.10 vs. Fc/Fc )
resulted in the formation of the neutral viologen quinone state charge transfer processes, optical and host-guest properties
in red-POP-V1 and red-POP-V2. Indeed, for mPOP-V1 that which can be exploited to control the adsorption of gases such
–
represents a model of the fully-reduced polymer with no Cl , as H
2
and CO
2
. The ability to access multiple switchable
–
1
the binding energy is significantly smaller (3.1 kJ mol vs 11.4 properties within the same material demonstrates significant
–
kJ mol for mPOP-V1(Cl) ). Upon one-electron oxidation to potential for the use of these systems in applications ranging
2
1
–
1
mPOP-V1(Cl)
3
(16.6 kJ mol ) or one-electron reduction to from molecular electronics to electrical swing adsorption
–1
[34-36]
mPOP-V1(Cl) (16.8 kJ mol ), the binding becomes somewhat processes.
-
stronger. The presence of the BF
4
counterion is presumed to
have a similar affect towards the binding energy of the ox-
-
POP-V1 polymer with CO
of the negatively charged ion with the partially positive carbon
of CO . It thus appears that the increased CO adsorption upon
2
as that of Cl due to the interaction
Experimental Section
2
2
All chemicals and solvents were used as obtained and used
[
without further purification. Tris(p-iodophenyl)amine, 1,3,5-
37]
polymer oxidation can be attributed to both an increase in the
binding, as well as an increase in the number of anionic
[
tris(p-ethynylphenyl)benzene, 1,1’-bis(4-bromophenyl)-4,4’-
38]
[
39]
[40]
2
binding sites. On the other hand, the decreased CO binding in
bipyridinium
chloride,
Pd(PPh
3
)
4
and
lithium
[33]
the reduced form may be a consequence of the removal of the
counter-ions from the polymer.
naphthalenide
were synthesized according to literature
[41]
procedures. Tris(p-ethynylphenyl)amine
dinitrophenyl)-4,4’-bipyridine-1,1’-diium chloride
synthesized according to modified literature procedures.
Acetonitrile and triethylamine were dried over CaH and
dimethylformamide dried over activated CaSO . Solution state
and C{ H} NMR spectra were recorded on Bruker
and 1,1’-bis(2,4-
[
42]
were
2
The CO isosteric heats of adsorption for all polymers are
indicative of a physisorption process (Figure 5d), which is in
agreement with our calculated complex structures. The
reduced polymers exhibited lower or comparable heats of
adsorption relative to the neutral systems whilst the enhanced
2
4
1
13
1
H
AVANCE200, AVANCE300 or AVANCE500 spectrometers
1
operating at 200, 300, 500 MHz for H and 50, 75, 125 MHz for
affinity for CO
was evident from the significantly higher heat of adsorption
for CO when compared to POP-V1 (-25 kJ/mol at low gas
2
by ox-POP-V1 (-32 kJ/mol at low gas loadings)
1
3
1
13
1
C respectively. H and C{ H} NMR chemical shifts were
referenced internally to residual solvent resonances. Spectra
were recorded at 298 and chemical shifts (δ), with
2
loadings).
K
1
13
uncertainties of ± 0.01 Hz for H and ± 0.05 Hz for C are
The single point selectivity improved as the temperature was
quoted in ppm. Coupling constants (J) are quoted in Hz and
1
1
increased due to the higher interaction of the quadrupolar CO
molecules with the surface of the polymer (Figure S14). In all
cases, the selectivity for CO over N increased upon reduction
of the framework at 295 whilst the selectivity was
significantly decreased for ox-POP-V1. For the neutral and
oxidized species, the selectivity for CO vs. N was enhanced
2
have uncertainties of ± 0.05 Hz for H- H. Deuterated solvents
were obtained from Cambridge Stable Isotopes and used as
received.
2
2
K
Synthesis of POP-V1: Tris(p-ethynylphenyl)amine (0.500 g, 1.57
2
2
mmol), 1,1’-bis(4-bromophenyl)-4,4’-bipyridinium chloride
upon increasing the temperature to 308 and 318 K. The
increased kinetic energy allowed the stronger electrostatic
(
0.679 g, 1.26 mmol) and Pd(PPh
were suspended in a mixture of DMF (5 mL) and triethylamine
5 mL) and stirred under nitrogen for 5 minutes prior to the
3 4
) (14.6 mg, 0.0126 mmol)
interaction of the polar CO
weaker interaction of the non-polar N
material. These results demonstrate the ability to tune the
selectivity parameters for CO /N as both a function of the
2
molecule to dominate over the
(
2
molecule with the
addition of CuI (4.80 mg, 0.0252 mmol). The reaction mixture
was observed to darken from a bright yellow suspension to a
dark yellow suspension. The reaction was then heated at 90 °C
for 40 hours to yield a dark red/brown solid. The reaction
mixture was cooled and filtered. The solid was washed
consecutively with DMF, chloroform, methanol, water,
methanol and acetone before being air dried. The solid was
then washed by soxhlet extraction with methanol until the
washings turned colorless before being dried under vacuum to
2
2
redox state and temperature in these materials.
Conclusions
8
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1
yield the product as a dark yellow-red solid (820 mg). Solid State NMR: The C CPMAS solid state NMR experiments
3
Elemental Analysis, found: C, 77.33; H, 4.33 and N, 5.35%.
were carried out on a wide-bore Bruker Biospin Avance III
solids-300 MHz spectrometer operating at a frequency of 75
1
3
Synthesis of POP-V2: 1,3,5-Tris(p-ethynylphenyl)benzene MHz for the C nucleus. Approximately 80 mg of samples
0.176 g, 0.464 mmol), 1,1’-bis(4-bromophenyl)-4,4’- were packed into 4 mm zirconia rotors fitted with Kel-f® caps
bipyridinium chloride (0.200 g, 0.371 mmol) and Pd(PPh (4.2 and spun in a double resonance H-X probehead at 8 kHz MAS
(
3 4
)
-6
13
1
mg, 3.71 × 10 mol) were suspended in a mixture of DMF (5 (Magic Angle Spinning). The C and H 90° radio frequency
1
mL) and triethylamine (5 mL) and stirred under nitrogen for 5 pulse lengths were optimized to 3.5 μs each. The C CPMAS
3
-6
minutes. CuI (1.4 mg, 7.42 × 10 mol) was then added and the spectra were acquired with 1 ms cross polarization contact
reaction mixture heated at 90 °C for 40 hours resulting in a time with a total suppression of spinning sidebands (TOSS)
1
color change from pale green to dark brown/green. The scheme, followed by H decoupling at 75 kHz field strength
1
3
reaction mixture was cooled and filtered and worked up as for using Spinal-64 decoupling. The C NQS (non-quaternary
POP-V1 to yield the product as a yellow solid (250 mg). carbon suppression) spectra were recorded by turning off the
1
Elemental Analysis, found: C, 83.45; H, 4.48 and N, 1.18%.
H decoupling for 40 μs during the TOSS period. For sufficient
signal to noise, ca. 10 k transient were acquired for each
Chemical Oxidation: The polymer (100 mg) was suspended in sample with recycle delays of 3.0 s in between to ensure
1
dry and distilled acetonitrile (2 mL) and the mixture thoroughly sufficient relaxation of the H magnetization. The spectra were
1
nitrosonium obtained at room temperature. The C chemical shifts were
3
degassed.
A
saturated
solution
of
tetrafluoroborate (10 mL) in acetonitrile (10 mL) was added referenced to the glycine CO peak at 176 ppm.
slowly dropwise with vigorous bubbling of nitrogen to yield an
immediate color change to dark brown. The mixture was left Electrochemistry: Solid state electrochemical measurements
to stir at room temperature with vigorous bubbling of nitrogen were performed using
for 30 minutes, the solid filtered off and dried. Electrochemical Analyser. Argon was bubbled through
solutions of 0.1 M [(n-C N]PF dissolved in distilled CH CN.
a Bioanalytical Systems Epsilon
4
H
9
)
4
6
3
Chemical Reduction: The polymer (20 mg) was suspended in The cyclic voltammograms (CVs) for POP-V1 were recorded
dry THF (2 mL) and a solution of lithium naphthalenide (0.1 M using a glassy carbon working electrode (1.5 mm diameter), a
+
in THF) added slowly dropwise under argon with stirring. The platinum wire auxiliary electrode and an Ag/Ag wire quasi
solid was observed to change color from yellow to brown. The reference electrode. Solid state samples were mounted on the
reaction mixture was stirred for 2 hours under argon before glassy carbon working electrode by dipping the electrode into
the solid was filtered, washed with THF and dried.
a paste made of the powder sample in acetonitrile. The CVs for
POP-V2 were recorded using a 1 mm wide strip of graphite as
Computational Calculations: Standard DFT calculations were the working electrode. A piece of platinum wire was adhered
[
43]
carried out with Gaussian 09. The molecules containing the to the top of the graphite strip with conductive silver paint.
viologen moiety capped with two triarylamines [mPOP-V1(Cl) The sample was immobilized on the working electrode by
or triarylbenzene [mPOP-V2(Cl) ] (Figure 3a), and with an smearing a paste of the sample suspended in acetonitrile to
n
]
n
appropriate number of chloride ions for charge balance, were the surface of the graphite strip and allowing the acetonitrile
–
employed to model the polymers in their neutral (2 Cl ), to evaporate. Ferrocene was added as an internal standard
–
oxidized (3 Cl ), singly-reduced (1 Cl ) and doubly-reduced (0 upon completion of each experiment. All potentials are quoted
–
–
Cl ) states. Geometries were obtained at the B-PW91/6-31G(d) in mV versus Fc/Fc .
+
[44-45]
level.
The vibrational frequencies of stationary points
were inspected to ensure that they corresponded to minima UV/Vis/NIR: UV/Vis/NIR spectroscopy was performed on a
on the potential energy surface. The UV/Vis spectra of the CARY5000 Spectrophotometer equipped with a Harrick Praying
model systems were computed with the TDA-DFT procedure, Mantis accessory, where dried BaSO
4
was used for the
i.e., time-dependent DFT with the Tamm–Dancoff baseline. The spectra of the chemically reduced species were
[46]
[47]
approximation
using the BMK functional
in conjunction recorded under an atmosphere of argon. Spectra are reported
function of
text correspond to those obtained with these TDA-BMK wavenumber (cm ). The optical band gap was estimated by
calculations. To examine the binding of CO with mPOP- drawing a tangential line along the slope to the curve and
V1(Cl) , we carried out constrained optimization (at the B- determining the wavelength (in nm) where this line
PW91/6-31G(d) level) of mPOP-V1(Cl) –CO complexes, in intersected the x-axis. The wavelength was then converted to
which the fully-optimized mPOP-V1(Cl) geometry is employed eV.
but frozen in the calculation. Refined single-point energies
with the 6-31G(d) basis set. Molecular orbitals reported in the as the Kubelka-Munk transform F(R) as
a
-1
2
n
n
2
n
[48]
were obtained with the MN12L procedure together with the Solid State Spectroelectrochemistry (Vis/NIR): In the solid state,
-31+G(d) basis set. Complexation energies were reported as the diffuse reflectance spectra of the electrogenerated species
6
–1
vibrationless values in kJ mol .
4 9 4 6 3
were collected in situ in a 0.1 M [(n-C H ) N]PF /CH CN
-1
electrolyte over the range 5000-25000 cm using a Harrick
Omni Diff Probe attachment and a custom built solid state
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DOI: 10.1039/C6TC00132G
ARTICLE
Journal Name
[
21]
spectroelectrochemical cell described previously.
The cell Thermal gravimetric analysis (TGA): TGA was performed under
+
consisted of a Pt wire counter electrode and an Ag/Ag quasi- a flow of nitrogen (0.1 L/min) on a TA Instruments Hi-Res
reference electrode. The solid sample was immobilized onto a Thermogravimetric Analyser from 25-700 °C at 1 °C/min.
0.1 mm thick Indium-Tin-Oxide (ITO) coated quartz slide
(which acted as the working electrode) using a thin strip of Gas adsorption: Gas adsorption measurements over the 0-1
Teflon tape. The applied potential (from –2.0 to 2.0 V) was bar range were conducted using a Micromeritics Accelerated
controlled using an eDAQ potentiostat. Continuous scans of Surface Area and Porosity (ASAP) 2020 instrument.
the sample were taken and the potential increased gradually Approximately 50-100 mg of the polymer sample was
until a change in the spectrum was observed.
degassed under vacuum at 70 °C for approximately 14 hours.
Nitrogen adsorption isotherms were measured at 77 K via the
Electron Paramagnetic Resonance (EPR) spectroscopy: EPR incremental dosing of nitrogen from 0-1 bar, and surface areas
spectroscopy (Bruker EMX-X band) was performed at room determined via the BET method using the ASAP2020 V4.01
2
temperature on freshly synthesized materials (20 mg). The software. CO isotherms at 298, 308 and 318 K were measured
microwave frequency was between 9.75 and 9.85 GHz for all on the same instrument, with temperature controlled by a
measurements and the microwave power well below that Julabo F25 Circulating Heating and Cooling bath. The enthalpy
required for saturation. All other parameters remained of adsorption for CO
2
was calculated using data from the three
constant: modulation frequency, 100 kHz; modulation CO isotherms and the Clausius-Clapeyron equation. An
2
amplitude, 1.00 G; sweep width, 100 G; and each spectrum estimation of the porosity distribution in the POPs was
was averaged over eight scans. The EPR signals obtained were obtained using the Tarazona Non-Local Density Functional
referenced against Strong Pitch to obtain the g-factor value.
Theory (with cylindrical pores) implemented within the
Micromeritics ASAP 2020 software (Version 4.0). The single
Solid State Spectroelectrochemistry (EPR): The procedure and component selectivity factors for CO
2
vs. N
2
for the POP
vs. 0.15 bar
2
A three- CO (0.1 bar other gases), which are industrially relevant
cell set up used were as previously reported for a solution systems were calculated using Eq. 1 at 0.75 bar N
2
[49]
state EPR spectroelectrochemical experiment.
electrode assembly consisting of simple narrow wires (A–M pressures for the capture of CO
Systems) as electrodes was used. Teflon coated platinum (0.20 gas streams.
and 0.13 mm coated and uncoated diameters, respectively)
2
from post combustion flue
and silver wires (0.18 and 0.13 mm coated and uncoated
(1)
diameters, respectively) were used for the working and quasi-
reference electrodes respectively, while a naked platinum wire
(
0.125 mm) attached to a small piece of rolled platinum mesh
~ 5 mm × 4 mm) played the role of the counter electrode. The
nCO = quantity of CO adsorbed (mmol/g)
2
2
(
nN = quantity of N adsorbed (mmol/g)
2
2
bottom 1 cm of the Teflon coated wires were stripped (using
an Eraser International Ltd., RT2S fine wire stripper). The
sample of interest was wrapped in a small piece of platinum
mesh (~ 5 mm × 3 mm) lengthwise and twisted to ensure the
sample remained immobilized. The exposed end of the
xCO = equilibrium partial pressure of CO (bar)
2
2
xN = equilibrium partial pressure of N (bar)
2
2
working electrode was carefully wrapped in a spiral manner Acknowledgements
around the platinum mesh where the working electrode was
We gratefully acknowledge support from the Australian
Research Council and the Science and Industry Endowment
Fund. We thank the EPSRC UK National Electron Paramagnetic
Resonance Service at the University of Manchester.
positioned lowest such that the redox product of interest was
generated at the bottom of the tube and was well separated
from the counter electrode. The electrodes were soldered to a
narrow three-core flexible wire/cable. The cell used was made
by flame sealing the tip of a glass pipette. The potential was
controlled with a portable μAutolab II potentiostat and the
EPR spectra obtained using an EMX Micro X-band EPR
spectrometer equipped with a 1.0 T electromagnet. The EPR
signals obtained were referenced against Strong Pitch to
obtain the g-factor value, and the response of the EPR sample
cavity and that of the sample holder and wire–setup was
checked prior each experiment. In all cases the samples were
EPR silent prior electrochemistry unless otherwise stated. A
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Table of Contents
The interplay between the electronic, spectral and host-guest properties of a donor-acceptor
polymer in its different redox states is demonstrated.