Synthesis and tunability of highly electron-accepting, N-benzylated "phosphaviologens"

Article abstract of DOI:10.1021/ja513258j

We report a structure-property study on phosphoryl-bridged viologen analogues with a remarkably low reduction threshold. Utilizing different benzyl groups for N-quaternization, we were able to confirm the p-benzyl substituent effect on the electronic tuna

Full text of DOI:10.1021/ja513258j

Article
pubs.acs.org/JACS
Synthesis and Tunability of Highly Electron-Accepting, N‑Benzylated
“Phosphaviologens”
Monika Stolar, Javier Borau-Garcia, Mark Toonen, and Thomas Baumgartner*
Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive Northwest, Calgary,
Alberta T2N 1N4, Canada
S
* Supporting Information
ABSTRACT: We report a structure−property study on phosphoryl-bridged viologen
analogues with a remarkably low reduction threshold. Utilizing different benzyl groups
for N-quaternization, we were able to confirm the p-benzyl substituent effect on the
electronic tunability of the system while maintaining the characteristic chromic
response of viologens with two fully reversible one-electron reductions. Due to the
considerably increased electron-acceptor properties of the phosphoryl-bridged
bipyridine precursor, N-benzylation was found to be very challenging and required
the development of new synthetic strategies toward the target viologen species. This
study also introduces a new and convenient way for the anion exchange of viologen
systems by utilizing methyl triflate. Finally, the practical utility of the new species was
verified in simplified proof-of-concept electrochromic devices.
chromic applications such as self-dimming mirrors,14 displays,15
INTRODUCTION
■
and, more recently, smart windows.16
The development of strongly electron-accepting π-conjugated
materials has become a rapidly evolving state-of-the-art research
While viologens themselves offer interesting electronic
properties, the incorporation of additional heteroatoms can
area in recent years, due to their considerable potential for
considerably enhance the electronic and optical properties of
application in organic electronics and the fact that suitable
the materials. Phosphorus is of particular interest in this context
acceptor materials are still quite rare.1 One particularly
due to its distinct pyramidal geometry, which leads to an
intriguing class of materials in this context is bipyridinium
energy-reduced lowest unoccupied molecular orbital (LUMO),
especially when incorporated into cyclic, ring-fused organo-
phosphorus structures, through negative hyperconjugation
species, which are formed upon N,N-diquaternization of 4,4′-
bipyridine, also known as viologens (Chart 1).2
between the σ* orbital of exocyclic substituents and the π-
Chart 1
conjugated main scaffold.17 We and others have been able to
confirm enhanced electron-accepting capabilities for the
resulting π systems, which make such species highly desirable
for electronic applications.18
In 2011, we first reported the phosphorus-bridged viologen,
N,N′-dimethyl-2,7-diazadibenzophosphole oxide P-MV (see
Supporting Information for nomenclature). While we observed
very similar redox behavior and electrochromic properties to
those of methyl viologen, we were able to significantly lower
The extraordinary properties of the viologens have found
widespread application in a host of most different fields that
span from photochemistry,3 electrochemistry,4 and solar energy
the reduction threshold for both redox steps by 500 mV
conversion5 to molecular wires.6 Viologens are frequently used
as electrochemical label7 or photochemical probe8 or in studies
through the addition of the central five-membered phosphole
oxide ring. The increased tendency for chemical reduction of
on the electron-transport processes or oxidative damage in
DNA.9 Many of these applications are largely based on the
the scaffold was confirmed via the significantly lowered LUMO
level as a result from effective σ*−π* hyperconjugation.19
viologens’ ability to undergo reversible redox chemistry giving
We have now focused our efforts toward further expanding
rise to three differently colored oxidation states (colorless,
purple, and red/orange)10,11 and the fact that the bipyridinium
the tunability of the reduction potentials of 2,7-diazadibenzo-
phosphole oxide 1 (Scheme 1), through functionalization at the
radical cations are among the most stable known organic
radicals.12,13 Moreover, the redox and electrochromic proper-
nitrogen atoms and modification of the counteranion. Herein
ties of viologens can usually be tuned via modification on the
N-alkyl group (R), as well as the counteranion (X−, Chart 1).2
Received: December 30, 2014
These features have also found practical utility in electro-
© XXXX American Chemical Society
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unable to detect the fully N-protonated scaffold. The only
detectable change was a gradual low-field shift of the pyridine
1H resonances as well as a small high-field shifted 31P NMR
signal upon addition of TFAH that at least confirmed a weak
association of the nitrogen centers with the acid (Supporting
Information). To overcome this considerable synthetic
challenge, neat reactions of 1 and the corresponding benzyl
bromides were carried out at 60 °C over 72 h that resulted in
the near quantitative formation of dicationic salts 2a−5 that
could be isolated in moderate yields (55−65%) after simple
filtration and precipitation from methanol into a diethyl ether/
methanol/chloroform mixture (Scheme 1). The formation of
the disubstituted symmetric products was confirmed by
multinuclear NMR spectroscopy. The most revealing indication
for the successful N-quaternization was the observed low-field
shift of the α-pyridine protons from 9.0/8.9 to 9.9/9.5 ppm,
respectively. In addition, the 31P NMR spectra of 2a−5 showed
a resonance at δ31P = 30.3 ppm (irrespective of the 4-benzyl
substituent) that is high-field shifted from that of 1 (cf.: 35.8
ppm).
Scheme 1. Synthesis of Benzylated 2,7-
Diazadibenzophosphole Oxides
a
a
Conditions: [a] MeOTf, 0 °C to RT, CH2Cl2;19 [b] benzyl bromide,
60 °C, 72 h; [c] 2,3,4,5,6-pentafluorobenzyl bromide, 60 °C, 72 h; [d]
MeOTf, 0 °C to RT, CH2Cl2; [e] MeOTf, 0 °C to RT, CH2Cl2
(carried out representatively with 2a).
To further accelerate the process, a neat reaction mixture was
exposed to microwave conditions at 60 °C, which successfully
resulted in the formation of the dicationic salt 2a within 15 min,
as opposed to the 72 h benchtop reaction.
we report the results of using a series of substituted benzyl
bromides for functionalization and electronic tuning. We
provide alternative synthetic methods for efficient modification
of the nitrogen centers via microwave conditions, along with
the tunability of the electronic and optical properties of the new
“phosphaviologens”. Finally, we report our proof-of-concept
efforts toward electrochromic devices.
While our new method proved feasible for several derivatives
reported here, 1 could not be N,N-dialkylated with 2,3,4,5,6-
pentafluorobenzyl bromide; the monobenzylated product 6a
was isolated instead. Even prolonged microwave conditions
with 2,3,4,5,6-pentafluorobenzyl bromide only led to the
formation of trace amounts of dibenzylated product, but we
were never able to complete the reaction to a quantity that
could be isolated and characterized. Monobenzylation of the
isolated product 6a was confirmed by the additional resonances
RESULTS AND DISCUSSION
■
Development of a Suitable Synthetic Strategy. While
solution-based N-benzylation of 4,4′-bipyridine has extensively
been reported to be the method of choice,20 it did not prove
feasible for obtaining the targeted phosphorus-bridged
dicationic salts at all. A series of solution-based synthetic
attempts only resulted in mixtures of the mono- and dicationic
salts, along with significant amounts of unreacted 2,7-
diazadibenzophosphole oxide 1, even after prolonged reaction
times (days) and an excess of the respective benzyl bromide.
This observation led us to determine the basicity of the
nitrogen centers. Titration of 2,7-diazadibenzophosphole oxide
with trifluoroacetic acid (TFAH) in CD3CN (Supporting
Information) revealed that the nitrogen centers in 1 have a
significantly decreased basicity due to the electron-withdrawing
effects of the phosphole oxide unit.17,19 Consequently, we were
1
in both the H and the 13C NMR spectra that supported an
unsymmetrically substituted scaffold as well as the 31P NMR
resonance at δ31P = 32.7 ppm. In order to obtain a dicationic
salt with 2,3,4,5,6-pentafluorobenzyl bromide (which is
evidently required for reversibly stable redox properties, see
Supporting Information), we quaternized the second nitrogen
center with excess methyl triflate in dichloromethane at room
temperature over 2 h to give the asymmetric viologen analogue
6b after a sequential anion exchange in situ (vide infra).
Successful methylation of the remaining pyridine nitrogen
center was clearly confirmed by additional resonances at δ =
4.52 and 48.6 ppm in the 1H and 13C NMR spectra,
respectively, as well as a high-field-shifted 31P resonance at
Table 1. Optical and Electronic Properties of Benzylated 2,7-Diazadibenzophosphole Oxides
a
a
b
c
d
e
compound
Ered1 [mV]
Ered2 [mV]
δEred [mV]
λmax [nm] (ε [L mol−1cm−1])
λmax [nm]
λmax [nm]
P-MV
2a
−602
−536
−533
−554
−537
−529
−538
−839
−1032
−966
−967
−992
−958
−940
−957
−1226
430
430
434
438
421
411
419
387
406, 581, 634
408
403
257 (6506), 294 (4936)
257 (10 700), 298 (8270)
268 (11 906)
415, 584, 636
f
f
f
f
2b
3
415 (5100) , 584 (3200) , 636 (3000)
414, 583, 635
402 (7350)
402
4
259 (10 978), 292 (8099)
259 (14 404), 293 (10 527)
286 (16 369)
415, 584, 636
402
5
415, 583, 636
398
6b
BnV
413, 581, 632
400
262 (23 852)
a
b
CV in DMF solution with tetrabutylammonium hexafluorophosphate (0.05 M) as supporting electrolyte, referenced to Fc/Fc+. Difference
c
d
e
between first and second reduction potentials. UV−vis absorption in methanol. Absorption maxima of radical species in CV solution. Absorption
maxima of neutral species in CV solution. Determined in a spectroelectrochemical cell.
f
B
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δ31P = 30.3 ppm, in line with the other diquaternized species
2a−5.
characterized azadibenzophospholes,17b as well as P-MV,19
and do not warrant any detailed discussion here.
Electrochemistry. The electrochemical properties of the
new phosphaviologen species were investigated using cyclic
voltammetry (CV), performed in DMF solutions; the results
are summarized in Table 1. All dicationic species showed two
reversible one-electron reductions (Figure 2A and Supporting
To investigate the potential impact of the counteranion on
the electrochemical behavior of the benzylated species, we also
developed an anion exchange on 2a using excess methyl triflate
to efficiently obtain 2b in dichloromethane at room temper-
ature overnight. Notably, this new method is superior to the
commonly applied anion exchange with silver triflate, as very
little purification (i.e., simple filtration) is needed due to the
volatility of the generated methyl bromide. Interestingly, the
anion seems to impact the solid-state properties, evident in a
charge transfer between anion and cation,2 with 2a being an
orange and 2b a white powder, respectively, but also the nature
of the existing ion pairs in solution; the 31P NMR spectrum of
2b in CD3OD showed a distinct high-field shift at δ31P = 27.7
ppm. Further indication with regard to distinct cation−anion
interactions in solution can be found in the absorption spectra
of 2a and 2b, showing considerable differences in their
extinction coefficients for the three main absorption peaks
that appear at the same wavelengths otherwise (Table 1). Likely
due to the considerable absorption properties of the bromide
anion, compared to those of triflate, the extinction coefficients
for 2a have values that are roughly 60% of those observed for
2b.
We were able to confirm the presence of several solid-state
cation−anion interactions via 4, for which we were able to
obtain single crystals suitable for X-ray crystallography from a
concentrated methanol solution. The anion−π interaction can
clearly be seen in the space-filling diagram of the molecular
structure of 4, where one bromide is interacting closely with the
phosphole ring π system at a distance of 3.2 Å while the other is
participating in H−Br interactions with three neighboring
hydrogen atoms at distances of 2.9−3.3 Å (Figure 1). In
Figure 2. Cyclic voltammogram of 2a (A) and Hammett plot vs δEred
(B).
Information) as previously observed with the dimethyl relative
P-MV.19 However, compared to the reduction features of P-
MV under the same electrochemical conditions,21 we were able
to obtain a 66 mV lower reduction threshold for both
reductions of 2a and a record 73 and 92 mV lower reduction
thresholds for the first and second reduction of 5, respectively.
In addition, the first reduction event of 2a was 303 mV lower
than the corresponding reduction of the parent analogue, 1,1′-
bis(phenylmethyl)-4,4′-bipyridinium bromide (BnV), while the
second reduction event occurred 260 mV lower under the same
conditions. The latter clearly supports the strong electronic
effect of the phosphoryl bridge on the viologen scaffold and the
resulting lowered LUMO energy level. As also observed in
related systems,17b,19 the DFT-derived LUMO for 2a,b largely
comprises the delocalized π* system of the conjugated
backbone with contribution of the exocyclic P substituents.
The LUMO clearly shows electron density contribution from
both the exocyclic σ* orbitals of the phenyl and the oxygen
substituents, confirming the existence of σ*−π* hyper-
conjugation. In comparison, the LUMO of the twisted BnV
only corresponds to its delocalized π* system and is as such
directly related to the LUMO of 2a, pending the phosphoryl
bridge (see Supporting Information for orbital visualizations).
Notably, the LUMO level for 2a (−4.05 eV) is lower than that
Figure 1. Solid-state structure of 4, as obtained via single-crystal X-ray
crystallography: (A) ellipsoids at 50% probability (H atoms omitted
for clarity); (B) space-filling view; (C) molecular packing along the b
axis of the unit cell.
addition, the trans orientation of the two 4-fluorbenzyl
substituents can be attributed to supramolecular packing
effects. As confirmed via DFT calculations (see Supporting
Information), the orientation of the benzyl substituents does
not have any noticeable effect on the electronic properties of
the main conjugated scaffold. Other pertinent metric
parameters of the main scaffold (see Supporting Information)
are largely similar to those of other related structurally
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of BnV (cf. −3.73 eV) but also that of P-MV (cf. −3.77 eV),19
suggesting that 2a is more easily reduced, which is in line with
the electrochemically obtained reduction potentials.
In order to further tune the electronic properties of the new
species, we investigated the effects of the substituent at the 4
position on the benzyl ring, ranging from an electron-donating
substituent (methyl) to two different electron-withdrawing
substituents (fluoro, trifluoromethyl). As mentioned above, due
to the lack of success in the synthesis of the diquaternized
pentafluorobenzyl species, we instead utilized the asymmetric
species 6b to investigate the strength of one electron-
withdrawing pentafluorobenzyl group. For the first reduction
potential, we observed a difference of 25 mV between 3 (the
most electron-donating substituent) and 5 (the most electron-
withdrawing substituent). Remarkably, the substitution entailed
a stronger effect on the second reduction event with a
difference of 52 mV between 3 and 5, demonstrating the
effective tunability of the new system through small functional
changes. In the case of the asymmetric pentafluoro species 6b,
we observed the first reduction at −538 mV, very similar to that
of 2a (cf. −536 mV). However, as seen with the other
compounds in this series, the second reduction event is more
strongly influenced by the quaternizing substituent, where we
see a lowered potential of −957 mV for 6b, compared to −966
mV for 2a. A smaller reduction potential difference (δEred) is
consequently observed with increasing electron-withdrawing
character in each species. Notably, δEred can also be correlated
reasonably well (R2 = 0.89) with the Hammett constants of the
4-benzyl substituents, providing a trend that may guide further
development of new phosphaviologenes with regard to
adaptable redox potentials (Figure 2B).
Figure 3. (A) UV−vis spectrum of isolated redox states of 2a. (B)
Chemical reduction of 2a. (C) Isolated first reduction of 2a. (D)
Isolated second reduction of 2a.
profiles were observed for the other species of this series (see
Supporting Information), highlighting the fact that while we
were able to tune the reduction potentials for these events by
modification of the 4-benzyl position, the photophysical
responses remain largely unaffected.
Notably, the reported compounds could also easily be
chemically reduced to observe the pronounced color changes
between the redox states in acetonitrile solution. Zinc dust was
used to obtain the radical species 2a′, evident in its deep blue-
purple color, which was air stable for several minutes at room
temperature and for several hours at −20 °C; sodium metal was
used to obtain the bright orange neutral species 2a″ (Figure
3B). Remarkably this species was still observed after the vial
was left exposed to air overnight.
Radical Characterization. The radical nature of the singly
reduced 2b′ (as well as the radical cation of P-MV, see
Supporting Information) was probed, for the first time, using
electron paramagnetic resonance (EPR) spectroscopy in a flat
cell at room temperature. To prepare the sample, 2b was
dissolved in dry and degassed acetonitrile; this solution was
then added to a flask containing zinc dust under a nitrogen
atmosphere. A small sample of the reduced species 2b′ in
solution was taken up by a syringe and filtered through an
Acrodisc 0.2 μm PTFE membrane prior to injection into the
flat cell. The resulting signal displayed a considerable amount of
hyperfine coupling (Figure 4A), suggesting delocalization of the
radical over a large portion of the molecular scaffold, as is also
seen in parent viologens.2 Both species, P-MV and 2b′, had
expected g-factor values of 2.034 and 2.031, respectively,
supporting the organic nature of the radical. To gain more
detailed insight into the nature of the radical, DFT calculations
were performed at the B3LYP/6-31G(d) level of theory using
the PCM solvation model and the spin density of the radical
was modeled (Figure 4B).22 The spin density calculations of
the radical species indicate there is a strong correlation between
the SOMO of the radical cation and the LUMO of the
dicationic species, further supporting the fact that the observed
reduction potentials are the result of the low energy level of the
LUMO.
As can be seen by the two reduction potentials for 2a and 2b,
respectively, the counteranion does not have any notable effect
on the redox properties of the main conjugated scaffold (ΔE =
1−3 mV). The latter is not overly surprising, however,
−
considering that the solution contains a large excess of PF6
from the electrolyte.
Spectroelectrochemistry. The photophysical properties
of the new species were investigated via UV−vis spectroscopy
in methanol for the dicationic salts (see Supporting
Information) as well as in DMF for the mono- and direduced
species in a spectroelectrochemical cell. While the absorption
profiles of the dicationic salts are featureless and only show
slight absorption in the UV region, confirming their optically
transparent nature, the reduced species offered some impressive
chromism. Using a spectroelectrochemical cell, we could isolate
each of the three redox states of 2a, providing a comparison of
absorption profiles (Figure 3A). When no potential was applied
to the cell, we observed the featureless absorption of the
dication. However, upon addition of a potential preceding the
first reduction threshold, the absorption features of the radical
species with increased absorption throughout the visible
spectrum could be seen. The production of the radical cation
species of 2a (2a′) was easily monitored over time, showing the
saturation of color and increased absorption in the visible
region of the optical spectrum (Figure 3C). Similarly, by
applying a potential beyond the first reduction and preceding
the threshold of the second reduction, we were able to observe
the absorption profile of the neutral species 2a″ in solution.
The conversion of the radical species 2a′ into the neutral
species 2a″ could be detected when monitoring the absorption
over a period of time while constant potential was applied to
the cell (Figure 3D). Interestingly, nearly identical absorption
The DFT data also provided strong support for the radical
being primarily delocalized and consequently stabilized over the
entire π-conjugated scaffold of the molecule, in line with the
observed EPR data. Using the calculated isotropic fermic
D
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solid state and thus the best candidate to give a colorless, fully
transparent device redox state. Figure 5A shows the solution-
Figure 5. (A) Solution-based electrochromic device with 2b. (B)
Reversible solution-based device with 2b.
Figure 4. (A) EPR spectrum of 2b′ (see Supporting Information for
simulation parameters). (B) Spin density of the cationic portion of 2b′
(B3LYP/6-31G(d) level of theory). (C) CV stability test of 2a. (D)
Absorption changes for electrochromic switching of 2a.
based cell used to test the electrochromic behavior of 2b. The
two pieces of FTO glass were sealed together with a UV-cured
gasket using a hardware gel. In order to further simplify the
active components in the electrochromic device, we tested a
cell without the use of a supporting electrolyte. To our
satisfaction, a color change without the addition of a supporting
electrolyte was observed, allowing us to develop electrochromic
cells simply made from our materials dissolved in an
appropriate solvent. However, while we were able to observe
the electrochromic behavior, we were not able to demonstrate
the reversibility within this particular device setting, as the
strongly colored radical would simply build up on the opposite
electrode but the entire cell would still appear to be the same
color. To circumvent this occurrence, a second setup was
constructed where the FTO was partially removed from each
side of the glass slide before the cell was sealed with the UV-
cured gasket. This second-generation device was then indeed
able to display the expected reversibility of the electrochromism
by alternating between positive and negative potentials (Figure
5B). Unfortunately we were not able to observe complete
discoloration at any given time, as the cell thickness was too
large (50 μm) and the colored radical species would diffuse
through the solution at a rate faster than the potential
switching. The movie in the Supporting Information
demonstrates the switching behavior but is sped up 32 times
to better visualize the redox switching. It should be mentioned
in this context that state-of-the-art electrochromic devices
require multilayer architectures and often a complex mixture of
functional materials that fulfill the roles of electrodes,
electrolyte, and active materials.24 While our simplified proof-
of-concept devices do not lend themselves for direct
comparison in performance with the former, our studies
nevertheless clearly establish the general dual applicability of 2b
as an electroactive component as well as an electrolyte in such a
device setting.
contact couplings and the delocalization information from the
spin density, the EPR spectrum of 2b′ was simulated and is in
good agreement with the experimentally observed EPR
spectrum.
Stability Tests and Application in Electrochromic
Devices. In order to test the viability of the new materials
for potential electrochromic applications, we performed an
electronic and photophysical stability test using CV and UV−
vis spectroscopy, respectively. Since all of the compounds show
similar electronic and photophysical properties, the tests were
representatively performed using 2a. Cyclic voltammetry was
performed under the same conditions as described above,
however, with a 100-scan cycle (Figure 4C). We observed
minor changes in the CV from the initial scan to scan 40, likely
due to diffusion effects present prior to reaching an equilibrated
state,23 but afterward stabilization upward to scan 100 is
evident, indicating good electronic stability of the material, even
after a large number of redox cycles. In addition, the minor
deviation throughout the redox cycles does not seem to affect
the reversibility of the redox events and is maintained
throughout, making these materials indeed viable for long-
term cycling in device applications. Moreover, the electro-
chromic switching of 2a was tested in the same spectroelec-
trochemical cell. We probed each of the three major absorption
peaks (415, 584, 636 nm) at separate intervals of 30 min, each
with a 30 s delay in the potential switch between dicationic and
radical species (Figure 4D). Reasonably good electrochromic
switching was observed with the exception of the peak at 415
nm, where a slight decrease in the transmittance change
occurred with an increased number of cycles. However, this
decreased switching characteristic can be attributed to either
diffusion of the radical species within the cell, which could leave
residual radical species in solution prior to the cell switch, or
different dynamic responses of the various species present in
the bulk mixture. This was also observed at 584 and 636 nm
but to a smaller extent, likely due to the decreased extinction
coefficients at these wavelengths.
CONCLUSIONS
■
We prepared a series of N,N-dibenzylated 2,7-diazadibenzo-
phosphole oxides via newly developed synthetic protocols that
allowed us to investigate the substituent effects on the
electronic and photophysical properties of the materials.
Furthermore, we developed a new method toward an efficient
anion exchange for viologen species. Importantly, while we
The proof-of-concept devices for this study, intended to
verify electrochromic behavior, utilized fluorine-doped tin oxide
(FTO)-coated glass as the electrodes and 2b as active
component, since the material is white/transparent in the
E
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were able to noticeably lower the reduction potentials and to
tune the electronic properties through introduction of either
electron-donating or electron-withdrawing groups at the para
position of the benzyl substituent, the various redox states
preserve their colors (colorless, blue/purple, orange) in each of
the species. This potentially opens the door for practical utility
of the new materials in energy-efficient electrochromic
applications. To this end, we designed simplified proof-of-
concept electrochromic cells using the dibenzylated phospha-
viologen 2b as a representative example. However, while we
have been able to achieve a simplistic device design with
materials showing considerably lowered reduction threshold
potential in the context of this initial study, an optimized
material and/or device design is necessary prior to more
sophisticated, large-scale applications. Corresponding studies
are currently underway. In addition, to further extend the
potential applications of our phosphaviologen materials beyond
dimming devices, such as multicolored displays, investigations
into obtaining differently colored redox states via modification
of the main molecular scaffold are also currently being carried
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ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details and characterization data of all new
compounds; X-ray crystallographic data for 4; comprehensive
DFT data for all compounds and their reduced relatives
(B3LYP/6-31G(d) level of theory); supporting movie showing
the electrochromic switching in a device setting. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*thomas.baumgartner@ucalgary.ca
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
(19) Durben, S.; Baumgartner, T. Angew. Chem., Int. Ed. 2011, 50,
7948−7952.
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(21) Note: the redox properties of viologens are strongly dependent
on the polarity of the solvent (ref 2). Since some of the new
benzylated species are poorly soluble in acetonitrile, the redox
properties of P-MV were reinvestigated in DMF solution to provide
for a more suitable point of reference.
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Wallingford, CT, 2009 (see Supporting Information for full reference).
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Canada
Foundation for Innovation is gratefully acknowledged. M.S.
thanks NSERC and Alberta Innovates−Technology Futures for
graduate scholarships. We thank Drs. T. C. Sutherland and C.-
C. Ling for access to their instrumentation as well as D. Fox and
W. White for support with the EPR spectroscopy.
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DOI: 10.1021/ja513258j
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX