Carbanionic route to electroactive carbon-centered anion and radical oligomers

Article abstract of DOI:10.1021/ol1017728

Figure Presented. The synthesis of poly-1,3-bisdiphenylene-2-phenyl allyl (BDPA) radicals via a new anionic oligomerization strategy is reported. The material displays a reversible reduction from the orange-red radical to the blue carbanion in solution.

Full text of DOI:10.1021/ol1017728

ORGANIC
LETTERS
2010
Vol. 12, No. 19
4324-4327
Carbanionic Route to Electroactive
Carbon-Centered Anion and Radical
Oligomers
Eric L. Dane and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts
AVenue, Cambridge, Massachusetts 02139
tswager@mit.edu
Received July 29, 2010
ABSTRACT
The synthesis of poly-1,3-bisdiphenylene-2-phenyl allyl (BDPA) radicals via a new anionic oligomerization strategy is reported. The material
displays a reversible reduction from the orange-red radical to the blue carbanion in solution.
We report electroactive conjugated materials containing a
carbon-centered radical based on the 1,3-bisdiphenylene-2-
phenylallyl (BDPA) radical (1)1 that can be reversibly
reduced to a delocalized carbanion.2 Our synthesis of these
materials is based on a new anionic oligomerization that
produces a soluble carbanionic polymer. The ability of
biradical 2 and oligomer 3 to be readily reduced to give
anionic materials is critical to several applications, including
battery anodes, in which polycarbanions with lithium coun-
terions are used for charge storage.3 Additionally, n-dopable
(electron-conducting) organic conjugated materials that
become conductive upon reduction are essential elements of
photovoltaic and field effect transistor devices.4 Conjugated
polymer semiconductors that are p-dopable are abundant, but
there remains a limited number of stable n-dopable materi-
als.4 Finally, 2 and 3 are electrochromic because they can
be switched between the orange-red colored radical and the
deep blue carbanion.5
(1) (a) Koelsch, C. F. J. Am. Chem. Soc. 1957, 79, 4439. (b) Kuhn, R.;
Neugebauer, F. A. Monatsh. Chem. 1964, 95, 3. (c) Azuma, N.; Ozawa,
T.; Yamauchi, J. Bull. Chem. Soc. Jpn. 1994, 67, 31.
The BDPA carbanion is remarkably stable for a material
containing no electronegative heteroatoms and can persist
(2) (a) Pacifici, J. G.; Garst, J. F.; Jansen, E. G. J. Am. Chem. Soc. 1965,
87, 3014. (b) Kuhn, R.; Fischer, H.; Neugebauer, F. A. Liebigs Ann. Chem.
1972, 654, 64. (c) Andrieux, C. P.; Merz, A.; Saveant, J.-M.; Tomahogh,
R. J. Am. Chem. Soc. 1984, 106, 1957. (d) Breslin, D. T.; Fox, M. A. J.
Phys. Chem. 1993, 97, 13341. (e) Plater, M. J.; Kemp, S.; Lattmann, E.
J. Chem. Soc., Perkin Trans. 1 2000, 971.
(4) (a) Babel, A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656.
(b) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Brdas, J.-L.;
Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436.
(3) (a) Nishide, H.; Oyaizu, K. Science 2008, 319, 737. (b) Oyaizu, K.;
Hatemata, A.; Choi, W.; Nishide, H. J. Mater. Chem. 2010, 20, 5404.
(5) Reviews: (a) Mortimer, R. J. Chem. Soc. ReV. 1997, 26, 147. (b)
Beaujuge, P. M.; Reynolds, J. R. Chem. ReV. 2010, 110, 268.
10.1021/ol1017728  2010 American Chemical Society
Published on Web 09/10/2010

Scheme 1. Synthesis of 2
for days under 1 atm oxygen in polar aprotic solvents, such
as a model system (Scheme 1). Previously reported bis-
BDPA biradicals have been limitted to those linked through
the phenyl ring.9 Bifluorene 6, synthesized by Pd-catalyzed
coupling of 4 and 5 followed by deprotection of the
9-positions, was reacted with 2.1 equiv of bromide 8 to obtain
dianion 2a. Protonation of 2a resulted in a complex mixture
of tautomers, E/Z double bond isomers, and rotamers (1H
and 13C NMR spectra included in Supporting Information).10
However, 2a offered a simplified 1H and 13C NMR spectrum
due to its increased symmetry.11 Figure 1 (top) shows a
as DMF and DMSO, when generated electrochemically.2a
The predominant decomposition pathway of the BDPA
carbanion under ambient conditions is via oxidation to the
radical.2a Previous electrochemical studies have found that
the BDPA radical and closely related derivatives with
substitution at the 4-position of the phenyl ring behave as
semiconductors with energy gaps of 1.5-1.7 eV in the solid
state.6
Building on Kuhn and Neugebauer’s synthesis of BDPA,1b
an anionic oligomerization method was pursued. In this
approach, the BDPA oligomer is synthesized as a polycar-
banion (3a), which can then be oxidized to the polyradical.
The incorporation of the BDPA radical into the main chain
of a conducting polymer or oligomer has not been previously
investigated. However, polyacetylenes with a pendant BDPA
connected through the 4-position of the phenyl ring have
been previously studied.7 In addition, there have been
extensive efforts reported in the literature to develop materials
based on the related triphenylmethyl radical and its deriva-
tives.8
1
portion of the H NMR spectrum of the carbanion of a
previously reported BDPA derivative with a bromide at the
4-position of the phenyl ring (Br-BDPA).1b Figure 1 (bottom)
1
shows a portion of the H NMR spectrum of 2a and the
proton assignments, which were determined using gCOSY
2D NMR. Oxidation of 2a with potassium ferricyanide
produces biradical 2, which shows an EPR signal very similar
to that of BDPA at room temperature in solution.12 Biradial
2 was observed in HRMS but could not be isolated as an
analytically pure compound.
On the basis of the success of the dimer synthesis,
monomer 9 was prepared, beginning with the condensation
To test the viability of an anionic oligomerization strategy
and to study the effect of linking two BDPA radicals through
the 2-position of a fluorene ring, dimer 2 was synthesized
(9) (a) Tukada, H. J. Am. Chem. Soc. 1991, 113, 8991. (b) Tukada, H.;
Mutai, K. Tetrahedron Lett. 1992, 33, 6665.
(10) (a) Dane, E. L.; Debelouchina, G. T.; Maly, T.; Griffin, R. G.;
Swager, T. M. Org. Lett. 2009, 11, 1871. (b) Dane, E. L.; Swager, T. M.
J. Org. Chem. 2010, 75, 3533.
(6) Eley, D. D.; Jones, K. W.; Littler, J. G. F.; Willis, M. R. Trans.
Faraday Soc. 1967, 902.
(7) Nishide, H.; Yoshioka, N.; Saitoh, Y.; Gotoh, R.; Miyakawa, T.;
Tsuchida, E. J. Macromol. Sci., Part A: Pure Appl. Chem. 1992, 29, 775.
(8) (a) Rajca, A. Chem. ReV. 1994, 94, 871. (b) Rajca, A.; Shiraishi,
K.; Vale, M.; Han, H.; Rajca, S. J. Am. Chem. Soc. 2005, 127, 9014. (c)
Crayston, J. A.; Devine, J. N.; Walton, J. C. Tetrahedron 2000, 56, 7829.
(d) Rajca, A. Chem.sEur. J. 2002, 8, 4834. (e) Veciana, J.; Rovira, C.;
Ventosa, N.; Crespo, M. I.; Palacio, F. J. Am. Chem. Soc. 1993, 115, 57.
(11) Carbanions for NMR study were generated by adding 2.0 equiv of
potassium tert-butoxide per acidic proton to a d6-DMSO solution of the
protonated precursor.
(12) Solution EPR spectra and effective magnetic moment measurements
are available in Figure S1 of Supporting Information. Solid-state EPR and
the magnetic properties of the materials will be a focus of future
investigations.
Org. Lett., Vol. 12, No. 19, 2010
4325

Scheme 2. Synthesis of 3
Figure 1.
Selected regions of 1H and 13C NMR of Br-BDPA anion
(a) and 2a (b) are shown. Spectra were collected in d6-DMSO (a,
20 °C; b, 40 °C).
of 2-bromofluorene and 4-carboxybenzaldehyde (Scheme 2).
The crude carboxylate salt was converted to a dibutylamide
via the acid chloride, and the E and Z isomers were separated
by chromatography for characterization. We have previously
shown that amides are compatible with the polymerization
conditions,10a and the n-butyl groups were chosen to provide
greater solubility to both the monomer and the oligomer.
After Pd-catalyzed coupling of 10-E/Z with boronic acid 5,
the alkene of 11-E/Z was dibrominated in acetic acid.
Hydrogen bromide was eliminated with sodium hydroxide
in refluxing ethanol with concurrent removal of the trimeth-
ylsilyl groups to provide 9-E and 9-Z, which could be
separated by chromatography.
Several conditions for oligomerization were screened, but
in all cases tert-butoxide was used as the base because it
deprotonates fluorene at the 9-position without attacking the
alkenyl bromide. The effects of solvent (THF, DMA) and
counterion (Li, Na, K, Mg, Ba, Zn) were explored (see Table
S1 in Supporting Information), and the extent of oligomer-
ization was analyzed on the basis of the Mn, as determined
by GPC (DMF) of the crude polyanions. Potassium tert-
butoxide in THF was chosen as the preferable method and
was used to synthesize 3b, which is a light yellow powder
that is readily soluble in CH2Cl2, THF, and chloroform (GPC
(THF): Mn ) 6.5 kDa, degree of polymerization (DP) ) 11).
The 1H and 13C NMR of 3b showed broadened signals that
are in agreement with the proposed structure. In the 13C
a E/Z isomers were synthesized in a 1:1 ratio but were isolated and
characterized separately.
spectrum, the resonance at 52.7 ppm, which is specific to
the carbon attached to the acidic proton of BDPA precursors,
supports the proposed structure (see Supporting Information).
To isolate the radical form of the oligomer, 3a was treated
with potassium ferricyanide. After precipitation into metha-
nol, a dark red powder (GPC (THF): Mn ) 9.4 kDa, DP )
16) was isolated. The higher Mn may be a result of 3 having
a larger persistence length in THF as compared to 3b.
Fractionation of the polymer during precipitation may also
contribute to the discrepancy. In addition to the expected IR
absorbances, the isolated oligomers 3 and 3b display a weak
ketone stretch at approximately 1720 cm-1 that we attribute
to 9-fluorenone end-groups (see Figure S2 in Supporting
Information).13 Oligomer 3 showed a featureless EPR
absorption in THF at room temperature (see Figure S1 in
the Supporting Information).
The UV-vis spectra of the protonated precursor, the
carbanions, and the radicals are presented in Figure 2.
(13) The results of combustion analysis for 3 and 3b are reported and
discussed in Supporting Information. Additionally, oligomer 3 showed no
detectable level (<0.1%) of bromide in combustion analysis.
4326
Org. Lett., Vol. 12, No. 19, 2010

Figure 3. Cyclic voltammograms of 2 (a) and 3 (b) showing the
reversible redox couple between the carbanion and the radical.
Conditions: (a) 0.10 M LiClO4 in DMF, Pt-button electrode, [2a]
≈ 1 mM, 2a formed by deprotonation with LiOt-Bu in DMF; (b)
0.10 M (n-Bu)4NPF6 in MeCN, Pt-wire electrode with 3 dropcast
from a chloroform solution.
Figure 2. Absorption spectra of 2a, 2b, 2 (a) and 3a, 3b, 3 (b) are
shown. All spectra were collected in THF except for 2a and 3a,
which were collected in DMSO purged with nitrogen.
Dianion 2a shows a strong absorbance at 614 nm in DMSO
similar to the BDPA anion absorbance at 600 nm in DMF.
The polyanion 3a is red-shifted and appears broadened.
Biradical 2 and polyradical 3 show strong absorbances near
490 nm and weak absorbances at 865 nm in THF, which
are comparable to the reported values for BDPA in dioxane
(485 and 860 nm).1b
Biradical 2 shows two closely spaced, reversible one-
electron reductions to form the dianion 2a (Figure 3).
Polyradical 3 displays a broad, reversible reduction peak,
presumably attributable to multiple radicals undergoing one-
electron reduction. The electrode displays a visible change
in color from reddish brown to deep blue upon reduction to
the blue polyanion. In general, 3 did not form quality films.
Future work will examine how cross-linking through the
solubilizing groups can aid in film formation.
a new anionic oligomerization strategy. The carbanions have
1
been characterized with H and 13C NMR. The absorption
spectra of the protonated, anionic, and radical forms of 2
and 3 are reported, as is the ability to reversibly perform
one-electron reductions to form the carbanions from the
radicals. Further work will investigate how to improve the
film forming abilities of 3 and investigate how changing
the connectivity between the fluorene rings affects the
electronic and magnetic properties of the materials.
Acknowledgment. This work was supported by the
National Science Foundation, DMR-1005810.
Supporting Information Available: Experimental details
and additional spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
In conclusion, materials that join BDPA radicals through
the 2-position of the fluorene ring have been synthesized via
OL1017728
Org. Lett., Vol. 12, No. 19, 2010
4327