Poly(pyridinium phenylene)s: Water-soluble N-type polymers

Article abstract of DOI:10.1021/ja906513u

(Chemical Equation Presented) Poly(pyridinium phenylene) conjugated polymers are synthesized by a cross-coupling and cyclization sequence. These polyelectrolytes are freely soluble in water and display high degrees of electroactivity. When reduced (n-doped) these materials display in situ conductivities as high as 160 S/cm. The high conductivity is attributed to the planar structure that is enforced by the cyclic structures of the polymer. The electron affinities are compared to PCBM, a C₆₀ based n-type material. We find that these polymers undergo excited state electron transfer reactions with other donor conjugated polymers and hence may find utility in photovoltaic devices.

Full text of DOI:10.1021/ja906513u

Published on Web 11/19/2009
Poly(Pyridinium Phenylene)s: Water-Soluble N-Type Polymers
Daisuke Izuhara and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received August 2, 2009; E-mail: tswager@mit.edu
Scheme 1. Synthetic Routes to P1, P2, and P3
Conjugated polymer semiconductors can be fabricated over large
areas by low-cost solution processing and hence offer economic
advantages in the production of photovoltaic cells,1 light emitting
diodes,2 and field-effect transistors.3 Although many varieties of
high performance p-type polymers are available,4 stable processable
n-type polymers remain largely elusive.5 Few studies have been
reported concerning heterojunctions between p/n-type polymers for
photovoltaic cells,6 mainly due to the limited high electron affinity
(EA) n-type polymers, and the polymer photovoltaic devices have
focused heavily on acceptor molecules, such as methanofullerene
phenyl-C61-butyric-acid-methyl-ester (PCBM, EA ) 4.2 eV).1b
Access to conjugated high electron affinity polymers remains a
critical challenge, and one of the most successful strategies has
been the construction of conjugated polymers based upon N-
heterocyclic electron-deficient aromatics.5a,7 However, development
of nitrogen-containing polyheterocycles with high electron affinities
and solubility in common solvents has met limited success.8
Table 1. Optical and Electrochemical Properties of P1, P2, and P3
b
b
λmax
(nm)
λonset
(nm)
Ega
(eV)
Ered
(V)
Eonset
(V)
EAc
(eV)
IPd
(eV)
We report herein the syntheses and electron-accepting properties
of a new class of water-solution processable n-type conjugated
polymers (P1, P2, P3) with pyridinium-phenylene units that exhibit
reversible electroactivity, useful electron affinities, and high electri-
cal conductivity. In these materials the electron-deficient pyridinium
rings are produced by an intramolecular cyclization that provides
low LUMO energies and a relatively planar structure for extended
π-electron delocalization. We further demonstrate electron transfer
quenching in bilayer donor/acceptor polymer heterojunctions8,9 with
the well-known p-type material, poly(2-methoxy-5-(2-ethylhexy-
loxy)-1,4-phenylene-vinylene) (MEH-PPV).10
polymer
P1
P2
P3
431
376
483
485
420
575
2.56 -0.56, -1.27 -0.40 4.00 6.56
2.95 -0.71, -0.90 -0.50 3.90 6.85
2.16 -0.59, -0.99 -0.26 4.14 6.30
a Eg: Band gap estimated from the band edge (λonset) of the absorption
b
spectra. Ered, Eonset: Formal and onset reduction potentials (vs SCE).
c EA: Electron affinity obtained based on EA ) Eonset + 4.4 (eV). d IP:
Ionization potential calculated from IP ) EA + Eg (eV).
polymers were soluble in common organic solvents (e.g., CHCl3,
THF), and thionyl chloride induced quaternizative cyclization gave
polyelectrolytes P1, P2, and P3.13 The polyelectrolytes are only
soluble in highly polar solvents, such as water and methanol, and
this feature allows for the facile formation of multilayer polymer
structures by simple spin-coating on top of polymers with orthogo-
nal solubilities.
The absorption and emission spectra of P1, P2, and P3 (Figure
1a-b) are significantly red-shifted relative to their respective
precursors.12 This is attributed to the two ethylene bridges enforcing
a planar conformation of the bis-pyridinium-phenylene segment.
Thin films of P3 displayed the smallest Eg (2.16 eV) as a result of
the donor-acceptor type structure as well as less steric hindrance.
The electron affinities (EA) of P1, P2, and P3 are estimated at
3.90-4.14 eV from the onset reduction potential in cyclic volta-
mmetry (CV) (Table 1).12,14 Interestingly, the EA values are higher
than those estimated in the same method for most conventional
polyheterocycles7a and are comparable to well-known electron
transporter PCBM (4.2 eV)1b or polybenzimidazobenzophenan-
throline (BBL) (4.0 eV).8 Additionally, P1, P2, and P3 exhibit
reversible electrochemical behavior as revealed in Figure 1c for
P1. The in situ conductivity measurements of P1 thin films (Figure
1c) on interdigitated microelectrodes reveal a narrow window of
high conductivity (σmax ) 36 S/cm at -0.96 V).15 Conductivity
measurements on thin films of P1 were made difficult by the need
Scheme 1 shows the synthetic routes to P1 and copolymers P2
and P3. We first synthesize a pyridyl precursor-polymer that is then
subjected to intramolecular nucleophilic substitution reactions to
form the pyridinium rings. A symmetrical monomer 3 provides a
head-to-head skeleton that was expected to display reversible
viologen-like redox behavior.11 The boronation of 2,5-bis(2-(tert-
butyldimethylsilyloxy)ethyl)-1,4-benzenedibromide (1) gives the
corresponding diboronic acid bis(pinacol) ester (2). Head-to-head
monomer 3, 1,4-bis[2-(5-bromopyridyl)]-2,5-bis(2-(tert-butyl dim-
ethylsilyloxy)ethyl) benzene, was synthesized by regioselective
Suzuki coupling of 2 with 2,5-dibromopyridine in 58% yield.
Yamamoto, Suzuki, and Stille coupling polymerizations yielded
high-molecular-weight siloxyethyl-substituted poly(pyridine phe-
nylene)s, P4, P5, and P6, in 65-95% yield.12 All the precursor
9
17724 J. AM. CHEM. SOC. 2009, 131, 17724–17725
10.1021/ja906513u  2009 American Chemical Society

C O M M U N I C A T I O N S
teristic PL emission spectrum of MEH-PPV (λem ) 585 nm, λex
)
507 nm) is strongly quenched (93%) in the bilayer structure with
the P1 layer (40 nm). These results clearly indicate that the electron
transfer occurs from the MEH-PPV (EA/IP ) 2.9/5.1 eV)8,10 to
the P1 (EA/IP ) 4.0/6.6 eV). Preliminary studies on bilayer organic
photovoltaic devices between P1 and poly(3-hexyl thiophene) gave
large open circuit voltages (Voc ) 1.2 V) but low short circuit
currents (7 µA/cm2). Field effect transistor devices of P1 were
investigated in air, and n-type behaviors with mobilities of 0.24
cm2/(V s) at gate voltages of 5-15 V and 3.4 cm2/(V s) for gate
voltages of 15-20 V were observed (Figure S10). Significant
experimentation is needed to understand the role of counterions in
these applications.
In conclusion, we report a promising class of water and/or
methanol soluble electron-accepting (n-type) conjugated polymers.
These materials display high EAs, reversible redox behavior, high
conductivities, good electron mobilities, and efficient quenching
of donor polymers.
Acknowledgment. This work was supported by the National
Science Foundation DMR-0706408 and TORAY Industries. The
authors thank Dr. Changsik Song and Dr. Moon-Ho Ham for
technical discussions.
Figure 1. (a) UV-vis absorption spectra of P1-3; thin films (continuous
line), water or methanol solutions (dashed line). (b) PL emission spectra of
P1-3 in water or methanol solutions. (c) Cyclic voltammogram, in situ
conductivity measurement. (d) Spectroelectrochemistry of P1 thin film.
Supporting Information Available: Experimental details and
characterization of the products. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) (a) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107,
1324. (b) Thompson, B. C.; Freche´t, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58.
(2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998,
37, 402.
(3) (a) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew.
Chem., Int. Ed. 2008, 47, 4070. (b) Murphy, A. R.; Freche´t, J. M. J. Chem.
ReV. 2007, 107, 1066.
Figure 2. (a) UV-vis absorption and (b) PL emission spectra (λex ) 507
nm) of thin films of MEH-PPV, P1, and a bilayer of MEH-PPV/P1.
(4) (a) McCullough, R. D. AdV. Mater. 1998, 10, 93. (b) McCulloch, I.; Heeney,
M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe,
D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.;
McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328. (c) Usta, H.; Lu,
G.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 9034.
(5) (a) Babel, A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656. (b)
Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li, Y.;
Zhu, D.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129, 7246.
(c) Letizia, J. A.; Salata, M. R.; Tribout, C. M.; Facchetti, A.; Ratner, M. A.;
Marks, T. J. J. Am. Chem. Soc. 2008, 130, 9679. (d) Usta, H.; Facchetti,
A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 8580.
to reduce with THF solutions of Na+-naphthalide. Less than optimal
redox states and decreased film quality from the solvent exposure
resulted in σ ) 1.1 S/cm, which is lower than our in situ
determination. The maximum in situ conductivities of P3 (σmax
)
160 S/cm) rivals values observed of well-known p-type poly(3-
alkyl thiophene)s, and P2 exhibits σmax ) 9 S/cm. The in situ
conductivity profile indicates that the ‘mixed valence’ state is
conductive and that the neutral (fully reduced) material is insulat-
ing.12 We investigated the reduction of P1 thin films deposited onto
ITO-coated glass electrodes by spectroelectrochemistry (Figure 1d).
The absorption spectra show a decrease of the original band gap
transition and the buildup of intragap energy states, which matches
well to the negative polaron-bipolaron model for charge delocal-
ized π-platforms.16
Optical absorption and photoluminescence (PL) spectra of thin
films of MEH-PPV, P1, and a MEH-PPV/P1 bilayer are shown in
Figure 2. It is noteworthy that high quality bilayer films can be
obtained by coating a water solution of P1 on the top of an MEH-
PPV thin film prepared from the chloroform solution, since MEH-
PPV neither dissolves nor swells in water. The absorption spectrum
of bilayer heterojunction is a superposition of those of the two
polymer layers, indicating no detectable ground state interaction
at the heterojunction interface. Figure 2b shows how the charac-
(6) (a) Granstrom, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.;
Friend, R. H. Nature (London) 1998, 395, 257. (b) Jenekhe, S. A.; Yi, S.
Appl. Phys. Lett. 2000, 77, 2635. (c) Kietzke, T.; Egbe, D. A. M.; Hrhold,
H.-H.; Neher, D. Macromolecules 2006, 39, 4018.
(7) (a) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater.
2004, 16, 4556, and references therein. (b) Zhu, Y.; Yen, C.-T.; Jenekhe.,
S. A.; Chen, W.-C. Macromol. Rapid Commun. 2004, 25, 1829.
(8) While an EA of 4.0 eV has been reported for BBL, it is noteworthy that
BBL was processed by a strong acid, rather than common solvents: Alam,
M. M.; Jenekhe., S. A. Chem. Mater. 2004, 16, 4647.
(9) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992,
258, 1474. (b) Alam, M. M.; Jenekhe, S. A J. Phys. Chem. B 2001, 105,
2479.
(10) Li, Y.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth. Met. 1999,
99, 243.
(11) Michaelis, M.; Hill, E. S. J. Gen. Physiol. 1933, 16, 859.
(12) See the Supporting Information.
(13) Zhang, S.-W.; Swager, T. M. J. Am. Chem. Soc. 2003, 125, 3420.
(14) Li, Y.; Ding, J.; Day, M.; Tao, Y.; Lu, J.; D’iorio, M. Chem. Mater. 2004,
16, 2165.
(15) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984,
106, 7389.
(16) (a) Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309. (b) Heeger,
A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W.-P. ReV. Mod. Phys. 1988, 60,
781.
JA906513U
9
J. AM. CHEM. SOC. VOL. 131, NO. 49, 2009 17725