Defining space around conducting polymers: Reversible protonic doping of a canopied polypyrrole

Article abstract of DOI:10.1021/ja034894+

A canopy-shaped pyrrole derivative 2 was prepared, in which a sterically demanding pendant group is juxtaposed to the pyrrole fragment to minimize interstrand π-π stacking interactions in the resulting polymer. Anodic polymerization of 2 afforded highly c

Full text of DOI:10.1021/ja034894+

Published on Web 05/16/2003
Defining Space around Conducting Polymers: Reversible Protonic Doping of
a Canopied Polypyrrole
Dongwhan Lee and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVe.,
Cambridge, Massachusetts 02139
Received February 27, 2003; E-mail: tswager@mit.edu
Scheme 1 a
Molecular wires built on π-extended organic frameworks have
proven useful for sensory signal amplification.1 In such functional
solids, binding of analytes can elicit either deconjugation of the
conducting polymer (CP) backbone or an energy mismatch between
juxtaposed redox-active sites.1a An important prerequisite for CP-
based sensing scheme is the presence of well-defined charge
transporting pathways, the stereoelectronic perturbation of which
translates into measurable changes in collective properties such as
conductivity. Equally crucial in this context is the availability of a
highly porous internal structure that facilitates passive diffusion of
analytes across the bulk material. In this communication, we report
a synthetic strategy targeting these two objectives. Here, a sterically
hindered canopy was integrated into the polypyrrole backbone to
provide free volume and to minimize cross-communication between
adjacent polymer strands. The reduced dimensionality of the charge-
transporting pathways in such constructs translates into a different
a (a) PhSH, NCS, CH2Cl2, -78 °C to rt. (b) mCPBA, CHCl3. (c) DBU,
CH2Cl2. (d) CNCH2CO2 Bu, BuOK, THF, reflux. (e) TFA, rt. (f) LiCl,
H2O, ethylene glycol, 190 °C. Structure of 2 was generated using 50%
probability thermal ellipsoids.
conductivity profile than is observed in the parent polypyrrole.
Notably, this porous polypyrrole derivative still retains high
conductivity, which can be reversibly modulated by protonic
doping.2
t
t
A synthetic strategy has been implemented that allows access
to a new class of sterically hindered pyrrole derivatives (Scheme
1). The prototypical molecule 2 features a C-shaped cavity defined
by the pyrrole ring and a phenyl canopy. Such an architectural
consideration is expected to minimize π-π stacking interactions
between adjacent CP strands in poly(2) (Scheme 1). Substitutions
on the â-positions of the pyrrole unit in 2 are also expected to
enhance structural homogeneity as well as chemical stability by
preventing interchain linkages and nucleophilic attack on the
polypyrrole backbone. Compound 2 was prepared in seven steps
from readily available materials (overall yield ) 32%) and
characterized by various methods including X-ray crystallography
(Scheme 1).3 Utilizing modified Barton-Zard approach,4 the pyrrole
fragment in 2 was synthesized from the olefinic double bond of
the kinetic Diels-Alder adduct of N-phenylmaleimide and cyclo-
pentadiene, 1.
Figure 1. Cyclic voltammogram (25 mV/s; dotted line) and in situ
conductivity measurement7 (5 mV/s, offset potential of 40 mV; solid line)
of poly(2) on 5-µm interdigitated Pt electrodes in MeCN with 0.1 M (Bu4-
N)PF6 as supporting electrolyte (left), and qualitative representations of
neutral (A), polaronic (B), and bipolaronic (C) states of the polypyrrole
backbone (right).
Anodic polymerization of 2 was carried out under ambient
conditions. Upon repeated potential sweeps between -500 and
+1000 mV vs Ag/AgNO3 (0.01 M) in MeCN, a quasi-reversible
redox wave arising from electrode-deposited poly(2) gradually
builds up. The scan rate dependence of poly(2) in monomer-free
electrolyte unambiguously establishes that the redox process at E1/2
) -95 mV originates from an electrode-bound redox-active
species.5 Spectroelectrochemical studies of poly(2) revealed proper-
ties reminiscent of those associated with the parent polypyrrole.
Despite the apparent similarities, the potential-conductivity
profiles of poly(2) are markedly different from those of the parent
polypyrrole system. As shown in Figure 1, the conductivity of poly-
(2) increased upon oxidation and maximizes at -90 mV (σmax
≈
40 S cm-1). Further oxidation resulted in a rapid drop, defining
finite potential windows of high conductivity. A similar potential-
conductivity profile was obtained upon return sweep. This buildup
and decay of the charge-transporting species in poly(2) matches
closely with the potential-dependent changes in the intensity of the
g ) 2 EPR signal,3 corroborating the notion that charge-delocalized
radical cation species (B, Figure 1) is the major participant in the
charge-transporting mechanism of poly(2). In contrast, the con-
1
Specifically, the build-up and decay of the S ) /2 paramagnetic
species as well as spectral shifts in the UV-vis spectra induced
by gradual oxidation of poly(2) could be explained by invoking
the polaron-bipolaron model of charge-delocalized π-platforms
(vide infra).3,6
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J. AM. CHEM. SOC. 2003, 125, 6870-6871
10.1021/ja034894+ CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
strands within poly(2) apparently limits the number of pathways
available for charge transport, facilitating a rapid and reversible
conductor-to-insulator transition. The sluggish response of poly-
pyrrole can be due to the slower diffusion of acids and bases in its
thin films and may also be due to its delocalized interchain charge
carriers that can potentially take alternative pathways upon en-
countering locally introduced energy barriers. If strong interstrand
electronic coupling in such a π-π stacked system attenuates the
perturbation introduced to the energy landscape of CP, it is a liability
for resistivity-based sensing. A preliminary experiment established
that the conductivity of poly(2) could be similarly modulated in
aqueous electrolyte solutions between pH ) 3 and pH ) 9. The
compatibility of the present system with aqueous media as well as
the potential for further elaborating the canopy module in the
prototypical monomer 2 offers opportunities for biologically and
environmentally important sensing applications.
In summary, by defining space around CP, we lowered the
dimensionality of the electronic structures responsible for charge
transport. A conceptual linkage can be drawn to a strategy recently
adopted for polymer light-emitting devices, in which luminescent
CP strands are encapsulated within insulating organic sheaths.12
Limited interstrand electronic interactions between such site-isolated
π-conjugated platforms facilitate an efficient exciton-to-photon
conversion, while high mobilities of charge-carrying species are
still being maintained.
Figure 2. Conductivity profiles (5 mV/s, offset potential of 40 mV) of
poly(2) (A-C) and polypyrrole (D-F) on 5-µm interdigitated Pt electrodes
in MeCN electrolyte solutions ([Bu4N)PF6] ) 0.1 M) of either TFA (10
mM, A, C, D, and F) or pyridine (10 mM, B and E). See the text for
experimental details.
ductivity of parent polypyrrole system increases upon oxidative
doping of the polymer and maximizes at the potential range
dominated by the EPR-silent bipolaronic states (C, Figure 1; Figure
2D).6,7b,c,8 An efficient interstrand charge hopping promoted by close
contacts between planar π-extended platforms facilitates charge
delocalization within polypyrrole, providing three-dimensional
electronic connectivity for bipolaron migration.9,10 In contrast,
limited interstrand electronic coupling in poly(2) apparently strength-
ens localization of the charge carriers and affords finite potential
windows of high conductivity dominated by polaronic charge
carriers (B, Figure 1).
Acknowledgment. This work was supported by the DoE and
Army Research Office’s Tunable Optical Polymers MURI program.
We thank Mr. H.-h. Yu for help in acquiring spectroelectrochemical
data and valuable discussions.
Supporting Information Available: Crystallographic data of 2
(CIF) and spectroelectrochemical data of poly(2) (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
References
Oxidation of the polypyrrole backbone of poly(2) formally
generates iminium functional groups (B and C, Figure 1). Depro-
tonation of these fragments would result in oxidized poly(2) having
varying degrees of deprotonation and contributions from quinonoid
resonance structures.11 Migration of polaronic charge carriers should
thus be significantly hampered across the polymer backbone
embedded with such local energy barriers. This postulation was
probed experimentally. When poly(2) was placed in an MeCN
electrolyte solution of trifluoroacetic acid (TFA, 10 mM), a well-
defined bell-shaped conductivity profile was obtained (Figure 2A).
Upon exposure to pyridine (10 mM), however, a significant (>70%)
drop in σmax was observed, followed by a featureless return sweep
(Figure 2B). The material remains insulating upon subsequent
sweeps. The conductivity profile of poly(2), however, was regener-
ated upon reexposure to TFA (Figure 2C). A similarly rapid and
reversible switching was effected when a sterically demanding
analogue of pyridine such as 2,3-cyclododecenopyridine was
used, demonstrating the highly porous nature of poly(2) accessible
to bulky analytes.
(1) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207. (b) McQuade, D.
T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537-2574.
(2) The paradigm of an orthogonal tuning of bulk conductivity by either
oxidative or protonic doping has been established for polyanilines: (a)
Chiang, J.-C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193-205. (b)
MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc. 1989,
88, 317-332. This concept has recently been extended to conducting
polymers having extended p-dihydroquinone segments: Yu, H.-h.; Xu,
B.; Swager, T. M. J. Am. Chem. Soc. 2003, 125, 1142-1143.
(3) See Supporting Information.
(4) (a) Barton, D. H. R.; Zard, S. Z. J. Chem. Soc., Chem. Commun. 1985,
1098-1100. (b) Barton, D. H. R.; Kervagoret, J.; Zard, S. Z. Tetrahedron
1990, 46, 7587-7598. (c) Abel, Y.; Montforts, F.-P. Tetrahedron Lett.
1997, 38, 1745-1748.
(5) All potentials are referenced to Fc/Fc+ unless otherwise noted.
(6) (a) Kaufman, J. H.; Colaneri, N.; Scott, J. C.; Street, G. B. Phys. ReV.
Lett. 1984, 53, 1005-1008. (b) Bre´das, J. L.; Street, G. B. Acc. Chem.
Res. 1985, 18, 309-315.
(7) (a) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc.
1984, 106, 7389-7396. (b) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J.
Am. Chem. Soc. 1990, 112, 7869-7879. (c) Schiavon, G.; Sitran, S.; Zotti,
G. Synth. Met. 1989, 32, 209-217.
(8) Reddinger, J. L.; Reynolds, J. R. AdV. Polym. Sci. 1999, 145, 57-122.
(9) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417-423.
(10) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bre´das, J.-L. AdV. Mater. 2001,
13, 1053-1067.
Polypyrrole subjected to similar conditions, however, displayed
(11) Deprotonation/protonation of the N-H groups in oxidized polypyrrole
and the accompanying changes in conductivity have previously been
reported: (a) Ingana¨s, O.; Erlandsson, R.; Nylander, C.; Lundstro¨m, I. J.
Phys. Chem. Solids 1984, 45, 427-432. (b) Nishizawa, M.; Matsue, T.;
Uchida, I. Anal. Chem. 1992, 64, 2642-2644. (c) Talaie, A. Polymer
1997, 38, 1145-1150.
(12) (a) Cacialli, F.; Wilson, J. S.; Michels, J. J.; Daniel, C.; Silva, C.; Friend,
R. H.; Severin, N.; Samor´ı, P.; Rabe, J. P.; O’Connell, M. J.; Taylor, P.
N.; Anderson, H. L. Nat. Mater. 2002, 1, 160-164. (b) Cardin, D. J.
AdV. Mater. 2002, 14, 553-563.
a significantly different behavior. Although the σmax (∼75 S cm-1
)
observed at the positive end of the potential scan (Figure 2D)
decreased substantially upon deprotonation (Figure 2E), polypyrrole
still retained significant conductivity (ca 35%) even after three scans
in a MeCN solution of pyridine (10 mM). Reprotonation by TFA
resulted in a partial increase in conductivity (Figure 2F), but it failed
to reproduce the original profile prior to deprotonation (Figure 2D).
A suppressed cross-communication between adjacent polymer
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