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
References
Oxidation of the polypyrrole backbone of poly(2) formally
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resonance structures.11 Migration of polaronic charge carriers should
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defined bell-shaped conductivity profile was obtained (Figure 2A).
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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.
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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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