Polaron‐induced conformation change in single polypyrrole chain: An intrinsic actuation mechanism

Abstract

Ab initio calculations show that a neutral polypyrrole chain in the ground state assumes a helical shape resulting from a novel bending mechanism, while upon oxidation the chain becomes planar within the polarons, an effect due to enhanced inter-ring bonding. This polaron-induced conformation change leads to an intrinsic potential for inducing macroscopic strains in the single chain, with implications for further theoretical studies and experiments. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem 102: 980 -985, 2005 Key words: polypyrrole; conjugated polymer; polaron; conformation; actuation mechanism; helix H elical structures have recently attracted much interest in the field of -conjugated polymers [1][2][3][4][5]. Helices may arise for a variety of reasons, such as hydrogen bonding [4], steric interactions between sidechains [5], twisting with a polymeric dopant, or polymerization in a chiral medium [2,3].It was observed via small-angle neutron scattering that upon oxidation, single poly-(3-alkylthiophene) chains undergo large conformational changes from coils to rods [6]. Two questions are of interest: i) How does a single conjugated polymer chain form coils or even helices? ii) What causes the chain conformation to change in the oxidation-reduction (redox) processes? Although the theory of solitons and polarons has been successful in explaining many electronic and optical properties of conjugated polymers [7], thus far a direct connection with chain conformation has not been established. In the classical picture of polarons, in which an electron or hole couples to an optical phonon, within the polaron the long bonds are shortened and short bonds lengthened, such that the total carbon backbone length is largely unchanged. Thus, it would appear that polarons would not play any significant role in processes giving rise to significant macroscopic strains. Yet this is what we find from ab initio calculations on single polypyrrole (Fig. 1) chains.We investigate the intrinsic connection between polaron and actuation by presenting three closely related results. These are: i) a neutral pyrrole (P) oligomer, starting from 2P, is nonplanar due to the preference of finite dihedral angles; ii) as a unique geometrical consequence of the five-membered pyrrole ring, alternating ϩ-ϩ-dihedral angles lead to helical ground-state structures, a mechanism which is entirely different from the energetically unfavorable helical defects [8] such as 180°rotation of alternating pyrrole units; iii) by coupling to the dihedral angles to favor a locally planar orientation in its quinoid core, a polaron is able to induce a macroscopic shape change in the oligomer. Our results thus suggest a possible nanoscale singlemolecule actuation mechanism driven by redox that is intrinsic to the pyrrole chain, in the sense that the counter ions and interchain interactions are not involved. A similar approach was adopted in the early discussions of charge transport [7] in these systems.In view of the intensive discussions of planarity [9 ...

Preparation of Rectangular WO3·H2O Nanotubes Under Mild Conditions

“…As the reaction continues at 60°C, the PANI molecules are further oxidized; a change of the conformation of the PANI occurs because the PANI chain in the ground state assumes a helical shape whereas upon oxidation the chain becomes planar (an effect caused by enhanced interring p-bonding). [26] The conformational variation inevitably causes torsional movement within the chains, [27] causing the nanosheets to roll up (Fig. 6b).…”

Controlling the Photoluminescence from a Laser Dye through the Oxidation Level of Polypyrrole

“…It is known that the polymer chains in reduced ppy present a neutral character and a nonplanar conformation. [20] Moreover, the polymeric matrix is more compact in its reduced form, [21] which would promote a shorter apparent distance between an embedded dye molecule and the polymer backbone. The dye molecules absorb the exciting radiation from the laser and emits within the wavelength range of the absorption of the ppy chromophore.…”

Nanostructural Aspects of Conducting‐Polymer Actuators

“…These calculations show a helical shape for neutral PPy, whereas upon oxidation the chains become planar. This creates the possibility of a single-molecule actuation mechanism and may explain the conformational changes seen in related polythiophene-type conducting polymers [117]. The ability to create molecular level actuators has been taken even further in the example of thiophene-based molecular muscles by Madden and Hunter.…”