Doping-induced Structural Changes

Doping by various guest species, as noted previously, induces a full scale cooperative structural reorganization within the host matrix. Since these side chain containing conducting polymer structures are initially more complex than those of the linearly unsubstituted materials, the overall structural response and location of the dopant ions is even more difficult to assess. Still recent scattering studies have found an enormous range of detailed physical behavior.

In particular the structural evolution of various P3AT's after p-type doping has been probed[,,,,132]. The most dramatic response is an unprecedented variation in the large interlayer d-spacing. P3AT's are found to undergo upwards of 20% expansions followed by $\sim25\%$ reductions in their interlayer repeats[,132]. There are also pronounced changes in the non-equatorial scattering profiles in both the peak positions and their intensities. In contrast there are only minimal changes in the intra-stack polymer polymer repeat. These structural characteristics aside there are other surprising properties. Both DBSA-PANI[133] and regioregular iodine-doped poly-(3-dodecylthiophenes)[7] can achieve remarkably high conductivities despite the limited proportion of electrically active regions. To understand the origin of these effects, determining the relative locations of the various constituents and the subsequent structural evolution is an issue of preeminent interest. In quasi-2D layered hosts, such as graphite intercalation compounds, the intercalant/dopant can only be situated between graphite layers. In these side chain substituted polymers the guest ions could conceivably intercalate between individual chains or, equally as well, lie off to the side of main chain stacks and nested amongst the side chains.

A number of suggestive models[,132,] have been proposed which address the most significant changes in the scattering data. While these models differ in their respective details, there are three key features which appear to be essential for replicating the structural evolution seen in the P3AT's:
1) The molecular dopants do not overtly disrupt the stacking of the polythiophene main chain backbones.
2) The dopants occupy sites which alter the overall orientation of the flexible side chains.
3) There are translational displacements by the polymer chains which are parallel to the chain axis.
A schematic model depicting these characteristic changes is shown in Fig. 18. This first feature preserves the nominal intrastack spacing at distances close to the nominal 3.8Å which, in turn, facilitates the availability of $\pi$-obital wavefunction overlap and hopping transport in a direction perpendicular to the main chain axes. The second characteristic is responsible for the dramatic changes in the interlayer repeat. The third feature functions to create quasi-one-dimensional dopant-ion galleries within the individual layers and enhances the uptake of the dopant ions. Detailed structure factors calculations have been used to further test and validate these defining characteristics in the iodine-P3AT complexes[132]. Analogous studies are not yet available for the whole range of model compounds.

 

Figure 18: Schematic models showing the local structural degrees of freedom necessary for iodine intercalation (doping) of poly(3-alkylthiophene) homopolymers. (a) Single chain structural parameters: main chain tilt (i.e., setting angle), $\theta_r$; side chain tilt, $\theta_d$; and side chain angular orientation, $\theta_b$. (b) Intralayer c-axis translations which produce quasi-one-dimensional ion channels perpendicular to both the chain axis and the interlayer spacing (along the a-axis as shown in Fig. 14).