Alkyl and Related Side Chain Substitutions

Among side chain substituted polymers the alkyl-substituted polythiophenes[43,6,44,45] are seen as a major milestone in the development of high-performance solution processible electronic polymers. Since the parent polymer, PT, already tends towards backbone planarity there is a strong cooperative self-assembly of poly(3-alkylthiophenes) into well ordered lamellar phases[46,47] as sketched in Fig. 3. For some applications this behavior has the desirable effect of enhancing interchain p$_z$ orbital overlap and thereby improving the electron transport properties. Increasing the interchain overlap also dramatically reduces the photoluminescence (PL) yield because non-luminescent interchain processes compete with intrachain singlet exciton formation and recombination. P3ATs in dilute solution generally have good PL yields but this property diminishes precipitously when these polymers become solid films[48].

Figure 3: Top: Basic P3AT molecular level packing motif that results in a lamellar phase. The chain axis repeat, $c$, is typically 7.8 Å and the intrastack spacing for the most common crystalline phase is 3.8 Å (or $b$/2) Bottom: Sketch of P3AT showing the various possible structural isomers and impact of steric packing constraints (large arrow).

This tendency to form layered structures is strongly reflected in P3AT X-ray diffraction profiles with a series of sharp evenly spaced low angle $(h00)$ reflections as shown in Fig. 4. P3ATs also afford an excellent opportunity to study the impact of structural disorder on charge transport[49]. Polymerization of P3AT monomers through ordinary methods results in the formation of three different possible repeating isomers: head-head (HH), head-tail (HT) and tail-tail (TT). There can be pronounced differences in the dyad sequencing or ``regioregularity'' depending on the specific synthesis procedure[50,51,52]. Materials incorporating a large proportion of repeating HT linkages (typically better than 90%) are termed regioregular (rR) while polymer samples have no distinguishable dyad repeat sequence are termed regioirregular (rIR). Highly rR samples maintain a high degree of backbone planarity (hence the smallest interband transition energies) and maximize interchain p$_z$ orbital overlap. These materials are known for extremely good charge mobilities and, as such, are candidates for field-effect transistor device applications[53]. A second nearest neighbor /H/-/H/ sequence (see Fig. 3, bottom) clearly introduces side chain steric interactions and packing constraints. rIR-P3ATs are less planar[54], tend to be more soluble in organics solvents and are therefore somewhat easier to process. Films of the rIR-P3ATs are also less ordered in the solid-state and exhibit poorer transport properties. Materials composed of alternating HH and TT sequences have also been synthesized[55,56] and, somewhat surprisingly, these are found to be highly disordered. In this case there is evidence for significant sulfur-alkyl steric interactions that force the backbone out of planarity.

In solid-state P3AT films the alkyl side chains are nearly planar as well except, as demonstrated by structure factor calculations[47] and modeling studies[57], for a pronounced torsional twist about the C-C linkage which anchors the backbone to the polythiophene main chain. This has the effect of tilting the side chains so that they themselves close pack into a secondary structure which loosely approximates the intrinsic packing of saturated hydrocarbons. Secondary complications, arising from the presence of alkyl side chains, include the appearance of crystalline polymorphism and mesomorphism. The latter is associated with formation of liquid crystal polymer (LCP) phases. Most often a 3.8 Å intermolecular intrastack repeat spacing is obtained but, depending on solvent, molecular weight and film forming conditions a very different X-ray pattern may be observed (Fig. 4 at right) and this phase is characterized by much larger interchain separations[58,59,60].

Figure 4: Left: Typical diffraction profiles for unoriented (a) poly(3-hexylthiophene), (b) poly(3-octylthiophene) and (c) poly(3-dodecylthiophene) films ($\sim $75% HT linkages). Also (c$'$) an equatorial $(hk0)$ profile and (c$''$) a meridional $(00\ell )$ profile from a uniaxially stretched film of poly(3-dodecylthiophene). The arrows identify weak $(h20)$ peaks associated with a full 3D ordering. Right: Comparison between calculated structure factor and powder diffraction data for poly(3-octylthiophene) in a second crystalline polymorph (a=15.3 Å, b=9.43 Å, c=8.07 Å and $\gamma $=72$^\circ $, from Ref. [59]). This model includes appreciable alkyl chain interdigitation from adjoining layers.

P3ATs also manifest a pronounced competition between structural ordering of the core components (i.e., the polythiophene backbone and adjacent CH$_2$ units) and that at the terminating alkyl side chain units. For decyl and somewhat longer side chains one can resolve distinctive peaks on the high angle side of the (020) reflection. These features are indicative of full 3D order and correlate with a secondary crystallization at the alkyl chain ends. This new characteristic is not necessarily commensurate with the intrastack chain spacing (which is perpendicular to both the lamellar spacing and the chain axis) and so perturbs the lamellar stacking of the core backbone. As evidence of this effect there are always systematic (h00) order dependent lineshape and peak width variations[61] as depicted in Fig. 5. This process is consistent with the classic effects of one-dimensional strain and paracrystallinity[62]. Increasing temperature first initiates melting at the alkyl chain ends and this destroys the layer to layer 3D ordering. Thus the high temperature LCP mesophase is still characterized by well defined layers but they are effectively free floating relative to one another. Each layer is therefore able to relax locally and so there is a distinctive reduction in the low order $(h00)$ peak widths. Often this narrowing is interpreted as an apparent increase in overall coherence length[63] but this will only be true if the lineshapes remain narrow in all resolved ($h00$) orders even after slow cooling back to the crystalline phase.

Figure 5: Equatorial (h00) X-ray diffraction profiles from uniaxially stretched (approximately 4:1 draw ratio) poly(3-dodecylthiophene) film at selected temperatures (and offset for clarity). The bottom arrow identifies a weak peak associated with the interdigitated crystalline polymorph (see Ref. [59]). Inset: (h00) peaks widths versus temperature spanning the transition from crystalline state to LCP mesophase.

This order-disorder transition (ODT) is nearly coincident with a rapid increase in the average torsional angle between adjacent thiophene units. This latter effect, in terms of optical properties, gives rise to a very pronounced thermochromism[64,65]. The low temperature phase, characterized by a more planar backbone, appears deep red while the higher temperature LCP phase is quite yellow in color. It may well be that there are actually two distinct LCP phases because, on cooling, the ODT is found to be extremely sensitive to the thermal history[66]. Levon and coworkers[67] have convincingly shown that modest undercooling correlates with a uniform appearance of a more orange film while stronger undercooling initiates nucleation and growth of red domains in the more orange background. Directly correlating the evolution in the optical properties with changes in the X-ray diffraction has been difficult to achieve. In terms of transport and device physics this sensitivity to thermal history is very important because of the presence of inhomogeneities and ``grain'' boundaries. Electron transport will be strongly modified by the presence of this heterogeneous structure.

Further control of conjugated polymer microstructure in general remains a key issue and there are important considerations at length scales beyond those of nearest-neighbor interactions and simple 3D packing. In solutions of a good solvent P3AT's are relatively coiled but as the solvent quality becomes poorer there are competing processes that impact the evolution to the solid state. Increasing $\pi $-conjugation lowers the electronic energy and so this tends to planarize the polymer chain locally. From an interfacial perspective it is energetically favorable to coil up more tightly in order to exclude solvent. Thus there is a clear impetus towards chain folding. Ihn et. al[68] have demonstrated that slowly precipitated P3ATs clearly exhibit a fine whisker type morphology that includes a chain folded structure. In these materials the chain axis direction is seen to be normal to the long axis of the whisker. Structures of this type tend to both limit the maximum effective conjugation length and reduce charge transport along the polythiophene backbone. In more recent work Ikkala and coworkers[69] have shown that it possible to use functionalization of the main chain and side chain substituents and selectively control the solution microstructure at larger length scales through an approach which exploits the property of molecular recognition.

Few other linear substituted electronic polymer have been studied as extensively or as thoroughly as the P3ATs. Although side chain addition almost always produces material with improved solubility, many of these alternative polymers are not especially well ordered. For rR-P3ATs there seems to be a number of complementary geometric factors working in its favor. The 7.8 Å intrachain spacing between adjacent alkyl chains along each side of the polymer is nearly optimal for introducing a $c/2$ staggering of the thiophene backbone and alkyl chains of the two nearest-neighbor chains (above and below). With a small setting angle tilt of the thiophene skeletal plane and a nominal 45$^\circ $ tilt (from twisting about the C-C bond to the thiophene ring), the net area per side chain becomes $7.8\times 3.8 \cos 15^\circ \cos 45^\circ $ or 20 Å$^2$ and comparable to that of polyethylene (within 10%).

In many phenylene ring containing polymers there are torsional degrees of freedom about the paraphenylene linkages. This introduces structure disorder, in terms of a conformational isomerism as shown in the Fig. 6 inset, and functions much in the same way as regioirregularity does in P3ATs. Many of these polymers exhibit some indications of a layered structure but the large broad peak centered about a $2\theta$ angle of 21$^\circ $ is a common telltale indicator of extensive side chain disorder. Figure 6 shows a series of X-ray diffraction curves from precipitated packed powders in one example copolymer thienyl-phenylene family. In this sample set there is a often a large proportion of small-angle scattering intensity onto which a single low angle peak is superimposed. As expected there is an inverse relationship between the peak position and side chain length. Extended annealing at high temperatures significantly increases the structural order[70] presumably by reducing disorder associated with conformational isomers. Liquid crystalline polymer phases were also observed. Ideally one would like to further develop design strategies which enhance the molecular level architecture and in so doing achieve controllable backbone conformations with and without the presence of regio specificity, tacticity and/or crystallization.

Figure 6: X-ray powder diffraction profiles from various indicated derivatives. All curves offset for clarity Inset: Chemical sketch of basic polymer structure and phenylene ring derived conformational isomerism. (Polymer samples courtesy of J.R. Reynolds.)

Winokur 2004-01-28