Polyacetylene

The best known member within this family of linear polymers, polyacetylene (PA)[18,19], has been the subject of numerous structural studies. One of the earliest reported structural references to polyacetylene[20] is in 1958. With the advent of the Shirakawa synthesis[21] high-quality fibrillar PA films (s-PA) first became available. In general these films (and PA samples prepared using alternate processes[22]) are found to be predominately crystalline in nature. Even the early studies of these films identified a rich interplay in the structural ordering. When s-PA is synthesized at relatively low temperatures the PA polymer chains are found to exist primarily in a cis conformation, as shown in Fig. 3, within an orthorhombic Pnma unit cell[23,24,25,26]. Of the two possible cis forms, cis-transoid or trans-cisoid, structural studies indicate a preference for the cis-transoid isomer. Thermal annealing[27] at ca. 150C or doping induces a solid state transformation to the highly conducting trans conformational form with associated changes in the crystal structure.

 

Figure 3: The two possible isomers for cis-polyacetylene.

Scattering studies of trans-PA have given rise to numerous debates concerning its three-dimensional (3D) ground state crystal structure. All experimental studies are consistent with the 2D p2gg equatorial packing of the PA chains. With respect to the 3D unit cell three different space groups, P2₁/a[28], P2₁/n[,30,31] and Pnam[32], have all been proposed. The difficulty in reaching a definitive answer can be attributed to the smallness of the monoclinic distortion, $\beta\approx 92^\circ$, incomplete thermal cis-trans conversion of test samples, limitations of the PA structural ordering within the films, and the subtle differences among the proposed structures.

In particular there has been considerable controversy, both experimental and theoretical[33,34,35,36], concerning the relative phase relationship between the two chains which contribute to the PA unit cell and in the degree of bond alternation between the C-C single and double bond lengths. From a theoretical standpoint PA is a very attractive model compound because the small number of CH units (four) which comprise the unit cell enables comprehensive total energy calculations. The P2₁/a structure has the two chains in an in-phase relationship while the P2₁/n structure yields an out-of-phase arrangement as shown in Fig. 4. An early analysis of the non-equatorial scattering intensities by Fincher et al.[] found strong evidence for P2₁/n symmetry with double and single bond lengths of 1.44Å and 1.36Å respectively. While other claims have been made in the interim period, the most recent studies available[31] continue to support this model with bond lengths close to those just stated.

 

Figure 4: Four different ``possible'' equilibrium trans-PA structures. (a) The two chains are in phase; (b) the chains are in phase but the second chain (on right) is rotated by $\pi$; (c) the chains are out of phase and the second chain is rotated by $\pi$; (d) the chains are out of phase. Adapted from Ref. [33].