Polydiacetylenes crystal structures

The two-step process of epitaxial polymerization has been applied to symmetrically substituted diacetylenes First, the monomers have been crystallized epitaxially on alkali halides substrates from solution and the vapor phase. The oriented monomer crystals are then polymerized under the substrate s influence by gamma-irradiation. The diacetylenes in this study are 2,4-hexadiyn-l,6-diol (HD) and the bis-phenylurethane of 5,7-dodecadiyn-l,12-diol (TCDU). The polydiacetylene crystal structures and morphologies have been examined with the electron microscope. Reactivity and polymorphism are found to be controlled by the substrate. 

Polydiacetylene crystals. The enhancement of x because of one-dimensional electron delocalization is strikingly corroborated in the polydiacetylene crystals. Their structure is that of a super alternated chain with four atoms per unit cell and the Huckel approximation yields four bands for the ir-electrons, two valence and two conduction bands. When depicted in the extended Jones zone, each pair can be viewed as arising by a discontinuity at the middle of the Brillouin zone of the polyene chain. The dominant contribution to X(2n 1) comes from the critical point at the edge of the extended Jones zone (initially at the center of the reduced B.Z.). The complete expressions are derived in (4,22) and calculated for different polydiacetylenes. We reproduce the values of x 2 for TCDU and PTS in table IV. The calculated values are in good agreement... 

Many aspects of the preparation and properties of polydiacetylenes are the subject of lively debate. This review presents recent results that bear on some of these controversies. First the relationship of diacetylene monomer crystal structure and solid-state reactivity is discussed. Secondly the temporal evolution of solvato-chromio transitions of soluble polydiacetylenes is displayed. Optical and Raman spectra reveal the occurrence of an intermediate form of the polymer. A model compatible with these results is described. 

This reaction, called a four-center photopolymerization, is a typical example of topochemical reactions used to prepare polymer crystals.5 The changes in higher-order structure during the reaction are shown in Table 2.5 . Various polydiacetylene crystals have also been prepared by solid-state photopolymerization of diacetylene monomer crystals, such as 1,6-dicarbazoyl-2,4-hexadiene. These syntheses have attracted considerable interest, since they can lead to organic materials of high conductivity or of nonlinear optical properties. 

Chain Packing and Crystal Structures. The chain packing and the suhmolecular arrangement of repeat units and pendant side groups of macromolecules in crystalline domains of polymers can be visualized using contact mode SFM. The resolution is in most cases not true resolution, since the area of the contact area (1 — few nm ) exceeds the molecular scale and must be considered lattice resolution instead. The first example of molecularly resolved structures of a polymer dates back to 1988, when Marti and co-workers reported on an SFM study on a polydiacetylene film (128). Examples for resolved chain packing and polymer crystal structure determination at the surface of semicrystalline polymers include poly(tetrafiuoroethylene) (PTFE) (129,130), polyethylene (PE) (131-133), polypropylene (PP) (134,135), poly(ethylene oxide) (PEO) (136), aramids (137,138), and poly(oxy methylene) (POM) (139). 

This agrees both with X-ray structure investigations on polydiacetylene crystals and with spectroscopic investigations on small oligomer radicals (19) which upon increasing the chain length convert from initially buta-trienic structures to PDA-type structures. Transitions from PDA to PBT in longer chains are therefore very improbable. ... 

Under favourable packing conditions, diacetylene single crystals exposed to heat, radiation, or pressure react to form high quality polydiacetylene single crystals. Structural aspects of monomer reactivity are fairly well understood (at least at a qualitative level) via models based on least motion principles. However, there remain many unanswered questions regarding the detailed chemistry and physics of reaction initiation, propagation, and termination. 

As a consequence of both the unusual backbone structures and the comparatively high perfection of (SN) and polydiacetylene crystals, electrical properties are observed which differ dramatically from conventional polymers. However, the conductive properties of (SN) and the photoconductive properties of the polydiacetylenes are still limited by phase imperfections. 

The diacetylene with R = R = paratoluylsulfonyloximethylene is termed TS6 (sometimes TS). It was shown by G. Wegner and his co-workers in a series of works, pubhshed in the early 1970s [1, 7], that large molecular crystals can be grown from a solution of TS6, e.g. in acetone, and that these monomer diacetylene crystals can be converted by a topo-chemical (or solid state) 1,4-addition reaction to the polydiacetylene single crystals (Fig. 9.2). The crystal structures of TS before and after the reaction have been investigated in detail by Kobelt and Paulus [8], Bloor et al. [8], and Enkelmann [9] and are sketched in Fig. 9.3 and in Tab. 9.1. 

Inspection of the known crystal structures of polydiacetylenes reveals that two common intermolecular interactions have been observed to establish the important spacing of the monomers. These are hydrogen bonding and, more commonly, n-n stacking [12] particularly of aromatic sulfonates. 

Scheme 5.3. Crystal structures illustrating the conversion of the host-guest compound 3—4 d = 4.97 A, Ri 4 = 3.38 A and < > = 43°) to the corresponding polydiacetylene.

As example of the use of exciton theory, the visible optical spectrum of two typical polydiacetylene crystals calculated by S. Suhai (PTS with an acetylene-like structure and TCDU with a butatriene-like structure) are presented using a first principle Greenes function formalism of charge transfer exciton theory. 

Polydiacetylenes (2) come closest to the model one-dimensional organic semiconductor and can be readily obtained in form of large, nearly defect-free single crystals so that a large number of experiments and measurements have been carried out on these materials. Their structure is shown in Figure 3 where also some typical side-groups R are indicated. On table I we summarize some measured va-... 

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

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