Diacetylenes structural studies

J. P. Aim6, M. Schott, M. Bertault, and L. Toupet, Study of reactive diacetylenes Structure of monomer and polymer pFBS crystals, Acta Crvst. B44 617 (1988). 

In addition to studies of diacetylene single crystals, current research, activities are focused on studies of the second X and third x order nonlinear optical responses of disubstituted diacetylene polymer films as active optical guided wave structures. Diacetylene polymers possess X values comparable to germanium(j 7). In the first stage, three major questions are being addressed ... 

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. 

Influence of subphase temperature, pH, and molecular structure of the lipids on their phase behavior can easily be studied by means of this method. The effect of chain length and structure of polymerizable and natural lecithins is illustrated in Figure 5. At 30°C distearoyllecithin is still fully in the condensed state (33), whereas butadiene lecithin (4), which carries the same numEer of C-atoms per alkyl chain, is already completely in the expanded state (34). Although diacetylene lecithin (6) bears 26 C-atoms per chain, it forms both an expanded and a condensed phase at 30°C. The reason for these marked differences is the disturbance of the packing of the hydrophobic side chains by the double and triple bonds of the polymerizable lipids. At 2°C, however, all three lecithins are in the condensed state. Chapman (27) reports about the surface pressure area isotherms of two homologs of (6) containing 23 and 25 C-atoms per chain. These compounds exhibit expanded phases even at subphase temperatures as low as 7°C. 

The most reliable information on the structure of the polymer has been derived from X-ray diffraction and Raman-spectroscopic studies. In a few cases it has been possible to determine in detail the structures of both the monomer and the corresponding polymer. From such measurements and other optical studies the process is considered to be 132 — 133 for a symmetric diacetylene. In polymerizable structures the diacetylene rods are inclined at about 45° to the translation (stack) axis, with the ends of each diacetylene moiety approaching the adjacent triple-bond systems to a distance of s4 A. The polymer is a planar system in extended conformation and having alternate R groups trans to one another. 

The extensive studies of the behavior of mixed monolayers or bilayers of di-acetylenic lipids and other amphiphiles parallel to some degree the studies of dienoyl-substituted amphiphiles. Since the dienoyl lipids do not contain a rigid diacetylenic group in the middle of the hydrophobic chains, they tend to be miscible with other lipids over a wide range of temperatures and compositions. In order to decrease the lipid miscibility of certain dienoyl amphiphiles, Ringsdorf and coworkers utilized the well-known insolubility of hydrocarbons and fluorocarbons. Thus two amphiphiles were prepared, one with hydrocarbon chains and the other with fluorocarbon chains, in order to reduce their ability to mix with one another in the bilayer. Of course it is necessary to demonstrate that the lipids form a mixed lipid bilayer rather than independent structures. Elbert et al. used freeze fracture electron microscopy to demonstrate that a molar mixture of 95% DM PC and 5% of a fluorinated amphiphile formed phase-separated mixed bilayers [39]. Electron micrographs showed extensive regions of the ripple phase (Pb phase) of the DM PC and occasional smooth patches that were attributed to the fluorinated lipid. In some instances it is possible to... 

An experimental study of diacetylene and HF in solid argon suggested both sorts of complexes (a and b in Fig. 6.6) were present and that they are of comparable stability. Correlated (MP2) calculations with a 6-31 -I- -l-G(d,p) basis set found the perpendicular complex (a in Fig. 6.6), wherein FH approaches one of the two triple bonds of diacetylene, is more stable than is complex b wherein C—H acts as proton donor. The electronic contributions to the binding energies of complexes a and b are calculated to be —3.8 and —2.6 kcal/rnol, respectively. However, these values are surely inflated by the failure to correct them for BSSE. One can conclude that the triple bond is a better proton acceptor than the alkynic C—H is a donor, at least when paired with the rather strong acid HF. The preference for this sort of geometry is confirmed by gas-phase measurements, and are valid also when HF is replaced by HCl . The importance of using a satisfactory level of theory for such complexes is reinforced by comparison with earlier SCF-level calculations which predicted a structure like b to be most stable. 

Although interesting within the framework of polymer physics and material science this would not be sufficient to attract so many workers from areas outside of conventional polymer research. Additional interest arouse because of the unusual structure of the polymers obtained via solid-state polymerization of diacetylenes and because of the mechanistic features related to its formation. Polydiacetylenes exhibit a fully conjugated and planar backbone in the crystalline state and are thus considered the prototype study object as far as the nature and physical behavior of polyconjugated macromolecules are concerned Theoretical discussions of the electronic structure of these polymers (2) lead to a description in terms of a wide band one-dimensional semiconductor... 

It is the main purpose of the following articles by V. Enkelmann, H. Bassler and H. Sixl to review the present status of polydiacetylene research from the point of view of structure and reactivity including all the details known on the mechanism of polymerization of various diacetylenes. The material science aspects will not be treated to the same depth with exception of the photopolymerization and its possible application in the contribution of H. Bassler. Similarly, the solution properties of polydiacetylenes are not touched upon. The interested reader is refered to the current literature where the problems encountered when studying the solutions and the recrystallization behaviour of polyconjugated macromolecules have just started to be discussed adding a new chapter to the statistical mechanics and hydrodynamic behaviour of macromolecules... 

A series of [n]paracyclophadiynes or [tw.nlparacyclophadiynes (12) with different numbers of methylenes was prepared for the study of the transannular electronic interaction between a diacetylene unit and a benzene ring . The absorption spectra of 12 show features which are obvious when compared with that of a reference compound 15c (Figure 1) (/) disappearance of vibrational fine structures with decrease of the methylene number, (//) bathochromic shift of the longest wavelength bands and (ii7) appearance of a new band at 233 nm for 12c and at 227 nm for 12d,... 

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