Diacetylene polymer crystals

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 ... 

Colourless diacetylene monomer crystals can be polymerized under heat, ultraviolet. X-ray or y-ray irradiation to form single-crystal, highly coloured polyacetylenes. The solid state reaction transforms the entire monomer crystal to polymer crystal without phase separation the polymer forms a solid solution with the monomer over the entire... 

Diacetylenes of the type shown in Scheme 13 can react under thermal or photochemical conditions to give crystalline polymers [80]. The change from colorless monomer to brightly colored polymer crystals of the same shape can occur... 

Upon exposure to heat, UV- or y-radiation diacetylenes are converted from a soluble monomer crystal which is transparent if pure, i.e., free of residual polymer, to a deeply colored polymer crystal. With a few exceptions, the latter is insoluble in all common solvents. The color arises from the lowest n-electron transition of the conjugated polymer backbone, which has its maximum near 600 nm. It is of excitonic origin 48-50) carries an oscillator strength of the order unity. Both insolubility and optical... 

This contribution gives a review of recent spectroscopic investigations concerning the photophysical and photochemical primary and secondary processes of the solid state polymerization reaction in diacetylene single crystals. It will be shown, that diacetylenes are an unique model system for the study of the reaction mechanism of a solid state chemical reaction which is characterized by a variety of reaction intermediates. The polymerization reaction in these crystals is of special importance, due to the resulting polymer single crystals, which exhibit extraordinary anisotropic physical properties. 

Fully conjugated and fully chain-aligned polymer single crystals with planar polymer backbone are obtained, which may have the alternative acetylene (ynene) or butatriene structures of Eq. (1). From our experiment we know that the acetylene structure is dominant in the polymer molecules. Up to now the best investigated diacetylene crystals are the TS-6 monomer crystals and the corresponding polymer crystals (poly TS-6). The substituents R and the notation of further diacetylene crystals discussed below are listed in Table 1. 

Figure 8. Top, optical absorption and the corresponding prompt fluorescence spectra of diacetylene polymer molecules middle, excitation and the corresponding delayed emission spectra and bottom, transient absorption for a partially polymerized (broad line) and fully polymerized (narrow line) diacetylene crystal.

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. 

Fig. 1.11 Below Two single crystals of the polydiacetylene paratoluyl-sulfonyl-oximethylene-diacetylene (TS6). Above three monomer crystals, illuminated with linearly polarised light. The polarisation direction of the light is horizontal, and the b axis of the polymer chains is oriented parallel to the long axis of the crystals. The polymer crystals strongly reflect light (below left) when the light...

Fig. 3.13 Projections of the TS-diacetylene monomers (above) and polymers (below) onto the plane of the polymer backbone in the poly-diacetylene crystal. The lattice constant b of the monomer crystal and the polymer crystal differ by only 0.2 A. Therefore, the crystal is not destroyed by the polymerisation the macroscopic polymer crystal is formed from the macroscopic monomer crystal [13].

Phase-matched second harmonic generation in single crystal polymers was first observed in 2-methyl-4-nitroaniline substituted diacetylene polymers. Subsequently, a number of other diacetylene structures... 

Besides single crystals, disubstituted diacetylene polymer thin films can be obtained for waveguide applications by evaporation, solidification, or deposition of the diacetylene monomers by the Langmuir-Blodgett approach, which allows film thicknesses to be controlled at the molecular level. These diacetylene films can be patterned using selective polymerization techniques such as developed for... 

If the crystal structures of the initial monomer and the polymer produced are crystallographically related, the polymerization is described as being topochemical or topotactic. Suitably substituted diacetylenes are so arranged in the crystal lattice that the conjugated triple bonds represent the steps of a ladder. Substituents linked by hydrogen bonding represent the runners of the ladder. A suitable substituent is, for example, —CH2—O—CO—NH—C6H5. A kind of shear takes place on polymerization, since the density difference between monomer and polymer crystals is small ... 

The polydiacetylenes and polytriacetylenes differ from polyacetylene because preorganization of the diacetylene and triacetylene is required for a successful polymerization (7). This remarkable observation was first recognized (8,9) in 1969 and marks the beginning of modern polydiacetylene and polytriacetylene chemistry. In a few cases, this topochemically controlled polymerization occurs from a crystal of the monomer to a crystal of the polymer, giving rare examples of macroscopic single polymer crystals (9). 

Macro Crystals.—Nearly defect-free crystals of polymers have been produced by direct polymerization of monomer at gas-solid or liquid-solid interface. Poly-(diacetylene) single crystals have been produced from irradiation of the crystalline monomer and involves the instantaneous polymerization and crystallization of... 

Enkelmann et al. reported the monomer structure of a diacetylene derivative and made clear the mechanism of the polymerization in 1980 as shown in Scheme 2.3 [6, 7]. Since the radiation polymerization usually occurs very quickly, it was difficult to analyze both structures of the monomer and the polymer. However, the 1,6-di (iV-carbazolyl)-2,4-hexadiyne (DCH) is stable and both the monomer and polymer crystal structures were successfully analyzed as shown in Fig. 2.6. Since the stacking distance of the monomer is 4.55 A and the produced polymer unit is 4.91 A, the very complicated SCSC transformation should occur in a monomer crystal. 

In a solid state polymerization reaction monomer diacetylene crystals are transformed to polymer crystals in successive reaction steps. Nearly perfect polymer single crystals are obtained thermally (kT) or by UV- or X-ray irradiation (hv) of the monomer crystals [1-3]. Within the class of diacetylene molecules (R-C=C-C=C-R) which show this unusual chemical reaction, the TSHD (with side groups R = -CH2SO2-0-CH2) is the best known representative, which is characterized by a variety of reaction intermediates [4-19]. The unconventional reactivity and the unusual properties of the polymer crystal have attracted the interest of both, physicists and chemists. The general feature of the low temperature photopolymerization reaction is shown schematically in Fig. 1 by example of the diradical DR-intermediates. 

MICROSTRUCTURES AND POLYMER CHAIN LENGTH IN DIACETYLENE SINGLE CRYSTALS... 

Application of Eqn. 1 to the data in Fig. 1 over the 1 to 10 eV range gave the absorption spectra for light polarised parallel and perpendicular to the b-direction for both the monomer and polymer crystals (3). The results in Fig. 2 show clearly that peaks A, A, and B must be associated with electronic transitions of the polymer backbone as they do not appear in the monomer spectra. In addition the direction of N for the transitions must lie approximately along the polymer chains as the absorption bands only appear for the E V b spectrum. Peak C Which a >ears in the absorption spectrum of the monomer but not the polymer h been identified with the diacetylene moiety of the monomer molecule. The rest of the features % re attributed to electronic transitions on the sidegroups. 

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