Polymer Single Crystals Diacetylenes

This is particularly true for the interesting members of the group of radical-ion salts, in which the acceptance or donation of an additional charge by an anorganic counterion occurs, such as in (FajJ PFg (Fig. 2.18), or by a metal ion as in the salts ofCu+(DCNQI)2 (Fig. 1.7). 

The computation of the structure of crystals with charge transfer is difficult, since here different, anisotropic intermolecular interactions contribute in the determination of the minimum energy. The crystal-growth conditions play a still stronger role than in crystals consisting of only one type of molecules, as do purity and the temperature. In general, the theoretical prediction of the structures of charge-transfer salts is hardly possible. 

There are also macroscopic polymer single crystals [8-11], which have a tt-electron system along the polymeric carbon chains (Fig. 1.11) e.g. the poly-diacetylene crystals (Fig. 1.10). They are formed from monomeric diacetylene crystals via a so called topochemical reaction (Fig. 2.19 and Chap. 3). Topochemical refers to reactions of molecules in crystals in which the reactivity of the molecules is less important than 

c) = 104°) and contains two fluoranthene molecules per unit cell. They are stacked along the a axis with two alternating spacings (0.328 nm and 0.333 nm). Two each of the fluoranthene molecules form the radical cation (Fa)2 PFg. The stacking axis a is perpendicular to the planes of the fluoranthene molecules. The PFg anions are located in the channels between the fluoranthene molecules. From [10]. 

The solid-state polymerisation of the diacetylenes can be initiated thermally and/or through irradiation with UV or y radiation and leads for many substituents R to macroscopic poly-diacetylene single crystals. For most substituents R, it is irreversible. Table 2.5 gives a selection of substituents R with which macroscopic diacetylene crystals have been successfully grown. Additional examples can be found in Enkelmann [11]. 

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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. 

Some important aspects of topochemical polymerizations can be understood by inspection of Eq. (1), All reactivity comes about by very specific rotations of the monomers and by 1,4-addition of adjacent units and an extended, fully conjugated polymer chain is formed. The unique feature of the topochemical polymerization of diacetylenes is the fact that in many cases the reaction can be carried out as a single phase process. This leads to macroscopic, defect-free polymer single crystals which cannot be obtained, in principle, by crystallization of ready-made polymers by conventional methods. Thus, polydiacetylenes are ideal models for the investigation of the behaviour of macromolecules in their perfect three dimensional crystal lattice. 

Macroscopic polymer single crystals with a high degree of perfection can be produced by the homogeneous topochemical polymerization of conjugated diacetylene monomers (1). The polydiacetylenes thus formed have a fully conjugated planar backbone and possess unusual optical and electronic properties... 

An interesting special case of crystal growth from solution are the macroscopic polymer single crystals of diacetylenene, the poly-diacetylene crystals. For their preparation, as a first step, macroscopic molecular crystals of substituted diacety-... 

Diacetylene derivatives are known to polymerize in a lattice controlled reaction. Polymerization proceeds via formation of a solid solution of the polymer in the monomer lattice. For a number of compounds the formation of polymer single crystals has been demonstrated... 

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. 

The synthetic method utilizes the three dimensional periodicity of a crystalline monomer phase as a template to determine the molecular and crystallographic structures of a polymer(4-12). In the ideal case, solid state reaction (initiated either thermally, mechanically, or by exposure to actinic radiation) transforms a monomer single crystal to a polymer single crystal with nearly the same dimensions and similar structural perfection. G. Wegner first demonstrated that certain diacetylene reactions closely approximate this ideal case(4) ... 

Phase separation occurs during certain diacetylene reactions, despite the fact that a continuous monomer-to-polymer single crystal transformation is not forbidden by symmetry considerations. For example, the diacetylene with substituent groups -CH2OH can be pol)nnerized to a limiting conversion of about 70% as a one-phase reaction. Annealing this partially pol3nnerized phase results in phase separation to produce a non crystalline polymer phase and the initial monomer phase(6). 

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 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. 

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... 

Poly diacetylenes. The polydiacetylenes (PDA s) are unique among highly conducting polymers discovered in the past years in that they can be obtained as highly perfect macroscopic single crystals.68 Upon solid-state polymerization of... 

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