Polydiacetylene crystals

The polydiacetylene crystals (1-4) most strikingly corroborate these conjectures. Along this line of thought is also shown that this electron-phonon interaction is intimately interwoven with the polymerisation process in these materials and plays a profound role there. We make the conjecture that this occurs through the motion of an unpaired electron in a non-bonding p-orbital dressed with a bending mode and guided by a classical intermolecular mode. Such a polaron type diffusion combined with the theory of non radiative transitions explains the essentials of the spectral characteristics of the materials as well as their polymerisation dynamics. ... 

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... 

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. 

Any theoretical approach to conduction in polydiacetylene crystals should therefore be capable of explaining these observations (especially 2), and should permit differentiation between the band and excitonic pictures. Numerous optical studies on fully and partially polymerized samples have revealed the following additional phenomena (see for example Bloor et al., 1974, 1976a, b Reimer et al., 1976). 

The perfection of the polymer chains in polydiacetylene crystals results in an absorption spectrum with a narrow electronic transition and sidebands due to vibrations of the backbone double and triple bonds. The absorption is... 

Owing to the mechanism of the topochemical reaction the polyconjugated polymer chain is of exceptional purity and stereochemical regularity. Polydiacetylene crystals are thus ideally suited to study the inherent optical and electrical properties of polyconjugated chains. These unique features have attracted considerable attention and in recent years the topochemical polymerization of diacetylenes has developed to... 

In a solid state polymerization reaction monomer crystals of diacetylene molecules (R-CiC-C=C-R) are converted to polydiacetylene crystals (1,2). The primary photochemical processes during the low-temperature photopolymerization reaction have been investigated by ESR (3,4) and optical absorption spectroscopy (5,6). A review ofthe spectroscopy of the intermediate states has been given by Sixl (V. A simple reaction scheme is shown in Figure 1. The reaction is characterized by the uv-photolnitiation of dira-dlcal dimer molecules. Chain propagation is performed by thermal addition of monomer molecules. Thus trimer, tetra-mer, pentamer etc. molecules are obtained. 

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. 

The reaction shown in Fig. 5 is an example of a topo-chemical polymerization in which conjugate, 1,4-addition of 1,3-diyne units take place in the crystalline state. The reaction is performed by irradiating the crystals with visible or ultraviolet light. X-rays or gamma quanta, or by annealing the crystals at a temperature below their melting point. The unreacted monomer is then removed by extraction with a suitable solvent and leaves behind a single deeply coloured polydiacetylene crystal. This unique polymerization process has been intensively studied and has been the subject of many reviews [15]. 

The Franz-Keldysh effects (Weiser and Horvath 1997) have been successfully used to distinguish the particle-hole continuum from exciton states in polydiacetylene crystals (Sebastian and Weiser 1981). 

Figure 10.2 illustrates the electroabsorption spectrum of phenyl-substituted traras-polyacetylene thin film (Liess et al. 1997). The feature at 2.0 eV is the red-shifted l Bu exciton. The feature at 2.5 eV is attributed to a dipole-forbidden state, namely the m Ag state. Unlike polydiacetylene crystals, disordered trans-polyacetylene thin film does not exhibit Pranz-Keldysh oscillations (described in Chapter 8) and therefore a definite assignment of a conduction band edge cannot made. However, because disordered polydiacetylene also does not exhibit Pranz-Keldysh oscillations, but a smeared-out feature similar to the one exhibited at 2.5 eV in Fig. 10.2 it is sometimes assumed that this feature does mark the band edge. Another interpretation is that this feature represents the n = 2 Mott-Hubbard exciton, described in Chapter 6, with the particle-hole continuum lying close in energy (possibly at 2.7 eV, which is three times the THG feature at... 

One of the interesting aspect of conjugated polymers is their ability to behave as electrical conductors (44) (see Electrically-Conducting Polymers). The metallic appearance of many polydiacetylene crystals suggests they would be good candidates as electrical conductors. However, these organic materials are insulators with conductivities less than 10 Polyacetylene also shows low conductivity... 

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. ... 

B.S. Elman, D.J. Sandman, M.K. Thakur, M. A. Newkirk, E.F. Kennedy, Ion Beam Modification of Polydiacetylene Crystals," presented at the International Conference on Ion Beam Modification of Materials, Cornell University, Ithaca, NY, July 16-20, 1984 B. S. Elman, M. K. Thakur, D. J. Sandman, M. A. Newkirk, and E. F. Kennedy, J. Appl. Phys., submitted B.S. Elman, D.J. Sandman, and M.A. Newkirk, Appl. Phys. Lett., in press. 

It is important not to lose perspective when considering the defect sensitive properties of polydiacetylene and (SN) crystals. Although (SN) produced by solid-state reaction is not well ordered the crystalline perfection of this polymer is much higher than for conventional organic polymers. Polydiacetylene crystals are obtainable... 

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