Polydiacetylene crystals optical properties

Polydiacetylenes are obtained as single crystals by topochemical solid-state polymerization of the monomer single crystal. These compounds have received considerable attention because of their one-dimensionally -conjugated structure. Their unique --electron structures, and therefore superior third-order nonlinear optical properties, have been extensively investigated. 

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... 

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. 

Microcrystals of some diacetylenes, prepared by the reprecipitation method, have been studied as dispersions in liquid media. Interesting behavior has been observed in the solid-state polymerization of diacetylene monomers and with the optical properties of polydiacetylene (PDA) microcrystals. First, the polymerization perfectly proceeded from one end to the other end of the diacetylene microcrystals. Next, the excitonic absorption peak position was found to shift to higher energy side with decreasing size of the PDA microcrystals. The size effect was observed even for crystals as large as 100 nm or more in contrast to conventional quantum effect of inorganic semiconductors where size effect is observed only for microciystals of less than about 10 nm size. In addition, since the microcrystal dispersions in water have low optical loss, the c tical Kerr shutter response of PDA microciystals could be measured, and the non-resonant value was estimated to be on the order of 10 to 10" esu in very low concentrations (ca. 10 M). 

In this scenario, polydiacetylenes were nevertheless interesting, as the mechanisms of their large non-linearity form a good basis to build on. Only a few modifications were well characterised and there was a general lack of measurements with single crystals due to problems with sample preparation. Therefore, the aim was to characterise systematically the influence of side groups on the optical properties, preferably in the macroscopically ordered, stable, and reproducible framework of good and - later on - thin crystals. 

Hankin, S.H.W. Sandman, D.J., "Recent Studies of Raman Spectroscopy of Polydiacetylene Crystals Poly- Ipudo", p. 661 in Chiang, L.Y. Garito, A.F. Sandman, D.J. (Eds.), Mat. Res. Soc. Symp. Proc., Vol. 247 Electrical, Optical, and Magnetic Properties of Organic Solid State Materials, Materials Research Society, Pittsburgh, Pennsylvania, USA (1992). 

As it was pointed out earlier, the optical and photoelectrical properties of the polydiacetylenes may be explained in the framework of ID crystal model in which the interaction between the electrons in the chain is much stronger than between the chains. The interaction energy differs by 100 fold. Considering the anisotropy of the optical and photoelectrical properties one can expect this fact. This was actually observed for monocrystals and films of the polydiacetylene. Absorption spectra of the monocrystal films are presented in Fig. 21 for varying light polarization [141]. 

The complex morphology of most conjugated polymers prevents direct measurement, and hinders understanding, of their intrinsic properties. Such problems do not occur for the polydiacetylenes, which can be prepared as macroscopic single crystals. Since polydiacetylenes can also be obtained in less perfect forms the effects of disorder can be studied. Spectroscopic techniques have been widely used to study the intrinsic properties of polydiacetylenes and their modification by disorder. The results obtained by optical and electron spin resonance spectroscopy are discussed. 

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. 

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... 

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