Organic Crystals of High Conductivity

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 simplest organic metals can in fact be produced starting from naphthalene and similar arenes such as pyrene, perylene, fluoranthene or triphenylene. 4-16 if these compounds are electro-chemically oxidized in an otherwise inert solvent containing a supporting electrolyte with anions of low nucleophili city, coloured crystals of high conductivity will grow at the anode. These crystals... 

As an example for the /3-(ET)2X family the crystal structure of the salt with X = Ig is shown in Fig. 2.14 [127, 128]. /9-(ET)2l3 was the second ambient-pressure organic superconductor (Tc 1.5 K) to be found [129]. The principal structure of most ET salts can be seen in Fig. 2.14. The ET molecules are arranged in columnar stacks within planes which are separated by anion layers. The 13-type structure is triclinic with the lattice parameters as given in Table 2.1. The ET molecules are stacked along the a- -b direction and are slightly dimerized. S-S contacts less than the van der Waals radii of 3.6 A are found in the ab plane only between the stacks and not within the stacks. However, due to their 2D connectivity (see Fig. 2.14b) the ET form sheets of high conductivity in the ab planes which are separated by insulating layers of linear Ig anions. 

Organic crystals with high dark conductivities have become an active and extensive area of research in recent decades. We refer the reader to the review articles [Ml, M4, M5] and [4]. 

In three-dimensional mixed-valence systems, electron transfer can manifest itself as electrical conduction, thermally activated. Most work continues to focus on the better known semiconducting materials such as silicon-boron or silicon nitride " (at low temperature), or organic crystals of the anthracene type (at high temperature),or redox polymer-coated electrodes. In the last-mentioned case, the importance of ion migration as well as electron transfer has recently been emphasized. In the mixed-valent Tl(I)3Tl(III)Cl6, conductivity and isotopic exchange studies have been taken to indicate that cation transfer is the principal charge-carrying mechanism, and not electron transfer as such. " Mossbauer... 

Because STM measures a quantum-mechanical tunneling current, the tip must be within a few A of a conducting surface. Therefore any surface oxide or other contaminant will complicate operation under ambient conditions. Nevertheless, a great deal of work has been done in air, liquid, or at low temperatures on inert surfaces. Studies of adsorbed molecules on these surfaces (for example, liquid crystals on highly oriented, pyrolytic graphite ) have shown that STM is capable of even atomic resolution on organic materials. 

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