Bifunctional dithia- and diselenadiazolyl radicals and their charge transfer salts

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George, Noel Andrew
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University of Guelph
Abstract

This thesis is an investigation of the use of neutral radicals to design molecular conductors. To this end a variety of bifunctional 1,2,3,5-dithiadiazolyl and 1,2,3,5-diselenadiazolyl diradicals (R(CN\sb2E\sb2)\sb2; E = S, Se) have been prepared with the intent of exploring their solid state structures and transport properties. These materials have also been combined with halogens to form a new class of charge-transfer (CT) salt. Phenylene bridged diradicals can be combined with halogens via solution and cosublimation methods to generate partially oxidized materials. Their crystal structures consist of stacks of bifunctional molecules interspersed with columns of halogens bridging the E-E bond of the CN\sb2E\sb2 (E = S,Se) rings. While these materials have significant lateral contacts and exhibit higher conductivities than their neutral radical counterparts, they are still susceptible to charge density wave driven distortions which result in a transition from metallic type conductivity to an insulating state. Efforts to improve inter- and intrastack contacts through replacement of the phenylene bridge with furan and pyridine leads to CT salts which exhibit alternative types of intermolecular contacts. In particular, the nitrogen of the pyridine ring competes with the halogen as a coordinating base, resulting in a solid state structure where each molecule possesses two CN\sb2S\sb2 rings with different oxidation states. Neutral dithiadiazolyl diradicals have a tendency to dimerize as discrete, non-stacking, dimers, or as molecular stacks linked through alternating long and short S-S contacts. Using substituted biphenyls which possess twisted phenyl rings, materials have been prepared in which the diradicals are prevented from cofacial dimerization. 4,4-biphenylbis(1,2,3,5-dithiadiazolyl) consists of perfectly superimposed, evenly spaced, stacks of diradicals, while the 2,2\sp′-dimethylbiphenyl bridged material shows an absence of stacking. However, in the latter case one end of each molecule is dimerized in a head-to-tail manner, an unprecedented mode of association for dithiadiazolyl radicals. Current projects focus on the preparation of molecular conductors based on dithiazolyl (C\sb2NS\sb2) rings with fused aromatic substituents. Quinoxaline dithiazolyl forms slipped stacks of molecules spaced 3.7 A apart, which coupled with weak intermolecular contacts, leads to a conductivity on the order of 10\sp−6 S cm\sp−1.

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Keywords
neutral radicals, molecular conductors, bifunctional, 1,2,3,5-dithiadiazolyl, 1,2,3,5-diselenadiazolyl, diradicals, solid state structure, transport properties
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