Dithiadiazolyl dimers

More recent findings show that 4-(2 -pyridyl)-l,2,3,5-dithiadiazolyl dimer 38 and 4-(2 -pyrimidyl)-1,2,3,5-dithia-diazolyl dimer 39 adopt similar coplanar arrangements to those already described (cf. compounds 14b, 15, 36, and 37)... 

Dithiadiazolyl dimers with exchange interactions 500 < 2J < 2000 cm have intramolecular diradicaloid behavior as a result of thermally accessible triplet state excitons, attributed to a weakening of the intradimer interaction (2006CC2277, 2007IC7436, 2007IC7756, 2008JCD4029). Rawson... 

The selenium analogue [PhCNSeSeN] and cyano-functionalized diselenadiazolyl radicals adopt cofacial dimeric structures, e.g., 11.4 (E = Se), with unequal Se Se interactions of ca. 3.15 and 3.35 A. In the latter case the radical dimers are linked together by electrostatic CN Se contacts.Tellurium analogues of dithiadiazolyl radicals (or the corresponding cations) are unknown, but calculations predict that the radical dimers, e.g., 11.4 (E = Te), will be more strongly associated than the sulfur or selenium analogues. ... 

Some of the most fascinating and promising radical systems at the present time incorporate lower row elements, such as dithiadiazolyl,23 dithiazolyl,24 and diselenolyl25 systems. These tend to dimerize in the solid state, but recent advances by multiple groups have found structural strategies to avoid (and sometimes26 utilize ) this behavior. 

The shape of the singly occupied molecular orbital (SOMO) supports the theory of head-to-tail interactions between two 1,3,2,4-dithiadiazolyl monomers to give the 1,2,3,5-dimer 11 via transition state 10 (Equation 2). 

Dithiadiazolyl radicals are of interest in the design of molecular conductors. Multifunctional radicals, for example, 1,3,5-C6H3 (CN2 82)3, have been prepared.Iodine doping increases the conductivity of these radical systems. Materials with unique magnetic properties are produced by introducing substituents that inhibit the dimerization process. ... 

A range of derivatives of 2 was examined as part of a systematic approach by Oakley. Planar derivatives, particularly those bearing functional groups, which favour in-plane interactions and the use of di- and tri-radical derivatives to favour lateral interactions, were successfully employed to generate n-stacked structures. However in all cases the systems were subject to a spin-Peierls or charge-density-wave (CDW) distortion of the structure. This distortion of the regular 71-stack opens up a band gap at the Fermi level [Figure 4(b)] and the conductivities of the pristine materials were poor, i.e. insulators or poor semiconductors. The inherently low conduetivity observed in these 7t-staeked dithiadiazolyl (DTDA) dimers arises in part from the band gap, but also... 

The stability of the 1,3,2,4-dithiadiazolyl radical is extremely sensitive to substituent and is susceptible to both thermal and photochemical isomerisation to the thermodynamically more stable 1,2,3,5-dithiadiazolyl radical (Section 12.1.1.1) via a bimolecular rearrangement." In a number of other instances the radicals appear indefinitely stable in dilute solution, but decomposition appears to occur at high concentrations. As a consequence few 1,3,2,4-dithiadiazolyl radicals have been isolated in the solid state. Of these the phenyl-ene-bridged fe/5(l,3,2,4-dithiadiazolyl) adopts a n -n dimeric structure in the solid state" while the mixed l,2,3,5-/l,3,2,4-fcw(dithiadiazolyl) has recently been reported (Figure 8)." This too adopts a n n bonding mode," although the dimers exhibit an unexpected behaviour with two sets of n -n interactions between 2 and 3 rather than the symmetric 2/2 and 3/3 dimer. 

Dimerization can be overcome by strategic substitution. A combination of steric and electronic modifications on the C4 aryl group has led to a small number of derivatives that are monomeric in the soHd state, e.g., dithiadiazolyl radicals 84—86 (Figure 11) and the a phase of radical 82 (2003AGE4782, 2004MOL771, 2010CEJ2741, 201IPRB104450). 

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