Quaterthiophene a-4T and Derivatives

The X-ray structure of a-4T single crystals grown from the vapor phase has been solved simultaneously by two groups during the course of this review [61, 62]. Nevertheless, data had been previously predicted on the basis of calculations by Gavezzotti and Filippini [52] and further refined by Porzio et al. from X-ray powder diffraetion data using the Rietveld full-profile analysis [53, 63]. 

Striking difference is that the two polymorphs have different packing modes, neighboring molecules being more or less staggered relative to each other. 

Ferro et al. recently used a molecular mechanics computational procedure to refine and understand the crystal structure of a-4T, a-6T and PT [84]. For both oligomers, their calculations maintain the essential features of the original structure proposed by Porzio et al. [53], i.e. the small deviations from co-planarity of the four rings and the lack of elements of molecular symmetry (centre or twofold axis). Both q-4T and a-6T have, in fact, a molecular centre of inversion which does not act on the crystal packing. 

The radical cation salts of a-4T(a-Me)2 can also be produced by electrochemical oxidation of the neutral oligomer in the presence of large size anions [85]. The use of an end-substituted oligomer prevents coupling reactions and affords the formation of black-needle crystals with anions such as AsF, TaFg and GaClJ. Nevertheless, 

System monoclinic monoclinic orthorhombic monoclinic triclinic monoclinic 

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