Synthesis of some 2,4- and 2,5-disubstituted thiophene systems and the effect of the pattern of substitution on the refractive indices, optical anisotropies, polarisabilities and order parameters in comparison with those of the parent biphenyl and dithienyl systems

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Abstract
The refractive indices, optical anisotropies, polarisabilities and order parameters of a number of phenylthiophenes and the parent dithienyl and biphenyl compounds with butylsulfanyl and cyano substituents have been measured. The presence of a thiophene ring leads to: (i) increased values of n∥ and n⊥ with the latter values normally being increased more so that the optical anisotropies are typically reduced, and (ii) increased molecular polarisabilities with the 2,5substituted thiophene compounds having higher values than the 2,4-substituted compounds. The dithienyl compound has the lowest order parameter and the highest molecular polarisability. The results are discussed in terms of the structural differences of the compounds, including differences in the pattern of substitution in the thiophene ring and in linearity, inter-annular torsion angle and the distribution of the permanent dipoles.

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