Effect of a structural modification of the bipyridinium core on the phase behaviour of viologen-based bistriflimide salts

We report on the synthesis and characterization of novel viologen-based low-melting salts of bistriflimide, namely 1,1'-di-n-alkyl-2,2',6,6'-tetramethyl-4,4'-bipyridinium, with lateral alkyl chains of 8, 10 and 12 carbon atoms. The thermotropic behaviour has been fully characterized by polarized optical microscopy, X-ray scattering and calorimetric measurements. Comparison with literature data shows that the increase of the effective volume of the rigid bipyridinium core, obtained by methylation, has a dramatic effect on the phase behaviour and on the transition temperature to the isotropic liquid. DFT calculations of the energy and structure of model clusters provide useful insights to interpret the experimental findings.