Mesomorphic and electrooptical properties of viologens based on non-symmetric alkyl/polyfluoroalkyl functionalization and on an oxadiazolyl-extended bent core†
Two different sets of ionic liquid crystals based on bistriflimide salts of non-symmetrically substituted polyfluorinated bipyridinium (viologens) and bent symmetrically substituted dialkyl-oxadiazolyl-bipyridinium have been synthesized, in order to study the effect on the mesomorphic and electrooptical properties of the non-symmetric functionalization (alkyl chain and fluoroalkyl chains of different lengths) on the two pyridinium rings and additionally the effect of a bent conjugated spacer among the two pyridinium units of the viologen. POM and DSC characterization show that the synthesized salts have a mesomorphic and, in some cases, polymesomorphic behaviour in a wide thermal range, also encompassing room temperature. Some of the compounds exhibit an SmA phase in addition to more ordered smectic phases at lower temperature. The presence of a fluorinated chain on one side seems to generally increase the stability of the SmA phase of the ionic liquid crystal compared to alkylated analogues of viologens. Moreover, the insertion of the bent oxadiazolyl spacer between the two pyridinium units, has a significant effect on the mesophase behaviour leading to dendritic textures recalling that of banana phases. Electrochemical characterization by cyclic voltammetry shows that the presence of a fluorinated moiety causes an easier reduction compared to typical alkyl viologens while the oxadiazolyl-bipyridinium derivatives have more negative reduction potentials. Spectroelectrochemical experiments show that in contrast to classic viologens showing a typical electrochromic band of their radical cation, the oxadiazolyl insertion between the two pyridinium moieties hampers electrochromism due to absence of resonance coupling between the N redox centers. Interestingly, electrochromism of the polyfluorinated viologens, besides being observed in solution is observed in the ionic liquid crystal smectic phase of some of the salts of this series, upon radical cation formation the spectrum exhibits a further electrochromic band in the near infrared range which is not observed in solution.