We report the synthesis and characterization of dimeric viologen salts (10,10 0-(alkane-1,n-diyl)bis(1-ethyl-4,40-bipyridinium) with n = 4–10) with bis(trifluoromethanesulfonyl)amide (bistriflimide,Tf2N-) as a counteranion. For n = 4, 5 and 6, and for the nonylviologen cation (1,10-dinonyl-4,40-bipyridinium) we also prepared salts with the totally inorganic dodecatungstosilicate anion,SiW12O40(4-), featuring a poly-charged surface and nanosized dimensions. The materials have beencharacterized by means of calorimetric techniques, X-ray diffraction and solid state NMR and acomparison is made with analogous monomeric viologen salts exhibiting smectic mesophases.A strong odd–even effect is observed in the melting points and in the thermal behaviour of thebistriflimide dimeric systems, similar to what was reported for dipolar calamitic liquid crystaldimers, although the studied viologen dimers are not mesomorphic. By increasing the size of thecounteranion we have observed a destabilization of the crystal phases and of the mesophases infavour of a glassy amorphous state. Implications on the design of novel ionic liquid crystals arediscussed. The electrochemical behaviour in solution has been investigated by cyclic voltammetrymeasurements: interestingly, the odd–even effect is clearly visible also in the redox potentials. Thespin-pairing of the viologen radical cations formed at each end of the dimer is responsible for theobserved redox trend. Insights on the structure of the spin-paired dimer have been obtained byDFT calculations.
|Numero di pagine||8|
|Rivista||Physical Chemistry Chemical Physics|
|Stato di pubblicazione||Published - 2012|
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