Structural studies of the intercalated smectic C phases formed by the non-symmetric α-(4-cyanobiphenyl-4′-yloxy)-ω-(4-alkylaniline- benzylidene-4′-oxy) alkane dimers using EPR spectroscopy

Abstract

Many examples of intercalated smectic C and A phases, exhibited by liquid crystal dimers, have been reported. While odd spacer dimers tend to form smectic C phases, even spacer dimers tend to give smectic A phases. The tendency to exhibit smectic C phases appears to be related to the bent nature of the dimer, in which the mesogenic groups are inclined to each other for the majority of the spacer conformations. The non-symmetric α-(4-cyanobiphenyl-4′-yloxy)-ω-(4-alkylanilinebenzylidene-4′-oxy)alkanes (CBOnO·m) do, however, exhibit intercalated smectic A and C phases, both formed by odd spacer dimers. As yet, no satisfactory explanation as to the difference between the intercalated smectic A and C phases, for odd spacer dimers, has been developed. Here, we report the results from angle-dependent EPR experiments and their analysis. Our results indicate that, within the intercalated smectic A phase, the mesogenic groups of the odd spacer dimers are, on average, parallel with the smectic layer normal. On entering the smectic C phase a tilt is observed which, for the CBO11O.6 dimer, is found to have a maximum value of 18°. A partial alignment of the tilt directions is also observed in the EPR magnetic field, which contrasts with the behaviour of the smectic C phase formed by monomeric liquid crystals. The results are compared with XRD studies on an aligned sample of the related dimer CBO9O.6.